Predicted and unpredictable responses of plant communities to human disturbance revealed with long-term data
Human dominance of the landscape during the Anthropocene has caused dramatic changes to plant communities via fragmentation, pollution, and the introduction of exotic species. The realization of these changes is sometimes assumed to be a rather straightforward progression whereby disturbance-tolerant species, especially invasive exotics, spread across a landscape resulting in a decline in native species and an overall homogenization of species composition. However, patterns of plant community composition in human-dominated landscapes do not always match this predicted response, and the results of attempts to establish links between human disturbance, landscape change, plant traits and plant community composition have been mixed, and sometimes contradictory. This is at least in part due to the long-lived nature of many plants and their ability to survive long periods of unsuitable conditions by lying dormant in the seed bank, leading to time lags between a disturbance and plant community response. We used legacy data and resurveys of 184 plots in a suburban landscape on southern Vancouver Island to test the predicted response of plant communities to human disturbance over 4 decades. We examined the trajectories of change in plot-level and beta diversity, and tested for relationships between landscape context and colonizations or extirpations of plants with specific traits.
Our results highlight the importance of long-term data for understanding plant community response to human disturbance. Local plant species richness actually increased over time in our plots, with both native and exotic species richness increasing on average. While plant communities in our sample have become homogenized over the past four decades, this homogenization was not correlated with the increase in local exotic species richness, instead being correlated with gains in common disturbance-tolerant natives. The traits of winners over the past four decades include exotic origin and disturbance- tolerance, and these traits predict colonizations based on landscape conditions within 500m of a plot. However, extirpations were much less frequent than colonizations and rarely predictable based on landscape conditions and plant traits. This may be the result of an extinction debt. Given that our understanding of the consequences for biodiversity loss on ecosystem function are based on local-scale experiments, it is important to test whether assumptions about how local-scale diversity shifts in response to human disturbance are valid. Long-term data highlight cases where this predicted progression is oversimplified.