Size matters sometimes: wall height and the structure of subtidal benthic invertebrate assemblages in south‐eastern Australia and Mediterranean Spain

Abstract

Aim Variation in the structure of shallow subtidal invertebrate assemblages was examined over three spatial scales; within reef, between reef and between continents. We sought to provide a context from which to examine and interpret ecological processes between continents. In addition, we predicted that variation in pattern would increase as the scale of examination increased.Location Reefs near Wollongong and within Jervis Bay in south-eastern Australia and Mediterranean reefs on the Costa Brava (Catalonia), north-eastern Spain.Methods We compared assemblages on vertical rock walls of two heights -short (< 2 m) and tall (> 3 m) in two temperate regions over the same depth range. Specifically we examined the diversity and cover of invertebrates, the cover and biomass of foliose and crustose algae, the size of invertebrate colonies and the biomass of urchins on short and tall walls (n ¼ 3) at each of two locations in each country.Results Foliose algae dominated rock walls in Spain and although invertebrate cover was high, colonies were generally very small. Two urchin species were commonly encountered on rock walls in Spain, Arbacia lixula and Paracentrotus lividus; their biomass was relatively low and did not differ significantly between short and tall walls. These findings contrasted strongly with south-eastern Australia, where foliose algae were almost completely absent. A single urchin species, Centrostephanus rodgersii occurred with extremely high biomass on short walls, which were dominated by grazer-resistant crustose calcareous algae. In contrast, the biomass of this urchin was low on tall walls, which were dominated by invertebrates, usually exceeding 95% in cover. Invertebrate colonies were significantly larger on both short and tall walls in south-eastern Australia relative to the Mediterranean. Findings within a country were consistent between the replicate rock walls and between locations. In contrast to our prediction, however, there was significant variation among walls within a location, but not among locations within a continent. Temporal variation in the structure of these assemblages was not examined, but appears limited.Main conclusions We conclude that submarine topography, i.e. the presence of short or tall rock walls, as a function of rock type and structure, has a marked impact on community structure in south-eastern Australia, but made little difference to the structure of the assemblage in Mediterranean Spain. The differences in structure we observed between walls of different heights in Australia were correlated with differences in the biomass of urchins and they appear to be major determinants of assemblage structure. Interactions among species are often reported from disparate parts of the globe with little or no reference to the structure of the assemblage of which they are a part; we contend

Structure and dynamics of sponge-dominated assemblages on exposed and sheltered temperate reefs

“…Previous studies show that many marine assemblages vary at a hierarchy of scales, from centimetres to 100s of kilometres and from days to years (Coleman 2002, Hewitt et al 2002, Davis et al 2003. Most of these types of studies have, however, been done in the intertidal or in relatively shallow water, and, consequently, our understanding of the spatial and temporal scales of variability in invertebrate assemblages in deep water is scant and cannot be readily predicted from previous work.…”

Establishment and persistence of species-rich patches in a species-poor landscape: role of a structure-forming subtidal barnacle

“…Urchin densities in nearby crevices were extremely high (usually averaging > 2 kg m -2 wet weight; Davis et al 2003). The cover of grazer-resistant crustose coralline algae on the vertical surfaces usually averaged > 80%, with sessile invertebrates accounting 188 Davis & Ward: Habitat structure and subtidal community development for the remaining 20% cover (Davis et al 2003). Sponges were the dominant sessile invertebrates on these rock faces and their cover was usually positively associated with the density of Austrobalanus (Davis & Ward 1999).…”

“…All study plots were established in the urchin-grazed barrens and showed evidence of intense grazing. Urchin densities in nearby crevices were extremely high (usually averaging > 2 kg m -2 wet weight; Davis et al 2003). The cover of grazer-resistant crustose coralline algae on the vertical surfaces usually averaged > 80%, with sessile invertebrates accounting 188 Davis & Ward: Habitat structure and subtidal community development for the remaining 20% cover (Davis et al 2003).…”

“…Unlike many temperate echinoids, this urchin is strongly reliant on shelter, spending the day in crevices, then emerging at night to forage (Andrew 1993). In these barrens, a low cover of sessile invertebrates manages to maintain space despite the often intense urchin grazing (Wright et al 1997, Davis et al 2003. Sponges, ascidians and bryozoans are particularly evident on vertical surfaces, usually in association with the large balanoid barnacle Austrobalanus imperator (Darwin, 1854; hereafter Austrobalanus).…”

Heart of glass: spicule armament and physical defense in temperate reef sponges

“…On subtidal reefs in New South Wales, Australia, the large sea urchin Centrostephanus rodgersii is the most conspicuous consumer, and occurs in densities up to 22 m -2 (Underwood et al 1991, Andrew & Underwood 1993, with biomass (wet weight) frequently exceeding 2 kg m -2 (Davis et al 2003). Grazing by this species removes virtually all macroalgae, leaving a substratum covered almost exclusively by crustose coralline algae (the barrens habitat).…”