Because dispersal is important in maintaining biodiversity and ecosystem function, the fragmentation of a large expanse of habitat into smaller patches can have numerous negative effects. Mediterranean climate regions are characterized by distinct wet and dry seasons, with rainfall confined to the winter. Streams in these regions experience low flows during the summer, which can result in stream fragmentation via disruption of hydrologic connectivity. Such temporal fragmentation not only affects the available habitat but also the movement rates of drifting invertebrates, which in turn, may negatively affect the fitness of insectivorous fishes oversummering in these habitats. To assess the effects of stream fragmentation on available habitat, predator ecology, and ecosystem processes, we studied 12 contiguous riffle-pool habitats in a small California stream (John West Fork) from May through October 2009. We measured the degree of stream connectivity by quantifying riffle dimensions weekly. The primary predator in this system is the insectivorous juvenile steelhead trout (Oncorhynchus mykiss). We PIT-tagged and tracked juvenile trout over the summer to estimate their movement, growth, and survival rates during this period. To better understand the effects of temporal habitat fragmentation on ecosystem function, we collected algal samples during four sampling dates to estimate primary productivity.
Results/Conclusions
Our preliminary results indicate that our study stream became increasingly fragmented as the summer progressed, and that this had effects on predator ecology. All 12 of the riffles and 5 of the pools in our 12 riffle-pool study reach dried up completely by late summer. Fish isolated in these pools suffered 100% mortality. PIT tag data indicate that there was significant movement of juvenile steelhead among our study pools prior to complete disruption of hydrologic connectivity, but that among-pool movement ceased once pools became isolated. Prior to longitudinal disruption, many steelhead moved into large pools that persisted throughout the summer and enjoyed high survival rates. However, of the 38 steelhead that we marked in total, 18 were not detected during the late summer, suggesting that mortality during the summer low flow period can be high. Next steps include quantifying temporal variation in primary productivity over the summer low flow season to elucidate the impact of stream fragmentation on temporal variation in primary productivity. In general, our results suggest that temporal habitat fragmentation has strong physical and biological effects, with implications across several levels of organization.
