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| Ecological processes that determine the abundance of species within ecological communities
vary across space and time. These scale-dependent processes are especially
important when they affect key members of a community, such as ecosystem engineers
that create shelter and food resources for other species. Yet, few studies have
examined the suite of processes that shape the abundance of ecosystem engineers.
Here, we evaluated the relative influence of temporal variation, local processes,
and latitude on the abundance of an engineering insect—a rosette-galling midge,
Rhopalomyia solidaginis (Diptera: Cecidomyiidae). Over a period of 3–5 years, we
studied the density and size of galls across a suite of local experiments that manipulated
genetic variation, soil nutrient availability, and the removal of other insects
fromthe host plant, Solidago altissima (tall goldenrod).We also surveyed gall density
within a single growing season across a 2,300 km latitudinal transect of goldenrod
populations in the eastern United States. At the local scale, we found that host-plant
genotypic variation was the best predictor of rosette gall density and size within
a single year. We found that the removal of other insect herbivores resulted in an
increase in gall density and size. The amendment of soil nutrients for four years had
no effect on gall density, but galls were smaller in carbon-added plots compared to
control and nitrogen additions. Finally, we observed that gall density varied several
fold across years. At the biogeographic scale, we observed that the density of rosette
gallers peaked at mid-latitudes. Using meta-analytic approaches, we found that the
effect size of time, followed by host-plant genetic variation and latitude were the best
predictors of gall density. Taken together, our study provides a unique comparison of
multiple factors across different spatial and temporal scales that govern engineering
insect herbivore density. | |
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