A test of the cross-scale resilience model: Functional richness in Mediterranean-climate ecosystems

Ecological resilience has been proposed to be generated, in part, in the discontinuous structure of complex systems. Environmental discontinuities are reflected in discontinuous, aggregated animal body mass distributions. Diversity of functional groups within body mass aggregations (scales) and redundancy of functional groups across body mass aggregations (scales) has been proposed to increase resilience. We evaluate that proposition by analyzing mammalian and avian communities of Mediterranean-climate ecosystems. We first determined that body mass distributions for each animal community were discontinuous. We then calculated the variance in richness of function across aggregations in each community, and compared observed values with distributions created by 1000 simulations using a null of random distribution of function, with the same n, number of discontinuities and number of functional groups as the observed data. Variance in the richness of functional groups across scales was significantly lower in real communities than in simulations in eight of nine sites. The distribution of function across body mass aggregations in the animal communities we analyzed was non-random, and supports the contentions of the cross-scale resilience model.     

Spatial Ecological Hierarchies: Coexistence on Heterogeneous Landscapes via Scale Niche Diversification

Spatially heterogeneous environments are generally characterized by nested landscape patterns with resource aggregations on several scales. Empirical studies indicate that such nested landscape patterns impose selection constraints on the perceptive scales of animals, but the underlying selection mechanisms are unclear. We investigated the selection dynamics of perceptive scale within a spatial resource utilization model, where the environment is characterized by its resource distribution and species differ in their perceptive scales and resource preemption capabilities. Using three model landscapes with various resource distributions, we found that the optimal perceptive scale is determined by scale-specific attributes of the landscape pattern and that the number of coexisting species increases with the number of characteristic scales. Based on the results of this model, we argue that resource aggregations on different scales act as distinct resources and that animal species of particular perceptive scales are superior in utilizing resource aggregations of comparable spatial extent. Due to the allometric relationship between body size and perceptive scale, such fitness difference might result in discontinuous body mass distributions.     

Natural selection on body size is mediated by multiple interacting factors: a comparison of beetle populations varying naturally and experimentally in body size

Body size varies considerably among species and among populations within species, exhibiting many repeatable patterns. However, which sources of selection generate geographic patterns, and which components of fitness mediate evolution of body size, are not well understood. For many animals, resource quality and intraspecific competition may mediate selection on body size producing large-scale geographic patterns. In two sequential experiments, we examine how variation in larval competition and resource quality (seed size) affects the fitness consequences of variation in body size in a scramble-competing seed-feeding beetle, Stator limbatus. Specifically, we compared fitness components among three natural populations of S. limbatus that vary in body size, and then among three lineages of beetles derived from a single base population artificially selected to vary in size, all reared on three sizes of seeds at variable larval density. The effects of larval competition and seed size on larval survival and development time were similar for larger versus smaller beetles. However, larger-bodied beetles suffered a greater reduction in adult body mass with decreasing seed size and increasing larval density; the relative advantage of being large decreased with decreasing seed size and increasing larval density. There were highly significant interactions between the effects of seed size and larval density on body size, and a significant three-way interaction (population-by-density-by-seed size), indicating that environmental effects on the fitness consequences of being large are nonadditive. Our study demonstrates how multiple ecological variables (resource availability and resource competition) interact to affect organismal fitness components, and that such interactions can mediate natural selection on body size. Studying individual factors influencing selection on body size may lead to misleading results given the potential for nonlinear interactions among selective agents.     

Connecting landscape structure and patterns in body size distributions

Understanding the interaction between community structure and landscape structure represents a pressing theoretical challenge of great applied importance considering the increasing structural modification of ecosystems through habitat loss and fragmentation. Dispersal ability and energetic demands coupled to body size determine the landscape structure experienced by an organism, which could essentially be fragmented for small individuals but continuous for large ones. Although discontinuities in species assemblages have been predicted and detected, no explicit association between habitat structure and body size distributions has been demonstrated. In this contribution, we propose that body size structure in local communities should reflect such different perceptions of landscape structure. To this end, we explore this association in a simple metacommunity located in the Atacama Desert, in northern Chile. Using graph theory we found that species of different size and trophic position (carnivores and herbivores) perceive the landscape at contrasting spatial scales. In each community (n = 31) we determined the observed and the expected body size distributions – in a random sample from the metacommunity of 18 727 individuals –, which allowed us to identify the body sizes at which an overrepresentation or underrepresentation of individuals occur. Such aggregations and discontinuities in body sizes were related, for carnivores, to patch location within the landscape, and to the internal banded vegetation pattern within patches for herbivores. Our study shows, for the first time, an empirical connection between the spatial distribution of communities, their local attributes, and the existence and locations of discontinuities and aggregations in body size distributions.     

Social personality trait and fitness

Several recent studies have explored various aspects of animal personality and their ecological consequences. However, the processes responsible for the maintenance of personality variability within a population are still largely unknown. We have recently demonstrated that social personality traits exist in the common lizard (Lacerta vivipara) and that the variation in sociability provides an explanation for variable dispersal responses within a given species. However, we need to know the fitness consequences of variation in sociability across environmental contexts in order to better understand the maintenance of such variation. In order to achieve this, we investigated the relationship between sociability and survival, body growth and fecundity, in one-year-old individuals in semi-natural populations with varying density. 'Asocial' and 'social' lizards displayed different fitness outcomes in populations of different densities. Asocial lizards survived better in low-density populations, while social females reproduced better. Spatiotemporal variation in environmental conditions might thus be the process underlying the maintenance of these personality traits within a population. Finally, we also discuss the position of sociability in a more general individual behavioural pattern including boldness, exploration and aggressiveness.     

Living in forest fragments reduces group cohesion in diademed sifakas (Propithecus diadema) in eastern Madagascar by reducing food patch size

Forest fragmentation is thought to threaten primate populations, yet the mechanisms by which this occurs remain largely unknown. However, fragmentation is known to cause dietary shifts in several primate species, and links between food resource distribution and within-group spatial dynamics are well documented. Thus, fragmentation has the potential to indirectly affect spatial dynamics, and these changes may present additional stresses to fragmented populations. I present the results from a 12-month study of Propithecus diadema at Tsinjoarivo, eastern Madagascar, including two groups in fragments and two in continuous forest. Instantaneous data on activity and spatial position were collected during all-day focal animal follows. Fragment groups had much lower cohesion, being more likely to have no neighbor within 5 and 10 m. For continuous forest groups, cohesion was highest in the rainy season (when food patches are large) and lowest in winter (when the animals rely on small-crowned mistletoes), and the chance of having no neighbor within 5 m was positively correlated with mistletoe consumption. Thus their decreased cohesion in fragment groups is inferred to result from their increased reliance on mistletoes and other small resources, which causes them to spread out among multiple patches. This scenario is consistent with the reduced body mass of subordinate individuals (males and immatures) in fragments, and suggests the occurrence of steeper within-group fitness gradients. Further research is necessary to determine whether these patterns apply to other primates; however, since fragmentation tends to cause the loss of the largest trees, many primates in fragments may lose their largest food resources and undergo similar behavioral shifts.     

Sampling bias affects the relationship between structural importance and species body mass in frugivore-plant interaction networks

Within a community, body mass variation among frugivore species is associated with: a) animal's ecological, physiological and functional traits; b) community-level biogeographic/climatic variables; c) anthropogenic factors, and methodological approaches. Furthermore, frugivore-plant relationships are highly context dependent; thus, variation in species attributes at the community level might determine interaction patterns. An interaction network approach is a useful tool to analyze the relationship between species attributes and species role in maintaining the network connectivity patterns (species structural importance). Particularly, the relationship between species body mass and interspecific interaction patterns could be determined by differences in the community properties and the environmental context of the network's geographic location. We tested the hypotheses that: i) the relationship between frugivore species body mass and its structural importance in the network is determined by the frugivore species body mass coefficient of variation (COV) in the community, and ii) frugivore body mass COV depends on the network context in terms of the local climate variables, level of human impact, and taxa considered within the sampling. We evaluated the relationship between species structural importance and its body mass in 28 frugivore-plant interaction networks from different parts of the world. Species structural importance was calculated as a general measure of centrality, which quantifies the generalization level of the species, the proximity of a species to other species in the network, and the importance of a species as a connector between different parts of the network. A meta-analysis approach was applied to evaluate the influence of local climate and community variables associated with each network on the relationship between species structural importance and its body mass. The relationship between centrality and species body mass was highly fluctuating between networks, and frugivore body mass COV was the variable that best explained this heterogeneity. Moreover, networks with both bird and mammal species showed the highest COV values. Our results show that when there is sufficient body mass variability among species in the community, the largest species take important roles in maintaining the network connectivity patterns. This suggests that the bias towards small species in networks studies may impact the magnitude of the frugivore species body mass COV and, therefore, conceal the importance to larger species in network topologies. Future research in frugivore-plant interaction networks should include the highest possible number of interacting species without limiting the samples to species within a particular body size or taxonomic group.     

Habitat and climate shape growth patterns in a mountain ungulate

Uptake and use of energy are of key importance for animals living in temperate environments that undergo strong seasonal changes in forage quality and quantity. In ungulates, energy intake strongly affects body mass gain, an important component of individual fitness. Energy allocation among life‐history traits can be affected by internal and external factors. Here, we investigate large‐scale variation in body growth patterns of Alpine chamois Rupicapra rupicapra rupicapra, in relation to sex, age, temperature, and habitat variations across 31 (sub)populations in the Central European Alps. Taking advantage of an exceptionally large dataset (n = 178,175) of chamois hunted over 27 consecutive years between 1993 and 2019 in mountain ranges with different proportions of forest cover, we found that (i) patterns of body mass growth differ between mountain ranges, with lower body mass but faster mass growth with increasing proportion of forest cover and that (ii) the effect of spring and summer temperatures on changes in body growth patterns are larger in mountain ranges with lower forest cover compared to mountain ranges with higher forest cover. Our results show that patterns of body mass growth within a species are more plastic than expected and depend on environmental and climatic conditions. The recent decline in body mass observed in Alpine chamois populations may have greater impacts on populations living above the treeline than in forests, which may buffer against the effects of increasing temperatures on life‐history traits.     

Fitness consequences of potential assortative mating inside and outside a hybrid zone in Chorthippus parallelus (Orthoptera: Acrididae): implications for reinforcement and sexual selection theory

We have examined the fitness consequences of random and potentially non-random matings within two populations taken from inside, and two from outside a hybrid zone in Chorthippus parallelus. When given the opportunity to mate non-randomly, females from all populations laid egg pods more quickly than females obliged to mate at random. A range of fitness parameters measured on the offspring did not show increased fitness following potential non-random mating for any population. However, in non-hybrid populations, the sons of non-randomly mated females had about twice the mating success of the sons of those females forced to mate at random, suggesting the existence of heritable variation for male reproductive success. Hybrid dysfunction did not occur amongst the offspring of randomly mated hybrid females, demonstrating that the lack of dysfunction within these populations is not due to the evolution of assortative mating within them.     

Mating patterns influence vulnerability to the extinction vortex

Earth's biodiversity is undergoing mass extinction due to anthropogenic compounding of environmental, demographic and genetic stresses. These different stresses can trap populations within a reinforcing feedback loop known as the extinction vortex, in which synergistic pressures build upon one another through time, driving down population viability. Sexual selection, the widespread evolutionary force arising from competition, choice and reproductive variance within animal mating patterns could have vital consequences for population viability and the extinction vortex: (a) if sexual selection reinforces natural selection to fix ‘good genes’ and purge ‘bad genes’, then mating patterns encouraging competition and choice may help protect populations from extinction; (b) by contrast, if mating patterns create load through evolutionary or ecological conflict, then population viability could be further reduced by sexual selection. We test between these opposing theories using replicate populations of the model insect Tribolium castaneum exposed to over 10 years of experimental evolution under monogamous versus polyandrous mating patterns. After a 95‐generation history of divergence in sexual selection, we compared fitness and extinction of monogamous versus polyandrous populations through an experimental extinction vortex comprising 15 generations of cycling environmental and genetic stresses. Results showed that lineages from monogamous evolutionary backgrounds, with limited opportunities for sexual selection, showed rapid declines in fitness and complete extinction through the vortex. By contrast, fitness of populations from the history of polyandry, with stronger opportunities for sexual selection, declined slowly, with 60% of populations surviving by the study end. The three vortex stresses of (a) nutritional deprivation, (b) thermal stress and (c) genetic bottlenecking had similar impacts on fitness declines and extinction risk, with an overall sigmoid decline in survival through time. We therefore reveal sexual selection as an important force behind lineages facing extinction threats, identifying the relevance of natural mating patterns for conservation management.     

Are most species small? Not within species-level phylogenies.

The robust macro-ecological observation that there are more small-bodied species implies that small-bodied organisms have experienced elevated net rates of diversification. We investigate the role of body size in creating non-random differences in rates of cladogenesis using a set of 38 species-level phylogenies drawn from a range of animal groups. We use independent contrasts to explore the relationship between body size and species richness within individual phylogenies and across related sets of phylogenies. We also carry out a meta-analysis looking for associations between body size and species richness across the taxa. We find little evidence for increased cladogenesis among small-bodied organisms within taxa, and no evidence for any consistent differences between taxa. We explore possible explanations for the inconsistency of our findings with macro-ecological patterns.     

Ecological biogeography of Malagasy non‐volant mammals: community structure is correlated with habitat

Aim To examine the relationship between ecoregions, as a proxy for regional climate and habitat type, and mammalian community structure, defined by species composition and richness (e.g. taxonomic structure) and ecological diversity (e.g. ecological structure) of non‐volant species. Location Madagascar. Methods Faunal lists of non‐volant mammal species occurring in 35 communities from five World Wildlife Fund ecoregions were collected from published and unpublished sources. Species were assigned to ecological groups defined by trophic status, locomotor habits, activity cycle and body mass. We used Mantel tests, cluster analysis and principal coordinates analysis to evaluate geographic patterning in taxonomic composition and species richness. We used stepwise multiple discriminant analysis to characterize patterns in the ecological diversity of the mammalian communities from each ecoregion. Communities from transitional habitats (e.g. representing more than one ecoregion) were used to test the predictive power of the analyses. Results Non‐volant mammal communities divided into clusters that correspond to ecoregions. There was a strong distance effect in the taxonomic structure of communities across the island and within both humid and dry forest communities, but this effect was weak within humid forest communities. Mammalian species richness was significantly lower in dry forest than in humid forest communities. The ecological structure of communities was also correlated with ecoregions. Changes in the relative percentages of omnivory, arboreal quadrupedalism, terrestrial/arboreal quadrupedalism and two body mass classes accounted for 98.1% of the variation in ecological structure. Transitional communities were projected in intermediate positions by the discriminant model. Main conclusions Our analysis demonstrates that the broad‐scale habitat and climate variables captured by the ecoregion model have shaped the assembly of non‐volant mammal communities in Madagascar over evolutionary time. The spatial pattern is consistent with ecological sorting of species ranges along environmental gradients. Historical processes, such as recent extinction and migration, may have also affected the structure of mammal communities, although these factors have played a secondary role.     

How Humans Differ from Other Animals in Their Levels of Morphological Variation

Animal species come in many shapes and sizes, as do the individuals and populations that make up each species. To us, humans might seem to show particularly high levels of morphological variation, but perhaps this perception is simply based on enhanced recognition of individual conspecifics relative to individual heterospecifics. We here more objectively ask how humans compare to other animals in terms of body size variation. We quantitatively compare levels of variation in body length (height) and mass within and among 99 human populations and 848 animal populations (210 species). We find that humans show low levels of within-population body height variation in comparison to body length variation in other animals. Humans do not, however, show distinctive levels of within-population body mass variation, nor of among-population body height or mass variation. These results are consistent with the idea that natural and sexual selection have reduced human height variation within populations, while maintaining it among populations. We therefore hypothesize that humans have evolved on a rugged adaptive landscape with strong selection for body height optima that differ among locations.     

The importance of reproductive interference in ecology and evolution: from organisms to communities

Knowledge of species interactions is vital to understand ecological and evolutionary patterns in nature. Traditional species interactions (e.g., competition, predation, symbiosis) have received a great deal of deserved attention and their general importance in shaping the evolution of populations and structure of communities is unquestioned. Recently, reproductive interference has been receiving attention as an important species interaction. Reproductive interference is defined as interspecific reproductive activities that decrease the fitness of at least one of the species involved in the interaction. Reproductive interference has the potential to affect the evolutionary trajectories of populations and structure of communities. Here, I comment on seven papers that make up this special feature on reproductive interference. Along the way I highlight key discoveries of these studies and areas of research that may contribute to our understanding of the causes and consequences of reproductive interference.     

A macroevolutionary explanation for energy equivalence in the scaling of body size and population density

Across a wide array of animal species, mean population densities decline with species body mass such that the rate of energy use of local populations is approximately independent of body size. This "energetic equivalence" is particularly evident when ecological population densities are plotted across several or more orders of magnitude in body mass and is supported by a considerable body of evidence. Nevertheless, interpretation of the data has remained controversial, largely because of the difficulty of explaining the origin and maintenance of such a size-abundance relationship in terms of purely ecological processes. Here I describe results of a simulation model suggesting that an extremely simple mechanism operating over evolutionary time can explain the major features of the empirical data. The model specifies only the size scaling of metabolism and a process where randomly chosen species evolve to take resource energy from other species. This process of energy exchange among particular species is distinct from a random walk of species abundances and creates a situation in which species populations using relatively low amounts of energy at any body size have an elevated extinction risk. Selective extinction of such species rapidly drives size-abundance allometry in faunas toward approximate energetic equivalence and maintains it there.     

Global population genetic structure and biogeography of the oceanic copepods Eucalanus hyalinus and E. spinifer

Although theory dictates that limited gene flow between populations is a necessary precursor to speciation under allopatric and parapatric models, it is currently unclear how genetic differentiation between conspecific populations can arise in open-ocean plankton species. I examined two recently distinguished sympatric, circumglobal sister species, Eucalanus hyalinus and Eucalanus spinifer, for population genetic structure throughout their global biogeographic ranges. Here I show that oceanic zooplankton species can be highly genetically structured on macrogeographic spatial scales, despite experiencing extensive gene flow within features of the large-scale ocean circulation. Mitochondrial DNA analyses of 450 and 383 individuals of E. hyalinus and E. spinifer, respectively, revealed that habitat discontinuities at the boundaries of subtropical gyres in the North and South Pacific, as well as continental land masses, acted as effective barriers to gene flow for both species. However, the impact of specific barriers on population genetic structure varied between the sister species, despite their close phylogenetic relationship and similar circumglobal biogeogeographic distributions. The sister species differed in their oceanographic distributions, with E. spinifer dominating oligotrophic waters of the subtropical gyres and E. hyalinus more abundant along central water mass boundaries and in frontal zones and upwelling systems. This species-specific difference in the oceanographic habitat is an important factor determining the historical and contemporary patterns of dispersal of the two species. I suggest that species-specific ecological differences are likely to be a primary determinant of population genetic structure of open-ocean plankton.     

路径依赖下的物种形成机制

生物科学几乎所有研究都需要物种概念作为基础, 生物多样性研究亦需要可操作的物种概念, 但现有物种概念存在不同程度的人为因素或难操作性, 对物种划分造成不利影响。本文引入“进化路径”这一概念, 说明适合度景观时刻变化着, 物种在每个进化时间点上依据瞬时适合度选择下一时刻的进化状态, 且总是沿着动态适合度景观中适合度增加的方向进化。基于演化博弈的方法, 以随机过程为例模拟物种的进化过程。进而提出路径依赖下的物种形成机制, 并在此基础上给出可操作的物种定义, 即: 针对基因、性状、生态过程等任一状态下两个群体内个体的多个变量做统计分析, 若群体之间同时在两个或多个维度状态下呈现出的不连续性d大于群体内变量呈现出的差异性σ_k, 则拥有相应变量的个体属于不同物种。     

Subtle but ubiquitous selection on body size in a natural population of collared flycatchers over 33 years

Understanding the magnitude and long‐term patterns of selection in natural populations is of importance, for example, when analysing the evolutionary impact of climate change. We estimated univariate and multivariate directional, quadratic and correlational selection on four morphological traits (adult wing, tarsus and tail length, body mass) over a time period of 33 years (≈ 19 000 observations) in a nest‐box breeding population of collared flycatchers (Ficedula albicollis). In general, selection was weak in both males and females over the years regardless of fitness measure (fledged young, recruits and survival) with only few cases with statistically significant selection. When data were analysed in a multivariate context and as time series, a number of patterns emerged; there was a consistent, but weak, selection for longer wings in both sexes, selection was stronger on females when the number of fledged young was used as a fitness measure, there were no indications of sexually antagonistic selection, and we found a negative correlation between selection on tarsus and wing length in both sexes but using different fitness measures. Uni‐ and multivariate selection gradients were correlated only for wing length and mass. Multivariate selection gradient vectors were longer than corresponding vector of univariate gradients and had more constrained direction. Correlational selection had little importance. Overall, the fitness surface was more or less flat with few cases of significant curvature, indicating that the adaptive peak with regard to body size in this species is broader than the phenotypic distribution, which has resulted in weak estimates of selection.     

Population structure is not a simple function of reproductive mode and larval type: insights from tropical corals

1. For a wide range of organisms, heritable variation in life-history characteristics has been shown to be strongly subject to selection, reflecting the impact that variation in characters such as genotypic diversity, duration of larval development and adaptations for dispersal can have on the fitness of offspring and the make-up of populations. Indeed, variation in life-history characteristics, especially reproduction and larval type, have often been used to predict patterns of dispersal and resultant population structures in marine invertebrates. 2. Scleractinian corals are excellent models with which to test this relationship, as they exhibit almost every possible combination of reproductive mode and larval type. Some general patterns are emerging but, contrary to expectations, genetic data suggest that while populations of broadcast spawning species may be genotypically diverse they may be heavily reliant on localized recruitment rather than widespread dispersal of larvae. 3. Here we use microsatellites to test the importance of localized recruitment by comparing the genetic structure of populations of two broadcast spawning corals with contrasting modes of reproduction and larval development; Goniastrea favulus is self-compatible, has sticky, negatively buoyant eggs and larvae and is expected to have restricted dispersal of gametes and larvae. In contrast, Platygyra daedalea is self-incompatibile, spawns positively buoyant egg-sperm bundles and has planktonic development. 4. Surprisingly, spatial-autocorrelation revealed no fine-scale clustering of similar genotypes within sites for G. favulus, but showed a non-random distribution of genotypes in P. daedalea. Both species showed similar levels of genetic subdivision among sites separated by 50-100 m (F(ST) = 0.03), suggesting that larval dispersal may be equivalent in both species. 5. Interestingly, as fragmentation has been considered rare in massive corals, our sample of 284 P. daedalea colonies included 28 replicated genotypes that were each unlikely (P < 0.05) to have been derived independently from sexual reproduction. 6. We conclude that the extreme life history of G. favulus does not produce unusually fine-scale genetic structure and subsequently, that reproductive mode and larval type may not be not good predictors of population structure or dispersal ability.     

Laboratory maintenance does not alter ecological and physiological patterns among species: a Drosophila case study

Large comparative studies in animal ecology, physiology and evolution often use animals reared in the laboratory for many generations; however, the relevance of these studies hinges on the assumption that laboratory populations are still representative for their wild living conspecifics. In this study, we investigate whether laboratory‐maintained and freshly collected animal populations are fundamentally different and whether data from laboratory‐maintained animals are valid to use in large comparative investigations of ecological and physiological patterns. Here, we obtained nine species of Drosophila with paired populations of laboratory‐maintained and freshly collected flies. These species, representing a range of ecotypes, were assayed for four stress‐tolerance, two body‐size traits and six life‐history traits. For all of these traits, we observed small differences in species‐specific comparisons between field and laboratory populations; however, these differences were unsystematic and laboratory maintenance did not eclipse fundamental species characteristics. To investigate whether laboratory maintenance influence the general patterns in comparative studies, we correlated stress tolerance and life‐history traits with environmental traits for the laboratory‐maintained and freshly collected populations. Based on this analysis, we found that the comparative physiological and ecological trait correlations are similar irrespective of provenience. This finding is important for comparative biology in general because it validates comparative meta‐analyses based on laboratory‐maintained populations.