Niche Differentiation Hypothesis Research Articles

Ecological Niche Characteristics of the Diets of Three Sympatric Rodents in the Meili Snow Mountain, Yunnan.

Understanding the dietary preferences and ecological niche characteristics of mammals not only reveals their adaptive strategies under environmental changes but also reveals the interspecific relationships and coexistence mechanisms among sympatric species. Nevertheless, such data are scarce for rodents inhabiting areas spanning a wide altitude range. This study employed DNA metabarcoding technology to analyze the stomach contents of Apodemus ilex, Apodemus chevrieri, and Niviventer confucianus, aiming to investigate their dietary compositions and diversity in the Meili Snow Mountain in Yunnan Province, China. Levins's and Pianka's indices were used to compare the interspecific niche breadth and niche overlaps. The results revealed the following: (1) Insecta (relative abundance: 59.4-78.4%) and Clitellata (relative abundance: 5.2-25.5%) were the primary animal food sources for the three species, while Magnoliopsida (relative abundance: 90.3-99.9%) constitutes their main plant food source. Considerable interspecific differences were detected in the relative abundance of primary animal and plant foods among the three species; (2) There was partial overlap in the genus-level animal food between A. ilex and N. confucianus (Ojk = 0.4648), and partial overlap in plant food between A. ilex and A. chevrieri (Ojk = 0.3418). However, no overlap exists between A. chevrieri and N. confucianus, either in animal or plant food; (3) There were no significant interspecific differences in the α-diversity of animal and plant foods among the three species. The feeding strategies and ecological niche variations of these rodents support the niche differentiation hypothesis, indicating that they have diversified in their primary food sources. This diversification may be a strategy to reduce competition and achieve long-term coexistence by adjusting the types and proportions of primary foods consumed.

Patterns of molecular evolution in a parthenogenic terrestrial isopod (Trichoniscus pusillus).

The "paradox of sex" refers to the question of why sexual reproduction is maintained in the wild, despite how costly it is compared to asexual reproduction. Because of these costs, one might expect nature to select for asexual reproduction, yet sex seems to be continually selected for. Multiple hypotheses have been proposed to explain this incongruence, including the niche differentiation hypothesis, the Red Queen hypothesis, and accumulation of harmful mutations in asexual species due to inefficient purifying selection. This study focuses on the accumulation of mutations in two terrestrial isopods, Trichoniscus pusillus, which has sexual diploid and parthenogenic triploid forms, and Hyloniscus riparius, an obligately sexual relative. We surveyed sex ratios of both species in an upstate New York population and obtained RNA-seq data from wild-caught individuals of both species to examine within- and between-species patterns of molecular evolution in protein-coding genes. The sex ratio and RNA-seq data together provide strong evidence that this T. pusillus population is entirely asexual and triploid, while the H. riparius population is sexual and diploid. Although all the wild T. pusillus individuals used for sequencing shared identical genotypes at nearly all SNPs, supporting a clonal origin, heterozygosity and SNP density were much higher in T. pusillus than in the sexually reproducing H. riparius. This observation suggests this parthenogenic lineage may have arisen via mating between two divergent diploid lineages. Between-species sequence comparisons showed no evidence of ineffective purifying selection in the asexual T. pusillus lineage, as measured by the ratio of nonsynonymous to synonymous substitutions (dN/dS ratios). Likewise, there was no difference between T. pusillus and H. riparius in the ratios of nonsynonymous to synonymous SNPs overall (pN/pS). However, pN/pS ratios in T. pusillus were significantly higher when considering only SNPs that may have arisen via recent mutation after the transition to parthenogenesis. Thus, these recent SNPs are consistent with the hypothesis that purifying selection is less effective against new mutations in asexual lineages, but only over long time scales. This system provides a useful model for future studies on the evolutionary tradeoffs between sexual and asexual reproduction in nature.

Source height and contact with terrestrial soil drive transplanted epiphyte performance

Abstract Epiphytes are characterized by their ability to survive without a root connection to the ground, but many basic life‐history traits and ecological trade‐offs of this unique aerial growth habit remain largely uncharacterized. Mortality causes are still not well understood, but falling from the host tree has been suggested as a leading cause of epiphyte mortality and community dynamics. Little empirical evidence exists for why epiphytes do not survive when forced to become terrestrial, and few studies exist that transplant epiphytes between high‐ and low‐forest strata to test trade‐offs between thriving in canopy environments and survival in the forest understorey. Here, we experimentally test two hypotheses regarding the drivers of epiphyte mortality in a cloud forest of central Panama. We test whether simple contact with terrestrial soil is deleterious to epiphytes, preliminarily testing the epiphyte enemy escape hypothesis, and test the vertical niche differentiation hypothesis, wherein epiphytes are specifically adapted for microsites throughout the vertical forest strata. By monitoring survival, leaf loss and health status of 270 transplanted epiphytes for a year and a half, we pinpoint the extent to which soil contact and height of origin regulate epiphyte performance. We found that contact with terrestrial soil itself was detrimental to epiphytes in situ, providing some of the first empirical data to explain why falling onto the ground, versus falling into the understorey, is particularly fatal to epiphytes. However, we also found that mortality rates vary substantially among taxonomic groups and among epiphytes that originally came from different height strata. Synthesis. Plants that are adapted for the canopy experience a trade‐off with higher mortality when in contact with terrestrial soil. Follow‐up studies should explore the role of terrestrial soil microbes and physiological constraints as potential drivers of decreased grounded epiphyte survival.

Sexual selection for flight performance in hummingbirds

AbstractAmong size-dimorphic animals, a few clades such as hummingbirds show “reversed” sexual size dimorphism: females tend to be the larger sex. What selects for this pattern? Sexual selection for flight performance could drive the evolution of smaller, more agile males, either for male-male combat or female choice for aerial courtship displays. Alternately, natural selection can select for female fecundity (e.g., egg size influences female body size), or sex differences in foraging niche could favor body size differences. The sexual selection hypotheses predict that dimorphism extends to other aspects of flight morphology (e.g., flight muscle size) whereas the natural selection hypotheses predict that male and female flight morphologies are isometric, and the niche differentiation hypothesis predicts that bill dimorphism is correlated with size dimorphism. We tested these predictions through phylogenetic comparative analyses of flight morphology, wingbeat frequency, and courtship behaviors, focused on 30 species within the “bee” hummingbird clade (tribe Mellisugini). There is no correlation between bill morphology and dimorphism. Relative to females, males tend to be smaller, have proportionately shorter wings and higher hovering wingbeat frequencies, but also longer keels and larger flight muscles. Male wingbeat frequencies are greatly elevated during aerial displays, and the species with the greatest wingbeat frequencies have the greatest dimorphism. Of the four hypotheses for dimorphism, the data best support the hypothesis that female choice for courtship displays has selected for aerial agility and small size in male hummingbirds.

Preliminary patterns of seasonal changes in species composition of surface‐active arthropods in a South African savannah

AbstractSurface‐active arthropods have limited dispersal abilities and their abundances are responsive to changes in the environment. Their sensitivity allows for the identification of aspects of environmental variation that are responsible for changes in community assemblages of these arthropods. Limited dispersal abilities allow for changes in the populations of surface‐active arthropods to be linked reliably to changes in their local environment. In this study, we tested the hypothesis that seasonal variation in resource availability drives assemblages of three groups of surface‐active arthropods (ants, beetles and spiders). Ten study sites were selected, where sampling was carried out continually during the wet, austral summer season (December 2018–February 2019) and then repeated at the same sites in the dry winter season (June 2019–August 2019) in a mesic savannah of Mpumalanga province, South Africa. Pitfall trapping was used to sample the surface‐active arthropods. We found 89 morphospecies of ants, 48% of which were members of the Myrmicinae, while 32% of the 163 morphospecies of beetles were the Scarabaeidae. There were 262 morphospecies of spiders, of these, 22% were from the Salticidae. We found similar abundance and species richness of ants between seasons but significantly greater abundance and species richness of beetles and spiders in summer than in winter. These results show that the impact of seasonal variation in resource availability differs among arthropod taxa. However, the species compositions of all three taxa were significantly different between summer and winter. These results can be explained by the ‘niche differentiation hypothesis’ coupled with the ‘resource‐ratio hypothesis’. Additionally, these results highlight the role that temporal and dietary niche differentiation play in driving the assemblage composition of surface‐active arthropods in a mesic savannah.

Testing the niche differentiation hypothesis in wild capuchin monkeys with polymorphic color vision

Abstract The polymorphic color vision system present in most North, Central, and South American monkeys is a textbook case of balancing selection, yet the mechanism behind it remains poorly understood. Previous work has established task-specific foraging advantages to different color vision phenotypes: dichromats (red-green colorblind) are more efficient foraging for invertebrates, while trichromats (color “normal” relative to humans) are more efficient foraging for “reddish” ripe fruit, suggesting that niche differentiation may underlie the maintenance of color vision variation. We explore a prediction of the niche differentiation hypothesis by asking whether dichromatic and trichromatic capuchin monkeys (Cebus imitator) diverge in their foraging activity budget, specifically testing whether dichromats forage more frequently for invertebrates and trichromats forage more frequently for “reddish” ripe fruit. To assess this, we analyze a large data set of behavioral scan samples (n = 21 984) from 48 wild adult female capuchins of known color vision genotype, dominance rank, and reproductive status, together with models of food conspicuity. We find no significant differences between dichromats and trichromats in the frequency of scans spent foraging for different food types but do find that nursing females forage less overall than cycling females. Our results suggest that the potential for color-vision-based niche differentiation in foraging time may be curtailed by the energetic requirements of reproduction, behavioral synchrony caused by group living, and/or individual preferences. While niche differentiation in activity budgets by color vision type is not apparent, fine-scale niche differentiation may be occurring. This research enhances our understanding of the evolutionary processes maintaining sensory polymorphisms.

Loss of natural coastline influences species diversity of anemonefish and host anemones in the Ryukyu Archipelago

Abstract Anemonefish and host anemones are distributed in the shallow waters of coral reefs, and thus their existence may be affected by coastal development. The Ryukyu Archipelago includes the Okinawa and Sakishima Islands; the former has experienced extensive land reclamation and drainage in coastal areas while the latter still mostly contains natural coasts. To consider the layout of protected areas needed to conserve diversity of host anemones and anemonefish in the Ryukyu Archipelago, the community structure and coexistence mechanisms of anemones and anemonefish were compared between Okinawa and Sakishima Islands. Six species of anemonefish and seven species of host anemone are distributed in both islands. Among 15 species‐pair interactions observed in six species of anemonefish in the Sakishima Islands, 14 were explained by the niche differentiation hypothesis and only one interaction was explained by the cohabitation hypothesis. In the Okinawa Islands, 13 interactions were explained by the niche differentiation hypothesis, one by the cohabitation hypothesis and one by the lottery hypothesis. For both host anemones and anemonefish, β‐diversity in the Okinawa Islands was lower and had a more nested structure than that of the Sakishima Islands. β‐Diversity and its nestedness did not differ between the two regions excluding study sites adjacent to coastlines, suggesting that differences in the coastal environment affected the diversity of anemones and anemonefish in the Ryukyu Archipelago. These results suggested that there is an urgent need to prioritize the conservation of reef edges, where species diversity is relatively higher than directly next to the terrestrial coastline in Okinawa Island. In the Sakishima Islands, where many natural coasts remain, a wider area from directly next to the coast out to the reef edge needs to be comprehensively protected to increase β‐diversity.

Multiple abiotic and biotic drivers of aboveground biomass shift with forest stratum

Insights into the underlying ecological mechanisms for diversity-biomass relationships across forest strata (i.e., overstorey and understorey) are crucial to understand the importance of vertical stratification on ecosystem function in natural forests. Yet, it remains unclear how multiple abiotic (i.e., soil nutrients) and biotic (i.e., biodiversity, functional identity and stand structural complexity) factors simultaneously determine aboveground biomass in each individual forest stratum and whole-community. To address this knowledge gap, we disentangled the relative effects of soil nutrients (soil fertility hypothesis), taxonomic, functional trait and evolutionary diversity (niche complementarity hypothesis), stand structural complexity (niche differentiation hypothesis based on tree sizes), and community-weighted mean trait values (mass ratio hypothesis) on aboveground biomass across forest strata and whole-community. We used forest inventory, functional traits and environmental factors datasets from 125 subtropical forest plots in Eastern China. Multiple linear regression models were performed for the selection of best predictors within each biotic group, and structural equation modelling was used to evaluating how multiple abiotic and biotic drivers determine aboveground biomass. In the overstorey, aboveground biomass was positively related to the community-weighted mean of tree height (i.e., functional dominance) and stand density but was negatively related to functional evenness on nutrient-rich soils. In the understorey, aboveground biomass was positively related to phylogenetic species richness and stand-level tree mean diameter (i.e., a proxy for forest growth) but was negatively related to Shannon’s species diversity on nutrient-poor soils. Understorey aboveground biomass was also determined by overstorey stand structure and functional dominance through direct and indirect effects via understorey biotic drivers. These results suggest that functional dominance and stand structural complexity are the main biotic drivers of overstorey aboveground biomass. Whereas, functional dominance and stand structural complexity of overstorey, soil nutrients, and niche complementarity among understorey species with the conservative strategy determine understorey aboveground biomass. Whole-community aboveground biomass might be resulting from the superior role of overstorey functional dominance. This study highlights that the niche complementarity, mass ratio and soil nutrients effects are important for driving aboveground biomass, but in different ways across overstorey and understorey strata in natural subtropical forests. We argue that the relative effect of biodiversity loss on aboveground biomass depends critically on the functional and evolutionary identity of the lost species in the specific forest stratum. Better insights can be gained into forest management and biodiversity conservation by considering overstorey and understorey strata separately in complex natural forests.

Patterns of coexistence of six anemonefish species around subtropical Okinawa-jima Island, Japan

The species diversity of anemonefish in tropical regions has been explained by the niche differentiation hypothesis or cohabitation hypothesis. However, anemonefish community structure has seldom been investigated in high-latitude areas. To understand the mechanism underlying species diversity of anemonefish in the northern limits of their distribution and to compare with previous results from tropical and other areas, we investigated the community structure of anemonefish around the western coast of Okinawa-jima Island, Japan. Seven species of host anemones and six species of anemonefish were distributed across the 12 study sites. The diversity index of anemonefish and the host anemone varied among sites; anemonefish diversity increased with diversity of host anemones, which was associated with the ratio of sandy areas and distance from shore. The density of each species of anemonefish increased with the density of the primary host anemone, which suggests that niche differentiation may be occurring. However, results of two species of host anemone (Stichodactyla mertensii, Heteractis magnifica) associated each with two species of anemonefish did not support the niche differentiation hypothesis. Forty-seven percent of S. mertensii were shared by Amphiprion clarkii and A. sandaracinos simultaneously, supporting the cohabitation hypothesis. Amphiprion ocellaris and A. perideraion used the same host H. magnifica, but never shared a host simultaneously. Moreover, anemone size and water depth did not differ between the two anemonefish species, suggesting the possibility of the lottery model. In subtropical Okinawa, 86.7% of species interactions supported the niche differentiation hypothesis, 6.7% supported the cohabitation hypothesis, and 6.7% supported the lottery hypothesis. The mechanisms of coexistence of anemonefish in subtropical regions include not only niche differentiation and cohabitation but also the lottery hypothesis, which has not been reported in tropical regions.

The prevalence of species-habitat association is not adequate for justifying the niche differentiation hypothesis

The hypothesis of niche differentiation with respect to resources is considered to be one of the most influential explanations for the maintenance of species diversity. The hypothesis has been examined extensively by testing its prediction of species-habitat association, which posits that the spatial distribution of species is highly correlated with environmental variables. However, we argue that widespread evidence of the species-habitat association lacks adequate rigor to justify the niche differentiation hypothesis. In this study, we tested whether and to what extent the observed species-habitat association could be caused by ecological processes other than niche differentiation, in a 20-ha subtropical forest plot. The niche differentiation hypothesis was evaluated by testing the species-habitat association and performing a cross-evaluation of the habitat-diversity expectation, which posits that a strong positive correlation exists between species diversity and habitat complexity. Failure to support the habitat-diversity expectation would at a minimum indicate that the niche differentiation hypothesis might not be the main underlying process of species distribution, despite prevalence of the species-habitat association in the same plot. Our analysis revealed that distributions of most species (86.11%) in the plot were significantly associated with at least one of eight topographical and soil nutrient variables. However, there was almost no significant positive correlation between species diversity and habitat complexity at various spatial scales in the same plot. The results indicate that additional caution is warranted when interpreting the species-habitat association from the niche differentiation perspective. A significant species-habitat association indicates only a species’ habitat preference. The association may reveal nothing about interspecific differences in habitat preference, which is a requirement of the niche differentiation hypothesis.

Seeing the forest from drones: Testing the potential of lightweight drones as a tool for long-term forest monitoring

Long-term ecological monitoring has contributed substantially towards advancements in theoretical and applied ecology. However, the costs to maintain a long-term monitoring site are enormous. Lightweight unmanned aerial vehicles (UAVs or drones) have been rapidly emerging as a new tool for local-scale monitoring. To evaluate the value of drone applications in long-term ecological studies, we combined drone-derived canopy variables, detailed ground-based stem-mapping data and topographic and edaphic variables from a 20-ha forest dynamics plot in a species-rich subtropical forest. Specifically, we evaluated the relative importance of these variables in explaining local-scale variation in forest stand and species measures. We found that drone-derived canopy variables contributed substantially towards explaining local patterns of biodiversity and more specifically in supporting a gap dynamics hypothesis in structuring observed forest biodiversity. Stand basal area was positively related with canopy closure, indicating the importance of protecting old-growth forests as carbon sinks. The importance of topographic and edaphic variables was also demonstrated, supporting a niche differentiation hypothesis in structuring patterns in biodiversity. Species-level analyses illustrated that light-demanding species were more strongly correlated with canopy variables than shade-tolerant species. We provide convincing evidence that drones can add substantial value to long-term ecological monitoring by providing low cost, high resolution data. Drones should be included in the ecologist's toolbox to complement traditional field surveys.

The Contribution of Rare Species to Community Phylogenetic Diversity across a Global Network of Forest Plots

Niche differentiation has been proposed as an explanation for rarity in species assemblages. To test this hypothesis requires quantifying the ecological similarity of species. This similarity can potentially be estimated by using phylogenetic relatedness. In this study, we predicted that if niche differentiation does explain the co-occurrence of rare and common species, then rare species should contribute greatly to the overall community phylogenetic diversity (PD), abundance will have phylogenetic signal, and common and rare species will be phylogenetically dissimilar. We tested these predictions by developing a novel method that integrates species rank abundance distributions with phylogenetic trees and trend analyses, to examine the relative contribution of individual species to the overall community PD. We then supplement this approach with analyses of phylogenetic signal in abundances and measures of phylogenetic similarity within and between rare and common species groups. We applied this analytical approach to 15 long-term temperate and tropical forest dynamics plots from around the world. We show that the niche differentiation hypothesis is supported in six of the nine gap-dominated forests but is rejected in the six disturbance-dominated and three gap-dominated forests. We also show that the three metrics utilized in this study each provide unique but corroborating information regarding the phylogenetic distribution of rarity in communities.

Overlap in soil water sources of savanna woody seedlings and grasses

ABSTRACTSeasonal availability of water is a key controlling factor in semi‐arid savanna vegetation structure, function and interactions. Understanding of woody plant interactions with grasses in savannas has long been underpinned by Walter's two‐layered niche differentiation hypothesis that postulates that grasses and trees source water from different depths. The Walter hypothesis persists in the literature, despite contrary evidence and a lack of quantitative empirical tests of the theory. We conducted a greenhouse experiment to determine the following: (1) whether tree seedlings and grasses obtain water from different depths on rocky and sandy soils; (2) whether interspecific competition affected tissue water content of clipped grasses; and (3) the influence of repeated grass clipping on soil moisture. Grass competition significantly reduced tree seedling rooting depth on both rocky and sandy substrates. Trees had significantly longer roots on rocky substrates than on sandy substrates for all combinations (trees only, trees with unclipped grasses and trees with clipped grasses). Results indicated a three‐tier soil moisture depletion pattern, with a top layer (15 cm) exclusively exploited by grasses, an intermediate zone (25–35 cm) utilised by both grass and tree seedling roots and deeper subsoil exclusively tapped by tree seedling roots. Our results are consistent with Walter's hypothesis, but we distinguished between three rather than two layers of tree and grass root interactions in acquiring soil moisture. Copyright © 2012 John Wiley & Sons, Ltd.

Niche differentiation of two sympatric species of Microdochium colonizing the roots of common reed.

BackgroundFungal endophyte communities are often comprised of many species colonizing the same host. However, little is known about the causes of this diversity. On the one hand, the apparent coexistence of closely related species may be explained by the traditional niche differentiation hypothesis, which suggests that abiotic and/or biotic factors mediate partitioning. For endophytes, such factors are difficult to identify, and are therefore in most cases unknown. On the other hand, there is the neutral hypothesis, which suggests that stochastic factors may explain high species diversity. There is a need to investigate to what extent each of these hypotheses may apply to endophytes.ResultsThe niche partitioning of two closely related fungal endophytes, Microdochium bolleyi and M. phragmitis, colonizing Phragmites australis, was investigated. The occurrences of each species were assessed using specific nested-PCR assays for 251 field samples of common reed from Lake Constance, Germany. These analyses revealed niche preferences for both fungi. From three niche factors assessed, i.e. host habitat, host organ and season, host habitat significantly differentiated the two species. M. bolleyi preferred dry habitats, whereas M. phragmitis prevailed in flooded habitats. In contrast, both species exhibited a significant preference for the same host organ, i.e. roots. Likewise the third factor, season, did not significantly distinguish the two species. Differences in carbon utilization and growth temperature could not conclusively explain the niches. The inclusion of three unrelated species of Ascomycota, which also colonize P. australis at the same locations, indicated spatio-temporal niche partitioning between all fungi. None of the species exhibited the same preferences for all three factors, i.e. host habitat, host organ, and time of the season.ConclusionsThe fungal species colonizing common reed investigated in this study seem to exploit niche differences leading to a separation in space and time, which may allow for their coexistence on the same host. A purely neutral model is unlikely to explain the coexistence of closely related endophytes on common reed.

Disproportionate bill length dimorphism and niche differentiation in wintering western sandpipers (Calidris mauri)

Western sandpipers ( Calidris mauri (Cabanis, 1857)) exhibit slight female-biased sexual size dimorphism (5%) but disproportionate bill length dimorphism (15.9%). We test two predictions of the niche differentiation hypothesis at two wintering sites in Mexico with uniform western sandpiper densities, and use sex ratio as an index of intersexual competition. First, to test whether bill length dimorphism is larger at sites where sex ratios are strongly male-biased, we develop a migrant-based null model to represent dimorphism (12%, based on the average of males and females) in the absence of competition. Relative to the null model, bill length dimorphism was significantly larger at the large site (Santa María: 13.4%) but not at the small site (Punta Banda: 12.7%). Second, we tested whether bill length dimorphism increases as sex ratio approaches 1:1. Although the sex-ratio difference between sites was only 5%, bill length dimorphism increased marginally in the predicted direction. Additional comparisons suggest a cline in bill length dimorphism that mirrors a latitudinal gradient in prey burial depth. While sexual size dimorphism in the western sandpiper likely derived from selection for different body size optima, intersexual competition for food on the wintering grounds appears to have promoted further divergence in bill length.

Habitat differentiation of sympatric Geonoma macrostachys (Arecaceae) varieties in Peruvian lowland forests

Among the hypotheses invoked to explain high species richness in tropical forests, the niche differentiation hypothesis has received observational and experimental support (Ashton 1969, Chesson 2000, Clark et al. 1999, Souza & Martins 2004, Svenning 2001, Terborgh & Mathews 1999). Habitat specialization with regard to edaphic factors and topography has been observed in several plant groups including trees, lianas, shrubs, ferns and palms (Clark et al. 1999, Ibarra-Manriquez & Martinez-Ramos 2002, Svenning 1999, Tuomisto & Ruokolainen 1993). Treefall gaps and light gradients have also been found to be important ecological factors affecting plant distribution and niche differentiation in the understory (Chazdon 1986, Poorter & Arets 2003, Terborgh & Mathews 1999). It is suggested that such habitat heterogeneity may not only maintain biodiversity but also may lead to the origin of new species through the process of parapatric speciation (Gentry 1989, Haffer 1997, Patton & Smith 1992).

GRASSROOTS ECOLOGY: PLANT–MICROBE–SOIL INTERACTIONS AS DRIVERS OF PLANT COMMUNITY STRUCTURE AND DYNAMICS

A growing body of research on plant-microbe interactions in soil is con- tributing to the development of a new, microbially based perspective on plant community ecology. Soil-dwelling microorganisms are diverse, and interactions with plants vary with respect to specificity, environmental heterogeneity, and fitness impact. Two microbial pro- cesses that may exert key influences on plant community structure and dynamics are mi- crobial mediation of niche differentiation in resource use and feedback dynamics between the plant and soil community. The niche differentiation hypothesis is based on observations that soil nutrients occur in different chemical forms, that different enzymes are required for plant access to these nutrients, and that soil microorganisms are a major source of these enzymes. We predict that plant nutrient partitioning arises from differential associations of plant species with microbes able to access different nutrient pools. Feedback dynamics result from changes in the soil community generated by the specificity of response in plant- microbe interactions. We suggest that positive feedback between plants and soil microbes plays a central role in early successional communities, while negative feedback contributes both to species replacements and to diversification in later successional communities. We further suggest that plant-microbe interactions in the soil are an important organizing force for large-scale spatial gradients in species richness. The relative balance of positive feedback (a homogenizing force) and negative feedback (a diversifying force) may contribute to observed latitudinal (and altitudinal) diversity patterns. Empirical tests of these ideas are needed, but a microbially based perspective for plant ecology promises to contribute to our understanding of long-standing issues in ecology, and to reveal new areas of future research.

Sexual dimorphism and trophic constraints: Prey selection in the European polecat (Mustela putorius)

Although widespread among mammals, sexual dimorphism raises several evolutionary and ecological issues. Despite strong sexual dimorphism (reaching the ratio 1.81), a study of diet and prey selection in polecats (Mustela putorius) revealed only minor differences in their feeding habits. There was a greater frequency of large-sized prey (mainly lagomorphs) in the summer diet of females than in that of males. The frequency of anurans (Rana dalmatina and Bufo bufo) in the diet did not differ significantly between the two sexes. Male prey predominated in the diet of both sexes. Although prey availability, as indicated by the trapping of small rodents and anurans, showed a predominance of males in populations, Ivlev's index for selectivity demonstrated selective predation on male prey exceeding availability both by male and female polecats. This selective predation by polecats may affect both population structure and population exchanges. My results suggest that sexual dimorphism of polecats was not linked to a different prey choice but results from independent intrasexual selective pressures, thus refuting the predictions of the trophic niche differentiation hypothesis. The wide size dimorphism reflects selection both for mating access in males and for food in females, illustrating the complementary influence of sexual selection and environmental constraints on sex divergence in growth.

Vertical niche differentiation of ectomycorrhizal hyphae in soil as shown by T-RFLP analysis.

• Niche differentiation for different soil substrates has been proposed as a mechanism contributing to ectomycorrhizal fungal diversity. This hypothesis has been largely untestable because of a lack of techniques to study the in situ distribution of ectomycorrhizal hyphae. • We developed a technique involving soil DNA extraction, PCR and terminal restriction fragment length polymorphism (T-RFLP) analysis for species identification to investigate the vertical distribution of fungal hyphae in four distinct layers of the forest floor (lower litter, F-layer, H-layer, and B-horizon) of a Pinus resinosa plantation. • Fungal communities differed markedly among the four layers. Cluster analysis suggested six different patterns of resource utilization: litter-layer specialists, litter-layer generalists, F-layer, H-layer, and B-horizon species, and multilayer generalists. Known ectomycorrhizal species were found in all six clusters. • This spatial partitioning observed among ectomycorrhizal fungi along a single, relatively simple substrate-resource gradient supports the niche differentiation hypothesis as an important mechanism contributing to ectomycorrhizal fungal diversity.

Intersexual niche differentiation in the blue tit (Parus caeruleus)

The intersexual niche differentiation hypothesis, according to which sexual size dimorphism (SSD) has evolved to facilitate niche separation between the sexes, has received equivocal support in studies of birds. One explanation for the inconsistency of the results is that other factors crucial for reproductive success, such as variation in food abundance among territories, may obscure food niche differentiation between parents. We studied the effects of SSD, food availability and food demands of broods on niche differentiation in the blue tit (Parus caeruleus). Parents attending experimentally enlarged broods delivered the same amount, but more diverse food items, to their offspring than parents attending reduced broods, although this effect was significant in only one of the two study years. The time-lag (deviation from the peak food abundance in habitat in days) and squared time-lag were the most important determinants of the diversity of food items delivered to young and their condition (fitness surrogate). SSD, as measured by principal component scores of morphological traits, did not explain any variation neither in the diversity or amount of food provided, nor in offspring condition. These results suggest that synchronization of breeding with peak food availability is the most important determinant of diversity of food delivered to young and, more importantly, Their condition. No evidence for intersexual food niche differentiation was found; at best, it is likely to constitute only a weak selection pressure on the maintenance or enhancement of SSD in this system.