Lights in the night: Using night-light surveys and mapping technology to estimate Florida’s alligator population

map
Map of sampling route within
St. Johns River Puzzle Lake AMU

Conservation of the American alligator (Alligator mississippiensis) is a Florida success story. In 1967, survival of the species was in doubt, and it was listed as an endangered species. Regulations enacted by both the state of Florida (and other southern states) and the U.S. Fish and Wildlife Service provided protection for the American alligator and allowed the species to recover. In 1987, the population was deemed recovered and was removed from the Endangered Species List, although it was given the status of “Similarity of Appearance (Threatened)” due to its similarity to the “Threatened” American crocodile (Crocodylus acutus). Down-listing of the alligator paved the way for Florida to hold its first statewide recreational harvest in 1988, which has continued annually and is internationally recognized as a model harvest program.

To manage alligator populations, it is important to have a reliable estimate of the number of alligators in Florida. Every year, alligator research and management staff survey and monitor alligator populations on approximately 130 Alligator Management Units (AMUs) located around the state. An AMU can be a lake with distinct boundaries, an impoundment, or a section of river bounded by landmarks such as between two bridges. Although these AMUs only account for about 10 percent of the total available alligator habitat in Florida, it is possible to extrapolate from these data to obtain a statewide estimate. By looking at trends of the alligator population as a whole, the FWC can better manage and conserve the alligator population, as well as identify critical alligator wetland habitat and long-term population changes or habitat losses.

 

alligator in grassDuring the spring and summer months, researchers head out at dusk to lakes, rivers, and marshes of varying sizes throughout Florida to conduct night-light surveys. Using spotlights, researchers survey all accessible habitat in an AMU from an airboat in order to count the alligators. Like most nocturnal animals, the eyes of an alligator reflect light at night, which makes them easy to spot when the light hits them. Once an eyeshine is detected, researchers will attempt to approach the alligator and determine the length of the animal (based on the head length) so that it can be placed in a size class (e.g., 2-3 foot or 7-8 foot). Alligators are placed in broader, unknown size categories (e.g., unknown 2-4 foot or unknown 6+ foot) when an exact size estimate cannot be obtained (e.g., if the animal submerges before it can be sized). This allows staff to account for wary alligators when conducting population modeling. Once data are collected and quality checks have been conducted, staff analyze the data using statistical programs to determine the population estimates and trends for different size categories (e.g., total population, adults, juveniles, etc.). Data from these night-light surveys are used to calculate alligator densities for approximately 400,000 hectares of alligator habitat distributed throughout the state.

During the fall and winter months, researchers turn their attention towards habitat evaluation. The total area of wetland habitat throughout the state is calculated using Geographic Information Systems (GIS) and the Florida Vegetation and Land Cover Map of 2003. All wetland habitats are put into one of three categories based on the size of contiguous habitat: 0-80 hectares (ha), 81-400 ha, and more than 400 ha. Researchers then exclude the first category, wetland habitats with less than 80 ha of contiguous habitat, due to their small size and uncertainty of water availability. This leaves researchers with two wetland habitat categories: medium-sized wetlands (those between 81 and 400 ha) and large wetlands (those greater than 400 ha). Staff also break down the wetland size categories by habitat types as delineated by the Florida Vegetation and Land Cover Map of 2003.

juvenile alligator
Juvenile alligator

With alligator density data from the night-light surveys and wetland habitat categories identified, researchers then assign alligator densities to the wetland habitats to obtain a rough estimate of the number of alligators in the state. Some habitats are assigned conservative density estimates due to the lack of survey data for those habitats, such as hardwood swamp, salt marsh, and cypress swamp. Once densities are estimated for each wetland habitat, researchers obtain their annual estimate. Based on the 2010 estimate, nearly 1.3 million alligators live in the state of Florida. The majority (74%) of these alligators were juveniles and subadults less than 6 feet in length. The total adult population was roughly 340,000 based on size distributions from the survey data.

The work does not stop here. Scientists are currently working on new, more robust spatial analyses. These new efforts will focus on modeling alligator night-light densities with habitat characteristics to better understand the relationships between populations and habitats. This new spatial analysis model will be updated every five years or when new GIS habitat data become available. Researchers also need to identify habitat characteristics – such as type, size, location in relation to permanent water, etc. – that are not surveyed and obtain more reliable density estimates for these areas.

Once the new population estimate is completed, researchers will publish their results for the scientific community. Results from this project may help other state agencies looking to improve the way they estimate alligator populations. For the Florida Fish and Wildlife Conservation Commission, alligator research staff will ­­continue to provide statewide population estimates and explore new ways to improve our understanding of alligator populations in Florida.

JN0-102  
JN0-360  
LX0-103  
LX0-104  
M70-101  
MB2-704  
MB2-707  
MB5-705  
MB6-703  
N10-006  
NS0-157  
NSE4  
OG0-091  
OG0-093  
PEGACPBA71V1  
PMP  
PR000041  
SSCP  
SY0-401  
VCP550  
102-400  
810-403  
200-120  
1z0-808  
300-206  
PEGACPBA71V1  
100-105  ,
000-105  
70-347  
642-999  
300-206  
300-320  
9A0-385  
70-413  
100-105  ,
OG0-093  
350-060  
350-029  
NSE4  
200-125  ,
220-901  
CAP  
220-901  
101  
JN0-102  
MB5-705  
70-410  
SY0-401  
400-101  
1Z0-803  
9L0-066  
100-101  
300-101  
ITILFND  
300-101  
M70-101  
70-480  
SY0-401  
70-417  
70-177  
200-125  ,
C_TFIN52_66  
JN0-102  
9L0-012  
70-980  
000-080  
N10-006  
2V0-621  
HP0-S42  
642-999  
1z0-808  
AWS-SYSOPS  
OG0-093  
PMP  
70-462  
70-413  
640-692  
NSE4  
1z0-434  
ICBB