Is sandy beach macrofauna only physically controlled? Role of substrate and competition in isopods
No Thumbnail Available
Date
1997
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Exposed sandy beaches have been defined as physically stressful environments, so that benthic populations living there are thought to be regulated mainly by physical factors, biological interactions being minimal. However, recent long-term studies indicate that potential intra-and interspecific interactions should also play a role in structuring populations and communities. This paper evaluates the role of sediment characteristics and potential interactions in determining the abundance and distribution patterns of the cirolanid isopods Excirolana armata and Excirolana braziliensis in sandy beaches of Uruguay. Results from concurrent field sampling and laboratory experiments showed that: (1) at a macroscale (between beaches), E. armata occurred only in beaches with fine sands, whereas E. braziliensis was observed in both fine and coarse sand beaches, reaching its highest density in the latter; (2) at a mesoscale (within beaches) and in sympatry (fine sands), both cirolanids showed maximum densities at different tidal heights, with E. braziliensis restricted to the upper beach levels; (3) both isopods showed a clear preference for fine sands, when tested in isolation or combined; (4) survivorship of E. armata was higher when tested in the preferred sediment under co-occurrence with E. braziliensis, which in turn presented higher survivorship in coarse sand, either in isolation or combined with E. armata; and (5) individual mean length of both species was consistently higher in allopatry, and similar body lengths were observed in sympatric populations. A geographical analysis of the abundance of E. brazilienis along Pan-American beaches showed that this isopod is most abundant in fine sands; this overall pattern supports conclusions derived from sediment preference experiments, implicating a greater niche breadth than that observed in Uruguayan beaches. It was concluded that E. armata could be defined as a high substrate-specific species in which intraspecific interactions would be of utmost importance in population regulation. However, distribution patterns off. braziliensis could not be explained by a simple animal-sediment relationship, and correlational evidence suggests that it is displaced by E. armata towards coarse sands and upper beach levels. Thus, potential biotic interactions cannot be discarded as a structuring force in sandy beach communities. (C) 1997 Academic Press Limited.
Description
Keywords
habitat preference, sediment preference, competition, isopods, sandy beach, Uruguay