Habitat loss decreases predator /prey ratios in a pine-bark beetle system

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 prey9/SE) were significantly reduced in isolated stands (0.389/0.09) as compared to those with large amounts of surrounding conifer habitat (1.639/0.41). 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.