No abstract is provided for this article.
In the prolonged absence of catastrophic disturbance, ecosystem retrogression occurs, which is characterized by declining soil nitrogen (N) and phosphorus (P) availability, increasing plant and soil N to P ratios, and reduced plant biomass and productivity. It is, however, largely unknown as to how the effects of plant communities on soil nutrients change during retrogression or might contribute to declining nutrient availability as retrogression proceeds. We studied a well‐characterized system of 30 lake islands in northern Sweden that collectively represent a 5,000‐year post‐fire retrogressive chronosequence. For each island, we established an experiment that involved full factorial removal of three plant functional groups (tree roots, dwarf shrubs and mosses), and of three species of dwarf shrub ( Vaccinium myrtillus , V. vitis‐idaea and Empetrum hermaphroditum ) . After 19 years, we took various measures of soil N and P availability, and measured foliar N and P for each dwarf shrub species, for each plot in the experiment. Although plant removal effects (and particularly removal of tree roots, shrubs and Vaccinium species) on below‐ground N and P measures sometimes changed during retrogression, this seldom happened in a way that explains the decline in nutrient availability and increase in N to P ratios that characterize ecosystem retrogression. The only exceptions were that the positive effects of tree roots on soil mineral N and P, and of V. myrtillus on soil mineral P, declined during retrogression. Plant removal effects on community‐level measures of shrub N and P varied greatly across the chronosequence, but these effects again did not align with the changes in soil nutrient availability or N to P ratios that characterize ecosystem retrogression. Synthesis . Our results suggest that retrogression, and associated changes in nutrient availability and soil N to P ratios, is driven mainly by longer‐term pedogenic processes as opposed to shorter‐term effects of plant communities on soil N and P availability. More generally, they illustrate the value of long‐term and large‐scale experimental manipulations of plant communities for showing how effects of biodiversity loss on ecosystem properties vary across contrasting ecosystems.
No abstract is provided for this article.
Trophic cascades are increasingly being regarded as important features of aboveground and belowground food webs, but the effects of aboveground cascades on soil food webs, and vice versa, remains essentially unexplored. We conducted an experiment consisting of model synthesised communities containing grassland plant and invertebrate species, in which treatments included soil only, soil+plants, soil+plants+aphids, and soil+plants+aphids+predators; predator treatments consisted of the lacewing Micromus tasmaniae and ladybird beetle Coccinella undecimpunctata added either singly or in combination. Addition of Micromus largely reversed the negative effects of aphids on plant biomass, while both of the predator species caused large changes in the relative abundances of dominant plant species. Predators of aphids also affected several components of the belowground subsystem. Micromus had positive indirect effects on the primary consumer of the soil decomposer food web (microflora), probably through promoting greater input of basal resources to the decomposer subsystem. Predator treatments also influenced densities of the tertiary consumers of the soil food web (top predatory nematodes), most likely through inducing changes in plant community composition and therefore the quality of resource input to the soil. The secondary consumers of the soil food web (microbe‐feeding nematodes) were, however, unresponsive. The fact that some trophic levels of the soil food web but not others responded to aboveground manipulations is explicable in terms of top‐down and bottom‐up forces differentially regulating different belowground trophic levels. Addition of aphids also influenced microbial community structure, promoted soil bacteria at the expense of fungi, and enhanced the diversity of herbivorous nematodes; in all cases these effects were at least partially reversed by addition of Micromus . These results in tandem point to upper level consumers in aboveground food webs as a potential driver of the belowground subsystem, and provide evidence that predator‐induced trophic cascades aboveground can have effects that trickle through soil food webs.
All terrestrial ecosystems consist of explicit aboveground and belowground biotic components. Although these have traditionally been considered in isolation from one another, there has been increasing recognition over the past decade or so that these components...
It is well known that large islands support more biodiversity than small ones. Now a seven-year study of a group of islands in lakes Uddjaure and Hornavan in northern Sweden shows that the size of an island is also important in determining susceptibility to biodiversity loss. Considerable recent attention has focused on predicting how the losses of species and functional groups influence ecosystem properties1,2, but the extent to which these effects vary among ecosystems remains poorly understood3,4. Island systems have considerable scope for studying how biotic and abiotic factors influence processes in different ecosystems, because they enable the simultaneous study of large numbers of independent replicate systems at ecologically meaningful spatial scales5,6,7. We studied a group of 30 islands in northern Sweden, for which island size determined disturbance history, and therefore vegetation successional stage and biotic and abiotic ecosystem properties. On each island we conducted a seven-year study that involved experimental removals of combinations of both plant functional groups and plant species. We show that although losses of functional groups and species often impaired key ecosystem processes, these effects were highly context-dependent and strongly influenced by island size. Our study provides evidence that the consequences of biotic loss for ecosystem functioning vary greatly among ecosystems and depend on the specific abiotic and biotic attributes of the system.
Many ecological dynamics occur over time-scales that are well beyond the duration of conventional experiments or observations. One useful approach to overcome this problem is extrapolation of temporal dynamics from spatial variation. We review two complementary variants of this approach that have been of late increasingly employed: the use of natural gradients to infer anthropogenic effects and the use of anthropogenic gradients to infer natural dynamics. Recent studies have considered a variety of naturally occurring gradients associated with climate, CO2, disturbance and biodiversity gradients, as well as anthropogenic gradients such as those created by biological invasions, habitat fragmentation and land abandonment. These studies show that natural gradients are useful in predicting long-term consequences of human-induced environmental changes, whereas anthropogenic gradients are helpful in inferring the mechanisms behind natural dynamics because covarying factors are often more clearly understood in anthropogenic gradients than in natural gradients. We classify these studies into several categories, each with different strengths and weaknesses, and outline how the limitations can be overcome by combining the gradient-based approach with other approaches. Overall, studies reviewed here demonstrate that the development of basic ecological concepts and the application of these concepts to environmental problems can be more effective when conducted complementarily than when pursued separately.
Despite their ubiquity, the role of ants in driving ecosystem processes both aboveground and belowground has been seldom explored, except within the nest. During 1995 we established 16 ant exclusion plots of approximately 1.1 x 1.1 m, together with paired control plots, in the understory layer of a boreal forest ecosystem in northern Sweden that supports high densities of the mound-forming ant Formica aquilonia, a red wood ant species of the Formica rufa group. Aboveground and belowground measurements were then made on destructively sampled subplots in 2001 and 2008, i.e., 6 and 13 years after set-up. While ant exclusion had no effect on total understory plant biomass, it did greatly increase the relative contribution of herbaceous species, most likely through preventing ants from removing their seeds. This in turn led to higher quality resources entering the belowground subsystem, which in turn stimulated soil microbial biomass and activity and the rates of loss of mass and carbon (C) and nitrogen (N) from litter in litterbags placed in the plots. This was accompanied by losses of approximately 15% of N and C stored in the humus on a per area basis. Ant exclusion also had some effects on foliar stable isotope ratios for both C and N, most probably as a consequence of greater soil fertility. Further, exclusion of ants had multitrophic effects on a microbe-nematode soil food web with three consumer trophic levels and after six years promoted the bacterial-based relative to the fungal-based energy channel in this food web. Our results point to a major role of red wood ants in determining forest floor vegetation and thereby exerting wide-ranging effects on belowground properties and processes. Given that the boreal forest occupies 11% of the Earth's terrestrial surface and stores more C than any other forest biome, our results suggest that this role of ants could potentially be of widespread significance for biogeochemical nutrient cycling, soil nutrient capital, and sequestration of belowground carbon.
Six samples of agricultural soil were maintained at five separate soil moisture contents (15, 25, 35, 45 or 55%) for 6 h or 3, 10 or 30 days. These samples were then amended with 14C-labelled fungal or bacterial tissue, and the k c values determined following chloroform fumigation. Soil moisture effects explained 12.5% of the variation in the data set for fungal k c factors, and 9.5% in the bacterial data set. The 15% moisture treatment always had lower k c values than for at least some of the other moisture contents, in both the fungal and bacterial data sets but the differences were usually small. The 25, 35, 45 and 55% treatments did not usually differ from each other. Duration of incubation had a much stronger effect, explaining 51.6 and 35.8% of variation in the fungal and bacterial data sets, with prolonged incubation depressing k c values. The significance of blocking effects in each case indicated that the six samples (blocking units) had different k c values. Analysis of k c values, determined for the entire microbial biomass using a bacterial:fungal ratio of 25:75, showed essentially similar responses to the treatment effects tested for. Values of k c, determined using a different bacterial:fungal ratio determined for each subsample, were almost identical to those using a bacterial: fungal ratio of 25:75.
Because soil nematode populations play a key role in regulating the turnover of microbial communities and respond to changes in environmental conditions, shifts in their abundance and composition can be useful indicators of soil conditions. In this study nematode communities and other ecosystem variables were investigated over 7 yr under an annual (Zea mays) and a perennial (Asparagus officinale) crop using three weed management practices (cultivation, herbicide application, mulching) which can be related to agricultural intensification. Crop productivity and soil conditions did not change significantly during the trial. All management practices influenced the nematode fauna but the greatest long-term effects were from sawdust mulching. In the mulched plots there was an initial flush of both total and bacterial-feeding nematodes but both subsequently declined, which was coincident with enhanced populations of top predatory nematodes. The apparent negative interaction between bacterial-feeding and predatory nematodes was also demonstrated through the former being significantly (P<0.001) negatively correlated with soil carbon, bacterial mass and weed biomass and the latter being positively correlated with the same variables. Herbicide use did not exert any consistent detrimental effects on nematode communities and the nematode fauna in the herbicide treated plots tended to have greater diversity (as indicated by the Shannon–Weiner index) than that in many of the other plots. The effects of cultivation varied, but under the perennial crop the greatest number of total and bacterial-feeding nematodes were commonly at 5–10 cm depth in cultivated plots. While most treatments had relatively little general effect on the composition of the nematode fauna over the study period, several important specific effects were only apparent after at least 3 yr. Thus to effectively evaluate the relative effects of different agricultural practices in the long-term it is necessary to sample until the ecosystem has achieved some degree of equilibrium rather than monitoring only initial cropping cycles.
Relatively few studies have considered how aboveground invasive consumers influence decomposer communities. We investigated the potential effects of three
Plants return a wide range of carbon (C) substrates to the soil system. The decomposition rate of these substrates is determined by their chemical nature, yet few studies have examined the relative ecological role of specific substrates (i.e., substrate identity) or mixtures of substrates. Carbon substrate identity and diversity may alter soil chemistry and soil community composition, resulting in changes in belowground ecosystem functions such as decomposition and nutrient transfer, creating feedbacks that may affect plant growth and the aboveground community. A laboratory experiment was set up in which eight C substrates of varying chemical complexity were added to a base soil singly, in pairs, fours, or with all eight together every four days over a 92-day period. After 92 days these soils were analyzed for changes in chemistry, microbial community structure, and components of ecosystem functioning. The identity of the added C substrates significantly affected soil chemistry, microbial basal and substrate-induced respiration, and soil microbial community structure measured by either the catabolic response profile (CRP) technique or phospholipid fatty acid composition. These belowground changes strongly affected the ability of the soil microflora to decompose cellulose paper, probably because of differential effects of the C substrates on soil energy supplies and enzyme activities. The addition of C substrates to soils also reduced plant growth compared to the unamended control soil, but less so in soils amended with a tannin than those amended with other substrates. Carbon substrate diversity effects saturated at low diversity levels, tended to have neutral or negative effects on ecosystem functions, and depended strongly on which C substrates were added. It increased CRP compound use but had little effect on other measures of the soil microbial community. Overall, results showed that the chemical nature of C substrates added to soil, and sometimes their diversity, can affect the soil microbial community and soil chemistry, which subsequently affect other ecosystem processes such as decomposition and plant growth. The identity and diversity of substrates that plants add to soil may therefore have important consequences for both above- and belowground ecosystem functions.
