On the relationship between niche and distribution

Applications of Hutchinson's n -dimensional niche concept are often focused on the role of interspecific competition in shaping species distribution patterns. In this paper, I discuss a variety of factors, in addition to competition, that influence the observed relationship between species distribution and the availability of suitable habitat. In particular, I show that Hutchinson's niche concept can be modified to incorporate the influences of niche width, habitat availability and dispersal, as well as interspecific competition per se . I introduce a simulation model called NICHE that embodies many of Hutchinson's original niche concepts and use this model to predict patterns of species distribution. The model may help to clarify how dispersal, niche size and competition interact, and under what conditions species might be common in unsuitable habitat or absent from suitable habitat. A brief review of the pertinent literature suggests that species are often absent from suitable habitat and present in unsuitable habitat, in ways predicted by theory. However, most tests of niche theory are hampered by inadequate consideration of what does and does not constitute suitable habitat. More conclusive evidence for these predictions will require rigorous determination of habitat suitability under field conditions. I suggest that to do this, ecologists must measure habitat specific demography and quantify how demographic parameters vary in response to temporal and spatial variation in measurable niche dimensions.     

草地群落物种多样性维持机制的研究Ⅱ物种实现的生态位

尽管为解释种类丰富的植物群落物种共存和多样性维持机制,生态学家位做了大量的努力并提出了许多假说和模型。但这一问题仍处在争议之中,需要更多的证据支持他们的观点或提出新的看法,使这一生物多样性难题不断地向前推进。以松赖平原物种丰富度较高的羊草－杂类草群落为对象,在土壤C、N、P、K和H2O等5个资源轴上,探讨了物种多样性与实现生态位的关系。结果表明：尽管物种生态位存在一定程度的分化,但多数物种的生态位是高度重叠的,物种生态位的分化在草地群落物种共存和多样性维持中,不是唯一的途径,认为应更加重视的物种在长期协同进化中所形成的生物学特性。     

Niche construction in the light of niche theory

Ecological niche construction, the process whereby an organism improves its environment to enhance its growth and persistence, is an important missing element of niche theory. Niche theory has mainly focused on niche-deteriorating processes, such as resource consumption, predation and competition, which have negative effects on population growth. Here, we integrate niche construction explicitly into modern niche theory. We use a graphical approach to analyse how a species' niche-improving impacts interplay with niche-deteriorating impacts to modify its response to the environment. In a model of two consumers that compete for one limiting resource and one predator, we show how niche construction modifies the traditional niche-deteriorating impacts of its agent or of competing species, and hence the potential for species coexistence. By altering the balance between intraspecific and interspecific competitive effects, niche construction can either generate net interspecific facilitation or strengthen interspecific competition. The adaptive benefit derived from niche construction also strongly affects the realized niche of a niche-constructing species.     

Competition and coexistence in plant communities

Few ecologists today doubt that competition is an important structuring factor in plant communities, but researchers disagree on the circumstances where it is most intense, and on which traits can be considered to contribute to competitive ability in different species. The distinction between a species' effect on resources and its response to reduced resource levels might help to solve these questions. Whereas classical competition theory predicts competitive exclusion of species with similar requirements, recent ideas stress that species diversity may be explained by a multitude of processes acting at different scales, and that similarities in competitive abilities often may facilitate coexistence.     

生态位分化与森林群落物种多样性维持研究展望

一直以来,生态学家和进化生物学家对森林群落物种多样格局及其形成机制持有不同的观点。虽然Robert Ricklefs将进化和生态过程整合的观点已经被群落生态学家广泛接受,但是区域物种进化历史以及局域群落微进化过程是否能够影响群落生态学过程以及这些过程如何影响群落结构和动态还有待商榷。经典的生态位理论同时强调了种间和种内生态位分化对群落多样性维持的影响。但是生态学家普遍认为种间差异足以代表群落内个体间的相互作用关系,并且由于进化过程导致的种内分化往往涉及较长的时间尺度,因此,虽然种内差异是自然选择的重要材料,物种对环境的适应性进化过程所导致的种内分化对群落构建的影响往往被生态学家所忽视。为此,通过回顾种间和个体生态位分化的研究历史,对两类研究分别进行简要阐述,强调在今后的群落生态学研究中需要考虑个体分化对局域群落构建的影响。     

The relationship between biogeography and ecology: envelopes,models, predictions

This paper reviews ideas on the relationship between the ecology of clades and their distribution. Ecological biogeography represents a tradition that dates back to ancient times. It assumes that the distribution of organisms is explained by factors of present environment, especially climate. In contrast, modern systematics, following its origins in the Renaissance, concluded with Darwin that ‘neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions’. In many cases, species distribution models – ecological niche models – based on the current environment of a species (its environmental envelope) fail to predict the actual distribution of the species. In particular, they often over‐predict distributions. In addition, a group's niche often varies in space and time. These results provide valuable evidence that Darwin was correct, and many ecologists now recognise that there is a problem with the niche theory of distribution. Current ecological processes explain distribution at smaller scales than do biogeographical and evolutionary processes, but the latter can lead to patterns that are much more local than many ecologists have assumed. Biogeographical phenomena often occur at a much smaller scale than that of the Wallacean regions. In areas that have been subjected to marine inundation or intense tectonism, many centres of endemism are only tens of kilometres across. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115 , 456–468.     

Coexistence and geographical distribution of Leguminosae in an area of Atlantic forest in the semi‐arid region of Brazil

Abstract Understanding the factors that affect plant species distribution and coexistence in areas with high plant species diversity is a challenge for ecologists. According to some authors, species occupy specific niches, but for others, species coexistence and geographical distribution patterns are random. Floristic composition of the family Leguminosae was studied on moist and dry slopes of the Baturité mountains in semi‐arid northeastern Brazil and was compared with findings for other plant formations elsewhere in Brazil. Substantial floristic differences were found between the moist windward and dry leeward slopes of the Baturité mountains despite their close geographical proximity. The leeward slope was slightly more diverse than the windward slope. Similarity analyses showed that the windward face is floristically allied to the Amazon forest, whereas the leeward slope is similar to other dry‐area formations of northeastern Brazil, such as thorny woodland (caatinga) and seasonal forests. The strong floristic differences that were observed between the windward and leeward slopes corroborate the theory of ecological niche conservatism, which holds that species occurrence is closely linked to environmental factors, such as temperature and precipitation.     

Potential mechanisms of coexistence in closely related forbs

The stable coexistence of very similar species has perplexed ecologists for decades and has been central to the development of coexistence theory. According to modern coexistence theory, species can coexist stably (i.e. persist indefinitely with no long‐term density trends) as long as species' niche differences exceed competitive ability differences, even if these differences are very small. Recent studies have directly quantified niche and competitive ability differences in experimental communities at small spatial scales, but provide limited information about stable coexistence across spatial scales in heterogeneous natural communities. In this study, we use experimental and observational approaches to explore evidence for niche and competitive ability differences between two closely related, ecologically similar and widely coexisting annual forbs: Trachymene cyanopetala and T. ornata. We experimentally tested for stabilizing niche differences and competitive ability differences between these species by manipulating species' frequencies, under both well‐watered and water‐stressed conditions. We considered these experimental results in light of extensive field observations to explore evidence of niche segregation at a range of spatial scales. We found little evidence of intra‐specific stabilization or competitive ability differences in laboratory experiments while observational studies suggested niche segregation across pollinator assemblages and small‐scale microclimate heterogeneity. Though we did not quantify long‐term stabilization of coexisting populations of these species, results are consistent with expectations for stable coexistence of similar species via a spatial storage effect allowing niche differences to overcome even small (to absent) competitive ability differences.     

Synchronized reproduction promotes species coexistence through reproductive facilitation

Theories for species coexistence often emphasize niche differentiation and temporal segregation of recruitment to avoid competition. Recent work on mutualism suggested that plant species sharing pollinators provide mutual facilitation when exhibit synchronized reproduction. The facilitation on reproduction may enhance species persistence and coexistence. Theoretical ecologists paid little attention to such indirect mutualistic systems by far. We propose a new model for a two-species system using difference equations. The model focuses on adult plants and assumes no resource competition between these well-established individuals. Our formulas include demographic parameters, such as mortality and recruitment rates, and functions of reproductive facilitation. Both recruitment and facilitation effects reach saturation levels when flower production is at high levels. We conduct mathematical analyses to assess conditions of coexistence. We establish demographical conditions permitting species coexistence. Our analyses suggest a “rescue” effect from a “superior” species to a “weaker” species under strong recruitment enhancement effect when the later is not self-sustainable. The facilitation on rare species may help to overcome Allee effect.     

Emergent neutrality drives phytoplankton species coexistence

The mechanisms that drive species coexistence and community dynamics have long puzzled ecologists. Here, we explain species coexistence, size structure and diversity patterns in a phytoplankton community using a combination of four fundamental factors: organism traits, size-based constraints, hydrology and species competition. Using a 'microscopic' Lotka-Volterra competition (MLVC) model (i.e. with explicit recipes to compute its parameters), we provide a mechanistic explanation of species coexistence along a niche axis (i.e. organismic volume). We based our model on empirically measured quantities, minimal ecological assumptions and stochastic processes. In nature, we found aggregated patterns of species biovolume (i.e. clumps) along the volume axis and a peak in species richness. Both patterns were reproduced by the MLVC model. Observed clumps corresponded to niche zones (volumes) where species fitness was highest, or where fitness was equal among competing species. The latter implies the action of equalizing processes, which would suggest emergent neutrality as a plausible mechanism to explain community patterns.     

Water source niche overlap increases with site moisture availability in woody perennials

Classical niche partitioning theory posits increased competition for and partitioning of the most limiting resource among coexisting species. Coexisting plant species may vary in rooting depth, reflecting niche partitioning in water source use. Our goal was to assess the soil water partitioning of woody plant communities across northern Arizona along an elevational moisture gradient using stem and soil water isotopes from two sampling periods to estimate the use of different water sources. We hypothesized that niche overlap of water sources would be higher and monsoon precipitation uptake would be lower at sites with higher moisture availability. Pairwise niche overlap of coexisting species was calculated using mixing model estimates of proportional water use for three sources. Across the moisture gradient, niche overlap increased with site moisture index (precipitation/potential evapotranspiration) across seasons, and site moisture index explained 37% of the variation in niche overlap of intermediate and deeper sources of water. Desert trees utilized more winter source water than desert shrubs, suggesting the partitioning of water sources between functional groups. However, seasonal differences in surface water use were primarily found at intermediate levels of site moisture availability. Our findings support classical niche partitioning theory in that plants exhibit higher overlap of water sources when water is not a limiting resource.     

Quantitative predictions of pollinators’ abundances from qualitative data on their interactions with plants and evidences of emergent neutrality

Making quantitative predictions of the effects of human activities on ecological communities is crucial for their management. In the case of plant–pollinator mutualistic networks, despite the great progress in describing the interactions between plants and their pollinators, the capability of making quantitative predictions is still lacking. Here, in order to estimate pollinator species abundances and their niche distribution, I propose a general method to transform a plant–pollinator network into a competition model between pollinator species. Competition matrices were obtained from ‘first principles’ calculations, using qualitative interaction matrices compiled for a set of 38 plant–pollinator networks. This method is able to make accurate quantitative predictions for mutualistic networks spanning a broad geographic range. Specifically, the predicted biodiversity metrics for pollinators – species relative abundances, Shannon equitability and Gini–Simpson indices – agree quite well with those inferred from empirical counts of visits of pollinators to plants. Furthermore, this method allows building a one‐dimensional niche axis for pollinators in which clusters of generalists are separated by specialists thus rendering support to the theory of emergent neutrality. The importance of interspecific competition between pollinator species is a controversial and unresolved issue, considerable circumstantial evidence has accrued that competition between insects does occur, but a clear measure of its impact on their species abundances is still lacking. I contributed to fill this gap by quantifying the effect of competition between pollinators. Particular applications of our analysis could be to estimate the quantitative effects of removing a species from a community or to address the fate of populations of native organisms when foreign species are introduced to ecosystems far beyond their home range.     

How variation between individuals affects species coexistence

Although the effects of variation between individuals within species are traditionally ignored in studies of species coexistence, the magnitude of intraspecific variation in nature is forcing ecologists to reconsider. Compelling intuitive arguments suggest that individual variation may provide a previously unrecognised route to diversity maintenance by blurring species‐level competitive differences or substituting for species‐level niche differences. These arguments, which are motivating a large body of empirical work, have rarely been evaluated with quantitative theory. Here we incorporate intraspecific variation into a common model of competition and identify three pathways by which this variation affects coexistence: (1) changes in competitive dynamics because of nonlinear averaging, (2) changes in species’ mean interaction strengths because of variation in underlying traits (also via nonlinear averaging) and (3) effects on stochastic demography. As a consequence of the first two mechanisms, we find that intraspecific variation in competitive ability increases the dominance of superior competitors, and intraspecific niche variation reduces species‐level niche differentiation, both of which make coexistence more difficult. In addition, individual variation can exacerbate the effects of demographic stochasticity, and this further destabilises coexistence. Our work provides a theoretical foundation for emerging empirical interests in the effects of intraspecific variation on species diversity.     

The shifting balance of facilitation and competition affects the outcome of intra- and interspecific interactions over the life history of California grassland annuals

Trait-based resource competition in plants, wherein more similar plants compete more strongly for resources, is a foundation of niche-based explanations for the maintenance of diversity in plant communities. Alternatively, neutral theory predicts that community diversity can be maintained despite equivalent resource requirements among species. We examined interactions at three life history stages (germination, survival, and juvenile-adult growth) for three native and three exotic California annual species in a glasshouse experiment. We varied plant density and species composition in small pots, with pots planted with either intraspecific seeds or in a three species mix of intra- and interspecific neighbors. We saw a range of facilitative, neutral, and competitive interactions that varied significantly by species, rather than by native or exotic status. There were more competitive interactions at the emergence and juvenile-adult growth stages and more facilitative interactions for survival. Consequently, the relative strength of competition in intraspecific versus mixed-species communities depended on whether we considered only the juvenile-adult growth stage or the entire life history of the interacting plants. Using traditional analysis of juvenile-adult growth only, all species showed negative density-dependent interactions for final biomass production. However, when the net effect of plant interactions from seed to adult was considered, which is a prediction of population growth, two native species ceased to show negative density dependence, and the difference between intraspecific and mixed-species competition was only significant for one exotic species. Results were consistent with predictions of neutral, rather than niche, theory for five of six species.     

Rethinking plant community theory

Plant communities have traditionally been viewed as either a random collection of individuals or as organismal entities. For most ecologists however, neither perspective provides a modern comprehensive view of plant communities, but we have yet to formalize the view that we currently hold. Here, we assert that an explicit re-consideration of formal community theory must incorporate interactions that have recently been prominent in plant ecology, namely facilitation and indirect effects among competitors. These interactions do not suppport the traditional individualistic perspective. We believe that rejecting strict individualistic theory will allow ecologists to better explain variation occurring at different spatial scales, synthesize more general predictive theories of community dynamics, and develop models for community-level responses to global change. Here, we introduce the concept of the integrated community (IC) which proposes that natural plant communities range from highly individualistic to highly interdependent depending on synergism among: (i) stochastic processes, (ii) the abiotic tolerances of species, (iii) positive and negative interactions among plants, and (iv) indirect interactions within and between trophic levels. All of these processes are well accepted by plant ecologists, but no single theory has sought to integrate these different processes into our concept of communities.     

福建琅岐岛台湾相思群落灌木层主要物种生态位与种间联结

为探究福建琅岐岛台湾相思(Acacia confusa)群落种间关系和群落演替动态,对其灌木层主要物种的生态位和种间联结特征进行了分析。结果表明,重要值和生态位宽度最大的物种为雀梅藤(Sageretia thea),其竞争优势、环境适应能力最强;灌木层主要物种的生态位相似性和生态位重叠较小,表明种间对环境资源的利用和要求存在较大差异,表现为较稳定的种间关系;灌木层主要物种的总体联结性为显著正联结,种对间正负联结比为1.72,检验显著率为7.35%,表明种对间联结程度较弱,独立性相对较强,物种竞争不激烈,群落处于相对后期且稳定的演替阶段;红锥(Castanopsis hystrix)与许多物种间存在垂直方向上对生境要求的互补性,生态位重叠较小,种间呈正联结关系;外来物种马缨丹(Lantanacamara)与福建胡颓子(Elaeagnus oldhamii)存在一定的竞争,与灌木层其他主要植物之间的资源竞争不激烈,其对该群落的影响相对较小。琅岐岛台湾相思群落目前已发展为地带性的顶级群落,物种竞争不激烈,种间关系较稳定;而海岛上恶劣的环境和外来物种的扩散会导致植物生境资源短缺,迫使物种聚集在...     

Resource use and co-existence: Experimental tests from a sequence of different-aged pastures

Summary Competitive relationships were estimated for plants from young and old pasture populations. Experimental treatments were conducted under both common garden field station conditions and actual grazed pasture conditions. The results suggest four conclusions. (1) Grazing and other pasture conditions did not prevent plant competition. (2) Interspecific competition appears to promote the use of different resources (niche divergence). (3) Intraspecific competition may lead to a broadening of resource use by a species and, thus, indirectly increase both niche overlap and interspecific competition. No evidence was found to support an alternative theory of co-existence in which interspecific competition is argued to promote a balancing of competitive abilities without changes to relative niche overlap. (4) Changes in resource use appears to occur within a few decades after pasture formation.     

Pollination niche overlap between a parasitic plant and its host

Niche theory predicts that species which share resources should evolve strategies to minimise competition for those resources, or the less competitive species would be extirpated. Some plant species are constrained to co-occur, for example parasitic plants and their hosts, and may overlap in their pollination niche if they flower at the same time and attract the same pollinators. Using field observations and experiments between 1996 and 2006, we tested a series of hypotheses regarding pollination niche overlap between a specialist parasitic plant Orobanche elatior (Orobanchaceae) and its host Centaurea scabiosa (Asteraceae). These species flower more or less at the same time, with some year-to-year variation. The host is pollinated by a diverse range of insects, which vary in their effectiveness, whilst the parasite is pollinated by a single species of bumblebee, Bombus pascuorum, which is also an effective pollinator of the host plant. The two species therefore have partially overlapping pollination niches. These niches are not finely subdivided by differential pollen placement, or by diurnal segregation of the niches. We therefore found no evidence of character displacement within the pollination niches of these species, possibly because pollinators are not a limiting resource for these plants. Direct observation of pollinator movements, coupled with experimental manipulations of host plant inflorescence density, showed that Bombus pascuorum only rarely moves between inflorescences of the host and the parasite and therefore the presence of one plant is unlikely to be facilitating pollination in the other. This is the first detailed examination of pollination niche overlap in a plant parasite system and we suggest avenues for future research in relation to pollination and other shared interactions between parasitic plants and their hosts.     

Pulsed resource availability changes dietary niche breadth and partitioning between generalist rodent consumers

Identifying the mechanisms that structure niche breadth and overlap between species is important for determining how species interact and assessing their functional role in an ecosystem. Without manipulative experiments, assessing the role of foraging ecology and interspecific competition in structuring diet is challenging. Systems with regular pulses of resources act as a natural experiment to investigate the factors that influence the dietary niches of consumers. We used natural pulses of mast‐fruiting of American beech (Fagus grandifolia) to test whether optimal foraging or competition structure the dietary niche breadth and overlap between two congener rodent species (Peromyscus leucopus and P. maniculatus), both of which are generalist consumers. We reconstructed diets seasonally over a 2‐year period using stable isotope analysis (δ¹³C, δ¹⁵N) of hair and of potential dietary items and measured niche dynamics using standard ellipse area calculated within a Bayesian framework. Changes in niche breadth were generally consistent with predictions of optimal foraging theory, with both species consuming more beechnuts (a high‐quality food resource) and having a narrower niche breadth during masting seasons compared to nonmasting seasons when dietary niches expanded and more fungi (a low‐quality food source) were consumed. In contrast, changes in dietary niche overlap were consistent with competition theory, with higher diet overlap during masting seasons than during nonmasting seasons. Overall, dietary niche dynamics were closely tied to beech masting, underscoring that food availability influences competition. Diet plasticity and niche partitioning between the two Peromyscus species may reflect differences in foraging strategies, thereby reducing competition when food availability is low. Such dietary shifts may have important implications for changes in ecosystem function, including the dispersal of fungal spores.     

Foraging profiles of sympatric lowland gorillas and chimpanzees in the Lopé Reserve, Gabon.

Comparison of the diets of sympatric gorillas and chimpanzees allows an analysis of niche separation between these two closely related species. Qualitatively, their diets are similar, being dominated by an equally diverse array of fruit species complemented with vegetative plant parts, seeds and insects. Gorillas eat more vegetative plant parts than do chimpanzees, but niche separation is most obvious in periods of fruit scarcity when the two species show different strategies that reduce competition for food. Their abilities to overcome mechanical and physical plant defences appear to differ, as gorillas are able to subsist entirely on abundant vegetative foods. Chimpanzees show social adjustment, foraging alone or in small groups, to reduce intra-specific competition for scarce fruit resources. Thus it seems that subtle physiological differences have far-reaching repercussions, defining potential evolutionary pathways for social organization and allowing sufficient niche separation between species.