Northern Everglades Canals: Alligator Population Sources or Sinks?
Matthew D. Chopp
Florida Fish and Wildlife Conservation Commission, Lake Panasoffkee, FL
H. Franklin Percival
U. S. Geological Survey, Florida Cooperative Fish and Wildlife Research Unit,
Gainesville, FL
Kenneth G. Rice
U. S. Geological Survey, Center for Water and Restoration Studies, Ft.
Lauderdale, FL
The Comprehensive Everglades Restoration Plan (CERP) includes the proposed
removal of canals that fragment the Everglades wetland landscape. Dense
populations of American alligators (Alligator mississippiensis) exist in canals
throughout the system. Clutch and hatchling survival in canal and interior
habitats at A.R.M. Loxahatchee National Wildlife Refuge (LNWR) were
calculated during 2000 and 2001 to investigate the effects of canal habitats on
alligator production. Data were collected from 112 nests and 779 hatchling
alligators. Individuals from 57 hatchling pods were recaptured during this study.
No sampled clutches in the interior experienced flooding during 2000 and 2001
(fig. 1). However, most clutches in canal habitats were flooded or partially
flooded (fig. 1). Nests were depredated by raccoons (Procyon lotor) at a higher
rate during 2001 than 2000 (fig. 2). The number of clutches successfully
producing at least one hatchling was greater during 2000 than 2001, and greater in
interior than canal habitats (fig. 3). Survival probability estimates for 2000-cohort
hatchlings for the first 6 and 13 months of life were 44% and 20%, respectively.
Mean production per nest after 13 months was 2.4 + 0.5 in interior and 0.8 + 0.8
in canal habitats.
Compared to nests in interior habitats, canal nests were subjected to a larger range
of water levels during clutch incubation and were more susceptible to flooding.
Flooding was found to be the greatest risk to canal nests at LNWR. The majority
of nests in LNWR interior habitats were on tree islands. High clutch mortality,
small pod size, and overall low mean hatchling survival at LNWR, resulted in
negligible production in canal habitats during 2000 and 2001. Comparatively
high adult densities in canals at LNWR, and likely other areas of the Everglades,
are sustained by immigration and high adult survival rates. Everglades canals
appear to be population sinks for alligators and canal removal will probably not
affect the overall population.
100
Clutch Flooding (%)
100
80
77
Canal 2000
Canal 2001
Interior 2000
60
Interior 2001
40
20
0
0
0
Habitat by Year
Figure 1. Percentage of American alligator clutches that
experienced flooding during incubation in canal (n = 13 and 30)
and interior (n = 35 and 24) habitats at A.R.M. Loxahatchee
National Wildlife Refuge during 2000 and 2001, respectively.
Nest Depredation (%)
100
Canal 2000
80
Canal 2001
58
Interior 2000
60
Interior 2001
40
20
12
0
3
0
Habitat by Year
Figure 2. Percentage of American alligator nests that experienced
depredation during incubation in canal (n = 13 and 29) and interior
(n = 33 and 24) habitats at A.R.M. Loxahatchee National Wildlife
Refuge during 2000 and 2001, respectively.
Nests Producing at Least 1
Hatchling (%)
100
Canal 2000
80
Canal 2001
Interior 2000
60
Interior 2001
40
20
92
57
38
10
0
Habitat by Year
Figure 3. Percentage of American alligator nests that successfully
produced at least one hatchling during the hatch event in canal (n =
13 and 30) and interior (n = 36 and 35) habitats at A.R.M.
Loxahatchee National Wildlife Refuge during 2000 and 2001,
respectively.
Matthew D. Chopp, Florida Fish and Wildlife Conservation Commission, 8864
CR 247, Lake Panasoffkee, FL 33638
Phone: 352-637-4208, email: matthew.chopp@fwc.state.fl.us