The distributions of most species of fish in dendritic drainages show distributions analogous to a pruned tree, with the trunk and perhaps some branches (tributaries) remaining.  However, some species show the near complement of that pattern, with presence or highest densities at the tips of the branches, as if the trunk had been pruned away, leaving only parts of the branches.  Such 'tip' distributions could hypothetically result from several agents, including negative impacts of 'trunk' species.  The biology of 'tip' species may differ qualitiatively from 'trunk' species in at least two ways.  First, because 'tip' species are spatially fragmented, ecological and genetic communication among local subpopulations of 'tip' species is potentially reduced relative to 'trunk' species.  Second, the hypothetically reduced communication may result in independent occurances of local adaptation.

Results/Conclusions

I will present results of studies with a stream fish species in Trinidad, West Indies, the killifish Rivulus hartii.  In this case, the 'tip' distribution is now known to be caused by invasions of 'trunk'-distributed predators into parts of watersheds.  However, while one might reasonably hypothesize that such fragmented populations would necessarily show reduced dispersal among fragments versus cases where the population was continuously spatially abundant, experimental studies have shown that intervening predators responsible for the now population density in the 'trunk' areas also cause higher rates of movement through that unfavorable habitat.  I will also report on studies comparing 'personality' of fish living in predator-benign 'tip' areas versus rare but present populations of fish in predator-intense 'trunk' sites, with results germane to local adaptation and communication among fragments.