Abstract
We explore the impact of habitat fragmentation on interactions between keystone resources of forest trees—oaks, genus Quercus (Fagaceae)—and an associated radiation of specialist cynipid gall wasps. Habitat fragmentation is predicted to have bottom‐up impacts on cynipid communities through impacts on host plant quality (plant vigor hypothesis). We explored the bottom‐up impacts on cynipid communities of habitat fragment size, fragment edge effects and presence of isolated oaks. We quantified temporal and spatial variation of leaves produced in the canopy to quantify plant vigor, and surveyed cynipid gall species abundance and richness over three years using 15 permanent forest patches and 25 isolated oaks in a fragmented oak woodland landscape in central Mexico. Cynipid gall abundance and species richness were higher in isolated oaks and small woodland fragments than in larger ones. Cynipid abundance and species richness were also higher along fragment edges in comparison with fragment interiors. This contrasts with patterns observed in other taxa. In addition, host plant quality was higher in isolated trees, in smaller fragments and along fragment edges. We therefore hypothesize that observed patterns in cynipid abundance and species richness are driven by changes in host plant quality due to forest fragmentation. Our data represent a baseline for longer‐term monitoring of fragmentation effects at a landscape scale. Further work is required to explore alternative potential explanations for observed patterns, including the estimation of potential top‐down impacts of fragmentation mediated by natural enemies.
Highlights
Oaks (Quercus species) are dominant late successional species in a wide range of habitats and offer key environmental services (Faivre-Rampant et al 2011)
No relationship between gall abundance and percentage of canopy cover was observed in small fragments (F 1⁄4 3.2, R2 1⁄4 0.12, P . 0.05), medium-sized fragments (F 1⁄4 4.8, R2 1⁄4 0.04, P . 0.05) and large fragments (F 1⁄4 2.2, R2 1⁄4 0.06, P . 0.05). We found that both gall wasp species richness and abundance increased with habitat fragmentation
Gall wasp species richness and abundance were higher along fragment edges that in the interior
Summary
Oaks (Quercus species) are dominant late successional species in a wide range of habitats and offer key environmental services (i.e., carbon sequestration, energy production and water cycle regulation) (Faivre-Rampant et al 2011). An important feature of some oak-cynipid systems is the ability of some oak species to support very rich communities, providing considerable resolution for analysis of habitat-associated changes in assemblage structure. Examples include Quercus turbinella in North America, which supports 20 species of cynipids wasps and Quercus robur and Quercus petraea in Europe, which support more than 70 species (Fernandes and Price 1988, Csoka et al 2005). Such host plant species have been called ‘‘super-hosts’’ (Araujo et al 2013). We analyze oak cynipid assemblages associated to ‘‘super-hosts’’ species to examine the impact of habitat fragmentation on Mexican oak communities
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