Genetic Consequences and Management Implications of Climate Change for the American Alligator (Alligator mississippiensis)

Authors: Wade A. Ryberg and A. Michelle Lawing

The American Alligator (Alligator mississippiensis) has been found in essentially its present form in the fossil record for 30 million years. This long relatively unchanged evolutionary history gives the impression that alligator habitat has remained so stable over time that there was little evolutionary pressure to change, but evidence from the fossil record, genetics research reviewed here, and models of climate change presented here, suggest otherwise. In particular, the lack of genetic variation range-wide indicated that alligators were forced into a single Pleistocene refugium, which, according to fossil evidence and model projections in this study, was most likely restricted to the paleo coastline of peninsular Florida. The apparent expansion of suitable habitat from the mid-Holocene to modern day resulted in the current distribution of alligator populations, which follow an isolation by distance model of genetic structure. These data support the theory that alligators were fully capable of tracking changes in their distribution in response to past climate change, rather than evolving absolute climate tolerances to persist. Indeed, the amount and location of suitable habitat available to alligators fluctuated greatly during the last 2.5 million years of glacial-interglacial cycles. Under future climate scenarios, models predicted that suitable alligator habitat will expand north, increasing the number and area of habitat patches, but also retract from the southern tips of both Florida and Texas. In the context of the fossil record, these results illustrated that regions without alligators for thousands of years could potentially be recolonized in the near future, and also that regions that contained alligators for millions of years, such as southern Florida, could include populations that may be difficult or potentially impossible to maintain over the next half-century. Furthermore, if the distributional response of alligators to climate change is constrained by natural and human barriers, then the rate of climate change may outpace the alligator's capacity to adjust in those areas, leading to rapid localized changes in the size and distribution of alligator populations. These results warn that the alligator could be highly vulnerable to future changes in climate in specific regions through its current distribution. Thus, while alligators have shown a remarkable capacity to adjust to long-term climate changes, the potential for alligators to respond to climate change over much shorter timescales (i.e., decades vs. millennia) as depicted here, may be dependent on human intervention. Several landscape conservation perspectives and active management strategies are discussed that could help preserve the adaptive potential of populations and maintain species resilience to climate change.