Testing large-scale conservation corridors designed for patterns and processes: comparative phylogeography of three tree species
Primary drainage basins and megaherbivore suitability underpin the design of conservation corridors aimed at preserving regional patterns and processes in the Albany Subtropical Thicket (AST) biome. We use comparative phylogeography to investigate the influences of these two features on population-level processes (long-distance seed dispersal) of three tree species.
Albany Subtropical Thicket biome, South Africa.
Three dominant and widespread AST tree species (Nymania capensis, Pappea capensis and Schotia afra) were sampled across their distribution within the biome. Phylogeographical patterns were established using chloroplast DNA sequences, tested against drainage basin configuration and compared with the different seed dispersal syndromes of each tree species.
Two species, N. capensis and P. capensis, show strong and significant evidence of population isolation within basins, whereas S. afra has no evidence of restricted gene flow across the AST. We argue that this contradicting pattern is driven by the different seed dispersal syndromes found within each species: N. capensis is wind-dispersed, P. capensis is bird-dispersed, whereas a range of large herbivores disperse S. afra seeds including African elephants.
The evolutionary processes that make primary drainage basins significant biogeographical units within the AST continue to operate at the intraspecific level due to a lack of long-distance interpopulation seed dispersal between basins. Thus, drainage basins in this system, and in other topographically complex landscapes, are important surrogates of biodiversity and evolutionary processes that should underpin conservation planning. However, we suggest that this surrogacy breaks down when a species is dispersed by large herbivores, which are ideal agents of long-distance dispersal in complex landscapes. These results support the two significant components of the planning decision for the AST megaconservancy network: (1) drainage basins as major biogeographical (and also now phylogeographical) units, and (2) establishing interbasin networks with the aim of allowing long-distance movement of large mammals, including elephants.