Status & Trends

This section presents updated information based on apple snail density sampling in Florida lakes, wetlands, and rivers.  Information posted (see bullets/links below) is the most recent data made available by Dr. Phil Darby for a given wetland.  All of the data shown below come from the research program headed by Phil Darby, Associate Professor, at the University of West Florida.  [Data from other sources will be presented as available.]  Although other methods have been used to estimate snail density or relative abundance, all of the density estimates reported here reflect 1-m² throw trap sampling (see Sampling Methods). 

Additional data are available for Lakes Tohopekaliga, Kissimmee, and Istokpoga, and will be added at a future date.

 

What Our Status and Trends Data Do and Do Not Represent

The way in which these areas were sampled, and the sites selected for apple snail sampling, reflect a variety of different project objectives.  Consequently, most of the data do not reflect random site selection that represent the snail population for an entire wetland and all its habitat types.  Starting in 2008, however, P. Darby et al. initiated some random site selection procedures on Lake Kissimmee and in the Upper St. Johns Marsh, and they hope to expand this effort to many other wetlands and lakes in the future.  In this process, they used GIS to generate 50 or more random points in the marsh, randomly selected a subset to be considered for sampling, and then visited the site to find the closest available wet prairie or slough-type habitat in which to sample.

Sample site selection for most studies conducted in the past was based, in part, on some sign of snail presence.  This is because method development and comparisons were in their early stages (see citations under Sampling Methods), and they could not compare methods if there were no snails present.  In other cases, the objective was comparing snail densities in different habitat types (see Karunaratne et al. 2006), and again no comparisons could be made if an area was generally devoid of snails.  Because of the importance of collecting data related to snail kites, avaliable data tend to come from those habitat types in which kites most often forage (e.g., emergent marsh, wet prairie and slough), and not in areas of dense vegetation (e.g., cattail, sawgrass) (even though we know these latter habitats do support apple snails [see Valentine-Darby et al. 2008]).

Some snail densities were estimated as part of kite foraging studies.  Presumably kites forage in portions of a wetland unit that support the greatest number of snails.  We know, however, that snail availability also affects where they forage; for example, kites avoid relatively dense vegetation, even within wet prairie habitats that support relatively high snail densities (Bennetts et al. 2006). As a result, some snail density estimates are biased in the sense that they represent where birds chose to forage, not a typical snail density that represents the wetland unit.  These density estimates from kite foraging areas likely represent higher snail densities than would be observed, on average, in randomly selected wet praire and sloughs in the same wetland unit.

In the status and trends presented below, we indicate whether site selection was random, based on signs of apple snail presence, or based on the presence of foraging kites.  We discuss the density estimates relative to what was reported as an approximate minimum snail density, 0.14 snails/m², to support kite foraging (Darby et al. 2006).  We also report the proportion of juvenile snails (those <20 mm shell length), if available, because kites tend to capture larger snails (Sykes et al. 1995).  In most cases we cannot report trends, because data were available for only one year.

 

Blue Cypress Water Management Area (BCWMA) (Upper St. Johns River Marsh)

Most Recent Period Sampled

  • September–October 2007.  The work was conducted by Phil Darby (Univ. of West Florida) for the St. Johns River Water Management District (SJRWMD).  In May 2007, BCWMA experienced some of its lowest water levels in recent years.  Due to the topography of the marsh, some locations went dry while others retained surface water.  The study looked at snail densities in these "dry" or "wet" areas in the fall of 2007, after the entire marsh was reflooded.  

Sampling Site Characteristics

  • Twelve sites were sampled in wet prairie habitat.  Sites were randomly selected from those areas that had been wet in May 2007 and those that had remained flooded.  Eleocharis cellulosa was the dominant emergent at all sites.  Among sites, the average number of emergent plant stems per 0.5 m² ranged from 13 to 47.  Of the traps that contained submerged plants, Utricularia spp. and Chara spp. were dominant.  Average water depths (±SE) in the 12 sites ranged from 47 (±1.0) to 72 (±0.8) cm (ground elevation generally increases from west to east in BCWMA East).

Observations

  • Among the twelve sites, snail densities ranged from 0.0 (in eight of the twelve sites) to 0.087 (±0.005) snails/m².  Snail densities were not different in sites that had been wet vs those that went dry.  Note that the selection of sites in this study was not based on the presence of foraging snail kites (as in many of the other sampling efforts presented in this section).
  • No foraging snail kites or limpkins were observed during the study. 

Discussion

  • Based on the findings of Darby et al. (2006), the snail densities found at BCWMA East would be considered too low to support sustained snail kite foraging.  Note, however, that Darby et al. (2006) examined sites with a range of snail densities, and if only low density sites are available at a given wetland, kites may forage there for exploratory purposes or because there is no alternative.
  • Recent trends can not be examined with only one year of data.  However, in 1996 P. Darby sampled snail densities at two locations close to one of the sites in this study. The site from this (2007) study was in the northeast portion of BCWMA East; the 2007 density was 0.029 (±0.004), and the nearby densities in 1996 were 0.18 and 0.48 snails/m².

 

Lake Okeechobee

Most Recent Period Sampled

  • Spring 2005.  The work was conducted by P. Darby, Univ. of West Florida, for the Florida Fish and Wildlife Conservation Commission (FFWCC).

Sampling Site Characteristics

  • Sampling sites in the southwest littoral zone of the lake were chosen based on historical occurrence of foraging and nesting snail kites.  Six sites were sampled in wet prairie habitat, with sites dominated by Eleocharis cellulosa and Panicum hemitomon (and including Sagittaria spp. and Pontederia cordata).  Water depths in the six sites ranged from approximately 40-60 cm in March-early May 2005. 

Observations

  • Zero snails were found in four of the sites; the remaining two sites had densities of 0.04 and 0.06 snails/m².  
  • Snail kites were observed in three of the sites during throw trap sampling.  Field staff observed from 2-5 kites foraging over several hours on different days, but no kites captured a snail. 
Discussion
  • The near absence of snails in the study sites likely reflected the impacts of a 1.4-yr drying event in 2000-2001 (for details of this event, see Steinman et al. 2002).  These sites were also dry for 17-19 weeks in both 2002 and 2004.  It should be noted that high water conditions in years before the 2000 drawdown may have contributed to a decline of snails in the middle to late 1990s.  Habitat conditions had deteriorated during this high water period (Steinman et al. 2002). 
  • In the spring of 2005, snail densities in the southwest littoral zone were too low to support snail kite foraging (based on observations and Darby et al. 2006).  Kite attempts at foraging on LOKEE in 2005 likely reflect diminished foraging opportunities regionally, such that birds were forced to attempt to forage in areas that supported few snails (i.e., they were exploring the region for food, and tried, but failed to find sufficient forage on LOKEE).
  • Recent trends based on snail density data can not be examined with only one year of data.  However, it can be said that snail densities in 2005 were likely much lower than they had been in the 1990s when Okeechobee was frequently used for snail kite foraging and nesting (e.g., Bennetts and Kitchens 1997). 

 

Lake Istokpoga

Most Recent Period Sampled

  • Spring 2008 and Spring 2009.  The work was conducted by P. Darby, Univ. of West Florida, for the South Florida Water Management District (SFWMD).  The objective was to estimate snail densities in Spring 2008, prior to a potential drying event, and follow up with a Spring 2009 sampling effort. 

Sampling Site Characteristics

  • Nine wet prairie sites were sampled primarily along the western shore of the lake and the marsh surrounding Bumblebee Island.  The sites were dominated by pickerelweed (Pontederia cordata) and spikerush (Eleocharis cellulosa), with tape grass (Valisneria americana) as the dominant SAV.  The outer edge of the littoral zone was often bordered by bulrush (Scripus californicus).  Water depths in the 9 sites were similar and ranged over time from 25-66 cm during the April-May 2008 and 2009 sampling period. 

Observations

  • Among the 9 sites sampled in Spring 2008, one site had zero snails.  Adult snail densities ranged from 0.05 (± 0.002) to 0.24 (± 0.014) snails/m² in seven of the sites, and one site had a density of 1.06 (± 0.09) snails/m².  The proportion of juvenile snails in one site was 64%, and between 0-34% in all other sites. 
  • Density estimates were reported for six of the nine sites in Spring 2009.  Adult densities in those six sites ranged from 0.02 (± 0.010) to 0.45 (± 0.028) snails/m². The proportion of juvenile snails in the sites ranged from 0-39%.  
  • The field crew observed kites foraging and nesting in the vicinity of four of the nine sites in 2008 and 2009.

Discussion

  • The dry down anticipated for spring and summer 2008 did not occur.  
  • Total snail density (adults and juveniles) from Spring 2008 to Spring 2009 decreased in all sites but one.  The two greatest percent declines in snail density from 2008 to 2009 (-70% and -84%) occurred in two sites that were noted as having received herbicide applications.  In one site density increased from 0.14 to 0.29 snails/m².
  • Based on the findings of Darby et al. (2006), in 2008 four of nine sites had densities exceeding (or meeting) the minimum estimated for supporting sustained snail kite foraging.  In 2009, only two of six sites had densities exceeding (or meeting) the minimum threshold.
  • According to information from USFWS, Vero Beach, snail kites established at least 10 nests on Lake Istokpoga during the 2008 kite nesting season.

 

Water Conservation Area (WCA) 1, (part of Loxahatchee National Wildlife Refuge)

Most Recent Period Sampled

  • February–May, 2002, 2003, and 2004 (different sites sampled each year).  The work was conducted by P. Darby, Univ. of West Florida, for the U.S. Fish & Wildlife Service (USFWS).

Sampling Site Characteristics

  • Three prairie-slough sites were sampled in WCA1 in 2002 (A, B, C), one in 2003 (Site D), and two mixed prairie sites (E, F) in 2003.  Site selection in 2002-2003 was based in part on presence of apple snail egg clusters along the sawgrass-prairie ecotone.  In 2004, four sites labeled ‘KN’ (for historic kite nesting sites) were sampled in wet prairie habitat.  The most common emergent plant species in most of the wet prairie sites (the central prairie sites) were Rhyncospora spp. and Eleocharis interstincta.  In the other sites (in the north/northeast), E. cellulosa, E.baldwinii, Paspalidium geminata, and Panicum hemitomon were more common.

Observations

  • Among the 10 sites sampled in three years, one site had no snails, five sites had densities ≤0.08 snails/m², and three sites had 0.12-0.14 snails/m².  The highest density recorded was 0.22 snails/m², which was in the only site over the three years that a kite was observed foraging.

Discussion

  • At the time of sampling, WCA1 had a limited forage base for snail kites.  All but two of 10 sites had snail densities <0.14 snails/m², the minimum density estimated by Darby et al. (2006) to support sustained snail kite foraging.  
  • No site was sampled in more than one year.  Trends can not be examined with only one year of data per site. 
  • Based on water chemistry studies reported by Glass and Darby (2008), low calcium and low pH, in part, explain the relatively low densities found in WCA1, especially in the central interior sites (A-F).  However, water management and associated loss of certain habitat types may also contribute to lower snail densities (e.g., Rynchospora prairies that used to be more common in WCA1).

 

Water Conservation Area (WCA) 2B

Most Recent Period Sampled 

  • Late April 2006.  The work was conducted by P. Darby (Univ. of West Florida) for the U.S. Fish & Wildlife Service (USFWS).   

Sampling Site Characterisitcs 

  • Three sites were sampled in WCA2B.  Sites were selected based on the presence of foraging snail kites (as part of an ongoing foraging study).  All three sites had wet prairie dominated by Eleocharis cellulosa (66% of all traps sampled), with Panicum the next most abundant emergent.  Emergent species stem counts averaged 36/m² (±16 SD).  Utricularia purpurea was the most common submerged species, followed by Bacopa sp. and Potamogeton sp.  The wet prairie was bordered by sawgrass in all three sites.  Depths were similar among the three sites with an overall average of 50 cm (±9 SD).  

Observations 

  • Among the three sites, snail density (adults and juveniles) ranged from 0.045 (±0.003) to 0.085 (±0.004) snails/m².  33% of the snails found in WCA2B were juveniles (<20mm in shell width). 

 
Discussion

  • Based on the findings of Darby et al. (2006), the snail densities found at the WCA2B sites would be considered too low to support sustained snail kite foraging.  Note, however, that Darby et al. (2006) examined sites with a range of snail densities, and if only low density sites are available at a given wetland, kites may forage there for exploratory purposes or because there is no alternative.
  • Recent trends can not be examined with only one year of data.    

 

Water Conservation Area (WCA) 3A

Most Recent Period Sampled 

  • Late April 2007.  The work was conducted by P. Darby (Univ. of West Florida) for the U.S. Fish & Wildlife Service (USFWS).   

Sampling Site Characterisitcs 

  • Six sites were sampled in WCA3A in 2005 and 2006, and 2 of these sites were sampled in 2007.  The six sites were sampled to examine snail density and egg production along a hydrologic gradient (from south/deeper water to north/shallower water). Two additional sites were sampled in 2006 based on the presence of foraging snail kites (as part of an ongoing foraging study).  All sites were wet prairie habitats.  Five of the six sites were dominated by Eleocharis cellulosa (60-85% of traps sampled per site), with Panicum the next most abundant emergent; in one of the six sites, Panicum was slightly dominant over Eleocharis.  Emergent species stem counts averaged 42/m² (±20 SD) to 64/m² (±28 SD).  In the six sites, Utricularia purpurea was the most common submerged species, followed by Bacopa sp., Potamogeton sp., and Chara sp.  Based on estimated ground elevations, the two southern sites (latitude 25° 46') are approximately 9 cm lower in elevation than the two middle latitude sites (25° 53'), which are about 14 cm lower than the two northern sites (25° 57').  


Observations 

  • In 2006 (the most recent year in which all 6 sites were sampled), snail density (adults and juveniles) ranged from 0.017 (±0.003) to 0.204 (±0.005) snails/m².  The proportion of juveniles (<20 mm in shell width) varied among the sites, from 0 to 49.5% of the snails captured.  The highest-density site (0.204) had the greatest proportion (49.5%) of juveniles. The two additional sites sampled in 2006 (the "kite sites") had snail densities of 0.173 (±0.007) and 0.388 (±0.018) snails/m², with 45% and 87%, respectively, as juveniles.    

 
Discussion

  • Based on the findings of Darby et al. (2006), the snail densities found in southwestern WCA3A in 2006 and 2007 would be considered too low to support sustained snail kite foraging.  Note, however, that Darby et al. (2006) examined sites with a range of snail densities, and if only low density sites are available at a given wetland, kites may forage there for exploratory purposes or because there is no alternative.  In WCA3A in 2006-2007, Darby et al. observed kites foraging in sites with relatively low adult snail densities (~0.1/m²), but where snails with shells < 20 mm were relatively abundant (up to 0.3/m²).  It may be that low adult densities drive some kites to forage on smaller snails than documented in earlier studies.
  • Sampling was initiated in southwestern WCA3A in 2002.  Typical adult snail densities in these sites in 2002-2003 ranged from 0.3 to 1.0 snails/m².  Darby et al. also monitored egg cluster production in these sites; they attritubed a 10-fold reduction in egg cluster production in 2003 (despite relatively abundant snails) to long periods of high water, especially during the peak breeding season (April-June 2003).  The following year adult snail densities in these sites fell to 0.0 to 0.3/m².  From 2004-2007, adult snail densities stayed below 0.3/m². WCA3A is considered to be in a state of recovery, with regards to snails, following the dramatic decline in 2004 (P. Darby, pers. comm.).

 

Water Conservation Area (WCA) 3B

Most Recent Period Sampled 

  • March–April 2006.  The work was conducted by P. Darby (Univ. of West Florida) for the U.S. Fish and Wildlife Service (USFWS).   

Sampling Site Characterisitcs 

  • Three sites were sampled in WCA3B.  Sites were selected based on the presence of foraging snail kites (as part of an ongoing foraging study).  All three sites had wet prairie dominated by Eleocaris cellulosa (94% of all traps sampled), with Panicum the next most abundant emergent.  Emergent species stem counts averaged 76/m² (±38 SD).  Bacopa sp. was the most common submerged species, with scattered Potamogeton sp., Utricularia purpurea, and Utricularia sp. (yellow-flowered species).  The wet prairie was bordered by sawgrass in all three sites.  Depths were similar among the three sites with an overall average of 31cm (±7 SD).

Observations 

  • Among the three sites, snail density ranged from 0.030 (±0.001) to 0.167 (±0.006) snails/m².  47% of the snails found in WCA3B were juveniles (<20mm in shell width). 

Discussion

  • Based on the findings of Darby et al. (2006), the adult snail densities found at the WCA3B sites would be considered too low to support sustained snail kite foraging.  However, if the adult plus juvenile densities are considered, the site with a density of 0.167 would exceed the preliminary figure suggested as the minimum snail density to support sustained kite foraging.  Note, however, that Darby et al. (2006) examined sites with a range of snail densities, and if only low density sites are available at a given wetland, kites may forage there for exploratory purposes or because there is no alternative.
  • Recent trends can not be examined with only one year of data.   

 

Grassy Waters Preserve

Most Recent Period Sampled 

  • Early May 2006.  The work was conducted by P. Darby (Univ. of West Florida) for the U.S. Fish & Wildlife Service (USFWS).   

Sampling Site Characterisitcs 

  • Three sites were sampled in Grassy Waters Preserve.  Sites were selected based on the presence of foraging snail kites (as part of an ongoing foraging study).  All three sites had wet prairie dominated by Eleocaris cellulosa (73% of all traps sampled), with Panicum the next most abundant emergent.  Emergent species stem counts averaged 60/m² (±27 SD). Utricularia purpurea was the most common submerged species.  The wet prairie was bordered by sawgrass in all three sites. Depths in the three sites had an overall average of 42 cm (±13 SD).

Observations 

  • Among the three sites, snail density (adults and juveniles) ranged from 0.045 (±0.003) to 0.449 (±0.020) snails/m².  78% of the snails captured in Grassy Waters were juveniles (<20 mm in shell width). 

 
Discussion

  • Although the overall snail densities are relatively high, the majority of snails captured were <20 mm in size.  Based on the findings of Darby et al. (2006), the adult snail densities found at the Grassy Waters sites would be considered too low to support sustained snail kite foraging.  The field sampling crew also noted that kites were frequently perch hunting, and in some cases captured snails along the sawgrass-prairie ecotone, where the crew did not sample.
  • Recent trends can not be examined with only one year of data.   

 

Literature Cited

  • Bennetts, R.E. and W.M. Kitchens.  1997.  The demography and movements of snail kites in Florida.  U.S.G.S. Biological Resources Division, Florida Cooperative Fish & Wildlife Research Unit.  Tech. Rep. No. 56.  169 pp.
  • Bennetts, R.E., P.C. Darby, and L.B. Karunaratne.  2006.  Foraging patch selection by snail kites in response to vegetation structure and prey abundance and availability.  Waterbirds 29(1): 88-94.
  • Darby, P.C., R.E. Bennetts, and L.B. Karunaratne.  2006.  Apple snail densities in habitats used by foraging snail kites.  Florida Field Naturalist 34(2): 37-47.
  • Glass, N.H. and P.C. Darby.  2008.  The effect of calcium and pH on Florida apple snail, Pomacea paludosa (Gastropoda: Ampullariidae), shell growth, and crush weight.  Aquatic Ecology DOI 10.1007/S10452-008-9226-3.
  • Karunaratne, L.B., P.C. Darby, and R.E. Bennetts.  2006.  The effects of wetland habitat structure on Florida apple snail density.  Wetlands 26(4): 1143-1150.
  • Sykes, P.W., Jr., J.A. Rodgers, Jr., and R.E. Bennetts.  1995.  Snail kite (Rostrhamus sociabilis).  In The Birds of North America, No. 171 (A. Poole and F. Gill, eds.).  The Academy of Natural Sciences, Philadelphia, and The American Ornithologists' Union, Washington, D.C.
  • Steinman A., K. Havens, and L. Hornung.  2002.  The managed recession of Lake Okeechobee, Florida: Integrating science and natural resource management.  Conservation Ecology 6: Article No.17 (on-line, http://www.ecologyandsociety.org/vol6/iss2/art17/).
  • Valentine-Darby, P.L., P.C. Darby, R.E. Bennetts, W.M. Kitchens, and H.F. Percival.  2008.  The use of mark-recapture to estimate Florida apple snail (Pomacea paludosa SAY) density in wetland habitats.  Florida Scientist 71(2):115-127.  
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