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Is sandy beach macrofauna only physically controlled? Role of substrate and competition in isopods
Defeo, O.; Brazeiro, A.; de Alava, A.; Riestra, G. (1997). Is sandy beach macrofauna only physically controlled? Role of substrate and competition in isopods. Est., Coast. and Shelf Sci. 45(4): 453-462. dx.doi.org/10.1006/ecss.1996.0200
In: Estuarine, Coastal and Shelf Science. Academic Press: London; New York. ISSN 0272-7714, more
Peer reviewed article  

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Keywords
Author keywords
    habitat preference; sediment preference; competition; isopods; sandybeach; Uruguay

Authors  Top 
  • Defeo, O.
  • Brazeiro, A.
  • de Alava, A.
  • Riestra, G.

Abstract
    Exposed sandybeaches 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 isopodsExcirolana armataandExcirolana braziliensisin sandybeaches of Uruguay. Results from concurrent field sampling and laboratory experiments showed that: (1) at a macroscale (between beaches),E. armataoccurred only in beaches with fine sands, whereasE. braziliensiswas 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, withE. braziliensisrestricted to the upper beach levels; (3) both isopods showed a clear preference for fine sands, when tested in isolation or combined; (4) survivorship ofE. armatawas higher when tested in the preferred sediment under co-occurrence withE. braziliensis, which in turn presented higher survivorship in coarse sand, either in isolation or combined withE. 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 ofE. braziliensisalong 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 thatE. armatacould be defined as a high substrate-specific species in which intraspecific interactions would be of utmost importance in population regulation. However, distribution patterns ofE. braziliensiscould not be explained by a simple animal-sediment relationship, and correlational evidence suggests that it is displaced byE. armatatowards coarse sands and upper beach levels. Thus, potential biotic interactions cannot be discarded as a structuring force in sandybeach communities.

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