Print PageDisturbance is widely considered to be a driving factor influencing species diversity, and human induced landscape disturbance has been dubbed a leading cause of biodiversity decline. Among the most prominent ecological predictions of how biodiversity responds to disturbance is that richness should be highest at some medium extent of disturbance: the intermediate disturbance hypothesis (IDH). Diversity is expected to be maximised by heterogeneous conditions created by moderate disturbance. Although IDH remains a guiding model for forest management regimes in many North American forests, the pervasiveness of peaked diversity at intermediate disturbance and the utility of the model continue to be hotly debated among ecologists.
The landmark Ecology meta-analysis of Mackey & Currie (2001) concluded that highest species richness at intermediate disturbance is rarely observed among studies, and explains little of the variation in richness across disturbance gradients. It found IDH was most rarely observed with anthropogenic disturbance, larger sample areas, and more levels of disturbance.
We attempted to test the IDH under conditions for which IDH is least likely to be observed and explained the least variation in richness in previous studies. We explored richness along a 0 – 100% continuum of human disturbed area in the largest spatial scaled dedicated study of IDH to our knowledge, representing an area of 380 000 km2. We sampled species richness of vascular plants in relation to human disturbance in the boreal ecoregion of northern Alberta, Canada. In each of 320 sites selected systematically throughout the region, we determined species occupancy within a one hectare area, and human footprint in a 18 km2 area.
Results/Conclusions
Our results firmly contradict the aforementioned meta-analysis, and generally support IDH. An intermediate disturbance (quadratic) model best explained species richness in relation to human footprint. This pattern varied by footprint type. Richness in relation to agricultural footprint was best explained by IDH, but richness increased linearly with forestry, linear features, and industry/commercial footprints, perhaps because sites of extensive human footprint were rare.
Support for IDH was generally consistent among plant growth form. Species richness of forbs/herbs, shrubs, and trees each peaked at intermediate anthropogenic disturbance, however Graminoids increased with anthropogenic disturbance.
We found markedly different diversity-disturbance relationships among native and exotic species. While native species richness peaked at intermediate disturbance, non-native species richness increased linearly with disturbance, a pattern masked by lumping these groups together.
