Limited impacts of extensive human land use on dominance, specialization, and biotic homogenization in boreal plant communities

Abstract

BackgroundNiche theory predicts that human disturbance should influence the assembly of communities, favouring functionally homogeneous communities dominated by few but widespread generalists. The decline and loss of specialists leaves communities with species that are functionally more similar. Evenness of species occupancy declines, such that species become either widespread of rare. These patterns have often been observed, but it is unclear if they are a general result of human disturbance or specific to communities that are rich in species, in complex, spatially heterogeneous environments where the problem has often been investigated. We therefore tested whether human disturbance impacts dominance/evenness of species occupancy in communities, specialism/generalism of species, and functional biotic homogenization in the spatially relatively homogeneous, species poor boreal forest region of Alberta, Canada. We investigated 371 boreal vascular plant communities varying 0 – 100% in proportion of human land use.ResultsRank species occupancy curves revealed high species dominance regardless of disturbance: within any disturbance class a few species occupied nearly every site and most species were found in a low proportion of sites. However, species were more widespread and displayed more even occupancy in intermediately disturbed communities than among communities of either low or high disturbance. We defined specialists and generalists based on turnover in co-occupants and thereby assessed impacts of human disturbance on specialization of species and community homogenization. Generalists were not disproportionately found at higher disturbance sites, and did not occupy more sites. Communities with greater human disturbance were not more functionally homogeneous; they did not harbor communities with more generalists.ConclusionsWe unexpectedly did not observe strong linkages between species specialism/generalism and disturbance, nor between community homogenization and disturbance. These results contrast previous findings in more species rich, complex or spatially heterogeneous systems and ecological models. We suggest that broad occupancy-based intercommunity patterns are insensitive to human land use extent in boreal vascular plants, perhaps because of ubiquity of generalists, low species richness, and history of natural disturbance. The poor sensitivity of these metrics to disturbance presents challenges for monitoring and managing impacts to biodiversity in this region.

Global patterns of terrestrial assemblage turnover within and among land uses

“…Furthermore, human-modified habitats have been shown to benefit a few widespread species at the expense of many narrowly distributed species (McKinney 2006, Schwartz et al 2006). On the other hand, differences in beta diversity among land uses are not always found, and beta diversity can even increase with human disturbance (Tylianakis et al 2005, Hawkins et al 2015, Mayor et al 2015. The response of beta diversity to land use (both magnitude and direction) varies among taxonomic groups (Fleishman et al 2003, Clough et al 2007, Norfolk et al 2015.…”

Effect of Anthropogenic Disturbance on Floristic Homogenization in the Floodplain Landscape: Insights from the Taxonomic and Functional Perspectives

“…Most of the studies at a local scale reported that anthropogenic disturbance caused floristic differentiation or absence of the variation of β-diversity [18,19,21], while this study suggested that human disturbance could induce taxonomic homogenization in floodplain landscapes. As revealed by the results of this study, non-native species invasion was responsible for the homogenization in urbanized floodplains.…”

The ranked species occupancy curves reflect the dominant process of species sorting: Evidence from forest/scrub communities

“…One of the few studies supporting the hypothesis pertaining to disturbed (pioneer) communities was an experiment performed in old fields (Martinez et al, 2015). However, contrary to the mentioned predictions, Mayor et al (2015) found only concave, exponential RSOCs associated with intermediately disturbed boreal forests (but restricted to low-fertility sites), whereas Gibson et al (2005) observed bimodal patterns in OFDs (i.e. sigmoidal RSOCs) through the course of early succession in old fields.…”