Niche Metrics Research Articles

Individual diet variation in a marine fish assemblage: Optimal Foraging Theory, Niche Variation Hypothesis and functional identity

Individual diet variation (i.e. diet variation among individuals) impacts intra- and inter-specific interactions. Investigating its sources and relationship with species trophic niche organization is important for understanding community structure and dynamics. Individual diet variation may increase with intra-specific phenotypic (or “individual state”) variation and habitat variability, according to Optimal Foraging Theory (OFT), and with species trophic niche width, according to the Niche Variation Hypothesis (NVH). OFT proposes “proximate sources” of individual diet variation such as variations in habitat or size whereas NVH relies on “ultimate sources” related to the competitive balance between intra- and inter-specific competitions. The latter implies as a corollary that species trophic niche overlap, taken as inter-specific competition measure, decreases as species niche width and individual niche variation increase. We tested the complementary predictions of OFT and NVH in a marine fish assemblage using stomach content data and associated trophic niche metrics. The NVH predictions were tested between species of the assemblage and decomposed into a between- and a within-functional group component to assess the potential influence of species' ecological function. For most species, individual diet variation and niche overlap were consistently larger than expected. Individual diet variation increased with intra-specific variability in individual state and habitat, as expected from OFT. It also increased with species niche width but in compliance with the null expectation, thus not supporting the NVH. In contrast, species niche overlap increased significantly less than null expectation with both species niche width and individual diet variation, supporting NVH corollary. The between- and within-functional group components of the NVH relationships were consistent with those between species at the assemblage level. Changing the number of prey categories used to describe diet (from 16 to 41) did not change the results qualitatively. These results suggest that, besides proximate sources, intra-specific competition favors higher individual diet variation than expected while inter-specific competition limits the increase of individual diet variation and of species niche overlap with species niche expansion. This reveals partial trophic resource partitioning between species. Various niche metrics used in combination allow inferring competition effects on trophic niches' organization within communities.

The peril of proportions: robust niche indices for categorical data

Summary Indices of niche breadth and niche overlap for categorical data are typically expressed in terms of proportions of resources use. These are unit‐sum constrained data; hence, direct application of standard general linear modelling methods to such indices can lead to spurious correlations and misleading inference. To overcome these limitations, we introduce a compositional data analysis (CoDA) approach and derive compositional expressions of niche breadth, niche overlap and specialization. Compositional data analysis is specifically devoted to the analysis of vectors of proportions (i.e. compositions) and represents the appropriate framework for the study of sets of data with unit‐sum constraint as those typically used in the calculation of niche indices. We show that compositional indices exhibit suitable statistical properties that make them flexible and robust, allowing downstream application of the full toolbox of multivariate analysis techniques to these estimators, a possibility not available with classical indices. In addition, we find that when characterizing niche breadth, niche overlap and specialization in terms of vectors of proportions, these concepts are naturally integrated in a coherent unifying framework. When data are categorical, we recommend the use of compositional indices for the statistical analysis of specialization metrics, niche breadth and niche overlap. We believe that the unified framework emerging from our compositional approach to niche metrics will allow a more thorough understanding of specialization at multiple levels of biological organization and provide novel insights in complex phenomena such as invasions and niche shifts.

Fine-Scale Ecological and Genetic Population Structure of Two Whitefish (Coregoninae) Species in the Vicinity of Industrial Thermal Emissions.

Thermal pollution from industrial processes can have negative impacts on the spawning and development of cold-water fish. Point sources of thermal effluent may need to be managed to avoid affecting discrete populations. Correspondingly, we examined fine-scale ecological and genetic population structure of two whitefish species (Coregonus clupeaformis and Prosopium cylindraceum) on Lake Huron, Canada, in the immediate vicinity of thermal effluent from nuclear power generation. Niche metrics using δ13C and δ15N stable isotopes showed high levels of overlap (48.6 to 94.5%) in resource use by adult fish captured in areas affected by thermal effluent compared to nearby reference locations. Isotopic niche size, a metric of resource use diversity, was 1.3- to 2.8-fold higher than reference values in some thermally affected areas, indicative of fish mixing. Microsatellite analyses of genetic population structure (Fst, STRUCTURE and DAPC) indicated that fish captured at all locations in the vicinity of the power plant were part of a larger population extending beyond the study area. In concert, ecological and genetic markers do not support the presence of an evolutionarily significant unit in the vicinity of the power plant. Thus, future research should focus on the potential impacts of thermal emissions on development and recruitment.

Climatic niche attributes and diversification inAnolislizards

AbstractAimThe aim of this study was to test the link between climatic niche dynamics and species diversification inAnolison islands and on the mainland. We tested the hypotheses that lineages in warmer climates and with narrow climate niches diversified more than lineages in cold climates and with broad climate niches. We also tested the hypothesis that species‐rich clades exhibit greater niche diversity than species‐poor clades.LocationNeotropics.MethodsWe collated occurrence records for 328Anolisspecies to estimate niche breadth, niche position and occupied niche space (as a proxy for niche diversity). We compared niche breadth between insular and mainlandAnolisspecies and amongAnolisclades, controlling for the potential confounding effect of range size. Using two approaches (clade‐based and QuaSSE) we explored the association between niche metrics and diversification rates inAnolislizards.ResultsWe found that CaribbeanAnolishad a narrower niche breadth and niche space occupation compared to mainland anoles after controlling for range size differences. There was a significant association between niche traits (mean niche position and niche breadth) and diversification in anoles. Anole lineages with narrow niche breadths and that occupy warmer areas exhibited higher speciation rates than those with broader niche breadths and that occupy cold areas. Similarly, clades with higher total diversification exhibit more niche diversity than clades with lower total diversification.Main conclusionsClimatic niche attributes play a role in anole diversification with some differences between mainland and insular anole lineages. Climatic niche differences between regions and clades likely are related to differences in niche evolutionary rates. This also suggests that climate plays a strong role in shaping species richness between and within mainland and islands.

Isotopic niche of two coastal dolphins in a tropical marine area: specific and age class comparisons

Niche differentiation is the process by which species evolve different forms of resource use, and is used to explain the co-occurrence in a variety of habitats. The Bayesian framework of isotopic niche through quantitative niche metrics was applied to estimate and compare the niche breadth of two sympatric coastal dolphinsPontoporia blainvilleiandSotalia guianensisin a tropical marine area. The standard ellipse areas (SEAs) based on species were quite similar, but the SEAs based on age class showed that the matures’ niche space is larger than the immatures’ for both dolphins. A probabilistic comparison of SEAs indicated that specific differences are negligible compared with age class differences. Trophic level measures (δ15N range) indicated that the dolphins are comparable as top predators, and that immature specimens have a lower range of trophic levels than mature ones. In terms of variability of food sources (δ13C range),S. guianensisshowed a larger value thanP. blainvilleiand mature specimens had larger δ13C range than immatures for both species. In general,P. blainvilleiandS. guianensiswere similar in the niche metrics, with SEAs overlap of 52.1 and 39.7%. The immature specimens showed reduced isotopic niche overlap between species (<3%). In conclusion,Pontoporia blainvilleiandS. guianensisspecimens have similar isotopic niches, but pronounced differences between immature and mature specimens, both intraspecific and interspecific. Isotopic niche and quantitative metrics along with previous data on stomach contents provide a strong representation of species niche and their relationships.

Influences of ecology and biogeography on shaping the distributions of cryptic species: three bat tales in Iberia

To determine what shapes the distributions of cryptic species, we aimed to unravel ecological niches and geographical distributions of three cryptic bat species complexes in Iberia, Plecotus auritus/begognae, Myotis mystacinus/alcathoe and Eptesicus serotinus/isabellinus (with 44, 69, 66, 27, 121 and 216 records, respectively), considering ecological interactions and biogeographical patterns. Species distribution models (SDMs) were built using a presence-only technique (Maxent), incorporating genetically identified species records with environmental variables (climate, habitat, topography). The most relevant variables for each species’ distribution and respective response curves were then determined. SDMs for each species were overlapped to assess the contact zones within each complex. Niche analyses were performed using niche metrics and spatial principal component analyses to study niche overlap and breadth. The Plecotus complex showed a parapatric distribution, although having similar biogeographical affinities (Eurosiberian), possibly explained by competitive exclusion. The Myotis complex also showed Eurosiberian affinities, with high overlap between niches and distribution, suggesting resource partitioning between species. Finally, E. serotinus was associated with Eurosiberian areas, while E. isabellinus occurred in Mediterranean areas, suggesting possible competition in their restricted contact zone. This study highlights the relevance of considering potential ecological interactions between similarly ecological species when assessing species distributions. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112,150–162.

Isotopic niche metrics as indicators of toxic stress in two freshwater snails

Descriptors of trophic niche and of food web structure and function have been suggested as integrative and sensitive endpoints of toxicant effects. In the present study, carbon and nitrogen stable isotope signatures were used to assess the effects of the dithiocarbamate fungicide thiram (35 and 170μg/L nominal concentrations) and of a petroleum distillate (0.01, 0.4, 2 and 20mg/L nominal loadings as Hydrocarbon Emulsion or Hydrocarbon Water Accommodated Fraction) on the trophic niche of two freshwater gastropods in artificial streams (Radix peregra) and ponds (Lymnaea stagnalis). Results were analyzed using classical univariate statistical methods and recently proposed uni- and multivariate metrics of the realized trophic niche of species. The trophic niche metrics were highly sensitive to both types of chemicals, but exposure resulted in different response patterns according to the nature of the tested compound. Thiram clearly affected gastropod trophic niche leading to a change in the food resources used and resulting in trophic niche expansion (i.e., increase of diversity of used resources, especially dead animals) or trophic niche contraction (i.e., decrease of diversity of used resources) across time. Both gastropod taxa exposed to hydrocarbons showed a clear trophic niche expansion. Trophic niche metrics therefore provide a promising way of investigating non-lethal effects of exposure to organic chemicals on aquatic invertebrates, and subsequent disturbances in food webs.

Divergence in the trophic niche of sympatric freshwater invaders

When present in sympatry, invasive species have the potential to amplify or mitigate their ecological impacts through their trophic interactions. Their trophic niches may overlap, limiting impacts to specific trophic levels or functional groups; alternatively, they may diverge, with this niche differentiation resulting in contrasting impacts between species on the ecosystem. Here, we tested the trophic consequences for the global freshwater invaders common carp Cyprinus carpio, signal crayfish Pacifastacus leniusculus and topmouth gudgeon Pseudorasbora parva when their populations were in sympatry and under varying population biomass across six adjacent and identical ponds. Through using corrected values of δ13C and δ15N, stable isotope niche metrics revealed that when the species were analysed together across all of the ponds, the output indicated their potential to share trophic resources. This was because niche overlap was evident at the species level: P. parva shared 19.6 and 30.4 % of their isotopic niche with C. carpio and P. leniusculus respectively. At the population level, however, the invaders had no niche overlap when present in sympatry and, instead, diverged in their trophic niche space, with C. carpio occupying the highest trophic levels, followed by P. parva and then P. leniusculus. We suggest that at the population level within in each pond, niche differentiation was facilitated by each species being plastic in their resource use, allowing their co-existence in ponds that may otherwise have limited their ability to co-exist through resource limitation.

Climate influences the global distribution of sea turtle nesting

AbstractAimTo understand whether climate limits current sea turtle nesting distributions and shapes the ecological niche of the terrestrial life‐history stage of these wide‐ranging marine vertebrates.LocationCoastlines world‐wide.MethodsI predicted the spatial distributions of nesting habitat under current climatic conditions for seven sea turtle species using information criteria and maximum entropy modelling. I also compared niche similarity among species using three niche metrics: I, Schoener's D and relative rank.ResultsSea turtles currently nest across their entire bioclimatic envelopes, with up to six species predicted to nest on a single beach. The Caribbean Sea, Gulf of Mexico and Australasia support high nesting diversity, with most regional areas supporting three to five species. Despite large overlap in nesting distributions among species, loggerhead and green turtles have the broadest environmental niches, while Kemp's ridley and flatback turtles have very narrow niches.Main conclusionsThe terrestrial nesting habitat of sea turtles is characterized by distinct climatic conditions, which are linked to the physical conditions necessary for eggs to hatch successfully and allow hatchlings to disperse from natal areas. Despite broad geographic patterns of overlap and similar embryonic tolerances to temperature and moisture among species, sea turtles partition habitat by nesting in different niche spaces. The tight link between current geographic patterns of nesting and climate, along with the dependence of developing embryos on nest microclimate, imply that regional or global changes in environmental conditions could differentially influence the distribution of sea turtle species under climate change. This could influence the adaptive potential of different populations, and predicting these responses before they occur will be important in mitigating the effects of climate change.

Using stable isotopes to test for trophic niche partitioning: a case study with stream salamanders and fish

Summary1. Stream salamanders and fish often co‐occur even though fish prey on and outcompete salamanders. However, the mechanisms that allow palatable salamanders to coexist with fish are unknown.2. We tested mechanisms in the field that promote coexistence between Idaho giant salamanders (Dicamptodon aterrimus) and stream salmonid fishes in headwater streams. Previous research in this system indicated that salamander dispersal did not promote coexistence with fish. We tested the hypothesis that D. aterrimus shift their diet when they occur with fish, facilitating coexistence through local niche partitioning.3. We used nitrogen and carbon stable isotopes to describe the trophic niche of D. aterrimus and fish in three co‐occurring populations of salamanders and fish and three populations of salamanders without fish. We used two approaches to quantify trophic niche partitioning with stable isotopes: 95% kernel density estimators and isotopic mixing models.4. We found that salamanders and fish were generalists that consumed aquatic invertebrates primarily, but both species were also cannibalistic and predatory on one another. We also found no support for trophic niche partitioning as a coexistence mechanism because there were no differences in the trophic niche metrics among salamander populations with and without fish.5. Although we did not identify mechanisms that facilitate salamander and fish coexistence, our empirical data and use of novel approaches to describe the trophic niche did yield important insights on the role of predator–prey interactions and cannibalism as alternative coexistence mechanisms. In addition, we found that 95% kernel estimators are a simple and robust method to describe population‐level measure of trophic structure.

Chemosynthetic trophic support for the benthic community at an intertidal cold seep site at Mocha Island off central Chile

We analyzed C and N stable isotope ratios of benthic fauna and their potential food sources at an intertidal methane seep site and a control site without emanation at Mocha Island (central Chile). The objective was to trace the origin of the main food sources used by the local heterotrophic fauna, based on the hypothesis that chemosynthetic production could be partially fueling the local food web at the seep site. Food sources sampled at both sites included macroalgae, particulate organic matter and bacteria-like filaments found growing over the red algae Gelidium lingulatum within the areas of active methane release. At the control site, located 11 km away from the gas emanation, fauna exhibited moderate δ 13C values ranging from −16.2‰ (in a nereid polychaete) to −14.8‰ (in a cirolanid isopod), which were consistent with those of the potential photosynthetic food sources sampled at this site (−20.2 to −16.5‰). δ 13C values of the photosynthetic food sources at the seep site similarly ranged between −25.4 and −17.9‰. However, a portion of the animals at this site were consistently more 13C-depleted, with δ 13C values close to that of the seeping methane (−43.8‰) and the bacteria-like filaments (−39.2 ± 2.5‰) also collected at this site. Specific examples were the Marphysa sp. polychaetes (δ 13C = −44.7 ± 0.6‰), the Schistomeringos sp. dorvilleid polychaetes (δ 13C = −42.9‰), and the tanaid crustacean Zeuxo marmoratus (δ 13C = −37.3 ± 0.2‰). The significantly higher δ 13C values of the herbivorous gastropod Tegula atra at the seep site (−29.3 ± 3.1‰) than at the control site (−12.6 ± 0.3‰) also indicated differences among sites of the preferred carbon sources of this species. Mixing model estimates indicate that at the seep site bacteria-like filaments could be contributing up to ∼60% of the assimilated diet of selected invertebrates. Furthermore, several indicators of trophic structure, based in isotopic niche metrics, indicate a considerably higher variety of basal resources used at the seep site and a consequent enhanced trophic diversity, compared with the non-seep site. The isotopic data gathered at the seep site clearly and unequivocally reflects the ingestion of a 13C-depleted primary food source, which could be the bacteria-like filaments observed at this site.

A framework for estimating niche metrics using the resemblance between qualitative resources

Despite the central importance of the niche concept for the ecological theory, current methods to quantify the species niche from qualitative resources, such as food or habitat types, remain insufficiently developed. Classically, information theory and diversity measures have formed the toolbox used for calculating resource niche metrics on species preference data for a set of qualitative resources. We provide a comprehensive framework that extends these classical approaches by incorporating the resemblance between resources into the calculation of resource niche metrics. This does not only allow estimation of the niche centre, breadth, overlap and displacement with greater accuracy, but also makes the estimates less influenced by the way the resources are subdivided. In addition, all niche metrics can be calculated while taking into account the variation in resource availability, and confidence intervals can be obtained by bootstrapping. We illustrate the utility of the framework with an analysis of dietary preferences in feral pigeons Columba livia.

Generalists, specialists and opportunists: niche metrics of Poicephalus parrots in southern Africa

There are distinct differences in the way in which Poicephalus parrots utilise food resources within their distributional range. Our study evaluated the niche metrics of all Poicephalus parrots studied thus far for evidence of linkages between body mass, distributional range and niche breadth. We calibrated the degree of dietary specialisation of Meyer's Parrots in the Okavango Delta, Botswana, over 12 months using a modified Hurlbert's expanded and standardised niche breadth index, and for comparison using Levins's niche breadth, with the food item calendars of the other Poicephalus parrots in southern Africa. Our study showed that Meyer's Parrot has the widest niche breadth of any Poicephalus parrot studied thus far, which likely plays a role in their wide distributional range. In addition, degree of dietary specialisation was inversely correlated with body mass, whereby the narrower the trophic niche, the larger the body size.

Nestedness, niche metrics and temporal dynamics of a metacommunity in a dynamic natural model system

Prediction of extinction and colonization rates for whole species assemblages emerges as an urgent task for ecology. We hypothesized that nestedness of species assemblage reflects differential ability of species to occupy sites and of sites to support species. If correct, a nested ordering of species and sites should condense long‐term dynamics of metacommunities. To test this we characterized the differential ability of species to use habitat (niche position and niche breadth) using eight surveys of invertebrate communities inhabiting 49 tropical rock pools. We examined temporal consistency of the nested rank of species and pools, and related them to species and pool characteristics to infer temporal dynamics of species composition. Invertebrate assemblages in the rock‐pools were significantly nested and species ranks were generally preserved over time. By contrast, pool ranks were usually conserved between adjacent years only but their similarity declined with time separating surveys. The nested species‐by‐pool matrix of the first survey served as a benchmark to assess individual species and local community changed in subsequent years. As hypothesized, benchmark cells with high state occupancy probability had low extinction rates in subsequent years. Moreover, species high in the nested matrix (also with high regional occupancy probability) were better survivors and colonizers relative to species that ranked low. The year‐to‐year dynamics were similar. Species with non‐marginal niche position retained high ranks in the matrix. Yet, niche position predicted only colonization rate of species. Niche breadth and species’ nested ranking, extinction risk, or ability to colonize a pool showed no relationship. Counter to the expectation, pool ranks did not predict species extinction and colonization rates. Apparently, even in dynamic systems, regional nested pattern remains consistent and the underlying extinction and colonization dynamics appear to be largely determined by the hierarchical order among species and much less by that among sites.

Effects of logging and non-native tree proliferation on the birds overwintering in the upland forests of north-western Italy

Factors associated with management and logging practices are of particular interest in the perspective of biodiversity management. In this study we focused on overwintering birds of two close and contrasting forest habitats: a 60–120 years old oak forest and a secondary 10–20 years old wood that has lost most of its original vegetation because of continuous logging and is now dominated by an introduced species, black locust Robinia pseudoacacia. We examined bird use of the forests over two levels: species–habitat relationships and foraging niches. The former were assessed by means of point counts centred in 50 m radius plots, whereas the latter by observations of foraging behaviour and plant species exploitation. Mean species richness, diversity and abundance of bark foragers was significantly greater in the old-growth stand, where trees were higher and larger but stem density was lower. Almost no wintering bird species occurred at higher density in the second-growth than in old-growth forest, and no foraging birds positively selected the black locust as substrate. Despite the observed differences in biodiversity metrics and plant species exploitation, most birds of the two stands showed few shifts in their foraging behaviour (in terms of foraging technique, substrate, relative horizontal and vertical position) and niche metrics (breadth and overlap). By considering results obtained with the two approaches, we suggest that biodiversity of altered secondary woods can be increased if current silvicultural practices are modified towards (1) maintaining oaks or trees prone to snag formation such as old poplars and (2) preventing black locust establishment in post-clearcut communities.

High aquatic niche overlap in the newts Triturus cristatus and T. marmoratus (Amphibia, Urodela)

We studied spatial niche metrics of large-bodied newts (Triturus cristatus and T. marmoratus) in three breeding ponds in western France. Adults and larvae were sampled with underwater funnel traps. Larvae were identified to the species with diagnostic microsatellite DNA markers. The distribution of adult T. cristatus and T. marmoratus across pond regions differed in one out of six cases, no differences were observed between larvae (two ponds studied). Niche overlap and niche breadth indices across resource states defined as pond regions or individual traps were high (Schoener's C: pond regions 0.60–0.98, traps 0.35–0.71; Levins' B: pond regions 0.71–0.98, traps 0.35–0.76). Adults of large-bodied newts significantly differed in resource use from small-bodied newts (T. helveticus). The results are discussed in view of the occurrence of interspecific breeding attempts, and the unpredictable ecological characteristics of newt breeding ponds.

Species diversity, niche metrics and species associations in harvested and undisturbed bogs

Abstract. Competition is considered an important force in structuring plant communities and in governing niche relations, but communities recovering from disturbance, may be less governed by species interactions and less orderly organized. To address this issue, we studied species richness, abundance and patterns of association between plant species at three spatial scales (1 m2, 1/25 m2, 1/625 m2) in two ombrotrophic mires in east‐central Sweden. One was at a secondary successional stage following peat extraction 50 yr ago and the other was undisturbed. Peat extraction leads to a change in hydrology which is slowly restored by the formation of new peat. Niche breadth and niche overlap along the gradient of height above the water table were calculated for the five common Sphagnum species occurring at both mires in an attempt to better understand differences in species co‐occurrence at each mire.Species cover differed between the mires, and the number of species per plot was higher in the undisturbed community at all scales, suggesting that the degree of species intermingling was greater than at the harvested site. At all scales, the number of non‐random associations was higher, and niche overlap lower among ecologically similar species (e.g. hollow Sphagnum species) in the undisturbed mire. These differences indicate that random events are important in colonization, and that biotic interactions between neighbours later result in a higher degree of non‐randomness.In addition, we surveyed a number of abandoned peat pit sites to test the effect of disturbance for species composition at a regional scale. Ombrotrophic peat pits contained several Sphagnum species normally associated with minerotrophic mires, and species of wooded mires occurred frequently in peat pits, making them more species‐rich than undisturbed bogs. There were also Sphagnum species new to, or rare in, this part of Sweden which indicates effective long‐distance dispersal. Even 50 yr after peat extraction had ceased, the vegetation had not recovered to its original composition.

Anatomies and lifestyles, morphometrics and niche metrics: Tools for studying primate evolution

In recent years, comparative observations of primate anatomies have been extended using measurements and computer analyses: morphometrics. A question then follows. Can we extend field observations of animal lifestyles using measurement and computer analysis, using, in other words, a new «niche metrics», for comparison with the morphometrics of animal anatomies? This has been attempted (Oxnard, Crompton & Lieberman, 1990) by «measuring» 17 lifestyle variables (on locomotion, environment and diet) for 28 prosimian primate species. These data have been analysed by multivariate statistical methods entirely similar to those used in morphometrics. The results define groups of prosimian species that seem not to be related directly with taxonomy and evolution, but with functional adaptation to lifestyle. Comparisons between the morphometric and niche metric results reveal a degree of similarity of species grouping in each that is apparently related to adaptation and remarkably detailed. But further study of both the morphometric and niche metric results suggests that questions pertaining to evolution may also be asked. Thus, both morphometric investigations seeking clusterings of anatomical variables and niche metric studies seeking clustering of higher taxonomic groups of species produce results that seem to be of developmental and phylogenetic rather than directly adaptive import. Through the combination of these two approaches, it may eventually be possible to partition both evolutionary and adaptive components, even though, of course, there must always be overlap between them.

Temporal Variation, Habitat Selection and Community Structure

Isodar theory, a model of density-dependent habitat selection, is used to examine how temporal variation affects community organization. An isodar is a plot of the density of a species in one habitat versus its corresponding density in a second habitat such that an individual's expected fitness in the two habitats is the same. Isodars can also be plotted by using the density in one habitat at different times. Ecological succession modifies habitat, the expected reproductive success of individuals living in the habitat, and thus, population density. These effects can be detected in the slopes and intercepts of isodars. It is possible, therefore, to use density estimates alone to predict the time-course of community structure with ecological succession. The proper protocol for this analysis requires replicated population estimates for each temporal stage as well as simultaneous estimates from adjacent undisturbed controls. Previous empirical studies on mice demonstrate the ability of such analyses to reveal the joint effects of temporal and spatial variation on community structure. Extension of the theory to evaluate the relative use of different habitats shows that niche width can increase, decrease, or remain constant with changes in population density. Niche metrics based on habitat are thus likely to give biased interpretations about density-dependent competitive interaction

Niche Relationships between Two Sympatric Liolaemus Lizards in a Fluctuating Environment: The "Lean" versus "Feast" Scenario

Liolaemus fuscus and L. lemniscatus are two similarly-sized, completely sympatric species of central Chile. In some localities they are also syntopic, sharing the same microhabitat (rocks). We studied the microhabitat, time, and food use of these two species throughout one year, computing standard niche metrics. Between-species similarities on these three niche dimensions were very high at all times of the year (mean = 82% for all three microhabitat, time, and food overlaps), slightly decreasing during winter. The feast seasons of plentiful food, when dietary overlaps reached 90 and 99% (spring and summer, respectively) were associated with the two species overlapping broadly in microhabitat (77 and 81%, for spring and summer, respectively) and activity time (92 and 91%, respectively). Lean times were associated with decreased overlap in both diet (59 and 81% for fall and winter, respectively) and activity time (87 and 58%, respectively), but not in microhabitat (71 and 97%, respectively). Our data provide limited support for the view that interspecific niche relationships in fluctuating environments may be understood within the lean versus feast times scenario.