Regional occupancy in unicellular eukaryotes: a reflection of niche breadth,habitat availability or size‐related dispersal capacity?

1. The distribution patterns of unicellular and multicellular organisms have recently been shown to differ profoundly, with the former probably being mostly cosmopolitan, whereas the latter are mostly restricted to certain regions. However, the within‐region distribution patterns of these two organism groups may be rather similar. 2. We predicted that the degree of regional occupancy in unicellular eukaryotes would be related to niche characteristics, dispersal ability and size, as has been found previously for multicellular organisms. The niche characteristics we considered were niche position, that measures marginality in species habitat distribution, and niche breadth, that measures amplitude in species habitat distribution. Niche characteristics were determined using Outlying Mean Index (OMI) analysis. 3. We found that the regional occupancy in our model group of unicellular eukaryotes, stream diatoms, was primarily a reflection of the niche position of a species or, more generally, habitat availability. Thus, non‐marginal species (i.e. species that occupied common habitat conditions across the region) tended to be more widely distributed than marginal species (i.e. species that were restricted to a limited range of rare habitat conditions). This finding was further supported by the general linear model, with niche position, niche breadth, maximum size and attachment mode as explanatory variables: niche position was by far the most important variable accounting for variability in regional occupancy, with significant amounts of additional variation related to niche breadth and maximum size of diatoms. 4. Thus, the degree of regional occupancy among unicellular eukaryotes may be primarily governed by habitat availability, supporting former findings for multicellular organisms.     

Determinants of local abundance and range size in forest vascular plants

Aim For a large set of forest herbs we tested: (1) whether there is a positive relationship between local abundance and geographical range size; (2) whether abundance or range size are affected by the niche breadths of species or niche availability; and (3) whether these are affected by the species life‐history traits. Location Northwestern Germany. Methods We measured abundance as mean density in 22 base‐rich deciduous forests and recorded range size as area of occupancy on four different spatial scales (local to national). Niche breadth was expressed in terms of habitat specificity (specialists, generalists) and of the ability to grow across a broad range of soil pH. The species’ pH niche position was used as a measure of the importance of habitat availability. As life‐history traits we used diaspore mass and number, plant height, seed longevity, lifespan/clonality, pollination mode, dispersal capability and flowering time. Results There were mainly no positive relationships between the abundance of species and their range size, as tested across species and across phylogenetically independent contrasts. Forest specialists were generally distributed less widely than generalists, but habitat specificity was not related to local abundance. Species with a broader pH niche breadth were more common, but the positive relationships between niche breadth and abundance or range size disappeared when accounting for sample size effects. Clonal species with few and heavy diaspores were most abundant, as well as early‐flowering species and those lacking dispersal structures. Local and regional range size were determined largely by habitat availability, while national range was positively affected by plant height and diaspore mass. Main conclusions Different processes determine the local density of species and their range size. Abundance within habitat patches appears to be related mainly to the species life histories, especially to their capacity for extensive clonal reproduction, whereas range size appears to be determined strongly by the availability of suitable habitat.     

Relationships between local population persistence, local abundance and regional occupancy of species: distribution patterns of diatoms in boreal streams

Aims We have two aims: (1) to examine the relationship between local population persistence, local abundance and regional occupancy of stream diatoms and (2) to characterize the form of the species–occupancy frequency distribution of stream diatoms. Location Boreal streams in Finland. There were three spatial extents: (1) across ecoregions in Finland, (2) within ecoregions in Finland, and (3) within a single drainage system in southern Finland. Methods Diatoms were sampled from stones (epilithon), sediment (epipelon) and aquatic plants (epiphyton) in streams using standardized sampling methods. To assess population persistence, diatom sampling was conducted monthly at four stream sites from June to October. The relationships between local population persistence, local abundance and regional occupancy were examined using correlation analyses. Results There was a significant positive relationship between local persistence and abundance of diatoms in epilithon, epipelon and epiphyton. Furthermore, local abundance and regional occupancy showed a significant positive relationship at multiple spatial extents; that is, across ecoregions, within ecoregions and within a drainage system. The relationships between occupancy and abundance did not differ appreciably among impacted and near pristine‐reference sites. The occupancy–frequency distribution was characterized by a large number of satellite species which occurred at only a few sites, whereas core species that occurred at most sites were virtually absent. Main conclusions The positive relationship between local population persistence and abundance suggested that a high local abundance may prevent local extinction or that high persistence is facilitated by a high local cell density. High local persistence and local abundance may also positively affect the degree of regional occupancy in stream diatoms. The results further showed that anthropogenic effects were probably too weak to bias the relationship between occupancy and abundance, or that the effects have already modified the distribution patterns of stream diatoms. The small number of core species in the species–occupancy frequency distribution suggested that the regional distribution patterns of stream diatoms, or perhaps unicellular microbial organisms in general, may not be fundamentally different from those described previously for multicellular organisms, mainly in terrestrial environments, although average global range sizes may differ sharply between these two broad groups of organisms.     

Phylogenies and traits provide distinct insights about the historical and contemporary assembly of aquatic insect communities

The assumption that traits and phylogenies can be used as proxies of species niche has faced criticisms. Evidence suggested that phylogenic relatedness is a weak proxy of trait similarity. Moreover, different processes can select different traits, giving opposing signals in null model analyses. To circumvent these criticisms, we separated traits of stream insects based on the concept of α and β niches, which should give clues about assembling pressures expected to act independently of each other. We investigated the congruence between the phylogenetic structure and trait structure of communities using all available traits and all possible combinations of traits (4095 combinations). To account for hierarchical assembling processes, we analyzed patterns on two spatial scales with three pools of genera. Beta niche traits selected a priori – i.e., traits related to environmental variation (e.g., respiration type) – were consistently clustered on the smaller scale, suggesting environmental filtering, while α niche traits – i.e., traits related to resource use (e.g., trophic position) – did not display the expected overdispersion, suggesting a weak role of competition. Using all traits together provided random patterns and the analysis of all possible combinations of traits provided scenarios ranging from strong clustering to overdispersion. Communities were phylogenetically overdispersed, a pattern previously interpreted as phylogenetic limiting similarity. However, our results likely reflect the co‐occurrence of ancient clades due to the stability of stream habitats along the evolutionary scale. We advise ecologists to avoid using combinations of all available traits but rather carefully traits based on the objective under consideration. Both trait and phylogenetic approaches should be kept in the ecologist toolbox, but phylogenetic distances should not be used as proxies of traits differences. Although the phylogenetic structure revealed processes operating at the evolutionary scale, only specific traits explained local processes operating in our communities.     

Ecological correlates of geographical range occupancy in North American birds

Aim  The degree to which a species is predictably encountered within its range varies tremendously across species. Understanding why some species occur less frequently within their range than others has important consequences for conservation and for analyses of ecological patterns based on range maps. We examined whether patterns in geographical range occupancy can be explained by species-level traits.
Location  North America.
Methods  We used survey data from 1993 to 2002 from the North American Breeding Bird Survey along with digital range maps produced by NatureServe to calculate range occupancy for 298 species of terrestrial birds. We tested whether species traits explained variation in range occupancy values using linear regression techniques.
Results  We found three species traits that together explained more than half of the variation in range occupancy. Population density and niche breadth were positively correlated with occupancy, while niche position was negatively correlated with occupancy.
Main conclusions  Our results suggest that high range occupancy will occur in species that are common at sites on which they occur, that tolerate a relatively wide range of ecological conditions and that tend to have ranges centred on areas with common environmental conditions. Furthermore, it appears that niche-based characteristics may explain patterns of distribution and abundance from local habitats up to the scale of geographical ranges.     

Relationships between distribution and abundance vary with spatial scale and ecological group in stream bryophytes

1. We examined whether the local abundance of stream bryophytes in a boreal drainage basin (Koutajoki system in northeastern Finland) correlated with their: (i) regional occupancy; (ii) provincial distribution in northwestern Europe; and (iii) global range size. We specifically tested whether aquatic and semi‐aquatic species differ in their distribution–abundance relationships. We also analysed the frequency distributions of occupancy at two spatial scales: within the focal drainage system and across provinces of northwestern Europe. 2. Regional occupancy and mean local abundance of stream bryophytes were positively correlated, and the relationship was rather strong in aquatic species but very weak in semi‐aquatic species. Local abundance was related neither to provincial distribution nor global distribution. 3. Species frequency distributions differed between regional occupancy and provincial distribution. While most species were rare with regard to their regional occupancy within the focal drainage system, most of the same set of species were common and occurred in most provinces in northwestern Europe. 4. The results indicate the presence of dominants (core species) and transients/subordinates (satellite species) among stream bryophytes, highlighting marked differentiation in life‐history strategies and growth form. The observed abundance–occupancy relationships suggest that dispersal limitation and metapopulation processes may govern the dynamics of obligatory aquatic stream bryophytes. In semi‐aquatic species, however, habitat availability may be more important in contributing to regional occupancy.     

Tuned in: plant roots use sound to locate water

It was recently suggested that beta diversity can be partitioned into contributions of single sites to overall beta diversity (LCBD) or into contributions of individual species to overall beta diversity (SCBD). We explored the relationships of LCBD and SCBD to site and species characteristics, respectively, in stream insect assemblages. We found that LCBD was mostly explained by variation in species richness, with a negative relationship being detected. SCBD was strongly related to various species characteristics, such as occupancy, abundance, niche position and niche breadth, but was only weakly related to biological traits of species. In particular, occupancy and its quadratic terms showed a very strong unimodal relationship with SCBD, suggesting that intermediate species in terms of site occupancy contribute most to beta diversity. Our findings of unravelling the contributions of sites or species to overall beta diversity are of high importance to community ecology, conservation and bioassessment using stream insect assemblages, and may bear some overall generalities to be found in other organism groups.     

Exploring stream communities in a tropical biodiversity hotspot: biodiversity,regional occupancy,niche characteristics and environmental correlates

Exploring and describing biodiversity and the mechanisms structuring it is fundamental to advancing ecology. This is particularly pertinent in understudied biogeographical regions, such as the Afrotropics, that are characterised by strong seasonal climatic shifts. We investigated the characteristics of stream biodiversity in the Niger Delta region of Nigeria, a tropical biodiversity hotspot, by examining patterns in 20 stream invertebrate communities across both the wet and dry seasons. For this, we took a multi-faceted approach accounting for the three levels of biodiversity (α, β and γ), including partitioning the nestedness and turnover components of β diversity, regional occupancy-abundance patterns, niche characteristics, and the environmental drivers of community structure. α diversity was low in these streams, with strong turnover between sites leading to high β diversity contributing to regional biodiversity, but there was little variation in communities between seasons. The proportion of sites occupied by taxa declined with increasing niche position, and decreasing niche breadth. Occupancy was predicted well by a combination of these two factors (niche position and breadth), but not mean local abundance, as the abundance-occupancy link was an upper-limit unimodal relationship. On average, community structure was linked more strongly to environmental variables in the wet season. Our findings demonstrate the clear role of spatial, but not temporal, turnover in assemblages, which likely reflects the environmental heterogeneity of this region. This is further supported by the fact that regional occupancy was mostly related to niche characteristics, particularly niche position. We emphasise the importance of continued basic and applied ecological work in this important biogeographic region to enable better protection of its biodiversity.     

Control of stream insect assemblages: roles of spatial configuration and local environmental factors

Abstract 1. Current views in ecology emphasise that community structure is the sum of multiple processes, with imprints of both regional and local drivers. However, the degree to which stream insect assemblages are structured by spatial configuration (complying with the dispersal‐based neutral hypothesis) and local environmental features (complying with the niche‐based species sorting hypothesis) has not been rigorously examined based on surveys in multiple years. 2. Stream sites in a boreal drainage system were surveyed during three consecutive years and the relative contribution of spatial configuration and local environmental variables to aquatic insect assemblage structure (characterised by both abundance and presence–absence data) was assessed. Separate analyses were conducted for mayflies (Ephemeroptera), stoneflies (Plecoptera), caddisflies (Trichoptera), and non‐biting midges (Diptera: Chironomidae) in each year. 3. There were no relationships between the spatial location and local environmental features of streams in Mantel tests, facilitating exploration of their independent effects on assemblage structure. The study found virtually no effects of spatial location on stream insect assemblages across the study drainage system, as evidenced by Mantel tests and canonical correspondence analyses (CCA). The environmental variables were also rather weakly associated with assemblage structure, with the total amount of explained variation ranging from 9.8% to 31.7% in the CCAs. There were no appreciable differences in the amount of environment‐related explained variation in assemblage structure between mayflies, stoneflies, caddisflies, and midges, but some between‐year differences were noticeable in most insect groups. The environmental variables that were significantly related to assemblage structure exhibited some between‐group and between‐year variability. In general, patterns shown by abundance and presence–absence data were highly similar. 4. It appears that stream insect assemblages comply with the niche‐based species sorting hypothesis in the context of metacommunity ecology. In contrast, the absence of spatial structuring suggests that stream insect assemblages do not comply with the neutral hypothesis, being not strongly dispersal limited at the within‐drainage basin scale.     

Trait means or variance—What determines plant species' local and regional occurrence in fragmented dry grasslands?

One of the few laws in ecology is that communities consist of few common and many rare taxa. Functional traits may help to identify the underlying mechanisms of this community pattern, since they correlate with different niche dimensions. However, comprehensive studies are missing that investigate the effects of species mean traits (niche position) and intraspecific trait variability (ITV, niche width) on species abundance. In this study, we investigated fragmented dry grasslands to reveal trait‐occurrence relationships in plants at local and regional scales. We predicted that (a) at the local scale, species occurrence is highest for species with intermediate traits, (b) at the regional scale, habitat specialists have a lower species occurrence than generalists, and thus, traits associated with stress‐tolerance have a negative effect on species occurrence, and (c) ITV increases species occurrence irrespective of the scale. We measured three plant functional traits (SLA = specific leaf area, LDMC = leaf dry matter content, plant height) at 21 local dry grassland communities (10 m × 10 m) and analyzed the effect of these traits and their variation on species occurrence. At the local scale, mean LDMC had a positive effect on species occurrence, indicating that stress‐tolerant species are the most abundant rather than species with intermediate traits (hypothesis 1). We found limited support for lower specialist occurrence at the regional scale (hypothesis 2). Further, ITV of LDMC and plant height had a positive effect on local occurrence supporting hypothesis 3. In contrast, at the regional scale, plants with a higher ITV of plant height were less frequent. We found no evidence that the consideration of phylogenetic relationships in our analyses influenced our findings. In conclusion, both species mean traits (in particular LDMC) and ITV were differently related to species occurrence with respect to spatial scale. Therefore, our study underlines the strong scale‐dependency of trait‐abundance relationships.     

IS TEMPORAL OCCURRENCE OF DIATOMS RELATED TO SPECIES TRAITS,LOCAL ABUNDANCE,AND REGIONAL DISTRIBUTION?1

Water quality in streams typically changes fast, and sensitive biological indicators are crucial for monitoring these changes in water quality. Diatoms are widely used in biological stream quality assessment. However, interannual and intra‐annual variation of diatom population densities is large, often hampering a reliable quality assessment. We studied the importance of different species traits on temporal occurrence of diatom species. We also examined whether temporal occurrence of diatoms is related to species' local abundance or regional distribution using a data set collected in Finnish streams. According to the general linear model (GLM), temporal occurrence of diatoms increased with increasing local abundance and regional distribution. Species that occurred more frequently also had larger niche breadths and nonmarginal niche positions. In addition, cell size and attachment ability were positively related to species temporal occurrence. Our results imply that abundant and widely distributed species with large niches and ability to attach sustain persistent populations in varying environmental conditions typical for streams. We suggest that future studies could concentrate on monitoring these common (abundant) species when detecting the possible changes in the biological state of streams. We advise, however, to consider a relatively large number of species as many of the most common species may have low indicator values.     

The roles of landscape context, niche breadth, and range boundaries in predicting species responses to habitat alteration

Extant species in human‐dominated landscapes differ in their sensitivity to habitat loss and fragmentation, although extinctions induced by environmental alteration reduce variation and result in a surviving subset of species with some degree of ‘resistance’. Here, we test the degree to which variable responses to habitat alteration are (1) essentially an inherent property of a taxon subject to constraints imposed by its geographical range, as suggested by Swihart et al. (2003), (2) a function of the landscape in which a species occurs, or (3) a function of spatial trends occurring on large scales. We used data collected on 33 vertebrate species during 2001–04 across the upper Wabash River basin, Indiana, in 35 square ‘landscapes’, each 23 km² in size. Six species of forest rodent, six species of grassland rodents, seven species of bats, eight species of aquatic turtles, and six species of amphibians were sampled at 504, 212, 590, 228, and 625 patches, respectively. The fraction of patches of primary habitat (e.g. forests for tree squirrels, wetlands for aquatic turtles) occupied by a target species was used as a response variable. On a basin‐wide scale, 47% of variation in proportional occupancy among species could be explained by taxon‐specific variables; occupancy rates were related positively to niche breadth and negatively to the proximity of a geographical range boundary. After controlling for species effects, landscape‐level occupancy rates varied significantly for 16 of 33 species, with variation partitioned among landscape variables alone (mean = 11% of variation), spatial trend variables alone (26%), and both variable sets jointly (8%). Among landscape variables, percentage forest cover positively affected occupancy rates of three bat species and a tree squirrel. Variation in occupancy rates among landscapes was consistent with large‐scale spatial trends for 13 species. Our findings demonstrate the general importance of niche breadth as a predictor of species responses to habitat alteration and highlight the importance of viewing the effects of habitat loss and fragmentation at multiple spatial scales.     

Disturbance‐driven variation in abundance–occupancy relationships over time in tropical forest fragments

A central problem in ecology is to understand spatial–temporal variation in abundance–occupancy relationship (AOR) and to identify the biological and anthropogenic drivers behind this variation. How AOR is influenced by ecological traits and anthropogenic disturbances is poorly understood. A data set of woody plants from eleven forest fragments around Kampala area, Uganda, recorded in 1990 and resampled in 2010 was used to analyse spatial variation in AORs at regional scales, variation between tree size classes and temporal change in these patterns. Slopes of the AORs for each forest were related to data on forest disturbances and effects of species traits on AOR at regional scales assessed. There were statistically significant positive interspecific AORs at regional scales. Over two decades, the strength of AOR and slopes increased at the regional scale. At local scale, slopes decreased, in correspondence with increasing disturbance. Species traits interacted with abundance to explain statistically significant variation in occupancy. Species successional status best explained occupancy variability. Incorporating species' traits and anthropogenic disturbance over time may lead to better understanding of the variation in interspecific AOR, and these results suggest that anthropogenic exploitation could be responsible for the changes in slope over time.     

Niche breadth,rarity and ecological characteristics within a regional flora spanning large environmental gradients

Aim Species specialization, which plays a fundamental role in niche differentiation and species coexistence, is a key biological trait in relation to the responses of populations to changing environments. Species with a limited niche breadth are considered to experience a higher risk of extinction than generalist species. This work aims to measure the degree of specialization in the regional flora of the French Alps and test whether species specialization is related to species rarity and ecological characteristics. Location This study was conducted in the French Alps region, which encompasses a large elevational gradient over a relatively limited area (26,000 km²). Methods Specialization was estimated for approximately 1200 plant species found in the region. Given the inherent difficulty of pinpointing the critical environmental niche axes for each individual species, we used a co‐occurrence‐based index to estimate species niche breadths (specialization index). This comprehensive measurement included crucial undetermined limiting niche factors, acting on both local and regional scales, and related to both biotic and abiotic interactions. The specialization index for each species was then related to a selection of plant typologies such as Grime strategies and Raunkiaer life‐forms, and to two measurements of plant rarity, namely regional area of occupancy and local abundance. Results Specialist species were mainly found in specific and harsh environments such as wetlands, cold alpine habitats and dry heathlands. These species were usually geographically restricted but relatively dominant in their local communities. Although none of the selected traits were sufficient predictors of specialization, pure competitors were over‐represented amongst generalist species, whereas stress‐tolerant species tended to be more specialized. Main conclusions Our results suggest that co‐occurrence‐based indices of niche breadth are a satisfactory method for inferring plant specialization using large species samples across very heterogeneous environments. Our results are an empirical validation of the tolerance–dominance trade‐off and also provide interesting insights into the long‐standing question of which biological properties characterize species with narrow niche breadth that are potentially threatened by global changes in the environment.     

Temporal variability of nestedness and idiosyncratic species in stream insect assemblages

Aims The nested subset pattern has been widely studied in the last 20 years, and recent syntheses have challenged the prevalence of this pattern in nature. We examined the degree of nestedness, its temporal variability and its environmental correlates in stream insects of a boreal drainage system. We also examined differences between nested and idiosyncratic species in site occupancy, niche position and niche breadth. Location Koutajoki drainage basin in northern Finland. Methods We used (i) nestedness analyses with three null models for testing the significance of nestedness; (ii) Spearman rank correlation to examine the correlates of nestedness; (iii) outlying mean index analysis to analyse the niche characteristics of species; (iv) and t‐test to examine differences in niche breadth, niche position and site occupancy of idiosyncratic and other nested species. Results Stream insect assemblages were significantly nested in each of the three study years. The maximally packed matrices were significantly nested according to the nestedness calculator based on null models I (species frequencies and site richness equiprobable) and II (species frequencies fixed and site richness equiprobable), but non‐significant based on a conservative null model III (species frequencies and site richness fixed to those of the observed matrix). The most important correlate of nestedness was stream size, whereas isolation, productivity (total phosphorus) and habitat heterogeneity exhibited non‐significant relationship with nestedness. Idiosyncratic species occurred, on average, at more sites than nested species, mirroring the restricted distributions of several nested species that were inclined towards species‐rich sites. Idiosyncratic and nested species also differed in niche position and niche breadth, with idiosyncratic species having, on average, less marginal niche positions and wider niches than nested species. Main conclusions Stream size correlated with nestedness, possibly because small streams were inhabited only by species able to persist under, or colonize shortly after, disturbances, while most species could occur at larger sites where disturbances are less severe. From the conservation perspective, our findings suggest that stream size really matters, given that sites with high species richness and many rare species are more likely to occur in larger streams. However, also the requirements of idiosyncratic species should be accommodated in conservation planning.     

Dynamics of distribution in animal communities: Theory and analysis

Theoretical and analytical problems of the dynamics of distribution and abundance in animal communities were examined. In many communities, species with low abundance and of limited spatial occurrence (i.e., rare species) typically form a conspicuous peak when a frequency distribution of the number of species is constructed with respect to the proportion of sites occupied within an area of distribution. Models of distribution dynamics, including a new model proposed here, were compared with a range of animal community data using a new procedure to assess single- and bi-modal patterns in frequency distributions of spatial occurrence. Data reveal that single-modality with an excess of rare species occurs more frequently than bimodality. Even when bimodality is detected, the mode representing wide-spread species is in the majority of cases smaller than that for rare species. Thus, a new model in which the rate of local extinctions is assumed to be negatively related to patch occupancy (or population abundance) is in better agreement with observed data than earlier models. Some problems of analysis, in particular model assumptions and testing, are discussed.     

The trait and host plant ecology of aphids and their distribution and abundance in the United Kingdom

Aim We characterized the annual populations of 170 aphid species by their log abundance, site occupancy and site continuity (i.e. the persistence of species in time) and used this information to make predictions about groups of species that displayed characteristic patterns. By doing so, we aimed to identify commonalities in functional traits (host‐alternation; mode of reproduction; life‐cycle plasticity; median body size) and host plant geographic range sizes that may indicate why some species are common and others not. Location The population dynamics of winged aphids at 27 locations in the United Kingdom were studied. Methods The annual numbers of aphids were studied using a dataset comprising over 11 million individuals across 509 site‐years. Traits and host plants were analysed using linear mixed effects models and nonlinear regression models. Results Linear mixed effects models showed that the fixed effects of host alternation and winter host plant area of occupancy were important in predicting log abundance, site occupancy and site continuity. Life‐cycle plasticity was also a significant effect, although not for log abundance. Relationships between site continuity, site occupancy and log abundance were strongly nonlinear. Site continuity always lagged site occupancy, indicating that species were less likely to retain previously occupied sites when abundances were low. Main conclusions Aphid traits are a better paradigm than taxonomic relatedness in explaining macroecological patterns. Host alternation induces an annual flux of migrants that engenders higher annual log abundances, consistent with the theory that species with high local densities tend to confer a much wider distribution than those with low densities. The abundance of aphids is monotonically related to the geographic range size of their winter host, suggestive of a strong bottom‐up effect (i.e. resource controlled). The areas of occupancy (AOO) of winter host plants constrain aphids to low abundances because a greater proportion of hosts are trees and shrubs which tend to have smaller AOO than herbs and grasses, the common summer host types.     

Traits help explain species' performance away from their climate niche centre

Aim

Climate change impacts on biota are variable across sites, among species and throughout individual species' ranges. Niche theory predicts that population performance should decline as site climate becomes increasingly different from the species' climate niche centre, though studies find significant variation from these predictions. Here, we propose that predictions about climate responses can be improved by incorporating species' trait information.

Location

Europe.

Methods

We used observations of plant species abundance change over time to assess variation in climate difference sensitivity (CDS), defined as how species performance (colonization, extinction and abundance change) relates to the difference of site climate from the mean temperature and precipitation of each species' range. We then investigated if leaf economics, plant size and seed mass traits were associated with the species' CDS.

Results

Species that performed better (e.g. increased in abundance) towards sites progressively cooler than their niche centre were shorter and had more resource-acquisitive leaves (i.e. lower leaf dry matter content or LDMC) relative to species with zero or the opposite pattern of temperature difference sensitivity. This result supports the hypothesis that if sites cooler than niche centres are more stressful for a species, then shorter stature is advantageous compared with taller species. The LDMC result suggests the environment selects for more resource-acquisitive leaf strategies towards relatively cooler climates with shorter growing seasons, counter to expectations that conservative strategies would be favoured in such environments. We found few consistent relationships between precipitation difference sensitivities and traits.

Main Conclusions

The results supported key a priori foundations on how trait-based plant strategies dictate species responses to climate variation away from their niche centre. Furthermore, plant height emerged as the most consistent trait that varied with species CDS, suggesting height will be key for theory development around species response to climate change.     

Evidence that niche specialization explains species-energy relationships in lake fish communities

1. Interspecific niche differences have long been identified as a major explanation for the occurrence of species-rich communities. However, much fieldwork studying variation in local species richness has focused upon physical habitat attributes or regional factors, such as the size of the regional species pool. 2. We applied indices of functional diversity and niche overlap to data on the species niche to examine the importance of interspecific niche differentiation for species richness in French lake fish communities. We combined this information with environmental data to test generalizations of the physiological tolerance and niche specialization hypotheses for species-energy relationships. 3. We found evidence for a largely non-saturating relationship (relative to random expectation) between species richness and functional evenness (evenness of spacing between species in niche space), while functional richness (volume of niche space occupied) peaked at moderate levels of species richness and niche overlap showed an initial decrease followed by saturation. This suggests that increased niche specialization may have allowed species to coexist in the most species-rich communities. 4. We tested for evidence that increased temperature, local habitat area, local habitat diversity and immigration affected species richness via increased niche specialization. Temperature explained by far the largest amount of variation in species richness, functional diversity and niche overlap. These results, combined with the largely non-saturating species richness-functional evenness relationship, suggest that increased temperature may have permitted increased species richness by allowing increased niche specialization. 5. These results emphasize the importance of niche differences for species coexistence in species-rich communities, and indicate that the conservation of functional diversity may be vital for the maintenance of species diversity in biological communities. Our approach may be applied readily to many types of community, and at any scale, thus providing a flexible means of testing niche-based hypotheses for species richness gradients.     

Positive relationship between regional distribution and local abundance in stream insects: a consequence of niche breadth or niche position?

A positive relationship between regional distribution and local abundance of species is almost ubiquitous macroecological pattern, yet the mechanisms behind this pattern remain poorly understood. I tested for the relationship between regional distribution and local abundance of stream insect species in a boreal drainage system, with a specific aim to examine if this relationship follows the mechanistic basis of either the niche-based (niche breadth and niche position) or metapopulation models. There was a positive relationship between regional distribution and local abundance of stream insects, and there also were significant relationships between distribution/abundance and niche breadth or niche position. These results thus suggest that widely distributed species tend to be, on average, locally more abundant, have wider niches and lower marginality of niche position with regard to environmental factors than species that have more restricted distributions. However, although significant, there was much unexplained variability around these relationships, suggesting that other mechanisms (e.g. metapopulation dynamics) besides differences in species' niches are likely to affect the distribution and abundance of stream insects, at least within a drainage system. The results thus showed that 1) although niche position was more consistently related to the positive distribution-abundance relationship, ecologists should not abandon niche breadth as a potential mechanism behind this relationship, and 2) that several mechanisms are likely to act in concert in determining the relationship between distribution and abundance of species.