Habitat Architecture and the Abundance and Body‐Size‐Dependent Habitat Selection of a Phytal Amphipod

Field and laboratory experiments were conducted on the effect of habitat architecture (the number, size, shape, and arrangement of habitable spaces and structures) created by benthic algae on the habitat selection of an abundant mobile amphipod, Gammarellus angulosus, on the central coast of Maine. Amphipod population density and body size were determined in algae of different morphologies that provided amphipods with different habitat architectures. The two primary components of habitat architecture in this system were spatial (the number and size of spaces between fronds) and structural (the number, length, and width of fronds). These were measured for algae of specific morphologies and for artificial plants that mimicked these morphologies. Field experiments using algae and algal mimic counterparts showed that there were significantly higher densities of amphipods in algae with branched and filamentous morphologies than in those algae with foliose and leathery macrophyte morphologies. There was also a significant correspondence between the body size of amphipods and both components of habitat architecture. Laboratory experiments using algae and algal mimics excluded food value, predation, competition, and physical disruption in experimental treatments. When these processes were excluded, patterns of abundance and body—size scaling to habitat dimensions were the same as those in the field. It appears, through the use of algal mimics, that the spatial component (space between fronds) is an important factor in determining amphipod demographic patterns in algae. Algal mimics of different surface rugosity and color indicate that tenacity and crypsis are also important components in habitat selection of amphipods.