Seasonal Patterns of Mixed Species Groups in Large East African Mammals

Mixed mammal species groups are common in East African savannah ecosystems. Yet, it is largely unknown if co-occurrences of large mammals result from random processes or social preferences and if interspecific associations are consistent across ecosystems and seasons. Because species may exchange important information and services, understanding patterns and drivers of heterospecific interactions is crucial for advancing animal and community ecology. We recorded 5403 single and multi-species clusters in the Serengeti-Ngorongoro and Tarangire-Manyara ecosystems during dry and wet seasons and used social network analyses to detect patterns of species associations. We found statistically significant associations between multiple species and association patterns differed spatially and seasonally. Consistently, wildebeest and zebras preferred being associated with other species, whereas carnivores, African elephants, Maasai giraffes and Kirk''s dik-diks avoided being in mixed groups. During the dry season, we found that the betweenness (a measure of importance in the flow of information or disease) of species did not differ from a random expectation based on species abundance. In contrast, in the wet season, we found that these patterns were not simply explained by variations in abundances, suggesting that heterospecific associations were actively formed. These seasonal differences in observed patterns suggest that interspecific associations may be driven by resource overlap when resources are limited and by resource partitioning or anti-predator advantages when resources are abundant. We discuss potential mechanisms that could drive seasonal variation in the cost-benefit tradeoffs that underpin the formation of mixed-species groups.     

Cascading extinctions, functional complementarity, and selection in two-trophic-level model communities: A trait-based mechanistic approach

The influence of diversity on ecosystem functioning and ecosystem services is now well established. Yet predictive mechanistic models that link species traits and community-level processes remain scarce, particularly for multitrophic systems. Here we revisit MacArthur's classical consumer resource model and develop a trait-based approach to predict the effects of consumer diversity on cascading extinctions and aggregated ecosystem processes in a two-trophic-level system. We show that functionally redundant efficient consumers generate top-down cascading extinctions. This counterintuitive result reveals the limits of the functional redundancy concept to predict the consequences of species deletion. Our model also predicts that the biodiversity-ecosystem functioning relationship is different for different ecosystem processes and depends on the range of variation of consumer traits in the regional species pool, which determines the sign of selection effects. Lastly, competition among resources and consumer generalism both weaken complementarity effects, which suggests that selection effects may prevail at higher trophic levels. Our work emphasizes the potential of trait-based approaches for transforming biodiversity and ecosystem functioning research into a more predictive science.     

Density‐dependent dispersal and the formation of range borders.

Knowledge about the mechanisms of range formation is crucial for scientifically based species conservation strategies in the face of ongoing global climate change. In recent years an increasing amount of studies have focused on the influences of density‐dependent dispersal on demographic and biogeographical patterns. However, it still remains unclear, to what extent and in what ways this strategy would affect the range formation of species. In order to fill this gap, we present a study using individual‐based simulations of a species with discrete generations living along a dispersal mortality gradient. We compare the evolution of range sizes for species following density‐dependent and density‐independent emigration. Furthermore we assess the influence of environmental stochasticity and Allee effects on range formation, as both processes are known to play an important role for dispersal evolution. We find that density‐dependent dispersal always results in much wider ranges than unconditional dispersal. Increasing environmental stochasticity, a predicted consequence of climate change, can remarkably expand the ranges of species living in such connectivity gradients if dispersal decisions are based on local population density. A strong Allee effect causes range contraction for both strategies, but the effect is considerably less dramatic under density‐dependent compared to density‐independent emigration. We strongly recommend accounting for these findings in future attempts to model species’ range shifts due to climate change.     

Switching behavior, coexistence and diversification: comparing empirical community-wide evidence with theoretical predictions

Theory shows that the presence of behavioural switching between alternative resources can contribute to coexistence when competitors differ in trophic-related traits. In addition, switching can generate disruptive selection on such traits in a low-diversity community, increasing the number of species. Both of these processes should produce communities in which species differ in their values of the trophic trait, and display corresponding differences in the time-course of their switching from one resource to another. Here we present evidence for widespread switching behaviour for a diverse Mediterranean scrubland bird community. We show that species differ in a beak character related to their relative use of insect and fruit resource channels, and that the timing of switching is correlated with the relative use of resources. These patterns are consistent with theoretical predictions, suggesting a possible role of switching behaviour in promoting avian coexistence and diversification.     

Marine biodiversity and ecosystem functioning: what's known and what's next?

Marine ecosystems are experiencing rapid and pervasive changes in biodiversity and species composition. Understanding the ecosystem consequences of these changes is critical to effectively managing these systems. Over the last several years, numerous experimental manipulations of species richness have been performed, yet existing quantitative syntheses have focused on a just a subset of processes measured in experiments and, as such, have not summarized the full data available from marine systems. Here, we present the results of a meta‐analysis of 110 marine experiments from 42 studies that manipulated the species richness of organisms across a range of taxa and trophic levels and analysed the consequences for various ecosystem processes (categorised as production, consumption or biogeochemical fluxes). Our results show that, generally, mixtures of species tend to enhance levels of ecosystem function relative to the average component species in monoculture, but have no effect or a negative effect on functioning relative to the ‘highest‐ performing’ species. These results are largely consistent with those from other syntheses, and extend conclusions to ecological functions that are commonly measured in the marine realm (e.g. nutrient release from sediment bioturbation). For experiments that manipulated three or more levels of richness, we attempted to discern the functional form of the biodiversity–ecosystem functioning relationship. We found that, for response variables related to consumption, a power‐function best described the relationship, which is also consistent with previous findings. However, we identified a linear relationship between richness and production. Combined, our results suggest that changes in the number of species will, on average, tend to alter the functioning of marine ecosystems. We outline several research frontiers that will allow us to more fully understand how, why, and when diversity may drive the functioning of marine ecosystems. Synthesis The oceans host an incredible number and variety of species. However, human activities are driving rapid changes in the marine environment. It is imperative we understand ecosystem consequences of any associated loss of species. We summarized data from 110 experiments that manipulated species diversity and evaluated resulting changes to a range of ecosystem responses. We show that losing species, on average, decreases productivity, growth, and a myriad of other processes related to how marine organisms capture and utilize resources. Finally, we suggest that the loss of species may have stronger consequences for some processes than others.     

Landscape scale conservation: resources, behaviour, the matrix and opportunities

Landscape scale conservation efforts are becoming more commonplace in conservation, with a move from single species to multi-species initiatives. These initiatives are reliant on modelling processes, largely underpinned by metapopulation models. We argue that generic models developed for individual species in particular landscapes over selected time periods may only be applicable to alternative landscapes and time periods in restricted circumstances. Variability in species responses to landscapes and environmental conditions is dependent on a range of species-specific intrinsic characteristics, dependent on their responses to resources, (including weather) and also individual states. We propose that the behavioural component of how species respond to resources needs to be taken into account in modelling species responses to landscape, and therefore how limited resources for conservation are deployed. Species behaviours are inherently complex. We argue that because of this complexity the conservation of the majority of species, especially of the least rare, may be best served if conservation effort is additionally focused on increasing landscape heterogeneity and disturbance. This may also facilitate persistence in the face of climate change. We suggest that heterogeneity should be promoted through agri-environment schemes.     

Scale-dependence of resource-biodiversity relationships

The functional relationship between resource availability and species richness is addressed at different spatial scales. We analyse the smaller, community, scale by using a multi-species contact process coupled to a heterogeneous landscape, i.e. a stochastic spatial model of individual behavior in a system with limited resources. Using percolation theory, the theory of competitive exclusion processes, and the results of Monte Carlo simulations we show that a unimodal resource-species relationship may be understood as a tradeoff between the availability and connectivity of resource patches. We then pose the question of how resource-species relationships may be scaled up to the larger, regional, level and discuss the theoretical basis for differences in behavior at different scales. Regional ecosystems are modeled as statistical aggregates of dynamically driven small-scale ecosystems. Observing a transition from a unimodal relationship at small scales to a monotonically increasing relationship at large scales is shown to be contingent on the presence of a resource-dependent species pool. Finally, we confirm our theoretical prediction of a transition via Monte Carlo simulations of regional landscapes and discuss the potential complicating effects of spatial correlations in the distribution of both resources and species.     

Nitrogen and carbohydrate storage in biennials originating from habitats of different resource availability

Four biennial species (Arctium tomentosum, Cirsium vulgare, Dipsacus sylvester and Daucus carota) which originate from habitats of different nutrient availability were investigated in a 2-year experiment in a twofactorial structured block design varying light (natural daylight versus shading) and fertilizer addition. The experiment was designed to study storage as reserve formation (competing with growth) or as accumulation (see Chapin et al. 1990). We show that (i) the previous definitions of storage excluded an important process, namely the formation of storage tissue. Depending on species, storage tissue and the filling process can be either a process of reserve formation, or a process of accumulation. (ii) In species representing low-resource habitats, the formation of a storage structure competes with other growth processes. Growth of storage tissue and filling with storage products is an accumulation process only in the high-resource plant Arctium tomentosum. We interpret the structural growth of low-resource plants in terms of the evolutionary history of these species, which have closely related woody species in the Mediterranean area. (iii) The use of storage products for early leaf growth determines the biomass development in the second season and the competitive ability of this species during growth with perennial species. (iv) The high-resource plant Arctium has higher biomass development under all conditions, i.e. plants of low-resource habitats are not superior under low-resource conditions. The main difference between high- and low-resource plants is that low-resource plants initiate flowering at a lower total plant internal pool size of available resources.     

Spatial distribution and habitat selection in coexisting species of mountain ungulates

One of the main objectives of community ecology is to understand the conditions allowing species to coexist, which requires identifying how co‐occurring species use and share space and resources. Species of the same trophic level, such as large herbivores, are of fundamental interest in that context because competition for resources is likely. Segregation in space or on some axes of the ecological niche are processes allowing coexistence, yet, both are seldom studied jointly. Based on annual spring censuses collected for 11 yr, we analysed the degree of overlap in spatial distribution among chamois Rupicapra rupicapra and mouflon Ovis gmelini musimon, 2 species of similar size that coexist on the same alpine pastures. We further investigated whether they differed in terms of habitat selection processes, and identified which environmental factors led to species being aggregated or segregated. The areas of intensive use for 2 species were more spatially aggregated than expected by chance. Habitat selection was studied using multivariate methods based on the niche concept, considering the presence of 1 species as an environmental variable for the other. Despite a large overlap in niches (88%), segregation was significant as chamois preferred meadows dominated by Sesleria and Carex sempervirens while mouflon selected meadows dominated by Carex ferruginea and avoided being close to areas affected by human activities. Importantly, habitat selection within each species was not affected by the presence of the other species. Coexistence between these 2 species and spatial overlap may be permitted because resource partitioning occurs at a fine temporal and/or spatial scale. We underscore that joint approaches of spatial and ecological processes are necessary to disclose the type of interaction (neutral, facilitation or competition) at play within a community.     

The carrying capacity for species richness

The idea that the number of species within an area is limited by a specific capacity of that area to host species is old yet controversial. Here, we show that the concept of carrying capacity for species richness can be as useful as the analogous concept in population biology. Many lines of empirical evidence indicate the existence of limits of species richness, at least at large spatial and phylogenetic scales. However, available evidence does not support the idea of diversity limits based on limited niche space; instead, carrying capacity should be understood as a stable equilibrium of biodiversity dynamics driven by diversity‐dependent processes of extinction, speciation and/or colonization. We argue that such stable equilibria exist even if not all resources are used and if increasing species richness increases the ability of a community to use resources. Evaluating the various theoretical approaches to modelling diversity dynamics, we conclude that a fruitful approach for macroecology and biodiversity science is to develop theory that assumes that the key mechanism leading to stable diversity equilibria is the negative diversity dependence of per‐species extinction rates, driven by the fact that population sizes of species must decrease with an increasing number of species owing to limited energy availability. The recently proposed equilibrium theory of biodiversity dynamics is an example of such a theory, which predicts that equilibrium species richness (i.e., carrying capacity) is determined by the interplay of the total amount of available resources, the ability of communities to use those resources, environmental stability that affects extinction rates, and the factors that affect speciation and colonization rates. We argue that the diversity equilibria resulting from these biodiversity dynamics are first‐order drivers of large‐scale biodiversity patterns, such as the latitudinal diversity gradient.     

Diversity of germ layer and axis formation among mammals

The class mammalia is composed of approximately 4800 extant species. This class is divided into three subclasses, the prototheria (monotremes), metatheria (marsupials), and eutheria. Surprisingly, there is relatively little knowledge about germ layer and axis formation in mammalian species. Most knowledge about these embryonic processes has been obtained from one species, the mouse, Mus musculus. Here we discuss major variations in germ layer and axis formation among mammals. We suggest that more studies of embryonic development in diverse mammalian species are required for an understanding of germ layer and axis formation to provide insights into human biology and disease.     

Trait plasticity in species interactions: a driving force of community dynamics

Evolutionary community ecology is an emerging field of study that includes evolutionary principles such as individual trait variation and plasticity of traits to provide a more mechanistic insight as to how species diversity is maintained and community processes are shaped across time and space. In this review we explore phenotypic plasticity in functional traits and its consequences at the community level. We argue that resource requirement and resource uptake are plastic traits that can alter fundamental and realised niches of species in the community if environmental conditions change. We conceptually add to niche models by including phenotypic plasticity in traits involved in resource allocation under stress. Two qualitative predictions that we derive are: (1) plasticity in resource requirement induced by availability of resources enlarges the fundamental niche of species and causes a reduction of vacant niches for other species and (2) plasticity in the proportional resource uptake results in expansion of the realized niche, causing a reduction in the possibility for coexistence with other species. We illustrate these predictions with data on the competitive impact of invasive species. Furthermore, we review the quickly increasing number of empirical studies on evolutionary community ecology and demonstrate the impact of phenotypic plasticity on community composition. Among others, we give examples that show that differences in the level of phenotypic plasticity can disrupt species interactions when environmental conditions change, due to effects on realized niches. Finally, we indicate several promising directions for future phenotypic plasticity research in a community context. We need an integrative, trait-based approach that has its roots in community and evolutionary ecology in order to face fast changing environmental conditions such as global warming and urbanization that pose ecological as well as evolutionary challenges.     

Speciation in rapidly diverging systems: lessons from Lake Malawi

Rapid evolutionary radiations provide insight into the fundamental processes involved in species formation. Here we examine the diversification of one such group, the cichlid fishes of Lake Malawi, which have radiated from a single ancestor into more than 400 species over the past 700 000 years. The phylogenetic history of this group suggests: (i) that their divergence has proceeded in three major bursts of cladogenesis; and (ii) that different selective forces have dominated each cladogenic event. The first episode resulted in the divergence of two major lineages, the sand- and rock-dwellers, each adapted to a major benthic macrohabitat. Among the rock-dwellers, competition for trophic resources then drove a second burst of cladogenesis, which resulted in the differentiation of trophic morphology. The third episode of cladogenesis is associated with differentiation of male nuptial colouration, most likely in response to divergent sexual selection. We discuss models of speciation in relation to this observed pattern. We advocate a model, divergence with gene flow, which reconciles the disparate selective forces responsible for the diversification of this group and suggest that the nonadaptive nature of the tertiary episode has significantly contributed to the extraordinary species richness of this group.     

植物边缘种群遗传多样性研究进展

边缘种群指地理分布边缘可检测到的一定数量的同种个体集合, 准确评价其遗传多样性对于理解第四纪冰期后气候变化对物种边缘扩展或收缩、遗传资源保护与利用以及物种形成等有重要意义。该文探讨了维持植物边缘种群遗传多样性的进化机制, 分析交配系统对物种边缘及其遗传多样性的影响, 比较了边缘与中心种群遗传多样性的差异及其形成的生态与进化过程, 并探讨了边缘种群遗传多样性与其所在的群落物种多样性的关系及理论基础。该文提出今后研究的重点是应用全基因组序列或转录组基因序列研究前缘-后缘种群之间或边缘-中心种群之间的适应性差异, 边缘种群与所在群落其他物种之间相互作用的分子机制, 深入解析边缘种群对环境的适应及边缘种群遗传多样性与群落物种多样性关系的生态与进化过程。     

Effects of Competition and Facilitation on Species Assemblage in Two Types of Tropical Cloud Forest

Competition and facilitation between tree individuals are two kinds of non-random processes influencing the structure and functioning of forest communities, but how these two plant-plant interactions change along gradient of resources or environments remains very much a matter of debate. We developed a null model to test the size-distance regression, and assessed the effects of competition and facilitation (including interspecific interactions, intraspecific interactions and overall species interactions) on each adult tree species assemblage [diameter at breast height (dbh) ≥5 cm] across two types of tropical cloud forest with different environmental and resource regimes. The null model test revealed that 17% to 27% tree species had positive dbh-distance correlations while 11% to 19% tree species showed negative dbh-distance correlations within these two forest types, indicating that both competition and facilitation processes existed during the community assembly. The importance of competition for heterospecific species, and the intensity of competition for both heterospecific and overall species increased from high to low resources for all the shared species spanning the two forests. The importance of facilitation for conspecific and overall species, as well as that the intensity of facilitation for both heterospecific and conspecific species increased with increasing low air temperature stress for all the shared species spanning the two forests. Our results show that both competition and facilitation processes simultaneously affect parts of species assemblage in the tropical cloud forests. Moreover, the fact that nearly 50% species assemblage is not detected with our approaches suggest that tree species in these tropical forest systems are assembled with multiple ecological processes, and that there is a need to explore the processes other than the two biotic interactions in further researches.     

How Drosophila change their combs: the Hox gene Sex combs reduced and sex comb variation among Sophophora species

SUMMARY Identification of the events responsible for rapid morphological variation during evolution can help understand how developmental processes are changed by genetic modifications and thus produce diverse body features and shapes. Sex combs, a sexually dimorphic structure, show considerable variation in morphology and numbers among males from related species of Sophophora , a subgenus of Drosophila . To address which evolutionary changes in developmental processes underlie this diversity, we first analyzed the genetic network that controls morphogenesis of a single sex comb in the model D. melanogaster . We show that it depends on positive and negative regulatory inputs from proximo-distal identity specifying genes, including dachshund, bric à brac , and sex combs distal . All contribute to spatial regulation of the Hox gene Sex combs reduced (Scr ), which is crucial for comb formation. We next analyzed the expression of these genes in sexually dimorphic species with different comb numbers. Only Scr shows considerable expression plasticity, which is correlated with comb number variation in these species. We suggest that differences in comb numbers reflect changes of Scr expression in tarsus primordia, and discuss how initial comb formation could have occurred in an ancestral Sophophora fly following regulatory modifications of developmental programs both parallel to and downstream of Scr .     

Effects of trophic similarity on community composition

Understanding how ecological processes determine patterns among species coexisting within ecosystems is central to ecology. Here, we explore relationships between species’ local coexistence and their trophic niches in terms of their feeding relationships both as consumers and as resources. We build on recent concepts and methods from community phylogenetics to develop a framework for analysing mechanisms responsible for community composition using trophic similarity among species and null models of community assembly. We apply this framework to 50 food webs found in 50 Adirondack lakes and find that species composition in these communities appears to be driven by both bottom‐up effects by which the presence of prey species selects for predators of those prey, and top‐down effects by which prey more tolerant of predation out‐compete less tolerant prey of the same predators. This approach to community food webs is broadly applicable and shows how species interaction networks can inform an increasingly large array of theory central to community ecology.     

Food plant diversity as broad-scale determinant of avian frugivore richness

The causes of variation in animal species richness at large spatial scales are intensively debated. Here, we examine whether the diversity of food plants, contemporary climate and energy, or habitat heterogeneity determine species richness patterns of avian frugivores across sub-Saharan Africa. Path models indicate that species richness of Ficus (their fruits being one of the major food resources for frugivores in the tropics) has the strongest direct effect on richness of avian frugivores, whereas the influences of variables related to water-energy and habitat heterogeneity are mainly indirect. The importance of Ficus richness for richness of avian frugivores diminishes with decreasing specialization of birds on fruit eating, but is retained when accounting for spatial autocorrelation. We suggest that a positive relationship between food plant and frugivore species richness could result from niche assembly mechanisms (e.g. coevolutionary adaptations to fruit size, fruit colour or vertical stratification of fruit presentation) or, alternatively, from stochastic speciation-extinction processes. In any case, the close relationship between species richness of Ficus and avian frugivores suggests that figs are keystone resources for animal consumers, even at continental scales.     

Pattern Formation by Competition: A Biological Example

We present a simple model based on a reaction-diffusion equation to explain pattern formation in a multicellular bacterium (Streptomyces). We assume competition for resources as the basic mechanism that leads to pattern formation; in particular we are able to reproduce the spatial pattern formed by bacterial aerial mycelium in the case of growth in minimal (low resources) and maximal (large resources) culture media.