2005. Does macrophyte fractal complexity drive invertebrate diversity, biomass and body size distributions? Á/ Oikos 111: 279 Á/290.Habitat structure is one of the fundamental factors determining the distribution of organisms at all spatial scales, and vegetation is of primary importance in shaping the structural environment for invertebrates in many systems. In the majority of biotopes, invertebrates live within vegetation stands of mixed species composition, making estimates of structural complexity difficult to obtain. Here we use fractal indices to describe the structural complexity of mixed stands of aquatic macrophytes, and these are employed to examine the effects of habitat complexity on the composition of freeliving invertebrate assemblages that utilise the habitat in three dimensions. Macrophytes and associated invertebrates were sampled from shallow ponds in southwest England, and rapid digital image analysis was used to quantify the fractal complexity of all plant species recorded, allowing the complexity of vegetation stands to be reconstructed based on their species composition. Fractal indices were found to be significantly related to both invertebrate biomass Á/body size scaling and overall invertebrate biomass; more complex stands of macrophytes contained a greater number of small animals. Habitat complexity was unrelated to invertebrate taxon richness and macrophyte surface area and species richness were not correlated with any of the invertebrate community parameters. The biomass Á/body size scaling relationship of lentic macroinvertebrates matched those predicted by models incorporating both allometric scaling of resource use and the fractal dimension of a habitat, suggesting that both habitat fractal complexity and allometry may control density Á/body size scaling in lentic macroinvertebrate communities.
Seaweed and seagrass communities in the northeast Atlantic have been profoundly impacted by humans, and the rate of change is accelerating rapidly due to runaway CO2 emissions and mounting pressures on coastlines associated with human population growth and increased consumption of finite resources. Here, we predict how rapid warming and acidification are likely to affect benthic flora and coastal ecosystems of the northeast Atlantic in this century, based on global evidence from the literature as interpreted by the collective knowledge of the authorship. We predict that warming will kill off kelp forests in the south and that ocean acidification will remove maerl habitat in the north. Seagrasses will proliferate, and associated epiphytes switch from calcified algae to diatoms and filamentous species. Invasive species will thrive in niches liberated by loss of native species and spread via exponential development of artificial marine structures. Combined impacts of seawater warming, ocean acidification, and increased storminess may replace structurally diverse seaweed canopies, with associated calcified and noncalcified flora, with simple habitats dominated by noncalcified, turf-forming seaweeds.
The number of species in an assemblage at a given point in time is a fundamental property of ecological systems, yet it is hard to quantify for many marine systems. We studied the performance of 6 techniques ('estimators') for extrapolating species richness from limited numbers of samples, using 3 datasets for which an absolute value for total species richness could be determined. We propose that the ideal estimator should always slightly overestimate species richness compared to any observed maximum species richness derived from sampling, as sampling error will always lead to underestimation of true richness. We quantified performance of the estimators relative to the sampled total richness in the assemblage across a range of efforts up to 80% of that required to achieve the asymptote of the species accumulation. We used 3 measures: bias (mean deviation of an estimate from the known richness), precision (variance of repeated estimates based upon a subset of the available pool of samples), and overall accuracy (a combination of bias and precision). No single estimator performed best in all cases, and estimator performance was affected by sampling effort. The estimator Chao1 performed best at intermediate sampling efforts, with LAG S ∞ also performing well at high relative effort. S ∞ consistently underestimated, whilst Chao2 and ICE both overestimated and displayed poor precision and accuracy, especially at intermediate sampling efforts and in datasets with uneven patterns of species incidence. Species abundance and incidence amongst samples of a dataset were shown to affect performance of most of the estimators, with the exception of the recently proposed S ∞ family of techniques. We conclude that Chao1 represents the best compromise choice of estimator, and that such nonparametric techniques may represent useful tools for rapid estimation of species richness for some marine assemblages, based on limited sampling effort.
1. Dispersal ability influences the distribution and abundance of organisms, but empirical investigations of the relationship between dispersal ability and the composition of ecological assemblages are scarce. Here, we compare between‐site variation in the species richness and community composition of actively and passively dispersing pond invertebrates. 2. Coleoptera (active dispersers) and microcrustacea (passive dispersers) were sampled over a season from 16 ponds within a 4‐km radius in south‐west England. Species richness and community composition were related to environmental variables using regression and Canonical Correspondence Analysis (CCA), respectively. 3. Coleopteran species richness was significantly and positively correlated with pond permanence and maximum area, whereas microcrustacean species richness was relatively equal across sites and did not correlate with environmental variables. The frequency of species' occurrence between sites was the same for both groups, which suggests that active and passive dispersers exhibited the same degree of dispersal. 4. Between‐site variation in community composition was non‐random for both groups, with pond permanence and area, together, explaining similar proportions of between‐site variation for Coleoptera. Permanence was correlated most strongly with microcrustacean community composition and a high proportion (25%) of microcrustacean species were more numerous in smaller, more ephemeral ponds. 5. These data suggest that, at small spatial scales, Coleoptera which can undertake multiple dispersal events, are more likely to colonise large, more permanent ponds than passively dispersing microcrustacea. For microcrustacea, other traits (in this case those permitting existence in ephemeral habitats) may over‐ride the influences of dispersal in driving between‐site variation in species composition.
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