Dispersal limitation and the spatial organization of communities and populations in alpine headwater streams

Abstract

The alpine zone encompasses all areas higher in elevation than the permanent tree line and is characterized by harsh climatic conditions and extreme topographic relief. In the Rocky Mountains, permanent alpine streams represent patches of stable aquatic habitat embedded in this relatively harsh matrix. Alpine stream insect communities are isolated from one another by the landscape and by physiological constraints confining them to the upper extent of the river continuum. The objective of my dissertation research was to use these spatially-structured systems to assess the effects of dispersal limitation on different biological levels across a relatively fine spatial scale in and around Rocky Mountain National Park, Colorado. On a more general level, comprehensive studies of alpine streams are lacking, and understanding these systems will be important to help assess the potential impact of climate change and other anthropogenic disturbance. Using a rapidly-evolving mitochondrial marker, I found evidence of significant geographic population genetic structure in two strictly-alpine black fly (Simuliidae) species: Prosimulium neomacropyga and Metacnephia coloradensis. The latter is rare, extremely habitat-specific (at productive lake outlets), and has several life history traits related to limited adult flight dispersal. Surprisingly, population genetic subdivision was significantly greater in P. neomacropyga, possibly because local populations of M. coloradensis are extraordinarily dense, allowing for a greater number of dispersal propagules. Across the study region, grouping the more highly structured P. neomacropyga populations according to "island" of contiguous alpine zone explained a significant proportion of total genetic variation, despite streams on such islands occupying different major watersheds. This structure is probably a signature of historic allopatric fragmentation driven by post-glacial climatic warming. Within alpine islands, I found strong isolation by distance, and geographic distance was highly correlated with more biologically realistic measures of isolation (such as steep ridgelines). I compared spatial population genetic structure of the more common P. neomacropyga to spatial structure of an entire benthic community under the framework of neutral theory, which yields equivalent spatial predictions at the population [genetic] and the community levels, assuming dispersal limitation. A pattern analogous to genetic isolation by distance known as "distance decay" is expected at the community level such that more isolated communities are more different than close ones. I found no evidence for this pattern, despite spatially implicit evidence that communities are dispersal-limited. Ultimately, I reasoned that neutral community theory is a useful heuristic tool, but most real communities probably do not meet all of the assumptions required for a practical application of the theory. I also made direct observations of the flight activity of the benthic insect community emerging from an alpine headwater reach of the S. Fork Cache la Poudre River in northern Rocky Mountain National Park. Using perpendicular transects of Malaise traps set to a distance of 60 meters in 2002 and 2003,1 found that most flight activity was concentrated near the stream and that most flying insects were generalists, as opposed to alpine zone specialists, which may be poorly adapted for dispersal because their habitat is relatively rare and stable. Sex ratios of most taxa were strongly skewed towards the sex that is traditionally more active in flight, suggesting that the more extreme climatic conditions of the alpine zone may intensify these differences. The prevalent westerly winds, however, did not affect the direction of flight activity, possibly because insects do not initiate flight when it is windy. Between years, emergence time varied drastically for most common taxa, probably due to different winter precipitation amounts. 2003 was a very snowy year compared to 2002, and a dense snowpack covered the stream until a much later date, apparently causing much later emergence. Because the emergence season is already short in the alpine zone, these differences have strong implications given continued climate change, under which winter precipitation is expected to increase.