Competitive interactions in macroinfaunal animals of exposed sandy beaches

Abstract

The influence of biotic interactions in structuring macroinfaunal communities of exposed sandy beaches, an unstable habitat characterized by strong physical forces, is generally considered negligible. We investigated the hypothesis that competitive interactions during burrowing could potentially affect the intertidal distribution and abundance of macroinfaunal animals of sandy beaches using two species of invertebrates, a hippid crab, Emerita analoga, and a bivalve, Mesodesma donacium, common along the coast of Chile. Spatial overlap in the intertidal distributions of these species was dynamic, varying with abundance, location, time of year and tide. Highest density zones of each species were often distinctly separated at low tide and spatial overlap in their distributions decreased significantly with increasing density, suggesting density dependence of the interactions. Negative relationships between densities of the two species at the smallest spatial scale examined also suggested active interactions among individuals. Over a tidal cycle, peak densities of the two species overlapped suggesting that interactions could occur frequently. Burrowing performance of E. analoga varied between size classes in three experimental densities of clams (5, 10 and 15 clams 0.008 m(-2)) and in controls with no clams. Burrowing times of large crabs were significantly longer (approximately twofold) in all densities of clams than in controls, while those of small crabs did not differ significantly among treatments and controls. Large crabs also displaced clams from the sand while burrowing suggesting that two mechanisms of direct interference can occur, both of which could increase exposure of individuals involved to active swash and transport across or along the beach with potentially negative consequences. Our results suggest that competitive interactions capable of affecting zonation and population and community biology on a number of scales can occur among burrowing macroinfauna on exposed sandy beaches. Those interactions could be more ecologically significant than previously appreciated and may contribute to patterns observed in community structure and zonation on sandy beaches. Our results illustrate the potential importance of negative biological interactions in a physically stressful environment.

Thriving and declining: climate variability shaping life-history and population persistence of Mesodesma donacium in the Humboldt Upwelling System

“…The enhanced ability of juveniles to cope with low salinities may represent an early physiological trait allowing larvae to settle in vacant areas by active or passive processes (e.g. Mann et al 1991, Shanks & Brink 2005) and thereby to reduce strong intraspecific and interspecific competition for food and habitat quality (Tarifeño 1980, Dugan et al 2004 and increasing recruitment success. However, contradictory evidence regarding spatial segregation related to salinity has been reported by Ortiz & Stotz (1996).…”

Patterns, processes and regulatory mechanisms in sandy beach macrofauna: a multi-scale analysis

“…Defeo et al (1997) reported maximum densities of Excirolana armata similar in allopatry and sympatry, but allopatric populations of E. braziliensis were more abundant lower on the shore than at sympatric locations, where distribution was restricted to upper beach levels. McLachlan (1998) and Dugan et al (2004) provided further evidence of the role of competitive interactions in experiments where changes in burrowing performance of one species were monitored at different densities of other species, even though these effects could be minimal at field densities (McLachlan 1998). Yu et al (2002) found that feeding habits in 4 sublittoral amphipods could explain variations in zonation; when dietary composition overlapped, spatial segregation reduced competition for food.…”

“…Active filtering of larvae during feeding activity was a potential explanatory mechanism. Dugan et al (2004) demonstrated negative relationships between the mole crab Emerita analoga and the surf clam Mesodesma donacium at small spatial scales. Spatial overlap in the intertidal distributions of these species varied with abundance in time and space.…”

Long or short? Investigating the effect of beach length and other environmental parameters on macrofaunal assemblages of Maltese pocket beaches

“…In fact, some authors, including, Dugan et al (2004), believe that biological interactions, including competitive ones, not only affect the distributions and zonation of intertidal macro-infauna but potentially also community structure. Studies by Veloso and Cardoso (2001) show that the abundance of the communities was also not related to any physical parameter and these authors suggested that certain biotic factors, such as food source, recruitment, predation and competition, are responsible for the differences in macrofaunal densities between beaches.…”