The tremendous diversity of species in ecological communities has motivated a century of research into the mechanisms that maintain biodiversity. However, much of this work examines the coexistence of just pairs of competitors. This approach ignores those mechanisms of coexistence that emerge only in diverse competitive networks. Despite the potential for these mechanisms to create conditions under which the loss of one competitor triggers the loss of others, we lack the knowledge needed to judge their importance for coexistence in nature. Progress requires borrowing insight from the study of multitrophic interaction networks, and coupling empirical data to models of competition.

Although observations of species-rich communities have long served as a primary motivation for research on the coexistence of competitors, the majority of our empirical

In contrast to many other biotic forces, such as competition and predation, the role played by habitat modification by plants and sessile animals in natural communities has not been given the experimental attention it deserves. To test the hypothesis that habitat modification by seaweed canopies can have direct positive effects on rocky intertidal communities, we quantified habitat amelioration by Ascophyllum nodosum canopies and its consequences on understory organisms in the Gulf of Maine, USA. At the upper and lower elevational borders of the algal canopy, we examined the recruitment, growth, and survivorship of common benthic organisms in canopy removal, canopy control, and shaded canopy removal plots intended to mimic canopy habitat modification. The algal canopy greatly reduced potential physical stresses, particularly at high tidal heights. Maximum daily rock temperatures were 5°–10°C lower and evaporative water loss was an order of magnitude less under the canopy than in canopy removal plots. The response of understory organisms to canopy removal, however, was species specific and somewhat idiosyncratic. Nonetheless, in general, at the high intertidal border of the canopy the recruitment, growth, and survival of understory organisms were enhanced by the canopy, whereas at the low intertidal border canopy effects were negative or neutral. Nearly half of the interactions we studied were positive in the high zone. In contrast to positive canopy effects on understory organism recruitment and growth at high tidal heights, consumer pressure was severe under the canopy, particularly at low tidal heights. Green crab predation is likely responsible for limiting understory mussel densities, while grazing by the snail, Littorina littorea, keeps understory substrate clear of algal recruits. The amelioration of harsh physical conditions by algal canopies can have strong direct positive effects in high rocky intertidal communities by enhancing organism recruitment, growth, and survival. These canopy effects, however, may often be offset by increased consumer pressure at low tidal heights. These types of habitat modification effects are likely to be pervasive in many other terrestrial and marine communities exposed to harsh physical conditions.