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Growth Rate o f an Invasive Apple Snail, Pomacea insularum (d'Orbigny) and its Potential Impact on Native Florida Ecosystems by Allegra Buyer A thesis Submitted to the Division of Natural Sciences New College of Florida In partial fu lfillment of the requirements for the degree Bachelor of Arts Under the sponsorship of Dr. Sandra Gilchrist Sarasota, FL April 2012
ii Acknowledgements: I would like to thank Dr. Gilchrist for helping me throughout this entire process and Dr. Weber, and Dr. McCord for being on my committee. I would also like to thank Dr. Cooper for the many hours spent assisting me with the statistical analysis of my results. Thank you Joel and everyone else at Pritzker for the support and advic e. I would also like to thank Dan Monhollon for his help collecting field data. Finally I would like to thank my friends and family for their support.
iii Table of Contents: Introduction............................................. ..........................1 1.1: Invasive Species...............................................1 1.2: Apple Snail Life History..................................3 1.3: Apple Snails as Exotic and Invasive Species...8 Methods................................ ............................................13 2.1: Collection and Hatching Methods................13 2.2: Density..........................................................15 2.3: Interaction.....................................................16 2.4: Fiel d Study....................................................18 Results..............................................................................21 3.1: Density Study................................................21 3.2: Interaction Study.............. .............................23 3.3: Field Study....................................................31 Discussion........................................................................33 4.1: The effect of density on P. insularum Growth Rates............ .......................................................33 4.2: Interaction between P. insularum and P. palu dosa.................................................................... 36 4.3: Field Study..................................................39 4.4 : Impact of P. insularum in the US...............40 Works Cited....................................................................44
iv List of Tables and Figures: Tables 1.1: Exotic Mollusks................................................ ....2 3.1: Surviving snails in Density Study......................21 3.2: Comparison of species in mixed tanks..............28 Figures Introduction.......................................................................1 1.1: Adult Apple Snail Shells.... ....................................3 1.2: Anatomy of Pomacea .............................................5 1.3: Egg clutches...........................................................6 1.4: Spread of P. insularum ........................................... 9 Methods............................................................................13 2.1: Map of airport pond.............................................13 2.2: Map of Celery Fields...........................................13 2.3: Location of Sarasot a............................................14 2.4: Hatching tank.......................................................14 2.5: Density set up ...................................................... 15 2.7: Interaction experiment............................. ............17 2.8 2.11: Oviposition sites..........................................19 2.12: Maturation of clutches.......................................20 Results..............................................................................21 3.1 3.2 De nsity study graphs......................................22 3.3 3.4: Single species growth rates ...................... ..... 25 3.5: Map of interaction experiment.............................26 3.6 3.7: Interaction study graphs ( P. insularum) .........27 3.8 3.9 : Sunlit tanks comparing species ...................... 29 3.10 3.11: Shaded tanks comparing species................30 3.12 3.13: Frequency of egg clutches.........................32
v Abstract: Pomacea insularum, an invasive apple snail species, has spread throughout Florida and other regions of the United States. It could potentially consume large amounts of vegetation and interact with the native Florida apple snail, Pomacea paludosa (Say). In this study, its growth rate under different densiti es was examined. Growth rates of juvenile P. insularum and P. paludosa were compared in separate and combined tanks. A possible lowered growth rate was noted for P. insularum at a higher density than for other species in the Pomacea genus. Pomacea insularu m grew faster than P. paludosa similar to other studies, but may have been limited by food resources. This species did not grow nearly as fast in tanks receiving less sunlight, which also had less algae and/or cyanobacteria. While P. insularum may be able to colonize certain areas faster than P. paludosa the level of predation could limit its spread. Long term studies need to be conducted in ecosystems that have both snails to determine the potential impact of P. insularum. Dr. Sandra Gilchrist Division of Natural Sciences
1 Chapter 1: Introduction 1.1: Invasive Species Invasive species are a worldwide problem, resulting in direct costs as high as $120 billion dollars a year (Pimentel et al., 2004). They have also contributed to the decline of native populations, and are on e of the largest causes of extinction (Clavero and Garcia Berthou, 2005). There are a number of different factors contributing to an being researched more thoroughly. Some of the most important factors, with supportive per season and in some studies number of seeds or eggs (Kolar and Lodge, 2001). In a comparative study of nati ve and invasive plants, most invasive plants did not outcompete the natives overall, but had more resources available or were found in altered habitat that helped them perform (Daehler, 2003). While ecologists have postulated many additional theories, suc h as high fecundity and growth rates, there are few quantitative data, especially available in numerous taxa with which to make generalizations (Strayer, 1999). Florida is a haven for many alien species because of its warm climate, allowing many species to thrive that cannot survive in many other locations in the United States (Pimentel et al., 2004). Many species come into Florida due to its position and the number of ports of entry; 85% of nonindigenous plants come through Miami (ISWG, 2002). Many inver tebrates have spread through ballast water on ships, and are also commonly introduced through the pet or aquaculture trades (Strayer, 1999). While plants are the most damaging and conspicuous problem, there are other organisms, like feral pigs and fruit fl ies, that have also caused damage, with hundreds of millions of dollars
2 spent on the control since 1980 (ISWG, 2002). Exotic invertebrates can have numerous effects on ecosystems (Table 1). Mollusks have been implicated in a reduction in water quality, lo ss of native populations, and habitat destruction (Strayer, 1999). In Florida, a more recently identified exotic species, Pomacea insularum also known as the island apple snail, has become established across the state, and may impact native plant and snai l species. Table 1: Exotic Mollusks found in the United States and their economic and environmental impacts. Species Range in USA Negative Effects Status in Florida or release? Perna viridis (Asian Green Mussel) West Coast of Florida as well as up to SC on East Coast 1 Clogs up power plants through fouling; may impact oyster fisheries and native mussel populations Established, but actual damaging effect has not been confirmed considered exotic 2 Lissachatina (or Achatina) fulica (Giant African Land Snai l) 3 Miami Dade County Can eat up to 500 plants, also destroys plastic and stucco; can carry meningitis 3 Population discovered in Miami, but not established statewide considered exotic Corbicula fluminea (Asian Clam) Located throughout eastern US 4 Biof ouling of water systems and pipes, causing millions of dollars in damage; also competes with native species Widespread throughout Florida considered exotic Marisa cornuarietis (Giant Rams Horn Snail) 5 Established in south Florida May actually be benef icial, has been used to control water hyacinth and hydrilla populations Exotic Melanoides tuberculatus (Red rimmed melania) 6 Established from Florida to Texas and found throughout US Biggest concern is the parasite it can carry; it has infected an endang ered fish, the fountain darter, in Texas 1: USGS, 2012a., 2: TBEP, 2011., 3: USDA, 2011., 4: Foster et al., 2012., 5: USGS, 2012b., 6: Benson, 2012.
3 1.2 Apple Snail Life History The Ampullaridae family consists of 120 species and ten different gen era (Cowie, 2002). This family contains the largest freshwater gastropods and is native to the Americas, Asia, and Africa (Cowie, 2002). However, they are also famous for their role as an invasive species in Asia. One species, Pomacea canaliculata Lamarck, has even been placed on the 100 Most Dangerous Invasives list (Lowe et al., 2000). The genus Pomacea also known as apple snails, is one of the most infamous, and is also native to the Americas. Pomacea paludosa the Florida apple snail, is the only one n ative to the United States, endemic to Florida, and is also found in a few other southern states. While P. paludosa is native to Florida, all other Pomacea spp. are from South America, generally the Amazonian regions of Brazil, Argentina, Paraguay, Uruguay and Bolivia (Hayes, et al., 2008). They live in warm, tropic environments, making Florida, Hawaii, and Texas the states with the largest invasion sites. The Department of Agriculture has also increased regulation of aquatic snail trade because of their threat, requiring permits for interstate trading and transportation (USDA, 2010). Figure 1.1: Adult apple snail shells: a. Pomacea diffusa. b. Pomacea haustrum c. Pomacea insularum d. Pomacea paludosa. Taken from Rawlings et al., 2007. Scale bar 5 cm.
4 There have been five identified established populations of non native apple snails in the US, four in the Pomacea genus and one in Marisa (Rawlings et al., 2007, Figure 1 .1). Pomacea canaliculata brought attention to apple snails beginning in the 1980's when it was introduced into Asia, and spread quickly. However, the study of apple snails has suffered from the inability to identify certain species properly, and P. canali culata was probably not the only species causing damage in Asia during that period, but part of group called the channeled apple snails (Hayes et al., 2008). Genetic analyses have made it easier for researchers to distinguish species, and in the process, m ore species have been identified. Pomacea insularum was only recently distinguished from P. canaliculata and is also considered a channeled apple snail (Rawlings et al., 2007). Because of this, the effects of P. insularum on non native habitats are unknow n, but possibly just as damaging as that of P. canaliculata. Pomacea lives only in freshwater habitats, and while types of habitat may vary slightly between species, many live in slow moving or stagnant water in places like wetlands, ponds, and canals (Co wie, 2002). Apple snails do have the ability survive drought periods by entering aestivation. Adult P. paludosa that have not yet reproduced has been shown to survive up to 12 weeks in dry areas with a survival rate of 70% (Darby et al., 2008). However, P. paludosa are affected by significant drought periods and population numbers can fluctuate greatly, with extremely low numbers during years of little water (Paul Gray, personal communication). Their habitat choices may depend on present vegetation and have been shown to prefer locations with stable emergent macrophytes on which to lay their eggs (Karunaratne et al., 2006). Apple snails are dioecious and oviparous (Cowie, 2002). Their life cycles are
5 poorly understood and many species may differ depending o n habitat and other factors. Pomacea paludosa lives up to one and a half years, reaching 40 to 70 mm in height at maturity, and 25 to 60 mm in diameter (Rawlings et al., 2007, Cowie, 2002). Sizes of apple snails, like P. canaliculata have been shown to va ry greatly in size, and habitat may play a role in their maximum size (Cowie, 2002). Pomacea insularum can reach 90 150 mm in length as adults (as reviewed in Cattau et al., 2007). They are known to bury in the mud, either during aestivation or as a defens e mechanism against predators (Snyder and Snyder, 1971). Pomacea spp. are amphibious, and have both gills and lungs ( Howells et al., 2006 Figure 1.2). They spend all of their time in the water except when they lay eggs, which are laid on vegetation or man made structures above the water (Burks et al., 2008). These eggs cannot be submerged or they will die. The number of eggs per clutch and the Figure 1.2: Anatomy of a male Pomacea Dorsal view with the mantle open and deflected. The gills and lungs are both shown. Taken from Fox, 2003.
6 number of clutches laid per season vary greatly. Pomacea paludosa lay one of the smallest clutches, with appro ximately 20 eggs per clutch (Cowie, 2002). Pomacea canaliculata lay about 200 300 eggs per clutch, and P. insularum lay on average over 2,000 eggs per clutch (Teo, 2004, Barnes et al., 2008). Hatching success of P. insularum has been calculated to be 70.8% in the field, although survival rates are probably much lower, potentially less than one percent (Barnes et al., 2008, Burlakova et al., 2010). The eggs of P. paludosa are much larger, as are the juvenile apple snails when they first hatch (A. Buyer, pers onal observation, Figure 1.3). While temperature has not been shown to affect the hatching rate, photoperiod may play a role, with eggs in less light having a lower hatching rate (Barnes et al., 2008). Eggs generally take 1 3 weeks to hatch (Burks et al., 2010). Figure 1.3: Images of egg clutches from five different apple snail species: a) P. haustrum b) P. diffusa c) P. canaliculata d) P. paludosa e) P. insularum. Taken from Rawlings et al., 2007. Scale bar: 5 cm The eggs are generally colorful, rangin g from red to pink (Cowie, 2002). Eggs lose their color as they mature (Rawlings et al., 2007). Apple snails have been observed
7 laying a clutch per week during the summer (Barnes et al., 2008). Florida apple snails lay from February November, and lay most of their eggs during the early summer months, April May or June (Darby et al., 2008). However, this can vary depending on hydroperiods; the peak laying period will occur later in areas that have later wet seasons (O'Hare, 2010). While it is thought that P insularum generally deposit their clutches during a similar timeframe, there have been no studies examining their peak laying season in any region. Pomacea spp. consume a number of different macrophytes and algae, although Morrison and Hay (2010) found that Utricularia spp. (bladderwort) is not only preferred, but when consumed more often, contributes to higher growth rates. In Lake Okeechobee, the consumption of assemblages of Utricularia and associated periphyton contributed to the highest growth rate s of P. paludosa followed by Eleocharis (Sharfstein and Steinman, 2001). Attempts to categorize plants food quality at the elemental level has not succeeded, but chlorophyll and phosphorous contents were correlated with higher snail growth rates (Sharfste in and Steinman, 2001). Pomacea insularum and P. canaliculata eat more than the native apple snails, which in turn also produces higher growth rates (Morrison and Hay, 2010). While it has been proposed that P. insularum could help control Hydrilla (an inv asive macrophyte) populations, densities of snails are unlikely to be high enough to actually make an impact and feeding on native species at similar levels also counteracts the effects of Hydrilla consumption (Baker et al., 2010). Apple snails have been o bserved feeding on dead organisms, other snails, or eggs; however, these observations generally lack concrete evidence (Cowie, 2002). Traditionally, the importance of the Florida apple snail has mostly been related to
8 its role as the almost sole prey of the snail kite. Apple snails are also food for ibis, alligators, turtles, fish, and raccoons (Snyder and Snyder, 1971; S. Gilchrist, personal observation). Many apple snail studies focus on its interaction with the snail kite, a Florida endangered subspeci es. The decline of the native apple snail has been noted in several studies, sometimes in conjunction with the growth of P. insularum and other times not (Corrao et al., 2006). While the native apple snail may be less available, snail kites have begun prey ing on P. insularum. The effects this may have are unknown, although scientists have begun studying the issue with mixed results. Cattau and colleagues (2010) found that juvenile kites especially had a harder time with larger invasive snails, with increase d amounts of dropped snails and longer handling times per snail, increasing the energy spent by the kites. Because starvation is a primary cause of death for snail kites, this could be a serious issue. However, other researchers think that an increased foo d supply for the snail kites may be beneficial (Paul Gray, personal communication). In Lake Toho, they have actually stopped attempting to eradicate Hydrilla because it serves as an important oviposition site for P. insularum and helps provide more food f or the kites (Riviera Lyles, 2010). Taro, an invasive macrophyte, is another oviposition site used by P. insularum and may be used at a higher proportion than other surfaces based on available sites (Burks et al., 2010). 1.3: Apple Snails as Exotic and Invasive Species Exotic apple snails were first noticed in Florida in the 1950's. Marisa cornuaretis L. was first m entioned scientifically in 1957 and was probably released through the pet trade (Rawlings et al., 2007). Channeled apple snails were discove red in 1978 in Palm
9 Beach County and while originally considered P. canaliculata now thought to have been P. haustrum which thus far has not become invasive, and while established in south Florida, has not expanded its range (Howells et al., 2006). Pomac ea canaliculata has established populations in California and Arizona, and there are reports of a possible population in northern Florida (Hardy, 2001, Howells et al., 2006). Pomacea insularum is c urrently thought to be the apple snail with the most potent ial impact o n Florida wetlands and may actually have only recently been established in Florida. The first official confirmation was in 2002, meaning that it has spread throughout Florida much quicker than originally anticipated Figure 1.4 (Rawlings et al. 2007). It is also found in Georgia Figure 1.4: The spread of Pomacea insularum in the southeastern United States. Not pictured is a small population in Arizona. Taken from Benson, A. J. 2011. and Texas where there is concern that rice growing regions may come under attack (Burlakova, 2010). Another concern is that snails in the United States matching Argentinean samples have been found, meaning that they may be more cold tolerant and could even survive frosts, allowing for a significantly expanded rang e as far north as South Carolina (Rawlings et al., 2007). Pomacea spp. has caused large amounts of
10 damage in Asia where apple snails have cost millions of dollars to rice farmers because of yield losses. Apple snails were brought to Asia in 1979 as a hi gh protein food source. However, they quickly escaped and spread through irrigation canals, infesting rice paddies and wetlands throughout many different countries. In Taiwan, where they were first introduced they spread throughout 155,444 ha in seven yea rs (Naylor, 1996). Their ability to spread and their voracious appetites for young rice seedlings led to extensive damage to agriculture systems. These systems were quickly invaded because highly altered and simpler ecosystems are easier to colonize (Burla kova et al., 2009). In the Philippines, yield losses were as high as 40% and at least 28 million dollars of damage in yield losses was done in 1990 (Naylor, 1996). While a lot of the focus has been on the agricultural damage because of the immense yield l osses Pomacea may also impact natural ecosystems. Carlsson and colleagues (2004) found that in wetlands with high P. canaliculata densities there was lower macrophyte biodiversity and plant coverage. Higher nitrogen and phosphorous levels were noted, and the higher levels of phytoplankton may trigger algal blooms. Pomacea could destroy wild rice strains, resulting in a loss of germplasm that could be valuable in the future (Naylor, 1996). Pomacea also carries nematodes and parasites, which have can infect humans and other mammals (Hollingsworth and Cowie, 2006). Right now, the potential impact of P. insularum on Florida ecosystems is unknown While Pomacea has caused extensive damage in Asian agricultural systems, this has mainly been attributed to P. ca naliculata (Rawlings et al, 2007, Hayes et al., 2008) However, P. insularum was only recently identified as a separate species, and has
11 been observed in many of these same agricultural systems It is also possible that apple snails are much more damaging to agricultural systems than to native wetlands, and the rice growing regions of Florida are certainly not as expansive as those in Asia While there are rice growing regions in the United States, the crop rotation and drainage methods may impede the abili ty of P. insularum to colonize these agricultural areas (Burlakova et al., 2010). Apple snails may perform better in altered ecosystems with little predators, whereas Florida has a number of natural predators due to the population of native apple snails. U nfortunately, P. insularum has a higher fecundity than P. canaliculata which has been shown to damage Asian natural wetlands under high densities (Burlakova et al., 2009). These high densities may also cause algal blooms in freshwater systems, which alre ady occur in Florida in saline environments, because of increased macrophyte consumption and increased nitrogen and phosphorous levels (Burlakova et al., 2009, Carlsson et al., 2004). Because t he spread of P. insularum may be quite recent and occurring at a more rapid rate than originally thought, densities may be higher in another decade or two. If densities and frequency of P. insularum continue to increase, they may be able to cause extensive damage by this time (Rawlings et al., 2007). It is also unknow n what impacts the P. insularum may have on native apple snail populations. Connor and colleages (2008) found the adult exotic P. canaliculata can slow the growth rate of P. paludosa which may hurt their reproductive abilities In the current experiment t he effect of density on growth rate of P. insularum is measured in the lab. Also, the effects of interaction between juvenile P. insularum and P. paludosa on growth rate are measured T he time frame of reproduction was also observed in the field by
12 exam ining the number of egg clutches deposited towards the end and beginning of their reproductive cycles Two different ponds were observed; one had only P. paludosa and the other only had P. insularum.
13 Chapter 2: Methods 2.1 Collection and Hatchin g Methods Apple snail eggs were collected from two different locations in Sarasota and Lakewood Ranch respectively. O ne pond (a retention pond next to the Sarasota Bradenton Airport off of University Blvd.) contain ed P. paludosa (Figure 2.1), and the oth er (at Celery Fields) had P. insularum (Figure 2.2) Egg clutches were taken from Figure 2.1: On the left is a view of the area surrounding the New College Campus a nd airport, with the pond circled, and the right a close up view of the pond with P. paludosa. A scale is in the bottom left corner and a compass in the top right corner. Taken with Google Earth. Figure 2.2: On the left is a view of the area surrounding C elery Fields, with the pond circled, and the right a close up view of the pond with P. insularum. A scale is in the bottom left corner and a compass in the top right corner. Taken with Google Earth.
14 vegetation by cut ting below the clutch, so they could be moved without damaging the eggs. The clutches were taken to Pritzker Marine Labor atory on the New Colle ge campus, Sarasota, Florida. The location of Sarasota is shown in Figure 2.3. E ggs were placed on crate materials above 56.78 liters of water in two 189.27 gallon tubs for hatching (Figure 2.4). The water was filtered by reverse osmosis (RO) and adjusted for specific gravity, with nine g of sodium chloride for every 18.92 liters of Figure 2.3: Map of Florida with the location of Sarasota labeled. Figure 2.4: Pomacea paludosa eggs above the hatching tank.
15 water. This recipe is used for other freshwater snails at the Pritzker Marine Biology Research Center. There was no filtration, just an airline tube placed in each tan k for aeration. Each hatching tank had calcified rock as well to raise the pH to approximately 8 After hatching, the snails fed on algae in the tanks for 14 days and then were fed organic spinach ad libitum Because these were underneath Pritzker, the li ght cycle and temperature were consistent with outdoor conditions. 2. 2: Density This experiment, as well as the competition experiment, was adapted from Conner and colleagues (2008). This study measured the effects of density on Pomacea insularum S nails were placed in four different treatments, with 2, 4, 8, or 16 snails per 9.4 liter tank, and three tanks per treatment, for a total of 90 snails. This amounts to 10, 20, 40, and 80 snails/m 2 Snails were measured at the time they were placed in the t ank, and every two weeks thereafter for a total of 8 measurements. At the time of original measurement, the Figure 2.5: Set up of the density study.
16 snails were approximately 18 days old. The snails were placed in tanks based on their size, with efforts made to keep the average starting size for each tank similar. The operculum width s w ere recorded to the closest tenth of a millimeter and weight in grams. The operculum width s and weight were recorded because it has been shown to be most accurate indicator of size (Youens and Burks, 2007). Water w as added to the tanks three times a week to account for evaporation W ater changes were made every two to three weeks or as needed. Dead snails and empty shells were removed and no new snails were added to the tanks. The average operculum width and weight of snails was calculated for each tank at each time period. These were graphed, as well as compared statistically. For the final measurements, the Mann Whitney U test was performed to determine difference in size by the end of the experiment. 2. 3: Inter action Snail operculum width and weight were recorded, and then each snail wa s marked with fingernail polish so that each snail could be identified separately per tank (Figure Figure 2.6: Four snails with nail polish. Six different colors were used, up to comb inations of two colors.
17 2.6). There were ten 189.27 liter tanks set up with 20 snails per tank. Two tanks contained P. paludosa, and two P. insularum. The remaining six tanks (50/50 treatment) had 10 snails of each species. The different treatment tanks were placed randomly, in case the position of the tanks had any effect on growth rate. The tanks were set up with tap water instead of RO water and sat for two weeks to allow for dechlorination Tanks also contained pond water solution ( 9 g of salt for 5 L ) and calcified rock to raise the pH. The tanks were pla ced under the building outdoors so the t emperature varied and was more or less consistent with the outside temperature. There was an airline and an airstone to provide aeration, but no filtration system Water changes were performed using tap water as needed to keep the water clean. Every two w eeks 10 snails from each tank were randomly chosen and measured, Figure 2.7: The interaction experiment tanks placed under the Pritzker Marine Laboratory.
18 so in the 50/50 treatment tanks, five of each species were chosen. Snails were chosen based on their location in the tank by splitting the tank into a numbered grid, and removing snails in th e numbered squares, which were randomized by http://randomizer.org/form.htm The snail polish did not always adhere to the snails, and Pomacea insularum grew so quickly that markings disappeared. E very effort was made to keep marking them. These snails wer e fed organic spinach ad libitum. Sp inach was placed at the bottom of the tank later in the experiment, when it was noticed that mostly natives were feeding on s pinach leaves floating on the surface Also, through discussion with another researcher, some t hink that native appl e snails reside in shallower regions of aquatic systems while the invasive snails live deeper, so every effort was made to be sure that all of the snails would have access to food. The average operculum width and weight was calculated separately for each species in all tanks for every time period. These were graphed, as well as compared statistically. For the first and final measurements, the Mann Whitney U test was performed to determine difference in size between species and tanks by the end of the experiment. Growth rates of P. insularum in t anks 5 and 7 were also compared over time using 2 way repeated measures ANOVA. 2.4 Field Study: A field study was also carried out to determine the period during which each species deposit thei r eggs. Starting the first week of November, five different sites at both ponds where it was known that the apple snails have already laid clutches were chosen to observe. Some of the sites had vegetation and others had man made structures. The water tempe ra ture, pH, and air temperature were recorded for the pond each week, and the
19 number of clutches at each site was recorded. Four clumps of vegetation were surveyed a t the airport pond with P. paludosa along with one large concrete structure (Figures 2.8 a nd 2.9). At the Celery Fields pond for P. insularum three concrete structures, one metal frame, and a small tree were surveyed (Figures 2.10 and 2.11). The depth at each site was recorded once. The general age of the clutch was noted, to d etermine if new clutches were still being laid. This was an estimate, based on the color of the eggs ( the eggs mature from pink to white see Figure 2.12) and whether Figure 2.10: A tire was one of the surveyed s ites at Celery Fields where the P. insularum deposited their clutches. Figure 2.11: This tree was another site surveyed at Celery Fields where the P. insularum deposited their clutches. Figure 2.7: This concrete structure was one site at the airport pond where many P. paludosa eggs were deposited. Figure 2.9: These are t he four clusters of vegetation at the airport pond where P. paludosa eggs deposited their eggs. Figure 2.8: This concrete structure was one site at the airport pond where many P. paludosa eggs were deposited.
20 eggs had begun to hatch The number of clutches between sites was also recorded, just to keep track of where and when they were deposited. Observations stopped mid December, and started again at the beginning of February. Figure 2.12: The age of P. paludosa eggs can be shown through the co lor change from pink to white. From Rawlings et al., 2007. Scale bar: 5 cm.
21 Chapter 3: Results 3.1 Density Study: By the end of the study, most tanks had lost a few snai ls through escape or death (Table 3.1). The tanks with 2 snails did not initially lose any subjects; however, one of the tanks wit h 4 snails lost three subjects. Animals in tanks of the same density did not generally grow at the same rate, possibly due to the loss of snails at different times during the experiment While snails in tanks with lower densities appeared larger, there was little significance to support the idea (Figures 3.1 and 3. 2). By Time 8, animals in tank 2 (20 snails/ m 2 ) were significan tly larger in both weight and width than all of the tanks with a density of 80 snails/ m 2 2 2 = 6.3224 and 2 2 = 6.3224 and p < .0119 for wid 2 2 = 6.3224 and p < .0119 for width), and one tank with a density of 40 snails/m 2 2 = 6.3224 and p < .0119 for 2 = 6.3224 and p < .0119 for width). By Time 8, animals in tank 7 (40 snail s/ m 2 ) were significantly larger than those in tank 4 (80 snails/ m 2 ) in both width and weight 2 2 = 6.3224 and p < .0119 for width). Tank Number # of Snails At Time 1 # of Snails at Time 4 # of Snails at Time 8 1 2 2 2 2 4 3 3 3 8 7 7 4 16 14 13 5 2 1 1 6 4 3 1 7 8 8 6 8 16 15 13 9 2 2 2 10 4 1 1 11 8 8 8 12 16 10 9 Table 3.1: The number of snails in each tank at times 1, 4, and 8.
22 Figure 3.1: The average operculum width of the snails in each tank for each time period P. insularum was measu red in the density experiment. Tanks 5, 6, and 10 were excluded because there was only one snail in each tank. Figure 3.2: The average weight of the animals in each tank for each time period P. insularum was measured in the density experiment. Tanks 5, 6, and 10 were excluded because there was only one snail in each tank.
23 For the tanks with only two snails each, 1 and 9 grew at the same rate in terms of weight, but 5 did not, possibly beca use the other snail died before a second measurement could be taken. The snail in tank 5 was the second largest by the end of the experiment, 11.1 mm wide and weighing 1.21 g. In the tanks with four snails, only tank 2 had three snails by the end, the othe r two only had one. Tank 6 was down to one snail by the fifth week, whereas tank 10 only had one snail left by the third week. The snail in tank 10 was the largest snail overall, and the snail in tank 6 was larger than the means of all of the other tanks e xcept 5 and 10. Tanks 3, 7, and 11 each had eight snails, and by the end they had 7, 6, and 8 snails respectively. Tank 3 was down to seven snails by the second measuring period, and Tank 7 had eight snails until the seventh measuring period, when two es caped from the tank. After losing those two, the growth rates in the tank rose considerably, leading to the remaining occupants having significantly larger size. Their growth rates differ ed and could not be combined. Tanks 4, 8, and 12 lost many of their s nails, but by time 8, the average weight and width within each tank were similar. Tanks 4 and 8 each had thirteen snails and Tank 12 only had ten. These tanks also had more snails that escaped the tanks. The mean operculum width and weight standard deviati on was 1 for snails in most tanks at time 8, but tanks 1, 2, and 9 were much higher, probably due to the low number of individuals. Tanks 2 and 9 consistently had higher standard deviations than the other tanks for the animal's mean width and weight by t ime 4, but the animals in tank 1 did not have a higher standard deviation until time 6. 3.2 Interaction Study: Pomacea insularum grew faster and larger than P. paludosa in the tanks with only
24 one species of snail. Only one of the P. paludosa tanks (Tank 8) had a positive weight growth rate over time, the animals in Tank 1 did not Animals in the P. insularum tanks (Tanks 5 and 7) grew quickly and at similar rates with no significant difference between their growth rates, using the 2 way ANOVA Repeated Mea sures test (p < .1500). However, only animals ( P. insularum ) in tank 5 were a significantly larger size by the last measurement, according to both width and weight; tank 7 was only significantly larger in terms of weight, not width (Figures 3.3 and 3.4). U sing th = .05, the Mann Whitney U test indicates that there was a significant difference among P. insularum in tank 5 and P. paludosa in tank 2 = 2 = 10.337 5 an d p < .0013, respectively). Pomacea insularum operculum width in tank 7 was not significantly different from P. paludosa in tank 8 (p < 2 = 6.3224 and p < .0119).
25 Figure 3.3: The average weight of the single species tanks from the interaction study, P. insularum in 5 and 7 and P. paludosa in 8. Figure 3.4: The average operculum width of the singl e species tanks from the interaction study, P. insularum in 5 and 7 and P. paludosa in 8.
26 T anks 6 10 received more sunlight and had more a lg ae and/or cyanobacteria growth. The growth rates of P. insularum in the mixed tanks differed significantly between the tanks with more and less sunlight. Interestingly, there was no difference between animals in tanks 5 and 7, even though 7 clearly had mor e access to sunlight (Figure 3.5). In the mixed tanks, there was a significant difference between the growth rates of P. insularum in tanks 2, 3, and 4 combined and tanks 6, 9, and 10 (Figures 3.6 and 3.7). Using the Mann Whitney U test, animals tanks 2, 3 and 4 were much smaller than those in tanks 6, 9, and 10 (p < .0001), which grew to be the largest and also had a steeper growth rate curve. Animals in tanks 5 and 7 were in the middle, and were significantly larger than those in tanks 2, 3, and 4 (p < 0003), but significantly smaller than animals in tanks 6, 9, and 10 ( p < .0001). The natives in each tank grew at similar rates, and there was no pattern between the growth rates of animals in different tanks. Figure 3.5: Map of the tanks in the interaction study. 50/50 refers to tanks that had both P. insularum and P. paludosa. The line at the top is the fence, so tanks 6 10 had more access to sunlight.
27 Figure 3.6: The average operculum width of P. insularum only across all tanks a nd time periods in the interaction study. Green represents the snails in mixed tanks receiving more sunlight, blue the P. insularum only tanks, and red the tanks that did not receive as much light. Figure 3.7: The average weight in grams of P. insularum only across all tanks and time periods in the interaction study. Green represents the snails in mixed tanks receiving more sunlight, blue the P. insularum only tanks, and red the tanks that did not receive as much light.
28 In the mixed tanks 6, 9, and 10, there was a significant difference between the sizes of P. insularum and P. paludosa at the first and the final measurement (Figures 3.8 and 3.9). Pomacea insularum was significantly smaller at the first measurement, a nd significantly lar Mann Whitney U test indicates that in tank 6 at time 1, P. insularum was significantly 2 2 = 14.2857 and p < .0002, respect ively). Pomacea insularum was also significantly smaller in tanks 9 and 10 at time 2 2 = 14.2965 and p < 2 2 = 12.6229 and p < .0004, respectively). U Whitney U test indicate that in tank 6 at time 6, P. insularum was significantly larger in terms of width and weight 2 2 = 9.375 and p < .0022, respectively). Pomacea i nsularum was also significantly larger in tanks 9 and 10 at time 6, according to both 2 2 2 = 7.7419 and 2 = 7.5469 and p < .0060, respectively). In tanks 2, 3, and 4, P. insularum did not grow larger than P. paludosa although between times five and six, their growth rate increased greatly and were approaching a similar size by the last measurement (Table 3.2, Figures 3.10 and 3.11). They were consistently much smaller unt il that last measurement however. Avg. Weight at Time 6 in mg (Std. Dev.) Avg. Operculum Width at Time 6 in mm (Std. Dev.) Tank #: P. insularum P. paludosa P. insularum P. paludosa 2 110 (.086) 140 (0.064) 5.32 (1.950) 6.72 (0.857) 3 100 (.017) 170 (0 .119) 5.85 (0.212) 6.92 (1.444) 4 200 (.003) 120 (0.036) 7.4 (0.141) 6.5 (0.809) Table 3.2: A comparison of the av erage sizes of P. insularum and P. paludosa in the mixed tanks 2, 3, and 4, which received less sunlight at time 6.
29 Figure 3.9: The average width of P. insularum (Inv) and P. paludosa (Nat) in the tanks receiving more sunlight, 6, 9, and 10 in the i nteraction study. The decrease in size of Nat10 is due to the random sampling and larger snails being chosen in earlier week. Figure 3.8: The average weight of P. insularum (Inv) and P. paludosa (Nat) in the tanks receiving more sunlight, 6, 9, and 10 in the inter action study. The decrease in size of Nat10 is due to the random sampling and larger snails being chosen in earlier week.
30 Figure 3.10: The average operculum width of P. insularum (Inv) and P. paludosa (Nat) in the tanks receiving less sunlight, 2, 3, and 4 in the interaction study. Figure 3.11: The average weight of P. insularum (Inv) and P. paludosa (Nat) in the tanks receiving more sunlight, 6, 9, and 10 in the interaction study.
31 3.3 Field Study: In the two ponds surveyed, P. insularum deposited eggs much later into the year than did P. paludosa. There was an increase i n clutches throughout November at Celery Fields pond, but declined by mid December (Figure 3.12). T here were only six newly deposited clutches by P. paludosa at the airport pond at the beginning of November and by mid November there were no new clutches ( Figure 3.13). However, there were more overall clutches at the airport pond (Figures 2.7 and 2.8). C lutches were laid in more places at Celery Fields. Pomacea insularum laid eggs on trees, short vegetation, rubber and concrete surfaces (Figures 2.9 and 2 .10). Pomacea paludosa only laid eggs on the five protruding spots, the large concrete structure and the very tall vegetation in the middle of the pond. While there was similar short vegetation in the airport pond, P. paludosa did not deposit any eggs on t hose plants. The ponds had different overall layouts as well, and P. paludosa laid their eggs over much deeper water than P. insularum. S ome of the clutches deposited by P. insularum were over areas that were dry at the time they were surveyed. The water t emperature was also generally slightly higher at the airport pond, with an average difference of 2.42 C
32 Figure 3.12: The number of P. insularum egg clutches at Celery Fields pond, separated by whether they are old or new. For a description of how that was determined see Figure 2.11 in Chapter 2. The first five dates are from November December, and the last three are February March. Figure 3.13: The number of P. paludosa egg clutches at the airport pond, separated by whether they are old or new. The first five dates are from November December, and the last three are February March.
33 Chapter 4: Discussion 4.1: The Effect of Density on Pomacea insularum Growth Rates Collected data showed a general pattern of large r growth of snails in tanks with a lower density. This is similar to other research on snail density, which has shown that apple snails below a certain density will have higher growth rates, but at higher densities, the growth rate will stabilize at a lowe r growth rate (Alves et al., 2006, Conner et al., 2008). High snail mortalities and some escapes made it difficult to analyze the results from the current study; however, a pattern does emerge of a certain density where growth is decreased, potentially whe n there were 40 snails m 2 This experiment should be replicated with many more tanks to produce more accurate results. A wide range of densities has been reported for gastropods and the difference may be due to different life history strategies. The m arine snail, Haliotus tuberculata L. has significant growth rates at densities above 2,500 m 2 and a density of 200 m 2 has been suggested for optimal growth in aquaculture (Mgaya and Mercer, 1995). The optimal growth of the terrestrial snail Helix aspers a M ller is around 133 m 2 and may even be commercially profitable at densities up to 500 m 2 (as reviewed in Dupont Nivel et al., 2000). All of the Pomacea data to date show much lower optimal growth rates, demonstrating that individuals may need large s pace to succeed (Conner et al., 2008). Connor and colleagues (2008) found that P. paludosa growth rate was significantly lower at densities of 16 m 2 but that there was no significant difference between growth rates at densities of 16 m 2 32 m 2 and 64 m 2 Pomacea canaliculata experiences lower growth rates even at a density of 2 m 2 so this experiment may not have had low enough densities (Tanaka et al., 1999). The data produced may show that P.
34 insularum is not affected by density until the density is much higher than that for P. paludosa. This could be important for a number of life history strategies. Species theoretically reproduce at rates that maximize their fitness (Lack, 1947; Both et al., 2000). The small size of neonate P. insularum may mak e them more vulnerable to certain pressures such as dry downs (Darby et al., 2008), cold weather (as reviewed in Tanaka, 1999), and predators (Burks, 2005; Carlsson, 2006), so a larger number of eggs may be necessary for their survival. Burlakova and colle agues (2010) estimated that only 0.4 to 0.7% of juvenile P. insularum survive and the survival rates of P. paludosa are unknown in the wild. However, in laboratory conditions P. insularum did have a higher survival rate (Morrison and Hay, 2010). In the cur rent study there appeared to be little difference in survival rates, although fewer P. insularum individuals survived in the tanks receiving less sunlight. Higher adult density may need more eggs as well. Pomacea canaliculata females individually lay fewer eggs if the adult density is higher (Tanaka et al., 1999). The relationship between density and clutch size is variable, and while some studies have shown that natural clutch size does maximize fitness, others have not (Both et al., 2000). Most studies on the impact of clutch sizes have been conducted on birds, although the number of studies on invertebrates has increased (Godfray et al. 1991). If P. insularum is able to have higher densities before growth rate is affected, this may also give them an adv antage in the wild, especially if densities are high due to two species occupying a pond. However, this may also make both snails more susceptible to predation. At lower densities, higher proportions of P. canaliculata will bury in the substrate in the pr esence of predators, but when there are higher densities and resources are less available, the snails are less likely to bury (Yoshie and Yusa, 2011). Many bird
35 species have low clutch sizes because higher numbers of young will attract more predators, lead ing to increased rates of predation (Godfray et al., 1991). While this does not apply as much to invertebrates, the large number of apple snail predators in Florida may be attracted by the large clutch sizes of P. insularum. Juvenile apple snails also eat more than adults per individual mass, and a larger density of juvenile P. insularum could result in even more vegetation consumption, although the variety of resources consumed may vary over time (Boland et al., 2008). With increased numbers of P. insularu m the growth rate of P. paludosa may be affected (Conner et al., 2008). Smaller P. canaliculata eat more than larger individuals, so smaller P. paludosa may also eat more (Carlsson and Bronmark, 2006). Lower growth rates have been shown to impact a number of biological functions, including a smaller adult size, which may lead to smaller eggs and a lower hatching rate (Tanaka et al., 1999). While the effect of higher density may be more extreme in Pomacea and may limit their population numbers due to the p ossible desire for more space or resources, this is certainly not true in agricultural systems. This highly modified habitat may allow for higher densities because of less predation and more food sources. The density of P. insularum was found to be as high as 130 m 2 and the juvenile survival rate was up to 10 times higher in these ecosystems (Burlakova et al., 2010). Burlakova and colleagues (2010) also found that juveniles were rare and the density was stable throughout the year. In the current experime nt, the snails began to escape the tanks, especially at higher densities, which may have been due to a need for more space. This movement was also noticed at the airport pond. Adult P. paludosa individuals had crawled out of the pond and died, but there wa s no sign of predation. The density of P. paludosa in the field is
36 generally less than 1 m 2 and it is unknown how that may interact with a higher P. insularum density rate. Laboratory experiments have also shown that with the proper diet with greatly inc reased protein, P. paludosa can be cultured at much higher densities, and the lower growth rates are limited to the first month (Garr et al., 2011). 4.2: Interaction between Pomacea paludosa and Pomacea insularum There were a few interesting results from the comparison of growth rates between juvenile P. insularum and P. paludosa While originally expecting to see a slower growth rate of the Florida apple snail in the mixed tanks, there was no difference in the growth rate of P. paludosa in any of the tan ks, with the exception of Tank 1, which did not have significant growth over time and is probably an anomaly. The population and density of the tanks seemed to have no effect on their growth rates, which may be good for Florida apple snail populations faci ng new invasions. However, because juvenile Florida apple snails have been shown to have lower growth rates with adult snails in the tank, and even lower rates with P. insularum adults, it may be that their growth rates would have decreased as the island a pple snails grew larger (Conner et al., 2008). Another issue may be that in the wild P. insularum clutches are much larger, and it is very likely that when they hatch in the vicinity of P. paludosa, the Florida apple snails would be vastly outnumbered and may face diminished food sources. Interestingly, P. insularum had very different growth rates between the tanks closest to the fence and further away. The most likely explanation for this difference is that the tanks closest to the fence, tanks 6 10, rec eived more sunlight and had higher levels of algae and/or cyanobacteria growth. Because neonate P. insularum are much
37 smaller than P. paludosa a more readily available food source may be important to their growth after hatching. In another study (Boland e t al., 2007), juvenile P. insularum ate more food when there was periphyton present, so periphyton may be an important part of their diet. The lack of algae/cyanobacteria did not affect the growth of the Florida apple snail, which may show that their grow th rates are less susceptible to food availability or that the periphyton grown in the sunlit tanks was not a food source, keeping their growth rates similar. Daehler (2003) noted that in many cases, native plants could outperform invasive plants in certai n situations such as low resource availability, so it is possible that the Florida apple snail may be better adapted for circumstances like this. Spinach was used in this experiment because it contains more vitamins and minerals than lettuce, which has bee n used in other studies (Nutrition Facts, 2012). However, because vegetation structure has been shown to be important for consumption rates, it is also possible that the spinach was harder for P. insularum to eat (Boland et al., 2007). Some researchers h ave proposed that Florida apple snails need large amounts of spacious habitat, so it may also be possible that they were able to grow faster in the tanks without as much sunlight because less island apple snails survived (Karunaratne et al., 2006, Conner e t al., 2008). However, they grew even faster in tanks with P. paludosa than in the tanks where they were held separately may show that under the right conditions, sharing space with the Florida apple snail especially at the beginning of the invasion may ev en enhance their growth rates in the wild, and due to their larger size, impact P. paludosa growth rates later. These enhanced growth rates may also allow P. insularum to reach sizes inhibiting some predation at an earlier age, which could lead to higher s urvival rates (Carlsson, 2006; Burks, 2005).
38 Comparing the growth rates between the species showed the island apple snail grew faster overall which has been shown in other studies (Morrison and Hay, 2010). While small amounts of calcium can result in low er shell strength (Glass and Darby, 2009), a small number of P. insularum shells from the interaction study at different sizes were crushed with a vice. No difference in crush weight noted, so higher growth rates may not affect shell strength when calcium is not limited. In the tanks receiving more sunlight, the island apple snails outgrew the Florida apple snail by the fourth measuring period. In the tanks further away, the island apple snails were approaching the same size as the Florida apple snails by the end of the experiment, but there were so few remaining that the lower density may have also increased growth rates. Garr and colleagues (2011) found that P. paludosa grew at lower rates at high densities for the first month, but in subsequent months the growth rates leveled out and all snails reached similar sizes, which may explain what occurred with the surviving P. insularum in low light conditions. Pomacea insularum also grew faster than P. paludosa when they were kept in separate tanks, and other studies have shown that an adult island apple snail can grow to weigh four times larger than Florida apple snails (Conner et al., 2008). Increase in size could result in increased vegetation consumption, which could certainly affect native ecosystems (Mor rison and Hay, 2010). However, Carlsson and Bronmark (2006) showed that smaller snails will actually eat more per weight, so the difference in size may not be quite as important. The increased fecundity of P. insularum may impact the ecosystem more, even i f not all the juveniles survive. 4.3: Field Study
39 Preliminary field data suggest the P. paludosa begins to breed earlier in the year, but also stops sooner than P. insularum. Burlakova and colleagues (2010) also found that P. insularum did not begin depo siting eggs until March, although that study occurred in Texas. Due to the potential effect of limited food on P. insularum the earlier hatching of P. paludosa young could allow them to consume more of the much needed food, limiting the growth of P. insul arum. This study did show that they may have slightly different diets, especially at the beginning, so they may not compete for the same resources. The placement of eggs also differed between the two species. Pomacea paludosa appeared to deposit eggs in s lightly more strategic places, whereas some of the clutches laid by P. insularum were carelessly placed. Pomacea insularum clutches were laid on vegetation that did not emerge more than a centimeter or two from the water or even over dry land where the wat er may have been when the pond level was higher. Clutches laid by P. insularum in Texas were shown to be preferentially laid on wild taro plants, and shorter plants such as alligator weed were avoided (Burks et al., 2010). In this study, concrete and other artificial substrates were the more favored places to lay eggs, but this may be because it was one of the best places to deposit eggs in these ponds. Predators were noted at both sites, including turtles, ducks, and herons. Studies in the lab have shown that juvenile P. insularum have a higher survival rate than P. paludosa (Morrison and Hay, 2010). However, this may not be the case in the wild, especially considering that P. paludosa does not appear to necessarily have a smaller density in natural enviro nments, has a shorter lifespan and deposits fewer eggs per clutch than P. insularum (Cowie, 2002) While P. canaliculata and P. insularum deposit multiple clutches per season, P. paludosa generally dies after reproduction
40 (Barnes et al., 2008, as reviewed in Darby et al., 2008). The higher survival rates of P. paludosa in what may have been low resource conditions in the interaction study may also provide evidence for a higher juvenile survival rate. Laying eggs earlier in the season would give adults more time to mature earlier and begin to reproduce during the peak laying season, allowing their young to survive the winter (as reviewed in Tanaka, 1999; O'Hare, 2011). This study should be conducted with numerous ponds and a wider variety of conditions to sh ow significant differences. 4.4: Impact of P. insularum in the United States One of the limiting factors on P. insularum in Florida may be the amount of natural predators. While some predators, such as the redear sunfish and potentially juvenile snail k ites, may not be able to eat larger snails (Burks, 2005, Cattau et al., 2010), others like adult snail kites and alligators are able to consume snails of all sizes (Cattau et al., 2010, Snyder and Snyder, 1971). If P. insularum does spread as far north as South Carolina, factors such as predation may not have the same impact. The success of channeled apple snails in Asia may also be partially due to less predation, especially because of high fishing and hunting rates (Carlsson, 2006). This could have unfor tunate consequences for states farther north, where some important apple snail predators, like snails kites and limpkins do not commonly live (ABA, 2012a and 2012b). However, there are other predators such as alligators, raccoons and other potential predat ors like fish (SREL, 2012). Invasive species can impact an environment more when they take up a role in the ecosystem that did not previously exist (Parker et al., 1999). Areas outside of Florida with established populations do not have apple snails except in springs, so they
41 may have an extremely detrimental effect on those ecosystems (Howells et al., 2006). An issue not only in Florida, but in the rest of the United States and the world has been the destruction or degradation of wetlands (Mitch and Gosse link, 2007). As wetlands are restored, dynamics may be changed (Zedler and Kercher, 2005). Pomacea paludosa has shown lower reproductive rates in restored wetlands with mid hydroperiods (O'Hare, 2010). This correlates with the other studies that have demon strated invertebrate's difficulty in colonizing restored habitat, especially gastropods, due to lower dispersal ability and inappropriate habitat (Lehtinen and Galatowitsch, 2001; Armitage et al., 2004). However, as the habitat improves, restored habitats may provide better substrate and allow for increased densities of snails (Spelke et al., 1995; Santiago and Judith, 2008). If P. insularum can grow faster at higher densities, it may be able to reproduce and colonize restored wetlands quickly, as long as t hey are not limited by resources. Because predation is one of the main limiting factors, there may be higher survival rates in restored areas, at least at the beginning of an invasion, similar to agricultural scenarios (Carlsson, 2006). Predators such as w ading birds have been shown to be attracted to the higher densities of snails, so in many cases the primary invasion by apple snails may balance out (Santiago and Judith, 2008). A beneficial impact by P. insularum on Florida ecosystems should not be rule d out either. Removing an invasive species may have unintended consequences as ecosystems change to deal with the invader and the invader may even replace the role of native species (Antonio and Meyerson, 2002). Decline of P. paludosa has been observed for years, and a species of apple snail that can prosper in Florida may be important. In this case, snail kite populations may be hurt and changes in macrophyte consumption could
42 affect overall wetland composition (P. Gray, personal communication; Carlsson, 2 006; Fang, 2010). While some studies have shown that P. insularum will consume more Florida native vegetation, there are few field data to show a negative impact on Florida ecosystems. A similar species, P. canaliculata is shown to be completely non in vasive in its native ecosystem where farms are not at risk, and in Asia, most of its impact has been on the agriculture (Joshi and Sebastien, 2006). Florida does not have extensive rice or taro farming, although there is some rice, watercress and waterches tnut production, but other states like Texas do, and as the snails spread, they may impact those agricultural systems (EDIS, 2012). There are many different directions one could take to increase knowledge of the effect P. insularum may have in the United States. Right now adult P. paludosa generally live at or less than a density of 1 m 2 and P. insularum lives at densities of 2 m 2 when they are in stable ecosystems. However, when both live in a pond, these densities may change, potentially impacting nat ive vegetation and apple snails. Pomacea insularum had a much lower survival rate and lower growth rates in tanks that did not receive as much sun and studies could be done to determine if juveniles of this species are limited more by food than potential competitors such as P. paludosa. Field studies are much harder to conduct due to the lack of reliability with current sampling techniques (Darby et al., 1999). However, long term studies on the impact of P. insularum in the wild will be the most effective way to study this potential problem. Many lab studies have researched the target vegetation of Pomacea and how this affects their growth rates, but data from the field would provide a more accurate picture of the island apple snail's impact.
43 It is possibl e that P. insularum could have a long term impact on P. paludosa populations that will not be measurable for many years. Pomacea insularum may grow faster under certain conditions around P. paludosa and may also have higher growth rates at higher densities Winter 2011 2012 was milder, may have affected the outcome of the field and the interaction studies. Because P. insularum adults have been shown to negatively impact juvenile P. paludosa growth rates, changes in their population structure could slowly oc cur over the next few decades. A higher density of P. insularum could also result in large ecosystem changes due to increased macrophyte or periphyton consumption. On the other hand, an increase in P. insularum may benefit an important species in Florida, the snail kite. In this situation, careful monitoring for many years will need to occur in order to determine the overall impact that P. insularum could have in Florida.
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