Collapse of a unique insular bird–snake relationship

Abstract

AbstractSeahorse Key (SHK) supports a population of Florida cottonmouth snakes, Agkistrodon conanti, which have a unique trophic association with colonially nesting waterbirds. Each spring >10 000 birds have nested on SHK and provided an influx of food in the form of fish carrion that snakes scavenge on the ground. Because of this allochthonous input of trophic resources, the cottonmouth population on SHK historically was dense, with snakes of large body size. Suddenly and unexpectedly, the entire waterbird colony abandoned its nests during April, 2015 and did not return. We examined data collected from 1999 to 2016 on the abundance and body size of SHK cottonmouths to quantify how the bird abandonment has affected the snake population. Our results show that abundance and body condition index (BCI) have decreased significantly, particularly on the west end of SHK where the bird rookery historically concentrated. Abundance of snakes (counts per survey) along a west beach transect was higher during the pre‐abandonment period (10.99 ± 8.11 SD) than during the post‐abandonment period (7.52 ± 4.64). Similarly, we observed a significant decrease in the BCI of snakes from the west end of SHK between the pre‐ (0.04 ± 0.10) and post‐abandonment periods (−0.12 ± 0.14). Loss of the bird rookery as a source of energy has negatively affected the SHK cottonmouth population. In contrast, numbers and body condition of cottonmouths appear to have increased on an adjacent island where ~3000 to 5000 waterbirds began nesting in 2015 and 2016.