Invariant size-frequency distributions along a latitudinal gradient in marine bivalves

Energetic constraints on body-size niches in a resource-limited marine environment

Body size of life on the Earth spans many orders of magnitude, and with it scales the energetic requirements of organisms. Thus, changes in environmental energy should impact community body-size distributions in predictable ways by reshaping ecological and niche dynamics. We examine how carbon, oxygen and temperature, three energetic drivers, impact community size-based assembly in deep-sea bivalves. We demonstrate that body-size distributions are influenced by multiple energetic constraints. Relaxation in these constraints leads to an expansion of body-size niche space through the addition of novel large size classes, increasing the standard deviation and mean of the body-size distribution. With continued Anthropogenic increases in temperature and reductions in carbon availability and oxygen in most ocean basins, our results point to possible radical shifts in invertebrate body size with the potential to impact ecosystem function.

Bad neighbors? Niche overlap and asymmetric competition between native and Lessepsian limpets in the Eastern Mediterranean rocky intertidal

The Eastern Mediterranean Sea hosts more non-indigenous species than any other marine region, yet their impacts on the native biota remain poorly understood. Focusing on mollusks from the Israeli rocky intertidal, we explored the hypothesis that this abiotically harsh habitat supports a limited trait diversity, and thus may promote niche overlap and competition between native and non-indigenous species. Indeed, native and non-indigenous assemblage components often had a highly similar trait composition, caused by functionally similar native (Patella caerulea) and non-indigenous (Cellana rota) limpets. Body size of P. caerulea decreased with increasing C. rota prevalence, but not vice versa, indicating potential asymmetric competition. Although both species have coexisted in Israel for >15 years, a rapid 'replacement' of native limpets by C. rota has been reported for a thermally polluted site, suggesting that competition and regionally rapid climate-related seawater warming might interact to progressively erode native limpet performance along the Israeli coast.

Diversity begets diversity in mammal species and human cultures

Across the planet the biogeographic distribution of human cultural diversity tends to correlate positively with biodiversity. In this paper we focus on the biogeographic distribution of mammal species and human cultural diversity. We show that not only are these forms of diversity similarly distributed in space, but they both scale superlinearly with environmental production. We develop theory that explains that as environmental productivity increases the ecological kinetics of diversity increases faster than expected because more complex environments are also more interactive. Using biogeographic databases of the global distributions of mammal species and human cultures we test a series of hypotheses derived from this theory and find support for each. For both mammals and cultures, we show that (1) both forms of diversity increase exponentially with ecological kinetics; (2) the kinetics of diversity is faster than the kinetics of productivity; (3) diversity scales superlinearly with environmental productivity; and (4) the kinetics of diversity is faster in increasingly productive environments. This biogeographic convergence is particularly striking because while the dynamics of biological and cultural evolution may be similar in principle the underlying mechanisms and time scales are very different. However, a common currency underlying all forms of diversity is ecological kinetics; the temperature-dependent fluxes of energy and biotic interactions that sustain all forms of life at all levels of organization. Diversity begets diversity in mammal species and human cultures because ecological kinetics drives superlinear scaling with environmental productivity.

Common latitudinal gradients in functional richness and functional evenness across marine and terrestrial systems

Functional diversity is an important aspect of biodiversity, but its relationship to species diversity in time and space is poorly understood. Here we compare spatial patterns of functional and taxonomic diversity across marine and terrestrial systems to identify commonalities in their respective ecological and evolutionary drivers. We placed species-level ecological traits into comparable multi-dimensional frameworks for two model systems, marine bivalves and terrestrial birds, and used global species-occurrence data to examine the distribution of functional diversity with latitude and longitude. In both systems, tropical faunas show high total functional richness (FR) but low functional evenness (FE) (i.e. the tropics contain a highly skewed distribution of species among functional groups). Functional groups that persist toward the poles become more uniform in species richness, such that FR declines and FE rises with latitude in both systems. Temperate assemblages are more functionally even than tropical assemblages subsampled to temperate levels of species richness, suggesting that high species richness in the tropics reflects a high degree of ecological specialization within a few functional groups and/or factors that favour high recent speciation or reduced extinction rates in those groups.

Twelve thousand recent patellogastropods from a northeastern Pacific latitudinal gradient

Body size distributions can vary widely among communities, with important implications for ecological dynamics, energetics, and evolutionary history. Here we present a dataset of body size and shape for 12,035 extant Patellogastropoda (true limpet) specimens from the collections of the University of California Museum of Paleontology, compiled using a novel high-throughput morphometric imaging method. These specimens were collected over the past 150 years at 355 localities along a latitudinal gradient ranging from Alaska to Baja California, Mexico and are presented here with individual images, 2D outline coordinates, and 2D measurements of body size and shape. This dataset provides a resource for assemblage-scale macroecological questions and documents the size and diversity of recent patellogastropods in the northeastern Pacific.

Lifespan, growth rate, and body size across latitude in marine Bivalvia, with implications for Phanerozoic evolution

Mean body size in marine animals has increased more than 100-fold since the Cambrian, a discovery that brings to attention the key life-history parameters of lifespan and growth rate that ultimately determine size. Variation in these parameters is not well understood on the planet today, much less in deep time. Here, we present a new global database of maximum reported lifespan and shell growth coupled with body size data for 1 148 populations of marine bivalves and show that (i) lifespan increases, and growth rate decreases, with latitude, both across the group as a whole and within well-sampled species, (ii) growth rate, and hence metabolic rate, correlates inversely with lifespan, and (iii) opposing trends in lifespan and growth combined with high variance obviate any demonstrable pattern in body size with latitude. Our observations suggest that the proposed increase in metabolic activity and demonstrated increase in body size of organisms over the Phanerozoic should be accompanied by a concomitant shift towards faster growth and/or shorter lifespan in marine bivalves. This prediction, testable from the fossil record, may help to explain one of the more fundamental patterns in the evolutionary and ecological history of animal life on this planet.

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.

Approaches to Macroevolution: 2. Sorting of Variation, Some Overarching Issues, and General Conclusions

Approaches to macroevolution require integration of its two fundamental components, within a hierarchical framework. Following a companion paper on the origin of variation, I here discuss sorting within an evolutionary hierarchy. Species sorting-sometimes termed species selection in the broad sense, meaning differential origination and extinction owing to intrinsic biological properties-can be split into strict-sense species selection, in which rate differentials are governed by emergent, species-level traits such as geographic range size, and effect macroevolution, in which rates are governed by organism-level traits such as body size; both processes can create hitchhiking effects, indirectly causing the proliferation or decline of other traits. Several methods can operationalize the concept of emergence, so that rigorous separation of these processes is increasingly feasible. A macroevolutionary tradeoff, underlain by the intrinsic traits that influence evolutionary dynamics, causes speciation and extinction rates to covary in many clades, resulting in evolutionary volatility of some clades and more subdued behavior of others; the few clades that break the tradeoff can achieve especially prolific diversification. In addition to intrinsic biological traits at multiple levels, extrinsic events can drive the waxing and waning of clades, and the interaction of traits and events are difficult but important to disentangle. Evolutionary trends can arise in many ways, and at any hierarchical level; descriptive models can be fitted to clade trajectories in phenotypic or functional spaces, but they may not be diagnostic regarding processes, and close attention must be paid to both leading and trailing edges of apparent trends. Biotic interactions can have negative or positive effects on taxonomic diversity within a clade, but cannot be readily extrapolated from the nature of such interactions at the organismic level. The relationships among macroevolutionary currencies through time (taxonomic richness, morphologic disparity, functional variety) are crucial for understanding the nature of evolutionary diversification. A novel approach to diversity-disparity analysis shows that taxonomic diversifications can lag behind, occur in concert with, or precede, increases in disparity. Some overarching issues relating to both the origin and sorting of clades and phenotypes include the macroevolutionary role of mass extinctions, the potential differences between plant and animal macroevolution, whether macroevolutionary processes have changed through geologic time, and the growing human impact on present-day macroevolution. Many challenges remain, but progress is being made on two of the key ones: (a) the integration of variation-generating mechanisms and the multilevel sorting processes that act on that variation, and (b) the integration of paleontological and neontological approaches to historical biology.

Climate change and body size shift in Mediterranean bivalve assemblages: unexpected role of biological invasions

Body size is a synthetic functional trait determining many key ecosystem properties. Reduction in average body size has been suggested as one of the universal responses to global warming in aquatic ecosystems. Climate change, however, coincides with human-enhanced dispersal of alien species and can facilitate their establishment. We address effects of species introductions on the size structure of recipient communities using data on Red Sea bivalves entering the Mediterranean Sea through the Suez Canal. We show that the invasion leads to increase in median body size of the Mediterranean assemblage. Alien species are significantly larger than native Mediterranean bivalves, even though they represent a random subset of the Red Sea species with respect to body size. The observed patterns result primarily from the differences in the taxonomic composition and body-size distributions of the source and recipient species pools. In contrast to the expectations based on the general temperature-size relationships in marine ectotherms, continued warming of the Mediterranean Sea indirectly leads to an increase in the proportion of large-bodied species in bivalve assemblages by accelerating the entry and spread of tropical aliens. These results underscore complex interactions between changing climate and species invasions in driving functional shifts in marine ecosystems.

Shell properties of commercial clam Chamelea gallina are influenced by temperature and solar radiation along a wide latitudinal gradient

Phenotype can express different morphologies in response to biotic or abiotic environmental influences. Mollusks are particularly sensitive to different environmental parameters, showing macroscale shell morphology variations in response to environmental parameters. Few studies concern shell variations at the different scale levels along environmental gradients. Here, we investigate shell features at the macro, micro and nanoscale, in populations of the commercially important clam Chamelea gallina along a latitudinal gradient (~400 km) of temperature and solar radiation in the Adriatic Sea (Italian cost). Six populations of clams with shells of the same length were analyzed. Shells from the warmest and the most irradiated population were thinner, with more oval shape, more porous and lighter, showing lower load fracture. However, no variation was observed in shell CaCO₃ polymorphism (100% aragonite) or in compositional and textural shell parameters, indicating no effect of the environmental parameters on the basic processes of biomineralization. Because of the importance of this species as commercial resource in the Adriatic Sea, the experimentally quantified and significant variations of mass and fracture load in C. gallina shells along the latitudinal gradient may have economic implications for fisheries producing different economical yield for fishermen and consumers along the Adriatic coastline.

Environmental and scale-dependent evolutionary trends in the body size of crustaceans

The ecological and physiological significance of body size is well recognized. However, key macroevolutionary questions regarding the dependency of body size trends on the taxonomic scale of analysis and the role of environment in controlling long-term evolution of body size are largely unknown. Here, we evaluate these issues for decapod crustaceans, a group that diversified in the Mesozoic. A compilation of body size data for 792 brachyuran crab and lobster species reveals that their maximum, mean and median body size increased, but no increase in minimum size was observed. This increase is not expressed within lineages, but is rather a product of the appearance and/or diversification of new clades of larger, primarily burrowing to shelter-seeking decapods. This argues against directional selective pressures within lineages. Rather, the trend is a macroevolutionary consequence of species sorting: preferential origination of new decapod clades with intrinsically larger body sizes. Furthermore, body size evolution appears to have been habitat-controlled. In the Cretaceous, reef-associated crabs became markedly smaller than those in other habitats, a pattern that persists today. The long-term increase in body size of crabs and lobsters, coupled with their increased diversity and abundance, suggests that their ecological impact may have increased over evolutionary time.

Body-size trends of the extinct giant shark Carcharocles megalodon: a deep-time perspective on marine apex predators

The extinct shark Carcharocles megalodon is one of the largest marine apex predators ever to exist. Nonetheless, little is known about its body-size variations through time and space. Here, we studied the body-size trends of C. megalodon through its temporal and geographic range to better understand its ecology and evolution. Given that this species was the last of the megatooth lineage, a group of species that shows a purported size increase through time, we hypothesized that C. megalodon also displayed this trend, increasing in size over time and reaching its largest size prior to extinction. We found that C. megalodon body-size distribution was left-skewed (suggesting a long-term selective pressure favoring larger individuals), and presented significant geographic variation (possibly as a result of the heterogeneous ecological constraints of this cosmopolitan species) over geologic time. Finally, we found that stasis was the general mode of size evolution of C. megalodon (i.e., no net changes over time), contrasting with the trends of the megatooth lineage and our hypothesis. Given that C. megalodon is a relatively long-lived species with a widely distributed fossil record, we further used this study system to provide a deep-time perspective to the understanding of the body-size trends of marine apex predators. For instance, our results suggest that (1) a selective pressure in predatory sharks for consuming a broader range of prey may favor larger individuals and produce left-skewed distributions on a geologic time scale; (2) body-size variations in cosmopolitan apex marine predators may depend on their interactions with geographically discrete communities; and (3) the inherent characteristics of shark species can produce stable sizes over geologic time, regardless of the size trends of their lineages.

Origins, bottlenecks, and present-day diversity: patterns of morphospace occupation in marine bivalves

It has long been known that species should not be distributed randomly in morphospace (a multidimensional trait space), even under simple models of evolution. However, recent studies suggest that position in morphospace can affect aspects of evolution such as the durations of clades and the species richness of their constituent taxa. Here we investigate the dynamics of morphospace occupancy in living and fossil marine bivalves using shell size and aspect ratio, two functionally important traits. Multiple lines of evidence indicate that the center of a family's morphospace today represents a location where taxonomic diversity is maximized, apparently owing to lower extinction rates. Within individual bivalve families, species with narrow geographic ranges are distributed throughout the morphospace but widespread species, which are generally expected to be extinction resistant, tend to be concentrated near the center. The morphospace centers of most species-rich families today (defined as the median value for all species in the family) tend to be close to the positions of the family founders, further suggesting an association between position in morphospace and net diversification rates. However, trajectories of individual subclades (genera) are inconsistent with the center of morphospace being an evolutionary attractor.

Origination and immigration drive latitudinal gradients in marine functional diversity

Global patterns in the functional attributes of organisms are critical to understanding biodiversity trends and predicting biotic responses to environmental change. In the first global marine analysis, we find a strong decrease in functional richness, but a strong increase in functional evenness, with increasing latitude using intertidal-to-outer-shelf bivalves as a model system (N = 5571 species). These patterns appear to be driven by the interplay between variation in origination rates among functional groups, and latitudinal patterns in origination and range expansion, as documented by the rich fossil record of the group. The data suggest that (i) accumulation of taxa in spatial bins and functional categories has not impeded continued diversification in the tropics, and (ii) extinctions will influence ecosystem function differentially across latitudes.

Size-frequency distributions along a latitudinal gradient in Middle Permian fusulinoideans

Geographic gradients in body size within and among living species are commonly used to identify controls on the long-term evolution of organism size. However, the persistence of these gradients over evolutionary time remains largely unknown because ancient biogeographic variation in organism size is poorly documented. Middle Permian fusulinoidean foraminifera are ideal for investigating the temporal persistence of geographic gradients in organism size because they were diverse and abundant along a broad range of paleo-latitudes during this interval (~275-260 million years ago). In this study, we determined the sizes of Middle Permian fusulinoidean fossils from three different paleo-latitudinal zones in order to examine the relationship between the size of foraminifers and regional environment. We recovered the following results: keriothecal fusulinoideans are substantially larger than nonkeriothecal fusulinoideans; fusulinoideans from the equatorial zone are typically larger than those from the north and south transitional zones; neoschwagerinid specimens within a single species are generally larger in the equatorial zone than those in both transitional zones; and the nonkeriothecal fusulinoideans Staffellidae and Schubertellidae have smaller size in the north transitional zone. Fusulinoidean foraminifers differ from most other marine taxa in exhibiting larger sizes closer to the equator, contrary to Bergmann's rule. Meridional variation in seasonality, water temperature, nutrient availability, and carbonate saturation level are all likely to have favored or enabled larger sizes in equatorial regions. Temporal variation in atmospheric oxygen concentrations have been shown to account for temporal variation in fusulinoidean size during Carboniferous and Permian time, but oxygen availability appears unlikely to explain biogeographic variation in fusulinoidean sizes, because dissolved oxygen concentrations in seawater typically increase away from the equator due to declining seawater temperatures. Consequently, our findings highlight the fact that spatial gradients in organism size are not always controlled by the same factors that govern temporal trends within the same clade.

Increased energy promotes size-based niche availability in marine mollusks

Variation in chemical energy, that is food availability, is posited to cause variation in body size. However, examinations of the relationship are rare and primarily limited to amniotes and zooplankton. Moreover, the relationship between body size and chemical energy may be impacted by phylogenetic history, clade-specific ecology, and heterogeneity of chemical energy in space and time. Considerable work remains to both document patterns in body size over gradients in food availability and understanding the processes potentially generating them. Here, we examine the functional relationship between body size and chemical energy availability over a broad assortment of marine mollusks varying in habitat and mobility. We demonstrate that chemical energy availability is likely driving body size patterns across habitats. We find that lower food availability decreases size-based niche availability by setting hard constraints on maximum size and potentially on minimum size depending on clade-specific ecology. Conversely, higher food availability promotes greater niche availability and potentially promotes evolutionary innovation with regard to size. We posit based on these findings and previous work that increases in chemical energy are important to the diversification of Metazoans through size-mediated niche processes.

Selectivity of terrestrial gastropod extinctions on an oceanic archipelago and insights into the anthropogenic extinction process

Anthropogenic impacts have led to widespread extinctions of species on oceanic islands but the nature of many of these extinctions remains poorly known. Here we investigate extinction selectivities of terrestrial gastropods from the Ogasawara archipelago in the northwest Pacific, where anthropogenic threats have changed over time, shifting primarily from the effects of habitat loss to predation by a variety of different predators. Across all of the islands, extinct species had significantly smaller geographic ranges compared with species that are still alive, but among the surviving species, ranges of those that are currently declining due to human impacts do not differ significantly from those that are not threatened. Extinctions were selective with respect to spire index (SI) of shells, a trait of potential functional importance, but the relationship between body size and extinction vulnerability varied among extinction agents, some of which were strongly size selective, whereas others were not. Overall, whereas anthropogenic impacts have resulted in nonrandom losses of phenotypic diversity, the patterns of selectivity are complex, vary among islands, and with the type of threat. As extinction agents have changed historically, so has the pattern of loss. Because of the changing nature of anthropogenic impacts, resiliency to one type of threat does not guarantee long-term survival of species and future patterns of biodiversity loss on these islands are likely to be different from those in the past.

Differential extinction and the contrasting structure of polar marine faunas

Background

The low taxonomic diversity of polar marine faunas today reflects both the failure of clades to colonize or diversify in high latitudes and regional extinctions of once-present clades. However, simple models of polar evolution are made difficult by the strikingly different faunal compositions and community structures of the two poles.

Methodology/Principal Findings

A comparison of early Cenozoic Arctic and Antarctic bivalve faunas with modern ones, within the framework of a molecular phylogeny, shows that while Arctic losses were randomly distributed across the tree, Antarctic losses were significantly concentrated in more derived families, resulting in communities dominated by basal lineages. Potential mechanisms for the phylogenetic structure to Antarctic extinctions include continental isolation, changes in primary productivity leading to turnover of both predators and prey, and the effect of glaciation on shelf habitats.

Conclusions/Significance

These results show that phylogenetic consequences of past extinctions can vary substantially among regions and thus shape regional faunal structures, even when due to similar drivers, here global cooling, and provide the first phylogenetic support for the “retrograde” hypothesis of Antarctic faunal evolution.

Congruence of morphologically-defined genera with molecular phylogenies

Morphologically-defined mammalian and molluscan genera (herein "morphogenera") are significantly more likely to be monophyletic relative to molecular phylogenies than random, under 3 different models of expected monophyly rates: approximately 63% of 425 surveyed morphogenera are monophyletic and 19% are polyphyletic, although certain groups appear to be problematic (e.g., nonmarine, unionoid bivalves). Compiled nonmonophyly rates are probably extreme values, because molecular analyses have focused on "problem" taxa, and molecular topologies (treated herein as error-free) contain contradictory groupings across analyses for 10% of molluscan morphogenera and 37% of mammalian morphogenera. Both body size and geographic range, 2 key macroevolutionary and macroecological variables, show significant rank correlations between values for morphogenera and molecularly-defined clades, even when strictly monophyletic morphogenera are excluded from analyses. Thus, although morphogenera can be imperfect reflections of phylogeny, large-scale statistical treatments of diversity dynamics or macroevolutionary variables in time and space are unlikely to be misleading.

Colloquium paper: patterns of biodiversity and endemism on Indo-West Pacific coral reefs

Diversity of the primary groups of contemporary Indo-West Pacific coral reef organisms, including mantis shrimps (stomatopod crustaceans), peaks in the Indo-Australian Archipelago (IAA), reaches a lower peak in East Africa and Madagascar [Indian Ocean continental (IOC)], and declines in the central Indian Ocean (IO) and Central Pacific (CP). Percent endemism in stomatopods (highest in the IAA, high in the IOC, lower in regions adjacent to centers, and moderate in the CP) correlates positively with species diversity (this varies with scale) and inversely with species body size. Because it constrains reproductive traits and dispersal, body size is a reliable indicator of speciation and extinction potential in reef stomatopods and probably most marine organisms. Assemblages are dominated by small-sized species in the IAA and IOC. Both speciation and extinction likely are high, resulting in especially high endemism (small ranges reflect both originating and disappearing species) in these regions. Rates of speciation exceed extinction, yielding centers of diversity (especially in the IAA). Dispersal slows speciation and extinction in areas adjacent to these centers. Body size declines toward the CP, especially in atoll environments. Here the wheels of speciation and extinction again spin rapidly but in the opposite direction (extinction > speciation), yielding low diversity and moderate endemism. We conclude that life histories, dispersal, and speciation/extinction dynamics are primary agents that mold patterns of diversity and endemism. Historical factors, currents, productivity, and species diversity itself (through ecological interactions) also influence these patterns, in some cases by altering body size.

A test of reproductive power in snakes

Reproductive power is a contentious concept among ecologists, and the model has been criticized on theoretical and empirical grounds. Despite these criticisms, the model has successfully predicted the modal (optimal) size in three large taxonomic groups and the shape of the body size distribution in two of these groups. We tested the reproductive power model on snakes, a group that differs markedly in physiology, foraging ecology, and body shape from the endothermic groups upon which the model was derived. Using detailed field data from the published literature, snake-specific constants associated with reproductive power were determined using allometric relationships of energy invested annually in egg production and population productivity. The resultant model accurately predicted the mode and left side of the size distribution for snakes but failed to predict the right side of that distribution. If the model correctly describes what is possible in snakes, observed size diversity is limited, especially in the largest size classes.

Patterns in body mass distributions: sifting among alternative hypotheses

Understanding how animals interact with their environment is critical for evaluating, mitigating and coping with anthropogenic alteration of Earth's biosphere. Researchers have attempted to understand some aspects of these interactions by examining patterns in animal body mass distributions. Energetic, phylogenetic, biogeographical, textural discontinuity and community interaction hypotheses have been advanced to explain observed patterns. Energetic and textural discontinuity hypotheses focus upon the allometry of resource use. The community interaction hypothesis contends that biotic interactions within assemblages of species are of primary importance. Biogeographical and phylogenetic hypotheses focus on the role of constraints on the organization of communities. This paper examines and organizes these various propositions about species body mass distributions and discusses the multiple competing hypotheses, how their predictions vary, and possible methods by which the hypotheses can be distinguished and tested. Each of the hypotheses is partial, and explains some elements of pattern in body mass distributions. The scale of appropriate application, relevance and interpretation varies among the hypotheses, and the mechanisms underlying observed patterns are likely to be multicausal and vary with scale.

Assessing the fidelity of the fossil record by using marine bivalves

Taxa that fail to become incorporated into the fossil record can reveal much about the biases of this record and provide the information needed to correct such biases in empirical analyses of the history of life. Yet little is known about the characteristics of taxa missing from the fossil record. For the marine Bivalvia, which have become a model system for macroevolutionary and macroecological analysis in the fossil record, 308 of the 1,292 living genera and subgenera (herein termed "taxa") are not recorded as fossils. These missing taxa are not a random sample of the clade, but instead tend to have small body size, reactive shell structures, commensal or parasitic habit, deep-sea distribution, narrow geographic range, restriction to regions exposing few Neogene marine sediments, or recent date of formal taxonomic description in the neontological literature. Most missing taxa show two or more of these features and tend to be concentrated in particular families. When we exclude the smallest taxa (<1 cm) and deep-sea endemics, date of published description and geographic range become the strongest predictors of the missing taxa; other factors are statistically insignificant or have relatively small effects. These biases might influence a variety of analyses including the use of fossil data in support of phylogenetic analyses, molecular clock calibrations, and analyses of spatial and temporal dynamics of clades and biotas. Clade inventories such as these can be used to develop protocols that minimize the biases imposed by sampling and preservation.

Global energy gradients and size in colonial organisms: worker mass and worker number in ant colonies

Body mass shapes processes from cell metabolism to community dynamics. Little is known, however, about how the average body mass of individuals varies among ecological communities. Ants alter colony mass by independently changing worker mass and/or worker number. In a survey of 49 ecosystems from tundra to tropical rainforest, average worker mass and worker number were uncorrelated (r(s) = 0.2, P > 0.14) and varied 100-fold. Data supported the hypothesis that higher mean monthly temperatures, T, reduce worker mass by increasing metabolic costs during worker development. In contrast, worker number was unimodal over a 1,000-fold gradient of net primary productivity (NPP, g of carbon per m2 per yr), a measure of organic carbon available to consumers. At the lowest levels of NPP colonies appeared to be carbon-limited; above 60 g of carbon per m2 per yr average worker number decreased to a global low. This decline in worker number with increasing NPP supports the hypothesis that abundant carbon ameliorates the Achilles heel of small taxa in competition with large taxa: their relatively high metabolic demands. Higher predation rates in resource-rich environments may also play a role in limiting worker number. In all, about half the global variation in worker mass and number was accounted for by gradients of NPP and T. Changes in global temperature and rainfall may thus mold gradients of ectotherm size, with consequences for the structure and function of the ecosystems.

Global energy gradients and size in colonial organisms: worker mass and worker number in ant colonies

Body mass shapes processes from cell metabolism to community dynamics. Little is known, however, about how the average body mass of individuals varies among ecological communities. Ants alter colony mass by independently changing worker mass and/or worker number. In a survey of 49 ecosystems from tundra to tropical rainforest, average worker mass and worker number were uncorrelated (r(s) = 0.2, P > 0.14) and varied 100-fold. Data supported the hypothesis that higher mean monthly temperatures, T, reduce worker mass by increasing metabolic costs during worker development. In contrast, worker number was unimodal over a 1,000-fold gradient of net primary productivity (NPP, g of carbon per m2 per yr), a measure of organic carbon available to consumers. At the lowest levels of NPP colonies appeared to be carbon-limited; above 60 g of carbon per m2 per yr average worker number decreased to a global low. This decline in worker number with increasing NPP supports the hypothesis that abundant carbon ameliorates the Achilles heel of small taxa in competition with large taxa: their relatively high metabolic demands. Higher predation rates in resource-rich environments may also play a role in limiting worker number. In all, about half the global variation in worker mass and number was accounted for by gradients of NPP and T. Changes in global temperature and rainfall may thus mold gradients of ectotherm size, with consequences for the structure and function of the ecosystems.

Insular Carnivore Biogeography: Island Area and Mammalian Optimal Body Size

Patterns of size variation in insular mammals have been used to support the claim that mammals have a single optimal body size. This hypothesis enjoys wide support, despite having been questioned on both theoretical and empirical grounds. It is claimed that species of optimal size maintain the highest population densities. Therefore these species are thought to inhabit the smallest islands, where larger and smaller species are generally absent. We sought such a pattern by testing how area affects the body sizes of the largest and smallest carnivore species on islands. Using data on carnivores from 322 islands, we found that the sizes of carnivores on small islands tend to be close to the order's mode. Furthermore, we found that the size distribution of carnivore species that inhabit islands resembles that of those whose range is entirely continental. We conclude that insular carnivores provide no support for theories proposing a single optimal size, and we suspect such theories are also flawed on theoretical grounds.

Similarity of Mammalian Body Size across the Taxonomic Hierarchy and across Space and Time

Although it is commonly assumed that closely related animals are similar in body size, the degree of similarity has not been examined across the taxonomic hierarchy. Moreover, little is known about the variation or consistency of body size patterns across geographic space or evolutionary time. Here, we draw from a data set of terrestrial, nonvolant mammals to quantify and compare patterns across the body size spectrum, the taxonomic hierarchy, continental space, and evolutionary time. We employ a variety of statistical techniques including "sib-sib" regression, phylogenetic autocorrelation, and nested ANOVA. We find an extremely high resemblance (heritability) of size among congeneric species for mammals over approximately 18 g; the result is consistent across the size spectrum. However, there is no significant relationship among the body sizes of congeneric species for mammals under approximately 18 g. We suspect that life-history and ecological parameters are so tightly constrained by allometry at diminutive size that animals can only adapt to novel ecological conditions by modifying body size. The overall distributions of size for each continental fauna and for the most diverse orders are quantitatively similar for North America, South America, and Africa, despite virtually no overlap in species composition. Differences in ordinal composition appear to account for quantitative differences between continents. For most mammalian orders, body size is highly conserved, although there is extensive overlap at all levels of the taxonomic hierarchy. The body size distribution for terrestrial mammals apparently was established early in the Tertiary, and it has remained remarkably constant over the past 50 Ma and across the major continents. Lineages have diversified in size to exploit environmental opportunities but only within limits set by allometric, ecological, and evolutionary constraints.

Empirical evidence for an optimal body size in snakes

The concept of optimal size has been invoked to explain patterns in body size of terrestrial mammals. However, the generality of this phenomenon has not been tested with similarly complete data from other taxonomic groups. In this study we describe three statistical patterns of body size in snakes, all of which indicate an optimal length of 1.0 m. First, a distribution of largest body lengths of 618 snake species had a single mode at 1.0 m. Second, we found a positive relationship between the size of the largest member of an island snake assemblage and island area and a negative relationship between the size of the smallest member of an island snake assemblage and island area. Best-fit lines through these data cross at a point corresponding to 1.0 m in body length, the presumed optimal size for a one-species island. Third, mainland snake species smaller than 1.0 m become larger on islands whereas those larger than 1.0 m become smaller on islands. The observation that all three analyses converge on a common body size is concordant with patterns observed in mammals and partial analyses of four other disparate animal clades. Because snakes differ so strikingly from mammals (ectotherms, gape-limited predators, elongate body shape) the concordant patterns of these two groups provide strong evidence for the evolution of an optimal body size within independent monophyletic groups. However, snakes differ from other taxonomic groups that have been studied in exhibiting a body size distribution that is not obviously skewed in either direction. We suggest that idiosyncratic features of the natural history of ectotherms allow relatively unconstrained distributions of body size whereas physiological limitations of endotherms constrain distributions of body size to a right skew.