Changes in rainfall predicted to occur with global climate change will likely alter rates of leaf-litter decomposition through direct effects on primary decomposers. In a field experiment replicated at two sites, we show that altered rainfall may also change how cascading trophic interactions initiated by arthropod predators in the leaf litter indirectly influence litter decomposition. On the drier site there was no interaction between rainfall and the indirect effect of predators on decomposition. In contrast, on the moister site spiders accelerated the disappearance rate of deciduous leaf litter under low rainfall, but had no, or possibly a negative, indirect effect under high rainfall. Thus, changes resulting from the more intense hydrological cycle expected to occur with climate change will likely influence how predators indirectly affect an essential ecosystem process.detrital food web ͉ litter decomposition ͉ rainfall ͉ spiders ͉ trophic cascade C limate-change models predict a more intense hydrological cycle with both increases in rainfall and increased length and severity of droughts (1, 2). Changes in rainfall will likely affect ecosystem processes such as primary production and nutrient release from decomposing litter caused by direct effects of altered rainfall on plants and primary decomposers, respectively. For example, low moisture can inhibit fungal growth and͞or activity (3, 4). Changes in rainfall may also alter how trophic interactions indirectly influence rates of ecosystem processes. Recent research has shown that climatic changes may have large impacts on how predators indirectly alter net primary production through trophic cascades (reviewed in refs. 5-8). In detritusbased food webs predators have the potential to influence indirectly the amount of leaf litter through trophic interactions that affect rates of decomposition. This chain of interactions is a trophic cascade (9-12) analogous to the classic cascade affecting living plants. In grassland systems litter in cages accessible to large arthropod predators exhibited lower rates of litter disappearance compared with predator-exclusion cages (13,14), although other similar experiments in grasslands revealed a negligible impact of these predators on decomposition rate (15). In a forest-floor system, experimentally reducing spider numbers also accelerated the rate of disappearance of a straw test litter (16). However, a longer-term experiment using natural canopy litter produced an opposite effect, with litter decomposing more rapidly at higher spider densities (12). This extreme variation in the sign of the indirect effect of spiders and other predators on decomposition, from negative through zero to positive, may at least partly reflect differences in abiotic factors between sites and years. The unexpected enhancement of decomposition by spiders in the forest-floor experiment occurred during a period of unusually low rainfall (12), which suggests that large changes in moisture, such as those predicted by climate-change models, may affect ...
Abstract. 1. A field experiment was conducted to determine how short‐term changes in moisture can alter activity‐densities of spiders and springtails.2. In a Kentucky forest 10 unfenced 4‐m2 plots were divided into two rainfall treatments. A clear roof over five plots excluded rainfall to simulate severe drought conditions (drought treatment). Water was sprayed on the five uncovered plots at a rate equal to two times the long‐term mean in order to establish the high‐rainfall treatment. Activity‐densities of Collembola and spiders were measured using pitfall traps designed to sample the top, middle, and bottom layers of leaf litter. The experiment ran from 20 July to 23 September 2001.3. Overall (i.e. litter layers pooled) activity‐density (mean number trapped each sampling date) of Collembola was ≈ 60% lower in drought plots than in plots receiving increased precipitation. Surprisingly, overall spider activity‐density was ≈ 1.6 times greater in the drought plots.4. Differences in rainfall affected the spatial stratification of Collembola and spiders in strikingly different ways. Activity‐densities of neither group differed between drought and high‐rainfall treatments in the bottom litter layer. Collembola activity‐density was three times greater in the top and middle litter layers in high‐rainfall plots than in drought plots. In contrast, spider activity‐density did not differ between treatments in the top layer, but activity‐density was decreased by 50% in the middle layer of high‐rainfall plots compared with drought plots.5. Three Collembola families (Sminthuridae, Tomoceridae, and Entomobryidae) accounted for most of the Collembola pattern. The spider response was due to altered activity‐density of one family of wandering spider, the Gnaphosidae.
Arthropods in the leaf litter layer of forest soils influence ecosystem processes such as decomposition. Climate‐change models predict both increases and decreases in average rainfall. Increased drought may have greater impacts on the litter arthropod community. In addition to affecting survival or behaviour of desiccation‐sensitive species, lower rainfall may indirectly lower abundances of consumers that graze drought‐stressed fungi, with repercussions for higher trophic levels. We tested the hypothesis that trophic structure will differ between the two rainfall scenarios. In particular, we hypothesized that densities of several broadly defined trophic groupings of arthropods would be lower under reduced rainfall. To test this hypothesis, we used sprinklers to impose two rainfall treatments during three growing seasons in roofed, fenced 14‐m2 plots and documented changes in abundance from initial, pre‐treatment densities of 39 arthropod taxa. Experimental plots were subjected to either LOW (fortnightly) or HIGH (weekly) average rainfall based upon climate models and the previous 100 years of regional weekly averages. Unroofed open plots, our reference treatment (REF), experienced higher than average rainfall during the experiment. The two rainfall extremes produced clear negative effects of lowered rainfall on major trophic groups. Broad categories of fungivores, detritivores and predators were more abundant in HIGH than LOW plots by the final year. Springtails (Collembola), which graze fungal hyphae, were 3× more abundant in the HIGH rainfall treatment. Taxa of larger‐bodied fungivores and detritivores, spiders (Araneae), and non‐spider predators were 2× more abundant under HIGH rainfall. Densities of mites (Acari), which include fungivores, detritivores and predators, were 1.5× greater in HIGH rainfall plots. Abundances and community structure of arthropods were similar in REF and experimental plots, showing that effects of rainfall uncovered in the experiment are applicable to nature. This pattern suggests that changes in rainfall will alter bottom‐up control processes in a critical detritus‐based food web of deciduous forests. Our results, in conjunction with other findings on the impact of desiccation on arthropods and fungal growth, suggest that drier conditions will depress densities of fungal consumers, causing declines in higher trophic levels, with possible impacts on soil processes and the larger forest food web.
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