Long term community dynamics of aphidophagous coccinellids in response to repeated invasion in a diverse agricultural landscape

Background/Question/Methods

Aphidophagous ladybeetles (Coleoptera: Coccinellidae) are important providers of herbivore suppression ecosystem services. In the last 30 years, the invasion of exotic coccinellid species coupled with observed declines in native species has led to renewed interest in the community dynamics and ecosystem function of this guild. Several hypotheses have been proposed to describe the mechanism of invasion coupled with native species declines (i.e. vacant niche exploitation, competitive displacement and habitat compression, intraguild predation), and the relationship of this decline to biodiversity and herbivore suppression. We use a 24 year dataset of a coccinellid community in nine habitats in southwestern Michigan to test for evidence supporting or refuting these hypotheses, and for changes in community function. Habitats were divided into annual crop, perennial vegetation, and forest for analysis, and coccinellid abundance data were subjected to Spearman rank correlation analyses, diversity analyses, and the herbivore suppression potential of the guild was estimated over time.

Results/Conclusions

Abundance of coccinellids varied widely between years and became increasingly exotic-dominated. More than 71% of 57,813 adult coccinellids captured over the 24 year study were exotic species. Correlation analyses indicated interactions between exotic and native species were not generalizable, suggesting that impact of exotic species on native coccinellids varies with their degree of interaction. Habitat use patterns by some native species (Adalia bipunctata and Coccinella trifasciata) changed during years when the exotic Harmonia axyridis reached high numbers, lending support to the habitat compression hypothesis, where native species retreat to ancestral habitats when invaders dominate cultivated habitats.

Shannon diversity increased slightly over time, but herbivore suppression potential of the community remained roughly constant over the course of the study. The relationship between Shannon diversity and herbivore suppression potential varied with habitat type: a positive relationship in forest and perennial habitats, but uncorrelated in annual habitats. An analysis of potential herbivore suppression indicated that although the composition of the coccinellid community has changed dramatically in the past 30 years in southwestern Michigan, its function appears to be relatively unchanged in both agricultural and natural habitats. However, coccinellid communities dominated by exotics may be less robust to changing land-use patterns.