Evolution of body size in food webs: does the energetic equivalence rule hold?

The energetic equivalence rule (EER), which is derived from empirical observations linking population density and body size and from the allometric law linking metabolism and body size, predicts that the amount of energy used by the various species should be independent of body size. Here, we examine this hypothesis using a model that allows entire food webs to emerge from coevolution of interacting species. Body size influences both individual metabolism and interactions among species in the model. Overall, population density does decrease with body size roughly following a power law whose exponent is variable. We discuss this variability in the light of empirical data sets. The emerging relationship between the flux of resources exploited by the various species and their body size follows a decreasing power law, thus contradicting the EER. Our model emphasizes the importance of considering the influence of body size on species interactions in attempting to explain large-scale patterns related to body size.     

Asymmetry patterns across the distribution range: does the species matter?

An important question in evolutionary ecology is whether different populations across a species range, from core to periphery, experience different levels of stress. The estimation of developmental instability has been proposed as a useful tool for quantifying the degree of environmental and genetic stress that individuals experience during their development. Fluctuating asymmetry, the unsigned difference between the two sides of a bilaterally symmetrical trait, has been suggested to reflect the levels of developmental instability in a population. As such, it has been proposed as a useful tool for estimating changes in developmental instability and in stress response in populations across a range of environmental conditions. Recent studies focusing mostly on birds have detected increasing fluctuating asymmetry from core to periphery across the distribution range, suggesting that peripheral populations may experience higher levels of environmental and/or genetic stress. Most of these comparisons were done for single taxa across a single gradient. However, different species are predicted to respond differently to environmental shifts across the range. We compared asymmetry patterns in wing morphology in populations of two Euchloe butterfly species across their opposing ranges in Israel. Contrary to the patterns observed in birds across the same gradient, bilateral asymmetry did not increase or shift towards the periphery in either of the butterfly species. If fluctuating asymmetry in these traits reflects levels of stress, these results may partly reflect the fact that the range of these two butterfly species is limited by the distribution of their host plant, rather than by abiotic environmental variables. In addition, developing pierids can diapause during harsh seasons and can persist in resource‐rich patches, thus minimizing the environmental stress perceived by developing individuals. We conclude that accounting for differences in species’ life histories and range‐limiting factors is necessary in order to better predict patterns of developmental instability across spatial and environmental gradients. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81 , 313–324.     

Patterns of size variation in bees at a continental scale: does Bergmann's rule apply?

Body size latitudinal clines have been widley explained by the Bergmann's rule in homeothermic vertebrates. However, there is no general consensus in poikilotherms organisms in particular in insects that represent the large majority of wildlife. Among them, bees are a highly diverse pollinators group with high economic and ecological value. Nevertheless, no comprehensive studies of species assemblages at a phylogenetically larger scale have been carried out even if they could identify the traits and the ecological conditions that generate different patterns of latitudinal size variation. We aimed to test Bergmann's rule for wild bees by assessing relationships between body size and latitude at continental and community levels. We tested our hypotheses for bees showing different life history traits (i.e. sociality and nesting behaviour). We used 142 008 distribution records of 615 bee species at 50 × 50 km (CGRS) grids across the West Palearctic. We then applied generalized least squares fitted linear model (GLS) to assess the relationship between latitude and mean body size of bees, taking into account spatial autocorrelation. For all bee species grouped, mean body size increased with higher latitudes, and so followed Bergmann's rule. However, considering bee genera separately, four genera were consistent with Bergmann's rule, while three showed a converse trend, and three showed no significant cline. All life history traits used here (i.e. solitary, social and parasitic behaviour; ground and stem nesting behaviour) displayed a Bergmann's cline. In general there is a main trend for larger bees in colder habitats, which is likely to be related to their thermoregulatory abilities and partial endothermy, even if a ‘season length effect’ (i.e. shorter foraging season) is a potential driver of the converse Bergmann's cline particularly in bumblebees.     

Phylogenetic comparative methods and the geographic range size – body size relationship in new world terrestrial carnivora

Most recent papers avoid describing macroecological relationships and interpreting then without a previous control of non-independence in data caused by phylogenetic patterns in data. In this paper, we analyzed the geographic range size – body size relationship for 70 species of New World terrestrial Carnivora (fissipeds) using various phylogenetic comparative methods and simulation procedures to assess their statistical performance. Autocorrelation analyses suggested a strong phylogenetic pattern for body size, but not for geographic range size. The correlation between the two traits was estimated using standard Pearson correlation across species (TIPS) and four different comparative methods: Felsenstein's independent contrasts (PIC), autoregressive method (ARM), phylogenetic eigenvector regression (PVR) and phylogenetic generalized least-squares (PGLS). The correlation between the two variables was significant for all methods, except PIC, in such a way that ecological mechanisms (i.e., minimum viable population or environmental heterogeneity- physiological homeostasis), could be valid explanations for the relationship. Simulations using different O-U processes for each trait were run in order to estimate true Type I errors of each method. Type I errors at 5% were similar for all phylogenetic methods (always lower than 8%), but equal to 13.1% for TIPS. PIC usually performs better than all other methods under Brownian motion evolution, but not in this case using a more complex combination of evolutionary models. So, recent claims that using independent contrasts in ecological research can be too conservative are correct but, on the other hand, using simple across-species correlation is too liberal even under the more complex evolutionary models exhibited by the traits analyzed here.     

Proximate causes of Rensch's rule: does sexual size dimorphism in arthropods result from sex differences in development time?

A prominent interspecific pattern of sexual size dimorphism (SSD) is Rensch's rule, according to which male body size is more variable or evolutionarily divergent than female body size. Assuming equal growth rates of males and females, SSD would be entirely mediated, and Rensch's rule proximately caused, by sexual differences in development times, or sexual bimaturism (SBM), with the larger sex developing for a proportionately longer time. Only a subset of the seven arthropod groups investigated in this study exhibits Rensch's rule. Furthermore, we found only a weak positive relationship between SSD and SBM overall, suggesting that growth rate differences between the sexes are more important than development time differences in proximately mediating SSD in a wide but by no means comprehensive range of arthropod taxa. Except when protandry is of selective advantage (as in many butterflies, Hymenoptera, and spiders), male development time was equal to (in water striders and beetles) or even longer than (in drosophilid and sepsid flies) that of females. Because all taxa show female-biased SSD, this implies faster growth of females in general, a pattern markedly different from that of primates and birds (analyzed here for comparison). We discuss three potential explanations for this pattern based on life-history trade-offs and sexual selection.     

Relationships between body size and geographical range size among Australian mammals: has human impact distorted macroecological patterns?

Extinction and artificial reduction in the size of geographical ranges of many species have occurred extensively across the globe because of human activities. In particular, Australian mammals have suffered heavily in the last two hundred years, with the highest number of reported cases of mammal extinctions anywhere. In the present study, we investigated the extent to which human impact has affected contemporary macroecological patterns in Australian terrestrial mammals. After examining patterns relating to body size and range size among the contemporary mammal fauna, we removed the effects of the last two hundred years of human impact by exploring patterns in the pre‐European assemblage. This permitted us to determine whether contemporary macroecological patterns are distortions of pre‐European patterns. In contrast to the expected pattern of a significant positive relationship between body size and range size, our results showed no significant association for the complete fauna in both cross‐species and phylogenetic analyses, even when data were corrected for species extinctions and range reductions. Analyses within families and among species with the same dietary strategy revealed three significant positive relationships (Macropodidae, Peramelidae, and herbivores) and one significant negative relationship (insectivores) within the contemporary assemblage that disappeared when the pre‐European assemblage was analysed. A positive relationship also emerged in the pre‐European assemblage for the Vombatidae that was not apparent in the contemporary fauna. Thus, correcting for human impact revealed important distortions among contemporary macroecological relationships that have been brought about by human‐induced range reduction and extinction. These findings not only provide further evidence that the Australian continent presents a unique and valuable opportunity with which to test the generality of macroecological patterns, but they also have important ramifications for the analysis and interpretation of contemporary macroecological datasets.     

What drives the evolution of body size in ectotherms? A global analysis across the amphibian tree of life

Aim

The emergence of large-scale patterns of animal body size is the central expectation of a wide range of (macro)ecological and evolutionary hypotheses. The drivers shaping these patterns include climate (e.g. Bergmann's rule), resource availability (e.g. ‘resource rule’), biogeographic settings and niche partitioning (e.g. adaptive radiation). However, these hypotheses often make opposing predictions about the trajectories of body size evolution. Therefore, whether underlying drivers of body size evolution can be identified remains an open question. Here, we employ the most comprehensive global dataset of body size in amphibians, to address multiple hypotheses that predict patterns of body size evolution based on climatic factors, ecology and biogeographic settings to identify underlying drivers and their generality across lineages.

Location

Global.

Time Period

Present.

Major Taxa Studied

Amphibians.

Methods

Using a global dataset spanning 7270 (>87% of) species of Anura, Caudata and Gymnophiona, we employed phylogenetic Bayesian modelling to test the roles of climate, resource availability, insularity, elevation, habitat use and diel activity on body size.

Results

Only climate and elevation drive body size patterns, and these processes are order-specific. Seasonality in precipitation and in temperature predict body size clines in anurans, whereas caecilian body size increases with aridity. However, neither of these drivers explained variation in salamander body size. In both anurans and caecilians, size increases with elevational range and with midpoint elevation in caecilians only. No effects of mean temperature, resource abundance, insularity, time of activity or habitat use were found.

Main Conclusions

Precipitation and temperature seasonality are the dominant climatic drivers of body size variation in amphibians overall. Bergmann's rule is consistently rejected, and so are other alternative hypotheses. We suggest that the rationale sustaining existing macroecological rules of body size is unrealistic in amphibians and discuss our findings in the context of the emerging hypothesis that climate change can drive body size shifts.     

Contest behavior in territorial male butterflies: does size matter?

Although intrasexual contests generally favor bigger or stronger individuals, the relevance of body size to war of attrition-typedisputes between weaponless animals such as butterflies isunclear. In this study I aimed to investigate the significanceof size in this context by studying territorial contests inHypolimnas bolina (L.), a species that exhibits consistentseasonal plasticity in body size. In this species adult ageis positively correlated with large size in spring but withsmall size in autumn. This shift allowed independent evaluationof the relevance of each variable (size and age) to intrasexualcontest success. Observation of a population of marked individualsindicated that only age appeared important, with the winnersof pair-wise contests significantly older than losers in both seasons, and with contests lasting longer when the age differencebetween the combatants was small. Age was also linked to residency;residents won 99% of all contests. This study suggests thatsize does not matter in these aerial disputes, but age andresidency do. It is not yet possible to determine whether olderbutterflies are intrinsically better competitors, or whether they simply have greater opportunity to find a vacant territory.     

Phenotypic plasticity of body size in a temperate population ofDrosophila melanogaster: When the temperature—size rule does not apply

A natural population ofDrosophila melanogaster in southern France was sampled in three different years and 10 isofemale lines were investigated from each sample. Two size-related traits, wing and thorax length, were measured and the wing/thorax ratio was also calculated. Phenotypic plasticity was analysed after development at seven different constant temperatures, ranging from 12‡C to 31‡C. The three year samples exhibited similar reaction norms, suggesting a stable genetic architecture in the natural population. The whole sample (30 lines) was used to determine precisely the shape of each reaction norm, using a derivative analysis. The practical conclusion was that polynomial adjustments could be used in all cases, but with different degrees: linear for the wing/thorax ratio, quadratic for thorax length, and cubic for wing length. Both wing and thorax length exhibited concave reaction norms, with a maximum within the viable thermal range. The temperatures of the maxima were, however, quite different, around 15‡C for the wing and 19.5‡C for the thorax. Assuming that thorax length is a better estimate of body size, it is not possible to state that increasing the temperature results in monotonically decreasing size (the temperature-size rule), although this is often seen to be the case for genetic variations in latitudinal clines. The variability of the traits was investigated at two levels—within and between lines—and expressed as a coefficient of variation. The within-line (environmental) variability revealed a regular, quadratic convex reaction norm for the three traits, with a minimum around 21‡C. This temperature of minimum variability may be considered as a physiological optimum, while extreme temperatures are stressful. The between-line (genetic) variability could also be adjusted to quadratic polynomials, but the curvature parameters were not significant. Our results show that the mean values of the traits and their variance are both plastic, but react in different ways along a temperature gradient. Extreme low or high temperatures decrease the size but increase the variability. These effects may be considered as a functional response to environmental stress.     

Are there latitudinal and altidudinal Rapoport effects in the geographic ranges of Andean passerine birds?

We analysed the range-sizes of 835 Andean passerine species (including 414 endemics and 421 non-endemics) to test for latitudinal and altitudinal Rapoport effects (LRE and ARE). We tested for positive range-size: latitude/altitude correlations using three different methods: (i) Rohde's mid-point method, (ii) species sorted out by altitude, and (iii) a phylogenetic comparative method (CAIC). Using Rohde's mid-point method, the mean latitudinal extent of species does not follow a Rapoport pattern, but the mean latitudinal occupancy of all passerines and non-endemics do increase with latitude. The latitudinal ranges of endemics sorted out by altitude follow a reverse Rapoport effect, but non-endemics support the pattern. CAIC confirms the latitudinal increase in the occupancy of non-endemics, but regressions have low coefficients of determination. The ARE is supported by the mean altitudinal extent of species, but the trend vanishes when controlling for geometric effects. Low-altitude species occupy about the same proportion of the available altitudinal space as do high-altitude ones. Our analyses suggest that latitude and altitude have low explanatory power for understanding the spatial variation in range-sizes at a continental scale. We show how different patterns can emerge from applying different criteria to the analysis of data.     

Genome size: does bigger mean worse?

Genome sizes vary enormously. This variation in DNA content correlates with effective population size, suggesting that deleterious additions to the genome can accumulate in small populations. On this view, the increased complexity of biological functions associated with large genomes partly reflects evolutionary degeneration.     

Does seasonality drive spatial patterns in demography? Variation in survival in African reed warblers Acrocephalus baeticatus across southern Africa does not reflect global patterns

Among birds, northern temperate species generally have larger clutches, shorter development periods and lower adult survival than similarly‐sized southern and tropical species. Even though this global pattern is well accepted, the driving mechanism is still not fully understood. The main theories are founded on the differing environmental seasonality of these zones (higher seasonality in the North). These patterns arise in cross‐species comparisons, but we hypothesized that the same patterns should arise among populations within a species if different types of seasonality select for different life histories. Few studies have examined this. We estimated survival of an azonal habitat specialist, the African reed warbler, across the environmentally diverse African subcontinent, and related survival to latitude and to the seasonality of the different environments of their breeding habitats. Data (1998–2010) collected through a public ringing scheme were analyzed with hierarchical capture‐mark‐recapture models to determine resident adult survival and its spatial variance across sixteen vegetation units spread across four biomes. The models were defined as state‐space multi‐state models to account for transience and implemented in a Bayesian framework. We did not find a latitudinal trend in survival or a clear link between seasonality and survival. Spatial variation in survival was substantial across the sixteen sites (spatial standard deviation of the logit mean survival: 0.70, 95% credible interval (CRI): 0.33–1.27). Mean site survival ranged from 0.49 (95% CRI: 0.18–0.80) to 0.83 (95% CRI: 0.62–0.97) with an overall mean of 0.67 (95% CRI: 0.47–0.85). A hierarchical modeling approach enabled us to estimate spatial variation in survival of the African reed warbler across the African subcontinent from sparse data. Although we could not confirm the global pattern of higher survival in less seasonal environments, our findings from a poorly studied region contribute to the study of life‐history strategies.     

Egg-size evolution in aquatic environments: does oxygen availability constrain size?

Selection against large eggs has been proposed for aquatic environments, putatively because large eggs should have more difficulty obtaining the required oxygen. Here, we use brown trout (Salmo trutta) eggs to provide an experimental test of this hypothesis. At high levels of dissolved oxygen (14 mg l(-1)), egg survival was high and independent of egg size. At low oxygen levels (2.3 mg l(-1)), survival decreased overall, and was higher for large-egged than small-egged siblings. Thus, contrary to conventional expectation, low oxygen levels selected for large rather than small eggs. A second experiment using Atlantic salmon (S. salar) eggs indicated that oxygen consumption increases relatively slowly with increasing egg mass (allometric constant = 0.44). The failure of the conventional 'bigger is worse during incubation' hypothesis may thus be due to the erroneous assumption that oxygen consumption increases at a greater rate with increasing egg mass than does the egg surface area that is available for oxygen diffusion. We also demonstrate, using data from Atlantic salmon, that nest-specific oxygen consumption decreases with increasing egg size, but that this effect is more pronounced for large than for small females. This may help to explain the positive correlation between adult body size and egg size observed in fishes that cluster their eggs.