Missouri Botanical Garden - 52nd Annual Systematics Symposium, St Louis, MO, October 2005.
Historical biogeographic methods and a priori hypotheses based on Australian freshwater fishes.
Freshwater aquatic systems are different biogeographically to other habitat types based on their distribution and connectivity pattern across the landscape. The fundamental unit for defining these systems is the drainage basin or catchment boundary. Because obligate freshwater aquatic organisms are only able to move via direct connections between habitats, drainage basins greatly limit opportunities for movement due to their isolated nature. Potential physical connectivity between drainage basins is largely limited to two factors: changes in sea level, which allow drainages to coalesce when sea level falls; and drainage rearrangements, which result in changed catchment boundaries.
The potential for both factors to influence and generate repeatable biogeographic patterns were a priori mapped across the Australian landscape to test their influence. Three freshwater fish groups were chosen for phylogeographic analysis based on their broad ranges and number of species. The groups were also chosen to maximize geographic overlap among taxa and provide maximum coverage of Australia.
When fish groups were combined, they showed little congruence in their patterns of divergence across Australia despite a high degree of co-occurrence. This suggests that shared biogeographic history within and among each group has been limited, and different biogeographic factors must be invoked to explain the observed patterns in each group. Two factors are proposed to explain this result. The first is that dispersal across freshwater biogeographic barriers may be more common than previously thought. This is due to the presence of low drainage divides which may allow fishes to move across catchment boundaries during high rainfall periods without drainage rearrangement. The second is that vicariance in freshwater systems does not appear to be a powerful structuring force. Although vicariant events are occurring on a regular basis, the fish fauna may not be equally affected by these events due to differences in ecology and climate.
Over-arching biogeographic hypotheses are inadequate to explain the independent histories of each group. Thus, patterns within each group require individual explanations. My research adds to the growing body of literature that suggests independent biogeographic patterns are common. Hence, finding broader explanations for patterns may be the exception rather than the rule.