J. M. Diamond's assembly rules model predicts that competitive interactions between species lead to nonrandom co-occurrence patterns. We conducted a meta-analysis of 96 published presence-absence matrices and used a realistic ''null model'' to generate patterns expected in the absence of species interactions. Published matrices were highly nonrandom and matched the predictions of Diamond's model: there were fewer species combinations, more checkerboard species pairs, and less co-occurrence in real matrices than expected by chance. Moreover, nonrandom structure was greater in homeotherm vs. poikilotherm matrices. Although these analyses do not confirm the mechanisms of Diamond's controversial assembly rules model, they do establish that observed co-occurrence in most natural communities is usually less than expected by chance. These results contrast with previous analyses of species co-occurrence patterns and bridge the apparent gap between experimental and correlative studies in community ecology.

Disturbance frequency, intensity, and areal extent may influence the effects of disturbance on biological communities. Furthermore, these three factors may have interacting effects on biological diversity. We manipulated the frequency, intensity, and area of disturbance in a full-factorial design on artificial substrates and measured responses of benthic macroinvertebrates in a northern Vermont stream. Macroinvertebrate abundance was lower in all disturbance treatments than in the undisturbed control. As in most other studies in streams, species density (number of species/sample) was lower in disturbed treatments than in undisturbed controls. However, species density is very sensitive to total abundance of a sample, which is usually reduced by disturbance. We used a rarefaction method to compare species richness based on an equivalent number of individuals. In rarefied samples, species richness was higher in all eight disturbed treatments than in the undisturbed control, with significant increases in species richness for larger areas and greater intensities of disturbance. Increases in species richness in response to disturbance were consistent within patches, among patches with similar disturbance histories, and among patches with differing disturbance histories. These results provide some support for Huston's dynamic-equilibrium model but do not support the intermediate-disturbance hypothesis. Our analyses demonstrate that species richness and species density can generate opposite patterns of community response to disturbance. The interplay of abundance, species richness, and species density has been neglected in previous tests of disturbance models.

Adult zebra mussels are generally thought of as sessile animals. However, when detached from their byssus, adult mussels exhibit creeping behavior similar to that of other bivalve species. Our study examined the effects of size, light, water hardness, temperature, and the presence of injured conspecifics on the movement of adult zebra mussels. Mussels were placed in aquaria and their movement was recorded after a 2-h interval. Mussels left visible trails in a thin layer of sand on the aquarium floor that allowed for accurate measurements of distance and direction of movement. The tendency to move and distance moved by the mussels was inversely proportional to size of the individual mussels. There was no significant effect of temperature on movement, though smaller mussels tended to move more at higher temperatures. Although distance traveled was not significantly different in light versus dark conditions, there was a significant effect of light on direction traveled. Zebra mussels were strongly negatively phototaxic. The presence of injured conspecifics decreased movement significantly, whereas water hardness did not have a significant effect on movement. The movement of adult zebra mussels is an under-appreciated phenomenon and deserves greater consideration for understanding dispersal vectors and development of antifouling methods to be used in the future.

1. We examined the effect of zebra mussel colonisation on invertebrate communities inhabiting soft sediments in two bays along the Vermont shoreline of Lake Champlain, U.S.A. 2. In summer 2001, we conducted manipulative experiments (addition and removal of zebra mussel colonies) with respective controls to assess the effect of colonies on invertebrate abundance, richness, and position within sediments (within colony versus underlying colony) and compared these data to comparative studies of natural communities in summer 2002. 3. Split core samples were taken two months after the manipulation and the following year so that we could quantify individuals and species inhabiting zebra mussel colonies separately from those in sediments underlying zebra mussel colonies and adjacent sediments lacking zebra mussels. 4. Zebra mussel-covered sediments supported more abundant and richer invertebrate communities than adjacent sediments lacking zebra mussels. 5. Abundance and richness patterns in zebra mussel-addition and removal treatments closely paralleled those in natural communities. 6. Despite severe oxygen depletion at the interface of underlying sediments and overlying zebra mussel colonies, most infaunal invertebrates responded positively to zebra mussel colonisation either by remaining in sediments underlying zebra mussel colonies or by migrating into zebra mussel colonies.