Does enemy loss cause release? A test of the enemy release hypothesis in a range-expanding insect

Background/Question/Methods Species' distributions are changing in response to global change. Interacting species, however, are not likely to move in concert, as species will necessarily differ in their capacity for a range shift. Enemies (predators, parasites, and pathogens), for example, often lag behind their range-expanding or introduced hosts, and this could result in hosts being released from top-down control. The enemy release hypothesis (ERH) is a popular explanation for the success of introduced species and can be broadly applied to climate-driven range shifts. The ERH is based on two assumptions: 1) that enemy abundance and/or richness is reduced in the invaded range, and 2) that this reduction causes host release. Most of the evidence for the ERH comes from correlational studies that observe enemy loss (1), and assume release (2). Furthermore, all tests of the ERH have focused on invasive plants. We tested the ERH for a gall-forming insect that was introduced just north of its native range where it is undergoing increased demographic success. This introduction is analogous to a climate-driven range shift such that the host was introduced into a habitat where it interacts with many of the same species as it does in its native range. We examined the first assumption of the ERH by conducting surveys of host (gall-former) abundance and enemy (parasitoid) abundance and richness at multiple sites in the native and invaded range. To examine the second assumption of the ERH, we conducted an enemy-exclusion experiment to compare the strength of top-down control between the native and invaded range. Results/Conclusions We found that host abundance was higher in the invaded range than in the native range. Parasitoid load was lower in the invaded range than in the native range, but only in one out of two years of surveys. Forthcoming results of differences in the parasitoid community between the two regions also will be presented. Our experiment did not support the second assumption of the ERH, however. Gall-former performance between exclosures and controls (i.e., top-down control) was higher in the invaded range than in the native range. This occurred because gall-former performance was higher in the absence of enemies (i.e., exclosures) in the invaded range than in the native range. Our results suggest that enemy release is not the sole cause of this gall-formers increased success, and other factors, such as abiotic factors and/or host plant interactions also likely play a role.