The distribution of a host-specific canopy parasite is linked with local species diversity in a northern temperate forest
Is the spatial distribution and density of the maple spindle gall mite Vasates aceriscrumena (MSGM) positively correlated with the distribution and density of its host? Is the distribution of MSGM influenced by non-host species and abiotic factors?
Temperate mixed hardwood forest stand, Haliburton Forest and Wildlife Reserve, Ontario, Canada.
We used the mapped locations of host and non-host trees to investigate the cause of spatial variation in the density of MSGM within an 8.8-ha forest plot in central Ontario, Canada. Gall densities were determined from fallen leaves, collected at 20-m intervals. We used Mantel and partial Mantel tests to compute the correlation between gall density and several spatially variable biotic and abiotic factors: (1) host density and basal area, (2) density of non-host stems, (3) overall stem density, (4) stem species diversity and (5) topography.
The density of leaf galls was weakly correlated with host density and basal area. Although the correlation with host density and basal area was statistically significant, leaf gall density was more strongly correlated with overall tree species richness and overall stem density. Gall densities were highest at the boundaries of neighbourhoods containing high and moderate sugar maple (Acer saccharum) densities. Partial Mantel tests indicated that the observed spatial correlations held when controlling for the potential influence of topography.
Based on the spatial relationships documented here, we speculate that the mechanism responsible for the correlation between the MSGM and non-host stems is parasite-induced host stress. Separate studies have established a strong negative impact of the MSGM parasite on sugar maple stem growth. We suggest that by weakening the competitive ability of its host, the parasite indirectly promotes local species diversity through competitive release. Given the high diversity and prevalence of leaf gall parasites in mixed hardwood stands, depression of host dominance by leaf parasites may represent an unexplored mechanism for the maintenance of species diversity in northern temperate forests.