Habitat loss decreases predator–prey ratios in a pine‐bark beetle system

Abstract

Increasing intensity of land use by humans has led to loss of natural habitats, resulting in isolation of remaining habitat fragments. Using a pine‐bark beetle ecosystem as a model, we tested the hypothesis that the ratio of abundance of predators to prey should decrease with increasing habitat loss at the landscape scale. We selected ten red pine (Pinus resinosa) sites, representing extremes of available habitat within a 2 km radius surrounding each stand. The bark beetle, Ips pini, and its coleopteran predators were sampled using baited multiple funnel traps. Effects of stand isolation were considerable; ratios of predators to prey (mean number of predators/number of prey±SE) were significantly reduced in isolated stands (0.38±0.09) as compared to those with large amounts of surrounding conifer habitat (1.63±0.41). The decline in ratio occurred both because there was: a) a lower abundance of predators (ca 0.5–0.8×) captured in isolated stands; and b) a significantly higher number of prey (ca 2.2×) captured in isolated stands. Isolation or loss of habitat, therefore, differentially affected the two trophic levels, supporting theoretical predictions. Reductions in predator abundance and, presumably, enemy‐caused mortality may lead to changes in the population dynamics of their prey species, possibly leading to increased outbreaks as habitat becomes increasingly isolated.