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.

Understanding the factors that determine community structure remains one of the most important issues in ecology. In this paper, we examine the role of flow velocities in governing community structure in marine intertidal communities. We broaden the traditional definition of ''bottom-up'' forces to include the delivery of nutrients, food, and larval resources to habitats and then test the hypothesis that, by controlling these fluxes as well as mediating predator effects, flow velocities leave strong bottom-up signatures on shoreline communities.We examined this hypothesis by quantifying community structure and dynamics at high and low flow sites in a tidal estuary in Maine. High flow sites were characterized by dense barnacle and mussel cover, while low flow sites had considerable bare space. High flow sites also had greater grazer and predator densities than low flow sites. Recruitment of all common shoreline organisms with planktonic larvae was greater at high flow sites, in direct proportion to the increased fluid flux there. Flow effects on the growth of the herbivorous and carnivorous components of the food web were less predictable. High flows increased the growth of barnacles, but not mussels, and increased the growth of the carnivorous gastropods that preyed on them. In contrast, high flows decreased the accumulation of benthic diatoms, but this was unrelated to the growth rates of herbivorous gastropods. High flow sites were universally characterized by low predation intensity and per capita predation rates on all three prey species.Our results show that the strength of top-down and bottom-up forces varies with flow rate in this estuary. Consumer stress (top-down) models accurately explain patterns we saw at low flow sites, but bottom-up processes predicted from nutrient/productivity models dominate at high flow sites. High consumer pressure is the dominant structuring force at low flow sites, whereas at high flow sites predators are inhibited, and individual recruitment and growth rates become the dominant structuring forces. We suggest that hydrodynamics may commonly decouple predation and resource processes in many aquatic systems when the physical process responsible for variation in top-down forces also acts as a strong bottom-up force.

Understanding the factors that determine community structure remains one of the most important issues in ecology. In this paper, we examine the role of flow velocities in governing community structure in marine intertidal communities. We broaden the traditional definition of ''bottom-up'' forces to include the delivery of nutrients, food, and larval resources to habitats and then test the hypothesis that, by controlling these fluxes as well as mediating predator effects, flow velocities leave strong bottom-up signatures on shoreline communities.We examined this hypothesis by quantifying community structure and dynamics at high and low flow sites in a tidal estuary in Maine. High flow sites were characterized by dense barnacle and mussel cover, while low flow sites had considerable bare space. High flow sites also had greater grazer and predator densities than low flow sites. Recruitment of all common shoreline organisms with planktonic larvae was greater at high flow sites, in direct proportion to the increased fluid flux there. Flow effects on the growth of the herbivorous and carnivorous components of the food web were less predictable. High flows increased the growth of barnacles, but not mussels, and increased the growth of the carnivorous gastropods that preyed on them. In contrast, high flows decreased the accumulation of benthic diatoms, but this was unrelated to the growth rates of herbivorous gastropods. High flow sites were universally characterized by low predation intensity and per capita predation rates on all three prey species.Our results show that the strength of top-down and bottom-up forces varies with flow rate in this estuary. Consumer stress (top-down) models accurately explain patterns we saw at low flow sites, but bottom-up processes predicted from nutrient/productivity models dominate at high flow sites. High consumer pressure is the dominant structuring force at low flow sites, whereas at high flow sites predators are inhibited, and individual recruitment and growth rates become the dominant structuring forces. We suggest that hydrodynamics may commonly decouple predation and resource processes in many aquatic systems when the physical process responsible for variation in top-down forces also acts as a strong bottom-up force.