Norman W. H. Mason

NEW MULTIDIMENSIONAL FUNCTIONAL DIVERSITY INDICES FOR A MULTIFACETED FRAMEWORK IN FUNCTIONAL ECOLOGY

Functional diversity is increasingly identified as an important driver of ecosystem functioning. Various indices have been proposed to measure the functional diversity of a community, but there is still no consensus on which are most suitable. Indeed, none of the existing indices meets all the criteria required for general use. The main criteria are that they must be designed to deal with several traits, take into account abundances, and measure all the facets of functional diversity. Here we propose three indices to quantify each facet of functional diversity for a community with species distributed in a multidimensional functional space: functional richness (volume of the functional space occupied by the community), functional evenness (regularity of the distribution of abundance in this volume), and functional divergence (divergence in the distribution of abundance in this volume). Functional richness is estimated using the existing convex hull volume index. The new functional evenness index is based on the minimum spanning tree which links all the species in the multidimensional functional space. Then this new index quantifies the regularity with which species abundances are distributed along the spanning tree. Functional divergence is measured using a novel index which quantifies how species diverge in their distances (weighted by their abundance) from the center of gravity in the functional space. We show that none of the indices meets all the criteria required for a functional diversity index, but instead we show that the set of three complementary indices meets these criteria. Through simulations of artificial data sets, we demonstrate that functional divergence and functional evenness are independent of species richness and that the three functional diversity indices are independent of each other. Overall, our study suggests that decomposition of functional diversity into its three primary components provides a meaningful framework for its quantification and for the classification of existing functional diversity indices. This decomposition has the potential to shed light on the role of biodiversity on ecosystem functioning and on the influence of biotic and abiotic filters on the structure of species communities. Finally, we propose a general framework for applying these three functional diversity indices.

An index of functional diversity

Functional diversity has been seen as the key to predicting the stability, invasibility, resource capture, nutrient cycling and productivity of communities. However, it has been unclear how to estimate it. Ten criteria for an index of functional diversity are developed. These include that it should reflect the range of characters present and the abundance of the species with those characters in the community, and be unaf- fected by the measurement units used or by the number of species. An index that meets all ten criteria, FD var , is investigated. It is based on the variance in characters, weighted by the abundance of the species with those characters. Tested with artificial and randomly generated data, it showed reasonable use of the 0 - 1 range (mean 0.60, range 0.0009 - 0.975) and intuitive behaviour. Tested with field data from eight sites in New Zealand, it gave a good spread of values (mean 0.65, range across sites 0.34 - 0.84), showed good ability to distin- guish between the communities and its performance was eco- logically intuitive. Illustrative correlations are made with mean annual temperature and soil fertility, determined by a bio- assay. FD var is recommended for general use.

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.

Functional characters combined with null models reveal inconsistency in mechanisms of species turnover in lacustrine fish communities

Functional characters have the potential to act as indicators of species turnover between local communities. Null models provide a powerful statistical approach to test for patterns using functional character information. A combined null model/functional character approach provides the ability to distinguish between the effect of competition and environmental filtering on species turnover. We measured 13 functional characters relating directly to resource use for the fish species found in French lakes. We combined this functional character data with a null model approach to test whether co-occurring species overlapped more or less than expected at random for four primary niche axes. We used an environmentally constrained null model approach to determine if the same mechanisms were responsible for species turnover at different sections of the altitudinal gradient. Functional diversity indices were used to examine the variation in functional character diversity with altitude, as a test of the hypothesis that competitive intensity decreases with increasing environmental adversity. The unconstrained null model showed that environmental filtering was the dominant influence on species turnover between lakes. In the constrained null model, there was much less evidence for environmental filtering, emphasising the strong effect of altitude on turnover in functional character values between local communities. Different results were obtained for low-altitude and high-altitude lake subsets, with more evidence for the effect of environmental filtering being found in the high-altitude lakes. This demonstrates that different processes may influence species turnover throughout an environmental gradient. Functional diversity values showed a slight decrease with altitude, indicating that there was only weak evidence that competitive intensity decreased with increasing altitude. Variation resource availability and environmental stress probably cause the observed turnover in functional characters along the altitudinal gradient, though the effects of dispersal limitation and species introductions in high-altitude lakes cannot be ruled out.

Does niche overlap control relative abundance in French lacustrine fish communities? A new method incorporating functional traits

1. The mechanisms that structure biological communities hold the key to understanding ecosystem functioning and the maintenance of biodiversity. Patterns of species abundances have been proposed as a means of differentiation between niche-based and neutral processes, but abundance information alone cannot provide unequivocal discrimination. 2. We combined species niche information and species relative abundances to test the effects of two opposing structuring mechanisms (environmental filtering and niche complementarity) on species relative abundances in French lacustrine fish communities. The test involved a novel method comparing the abundance-weighted niche overlap within communities against that expected when relative abundances were randomized among species within the community. 3. Observed overlap was consistently significantly lower than expected at random for two (swimming ability and trophic status) of four primary niche axes across lakes of differing physical environments. Thus, for these niche axes, pairs of abundant species tended to have relatively low niche overlap, while rare species tended to have relatively high niche overlap with abundant species. 4. This suggests that niche complementarity may have acted to enhance ecosystem function and that it is important for species coexistence in these fish communities. The method used may be easily applied to any sort of biological community and thus may have considerable potential for determining the generality of niche complementarity effects on community structure.

Is the abundance of species determined by their functional traits? A new method with a test using plant communities

The relation between functional traits and abundance of species has the potential to provide evidence on the mechanisms that structure local ecological communities. The niche-limitation/limiting-similarity hypothesis, derived from MacArthur and Levins’ original concept, predicts that species that are similar to others in terms of functional traits will suffer greater competition and hence be less abundant. On the other hand, the environment-filtering/habitat-optimum hypothesis predicts that groups of species with functional traits that are close to the optimum for that environment, and are therefore similar to other species, will be more abundant. We propose a new niche-assembly model for predicting the relative abundance of species in communities from their functional traits, which can detect the patterns that would be expected from either of these hypotheses. The model was fitted to eight plant communities sampled in the Lake Ohau district of New Zealand. For seven of the sites, the patterns could not be distinguished from that expected under a null model. However, in one site there was highly significant departure from the null model in the direction expected from the niche-limitation hypothesis. The site was probably the most productive of those examined. It is possible that competition for light rather than belowground resources, or faster recovery from disturbance, allowed greater predictability. Surprisingly, the predictability was seen when just the presences of a species’ neighbours in trait space were taken into account, but not when the potential effects of those neighbours were weighted by their abundance. For three of the four model types, the effects of species on each other were consistently negative: a significant trend. These results contradict the various neutral models of ecological communities.

Do plant modules describe community structure better than biomass? A comparison of three abundance measures

Abstract Plants are composed of modules, such as shoots and leaves. However, modules have been overlooked as potential abundance measures in describing plant communities. We sampled 8 communities, to examine whether module counts gave different conclusions from biomass on community structure, and better discriminated between communities. Different abundance measures – number of leaves, number of shoots and photosynthetic biomass – gave different results for between-site comparisons of evenness and rank consistency. Since evenness is intended to represent a feature of the whole community, it should vary more between communities than within; module-based abundance achieved this better than biomass. It is speculated that the restrictions on species co-occurrences when plant communities assemble may sometimes be based on the number of modules rather than on biomass. Abbreviation: PSU = Photosynthetic unit.