Territory size decreases minimally with increasing food abundance in stream salmonids: Implications for population regulation.

How the local density of territorial animals responds to changes in food abundance will depend on the flexibility of territory size. Quantitative estimates of territory size over a broad range of food abundance are relatively rare because of the difficulty of measuring food abundance in the wild. Stream salmonids are an ideal model system for investigating flexibility in territory size, because food abundance can be quantified in the field and manipulated in the laboratory. We conducted a meta-analysis to test whether territory size decreases with increasing food abundance, and a mixed model analysis to test among three competing predictions: with increasing food abundance, territory size will be (1) fixed-the slope of a regression of log territory size vs. log food abundance = 0; (2) flexible and decreasing, as if individuals are defending a fixed amount of food-a slope = -1; and (3) initially compressible, but with an asymptotic minimum size-a slope between 0 and -1. We collected data from 16 studies that manipulated or measured food abundance while monitoring changes in territory size of young-of-the-year salmonids; 10 were experimental laboratory studies, whereas six were observational field studies. Overall, territory size decreased significantly with increasing food abundance; the weighted average correlation coefficient was -0.31. However, the estimated slope of the relationship between log territory size and log food abundance was only -0.23, significantly different from 0, and also significantly shallower than -1. Our estimated slope suggests that attempts to increase the density of territorial salmonids by increasing food abundance and reducing territory size will be inefficient; a 20-fold increase in food abundance would be required to double population density. Our analysis may also have implications for other species with a territorial mosaic social system-i.e. contiguous territories. In these social systems, social inertia will dampen any effects of changes in food abundance on the local density of settlers, compared to non-territorial species or those with non-contiguous territories.