SPATIAL SCALE AND THE AGGREGATION OF STREAM MACROINVERTEBRATES ASSOCIATED WITH LEAF PACKS

1. An experimental field study examined the aggregation of stream macr oinvertebrates associated with leaf packs over different spatial scale s (several metres-km) (extent), at different patch sizes (grain) and t emporal scales (2 and 4 weeks). 2. Standardized leaf packs were constr ucted and set in eighteen blocks of nine equally spaced packs in glide areas over a 2 km stretch of a wooded stream. The distribution of mac roinvertebrates colonizing the artificial leaf packs was investigated to examine the extent of both intraspecific and interspecific aggregat ion across leaf packs. 3. All major colonizing taxa were intraspecific ally aggregated across the leaf packs. Aggregation decreased with incr easing patch size (grain) (from pack to block), and also decreased wit h decreasing spatial extent (from 2 km stretch to within-block scale) with patch size held constant. Interspecific associations among all ma jor taxa were not common on most occasions at the short temporal scale , although the proportion of significant associations tended to increa se somewhat over time and with spatial scale, but did not exceed 42% o f all possible associations. The vast majority of significant associat ions were positive rather than negative. 4. The influence of heterogen eity in a number of environmental variables measured for each leaf pac k (accumulated detritus and sediment, leaf mass, flow and depth) on th e distribution of invertebrates was considered, but this could only pa rtially explain the variation in macroinvertebrate abundance across le af packs. 5. The roles of intrinsic aggregation and stochastic process es were examined as alternative explanations for the distribution patt erns observed. It is apparent from this study that intrinsic aggregati on, in concert with resource partitioning, influences the community st ructure of stream macroinvertebrates associated with leaf packs. These findings may also have implications for the distribution of taxa in t he benthos as a whole.