AMERICAN ALLIGATORS. Trends in Alligator Relative Density Introduction

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1 AMERICAN ALLIGATORS Trends in Alligator Relative Density Introduction Alligators integrate biological responses of hydrological operations throughout their life. Their reproduction and survival is dependent upon suitable hydrologic patterns and current populations in Everglades marshes and estuaries are suppressed because of altered hydrology and salinity (Mazzotti and Brandt 1994, RECOVER 2009, Shinde et al. 2013, but see for documentation in review). In addition, alligators play a key role in shaping plant and animal communities through creation of deeper water (alligator holes and trails) and higher ground (nest sites) habitats. They also act as both prey (when they are small) and top predators and have a role in enhancing wading bird nesting success in areas where they may inhibit predation in colonies (Burtner 2011). Restoration of hydrological and salinity patterns that more closely mimic what occurred naturally, including less frequent and intense dry downs in areas that currently dry down almost every year, and changes in water levels that correspond to seasonal patterns are expected to result in an increase in relative density of alligators and alligators that are in better health as measured by body condition. In this summary, information from WY 2003 through WY 2012 is provided on patterns of alligator relative density in six areas that reflect different hydrological patterns. A water year begins on May 1 and ends on April 30 of the subsequent year. Methods Alligators are surveyed for relative density twice during spring and twice during fall according to protocols established and documented for the MAP (Mazzotti et al. 2010; Figure 6-64). Here we report on data from six areas: the WCA 1 (Lox on map), WCA 2A (WCA2A), northern WCA 3A (WCA3A-Tower), central WCA 3A (WCA3A-HD), WCA 3A southwest (WCA3A-N41), and northeastern SRS in ENP (Frog City). We selected these areas to report on because they represent contrasting hydrology with Lox, WCA3A-HD, and WCA3A-N41 being longer hydroperiod sites and WCA2A, WCA3A-Tower, and Frog City being shorter hyrodoperiod sites (Table 6-15 and Figure 6-65). Our analysis uses data from WY 2003 through WY 2012 to look at the ten-year trend and trends in five-year increments System Status Report Final

2 Figure Location of alligator survey routes and gauges used for WY 2003 WY 2012 analysis of trends in alligator relative density. Results for the Shark River route are presented in the Southern Coastal Systems chapter System Status Report Final

3 Table Hydrologic characteristics for WY 2003 WY 2012 for six survey areas listed from longest to shortest average hydroperiod. Gages used for analysis are 3-65, 1-7, 3-64, 2-17, 3-63, and NESRS1, respectively (Figure 6-64). Hydroperiod was calculated as number of days in each water year where the gage reading was equal to or above 15 cm (6 inches), below which is the depth that makes it harder for alligators to move around the marsh to feed and mate (Rice et al. 2004; Shinde et al. 2013). Average days since dry and average length dry are in reference to survey date. WCA3A-N41 Lox WCA3A-HD WCA2A WCA3A-Tower Frog City Average Water Year Hydroperiod (Days 15 cm) Average Days Since Dry Average Length Dry Number of water years in last ten (WY 2003 WY 2012) that it was dry Number of water years in last ten (WY 2003 WY 2012) that it was dry during courtship and mating (April 16 May 15) Number of water years during WY 1993 WY 2002 that it was dry Figure Yearly hydroperiod by water year for six areas of the Everglades where alligators are sampled (see Figure 6-64 for locations). Hydroperiods are calculated through the eyes of an alligator with 15 cm considered dry System Status Report Final

4 Multiple regression analysis was used to examine trends in non-hatchling alligator abundance (also called total population). Trends from WY 2003 WY 2012 and in five-year increments were looked at starting in The model regresses log-transformed counts of alligators per km (dependent variable) on water year, season (fall and spring), transect, average water depth (AWD), and average water temperature (AWT): Log((n+1)/transect length)=water year+season+transect+awd+awt (5) Where n is the count of alligators, and AWD and AWT are the average of the values measured on the surveys. Results There were significant negative trends from WY 2003 to WY 2012 (p < 0.05) for WCA2A, Tower, and Frog City (the three shorter hydroperiod sites) and no significant trend for N41, HD, and Lox, the longer hydroperiod sites (Figure 6-66 and Table 6-16). Figure Average alligators/km of two spring and two fall surveys by water year for five areas where alligators are monitored. Lox is not included on the graph because densities are much higher (ranging from yearly averages of alligators/km) System Status Report Final

5 Periods of decline varied by area (Table 6-16). Negative trends for WCA2A were observed in three of four of the five-year increments from 2003 through Negative trends were observed for WCA3A- Tower in the five-year increments from 2006 through 2011 and negative trends were observed for Frog City in all five-year increments from 2006 through The WCA3A-HD site showed a negative trend in one five-year increment from 2004 through Table Modeled percent change per water year in trends in relative density of nonhatchling alligators. Empty cells indicate that a significant trend did not occur in that increment. Water Year Increment WCA3A-N41 Lox WCA3A-HD WCA2A WCA3A-Tower Frog City % -15.1% -12.6% % % -37.1% % -18.7% -17.0% % -13.2% % Discussion With the exception of the WCA 1 (Lox) route, all of the areas surveyed have alligator abundance below restoration targets > 1.7 alligators per km. The southern WCA 3A (N41) and the central WCA 3A (HD) routes fluctuate around the 1.7 alligators per km with yearly averages ranging from 1.2 to 2.4 alligators per km. These three areas (Lox, N41, and HD) are also the three areas that have not experienced declines since 2003, have the longest hydroperiods, and experience less frequent and intense dry downs. In contrast, in the other three areas (WCA 2A, northern WCA 3A [Tower], and Northeastern SRS in ENP [Frog City]), alligator densities were low and have declined since These areas have shorter hydroperiods and experience more frequent and intense dry downs. These data support the hypotheses that multi-year hydroperiods are important for maintaining alligator populations in the Everglades and are consistent with the hypothesis that dry downs on average of once every five years would be optimal. Repeated and intense dry downs affect both the ability of alligators to reproduce if they occur during April May and the survival of hatchling and juvenile alligators regardless of when they occur. Based on the information above, areas that experience dry downs that last longer than two months (60 days) or repeatedly occur at intervals more frequently than once every five years are not likely to support populations of alligators that are at or approaching restoration targets System Status Report Final

6 It is fortunate that there is at least ten years of data over various hydrologic conditions that allow a comprehensive review of patterns of change and better definition of appropriate hydrologic targets. Because the hypothesis is that dry down events on average of once every five years is optimal, a minimum of five years of data ae needed before and after the event to test this. In addition, long-term data give a big picture perspective on system responses and allow temporal lags to be incorporated into ecosystem responses. Some monitoring attributes will respond within months, such as alligator body condition and relative density. Others, such as growth and survival, are measured over years. Numbers of nests and clutch size respond over decades. Only long-term monitoring programs will tell if increases in body condition and relative density today will lead to increased reproduction in ten years. REFERENCES Abtew, W., C. Pathak, R.S. Huebner and V. Ciuca Chapter 2: Hydrology of the South Florida Environment. In 2008 South Florida Environmental Report Volume I, South Florida Water Management District, West Palm Beach, FL. Abtew, W., C. Pathak, R.S. Huebner and V. Ciuca Chapter 2: Hydrology of the South Florida Environment South Florida Environmental Report Volume I, South Florida Water Management District, West Palm Beach, FL. Abtew, W., C. Pathak, R.S. Huebner and V. Ciuca Chapter 2: Hydrology of the South Florida Environment South Florida Environmental Report Volume I, South Florida Water Management District, West Palm Beach, FL. Armentano T.V., D.T. Jones, M.S. Ross and B.W. Gamble Vegetation pattern and process in tree islands of the southern Everglades and adjacent areas. Pages in: Sklar, F.H. and A. van der Valk (eds.), Tree Islands of the Everglades, Kluwer Academic Publishers, Dordrecht, The Netherlands. Botson, B.A., J.A. Klassen and D.E. Gawlik Dry Season Prey Trophic Concentrations. Annual report to the United States Army Engineer Research and Development Center, Vicksburg, MS. Bradley, A.P The use of the area under the ROC curve in the evaluation of machine learning algorithms. Pattern Recognition 30: Bramburger, A., J. Munyon and E. Gaiser Water quality and wet season diatom assemblage characteristics from the Tamiami Trail pilot swales sites. Phytotaxa 127: Burtner, B Symbiosis between long legged wading birds (Ciconiiformes) and alligators (Alligator mississippiensis)? Testing the nest protector hypothesis. Unpublished MS thesis, University of Florida, Gainesville FL. Chastant, J.E., R.A. Botta and D.E. Gawlik Wading bird colony location, size, and timing on Lake Okeechobee. Annual Report to United States Army Engineer Research and Development Center, Vicksburg, MS System Status Report Final