Human landscape disturbance outweighs local disturbance in predicting boreal biodiversity

Background/Question/Methods

The ongoing, global, human-driven mass extinction intensifies our need to understand the relationship of biodiversity to anthropogenic disturbance.  However both species richness and disturbance depend on scale, complicating our understanding.  Studies of the impacts of disturbance on diversity have been complicated by the dependence of diversity and habitat loss on spatial scale as formulated by the species-area relationship and island biogeography theory.  Although the effect of scale on diversity has been clarified by the species-area relationship, the effect of scale-dependent disturbance on diversity is rarely considered.  Here, we hold the area at which species richness is measured constant to control for the scale dependence of richness but investigate the effects of disturbance scale on species richness.

We sampled vascular plant species richness in relation to human disturbance in the boreal ecoregion of northern Alberta, Canada.  We explored richness along a 0 – 100% continuum of human footprint in the largest spatial scaled study of this kind (area = 380 000 km²).  In each of 320 sites selected systematically throughout the region, we determined species occupancy within a one hectare area, and human footprint extent at six contiguous spatial scales from 1 ha (‘local’) to 18 km² (‘landscape’).   We thereby partitioned the relative roles of direct and indirect disturbance on species richness.  We expected that the direct, local disturbance of sites would have a major impact on site diversity, and that any additional effects of indirect disturbance at broader scales could be attributed to broader landscape processes of community assembly.   

Results/Conclusions

Unexpectedly, we find that the impacts of indirect human disturbance on local species richness far outweigh the impacts of direct, local human disturbance.  Nearly twice the variance in local richness was explained by regional disturbance as by local disturbance, even after excluding edge effects. These results suggest that indirect processes, such as dispersal, can outweigh local processes such as environmental filtering in assembling communities.  Our findings indicate that boreal plant biodiversity may be influenced by disturbance at larger scales and greater distances than previously thought.

Local species richness was affected by human disturbance at multiple spatial scales; diversity-disturbance relationships depend on the scale at which disturbance is measured.  Species richness predicted from disturbance in the surrounding landscape was up to three times higher than when predicted form local disturbance.  However, the richness-disturbance relationships were consistently significant and hump-shaped at all spatial scales of disturbance, supporting the intermediate disturbance hypothesis.