Ecosystem Process Rates Research Articles

Ecosystem process rate increases with animal species richness: evidence from leaf‐eating, aquatic insects

Effects of species number and identity on the breakdown rate of leaf litter were estimated in a laboratory experiment using leaf‐eating insects, three species of Plecoptera, as detritivores. We found significant differences between the different species on this process in single‐species experiment, but not when animal biomass was accounted for. When species were combined the effect of species identity was strongly reduced and rendered insignificant, whereas the number of species had a significant effect. This shows that rates of ecosystem processes may benefit from species richness even when all species belong to the same guild, which is in contrast to hypotheses predicting redundancy within guilds. Facilitation between species and negative interactions, where intraspecific interactions are greater than interspecific interactions, are two potential mechanisms which could explain increasing decomposition rates with species richness.

SPATIAL HETEROGENEITY OF STREAM WATER NUTRIENT CONCENTRATIONS OVER SUCCESSIONAL TIME

Nutrient availability in ecosystems is patchy both in space and in time. Whereas temporal trends have often been studied, less information exists on spatial patterns of nutrient availability, particularly in aquatic ecosystems. The goals of this study were (1) to describe and quantify patterns of nutrient concentration in surface waters of an arid land stream and (2) to compare spatial patterns of nutrient availability across nutrients and over a successional sequence. We describe changes in the spatial pattern of stream water nutrient concentrations over successional time (between floods) using quantitative measures of heterogeneity. Samples were collected from the middle of the channel every 25 m over a 10-km section of a Sonoran Desert stream during three periods: early succession (2 wk post-flood), middle succession (2 mo post-flood), and late succession (9 mo post-flood). Nutrient concentrations were extremely variable in space (coefficients of variations as high as 145%). Coefficients of variation increased over successional time and were consistently greater for nitrate-nitrogen than for soluble reactive phosphorus. Semi-variogram analysis showed that nutrient concentrations were spatially dependent on all dates, but to different degrees and over different distances. The distance over which nutrient concentrations were spatially dependent, as measured by the semi-variogram range, tended to decrease with successional time. The strength of spatial dependence, as measured by the slope of the ascending limb of the semi-variogram, increased with successional time. The limiting nutrient, nitrogen, was consistently more spatially heterogeneous than phosphorus or conductivity, both in terms of patch size (range) and strength of spatial dependence. In streams, downstream transport combined with nutrient transformation produces patches of similar nutrient concentrations that are elongated compared with nutrient patches in terrestrial soils. Variation in nutrient concentration is likely to affect the spatial distribution of organisms and rates of ecosystem processes.

Spatial Heterogeneity of Stream Water Nutrient Concentrations over Successional Time

Nutrient availability in ecosystems is patchy both in space and in time. Whereas temporal trends have often been studied, less information exists on spatial patterns of nutrient availability, particularly in aquatic ecosystems. The goals of this study were (1) to describe and quantify patterns of nutrient concentration in surface waters of an arid land stream and (2) to compare spatial patterns of nutrient availability across nutrients and over a successional sequence. We describe changes in the spatial pattern of stream water nutrient concentrations over successional time (between floods) using quantitative measures of heterogeneity. Samples were collected from the middle of the channel every 25 m over a 10-km section of a Sonoran Desert stream during three periods: early succession (2 wk post-flood), middle succession (2 mo post-flood), and late succession (9 mo post-flood). Nutrient concentrations were extremely variable in space (coefficients of variations as high as 145%). Coefficients of variation increased over successional time and were consistently greater for nitrate-nitrogen than for soluble reactive phosphorus. Semi-variogram analysis showed that nutrient concentrations were spatially dependent on all dates, but to different degrees and over different distances. The distance over which nutrient concentrations were spatially dependent, as measured by the semi-variogram range, tended to decrease with successional time. The strength of spatial dependence, as measured by the slope of the ascending limb of the semi-variogram, increased with successional time. The limiting nutrient, nitrogen, was consistently more spatially heterogeneous than phosphorus or conductivity, both in terms of patch size (range) and strength of spatial dependence. In streams, downstream transport combined with nutrient transformation produces patches of similar nutrient concentrations that are elongated compared with nutrient patches in terrestrial soils. Variation in nutrient concentration is likely to affect the spatial distribution of organisms and rates of ecosystem processes.

Expert Estimates about Effects of Biodiversity on Ecosystem Processes and Services

Attempts to assess the functional importance of biodiversity in ecosystems continue to stimulate debate on the potential impacts of biodiversity losses. However, finding experimental setups to realistically simulate current and anticipated species loss turns out to be surprisingly difficult. Uncertainties concern the relevance of the present empirical evidence for large-scale natural and agricultural situations and the relation between observed specific processes and ecosystem services that provide a basis for human life support. Since ecosystem effects of biodiversity loss may be experimentally less tractable than other global change issues we were interested in current expert opinions. The resulting survey provides what might be the best-available answers to urgent questions regarding the certainties and uncertainties of the functional role of biodiversity in ecosystems. The respondents generally expected that: (1) ecosystem process rates are strongly correlated with biological diversity, and (2) these same processes are (although to a varying extent) important for the delivery of humanly defined ecosystem services by natural systems. The survey thus suggests caution in reducing ecosystem-level biodiversity, but it also underscores the opportunities for land management that may arise from the recognition of biodiversity effects in ecosystems. A caveat regards the possible bias of participants toward scientists who are particularly concerned about environmental change.

Mesoscale Disturbance and Ecological Response to Decadal Climatic Variability in the American Southwest

Climatic variables such as radiation, temperature and precipitation determine rates of ecosystem processes from net primary productivity to soil development. They predict a wide array of biogeographic phenomena, including soil carbon pools, vegetation physiognomy, species range, and plant and animal diversity. Climate also influences ecosystems indirectly by modulating the frequency, magnitude, and spatial scales of natural disturbances (Clark 1988; Overpeck et al. 1990; Swetnam 1993).

Trophic structure stability and extinction dynamics of beetles (Coleoptera) in tropical forest fragments

A first analysis of the stability of trophic structure following tropical forest fragmentation was performed in an experimentally fragmented tropical forest landscape in Central Amazonia. A taxonomically and trophically diverse assemblage of 993 species of beetles was sampled from 920 m 2 of leaf litter at 46 sites varying in distance from forest edge and fragment area. Beetle density increased significantly towards the forest edge and showed non-linear changes with fragment area, due to the influx of numerous disturbed-area species into 10 ha and 1 ha fragments. There was a marked change in species composition with both decreasing distance from forest edge and decreasing fragment area, but surprisingly this change in composition was not accompanied by a change in species richness. Rarefied species richness did not vary significantly across any of the sites, indicating that local extinctions of deep forest species were balanced by equivalent colonization rates of disturbed-area species. The change in species composition with fragmentation was non-random across trophic groups. Proportions of predator species and xylophage species changed significantly with distance from forest edge, but no area-dependent changes in proportions of species in trophic groups were observed. Trophic structure was also analysed with respect to proportions of abundance in six trophic groups. Proportions of abundance of all trophic groups except xylomycetophages changed markedly with respect to both distance from forest edge and fragment area. Local extinction probabilities calculated for individual beetle species supported theoretical predictions of the differential susceptibility of higher trophic levels to extinction, and of changes in trophic structure following forest fragmentation. To reduce random effects due to sampling error, only abundant species ( n ≥ 46) were analysed for extinction probabilities, as defined by absence from samples. Of these common species, 27% had significantly higher probabilities of local extinction following fragmentation. The majority of these species were predators; 42% of all abundant predator species were significantly more likely to be absent from samples in forest fragments than in undisturbed forest. These figures are regarded as minimum estimates for the entire beetle assemblage because rarer species will inevitably have higher extinction probabilities. Absolute loss of biodiversity will affect ecosystem process rates, but the differential loss of species from trophic groups will have an even greater destabilizing effect on food web structure and ecosystem function.

Testing Forest‐BGC Ecosystem Process Simulations Across a Climatic Gradient in Oregon

Field measurements from the Oregon Transect Ecological Research project (OTTER) were used to validate selected process simulations in the FOREST—BGC ecosystem model. Certain hydrologic, carbon, and nitrogen cycle process simulations were tested in this validation, either comparatively across sites, or seasonally at single sites. The range of simulated ecosystem—process rates across the OTTER sites was large; annual evapotranspiration (ET) ranged from 15 to 82 cm, net photosynthesis (as carbon) from 2.2 to 22.8 Mg/ha, and decomposition (as carbon) from 1.0 to 7.2 Mg°ha—1°yr—1. High correlations between predicted and measured data were found for: aboveground net primary production, R2 = 0.82; 100—yr stem biomass, R2 = 0.79; and average leaf nitrogen concentration, R2 = 0.88. However, correlations for pre—dawn leaf water potential and equilibrium leaf area index (LAI) were much lower, because successful simulation of these variables requires accurate data for soil water—holding capacity. Defining the water—holding capacity of the rooting zone and the maximum surface conductance for photosynthesis and transpiration rates proved to be critical system variables that defied routine field measurement. Although many other processes are simulated in FOREST—BGC, no other processes had repeated field data sets for validations. Problems in parameterizing the model from disparate data sets are also evaluated, with suggestions for using ecosystem modeling in future integrated research programs.

Long‐Term Soil Water Dynamics in the Shortgrass Steppe

To assess the temporal and spatial dynamics of soil water at a shortgrass steppe site in northcentral Colorado, we evaluated the precipitation regime for a 33—yr period and ran a simulation model for this period. Small precipitation events accounted for a large fraction of the total number of events and represented a source of water with small interannual variability. The difference between wet and dry years was related to the occurrence of a few large events. Average daily precipitation was concentrated during the warmest months of the year with a maximum in late spring. Water in the surface soil layers had a short residence time and no seasonal pattern. Intermediate layers reflected the seasonal pattern of precipitation. Maximum soil water availability occurred in the late spring, but this was also the period with the highest interannual variability. The wettest layer was at 4—15 cm of depth. The frequency of wet conditions decreased above this layer because of the strong influence of evaporation and below because recharge was infrequent. No deep percolation events were recorded. During dry years distribution of soil water was very shallow and during wet years wet conditions reached to depths of 120—135 cm. This shallow distribution of soil water matches the distribution of processes and structural elements in the steppe suggesting that there is a cause—effect relationship between them. We speculate that the pattern of water availability interacts with biotic constraints and determines the rate of ecosystem processes. The depth distribution of water in dry and wet years is compared to the root distribution of grasses, shrubs, herbs, and succulents to suggest the response of each group to modal and extreme conditions. Comparison of long—term soil water patterns and traits of the major species allows us to suggest why Bouteloua gracilis is the dominant species in the shortgrass steppe.

Monitoring of the functioning of ecosystems

The theoretical basis, the content and the possible applications of the program 'Monitoring of ecosystems' are considered. A functional definition of ecosystem is proposed, and some features of ecosystems are defined on this basis. The program 'Monitoring of ecosystems' is visualized as a large-scale program of simple measurements of the rates of the main ecosystem processes. The expected results can be utilized in two ways: (1) a comparative knowledge of ecosystem functioning provides the fundamentals of geography of ecosystems, and (2) the constant, repeatable evaluation of a given ecosystem function provides a basis for its rational management.