Biotic resistance is the ability of a local community to limit or halt the establishment of an exotic species through predation and/or competition. Because island ecosystems are particularly vulnerable to the negative effects of exotic species, biotic resistance is theorized to be a valuable ecosystem service, but tests of the strength of biotic resistance in island systems are scant, especially for terrestrial vertebrate communities.
To evaluate the effectiveness of predation-based biotic resistance on islands, three criteria must be considered: 1) the ability of local predator species to overcome the exotic species’ antipredator traits; 2) spatial variation in effective predators across habitat types; 3) the strength of predation on an exotic species relative to the likelihood of it being transported to the island.
In the Caribbean, anole lizards are one of the most well-documented species groups being transported unintentionally from island to island via cargo shipping containers, and at present 22 anole species are responsible for 40 exotic colonizations across Caribbean islands. Beyond the spread of exotic species, Caribbean islands are also experiencing the conversion of natural areas into urban habitats, which can alter the composition of effective predator species in each habitat type.
We used clay-covered 3D printed models of anole lizards as a proxy to test predation rates across the Caribbean island of Curacao. Models varied in two traits that influence predator encounter and detection rates: perch height (high or low) and body color (green or brown). These traits also correspond to exotic anole species that vary in their likelihood of reaching Curacao through direct and indirect shipping routes. We placed models in both natural and urban habitat types and measured spatial variation in both predator compositions and predation rates.
Although predator composition varied by habitat type, models were attacked in both urban and natural habitats by lizards, mammals, and birds. The anole species with the highest likelihood of reaching Curacao as estimated from shipping connectivity networks are the species from neighboring Aruba and Bonaire, which have similar traits to the native species on Curacao. Attack rates on clay models with these traits may not be high enough to qualify as effective biotic resistance. Furthermore, exotic mammalian predators (rats, cats and dogs) exhibited high attack rates in urban environments which may help limit the establishment of exotic species while simultaneously reducing the population of native species.