The American alligator (Alligator mississippiensis) is an iconic species of cultural, ecological, and economic importance in the southeastern U.S. Unregulated harvest through the early 20th century induced drastic range-wide population declines. Though populations have stabilized, habitat loss and degradation remain prominent conservation threats.

South Carolina began monitoring alligator populations in 1971 and recently implemented a public lands harvest program. South Carolina’s landscape is rapidly changing, therefore, identifying habitat types that influence alligator abundance and distribution, past and present, is necessary for successful management and broad-scale conservation planning.

Introduction

Figure 2. Study area alligator nightlight survey routes conducted from (a) 1971-1977 (N=8 routes) and (b) 2009-2013 (N=15 routes).

Land Cover Influence on American Alligator Densities Derived from Nightlight Survey Data (1971-2013)

Abigail J. Lawson

School of Agricultural, Forest, and Environmental Sciences, Clemson University, Clemson, SC, 29634

I thank the South Carolina Department of Natural Resources and the U.S. Geological Survey for funding support. I also thank Mark Bara, Tom Murphy, Walt Rhodes, Brad Taylor, and the many field technicians who have contributed to this project. Chabreck, R.H. 1966. Methods of determining the size and composition of alligator population in Louisiana. Proceedings of 20th

Annual Conference, Southeastern Association of Game and Fish Commission 20:105-112.

Digitized and buffered survey routes (1, 3, 5, 10 km) in ArcGIS (Fig. 4)

Reclassified historical and 2006 National Land Cover Databases (NLCD) to 6 broad land cover categories (Fig. 5)

Three regression model sets evaluating: adult, juvenile, and total (adults + juveniles + unknown) densities (alligators/km)

oCovariates: Proportion of land cover category within buffered polygons, survey period (historic vs. current), and year

Table 1. The three best-supported regression models for each model set. Covariate superscript denotes the effect sign, k denotes number of model parameters.

Figure 6. Proportion of reclassified land cover category within the study area for each survey period. Percentages above the bars are the percent change among periods.

Average total alligator densities: o Historic: 0.48 ± 0.54 (SE) alligators/ km (N = 40 surveys) o Current: 2.71 ± 2.32 (N = 39; 2 replicates = 1 survey)

Boat-based nightlight surveys conducted on major water bodies within South Carolina’s alligator distribution (Fig. 1,3)

Alligators classified as adult (≥6 ft.), juvenile (<6 ft.), or unknown based on estimated total length (Chabreck 1966)

Two survey time periods:

Study Area and Field Methods

Figure 1. The fall line bounds the alligator’s inland distribution.

Study area includes all counties within the alligator’s South

Carolina distribution.

Fall line Study area

Questions and Hypotheses What habitat features and spatial scale best describe alligator densities in

rivers and lakes?

Do habitat associations differ for adult and juvenile alligators?

How have alligator densities and available habitat changed over time?

H1: Density will be positively associated with natural habitat features and negatively associated anthropogenic features at all spatial scales

H2: Juvenile and adults differ in behavior and energetic requirements, which will generate age-related variation in habitat associations

H3: Alligator densities have likely increased through enhanced legal protection, though available habitat has likely decreased.

SCDNR Alligator Survey Routes

Historic Current

(a) (b)

Analysis Methods

Buffer Distances

1 km

River

5 km

3 km

10 km

Figure 4. Digitized survey route example (Black River) in ArcGIS. Basemap: Esri.

Figure 5. Study area reclassified land cover in 1978 (left) and 2006 (right). Map projection: NAD 1983 UTM Zone 17N.

Results

Adjust survey densities to reflect detectability Evaluate seasonal differences in habitat associations Incorporate within-water body characteristics and finer-

scale land cover classes, particularly wetlands

Alligator densities increased significantly between survey periods; more monitoring is needed

Explanatory habitat features differed between juveniles and adults as expected, similarities between adults and total suggest majority of unknown individuals are likely adults

Larger spatial scales more informative of habitat associations, consistent negative anthropogenic influence

Conclusions

1971 -1977: Historic surveys followed 1967 endangered species listing (Fig. 2a)

2009 – 2013: Current survey period began after 2008 inaugural public harvest (Fig. 2b)

Land Cover Category

1978 2006 90 120 0 15 30 60

Kilometers

Figure 3. Alligators are detectable during nightlight surveys by their eyeshine.

0.00

0.10

0.20

0.30

0.40

Water Urban Barren Land Forest Agriclture/Range Wetland

Water Barren Land

Wetland Urban Agri./Range

Forest

Prop

ortio

n of

Stu

dy A

rea

Land

Cov

er (±

SE) 2006

1978 -10%

-12% +23%

+87%

-37% -47%

Models ∆AIC AIC k

Juveniles Period+ + Urban- 10km Period+ + Urban- 5km Period+ + Urban- 3km

0.00 1.42 4.00

108.60 108.61 109.49

4 4 4

Adults Period+ + Agri- 10km Period+ + Agri- 5km Period+ + Agri- 3km

0.00 0.63 1.18

104.47 105.10 105.65

4 4 4

Total Period+ + Agri- 5km Period+ + Agri- 10km Period+ + Agri- 3km

0.00 0.01 0.89

305.58 306.01 308.59

4 4 4

Acknowledgements and Literature Cited

No overlap in habitat variables among juveniles and adults, commonalities between adults and total (Table 1)

Larger spatial scales best supported, negative influence of anthropogenically altered habitats (Table 1)

Extensive urban and exurban expansion, forest land conversion in the last three decades (Fig. 5, 6)

Future Directions