Niche Axes Research Articles

The macroevolution of climatic niches and its role in ant diversification

1. Investigating how climatic niches change over evolutionary timescales is a necessary step to understanding the current distribution of lineages, yet few studies have addressed this issue using comprehensive datasets. In this study, the evolution of ant climatic niches is investigated at a global scale based on bioclimatic data associated with 163 481 ant occurrence records. The resulting dataset was subjected to principal component analysis, and the scores obtained were used to characterise the main axes of ant climatic niche evolution.2. Principal component axis 1 (PC1) reflected variation in average temperature and seasonality – consistent with typical tropical/temperate gradients – whereas PC2 was associated with varying levels of aridity. Evolution along these two niche axes was markedly different: differences in the amount of explained variance between PC1 (65%) and PC2 (19%) suggest that climatic niche evolution was nearly three times more pronounced along a tropical–temperate climate axis.3. There was statistically significant phylogenetic signal on PC1, with genera occupying more tropical conditions diversifying at a faster rate, yet neither of these results is significant on PC2. In addition, most of the ancient ant lineages are associated with conditions of low seasonality and high temperatures.4. These results provide partial support for the tropical conservatism hypothesis as an explanation for geographical patterns of ant species richness.

Separate histories in both sides of the Mediterranean: phylogeny and niche evolution of ocellated lizards

AbstractAimThe ocellated lizards of the genus Timon, comprising six species that are distributed across continental Asia, Europe and Africa, offer an interesting model to study the evolution of ecological niches through comparative phyloclimatic analysis. Our study provides insights into the evolutionary history of the ocellated lizards and helps to understand the role of climatic niche evolution during the speciation process.LocationEastern and western margins of the Mediterranean basin.MethodsA dated molecular phylogeny was estimated based on three mitochondrial and two nuclear genes. Using multivariate statistics, species distribution models were developed to characterize the Grinnellian niches of all species. Subsequently, ancestral environmental niche occupancy of each taxon was reconstructed using niche occupancy profiles. Niche divergence among species was quantified by computing multivariate niche overlaps via two‐dimensional and n‐dimensional approaches.ResultsPhylogenetic analysis supports that the ancestor of Timon diverged into the eastern and western groups following multiple vicariance events that shaped the current distribution pattern of Palaearctic lizards. High complexity in the ecological niche evolution between the Mediterranean and non‐Mediterranean climatic regions was detected. The generally low niche divergence among members of the eastern group and the remarkable climatic divergence within the western group highlight an important role of temperature seasonality in a Mediterranean and Atlantic climate context. The results also suggest niche conservatism in terms of microhabitats described by vegetation cover.Main conclusionsThe ocellated lizards provide an interesting example of a vertebrate radiation where niche shift (with or without vicariance) and niche conservatism alternate in different niche axes shaping current biogeographical patterns.

Coexistence and emergent neutrality generate synchrony among competitors in fluctuating environments

Many competitive communities exhibit a puzzling amount of species diversity. In this study, we model a community of symmetric competitors in a fluctuating environment. We use biologically realistic temperature-dependent growth curves with a widely hypothesized trade-off between maximum growth and nice breadth to control the shapes of the curves of different species. We perform three analyses of the community dynamics to investigate the role of environmental fluctuations in community composition and species diversity. We initiate communities with equal abundances of all species and randomize the temperature fluctuations so that there is no correlation between species responses, only noise. We initiate single populations and allow other species to randomly invade the community. We also knock out extant species one by one from an established community and allow them to reinvade after the remaining species have adjusted. We find that competitors with sufficiently different temperature niches coexist via temporal niche differentiation. We also find long-term persistence of species that are very similar to a dominant competitor. This creates communities with species clumped along a temperature niche axis, with stable coexistence between groups and near neutrality within groups. The near neutrality results in interspecific synchrony within the groups, providing an explanation for the maintenance of high diversity in competitive communities where synchrony is commonly observed.

Niche partitioning along the diet axis in a colour polymorphic population of Eastern Red-backed Salamanders, Plethodon cinereus

Eastern Red-backed Salamanders are colour polymorphic and have become a model system for examining ecological separation and the mechanisms for potential divergence in sympatry. Morphs of this species are differentiated along several niche axes including temperature optima, territorial behaviour, and response to predators. We were interested in whether temporal variation in ecological conditions would affect foraging behavior and ultimately diet. The goal of our study was to compare the diets of striped and unstripedP. cinereusover a range of seasonal conditions to better understand if the reported differences observed in the fall season at our field site remain consistent through the active season of this species. Diet differences between the two morphs were greatest during spring and fall when salamanders are most abundant at the surface. These diet differences were driven largely by two prey categories. In the spring, striped salamanders ate more oribatid mites and in the fall they consumed more entomobryomorph Collembola. Trade-offs associated with territory acquisition coupled with physiological and morphological differences may explain the observed seasonal niche partitioning related to the diet in this population.

Resource partitioning and niche segregation in a steppe bird assemblage

Niche theory predicts that coexisting species will partition resources to limit the effects of interspecific competition. We examined microhabitat partitioning in six sets of steppe birds associated to agroecosystems in central Spain (female and male Great Bustards Otis tarda, female and male Little Bustards Tetrax tetrax, Red-legged Partridges Alectoris rufa and Eurasian Stone-curlews Burhinus oedicnemus) to estimate realized niche breadth, overlap and segregation. Principal Components Analysis on data from used and random microhabitat locations produced two axes we retained for analysis related with two key factors: cover-visibility and food availability. Non-parametric kernel density functions were calculated for each of the PCA axes and species (or sexes), and niche overlap estimated as the area shared between species’ density functions. Null models were run to evaluate overlap significance. In analyses of microhabitat selection by the six sets of birds, 13 out of 15 pairs had significant resource partitioning and niche segregation, except for the pairs partridge and female Great Bustard and the two sexes of Great Bustard. Eurasian Stone-curlew showed wider trophic niche breadth, although segregated from the other species, probably because of its higher invertebrate requirements. Great and Little Bustards segregated in both niche axes, selecting microhabitat according to their body size. Accessibility to food resources and shelter seems to be similar for partridges and female Great Bustards, overlapping in their selection, which may indicate the existence of segregation in other niche factors (e.g., feeding habits). Great Bustard males showed niche overlap with females. Little Bustard males showed feeding microhabitat selection patterns similar to those of females, although they preferred more open microhabitats to meet their sexual display requirements. The entire assemblage had significantly less overlap than expected by chance, suggesting that differential microhabitat selection and realized niche partitioning may explain coexistence in steppe bird communities. Our results suggest that the maintenance of different microhabitat structure should be a priority in the management of agricultural environments.

The Curious Case of the Camelthorn: Competition, Coexistence, and Nest-Site Limitation in a Multispecies Mutualism.

Myrmecophyte plants house ants within domatia in exchange for protection against herbivores. Ant-myrmecophyte mutualisms exhibit two general patterns due to competition between ants for plant occupancy: (i) domatia nest sites are a limiting resource and (ii) each individual plant hosts one ant species at a time. However, individual camelthorn trees (Vachellia erioloba) typically host two to four ant species simultaneously, often coexisting in adjacent domatia on the same branch. Such fine-grain spatial coexistence brings into question the conventional wisdom on ant-myrmecophyte mutualisms. Camelthorn ants appear not to be nest-site limited, despite low abundance of suitable domatia, and have random distributions of nest sites within and across trees. These patterns suggest a lack of competition between ants for domatia and contrast strongly with other ant-myrmecophyte systems. Comparison of this unusual case with others suggests that spatial scale is crucial to coexistence or competitive exclusion involving multiple ant species. Furthermore, coexistence may be facilitated when co-occurring ant species diverge strongly on at least one niche axis. Our conclusions provide recommendations for future ant-myrmecophyte research, particularly in utilizing multispecies systems to further our understanding of mutualism biology.

Exploring the nature of ecological specialization in a coral reef fish community: morphology, diet and foraging microhabitat use.

Patterns of ecological specialization offer invaluable information about ecosystems. Yet, specialization is rarely quantified across several ecological niche axes and variables beyond the link between morphological and dietary specialization have received little attention. Here, we provide a quantitative evaluation of ecological specialization in a coral reef fish assemblage (f. Acanthuridae) along one fundamental and two realized niche axes. Specifically, we examined ecological specialization in 10 surgeonfish species with regards to morphology and two realized niche axes associated with diet and foraging microhabitat utilization using a recently developed multidimensional framework. We then investigated the potential relationships between morphological and behavioural specialization. These relationships differed markedly from the traditional ecomorphological paradigm. While morphological specialization showed no relationship with dietary specialization, it exhibited a strong relationship with foraging microhabitat specialization. However, this relationship was inverted: species with specialized morphologies were microhabitat generalists, whereas generalized morphotypes were microhabitat specialists. Interestingly, this mirrors relationships found in plant-pollinator communities and may also be applicable to other ecosystems, highlighting the potential importance of including niche axes beyond dietary specialization into ecomorphological frameworks. On coral reefs, it appears that morphotypes commonly perceived as most generalized may, in fact, be specialized in exploiting flat and easily accessible microhabitats.

Management-Induced Niche Shift? The Activity of Cheetahs in the Presence of Lions

Habitat loss, persecution and population declines have resulted in the restriction of many large carnivores, including cheetahs (Acinonyx jubatus), to fenced reserves. These reserves are often small and the likelihood of interference competition between dominant and subordinate predators is increased, while the possibility of spatial avoidance is reduced. Given artificial space limitations, subordinate predators may reduce competitive interactions along niche axes other than habitat type such as time, coined a ‘management-induced niche shift’. We collected data on the activity patterns of male and female cheetahs using continuous follows and satellite GPS collars on two small, fenced reserves in the Eastern Cape Province of South Africa, one with lions (Panthera leo) and one without lions. There was a significant difference in the activity patterns of male and female cheetahs between the two reserves. In the presence of lions, nocturnal activity of cheetahs was greatly reduced, particularly for females. T...

Resource partitioning along multiple niche axes drives functional diversity in parrotfishes on Caribbean coral reefs.

The recent loss of key consumers to exploitation and habitat degradation has significantly altered community dynamics and ecosystem function across many ecosystems worldwide. Predicting the impacts of consumer losses requires knowing the level of functional diversity that exists within a consumer assemblage. In this study, we document functional diversity among nine species of parrotfishes on Caribbean coral reefs. Parrotfishes are key herbivores that facilitate the maintenance and recovery of coral-dominated reefs by controlling algae and provisioning space for the recruitment of corals. We observed large functional differences among two genera of parrotfishes that were driven by differences in diet. Fishes in the genus Scarus targeted filamentous algal turf assemblages, crustose coralline algae, and endolithic algae and avoided macroalgae, while fishes in the genus Sparisoma preferentially targeted macroalgae. However, species with similar diets were dissimilar in other attributes, including the habitats they frequented, the types of substrate they fed from, and the spatial scale at which they foraged. These differences indicate that species that appear to be functionally redundant when looking at diet alone exhibit high levels of complementarity when we consider multiple functional traits. By identifying key functional differences among parrotfishes, we provide critical information needed to manage parrotfishes to enhance the resilience of coral-dominated reefs and reverse phase shifts on algal-dominated reefs throughout the wider Caribbean. Further, our study provides a framework for predicting the impacts of consumer losses in other species rich ecosystems.

Phylogeny and biogeography of the American live oaks (Quercus subsection Virentes): a genomic and population genetics approach.

The nature and timing of evolution of niche differentiation among closely related species remains an important question in ecology and evolution. The American live oak clade, Virentes, which spans the unglaciated temperate and tropical regions of North America and Mesoamerica, provides an instructive system in which to examine speciation and niche evolution. We generated a fossil-calibrated phylogeny of Virentes using RADseq data to estimate divergence times and used nuclear microsatellites, chloroplast sequences and an intron region of nitrate reductase (NIA-i3) to examine genetic diversity within species, rates of gene flow among species and ancestral population size of disjunct sister species. Transitions in functional and morphological traits associated with ecological and climatic niche axes were examined across the phylogeny. We found the Virentes to be monophyletic with three subclades, including a southwest clade, a southeastern US clade and a Central American/Cuban clade. Despite high leaf morphological variation within species and transpecific chloroplast haplotypes, RADseq and nuclear SSR data showed genetic coherence of species. We estimated a crown date for Virentes of 11Ma and implicated the formation of the Sea of Cortés in a speciation event ~5Ma. Tree height at maturity, associated with fire tolerance, differs among the sympatric species, while freezing tolerance appears to have diverged repeatedly across the tropical-temperate divide. Sympatric species thus show evidence of ecological niche differentiation but share climatic niches, while allopatric and parapatric species conserve ecological niches, but diverge in climatic niches. The mode of speciation and/or degree of co-occurrence may thus influence which niche axis plants diverge along.

Spatial and isotopic niche partitioning during winter in chinstrap and Adélie penguins from the South Shetland Islands

Closely related species with similar ecological requirements should exhibit segregation along spatial, temporal, or trophic niche axes to limit the degree of competitive overlap. For migratory marine organisms like seabirds, assessing such overlap during the non‐breeding period is difficult because of long‐distance dispersal to potentially diffuse foraging habitats. Miniaturization of geolocation devices and advances in stable isotope analysis (SIA), however, provide a robust toolset to quantitatively track the movements and foraging niches of wide ranging marine animals throughout much of their annual cycle. We used light‐based geolocation tags and analyzed stable carbon and nitrogen isotopes from tail feathers to simultaneously characterize winter movements, habitat utilization, and overlap of spatial and isotopic niches of migratory chinstrap (Pygoscelis antarctica) and Adélie (P. adeliae) penguins during the austral winter of 2012. Chinstrap penguins exhibited a higher diversity of movements and occupied portions of the Southern Ocean from 138° W to 30° W within a narrow latitudinal band centered on 60° S. In contrast, all tracked Adélie penguins exhibited smaller‐scale movements into the Weddell Sea and then generally along a counter‐clockwise path as winter advanced. Inter‐specific overlap during the non‐breeding season was low except during the months immediately adjacent to the summer breeding season. Intra‐specific overlap by chinstraps from adjacent breeding colonies was higher throughout the winter. Spatial segregation appears to be the primary mechanism to maintain inter‐ and intra‐specific niche separation during the non‐breeding season for chinstrap and Adélie penguins. Despite low spatial overlap, however, the data do suggest that a narrow pelagic corridor in the southern Scotia Sea hosted both chinstrap and Adélie penguins for most months of the year. Shared occupancy and similar isotopic signatures of the penguins in that region suggests that the potential for inter‐specific competition persists during the winter months. Finally, we note that SIA was able to discriminate eastward versus westward migrations in penguins, suggesting that SIA of tail feathers may provide useful information on population‐level distribution patterns for future studies.

Area, climate heterogeneity, and the response of climate niches to ecological opportunity in island radiations of Anolis lizards

AbstractAimRates of climate niche evolution underlie numerous fundamental ecological processes and patterns. However, while climate niche conservatism varies markedly among regions and clades, the source of this variation remains poorly understood. We tested whether ecological opportunity can stimulate radiation within climate niche space at biogeographic scales, predicting that rates of climate niche evolution will scale with geographic area and climate heterogeneity.LocationCaribbeanMethodsWe quantified two temperature axes (mean temperature and temperature seasonality of species' localities) of the climate niche for 130 Anolis species on Cuba, Hispaniola, Puerto Rico, Jamaica and the northern and southern Lesser Antilles. Using a species‐level phylogeny, we fitted macroevolutionary models that either constrained rates of climate niche evolution or allowed them to vary among regions. Next, we regressed region‐specific evolutionary rates against area, species richness and climate heterogeneity. We evaluated whether results were robust to uncertainty in phylogenetic and biogeographic reconstructions and the assumed mode of evolution.ResultsFor both niche axes, an Ornstein‐Uhlenbeck model that allowed the net rate of evolution (σ2) to vary among island groups fit the data considerably better than a single‐rate Brownian motion model. Nagelkerke pseudo‐R2 values of the fit of these OU models to mean temperature and seasonality axes were 0.43 and 0.66, respectively. Evolutionary rates for both axes were higher in larger areas, which also have more species. Only the rate of mean occupied temperature evolution was positively related to climate heterogeneity, and only after accounting for region size.ConclusionsRates of climate niche evolution scale consistently with the area available for radiation, but responses to climate heterogeneity vary among niche axes. For the mean temperature axis, climate heterogeneity generated additional opportunities for radiation, but for seasonality it did not. Overall, the physical setting in which a clade diversifies can influence where it falls on the evolutionary continuum, from climate niche conservatism to radiation.

Adult feeding moths (Sphingidae) differ from non-adult feeding ones (Saturniidae) in activity-timing overlap and temporal niche width.

According to classic ecology, resource partitioning by segregation along at least one of the three main niche axes (time, food, and space) must take place for the coexistence of species with similar ecological requirements. We used nocturnal light traps to investigate the assemblage structuration of two moth families: Sphingidae (23 species) and Saturniidae (13 species). Because competition for food among adults potentially occurs only among sphingids, only for this family did we expect less overlap of diel activity patterns than expected by chance and also a greater temporal niche width compared to saturniids. Moreover, we expected a greater number of sphingid species pairs to differ in activity timing compared to saturniid pairs. We also hypothesized that in the case of a lack of temporal structuration, sphingids would be morphologically structured in relation to proboscis length. Contrary to what we expected, both families overlapped their activity patterns more than expected by chance alone and sphingid moths were not morphologically structured. Nevertheless, there were 173 significant pairwise differences in temporal activity between sphingids, contrasting with no interspecific differences between saturniids. Sphingid species also showed a wider temporal niche width than saturniids, as expected. Predation risk and abiotic factors may have caused the overall similarities in activity patterns for both families. The temporal niche seemed not to be determinant for the assemblage structuration of moths as a whole for either of the studied families, but segregation along the temporal niche axis of some potentially competing species pairs can be a relevant factor for the coexistence of nectar-feeding species.

Fine‐scale hydrological niche differentiation through the lens of multi‐species co‐occurrence models

Summary Theory suggests spatial heterogeneity can facilitate species co‐occurrence at fine scales, but environmental data are rarely collected at sufficiently high resolution to test this empirically. Whilst there is emerging evidence that subtle variation in soil hydrology represents a fundamental fine‐scale niche axis within plant communities, this is largely derived from studies of soil hydrology in isolation from other environmental factors. We assessed the comparative importance of fine‐scale hydrological niche differentiation for species co‐occurrence using a high‐resolution study of soil hydrology and other edaphic variables, coupled with a long‐term (24 years) data set of herbaceous plant plots in a heathland community in south‐east Australia. For the analysis, we employed novel latent variable models (LVMs), which offer an explicit, model‐based approach to partitioning out the different drivers of species co‐occurrence patterns. Whilst the regression component of an LVM models the species‐specific environmental responses, the latent variable component can be used to identify residual patterns of co‐occurrence, which may be attributable to unmeasured factors and/or biotic interactions. Relative to a host of plant resources, non‐resource factors and ‘unmeasured’ latent variables, soil hydrology emerged as the best predictor of negative co‐occurrences within the community, with the dominant species exhibiting strongly differentiated responses across a comparatively narrow moisture gradient. Nevertheless, strong species‐specific responses to environmental variability only emerged at scales greater than those at which plants may be expected to compete for resources, throwing doubt on the direct role of spatial heterogeneity as a mechanism for local‐scale coexistence. Synthesis. This study confirms the vital role of hydrological niches for the maintenance of within‐community plant diversity, but also highlights the need for more rigorous analysis of scale dependencies to better understand the underlying coexistence mechanisms at play. In addition, it illustrates the inferential gains made possible with model‐based approaches to the analysis of species co‐occurrence. r code illustrating model fitting and inference is provided as a supplement.

Elevation, moisture and shade drive the functional and phylogenetic meadow communities’ assembly in the northeastern Tibetan Plateau

Despite a long history of alpine meadows studies, uncertainty remains about the importance of environmental factors in structuring their assembly. We examined the functional and phylogenetic structure of 170 alpine Tibetan meadow communities in relation to elevation, soil moisture and shade. Functional community structure was estimated with both communityweighted mean (CWM) trait values for specific leaf area (SLA), plant height and seed mass and functional diversity (Rao’s quadratic index) for their traits individually and in combination (multivariate functional diversity). We found that shade induced by woody plants significantly increased the phylogenetic diversity and functional diversity of SLA of co-occurring species, suggesting that woody plants behave as “ecosystem engineers” creating a different environment that allows the existence of shade tolerant species and thereby facilitates the coexistence of plant species with different light resource acquisition strategies. We also found evidence for a clear decrease in phylogenetic diversity, CWM and functional diversity related to plant height in the two extreme, both the dry and wet, soil moisture conditions. This indicates that both drought and excess moisture may act as environmental filters selecting species with close phylogenetic relationships and similar height. Moreover, we detected significant decreases in both CWM and functional diversity for seed mass along elevational gradients, suggesting that low net primary productivity (NPP) limits seed size. Finally, because of different individual trait responses to environmental factors, the multivariate functional diversity did not change across environmental gradients. This lack of multivariate response supports the hypothesis that multiple processes, such as environmental filtering, competition and facilitation, may operate simultaneously and exert opposing effects on community assembly along different niche (e.g., water use, light acquisition) axes, resulting in no overall functional community structure change. This contrast between individual and multivariate trait patterns highlights the importance of examining individual traits linked with different ecological processes to better understand the mechanisms of community assembly.

Low Food Availability Narrows the Tolerance of the Copepod Eurytemora affinis to Salinity, but Not to Temperature

Invasive species perturb food webs, often decreasing resource availability for resident taxa. Low resource availability may interact with abiotic factors to restrict niches, particularly niche axes that influence metabolic demand. The San Francisco Estuary (SFE) provides a case study, as it has low phytoplankton concentrations, likely due to invasive bivalves. We conducted two laboratory experiments to examine how phytoplankton concentrations influence the salinity and temperature tolerance of Eurytemora affinis, an important prey taxon for threatened SFE fish. We found that decreased algal concentration narrowed the tolerance of E. affinis to salinity, but not to temperature. A third experiment revealed that when food concentration was relatively low (chlorophyll-a of 15 μg L−1) and salinity was elevated (8 vs 4), E. affinis did not compensate for osmotic stress by increasing consumption, halving its growth rate. However, when algal concentration was elevated (chlorophyll-a 55 μg L−1), E. affinis consumed three times more algae at a salinity of 8 vs 4, allowing copepods to grow equally at both salinities. Our interpretation is that while E. affinis can compensate for increased metabolic demand as temperature increases at low food concentrations, it can only compensate for elevated metabolic demand at hypo- or hyperosmotic salinities when food concentrations are high. We therefore propose the hypothesis that low food concentrations narrow the realized salinity, but not the realized thermal, niche of E. affinis.

Temperate snake community in South America: is diet determined by phylogeny or ecology?

Communities are complex and dynamic systems that change with time. The first attempts to explain how they were structured involve contemporary phenomena like ecological interactions between species (e.g., competition and predation) and led to the competition-predation hypothesis. Recently, the deep history hypothesis has emerged, which suggests that profound differences in the evolutionary history of organisms resulted in a number of ecological features that remain largely on species that are part of existing communities. Nevertheless, both phylogenetic structure and ecological interactions can act together to determine the structure of a community. Because diet is one of the main niche axes, in this study we evaluated, for the first time, the impact of ecological and phylogenetic factors on the diet of Neotropical snakes from the subtropical-temperate region of South America. Additionally, we studied their relationship with morphological and environmental aspects to understand the natural history and ecology of this community. A canonical phylogenetical ordination analysis showed that phylogeny explained most of the variation in diet, whereas ecological characters explained very little of this variation. Furthermore, some snakes that shared the habitat showed some degree of diet convergence, in accordance with the competition-predation hypothesis, although phylogeny remained the major determinant in structuring this community. The clade with the greatest variability was the subfamily Dipsadinae, whose members had a very different type of diet, based on soft-bodied invertebrates. Our results are consistent with the deep history hypothesis, and we suggest that the community under study has a deep phylogenetic effect that explains most of the variation in the diet.

Ecological consequences of contrasting dispersal syndromes in New World Ephedra: higher rates of niche evolution related to dispersal ability

In this study we selected the New World species of Ephedra to understand the ecological consequences of different dispersal syndromes. The twenty‐three species of Ephedra in the New World have a disjunct distribution in North and South American arid and semi‐arid habitats, exhibiting three dispersal syndromes related to dispersal by birds, wind and rodents. Using DNA sequence data we inferred phylogenetic relationships and lineage divergence times, and used these estimates to test different ecological assumptions. Using comparative methods we tested for correlations between dispersal syndromes and a set of ecological variables (niche breadth, niche evolution, distributional ranges and niche position). We found that speciation events in the New World coincided with the expansion of arid habitats in this region. We suggest that the bird dispersal syndrome is related with higher rates of climatic niche evolution for all variables used, including aridity index, mean annual temperature and mean annual precipitation. Distribution ranges were correlated with niche breadth, they were however not significantly different between dispersal syndromes. Species inhabiting the extremely arid regions on niche axes had narrower niche breadths. We conclude that species whose seeds are dispersed by birds have colonized a broader set of habitats and that those with wind and rodent dispersal syndromes might have promoted the colonization of more arid environments.

Co‐existence in a species‐rich grassland: competition, facilitation and niche structure over a soil depth gradient

AbstractQuestionNiche differentiation may promote species co‐existence because it precludes competitive exclusion. However, in stressful environments, facilitation may predominate, and large niche overlaps may result from niche widening and positive spatial associations. We analysed the niche structure of a species‐rich system over a soil depth gradient (a niche axis positively correlated with water availability) and tested for various patterns expected from competition or facilitation.LocationSemi‐arid grassland, Oaxaca, Mexico.MethodsWe recorded species abundance and soil depth in 1200 0.1 × 0.1 m quadrats from four sites. Community structure was described through correspondence analysis and correlated with soil depth. We measured niche breadth (Levine's index), position (mean depth) and overlap (Hurlbert's index). We used a null model to estimate the expected overlap if interactions play no role in determining community structure. We tested whether overlap changed along the depth gradient. We also analysed if niches become narrow with higher competition.ResultsData supported three predictions derived from competition: most species’ niches were relatively narrow and differed in their position along the gradient, a pattern reflected in a changing community structure; realized niches became narrower where competition was stronger; and niche overlap was smaller than expected by chance. Facilitation had no support: large niche overlaps expected from stress amelioration were as frequent as predicted by chance. Overlap increased in the most stressful (shallow) soils, as expected if facilitation predominates there, but also in the deepest ones where evidence for competition was strong, suggesting an artifact at the extremes of the gradient.ConclusionsIn this grassland, competition seems to restrict species distribution, abating richness in the deepest soils and relegating poor competitors (succulent and poikilohydric species) to the shallow soils. There, drought tolerance rather than facilitation determined community composition and richness. Our results support the idea that hydrological niches play an important role in maintaining plant diversity because (1) differentiation in hydrological requirements (small niche overlaps over the water‐use axis) reduces competition, and (2) spatial heterogeneity and stress create competition‐free patches. Our results also support the hypothesis that facilitation is rare when stress is caused by resource (in this case, water) scarcity.

Life-history traits, abiotic environment and coexistence: The case of two cryptic rotifer species

Trade-offs are important in life-history evolution and coexistence of competitors. However, how alternative life-history optima might promote competitor coexistence has received little attention. The rotifers Brachionus plicatilis and Brachionus manjavacas are two cryptic species with a considerable ecological overlap in relation to biotic factors. These species often co-occur in temporal brackish ponds characterised by environmental fluctuations. In these rotifers, sexual reproduction results in diapausing stages, which are able to survive through adverse conditions but constrain current population growth. In the present work, the differential responses of both cryptic rotifer species to varying salinities and temperatures in terms of population growth, sexual reproduction and diapause patterns (i.e., investment, viability and hatchability of diapausing eggs) were investigated to assess their possible roles in species coexistence. B. plicatilis and B. manjavacas showed differential responses to salinity and temperature and differed consistently in traits involved in diapause. The niche overlap between B. plicatilis and B. manjavacas decreased dramatically when the temperature and salinity response niche axes were added to a number of previously studied biotic niche axes. The results suggest that B. plicatilis exploits ephemeral and favourable conditions (low salinity) to invest in diapause, which would accelerate its exclusion from the water column but promote its long-term persistence. The findings show that cryptic species may evolve different compromises among life-history traits, allowing ecological divergence, and that ecological divergence may be uncoupled from morphological divergence for some factors in the abiotic environment.