Linking landscape patterns of resource distribution with models of aggregation in ovipositing stream insects

1. Many species rely on resources that are discrete patches distributed across the landscape. Models of the coexistence of such taxa often require intraspecific aggregation of individuals on some patches, brought about by choices made during dispersal and patch location. Few studies have employed spatially explicit measures of dispersion to detect whether and how any aggregation of individuals is influenced by the dispersion of resource patches in natural landscapes. 2. For two species of caddisfly (Hydrobiosidae) in two stream stretches, we mapped the spatial distribution of potential and exploited patches (emergent rocks as oviposition sites) and enumerated egg masses on each rock. Point pattern analysis, based on Ripley's K-function, was used to describe the spatial pattern of potential and exploited oviposition sites, and to test hypotheses about how site selection behaviour of females influences dispersion. 3. Emergent rocks were clumped and equally abundant in both stretches. As documented previously, egg masses of Ulmerochorema and Apsilochorema were aggregated within oviposition sites. Both species showed velocity-specific preferences for oviposition sites: oviposition of Ulmerochorema occurred most often on rocks in fast flowing water (>0.30 m s(-1)); oviposition of Apsilochorema on rocks in slow water (<0.30 m s(-1)). 4. Our perception of how caddisflies deposit eggs into the stream landscape varied with the neutral landscape model. The most informative neutral landscape compared the spatial pattern of oviposition sites with the underlying pattern of emergent rocks, while constraining the random selection of rocks to reflect species' velocity preferences. Oviposition sites of Ulmerochorema were clumped, over and above the background pattern of emergent rocks. Oviposition sites of Apsilochorema were over-dispersed. 5. Hydrobiosid caddisflies appear to make decisions about egg-laying at different scales. We propose a mechanistic explanation, based on what we know about the oviposition behaviour of these species, to explain how the observed patterns might arise. Our results emphasize the importance of quantifying resource patch use in natural (rather than artificial) landscapes.