Body Size Frequency Distributions Research Articles

Bayesian hierarchical modelling of size spectra

Abstract A fundamental pattern in ecology is that smaller organisms are more abundant than larger organisms. This pattern is known as the individual size distribution (ISD), which is the frequency distribution of all individual body sizes in an ecosystem. The ISD is described by a power law and a major goal of size spectra analyses is to estimate the exponent of the power law, λ. However, while numerous methods have been developed to do this, they have focused almost exclusively on estimating λ from single samples. Here, we develop an extension of the truncated Pareto distribution within the probabilistic modelling language Stan. We use it to estimate multiple λs simultaneously in a hierarchical modelling approach. The most important result is the ability to examine hypotheses related to size spectra, including the assessment of fixed and random effects, within a single Bayesian generalized mixed model. While the example here uses size spectra, the technique can also be generalized to any data that follow a power law distribution.

Mammalian body size evolution was shaped by habitat transitions as an indirect effect of climate change

AbstractAimBody size evolution has long been hypothesized to have been driven by factors linked to climate change, but the specific mechanisms are difficult to disentangle due to the wide range of functional traits that covary with body size. In this study, we investigated the impact of regional habitat changes as a potential indirect effect of climate change on body size evolution.LocationEurope and North America.Time periodThe Neogene (~23–2 million years ago).Major taxaFive orders of terrestrial mammals: Artiodactyla, Carnivora, Perissodactyla, Proboscidea and Primates.MethodsWe compared the two continental faunas, which have exceptional fossil records of terrestrial mammals and underwent different processes of habitat transition during the Neogene. Using Bayesian multilevel regression models, we assessed the variation in the temporal dynamics of body size diversity among ecographic groups, defined by their continent of occurrence and dietary preference.ResultsModel comparisons unanimously supported a combined effect of diet and continent on all metrics of body size frequency distributions, rejecting the shared energetic advantage of larger bodies in colder climates as a dominant mechanism of body size evolution. Rather, the diet‐specific dynamics on each continent pinpointed an indirect effect of climate change – change in habitat availability, and thus the resource landscape as a key driver of mammalian evolution.Main conclusionsOur study highlights dietary preference as a mechanistic link between mammalian evolution and habitat transition mediating an indirect climate‐change effect and demonstrates the complexity of climatic influence on biodiversity. Our findings suggest that the intensified habitat modification today likely poses a bigger threat than climate change in itself to living mammals, and perhaps all endotherms.

The geographical diversification in varanid lizards: the role of mainland versus island in driving species evolution.

Monitor lizards (Varanidae) inhabit both the mainland and islands of all geological types and have diversified into an exceptionally wide range of body sizes, thus providing an ideal model for examining the role of mainland versus island in driving species evolution. Here we use phylogenetic comparative methods to examine whether a link exists between body size-driven diversification and body size-frequency distributions in varanid lizards and to test the hypothesis that island lizards differ from mainland species in evolutionary processes, body size, and life-history traits (offspring number and size). We predict that: 1) since body size drives rapid diversification in groups, a link exists between body size-driven diversification and body size-frequency distributions; 2) because of various environments on island, island species will have higher speciation, extinction, and dispersal rates, compared with mainland species; 3) as a response to stronger intraspecific competition, island species will maximize individual ability associated with body size to outcompete closely-related species, and island species will produce smaller clutches of larger eggs to increase offspring quality. Our results confirm that the joint effect of differential macroevolutionary rates shapes the species richness pattern of varanid lizards. There is a link between body size-driven diversification and body size-frequency distributions, and the speciation rate is maximized at medium body sizes. Island species will have higher speciation, equal extinction, and higher dispersal rates compared with mainland species. Smaller clutch size and larger hatchling in the island than in mainland species indicate that offspring quality is more valuable than offspring quantity for island varanids.

Reproductive life-history strategies in a species-rich assemblage of Amazonian electric fishes.

The reproductive biology of only a small fraction of Neotropical freshwater fishes has been described, and detailed comparative studies of reproductive life-history variation in the Neotropical ichthyofauna are lacking. Here we describe interspecific variation in reproductive life history for a multi-species assemblage of the electric knifefish genus Brachyhypopomus (Hypopomidae: Gymnotiformes: Ostariophysi) from Amazonian floodplain and terra firme stream systems. During a year-round quantitative sampling program, we collected and measured key life-history traits from 3,410 individuals. Based on oocyte size distributions, and on circannual variation in gonadosomatic indices, hepatosomatic indices, and capture-per-unit-effort abundance of reproductive adults, we concluded that all species exhibit a single protracted annual breeding season during which females spawn fractionally. We found small clusters of post-larval individuals in one floodplain species and one terra firme stream species, but no signs of parental care. From analyses of body size-frequency distributions and otolith growth increments, we concluded that five species in our study area have approximately one-year (annual) semelparous life history with a single reproductive period followed by death, while two species have a two-year iteroparous life history, with breeding in both year-groups. Despite predictions from life-history theory we found no salient correlations between life history strategy (semelparity or iteroparity) and habitat occupancy (floodplain or terra firme stream). In the iteroparous species B. beebei, we documented evidence for reproductive restraint in the first breeding season relative to the second breeding season and argue that this is consistent with age-regulated terminal investment.

Exploring the Effects of Land-Cover Configuration, Body Size and Trophic Diversity on the Avifauna Richness of Prince Edward Island, Canada

We explored the effects of land-cover configuration, body size and trophic diversity in determining avian species richness on Prince Edward Island, Canada. Data on avian species richness were obtained from the Maritime Breeding Bird Atlas data. Prince Edward Island was divided into 97 sampling cells of 10 × 10 km. Land-cover metrics were calculated using a forest inventory database, Fragstats and ArcView version 8.1. The relationships between avian species richness and explanatory variables were explored using correlation analysis, mixed forward-backward stepwise analysis, generalized linear models and Akaike’s information criterion. Models predicted between 27% and 63% of the variability in species richness, attributing substantial explanatory power to both the average body size and the range of body size spanned by the avian community. The body-size frequency distribution showed that avian communities were dominated by species weighing between 50 and 80 g. Habitat metrics associated with forests were more important to the avifauna than those related to agriculture. Avian species richness also decreased with both the fragmentation and isolation of wetlands. The total area covered by the human infrastructure land-cover and its subdivision were also important. Clearly, body size plays a key role in determining the diversity of birds on Prince Edward Island. In particular, species weighing 50 - 80 g appear to have sufficient resources to be successful on Prince Edward Island’s landscapes. Our findings also highlighted the importance of controlling the expansion of human infrastructure and both the fragmentation and reduction in size of wetlands to maintain avian species richness patterns.

Snake body size frequency distributions are robust to the description of novel species

AbstractThe accurate interpretation of body size frequency distributions (BSFDs) has important implications for understanding large‐scale ecological and evolutionary patterns because they are shaped by ecological and evolutionary processes. However, incomplete species sampling, resulting either from a paucity of reliable body size data or because those species have not yet been detected/described, also has the potential to influence BSFD shape and skew when missing species are disproportionately large or small. Missing species may thus influence the skew of a BSFD, misleading subsequent interpretations. In contrast to many vertebrate taxa, the BSFDs of snakes appears to be log‐normal, with no significant skew. However, the influence of imperfect species sampling on these distributions has however not been previously evaluated. We collated body size data and date of description for 300 species of elapid, and 264 species of viperid snakes and assessed the shape and skew of the BSFD of each clade. Next we tested the hypothesis that date of description was related to body size in each clade. We show that the BSFDs of elapid and viperid snakes are both log‐normal, with no significant skew. Moreover we demonstrate size‐related differences in the probability of detection (as measured by year of description) among elapids and vipers: in both lineages, species with larger body size tend to have been described earlier. However, simulations testing the effect of the addition of novel, small‐bodied species revealed that the distributions of elapids and vipers are not highly vulnerable to becoming significantly skewed in the future. Our results support current interpretations of BSFDs in snakes that suggest a lack of size‐associated biases in the speciation and extinction rates of this clade.

Body sizes and diversification rates of lizards, snakes, amphisbaenians and the tuatara

AbstractAimSize is one of the most important and obvious traits of an organism. Both small and large sizes have adaptive advantages and disadvantages. Body size–frequency distributions of most large clades are unimodal and right skewed. Species larger than the mean or range midpoint of body sizes are relatively scarce. Theoretical models suggest evolutionary rates are higher in small organisms with short generation times. Therefore diversification rates are usually thought to be maximal at relatively small body sizes. Empirical studies of the rates of molecular evolution and clade diversification, however, have usually indicated that both are unrelated to body size. Furthermore, it has been claimed that because snakes are longer than lizards, the size–frequency distribution of all squamate species is bimodal overall. We examined the shape of the size–frequency distribution of nearly all Squamata and Rhynchocephalia species, and investigated how size affected diversification rates.LocationGlobal.MethodsWe collected data on maximum body length for 9805 lepidosaur (squamates and the tuatara) species (99.7% of all species) and converted them to mass using clade‐specific allometric equations. Using methods that test for relationships between continuous traits and speciation and extinction rates on a large, dated phylogeny (4155 species), we investigated the relationship between diversification rates and body size.ResultsLiving squamates span six orders of magnitude in body size, eight when giant extinct snakes and mosasaurs are included. The body size–frequency distributions of snakes and lizards separately, and of all lepidosaur species combined, are unimodal and right skewed. Nonetheless, we find neither linear nor hump‐shaped relationships between size and diversification rates, except in snakes, where speciation and diversification are hump shaped.Main conclusions Despite a clear modality and skew in the body sizes of lepidosaurs, we find little evidence for faster diversification of modal‐sized taxa, perhaps implying that larger‐sized clades are relatively young.

About the Cover.

Next article FreeAbout the CoverPDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinked InRedditEmailQR Code SectionsMoreCoverThe sea star Pisaster ochraceus is a dominant rocky intertidal predator that lives along the west coast of North America. It plays a large role in determining local patterns of biodiversity via its interactions with mussels in the genus Mytilus. The cover image depicts juvenile P. ochraceus (the maroon, purple, and ochre sea stars) foraging among Mytilus trossulus (the bluish-black mussels) on a floating dock at the Hakai Beach Institute on Calvert Island, British Columbia. Several empty mussel shells are evidence of recent sea star predation events. The Pisaster-Mytilus interaction is famous for providing the inspiration for the “keystone predator” concept, and is now widely featured in textbooks.As a well-studied predator-prey pair, Pisaster and Mytilus make an ideal model system for addressing ecological and behavioral theory relating to the effects of body size. Body size of both predator and prey are key attributes of trophic interactions, as differences in body size can determine ease of prey capture, time required to handle and consume the prey, and number of prey required to meet energetic demands. On pages 192–200 of this issue, Gooding and Harley explore the ways in which variation in body size can affect the relationship between the predator P. ochraceus and its preferred prey in wave-protected waters, Mytilus trossulus.Gooding and Harley found that predation rates, expressed as mussels consumed per unit time, increased with increasing sea star size and decreased with increasing mussel size. Larger sea stars preferred to consume larger mussels. For juvenile sea stars, in which size relationships could be worked out in the greatest detail, prey-size preference corresponded with mussel size classes that allowed Pisaster to consume the most tissue in the least amount of time. Overall, the study found that even small changes in predator or prey body size could play a large role in predator energetics (tissue consumed per unit time) and prey population dynamics (individuals consumed per unit time). Because sea star and mussel growth rates and body size frequency distributions are affected by temperature and ocean acidification, changes in body size and the accompanying shifts in predation rates may be one important pathway through which global change alters community structure.Credits: Photo, Christopher D.G. Harley, University of British Columbia; cover design, Beth Liles, Marine Biological Laboratory. Next article DetailsFiguresReferencesCited by The Biological Bulletin Volume 228, Number 3June 2015 Published in association with the Marine Biological Laboratory Article DOIhttps://doi.org/10.1086/BBLv228n3cover © 2015 by Marine Biological Laboratory. All rights reserved.PDF download Crossref reports no articles citing this article.

Body size diversity and frequency distributions of Neotropical cichlid fishes (Cichliformes: Cichlidae: Cichlinae).

Body size is an important correlate of life history, ecology and distribution of species. Despite this, very little is known about body size evolution in fishes, particularly freshwater fishes of the Neotropics where species and body size diversity are relatively high. Phylogenetic history and body size data were used to explore body size frequency distributions in Neotropical cichlids, a broadly distributed and ecologically diverse group of fishes that is highly representative of body size diversity in Neotropical freshwater fishes. We test for divergence, phylogenetic autocorrelation and among-clade partitioning of body size space. Neotropical cichlids show low phylogenetic autocorrelation and divergence within and among taxonomic levels. Three distinct regions of body size space were identified from body size frequency distributions at various taxonomic levels corresponding to subclades of the most diverse tribe, Geophagini. These regions suggest that lineages may be evolving towards particular size optima that may be tied to specific ecological roles. The diversification of Geophagini appears to constrain the evolution of body size among other Neotropical cichlid lineages; non-Geophagini clades show lower species-richness in body size regions shared with Geophagini. Neotropical cichlid genera show less divergence and extreme body size than expected within and among tribes. Body size divergence among species may instead be present or linked to ecology at the community assembly scale.

Body size shifts in philippine reef fishes: interfamilial variation in responses to protection.

As a consequence of intense fishing pressure, fished populations experience reduced population sizes and shifts in body size toward the predominance of smaller and early maturing individuals. Small, early-maturing fish exhibit significantly reduced reproductive output and, ultimately, reduced fitness. As part of resource management and biodiversity conservation programs worldwide, no-take marine protected areas (MPAs) are expected to ameliorate the adverse effects of fishing pressure. In an attempt to advance our understanding of how coral reef MPAs meet their long-term goals, this study used visual census data from 23 MPAs and fished reefs in the Philippines to address three questions: (1) Do MPAs promote shifts in fish body size frequency distribution towards larger body sizes when compared to fished reefs? (2) Do MPA size and (3) age contribute to the efficacy of MPAs in promoting such shifts? This study revealed that across all MPAs surveyed, the distribution of fishes between MPAs and fished reefs were similar; however, large-bodied fish were more abundant within MPAs, along with small, young-of-the-year individuals. Additionally, there was a significant shift in body size frequency distribution towards larger body sizes in 12 of 23 individual reef sites surveyed. Of 22 fish families, eleven demonstrated significantly different body size frequency distributions between MPAs and fished reefs, indicating that shifts in the size spectrum of fishes in response to protection are family-specific. Family-level shifts demonstrated a significant, positive correlation with MPA age, indicating that MPAs become more effective at increasing the density of large-bodied fish within their boundaries over time.

Is the island rule general? Turtles disagree

AbstractAimThe island rule describes a trend toward dwarfism of large animals and gigantism of small animals on islands. Studies of vertebrates, including reptiles, report conflicting results regarding the generality of this evolutionary pattern. We studied the size evolution of insular turtles at the intra‐ and interspecific levels, as well as the clade level. We examined the evolutionary patterns, the relationship of habitat preferences and insular turtle size and whether the island rule extends to turtles.LocationIslands world‐wide and adjacent continents.MethodsWe collected data from the literature and expanded our database by measuring turtle specimens in museum collections. We used reduced major axis regression to test the island rule at the inter‐ and intraspecific levels, after accounting for phylogenetic effects. In addition, we compared body size frequency distributions of insular versus mainland turtles at the clade level.ResultsOur analyses revealed no support for the island rule in turtles. Intraspecifically, we found no systematic pattern of body size evolution. Interspecifically, large turtle species tend to become larger on islands, in contrast to the island rule predictions. Islands generally harbour larger turtles than mainland regions. This pattern is especially apparent when recently extinct species are considered. We found no impact of habitat preferences on the evolution of body size on islands.Main conclusionsTurtles on islands do not follow the island rule. We suggest that both physiological and ecological factors drive turtle body size evolution on islands, and their tendency for insular gigantism. Large turtles can survive longer periods of water or food shortage on small oceanic islands. Increased intraspecific competition and reduced interspecific competition and predation on islands may favour large body size. A non‐mutually exclusive explanation is that the presence of giant tortoises on islands may reflect founder effects rather than evolution – large turtles are better island colonizers.

Vanishing Clams on an Iberian Beach: Local Consequences and Global Implications of Accelerating Loss of Shells to Tourism

Multi-decadal increase in shell removal by tourists, a process that may accelerate degradation of natural habitats, was quantified via two series of monthly surveys, conducted thirty years apart (1978–1981 and 2008–2010) in one small embayment on the Mediterranean coast of the Iberian Peninsula. Over the last three decades, the local tourist arrivals have increased almost three-fold (2.74), while the area has remained unaffected by urban encroachment and commercial fisheries. During the same time interval the abundance of mollusk shells along the shoreline decreased by a comparable factor (2.62) and was significantly and negatively correlated with tourist arrivals (r = −0.52). The strength of the correlation increased when data were restricted to months with high tourist arrivals (r = −0.72). In contrast, the maximum monthly wave energy (an indirect proxy for changes in rate of onshore shell transport) was not significantly correlated with shell abundance (r = 0.10). Similarly, rank dominance of common species, drilling predation intensity, and body size-frequency distribution patterns have all remained stable over recent decades. A four-fold increase in global tourist arrivals over the last 30 years may have induced a comparable worldwide acceleration in shell removal from marine shorelines, resulting in multiple, currently unquantifiable, habitat changes such as increased beach erosion, changes in calcium carbonate recycling, and declines in diversity and abundance of organisms, which are dependent on shell availability.

Body size distributions at community, regional or taxonomic scales do not predict the direction of trait‐driven diversification in snakes in the United States

AbstractAimWe determine whether trait‐driven diversification yields similar body size distributions for snakes in local, regional and phylogenetic assemblages.LocationUnited States, North America.MethodsUsing total length and mass, we examine body size frequency distributions (BSFD) across 79 sites and respective biomes to determine if these areas represent random subsamples from the source pools of taxon body sizes. Using QuaSSE, we determine if the most probable model of trait‐driven diversification in the three most common groups of snakes in North America, the ratsnakes, pitvipers and watersnakes, is similar to the predicted regional BSFD.ResultsBSFD of snakes at the community, biome, regional and clade scales show symmetric distributions of body size. These patterns may simply be generated from random statistical subsampling. Speciation rates are not highest at or near the modal body size and simulations show that linear trait‐driven models can still yield highly symmetric distributions of body size.Main conclusionsIn this study region, processes such as competition due to size do not alter BSFD from one scale to the other. It is likely that rates of speciation are not highest at the mode in snakes and trait‐driven diversification is not likely to account for a regional pool of body sizes from which local communities are drawn, although persistence of modal body sizes through time could yield regional BSFD.

Size frequency distributions of key epibenthic organisms in the eastern Chukchi Sea and their correlations with environmental parameters

Epibenthic communities play a key role in ecosystem functioning in Arctic shelf Seas, such as in the Chukchi Sea in the Pacific Arctic. These communities, however, are patchily distributed and are influenced by various environmental parameters. Along with taxonomic composition, another community aspect that may vary spatially and be influenced by the environment is the distribution of organism sizes. This study presents the first size frequency distributions of nine epifaunal taxa that were determined to be dominant in the eastern Chukchi Sea in July/August 2009 and 2010, including male, female and gravid Chionoecetes opilio and Hyas coarctatus crabs, the gastropods Neptunea spp., Plicifusus spp., Colus spp., and Cryptonatica spp., and the echinoderms Gorgonocephalus spp., Leptasterias spp., and Echinarachnius parma. Some abundant taxa exhibited a wide range of sizes (i.e. C. opilio, Neptunea spp., and Leptasterias spp.), while others had a much smaller size range (i.e. Cryptonatica spp. and E. parma). We also found that size distributions of these taxa correlated with various combinations of the environmental parameters that have been shown to be important in structuring the general distribution patterns for the epibenthic invertebrate communities in the study area, including percent total sediment organic carbon, sediment chlorophyll a, temperature, latitude, sediment grain size 2 and 4phi, pH, and dissolved oxygen. Our findings present benchmark information that is needed to detect future alterations in body-size frequency distributions that are likely to happen in response to the predicted climate and environmental changes in the Chukchi Sea region.

Scale effects on the body size frequency distributions of African birds: patterns and potential mechanisms

AbstractAimTo describe and analyse the body size frequency distributions (BSFDs) of avian assemblages at several spatial scales in the Afrotropics. We also tested if the variation in median body size across assemblages at different spatial scales was related to environmental variables and whether purely stochastic processes could explain BSFDs.LocationThe Afrotropical biogeographic realm.MethodsAvian body masses for 1960 species where analysed at continental, biome, ecoregion and local spatial scales with standard metrics. Variation in median assemblage body size was modelled as a function of environmental and spatial explanatory variables to assess non‐random assemblage structure. We tested if BSFDs of smaller spatial scale distributions are random subsets of the larger spatial scale assemblages in which they are embedded, and used three different null model randomizations to investigate the influence of stochastic processes on BSFDs.ResultsThe African avifauna's continental BSFD is unimodal and right‐skewed. BSFDs generally become less skewed and less modal with decreasing spatial scale. The best‐fit model explained 71% of median body size values at the ecoregion scale as a function of latitude, latitude2, longitude, species richness and species range size. BSFDs at smaller scales show non‐random assembly from larger scale BSFDs distributions.Main conclusionAfrican avifaunal BSFDs are quantitatively dissimilar to African mammal BSFDs, which are bimodal at all spatial scales. Much of the change in median body size with spatial scale can be captured by a range‐weighted null model, suggesting that differential turnover between smaller‐ and larger‐bodied species might explain the shift in the central tendency of the BSFD. At the local scale, energy may well contribute to structuring BSFDs, but this pattern is less pronounced at larger spatial scales.

Intraspecific predation in the Liolaemus lizard radiation: a primer

Intraspecific predation (cannibalism) occurs in a number of lizard species, although the incidence of this trophic behaviour appears to be infrequent. Within the South AmericanLiolaemuslizard evolutionary radiation, the primary component of the carnivorous diet (herbivory has also evolved in several species) includes a diversity of arthropods and other invertebrates. However, a number of cases of saurophagy, mostly cannibalism, have also been reported in different species. Here, I review for the first time the occurrence of cannibalism in this prominent group of lizards, one of the largest and more ecologically diverse living adaptive radiations. Adults of fiveLiolaemusspecies have previously been reported to feed on conspecific hatchlings, and one additional case (L. zapallarensis) is reported in this paper. In general, cannibalism is found in species located at the right side of the body size frequency distribution, while no cases have been reported on the far left range of body sizes, suggesting that cannibalism is more common in large species. Only one species of medium size (L. darwinii) is known to feed on conspecifics. In addition, males are responsible for 83% of the cases of cannibalism, while only one case of female cannibalism is known (L. chiliensis). Finally, no clear phylogenetic signal can be inferred from these observations, as the six reported cannibal species belong to four of the five mainLiolaemusclades.

Spatial Dynamics of Body Size Frequency Distributions for North American Squamates

Scale dependent patterns of body size frequency distributions (BSFDs) have been explained by competition and an evolutionarily optimal body size in mammals. We test these ideas in a vertebrate group that is a model for competition and evolutionary studies by assessing the scale-dependence of BSFDs. BSFDs (body size defined as maximum total length) of North American squamates were assembled for the entire continent, biomes within the continent and local habitat patches within biomes. We described these distributions using skewness, kurtosis, interquartile range (IQR), and an index of evenness. We compared these parameters among spatial scales using Kolmogorov–Smirnov tests and bootstrap simulations. We assessed the relationship between body size and species richness using correlations (Pearsons and Spearmans R). The North American BSFD is bimodal, with a primary mode (240 mm) corresponding to lizards and small snakes and a secondary mode (912 mm) to snakes. Squamate BSFDs varied in a scale dependent fashion for some biomes and local habitat patches for kurtosis (12% of local patches and 10% of biomes more platykurtic), skewness (30% of biomes skewed to the right) and IQR (12% of patches increased). The index of evenness of BSFDs did not vary with spatial scale. Body size of biomes and local habitat patches closely resembles the North American BSFD as species richness increases. We found limited statistical support for the scale-dependency of North American squamate BSFDs (only 12–30% of patches or biomes conformed to the predicted pattern). These results suggest that the mechanisms implicated in scale-dependent patterns of BSFDs for mammals, geographic turnover of modal-sized species and competition within local assemblages may be of diminished importance in squamates. As geographic turnover of modal-sized species is theoretically linked to an evolutionarily optimal body size, this may suggest that optimal size theory is not adequate to predict spatial scaling of BSFDs in squamates.

Intraspecific Body Size Frequency Distributions of Insects

Although interspecific body size frequency distributions are well documented for many taxa, including the insects, intraspecific body size frequency distributions (IaBSFDs) are more poorly known, and their variation among mass-based and linear estimates of size has not been widely explored. Here we provide IaBSFDs for 16 species of insects based on both mass and linear estimates and large sample sizes (n≥100). In addition, we review the published IaBSFDs for insects, though doing so is complicated by their under-emphasis in the literature. The form of IaBSFDs can differ substantially between mass-based and linear measures. Nonetheless, in non-social insects they tend to be normally distributed (18 of 27 species) or in fewer instances positively skewed. Negatively skewed distributions are infrequently reported and log transformation readily removes the positive skew. Sexual size dimorphism does not generally cause bimodality in IaBSFDs. The available information on IaBSFDs in the social insects suggests that these distributions are usually positively skewed or bimodal (24 of 30 species). However, only c. 15% of ant genera are polymorphic, suggesting that normal distributions are probably more common, but less frequently investigated. Although only 57 species, representing seven of the 29 orders of insects, have been considered here, it appears that whilst IaBSFDs are usually normal, other distribution shapes can be found in several species, though most notably among the social insects. By contrast, the interspecific body size frequency distribution is typically right-skewed in insects and in most other taxa.

Variation in scale shape among alternative sympatric phenotypes of Arctic charr Salvelinus alpinus from two lakes in Scotland

Landmark-based geometric morphometric analysis was used to detect differences in scale shape between ecologically distinct phenotypes of Arctic charr Salvelinus alpinus coexisting in the same lake. Relative warp analysis and standard multivariate analyses of the partial warps, obtained after a Procrustes superimposition, showed that scale landmarks were efficient in discriminating among two closely related alternative phenotypes within each of the two lakes. In Loch Tay, S. alpinus exhibited a bimodal body size-frequency distribution among sexually mature fish, whereas in Loch Awe, S. alpinus are unimodal in body size but segregated into two distinct spawning phenotypes. In both lakes, alternative phenotypes showed significant differences in foraging ecology, habitat use and life history. It is probable that differences in scale shape reflect differences in ecology of these forms.

Body size variation in insects: a macroecological perspective

Body size is a key feature of organisms and varies continuously because of the effects of natural selection on the size-dependency of resource acquisition and mortality rates. This review provides a critical and synthetic overview of body size variation in insects from a predominantly macroecological (large-scale temporal and spatial) perspective. Because of the importance of understanding the proximate determinants of adult size, it commences with a brief summary of the physiological mechanisms underlying adult body size and its variation, based mostly on findings for the model species Drosophila melanogaster and Manduca sexta. Variation in nutrition and temperature have variable effects on critical weight, the interval to cessation of growth (or terminal growth period) and growth rates, so influencing final adult size. Ontogenetic and phylogenetic variation in size, compensatory growth, scaling at the intra- and interspecific levels, sexual size dimorphism, and body size optimisation are then reviewed in light of their influences on individual and species body size frequency distributions. Explicit attention is given to evolutionary trends, including gigantism, Cope's rule and the rates at which size change has taken place, and to temporal ecological trends such as variation in size with succession and size-selectivity during the invasion process. Large-scale spatial variation in size at the intraspecific, interspecific and assemblage levels is considered, with special attention being given to the mechanisms proposed to underlie clinal variation in adult body size. Finally, areas particularly in need of additional research are identified.