Abstract
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 from the 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.
Highlights
Understanding patterns in the abundance and the distribution of species across space and time is one of the core issues in ecology (Brown et al, 1995; Holt & Keitt, 2005)
Ramets were collected from local S. altissima patches growing in fields surrounding the study site, and we identified each ramet as a unique genotype by means of amplified fragment length polymorphisms (AFLPs)
While a growing number of studies have focused on the cascading effects of host-plant genetic variation (Whitham et al, 2006; Hersch-Green et al, 2011), whether the strength or direction of those effects remains consistent over space (Tack & Roslin, 2011) or time (e.g., Keith et al, 2010) remains an open question
Summary
Understanding patterns in the abundance and the distribution of species across space and time is one of the core issues in ecology (Brown et al, 1995; Holt & Keitt, 2005). How to cite this article Crutsinger et al (2013), Local and latitudinal variation in abundance: the mechanisms shaping the distribution of an ecosystem engineer. Ecosystem-engineering insects, such as leaf tiers or leaf rollers, stem borers, and gall makers can create shelters for a suite of colonizing species that secondarily use these microhabitats (Lill & Marquis, 2003; Marquis & Lill, 2007; Calderon-Cortes et al, 2011; Crawford et al, 2007; Crutsinger et al, 2009; Vieira & Romero, in press). As many engineering insect species are herbivores, variation in abundance and quality of their host plants may predict susceptibility to attack. Many host plants can vary considerably within species in their susceptibility to insect herbivores. Genetic variation within host-plant species can have strong effects on the population dynamics of associated herbivore species (Egan & Ott, 2007; Tack & Roslin, 2011; Underwood, 2012)
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