Variation In Ecosystem Function Research Articles

Dendrograms and measuring functional diversity

Patterns and changes in functional diversity can inform about spatial and temporal variation in trait diversity, about the processes that drive assembly, and whether assemblages are likely to contain redundant species. We recently provided a new measure (termed FD) and detailed its advantages over previous ones. Since then an increasing amount of research effort has been directed towards both developing appropriate measures of functional diversity and critiquing previous ones, including FD. Podani and Schmera (2006) attempt to do both, though here we argue that they accomplish neither. First, they suggest that a particular distance measure and clustering method are appropriate. We suggest that this is not the case, and show that they may have little effect on quantitative patterns in FD. Second, they suggest that values of functional diversity must be insensitive to the number of functional traits used. We do not agree because we can envisage no relevant ecological question. Third, they observe that we originally defined an FD of zero for an empty assemblage, whereas it is more appropriate for single species assemblages to have FD of zero. We agree. Their solution, however, is to create a measure of functional diversity which violates set monotonicity. Our solution is a revised version of FD for which single species assemblages have FD=0, and which does not violate set monotonicity. In conclusion, we are confident that FD behaves appropriately and note that it remains the measure of functional diversity with greatest power to explain variation in ecosystem functioning.

Dendrograms and measuring functional diversity

Patterns and changes in functional diversity can inform about spatial and temporal variation in trait diversity, about the processes that drive assembly, and whether assemblages are likely to contain redundant species. We recently provided a new measure (termed FD) and detailed its advantages over previous ones. Since then an increasing amount of research effort has been directed towards both developing appropriate measures of functional diversity and critiquing previous ones, including FD.Podani and Schmera (2006)attempt to do both, though here we argue that they accomplish neither. First, they suggest that a particular distance measure and clustering method are appropriate. We suggest that this is not the case, and show that they may have little effect on quantitative patterns in FD. Second, they suggest that values of functional diversity must be insensitive to the number of functional traits used. We do not agree because we can envisage no relevant ecological question. Third, they observe that we originally defined an FD of zero for an empty assemblage, whereas it is more appropriate for single species assemblages to have FD of zero. We agree. Their solution, however, is to create a measure of functional diversity which violates set monotonicity. Our solution is a revised version of FD for which single species assemblages have FD=0, and which does not violate set monotonicity. In conclusion, we are confident that FD behaves appropriately and note that it remains the measure of functional diversity with greatest power to explain variation in ecosystem functioning.

Assembly of weed communities along a crop diversity gradient

Summary Increasing cropping system diversity is one strategy for reducing reliance on external chemical inputs in agriculture and may have important implications for agro‐ecosystem functions related to the regulation of weed populations and community assembly. However, the impacts of cropping system diversity on weed communities have not been evaluated formally in a study comparable with those performed in experimental grasslands, where much of the evidence regarding diversity–ecosystem function has been reported. We performed a field experiment in Michigan, USA, in which we manipulated the number of crop species grown in rotation and as winter cover crops over a 3‐year period and in the absence of fertilizer and pesticides, to determine the impact of crop diversity on the abundance, composition and structure of the weed community. Crop diversity treatments consisted of three row‐crops, corn Zea mays L., soybean Glycine max (L.) Merr. and winter wheat Triticum aestivum L., grown in continuous monoculture and in 2‐ and 3‐year annual rotations with and without cover crops (zero, one or two legume/small grain species). Weed communities were measured each year at peak biomass, with soil resources and light availability being measured over the course of the growing season in the final year of the study. The effects on weed communities of the crop diversity treatments were dependent on rotation phase. In winter wheat, weed abundance and diversity (species richness, H′ and D) were lowest in the two highest crop diversity treatments. Across all phases of the rotation, weed community structure was affected more by crop identity than crop diversity per se. In general, the effects of crop diversity on weed communities were mainly the result of the presence of cover crops, which had strong effects on soil resource and light levels, particularly in winter wheat. Synthesis and applications. Increasing crop diversity in the absence of external chemical inputs can result in changes in soil resource availability without a concomitant increase in the abundance of weeds or a shift to weed communities that are more difficult to manage.

Functional diversity within a morphologically conservative genus of predators: implications for functional equivalence and redundancy in ecological communities

Summary The idea that sets of species may have similar effects on population, community or ecosystem processes is a prevalent theme in many areas of ecology, especially in the context of biodiversity and ecosystem function. If indeed species are functionally equivalent, limiting similarity suggests that it should be closely related, morphologically similar species using similar resources in a similar manner. We assayed the functional equivalence of three congeneric, morphologically similar predatory fish species (genus Enneacanthus). Functional equivalence was evaluated using aspects of both effects of fish on a variety of prey responses and the growth responses of the fish themselves as a measure of energy consumption. Fish were matched by initial size to control for effects of body size. A strict definition of functional equivalence based on niche theory was used to delineate it from the alternative of functional diversity. Based on observed effects on larval anurans, only a single species pair could roughly be judged functionally equivalent, but these two species showed the greatest differences in growth rate and, hence, metabolic demand. Using the criterion of relative yield total, again, only a single pair could roughly be judged equivalent, however, members of this alternative species pair were dramatically different in their effects on larval anurans. Thus, as previously shown for a more diverse set of species, grouping of species by similarity in effects depends upon the specific response variable. Overall range of effects produced on a variety of response variables was surprising, given the similarity in morphology and autecology, strong phylogenetic affinity, and the fact that neither predator size nor growth explained significant variation. Each species appears to be interacting with the environment in a different manner, either as a consequence of differences in metabolic demand or differences in preferences or efficiency with regard to prey types. Observed responses are consistent with the predictions of niche theory and support an alternative explanation for observed relationships between diversity and ecosystem function. Our work suggests that functional equivalence may be uncommon, difficult to predict a priori, and that functional diversity, not functional equivalence, may underlie observed diversity–ecosystem function relationships.

Effects of Plant Species Diversity and Composition on Nitrogen Cycling and the Trace Gas Balance of Soils

Experiments addressing the role of plant species diversity for ecosystem functioning have recently proliferated. Most studies have focused on plant biomass responses. However, microbial processes involved in the production of N2O and the oxidation of atmospheric CH4 could potentially be affected via effects on N cycling, on soil diffusive properties (due to changes in water relations and root architecture) and by more direct interactions of plants with soil microbes. We studied ecosystem-level CH4 and N2O fluxes in experimental communities assembled from two pasture soils and from combinations of 1, 3, 6, 8 or 9 species typical for these pastures. The soils contrasted with respect to texture and fertility. N2O emissions decreased with diversity and increased in the presence of legumes. Soils were sinks for CH4 at all times; legume monocultures were a smaller sink for atmospheric CH4 than non-legume monocultures, but no effect of species richness per se was detected. However, both the exchange of CH4 and N2O strongly depended on plant community composition, and on the interaction of composition with soil type, indicating that the functional role of species and their interactions differed between soils. N2O fluxes were mainly driven by effects on soil nitrate and on nitrification while soil moisture had less of an effect. Soil microbial C and N and N mineralisation rates were not altered. The driver of the interactive soil type×plant community composition-effects was less clear. Because soil methanotrophs may take longer to respond to alterations of N cycling than the 1/2 year treatment in this study, we also tested species richness-effects in two separate 5-year field studies, but results were ambiguous, indicating complex interactions with soil disturbance. In conclusion, our study demonstrates that plant community composition can affect the soil trace gas balance, whereas plant species richness per se was less important; it also indicates a potential link between the botanical composition of plant communities and global warming.

The emerging role of genetic diversity for ecosystem functioning: Estuarine macrophytes as models

Recent experiments in macrophyte dominated communities on the relationship between biological diversity and ecosystem functioning suggest that effects and mechanisms of genetic-genotypic and species diversity are analogous. As previously shown for species diversity, genotypic diversity enhances ecosystem productivity and recovery from disturbance. These findings generalize ecological theory, and provide an empirical basis for explicitly considering the maintenance of genetic or genotypic diversity for conservation strategies. Macrophyte systems such as seagrasses or salt-marshes may be excellent systems to test the interaction between diversity across several (genetic versus species) levels of biological organization because they are relatively species poor while simultaneously allowing the manipulation of genotypic diversity by taking advantage of clonal propagation in many species.

BIOGEOCHEMICAL AND METABOLIC RESPONSES TO THE FLOOD PULSE IN A SEMIARID FLOODPLAIN

Flood pulse inundation of riparian forests alters rates of nutrient retention and organic matter processing in the aquatic ecosystems formed in the forest interior. Along the Middle Rio Grande (New Mexico, USA), impoundment and levee construction have created riparian forests that differ in their inter-flood intervals (IFIs) because some flood- plains are still regularly inundated by the flood pulse (i.e., connected), while other flood- plains remain isolated from flooding (i.e., disconnected). This research investigates how ecosystem responses to the flood pulse relate to forest IFI by quantifying nutrient and organic matter dynamics in the Rio Grande floodplain during three years of experimental flooding of the disconnected floodplain and during a single year of natural flooding of the connected floodplain. Surface and subsurface conditions in paired sites (control, flood) established in the two floodplain types were monitored to address metabolic and biogeochemical responses. Compared to dry controls, rates of respiration in the flooded sites increased by up to three orders of magnitude during the flood pulse. In the disconnected forest, month-long experimental floods produced widespread anoxia of four-week duration during each of the three years of flooding. In contrast, water in the connected floodplain remained well ox- ygenated (3-8 ppm). Material budgets for experimental floods showed the disconnected floodplain to be a sink for inorganic nitrogen and suspended solids, but a potential source of dissolved organic carbon (DOC). Compared to the main stem of the Rio Grande, flood- water on the connected floodplain contained less nitrate, but comparable concentrations of DOC, phosphate-phosphorus, and ammonium-nitrogen. Results suggest that floodplain IFI drives metabolic and biogeochemical responses dur- ing the flood pulse. Impoundment and fragmentation have altered floodplains from a mosaic of patches with variable IFI to a bimodal distribution. Relatively predictable flooding occurs in the connected forest, while inundation of the disconnected forest occurs only as the result of managed application of water. In semiarid floodplains, water is scarce except during the flood pulse. Ecosystem responses to the flood pulse are related to the IFI and other measures of flooding history that help describe spatial variation in ecosystem function.

Biodiversity and ecosystem functioning in aquatic microbial systems: a new analysis of temporal variation and species richness‐predictability relations

Studies of microbial communities from aquatic ecosystems provide important insights into relations between various aspects of ecosystem functioning and changes in biodiversity. Aquatic microbial systems provide a valuable counterpoint to studies of terrestrial systems, because patterns reflect consequences of interactions occurring over many generations of community development, and are unlikely to represent artifacts of the initial conditions established in experimental communities. In this paper we re‐analyse our previously published data to separate the contributions of temporal and spatial variation to overall variation in ecosystem functioning. A new analysis based on re‐sampling confirms a negative relationship between richness and the variability of one ecosystem process, carbon dioxide flux. The negative relationship reflects high variation among communities of low species richness, rather than high temporal variation within communities of low richness. We also review the various transformations and summary statistics proposed as alternate measures of variability in ecosystem functioning, to point out that different measures are often appropriate for different kinds of data. Finally, we conclude that arguments about the cosmopolitan distribution of microbes do not preclude the existence of important relations between microbial species richness and ecosystem functioning.

Sensitivity of soil processes in northern forest soils: are management practices a threat?

There is evidence that forest management practices influence soil-decomposer communities. It is also established that changes in the trophic structure and composition of these communities can induce changes in soil-nutrient dynamics, thereby affecting plant growth. Whether forest productivity is affected by management-induced changes in, e.g. soil faunal structure, is, however, yet to be shown. The aim of this study was (1) to determine the resolution of the ecological hierarchy (e.g. species, functional groups, trophic levels) at which a change in soil fauna would alter biotically-controlled processes in soils, and (2) to examine the sensitivity of soil fauna of the boreal forest floor to various kinds of forest management practices. A review of laboratory miniecosystem experiments carried out at the University of Jyväskylä is presented to examine the diversity-ecosystem function relationship. The response of tree growth to manipulation of soil-faunal composition was measured. A field experiment was conducted in central Finland in spruce stands, including several stand management treatments in addition to the untreated controls. The fellings took place in winter 1996, and various groups of soil animals have been sampled since 1995. Laboratory experiments revealed that soil processes and plant growth are largely insensitive to changes taking place at the species level of soil fauna. Some important keystone species may exist, but a change in the functional group architecture seems to be a prerequisite for altered rates in soil processes. Predators high up in the detrital food web had no detectable influence on any of the ecosystem-level processes. In the field, all of the faunal groups studied proved to be highly insensitive to the stand management practices. As compared to the untreated controls, numbers of enchytraeid worms, collembolans and most of the macroarthropods in the managed stands were not significantly different. It is concluded that management practices with minor impacts on the soil organic layer, which buffers soil biota against drastic changes in their environment, have little influence on biotically-controlled soil processes.

Testate amoebae and the fossil record: issues in biodiversity

SummaryBiodiversity assessment is broadly divided into a consideration of species diversity, genetic diversity and ecosystem diversity. Individual species are readily defined for many groups of organisms and their relationship with fossil taxa found in the Late Quaternary is clear. Testate amoebae are a group of protozoa which form external tests that are preserved in peat and lake sediments. The higher taxa levels are defined by the morphology of the soft pseudopodia which are not preserved in fossil specimens but many lower taxa can be recognized by test characteristics. Two sets of problems occur in the assessment of fossil testate amoebae biodiversity, related to species definitions based on morphological criteria in living specimens, and the preservation and study of fossil tests. The biological species concept cannot be readily applied to testate amoebae and morphospecies are therefore defined on the basis of the characteristics of pseudopodia and tests. However, the accepted degree of intraspecific morphological variation appears to be variable between authorities and the number of described species is therefore poorly defined. In addition, there is likely to be no straightforward relationship between species diversity and genetic diversity. Further problems of species differentiation are encountered in fossil specimens so that the assessment of biodiversity in fossil populations is even more problematic. The variable results which can be obtained by the adoption of different sets of species definitions are illustrated with respect to fossil faunas from peat profiles in Canada and the UK. The biodiversity of living testate amoebae and other groups of lower organisms is poorly known and further consideration needs to be given to the definition and description of both living and fossil specimens if these problems are to be resolved. The assessment of complete ecosystem biodiversity is not possible without this and investigations which consider the role of diversity in ecosystem functioning cannot even begin.