A multitude of biologically significant environmental changes are projected to occur as a consequence of anthropogenic climate change (Solomon et al., 2007). How will life and biodiversity on Earth respond to the current and projected climate change? Scientists and policy makers recognise that this is one of the most important questions in science at the moment, since predicting ecosystem level responses to change are a fundamental requirement for the future management of biodiversity, agriculture and fisheries (Milenium Ecosystem Assessment, 2005, UN convention on Biological Diversity, CBD).
Predicted increase in ocean temperatures is one of the most important impacts of climate change, as temperature influences physiological and ecological processes across biological scales, from genes to ecosystems. Our current knowledge of the observed and expected biological changes, the ecophysiological and genetic mechanisms that drive them on land far exceeds that from ocean systems. The inaccessibility of aquatic habitats, the large cost involved and the challenging nature of marine research contributes to the gap in the knowledge.
To date, predictions of responses to change in animals have been primarily at the species level and based around 2 approaches. The first uses current species range (climate envelops) and then predicts future ranges by assessing where similar conditions are likely to be from climate models. The second approach evaluates an organism’s physiological capacity to cope with experimentally altered conditions in the laboratory. Although widely used across studies (Kruuk et al., 2008; Kopp et al., 2014), both approaches have significant problems. Species ranges lack the knowledge of adaptation rates and genetic and functional tolerance difference within and between populations. Similarly, the conclusions that can be drawn from using physiological approaches are limited, because they predominantly evaluate small numbers of species and because experimental rates of change are usually much faster than natural change. There is therefore a key requirement to develop an approach that will improve our understanding of assemblage or community level processes and responses. One of the parameters that need to be considered when developing approaches to understanding how communities or assemblages will respond to environmental change, is the identification of the most vulnerable stages of the population. The loss of such stages could have a great impact in the overall biodiversity. In this context, early life history stages have been identified as the most vulnerable stages to change (Pedersen et al., 2008): the largest mortality across life histories occurs in early development and recruitment. The small body size, reasonably high (mass-specific) metabolic rates and lower energy reserves, decreases the capability of early life stages to select and migrate towards a suitable habitat, further increasing their vulnerability to climate change (Rijnsdorp et al., 2009). Such characteristics will also increase their mortality during periods of adverse environmental condition e.g. periods of food shortage.
Studies of temperature tolerances of developmental stages in the laboratory are relatively rare, but those that have been conducted do not often show marked differences from temperature limits of adults. In this context, Stanwell-Smith & Peck (1998) showed that over 80% of embryos developed normally in 3 species of Antarctic echinoderms at temperatures up to 3??C, but not above. Different species will vary in their responses to warming and this will affect their competitive abilities and fitness, potentially through variations in aerobic capacity and functional capability (P??rtner et al. 2007). Small changes in balance in early life history and colonisation stages in marine species are likely to give very large changes in community structure and ecosystem balance. These factors, coupled with the very high ecologically-driven mortality in early life history stages, mean that investigations of warming effects on recruitment and early community development in marine benthic groups are an essential step towards understanding ecosystem level responses to change.
2. The Antarctic sessile marine benthos
Sublittoral sessile epifaunal assemblages are often characterised by similar group of organisms, potentially allowing comparisons of the effects of environmental variation on basic biological and ecological processes to be made between regions. The Antarctic benthos has been studied from the earliest expeditions and there has been much interest in the rates of biological and ecological processes in relation to other latitudes (See reviews by Clarke, 1991). Antarctic seas are particularly relevant to such studies because environmental seasonality is at it’s most extreme and seawater temperatures are extremely low and stable throughout the year (Clarke & Leakey, 1996). However, with notable exceptions (Dayton et al., 1974; Stanwell-Smith & Barnes, 1997; Bowden et al., 2006), the majority of studies up to date are largely based on the description of the instantaneous pattern observed, from which the rates and causal mechanisms of underlying processes are largely inferred. There is therefore no empirical data that explains the mechanisms and ecological processes involved. The latter is primarily a consequence of sampling difficulties associated with the slow rates of biological processes in the region and loss of equipment through ice impacts during prolonged deployment.
Existing studies of benthic assemblages in Antarctica suggest three distinctive characteristics. First, growth in most taxa is slow by comparison with similar taxa at lower latitudes (Pearse et al., 1991; Clarke et al., 2004). This may be due to physiological limitations associated with low temperature, food limitation due to temporally-limited primary production or a combination of these factors (Clarke et al., 2004). Second, growth in many taxa is highly seasonal, primarily restricted to the summer period of primary production. Notable exceptions include shell growth in brachiopods and 1 infaunal bivalve species, which take place during winter (Peck et al., 1997; Peck et al., 2000). Third, the principal factor determining sessile assemblage structure in nearshore waters is the gradient of decreasing physical disturbance by ice with increasing depth (Gutt, 2001).
Despite the latitudinal diversity gradient hypothesis, by which species richness decreases away from the tropics, life on the Southern Ocean shelves is highly abundant and rich (Clarke & Johnston, 2004). Cheilostome bryozoans together with spirorbid polychaetes are the most abundant group in the Antarctic, in terms of number of recruits and areal coverage (Barnes et al., 2006). There is a high variability of areal cover. Using settlement plates, Osman (1997) and Greene et al., (1983) reported significant declines in areal coverage after initial rapid colonisation. A later study by Todd (1998) identified that differences in area coverage were dependent on the season of initial immersion. Thus, the timing of the first immersion indicates that timing of disturbance and hence, the availability of free space, is an important factor in determining the taxonomic composition of Antarctic assemblages. This effect is primarily caused by the pre-emption of space by initial recruits, which settle in high densities and grow rapidly (E.g. Ascidia sp. in Sutherland 1974). In the Antarctic however, restricted and strongly seasonal growth may negate the potential for such ‘lottery’ effects to be generated by variations in the timing of disturbance.
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