Niche Limitation Research Articles

The ontogeny of tolerance curves: habitat quality vs. acclimation in a stressful environment.

Stressful environments affect life-history components of fitness through (i) instantaneous detrimental effects, (ii) historical (carry-over) effects and (iii) history-by-environment interactions, including acclimation effects. The relative contributions of these different responses to environmental stress are likely to change along life, but such ontogenic perspective is often overlooked in studies of tolerance curves, precluding a better understanding of the causes of costs of acclimation, and more generally of fitness in temporally fine-grained environments. We performed an experiment in the brine shrimp Artemia to disentangle these different contributions to environmental tolerance, and investigate how they unfold along life. We placed individuals from three clones of A. parthenogenetica over a range of salinities during a week, before transferring them to a (possibly) different salinity for the rest of their lives. We monitored individual survival at repeated intervals throughout life, instead of measuring survival or performance at a given point in time, as commonly done in acclimation experiments. We then designed a modified survival analysis model to estimate phase-specific hazard rates, accounting for the fact that individuals may share the same treatment for only part of their lives. Our approach allowed us to distinguish effects of salinity on (i) instantaneous mortality in each phase (habitat quality effects), (ii) mortality later in life (history effects) and (iii) their interaction. We showed clear effects of early salinity on late survival and interactions between effects of past and current environments on survival. Importantly, analysis of the ontogenetic dynamics of the tolerance curve reveals that acclimation affects different parts of the curve at different ages. Adopting a dynamical view of the ontogeny of tolerance curve should prove useful for understanding niche limits in temporally changing environments, where the full sequence of environments experienced by an individual determines its overall environmental tolerance, and how it changes throughout life.

Range limits and niche limits

Biogeography Do the geographical range limits of species reflect the limits of their ecological niches, or are they determined more by limits on their dispersal? These questions have long bothered ecologists, and they matter in the context of shifting distributions in a changing climate. Lee-Yaw et al. synthesized cases of 40 species (mainly plants) experimentally transplanted beyond their normal range limits, and they compared the results with niche models of habitat suitability. They found that for three-quarters of the species, range limits matched their niche limits, suggesting that most species are not dispersal-limited. The authors note, however, that dispersal limitation is likely to assume greater importance as habitats become increasingly fragmented by human activity. Ecol. Lett. 10.1111/ele.12604 (2016).

A synthesis of transplant experiments and ecological niche models suggests that range limits are often niche limits.

Global change has made it important to understand the factors that shape species' distributions. Central to this area of research is the question of whether species' range limits primarily reflect the distribution of suitable habitat (i.e. niche limits) or arise as a result of dispersal limitation. Over-the-edge transplant experiments and ecological niche models are commonly used to address this question, yet few studies have taken advantage of a combined approach for inferring the causes of range limits. Here, we synthesise results from existing transplant experiments with new information on the predicted suitability of sites based on niche models. We found that individual performance and habitat suitability independently decline beyond range limits across multiple species. Furthermore, inferences from transplant experiments and niche models were generally concordant within species, with 31 out of 40 cases fully supporting the hypothesis that range limits are niche limits. These results suggest that range limits are often niche limits and that the factors constraining species' ranges operate at scales detectable by both transplant experiments and niche models. In light of these findings, we outline an integrative framework for addressing the causes of range limits in individual species.

The Niche Limitation Method (NicheLim), a new algorithm for generating virtual species to study biogeography

Virtual species are simplified models of real species that codify the response of those species to the climatic conditions. Virtual species have been used to quantify the response of species to climatic changes, to predict potential shifts in species’ geographic ranges and to test the methods used to predict the geographic ranges of species (Ecological Niche Models). Today, there are different methods used to construct virtual species for biogeographic analysis. All of those methods combine partial suitabilities across variables (temperature, precipitation, etc.) to create one multi-variable habitat suitability index. The normal procedure for combining partial suitabilities is to sum or multiply the individual layers. However, this procedure might yield misleading results. In this paper, we run several analyses that indicate that those methods underestimate the role of the limitation factors (factors with unsuitable conditions that should decrease the habitat suitability index to zero). To solve this problem we programmed an algorithm—the Niche Limitation algorithm (NicheLim). NicheLim uses the same philosophy as the BIOCLIM model: species have independent physiological tolerances to the environmental variables. This means that we must first transform each continuous layer into a presence-absence variable, and then combine them. Here, we discuss the current framework for constructing virtual species in biogeography and its main drawbacks. We then explain the improvements of using NicheLim to create our virtual species and test biogeographic hypotheses.

Can't see the trees for the forest: complex factors influence tree survival in a temperate second growth forest

Forest decline is a widespread, well‐recognized problem, but studies reporting decreases in tree survival have been largely limited to relatively rare old‐growth forests or low‐diversity systems, and to models which are species‐aggregated or cannot easily accommodate yearly climate variables. We created survival models for a multispecies second‐growth forest in the Sierra Nevada of California using a hierarchical state‐space framework. We accounted for a mosaic of measurement intervals and random plot variation, and we directly included yearly stand development variables alongside climate variables and topographic proxies for nutrient, water, and light availability. Our model captured the expected dependence of survival on tree size, but revealed different relationships between size and survival for each species. At the community level, including stand development variables accounted for decreasing survival time trends, but species‐specific models demonstrated a diversity of factors influencing survival, including time trends, fundamental niche limitations, and the impact of competition. Our results on time trends and competitive performance showed local exceptions to existing theories of Sierran forest dynamics, with some shade‐tolerant species increasing in survival over time and others performing better than expected under more crowded conditions. Within species, low survival was concentrated in susceptible subsets of our population and single estimates of annual survival rates did not reflect this heterogeneity in survival.

Temperature-related geographical shifts among passerines: contrasting processes along poleward and equatorward range margins.

Climate change is causing widespread geographical range shifts, which likely reflects different processes at leading and trailing range margins. Progressive warming is thought to relax thermal barriers at poleward range margins, enabling colonization of novel areas, but imposes increasingly unsuitable thermal conditions at equatorward margins, leading to range losses from those areas. Few tests of this process during recent climate change have been possible, but understanding determinants of species’ range limits will improve predictions of their geographical responses to climate change and variation in extinction risk. Here, we examine the relationship between poleward and equatorward range margin dynamics with respect to temperature‐related geographical limits observed for 34 breeding passerine species in North America between 1984–1988 and 2002–2006. We find that species’ equatorward range margins were closer to their upper realized thermal niche limits and proximity to those limits predicts equatorward population extinction risk through time. Conversely, the difference between breeding bird species’ poleward range margin temperatures and the coolest temperatures they tolerate elsewhere in their ranges was substantial and remained consistent through time: range expansion at species’ poleward range margins is unlikely to directly reflect lowered thermal barriers to colonization. The process of range expansion may reflect more complex factors operating across broader areas of species’ ranges. The latitudinal extent of breeding bird ranges is decreasing through time. Disparate responses observed at poleward versus equatorward margins arise due to differences in range margin placement within the realized thermal niche and suggest that climate‐induced geographical shift at equatorward range limits more strongly reflect abiotic conditions than at their poleward range limits. This further suggests that observed geographic responses to date may fail to demonstrate the true cost of climate change on the poleward portion of species’ distributions. Poleward range margins for North American breeding passerines are not presently in equilibrium with realized thermal limits.

The importance (or lack thereof) of niche divergence to the maintenance of a northern species complex: the case of the long-toed salamander (Ambystoma macrodactylum Baird).

The relative importance of ecological vs. non-ecological factors for the origin and maintenance of species is an open question in evolutionary biology. Young lineages--such as the distinct genetic groups that make up the ranges of many northern species--represent an opportunity to study the importance of ecological divergence during the early stages of diversification. Yet, few studies have examined the extent of niche divergence between lineages in previously glaciated regions and the role of ecology in maintaining the contact zones between them. In this study, we used tests of niche overlap in combination with ecological niche models to explore the extent of niche divergence between lineages of the long-toed salamander (Ambystoma macrodactylum Baird) species complex and to determine whether contact zones correspond to (divergent) niche limits. We found limited evidence for niche divergence between the different long-toed salamander lineages, substantial overlap in the predicted distribution of suitable climatic space for all lineages and range limits that are independent of niche limits. These results raise questions as to the importance of ecological divergence to the development of this widespread species complex and highlight the potential for non-ecological factors to play a more important role in the maintenance of northern taxa.

Are Species’ Range Limits Simply Niche Limits Writ Large? A Review of Transplant Experiments beyond the Range

Many species' range limits (RL) occur across continuous environmental gradients without obvious barriers imposing them. Such RL are expected to reflect niche limits (NL) and thus to occur where populations cease to be self-sustaining. Transplant experiments comparing fitness within and beyond species' ranges can test this hypothesis, but interpretive power depends strongly on experimental design. We first identify often overlooked aspects of transplant design that are critical to establishing the causes of RL, especially incorporating transplant sites at, and source populations from, the range edge. We then conduct a meta-analysis of published beyond-range transplant experiments (n = 11 tests). Most tests (75%) found that performance declined beyond the range, with the strongest declines detected when the measure of performance was lifetime fitness (83%), suggesting that RL commonly involve niche constraints (declining habitat quality). However, only 46% supported range limits occurring at NL; 26% (mostly geographic RL) fell short of NL with self-sustaining transplants beyond the range, and 23% (all elevational RL) exceeded NL with range-edge populations acting as demographic sinks. These data suggest an important but divergent role for dispersal, which may commonly constrain geographic distributions while extending elevational limits. Meta-analysis results also supported the importance of biotic interactions at RL, particularly the long-held assertion of their role in causing low-elevation and equatorial limits.

Climatic niche at physiological and macroecological scales: the thermal tolerance–geographical range interface and niche dimensionality

AbstractAimUnder theHutchinsonian concept of the realized niche, biotic interactions and dispersal limitation may prevent species from fully occupying areas that they could tolerate physiologically. This can hamper the translation of physiological limits into climatically defined range limits and distorts inferences of evolutionary changes of the adaptive limits (i.e. niche conservatism). In contrast, heritable physiological limits should conform more closely to the position of the niche in the climatic hyperspace. Here, we hypothesize that a measure of niche position in the climatic hyperspace is more reliable than niche boundaries to capture the variability and evolutionary pattern of physiological tolerance.LocationNeotropics andPalaeartic.MethodsWe used phylogenetic and non‐phylogenetic regressions to test the relationships between physiological requirements and macroecological niche features (i.e. based on known species distributions) among anurans. We use larval critical thermal maximum (CTmax) as a measure of physiological response and maximum temperature (Tmax), temperature variability (Tvar) and the position and breadth of niche in climatic hyperspace as measures of the realized niche in geographical space. We also compare evolutionary rates among these parameters using the phylogenetic signal representation curve.ResultsCTmaxis better correlated with niche position (r2 = 0.414) than withTvar, andCTmaxis unrelated to eitherTmaxor niche breadth.CTmaxand macroecological niche position also show similar and rapid evolutionary rates, i.e. faster thanBrownian motion, whereasTmaxandTvarevolve more slowly and niche breadth evolves at random.Main conclusionsThe transferability between thermal tolerance and realized climatic niche limits is weak. Only macroecological niche position in the multivariate climatic hyperspace correlates with physiological tolerance. It thus appears to be more suitable for describing the variability and evolutionary pattern of the species' adaptive limits. We link these results to ‘niche dimensionality’, in that multiple interacting factors outweigh single factors in demarcating the species' realized climatic niche, thereby determining the conserved upper thermal limits of the species.

Diversity of rodent and shrew assemblages in different vegetation types of the savannah biome in South Africa: no evidence for nested subsets or competition

AbstractIdentifying nonrandom species composition patterns predicted by assembly rules has been a central theme in community ecology. Few studies have investigated the prevalence of multiple drivers on species composition patterns in small mammal assemblages in the Old World. This study investigated seasonal changes in rodent and shrew diversity in eleven savannah vegetation types in South Africa. We tested whether species composition patterns are nonrandom with respect to predictions from Diamond's assembly rules, niche limitation hypothesis and nestedness hypothesis. Species richness estimators indicated that inventories for the rodents (80%) and shrews (100%) were relatively complete. Rodent (n = 11 species) diversity and shrew (n = 5 species) diversity were highest in summer and lowest in autumn. Rodent richness was highest in the Terminalia sericea bushveld and woodlands and lowest in the Drypetes arguta sand forest, whilst shrew richness was highest in the T. sericea bushveld and woodlands and lowest in the Acacia nilotica/Dichrostachys cinerea open shrub savannah. We found no support for the predictions of competition and nestedness hypotheses and suggest that this was probably due to the high seasonal and annual variability in rodent and shrew diversity.

Fit-for-purpose: species distribution model performance depends on evaluation criteria - Dutch Hoverflies as a case study.

Understanding species distributions and the factors limiting them is an important topic in ecology and conservation, including in nature reserve selection and predicting climate change impacts. While Species Distribution Models (SDM) are the main tool used for these purposes, choosing the best SDM algorithm is not straightforward as these are plentiful and can be applied in many different ways. SDM are used mainly to gain insight in 1) overall species distributions, 2) their past-present-future probability of occurrence and/or 3) to understand their ecological niche limits (also referred to as ecological niche modelling). The fact that these three aims may require different models and outputs is, however, rarely considered and has not been evaluated consistently. Here we use data from a systematically sampled set of species occurrences to specifically test the performance of Species Distribution Models across several commonly used algorithms. Species range in distribution patterns from rare to common and from local to widespread. We compare overall model fit (representing species distribution), the accuracy of the predictions at multiple spatial scales, and the consistency in selection of environmental correlations all across multiple modelling runs. As expected, the choice of modelling algorithm determines model outcome. However, model quality depends not only on the algorithm, but also on the measure of model fit used and the scale at which it is used. Although model fit was higher for the consensus approach and Maxent, Maxent and GAM models were more consistent in estimating local occurrence, while RF and GBM showed higher consistency in environmental variables selection. Model outcomes diverged more for narrowly distributed species than for widespread species. We suggest that matching study aims with modelling approach is essential in Species Distribution Models, and provide suggestions how to do this for different modelling aims and species’ data characteristics (i.e. sample size, spatial distribution).

Relationship between fundamental and realized ecological niches

Since species are formed in course of evolutionary process, their ecological niches are formed in the evolutionary process, too. Species exist in a state of evolutionary stasis diring hundreds of thousands and millions years. Stasis in sustained mainly by counterbalance of vectors of directional selection. Niche can be viewed as a multidimensional structure. Multitude of environmental factors acts upon every population, which cause elimination and, by that, selection for adaptation to each eliminating factor. Different directions of these vectors of selection lead to their counteractions; selection in one direction is interfered by selection in an opposite direction. The counterbalance of vectors of selection interferes with progressive evolution thus supporting stasis. During species existence in a stasis condition it endures a whole set of various deterioration of environment. Such deteriorations lead to imbalance of selective processes. Unbalanced vectors of selection form adaptations to extreme conditions of existence. Such adaptations are superfluous as for usual conditions; but they define fitness borders and, by that, borders of a fundamental niche. Realized niche, as well as fundamental one, is a multidimensional structure. Each population occupies a subniche of the specific realized niche. Thus, it occupies habitats where conditions are as close to an ecological optimum as can be admitted by the conditions in the given part of the areal. The sum of all subniches of populations--the specific realized niche--coincides with a part of fundamental niche because only the part of adaptive possibilities of a species sufficient for existence in the given environment is used. Interspecific competition, even when it is capable to restrict consumption of limiting resources, is not the reason of the realized niche limitation. Restriction of one or two of niche parameters does not influece all others parameters of its multidimensional space.

The evolutionary loss of aerenchyma limits both realized and fundamental ecohydrological niches in the Cape reeds (Restionaceae)

Summary The basic physiological mechanisms that determine the niche limits, and so the local ranges, of species are often poorly understood. Here, we assessed whether the presence of root aerenchyma can account for both the fundamental and realized ecohydrological niche in the Restionaceae. We documented the ability to make aerenchyma of almost 300 of the c. 350 South African Restionaceae species with the help of field observations, cultivation experiments and herbarium collections, and correlated this information with the ecohydrological niche of the species. We used a diversity of sampling approaches to account for variation in both aerenchyma and habitat at individual, population and species level. Tolerance of waterlogging in species with and without aerenchyma was investigated experimentally. To account for phylogenetic relatedness, the evolution of both aerenchyma and the ecohydrological niche was explored in the context of an almost complete species‐level phylogeny using both parsimony and maximum likelihood optimization methods. We showed that the presence of aerenchyma was correlated with the ecohydrological niche at population and at species level, and after phylogenetic correction. Most individuals of non‐aerenchymatous species died when waterlogged, whereas most species with aerenchyma were waterlogging tolerant. This indicates that waterlogging acts as an environmental filter that excludes non‐aerenchymatous species from these conditions. The absence, and to a certain degree also the presence, of aerenchyma predicts both the field‐observed realized and experimentally determined fundamental ecohydrological niche. Aerenchyma was ancestrally present as inferred by both parsimony and maximum likelihood reconstruction and showed a strong phylogenetic signal with frequent losses but few gains. The ecohydrological niche was evolutionary more labile, with niche changes in many evolutionary lineages. Changes from wetland to dry habitats were reconstructed in species with aerenchyma, whereas the reverse change was never inferred for species that had lost aerenchyma. Synthesis. Our study provides evidence that functional traits can effectively predict species niches, and evolution of these may constrain the habitats available to the clades. We underpin the importance of understanding the causal driver of the local distribution of species for making robust predictions of species range shifts under climate change.

Is regional species diversity bounded or unbounded?

Two conflicting hypotheses have been proposed to explain large-scale species diversity patterns and dynamics. The unbounded hypothesis proposes that regional diversity depends only on time and diversification rate and increases without limit. The bounded hypothesis proposes that ecological constraints place upper limits on regional diversity and that diversity is usually close to its limit. Recent evidence from the fossil record, phylogenetic analysis, biogeography, and phenotypic disparity during lineage diversification suggests that diversity is constrained by ecological processes but that it is rarely asymptotic. Niche space is often unfilled or can be more finely subdivided and still permit coexistence, and new niche space is often created before ecological limits are reached. Damped increases in diversity over time are the prevalent pattern, suggesting the need for a new 'damped increase hypothesis'. The damped increase hypothesis predicts that diversity generally increases through time but that its rate of increase is often slowed by ecological constraints. However, slowing due to niche limitation must be distinguished from other possible mechanisms creating similar patterns. These include sampling artifacts, the inability to detect extinctions or declines in clade diversity with some methods, the distorting effects of correlated speciation-extinction dynamics, the likelihood that opportunities for allopatric speciation will vary in space and time, and the role of undetected natural enemies in reducing host ranges and thus slowing speciation rates. The taxonomic scope of regional diversity studies must be broadened to include all ecologically similar species so that ecological constraints may be accurately inferred. The damped increase hypothesis suggests that information on evolutionary processes such as time-for-speciation and intrinsic diversification rates as well as ecological factors will be required to explain why regional diversity varies among times, places and taxa.

Evidence for assembly rules: limiting similarity within a saltmarsh

Summary1. We sought evidence for limiting similarity, a basic aspect of community structure, in three zones of a saltmarsh. Sampling was conducted at three spatial scales (grains), from a single point up to the scale of several square metres. Twenty‐three functional traits, related to the structure of the shoot and root systems and to nutrient status, were measured on each species present, separately in each community.2. Patterns of association between plant species were compared with those expected under a null model, to assess whether plants with similar functional traits tended to coexist or to separate, i.e. whether there was environmental filtering or limiting similarity. A patch null model was used, a type that tends to be conservative but that avoids spurious evidence of limiting similarity caused by environmental pseudoreplication. One overall and four univariate test statistics were calculated, to capture possible patterns in trait space whilst minimising the problem of multiple testing.3. In the Shrub community, overall evidence for even spacing of co‐occurring species in functional‐trait space, the pattern expected from the theory of limiting similarity, was seen at the area scale. In univariate tests in that community, there was evidence for even spacing in leaf lobation and leaf succulence, especially at small scales.4. In the Rush community, there was significant evidence for limiting similarity in several traits, especially those related to canopy interactions, but also in some root characteristics. However, clustering in other traits, presumably owing to microenvironmental filtering, reduced overall tests for limiting similarity to ‘marginal significance’ (0.1 > P > 0.05).5. In the species‐poor and salt‐stressed Salt turf, significant departures from the null model were sporadic and not consistent, although chlorophyll characters and leaf nitrogen concentration tended to be clustered.6. Synthesis: There was evidence for niche limitation in two of the communities – Rush and Shrub – apparently based on canopy interactions in both cases and perhaps also root interactions in the latter community. Limiting similarity can be an important force in community assembly. However, in situations when it cannot be demonstrated, we do not know whether trait‐based competition is absent or whether its signal is overwhelmed by other processes.

Quantifying the Impact of Gene Flow on Phenotype-Environment Mismatch: A Demonstration with the Scarlet MonkeyflowerMimulus cardinalis

Geographic range margins offer testing grounds for limits to adaptation. If range limits are concordant with niche limits, range expansions require the evolution of new phenotypes that can maintain populations beyond current range margins. However, many species' range margins appear static over time, suggesting limits on the ability of marginal populations to evolve appropriate phenotypes. A potential explanation is the swamping gene flow hypothesis, which posits that asymmetrical gene flow from large, well-adapted central populations prevents marginal populations from locally adapting. We present an empirical framework for combining gene flow, environment, and fitness-related phenotypes to infer the potential for maladaptation, and we demonstrate its application using the scarlet monkeyflower Mimulus cardinalis. We grew individuals sampled from populations on a latitudinal transect under varied temperatures and determined the phenotypic deviation (PD), the mismatch between phenotype and local environment. We inferred gene flow among populations and predicted that populations receiving the most temperature- or latitude-weighted immigration would show the greatest PD and that these populations were likely marginal. We found asymmetrical gene flow from central to marginal populations. Populations with more latitude-weighted immigration had significantly greater PD but were not necessarily marginal. Gene flow may limit local adaptation in this species, but swamping gene flow is unlikely to explain its northern range limit.

Mowing and fertilizer effects on seedling establishment in a successional old field

Aims We used a 10-year field experiment that consisted of mowing and fertilizer treatments to evaluate the role of niche limitation in seedling establishment of species from different functional groups and of varying local abundance in an old field undergoing succession. Methods Seedlings of nine different species were planted into a successional field subjected to mowing and fertilizer treatments for 10 years that resulted in different plant communities and resource availability. Species representative of the factorial combination of three functional groups (C4 grasses, C3 grasses and legumes) and three abundance categories (abundant, present, or absent in the old field) were planted in four treatments resulting from the factorial combination of annual spring mowing (mowed and unmowed) and fertilizer application (annually fertilized and unfertilized). Survivorship, relative growth rate (RGR) and biomass were measured to determine the role of niche limitation on recruitment and growth.

Ecological assembly rules in plant communities—approaches, patterns and prospects

Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co-occur randomly but are restricted in their co-occurrence by interspecific competition. This concept can be redefined in a more general framework where the co-occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive. Here we present a survey and meta-analyses of 59 papers that compare observed patterns in plant communities with null models simulating random patterns of species assembly. According to the type of data under study and the different methods that are applied to detect community assembly, we distinguish four main types of approach in the published literature: species co-occurrence, niche limitation, guild proportionality and limiting similarity. Results from our meta-analyses suggest that non-random co-occurrence of plant species is not a widespread phenomenon. However, whether this finding reflects the individualistic nature of plant communities or is caused by methodological shortcomings associated with the studies considered cannot be discerned from the available metadata. We advocate that more thorough surveys be conducted using a set of standardized methods to test for the existence of assembly rules in data sets spanning larger biological and geographical scales than have been considered until now. We underpin this general advice with guidelines that should be considered in future assembly rules research. This will enable us to draw more accurate and general conclusions about the non-random aspect of assembly in plant communities.

Performance and reproduction of an exotic invader across temperate forest gradients

Widespread colonization by invasive species often obscures their underlying niche requirements. A robust inference into habitat requirements demands direct measures of invasive species performance linked with associated environmental conditions. In the context of general ecological theory, we investigated the niche requirements of Microstegium vimineum, an invasive grass in the U.S. that overruns native vegetation in forest understories. We examined M. vimineum's performance and reproduction as a function of environmental drivers across forested and unforested habitats along a 100-km regional and climatic gradient in the southeastern U.S. from the southern Appalachian Mountains to the Georgia piedmont. We then measured M. vimineum performance and reproduction in response to direct environmental drivers (diffuse light, litter cover, soil moisture, herbaceous cover, soil pH, clay content and temperature) in paired invaded and uninvaded plots. Lastly, we experimentally investigated recruitment in the context of experimental and natural disturbances. We find that all habitats are not equally suitable for M. vimineum—even those within which it occurs—and that the environmental conditions associated with roadsides and waterways are most suitable for M. vimineum persistence and spread. Microstegium vimineum's soil moisture, light and leaf litter requirements may delineate the boundaries of suitable habitat for the exotic invader. Significant decreases in M. vimineum recruitment, performance and reproduction along these environmental gradients suggest its potential niche limitations. Nevertheless, we also find significant dispersal limits on M. vimineum populations not subject to conspicuous overland water flow. We discuss our findings in the context of spread, impact and management of invasive species.

Niche Dimensions in Fishes: An Integrative View

Current shifts in ecosystem composition and function emphasize the need for an understanding of the links between environmental factors and organism fitness and tolerance. The examples discussed here illustrate how recent progress in the field of comparative physiology may provide a better mechanistic understanding of the ecological concepts of the fundamental and realized niches and thus provide insights into the impacts of anthropogenic disturbance. Here we argue that, as a link between physiological and ecological indicators of organismal performance, the mechanisms shaping aerobic scope and passive tolerance set the dimensions of an animal's niche, here defined as its capacity to survive, grow, behave, and interact with other species. We demonstrate how comparative studies of cod or killifish populations in a latitudinal cline have unraveled mitochondrial mechanisms involved in establishing a species' niche, performance, and energy budget. Riverine fish exemplify how the performance windows of various developmental stages follow the dynamic regimes of both seasonal temperatures and river hydrodynamics, as synergistic challenges. Finally, studies of species in extreme environments, such as the tilapia of Lake Magadi, illustrate how on evolutionary timescales functional and morphological shifts can occur, associated with new specializations. We conclude that research on the processes and time course of adaptations suitable to overcome current niche limits is urgently needed to assess the resilience of species and ecosystems to human impact, including the challenges of global climate change.