The population ecology of Rattus exulans on Tiritiri Matangi Island, and a model of comparative population dynamics in New Zealand

Spatial heterogeneity of mesopredator release within an oceanic island system

Predator-prey communities are ubiquitous in ecology, but introduced predators can drive native species to extinction within island systems, prompting the eradication of such exotics. Ecological theory predicts that elimination of top-introduced predators from islands can lead to the counterintuitive decline of native prey populations through the ecological release of smaller introduced species in a process termed "mesopredator release." We show, in accordance with mesopredator release theory and counter to conservation goals for a New Zealand island reserve, that initial eradication of cats on Little Barrier Island led to reduced breeding success of Cook's petrels, which also are vulnerable to predation by a mesopredator, the Pacific rat. The rat's impact on prey productivity varied with elevation within the island. Rat eradication was followed by a rise in petrel productivity, in support of both ecological theory and practical conservation management goals. It appears that interactions among introduced predators, native prey, and environmental gradients can drive counterintuitive and spatially heterogeneous responses to predator eradications from islands. Location-specific, ecosystem-level understanding is essential for predicting the outcomes of such restoration management techniques.

Population ecology of Norway rats (Rattus norvegicus) and interference competition with Pacific rats (R. exulans) on Raoul Island, New Zealand

Norway rats (Rattus norvegicus) are sympatric with, and more often trapped, than the smaller Pacific rat (R. exulans) on Raoul Island, New Zealand. Rats were removed from a four-hectare grid by trapping and poisoning in the winters of 1994, 1995 and 1996. Pacific rats were trapped in increasing numbers only after Norway rats were removed. Norway rats also ate significantly more bait than Pacific rats. Competitive interference of Pacific rats by Norway rats was apparent, which casts doubt on the ability to accurately monitor individual species abundance within assemblages of rat species and to effectively manage them in control grids. Snap-trapping lines provided baseline data on the abundance of the two species before, during and after the removal grids were operated. Maximum abundances of rats were recorded in late summer and autumn following spring and summer breeding.