Spatial heterogeneity of mesopredator release within an oceanic island system

Abstract

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.

Fig. 1.

Proportion of study burrows at high- and low-altitude study sites (±SE) fledging chicks during successive predator regimes on Little Barrier Island. Values for the high-altitude study sites are represented by squares (“Thumb” site) and circles (“Orau” site). Values for the low-altitude “Valley” study site are represented by triangles. Temporal differences in breeding success at high-altitude study sites are significant (F = 35.67, df = 2, P < 0.001). Temporal difference in breeding success at low study site is not significant.

Fig. 2.

Logistic model predicted probabilities (±SE) of Cook's petrel breeding success on Little Barrier Island. (A) High-altitude study sites during cat and Pacific rat, Pacific rat, and no predator regimes (data 1972–2007) (Table 1, model 2). (B) High- and low-altitude study sites during Pacific rat and no predator predation regimes (data 2003–2007) (Table 1, model 4).