Size–Abundance Relationships of Freshwater Macroinvertebrates in Two Contrasting Floodplain Channels of Rhone River

Body size is perhaps the most fundamental property of an organism and its relationship with abundance is one of the most studied relationships in ecology. Although numerous studies have examined these relationships in local communities, few have investigated how they vary at different temporal and spatial scales. We investigated the relationship between body size and abundance of local macroinvertebrate communities in two floodplain channels of the French upper Rhone River. The two channels differ in their vegetation coverage (high vs. low vegetation) and hydrological regimes. The shapes of the size–abundance relationship were similar between channels on a yearly basis but differed when compared between months. The variation in local size–abundance relationships between months was related to variation in the functional diversity across time. Our findings suggest that local size–abundance relationships are able to quantitatively describe temporal changes in community structure, showing the importance of relating diversity with ecosystem function in a more realistic context.

Increased energy differentially increases richness and abundance of optimal body sizes in deep-sea wood falls

Theoretical and empirical studies suggest that the total energy available in natural communities influences body size as well as patterns of abundance and diversity. But the precise mechanisms underlying these relationships or how these three ecological properties relate remain elusive. We identify five hypotheses relating energy availability, body size distributions, abundance, and species richness within communities, and we use experimental deep-sea wood fall communities to test their predicted effects both on descriptors describing the species-richness-body-size distribution, and on trends in species richness within size classes over an energy gradient (size-class-richness relationships). Invertebrate communities were taxonomically identified, weighed, and counted from 32 Acacia sp. logs ranging in size from 0.6 to 20.6 kg (corresponding to different levels of energy available), which were deployed at 3,203 m in the Northeast Pacific Ocean for 5 and 7 yr. Trends in both the species-richness-body-size distribution and the size-class-richness distribution with increasing wood fall size provide support for the Increased Packing hypothesis: species richness increases with increasing wood fall size but only in the modal size class. Furthermore, species richness of body size classes reflected the abundance of individuals in that size class. Thus, increases in richness in the modal size class with increasing energy were concordant with increases in abundance within that size class. The results suggest that increases in species richness occurring as energy availability increases may be isolated to specific niches, e.g., the body size classes, especially in communities developing on discrete and energetically isolated resources such as deep sea wood falls.

Parasitism alters three power laws of scaling in a metazoan community: Taylor's law, density-mass allometry, and variance-mass allometry

How do the lifestyles (free-living unparasitized, free-living parasitized, and parasitic) of animal species affect major ecological power-law relationships? We investigated this question in metazoan communities in lakes of Otago, New Zealand. In 13,752 samples comprising 1,037,058 organisms, we found that species of different lifestyles differed in taxonomic distribution and body mass and were well described by three power laws: a spatial Taylor's law (the spatial variance in population density was a power-law function of the spatial mean population density); density-mass allometry (the spatial mean population density was a power-law function of mean body mass); and variance-mass allometry (the spatial variance in population density was a power-law function of mean body mass). To our knowledge, this constitutes the first empirical confirmation of variance-mass allometry for any animal community. We found that the parameter values of all three relationships differed for species with different lifestyles in the same communities. Taylor's law and density-mass allometry accurately predicted the form and parameter values of variance-mass allometry. We conclude that species of different lifestyles in these metazoan communities obeyed the same major ecological power-law relationships but did so with parameters specific to each lifestyle, probably reflecting differences among lifestyles in population dynamics and spatial distribution.

Predicting free-space occupancy on novel artificial structures by an invasive intertidal barnacle using a removal experiment

Artificial structures can create novel habitat in the marine environment that has been associated with the spread of invasive species. They are often located in areas of high disturbance and can vary significantly in the area of free space provided for settlement of marine organisms. Whilst correlation between the amount of free space available and recruitment success has been shown in populations of several marine benthic organisms, there has been relatively little focus on invasive species, a group with the potential to reproduce in vast numbers and colonise habitats rapidly. Invasion success following different scales of disturbance was examined in the invasive acorn barnacle, Austrominiusmodestus, on a unique art installation located in Liverpool Bay. Population growth and recruitment success were examined by comparing recruitment rates within disturbance clearings of 4 different sizes and by contrasting population development with early recruitment rates over a 10 week period. Disturbed areas were rapidly recolonised and monocultures of A. modestus formed within 6 weeks. The size of patch created during disturbance had no effect on the rate of recruitment, while a linear relationship between recruit density and patch size was observed. Density-dependent processes mediated initial high recruitment resulting in population stability after 8-10 weeks, but densities continued to greatly exceed those reported in natural habitats. Given that artificial structures are likely to continue to proliferate in light of climate change projections, free-space is likely to become more available more frequently in the future supporting the expansion of fast-colonising species.

Animal body size distributions: patterns, mechanisms and implications

Documenting the shape of the frequency distribution of species body sizes for an animal taxon appears at first sight a straightforward task. However, a variety of patterns has been reported, and a consensus is only now being reached through an understanding of how potential biases may affect observed shapes of distributions. A new body of evidence suggests that, at large scales, size distributions are right-skewed, even on logarithmic axes. If body size distributions can be described with certainty, this will allow assessment of the mechanisms proposed to generate them, and will be an important step towards understanding the structure and dynamics of animal assemblages.

Differential rates of morphological divergence in birds

There are more small-bodied bird species than there are large-bodied, even on a logarithmic scale. In birds this pattern, which is also found in other higher taxa, appears not to be due to neutral evolution. It has often been suggested that the skew of body size frequency distributions is the result of a relationship between body size and the net rate of speciation, but phylogenetic analyses so far have rejected the hypothesis that small-bodied species are subject to higher net rates of speciation. On the contrary, we show that there exists a relationship between body size and its own evolutionary variability: avian families of small body size show less interspecific variation in body size than large-bodied families of similar age and species richness.

Size-abundance relationships in an Amazonian bird community: implications for the energetic equivalence rule

We studied size-abundance relationships in a species-rich Amazonian bird community and found that the slope of the logarithmic relationship between population density and bodymass (b = -0.22) is significantly shallower than expected under Damuth's energetic equivalence rule (EER), which states that population energy use (PEU) is independent of species body mass. We used estimates of avian field metabolic rates to examine the logarithmic relationship between PEU and body mass and its variation among ecological guilds. The relationship for all species had a significantly positive slope (b = 0.46), indicating that PEU of larger species was greater than that of smaller species. Analyses of guilds revealed significant variation. The slopes of the frugivore-omnivore, insectivore, and granivore guilds were all significantly positive, with that of the frugivore-omnivore guild being the steepest. In contrast, PEU did not vary significantly with species body mass among raptors. These results were confirmed, in analyses using both species values and phylogenetically independent contrasts, and the results do not support the EER in this community. The spatial distribution of resources and mechanisms of interference competition within guilds may explain why most patterns differed from the predictions of the EER. Other sources of variation, including the effects of scale, are also discussed.

The allometry of density within the space used by populations of mammalian Carnivora

The relationship between body mass and population density has been used to develop theory of how energy is used in ecosystems. The usual allometric density slope, –0.75, was reduced to near zero among species of mammalian Carnivora after Smallwood and Schonewald and Blackburn and Gaston adjusted density estimates by the sizes of the corresponding study areas. In this paper, I restricted the allometric analysis to density estimates made at or near the threshold area, which is the species-specific minimum area likely to support a population. I excluded densities estimated from subpopulations and "megapopulations", thereby removing biases of study design that had previously confused the allometry of population density. Density at threshold area declined with increasing body mass. The population's mass density did not relate to threshold area, within which carnivore species averaged 9 kg/km2. The spatial intensity of oxygen consumption did not relate to body mass, but assuming that species with smaller threshold areas occur at more locations than species with larger threshold areas, one must conclude that smaller bodied species use more energy from the environment than do larger bodied species. Furthermore, threshold area and density at threshold area were most responsive to female brain mass, which provides an ecological allometry that links spatial scale, sensory perception, parental care, life-history attributes, basal metabolic rate, and body mass.

Is the relationship between population density and body size consistent across independent studies? A meta-analytical approach

The Energetic Equivalence Rule (EER) is a controversial issue in ecology. This rule states that the amount of energy that each species uses per unit of area is independent of its body size. Here, we perform a meta-analytical procedure to combine and compare the slopes of population density and body size relationships across independent studies of mammals and birds. We then compared a distribution of 50,000 bootstrap combined slopes with the expected slope (b = -0.75) under the EER. The combined slopes obtained for mammals and birds separately were -0.755 and -0.321, respectively. The homogeneity hypothesis (i. e. within studies the slopes differ by no more than would be expected due sampling variation) was rejected in both cases. So, EER cannot be supported since the use of an exponent of -0.75 is, in fact, an oversimplification. Significant heterogeneity of slopes within each group (mammals and birds) is an indicator of inferential problems related with variation in body size, spatial scale, the regression model adopted and phylogenetic relationships among species. So, we consider that questions regarding the estimation and validity of slopes is the next challenge of density-body size relationship studies.

Relations between body size, abundance and taxonomy of birds wintering in Britain and Ireland

We examine the relation between body size, abundance, and taxonomy in the wintering bird assemblages in Britain and Ireland. The regression slope of abundance on body size across species in both assemblages is not significantly different from that predicted by an ‘energetic equivalence rule’, but the proportion of the variance in abundance explained by body size is very low. Previous work on breeding bird assemblages has found the novel relation that the correlation between size and abundance across species within a tribe is itself positively correlated with the degree of taxonomic isolation of the tribe from other tribes in the bird fauna. We show that the same relation holds within bird tribes in the two wintering assemblages. Furthermore, evidence for this relation is found by using two different measures of bird abundance, despite these two abundance measures showing very different correlations with body size across species. Although these patterns in the data are consistent, some are not formally statistically significant ( p = 0.089 or greater). Excluding coastal, stocked, feral and recently colonizing species increased the significance of time since origin of a tribe on species abundances. We conclude that the relation between size and abundance in bird tribes is somehow related to bird taxonomy. While acknowledging the unlikely nature of such an effect, we tentatively propose hypotheses for two mechanisms that could produce the observed patterns.

Population abundance and body size in animal assemblages

Studies of the relationship between body mass and population abundance for terrestrial and aquatic animal species based on pooling data from many taxa and assemblages suggest that abundance scales with mass to the —0.75 power. Because metabolic rate scales with mass as (plus) 0.75, this result has been taken as evidence that all species in assemblages use equal amounts of energy. The evidence for ‘energetic equivalence’ is, however, equivocal, because within many individual assemblages the scaling of abundance on mass differs significantly from —0.75. Here, we present a summary of patterns of size and abundance in a number of different terrestrial, freshwater and marine animal assemblages, with the aim of discovering whether there is any generality in size-abundance patterns within assemblages, and whether any generality might hold across terrestrial, freshwater and marine environments.

Phylogeny and relations among abundance, geographical range and body size of British breeding birds

Links between bird phylogeny and abundance, geographical range and body size relations were examined with use of a newly published data set on the numbers and distribution of British breeding birds. There was a negative correlation between abundance (and geographical range) and body size across species, but no significant correlations within non-passerine and passerine taxa considered separately. Abundance correlated positively with geographical range across species and within non-passerines and passerines. Three measures of phylogenetic relatedness of bird tribes were considered, termed ‘rootedness’, ‘date of origin’ and ‘radiation d ate’. The date at which a tribe originated (measured as rootedness or date of origin) had a consistent but weak influence on the form of the relation between abundance and body size. Phylogeny was not implicated in the relation between geographical range and body size. Phylogenetically isolated tribes were more likely to show a positive correlation between abundance and body size than more recently evolved tribes. Results are discussed in the context of previous studies of both regional and local bird assemblages and the hypotheses suggested to explain associations with phylogeny.

Species richness and population dynamics of animal assemblages. Patterns in body size: abundance space

Links between population dynamics, the relative abundance of species and the richness of animal communities are reviewed within the framework ol a simple conceptual model, based on body size and abundance. Populations of individual species occupy positions in this body size: abundance space. Problems of relative abundance and absolute species-richness revolve around a number ol simple questions, including: what determines the upper and lower bounds (maximum and minimum population densities) of species in the assemblage; what determines the overall density of points (number of species) within these bounds; and how are the vertical and horizontal partitioning rules between species decided? The answers to these, and related questions, are briefly reviewed.

Relations between abundance, body size and species number in British birds and mammals

British birds and mammals are compared in terms of their frequency distributions of abundance and body mass and in respect of the relation between abundance and body mass. Body masses of non-flying mammals are greater than those of resident birds which are, in turn, heavier than migrants; bats are lightest. The frequency distribution of masses are close to log-Normal for each of these groups, though their variances and skews differ. Differences in mean abundances (which are log-Normally distributed) parallel those in body mass. In each group, abundance declines with body mass: the exponent of the relation is close to the value of —0.75 predicted by the ‘energetic equivalence’ rule though not significantly different from the value of — 1.0 predicted by the ‘biomass equivalence’ rule. At comparable masses, species of non-flying mammals are more abundant than resident birds, migrant birds and bats by approximately 45, 300 and 200 times, respectively. The similarity between birds and bats in this regard may be no more than coincidental but it may be related to ecological similarities related to flight. The metabolic rates of non-flying mammals may be generally lower than those of birds and bats but not sufficiently to account for their much greater abundances.

Parasite prevalence and host sample size

Parasite prevalence is a summary statistic familiar to biologists. However, that there is an interspecific relationship between prevalence and sample size (the number of host individuals examined for parasites) is not widely appreciated. In this article, Richard Gregory and Tim Blackburn present some examples of this negative relationship, explain the mechanisms that underlie this pattern and discuss the potential problems this association might create for biological studies.

Scaling Population Density to Body Size in Rocky Intertidal Communities

Interspecific comparisons of animal population density to body size has been the subject of active research in the last decade, especially for terrestrial animals when considering particular taxa or taxonomic assemblages. Studies of rocky intertidal communities showed that animal population density scales with body size to the -0.77 power. This relation held within local communities representing a broad array of animal taxa and was not affected by a dramatic alteration in the network of between-species interactions, as revealed by two long-term human exclusion experiments.