Mechanisms of species coexistence in a lawn community: mutual corroboration between two independent assembly rules

Assembly rules are measures of community structure that link observed patterns with ecological processes, and as such may help to elucidate the mechanisms by which species coexist. We apply two approaches to a lawn community - limiting similarity and guild proportionality - hoping that agreement between them might give robust conclusions. We tested for agreement between these two assembly rules using functional characters that are related to two aspects of species function - light capture and response to defoliation. We combined point quadrat data and a null model approach to test for limiting similarity - a tendency for species differing in functional characters to co-occur more often than expected at random - in turves extracted from the lawn community. Examining the variance in the characters of the species co-occuring at each point, evidence for limiting similarity was found for mowing removal (the proportion of leaf area removed in mowing events). There was greater variation between the species co-occurring at a point than expected at random (i.e., under an appropriate null model). However, no such evidence was found for characters related uniquely to light capture, such as specific leaf area and pigment concentrations. In a previous study in the same community, “intrinsic” guilds had been determined from co-occurrences within the lawn community and from a competition experiment, as those effective in determining species assembly and co-existence. These intrinsic guilds are shown by t-test to differ in the proportion of their biomass removed in mowing (MRI), which is of course related to the height at which their leaf area is held. However, again no differences were seen in characters related uniquely to light capture. Thus, the two different approaches to assembly rules - guild proportionality and limiting similarity - agree that differences in response to mowing are responsible for species co-existence in the lawn community. The agreement between these two approaches, tested on independent datasets of quite different type from the same community, gives possibly the strongest evidence so far that niche differentiation may be responsible for local co-existence in plant communities. However, although MRI is related to this co-existence the lack of correlation with light-capture characters leads to speculation that the effects might be via the below-ground behaviour of the species

Functional diversity: back to basics and looking forward

Functional diversity is a component of biodiversity that generally concerns the range of things that organisms do in communities and ecosystems. Here, we review how functional diversity can explain and predict the impact of organisms on ecosystems and thereby provide a mechanistic link between the two. Critical points in developing predictive measures of functional diversity are the choice of functional traits with which organisms are distinguished, how the diversity of that trait information is summarized into a measure of functional diversity, and that the measures of functional diversity are validated through quantitative analyses and experimental tests. There is a vast amount of trait information available for plant species and a substantial amount for animals. Choosing which traits to include in a particular measure of functional diversity will depend on the specific aims of a particular study. Quantitative methods for choosing traits and for assigning weighting to traits are being developed, but need much more work before we can be confident about trait choice. The number of ways of measuring functional diversity is growing rapidly. We divide them into four main groups. The first, the number of functional groups or types, has significant problems and researchers are more frequently using measures that do not require species to be grouped. Of these, some measure diversity by summarizing distances between species in trait space, some by estimating the size of the dendrogram required to describe the difference, and some include information about species' abundances. We show some new and important differences between these, as well as what they indicate about the responses of assemblages to loss of individuals. There is good experimental and analytical evidence that functional diversity can provide a link between organisms and ecosystems but greater validation of measures is required. We suggest that non-significant results have a range of alternate explanations that do not necessarily contradict positive effects of functional diversity. Finally, we suggest areas for development of techniques used to measure functional diversity, highlight some exciting questions that are being addressed using ideas about functional diversity, and suggest some directions for novel research

Combinatorial functional diversity : an information theoretical approach

A new approach to the measurement of functional diversity based on two-state nominal traits is developed from the florula diversity concept of P. Juhász-Nagy. For evaluating functional diversity of an assemblage, first a traits by species matrix is compiled. Various information theory functions are used to examine structural properties in this matrix, including the frequency distribution of trait combinations. The method is illustrated by actual examples, the first from plant communities prone to fire in Spain, and the second from running water invertebrate assemblages in Hungary. The results suggest that of the various functions used the standardized joint entropy, termed combinatorial functional evenness supplies most meaningful results. In plant communities, high fire recurrence decreased combinatorial functional evenness, while this measure for freshwater assemblages was uncorrelated with stream width and negatively correlated with the degree of human impact. Stream width is negatively correlated with the number of manifested functional combinations. In both case studies, combinatorial functional evenness has an inverse relationship to species richness - i.e., fewer species have a larger chance to produce equiprobable functional combinations.Peer Reviewe

Assessing the impacts of mining activities on zooplankton functional diversity

Abstract Aim Mining activities generate countless environmental impacts, including heavy-metal contamination, sorting and increased turbidity. In aquatic ecosystems these impacts can drastically affect the initial links of the food chain, such as zooplankton. Methods To evaluate how the different mining activities can influence the structure and functional diversity of zooplankton, we investigated the geochemical characteristics of the water and sediment in two small impoundments impacted by different mining activities (kaolin and iron extraction). We also explored zooplankton composition, species diversity and functional diversity (feeding guilds taxa). Results As expected, the water and the sediment of both of the reservoirs showed high concentrations of trace elements, particularly Al, Ba, Fe, Mg, Mn, Sr and Zn. Zooplankton biomass and diversity were markedly reduced (< 12 &#956;g.DW.L-1 and H&#8217; < 1.5, respectively), and negatively correlated with turbidity and total suspended solids. Small microphages dominated the trophic composition of zooplankton, and an alternation of trophic guilds was not observed, since the dynamics of raptorial organisms was essentially linked to the temporal fluctuation of a single species of rotifer (Polyarthra cf. dolichoptera). Conclusions In addition to changes in the aquatic habitat and zooplankton composition, the functional niches were also affected by the mining impacts. The use of the functional diversity analysis can emerge as a valuable approach to understand how zooplankton communities respond to drastic environmental changes