Multi-taxa community response to disturbance: taxonomic vs. functional diversity responses to forest management
Diversity is thought to impart resistance to disturbance upon ecological communities, and forest management objectives often include diversity. However, these objectives tend to be defined in terms of species rather than function, and studies often focus on a single taxon. We examined the effects of taxonomic and functional diversity on the resistance of four animal taxa to a range of replicated forest harvest intensities. Abundances of these taxa, including bats, birds, wood-boring beetles, and lepidopterans, were sampled and estimated at the species level each spring/summer for up to three years before (2006–2008) and six years after (2009–2014) canopy removal. Functional diversity, including functional richness, functional evenness, and functional divergence, were computed for each taxon separately based on species-averaged body size and resource acquisition traits. We estimated taxonomic resistance as the Euclidean distance from a disturbed community to the reference in species space. Functional resistance was estimated as the degree of overlap between the disturbed and reference communities in functional trait space. We expected pre-harvest functional diversity metrics to be better indicators of resistance than taxonomic diversity. This study is a component of the Hardwood Ecosystem Experiment, a long-term, landscape-scale forest management study located in south-central Indiana.
Total species richness of the focal taxa, recorded 2006–2014, included eight bats, 91 birds, 121 wood-boring beetles (Cerambycidae), and 376 lepidopterans. Lepidopteran species richness decreased from before to after harvest commensurate with harvest intensity, so that among-harvest differences were significant in the first year after harvest. For example, mean species richness in 4 ha openings decreased from 98 before harvest to 35 after harvest. Functional richness of lepidopterans also differed among harvest types, and decreased from 0.7 to 0.5 in 4 ha openings. This drop in functional richness corresponded with an increase in the relative abundance of generalist feeders. Within two years of harvest, mean bird species richness increased from 15 to 25 in 4 ha openings, while functional richness increased from 0.18 to 0.32. Both taxonomic and functional resistance of lepidopterans was low immediately following harvest, but impacted communities were approaching those of the reference communities within four years. For birds, taxonomic resistance was low but functional resistance was high, suggesting that species composition changed after harvest, but functional trait composition was not greatly affected. Bat and beetle community analyses are ongoing and will also be discussed.