Changes in habitat complexity negatively affect diverse gastropod assemblages in coralline algal turf

Abstract

The physical structure of a habitat generally has a strong influence on the diversity and abundance of associated organisms. I investigated the role of coralline algal turf structure in determining spatial variation of gastropod assemblages at different tidal heights of a rocky shore near Sydney, Australia. The structural characteristics of algal turf tested were frond density (or structural complexity) and frond length (the vertical scale over which structural complexity was measured). This definition of structural complexity assumes that complexity of the habitat increases with increasing frond density. While frond length was unrelated to gastropod community structure, I found significant correlations between density of fronds and multivariate and univariate measures of gastropod assemblages, indicating the importance of structural complexity. In contrast to previous studies, here there were negative relationships between the density of fronds and the richness and abundance of gastropods. Artificial habitat mimics were used to manipulate the density of fronds to test the hypothesis that increasing algal structural complexity decreases the richness and abundance of gastropods. As predicted, there were significantly more species of gastropods in loosely packed than in tightly packed turf at both low- and mid-shore levels. Despite large differences between gastropod assemblages at different tidal heights, the direction and magnitude of these negative effects were similar at low- and mid-shore levels and, therefore, relatively independent of local environmental conditions. These novel results extend our previous understanding of the ecological effects of habitat structure because they demonstrate possible limitations of commonly used definitions of structural complexity, as well as distinct upper thresholds in the relationship between structural complexity and faunal species richness.

Physiological responses of the calcifying rhodophyte, Corallina officinalis (L.), to future CO2 levels

“…If increased CO 2 levels weaken the skeletal structure of C. oYcinalis, the potential combination of increased grazing pressure, slower growth, higher UV stress, and less recruitment will likely cause a phase shift in C. oYcinalis communities toward more Xeshy, non-calcifying macroalgae, and could even amplify changes in competition between non-calcifying algae. While both calcifying and non-calcifying algae provide important habitat and shelter for many marine organisms, erect calcifying algae such as C. oYcinalis contribute to the strength of the intertidal community structure and provide refugia for organisms in environments with high wave action (Stewart 1982;Coull and Wells 1983;Kelaher 2002Kelaher , 2003-all important ecological roles that could be interrupted under high CO 2 conditions.…”

“…Corallina spp. often form extensive macroalgal turfs that cover large areas of the intertidal and provides substratum, habitat, and refugia for a number of important marine organisms (Coull and Wells 1983;Hicks 1986;Akioka et al 1999;Kelaher 2002Kelaher , 2003. On the rocky coast of the island of Helgoland in the North Sea, C. oYcinalis is abundant and is an important species in the macroalgal community.…”

Structural variation in the brown alga Sargassum cymosum and its effects on associated amphipod assemblages

“…In contrast, Sanchez-Moyano et al (2001) reported that variation in the associated fauna was related to the life cycle of Caulerpa prolifera. In within-species comparisons, the greatest differences in the species richness and composition of the associated fauna have been associated with size-related characteristics of macroalgae (Russo, 1990;Attrill et al;, Kelaher, 2003a.…”

Effect of habitat architecture on mobile benthic macrofauna associated with patches of barnacles and ascidians

“…The relatively complex architecture of their reefs and beds influences other species to the same extent as their direct effects (Jones et al 1994). Variation in habitat structure (either of biogenic or abiotic nature) generates abundance and diversity patterns in numerous systems studied (see Beck 2000, Kelaher 2003, Hewitt et al 2005, and references therein) affecting recruitment and survival of benthic organisms. McCoy & Bell (1991) suggested quantifying habitat structure in terms of 'habitat complexity' and 'habitat heterogeneity'.…”

“…These factors stand for the absolute abundance of structural components and the relative abundance of different compo-nents at a certain spatial scale, respectively. With rare exceptions (Kelaher 2003), increased habitat complexity leads to an increase in diversity of the associated assemblages, which is generally believed to result from expansion of range of possible niches (MacArthur & MacArthur 1961).…”