Abstract
Edge effects play key roles in the anthropogenic transformation of forested ecosystems and their biota, and are therefore a prime field of contemporary fragmentation research. We present the first empirical study to address edge effects on the population level of a widespread galling herbivore in a temperate deciduous forest. By analyzing edge effects on abundance and trophic interactions of beech gall midge (Mikiola fagi Htg.), we found 30% higher gall abundance in the edge habitat as well as lower mortality rates due to decreased top-down control, especially by parasitoids. Two GLM models with similar explanatory power (58%) identified habitat specific traits (such as canopy closure and altitude) and parasitism as the best predictors of gall abundance. Further analyses revealed a crucial influence of light exposure (46%) on top-down control by the parasitoid complex. Guided by a conceptual framework synthesizing the key factors driving gall density, we conclude that forest edge proliferation of M. fagi is due to a complex interplay of abiotic changes and trophic control mechanisms. Most prominently, it is caused by the microclimatic regime in forest edges, acting alone or in synergistic concert with top-down pressure by parasitoids. Contrary to the prevailing notion that specialists are edge-sensitive, this turns M. fagi into a winner species in fragmented temperate beech forests. In view of the increasing proportion of edge habitats and the documented benefits from edge microclimate, we call for investigations exploring the pest status of this galling insect and the modulators of its biological control.
Highlights
Forest fragmentation is ranked amongst the leading mechanism behind the current global biodiversity crisis and belongs to the most pervasive impacts of human land use [1,2]
To cover a range of habitat traits and environmental variables with potential impacts on the interactions involved in our galling insect–host plant system, we considered habitat type, vegetation density and canopy closure as independent explanatory variables for variation in gall abundance
In the edge-influenced understorey a typical beech leaf showed a significant 30% increase in gall density compared to the forest interior (GLM: Df = 22; F = 7.15; Deviance = 0.037; p = 0.014)
Summary
Forest fragmentation is ranked amongst the leading mechanism behind the current global biodiversity crisis and belongs to the most pervasive impacts of human land use [1,2]. The disruption of trophic interactions has been repeatedly identified as a key mechanism involved in edge-mediated loss of ecosystem function or biodiversity [9,10,11] yet the understanding of the underlying causal processes is often sketchy. Generalist herbivores benefit from favorable micro-environmental conditions, increased food quantity and quality, including higher nutrition content and less defense capacity of plant tissue [8]. They may profit from a disrupted top-down regulation, yet this is less well documented [11]. While several studies showed a clear profit of generalist herbivores to edge-driven changes, empirical evidence on specialized herbivores is less clear [8]
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