Impact of fish introduction on planktonic food webs in lakes of the Patagonian Plateau

Patagonia fishless ponds have been stocked with fishes for recreational purposes since early in the 20th century. We carried out a summer plankton sampling in 18 Patagonian lakes; 12 fishless, 5 with introduced fishes and 1 with endemic fish fauna. The lakes are situated on a latitudinal gradient from 39° to 49°S. Zooplankton and phytoplankton composition, phytoplankton relative abundance, and zooplankton body size and mouthpart morphology were analyzed. Results showed differences between lakes with and without fishes; in the presence of fish zooplankton size spectrum tended to be narrower because of the disappearance of Daphnia and large centropagid copepods. Zooplankton composition changed: centropagid species richness decreased and rotifers dominated. Contrarily, in fishless lakes 3 or 4 centropagid species, differing markedly in body size and exploiting different food niches, were observed co-occurring. These changes in zooplankton seemed to cascade down to phytoplankton. Fish introduction increased the phytoplankton similarity in lakes even belonging to different basins in a latitudinal gradient. Indeed, cyanobacteria dominated only in lakes with introduced fishes. Probably the elimination of Daphnia favored cyanobacteria proliferation due to nutrient rebalance. As a consequence, water quality decreases and the value of sport fisheries is reduced. Fish introduction in Patagonia is a practice that should be re-evaluated by governments and NGOs due to its potentially negative impact on lakes and local economies.     

Evaluating macroinvertebrate responses to recovery from acidification in small lakes in Ontario,Canada

The biomonitoring program of Environment Canada examines food chains in small Ontario lakes to interpret ecological responses of waterfowl and their foods to changing acid deposition. Macroinvertebrates and fish were sampled in three acid-sensitive regions: Muskoka (1991; N=20), Algoma (1992; N=20), and Sudbury (1994; N=22). Small lakes (<11 ha;=" important=" breeding=" habitat=" for=" waterfowl)=" were=" chosen=" to=" cover=" the=" range=" of=" ph=" in=" each=" region,=" and=" include=" those=" with=" and=" without=" fish.=" in=" all=" regions,=" macroinvertebrate=" taxonomic=" richness=" (particularly=" nekton=" and=" benthos)=" was=" greater=" in=" fishless=" lakes=" compared=" to=" lakes=" with=" fish.=" among=" fishless=" lakes,=" taxonomic=" richness=" (especially=" benthos)=" was=" positively=" correlated=" with=" ph,=" although=" regional=" differences=" were=" evident.=" previous=" studies=" near=" sudbury=" have=" shown=" that=" several=" benthic=" groups=" have=" distribution=" and=" abundance=" patterns=" with=" respect=" to=" ph=" (trichoptera,=" ephemeroptera,=" hirudinea,=" amphipoda,=" and=" gastropoda).=" those=" patterns=" continue=" near=" sudbury,=" and=" were=" also=" strongly=" apparent=" in=" algoma.=" in=" all=" regions,=" the=" number=" of=" acid-sensitive=" taxa=" per=" lake=" is=" related=" to=" ph,=" and=" should=" increase=" as=" lakes=" recover=" from=" acidification.=" however,=" predicting=" macroinvertebrate=" responses=" to=" recovery=" must=" consider=" concurrent=" effects=" of=" fish,=" as=" they=" are=" a=" dominant=" factor=" structuring=" these=">     

Predation history and vulnerability: Conservation of the stickleback adaptive radiation

Recent adaptive radiations offer special insight into the processes that generate biodiversity. The loss of unique members of such radiations undermines their collective evolutionary potential, as well as the ability of scientists to compare divergent populations, thereby devaluing the entire radiation as a system of study. To illustrate our case, we consider the adaptive radiation of the threespine stickleback fish (Gasterosteus aculeatus) in light of anthropogenic threats. Specifically, we consider the effects of stocking with rainbow trout, Onchorynchus mykiss, on populations of stickleback in lakes of two types in the Cook Inlet Region of Alaska, USA: (1) those in which salmonids are native and (2) those historically devoid of predatory fish. Many populations in this second class exhibit reduced pelvic armor that is, in part, a consequence of the historical absence of piscine predators that prey upon stickleback. Long-term trapping data from 77 lakes show that stocking in lakes where trout are not native is associated with large fluctuations in stickleback abundance, relative to lakes in which trout are native, or to lakes in which trout are neither native nor stocked. Pelvic-reduced populations appear to be at most risk from stocking. We discuss how these populations have responded to, and may yet respond to such threats, and how the information we have on these populations can be used to identify evolutionarily significant units (ESUs: Ryder, 1986), and certainly, distinct populations segments (DPSs: [Green, 2005], [Moritz, 2002] and [Waples, 1991]) deemed worthy of protection under Section 3(15) of the 1973 US Endangered Species Act as amended in 1978. Finally, we consider relevance of our results to the conservation of other recent adaptive radiations.     

An update on the lake acidification mitigation project (LAMP)

Limestone addition is a management tactic to mitigate acidic conditions in lakes. The Lake Acidification Mitigation Project is a long-term integrated project to study the ecological effects of liming and develop the technical information necessary for an effective liming program. Three Adirondack lakes have been limed: Cranberry Pond, a small, 60-day hydraulic residence time fishless lake; Woods Lake, a larger fishless lake with longer residence time (214 days); and Little Simon Pond, a large lake (160 ha) containing populations of brook trout, lake trout and other fishes, with a residence time of 450 days. Woods Lake has been limed twice and the stocked brook trout fishery has been maintained; Cranberry has been limed once and then allowed to reacidify with loss of the stocked fishery. Little Simon Pond showed no ill effects of liming on the extant populations. In general, short term evaluation of ecological effects has shown no major deleterious effects of liming. Methods for predicting reacidification are extremely accurate and use of different limestone particle-size fractions is recommended to achieve longer term in-lake neutralization.     

Changes of planktivore fauna and development of zooplankton after liming of the acidified Lake Gårdsjön

The quantitative dynamics of the zooplankton community was studied during a period of 13 years in the manipulated Lake Gårdsjön and an acidified reference lake. Lake Gårdsjön was acid during the first 3 years, limed and fishless the following 4 years and stocked with brown trout for a period of 6 years. During the final 2 years rainbow trout was added. Changes in water chemistry and phytoplankton standing stock are associated with the manipulations. Liming had a positive effect on the overall species richness and diversity among rotifers and cladocerans. Changes in the zooplankton community during the different phases of the lake restoration are discussed in relation to the development of the planktivorous fauna of the lake, phantom midge larvae, aquatic hemipterans and fish. It can be concluded that liming creates a potential for the restoration of the zooplankton diversity, but the actual structure of the zooplankton community in limed lakes is strongly influenced by predatory and competitive interactions.     

Does natural acidity mediate interactions between introduced brown trout, native fish, crayfish and other invertebrates in West Coast New Zealand streams?

The presence of introduced brown trout (Salmo trutta) on the distribution of native crayfish (Paranephrops planifrons), native galaxiid fishes (Galaxias spp.) and invertebrate fauna was investigated in 18 West Coast New Zealand streams (8 with trout and 10 without trout) differing in chemical characteristics. Gut contents of trout, crayfish and eels were also examined to evaluate whether competition or predation could be linked to the patterns found. Abundances of crayfish and galaxiids were significantly lower in streams with trout, but in streams without trout, substrate size was one of the most important factors determining crayfish abundance. In contrast to crayfish, other macroinvertebrates were more abundant in trout streams than streams without trout and significantly more taxa were found in streams with trout. Macroinvertebrate abundance was related to environmental factors, such as pH, substrate, depth and total nitrogen. Gut content analysis showed an overlap in diet (mostly invertebrates) between trout and eels. Crayfish, however, had a more omnivorous diet where detritus was the most frequently occurring food material. Differences in chemical characteristics marked the streams with and without trout. Thus, trout were not present in streams with pH < 6.0. Crayfish and galaxiids were present in streams with pH ranging from 4.1 to 7.9, and those with pH < 6.0 may function as trout-free refuges where larger populations of these species may persist. However, other macroinvertebrate taxa may be more negatively affected by acidification than by trout. By protecting naturally acidic, brown water streams, New Zealand crayfish and galaxiid fish populations can be conserved within geographic areas where trout are present.     

Metals in stocked Lake Trout (Salvelinus namaycush) in lakes near Sudbury,Canada

Muscle tissue from stocked lake trout (Salvelinus namaycush) from lakes near the smelters at Sudbury, Canada was analyzed for Al, Cu, Fe, Hg, Mn, Ni, and Zn. Metals in tissue were not abnormally high compared with fish from lakes farther away from the smelters. Tissue concentrations of Al, Fe, Mn, and Zn declined significantly after stocking. Because metal concentrations were lower in hatchery water than the study lakes, this decline in tissue metal suggests that diet plays a prominent role in the uptake of these metals in lake trout. Only Hg showed evidence of accumulation to levels that might be harmful for human consumption, and then only in a few older, larger trout. In contrast, tissue Mn and Zn were inversely correlated with trout weight. Fluctuations, independent of age, body weight or time of year, were observed in tissue concentrations of Al, Cu, and Zn. Annual cycles, independent of pH and alkalinity were observed in tissue concentrations of Fe, Hg, Mn, and Ni. Bioaccumulation of metal proved to be an unfounded concern for the rehabilitation of fish populations in lakes near Sudbury.     

Liming of lakes,rivers and catchments in Norway

Despite a slight reduction in the level of acidic deposition in Norway, acidification of lakes and rivers continues. The Norwegian Liming Project (1979–84) demonstrated that lime treatment can be an effective measure against acidification of watercourses given appropriate adaptation to local conditions. Liming in Norway is difficult because of (1) large amounts of precipitation, (2) short retention time of lakes, and (3) episodic changes in water chemistry. In 1988 NOK 14 mill. has been allocated to operational liming and research. We report here on chemical and biological responses from lime treatment of a lake, a river and a catchment. Lake Store Hovvatn was limed in 1981 and successfully stocked with brown trout. Before reliming in 1987, fish growth had ceased, but increased post liming. The River Audna has been continuously limed since 1985. Sea trout fisheries have improved, and the stocking of Atlantic salmon smolts at the mouth of the river in 1986 has already resulted in the return of spawners. Liming of the entire terrestrial catchment to the pond Tjønnstrond in 1983 by helicopter was also successful; stocked brown trout have survived to the present.     

Status of Fish and Macroinvertebrate Communities in a Watershed Experiencing High Rates of Fossil Fuel Extraction: Tenmile Creek, a Major Monongahela River Tributary

Over the summer and fall seasons, 2006?C2010, we surveyed the fish and macroinvertebrate communities of the Tenmile Creek basin in southwestern Pennsylvania, an area undergoing accelerated extraction of energy resources??historically coal and more recently natural gas associated with the Marcellus formation. Tenmile Creek, its major South Fork (SF), and numerous tributaries drain a basin of 875?km². The drainage network is characterized as warm-water, low-gradient, and net alkaline. The purpose was to provide synoptic baseline data on water quality and the resident aquatic communities in terms of species richness, stress tolerance, and trophic structure. Overall, we sampled 20 stations on the 2 main branches and 1 on each of the 12 tributaries. We collected 26,375 fishes representing nine families and 54 species/hybrids along with 989 macroinvertebrates from 25 separate taxa. The parameter which defines water quality here is specific conductance which ranges from natural background levels of about 400???S/cm on the minimally impaired Tenmile mainstem to 4,500???S/cm on its SF. Diverse fish and macroinvertebrate communities were documented at levels of specific conductance exceeding 1,000???S/cm, well above the 300???S/cm criterion to protect aquatic life proposed by the US Environmental Protection Agency for streams in the Central Appalachian Region. South Fork fish communities exhibit impairment at levels of specific conductance approaching the maximum observed here.     

Liming the acid lake Hovvatn,Norway: A whole-ecosystem study

Hovvatn, a 1-km² chronically-acidified lake in southernmost Norway, was treated with 200 tonne of powdered limestone in March 1981. An additional 40 tonne were added to a 0.046 km² pond (Pollen) draining into Hovvatn. The lakes were stocked with brown trout in June 1981 and in each subsequent year. At ice-out pH rose from 4.4 to 6.3 (Hovvatn) and 7.5 (Pollen), Ca and alkalinity increased, and total Al decreased by 120 μg L^?1. None of the other major ions exhibited significant changes in concentration. Total organic Cand Pincreased after liming. The phytoplankton community was dominated by chrysophytes and did not change significantly following liming. The zooplankton community was typical of acid lakes prior to liming. There was a clear succession in species dominance following treatment, although no new species immigrated to the lakes. Zoobenthos changed from a community characterized by low abundance and reduced number of species to increased abundances of oligochaetes, mayflies and chironomids. Hovvatn and Pollen were barren of fish prior to stocking. The stocked fish showed remarkably high growth rate during the first years. Liming apparently improved conditions for zoobenthos, enhancing the processing of fine detritus which in turn resulted in elevated levels of TOC and P in the lakewaters during the first year after liming. The “oligotrophication” process typical of acid lakes was temporarily reversed by liming. The interactions between groups of organisms in Hovvatn and Pollen indicates that many years are required before a new steady-state can be attained following liming.     

Biotic Effects in Planktonic Crustacean Communities in Acifified Swedish Forest Lakes after Liming

In Sweden, approximately 16 000 of a total of about 85 000 lakes have been acidified due to acidic deposition. Of these about 8000 have been treated with limestone powder in order to detoxify the acidified waters and protect sensitive fauna. The present study was performed in ten lakes in the southern part of the country. The lakes belong to four different catchments and were in different stages of acidification at the time of lime treatment. The composition of the zooplankton and fish communities also differed and three lakes were empty of fish at the beginning of the studies. Quantitative sampling of planktonic crustaceans was performed during the ice free season between 1976–87 in five of the lakes and between 1977–87 in the other five. After treatment the pH increased significantly in all lakes except one. The average number of crustacean taxa found per sampling occasion increased in all lakes. Increases were statistically significant in four of the lakes. In the lakes empty of fish, increased abundances of chaoborids inhibited, by predation, the increase of species richness. Species richness increased after the introduction of fish and the subsequent reduction of the chaoborids. At the end of the study, more taxa were found in the limed study lakes than in non-treated west coast lakes with an alkalinity of 0.04–0.10 meq L^-1. Most species normally occurring in oligotrophic forest lakes were found. It was shown that the water quality after liming made the occurrence of sensitive species possible and that predation from fish and interactions within the zooplankton assemblage were of great importance to the species composition and structure of the zooplankton community.     

Liming of acidified lakes: Induced long-term changes

This study presents data concerning long-term trends after neutralization of four acidified lakes in two regions on the Swedish west coast. Neutralization was achieved by a di-Ca-silicate with 52% CaO and about 11.5% MgO. Between 61 and 74% of the spread lime product dissolved during a 5 to 7 yr period. The liming increased pH, from a range of 4.5 to 5.2 to near neutral and restored alkalinity in the range of 0.2 to 0.3 meq l^?1 and the Ca-content became 3 to 4 times higher than before liming. In two lakes transparency decreased significantly presumably due to changed phytoplankton composition. These changes successively declined due to dilution and continuous acid loading. The changes in water chemistry and development of stocked brown trout (Salmo trutta) populations initiated biotic changes. Phyto- and zooplankton communities reacted both instantly and later with successions in species composition. Changes of benthic macroinvertebrate species occured over several years, but some pelagic species, e.g. corixids were rapidly reduced due to predation of fish. Observed changes were predominantly due to expanding populations of species present at very low abundances even during acid state of the lakes. Some organisms found during preacid state of the lakes did not establish new populations and this process may need a prolonged time with favorable conditions. Reacidification towards the end of the study period significantly stressed the brown trout population and also favored expansion of the filamentous algaMougeotia sp. andSphagnum sp. that almost vanished during the first year after liming. Decreasing concentration of total P was not influenced by neutralization and may be mostly dependent on negative changes in the soils surrounding the lakes. If generally valid, this process may be an important factor for the oligotrophication of lakes in areas where acid deposition is high.     

Hybridization mechanisms between the endangered marble trout (Salmo marmoratus) and the brown trout (Salmo trutta) as revealed by in-stream experiments

Populations of the marble trout (Salmo marmoratus) are critically declining due to introgression by brown trout (S. trutta) strains. Hybrids between the two forms are fertile and presently predominant in most rivers of the species’ range.The involved hybridization mechanisms have been studied through two large scale in-stream experiments (Driselpoh and Stopnikarca) as 50% of each species have been stocked at the age of one year in fishless streams, each fish being individually marked. Diagnostic molecular markers were applied to test a partial reproductive isolation between the two species. Emphasis was put on survival and growth patterns of stocked fish (parental generation) as well as on fish hatched within the stream (F₁ generation).No evidence of a partial reproductive barrier between the two species was observed. Survival of the parental generation depended on the year in both streams, as well as on the species in Stopnikarca, but was identical for both species in Driselpoh. In both streams survival of instream hatched individuals measured from 0+ to 2+ was lower for brown trout. In Driselpoh, F₁ 0+, 1+ and 2+ hybrids were larger than pure individuals. Larger hybrids were only observed in Stopnikarca when analyses focused on individuals in inter-specific competition suggesting that heterosis and stress effects may explain the observed size differences.Our results point out important ecological differences between marble and brown trout and have shown that hybridization can easily take place. The findings indicate that high F₁ hybrid presence, survival and heterosis effects may impede marble trout rehabilitation in the area.     

Removal of nonnative fish results in population expansion of a declining amphibian (mountain yellow-legged frog, Rana muscosa)

The mountain yellow-legged frog (Rana muscosa) was once a common inhabitant of the Sierra Nevada (California, USA), but has declined precipitously during the past century due in part to the introduction of nonnative fish into naturally fishless habitats. The objectives of the current study were to describe (1) the effect of fish removal from three lakes (located in two watersheds) on the small, remnant R. muscosa populations inhabiting those lakes, and (2) the initial development of metapopulation structure in each watershed as R. muscosa from expanding populations in fish-removal lakes dispersed to adjacent habitats. At all three fish-removal lakes, R. muscosa population densities increased significantly following the removal of predatory fish. The magnitude of these increases was significantly greater than that observed over the same time period in R. muscosa populations inhabiting control lakes that remained in their natural fishless condition. Following these population increases, R. muscosa dispersed to adjacent suitable (but unoccupied) sites, moving between 200 and 900 m along streams or across dry land. Together, these results suggest that large-scale removal of introduced fish could result in at least partial reversal of the decline of R. muscosa. Continued monitoring of R. muscosa at the fish-removal sites will be necessary to determine whether the positive effects of fish eradication are sustained over the long-term, especially in light of the increasingly important role played by an emerging infectious disease (chytridiomycosis, caused by Batrachochytrium dendrobatidis) in influencing R. muscosa populations.     

Fish species distribution in relation to water chemistry in selected Maine lakes

We examined the possible effects of acidity on the number of fish species in 22 lakes in Maine, ranging in pH from 4.4 to 7.0 (mean values). We caught no fish in three lakes with pH <5.0, but collected 1 to 9 species in the remaining 19 lakes (pH 5.4 to 7.0). Brook trout (Salvelinus fontinalis), golden shiners (Notemigonus crysoleucas), and white suckers (Catostomus commersoni) were ubiquitous, but no common shiners (Notropis cornutus) or creek chubs (Semotilus atromaculatus) were collected from lakes with pH below 6.0 and 5.9, respectively. The fishless lakes differed from the others primarily in water chemistry variables related to acidity, i.e., pH, Al, and concentration of divalent cations. Among lakes that contained fish, the factors related to the number of species collected were lake surface area and maximum depth, which may be related to habitat quantity and diversity. Cluster analysis identified two distinct fish species groups — depauperate and cyprinid-sucker — but multiple comparison analysis failed to relate any measured chemical or physical variable to these two groups, probably because water chemistry was suitable for reproduction by these species.     

Responses of Riverine Macroinvertebrates to Zinc in Natural Streams: Implications for the Japanese Water Quality Standard

We conducted a field survey in Japanese streams and rivers to generate useful information about whether the Japanese water quality standard for zinc (Zn) in the freshwater environment (30 μg/L) is over- or underprotective of the populations and communities of lotic benthic macroinvertebrates. Many macroinvertebrate populations were reduced and community characteristics were altered at two polluted sites with Zn concentrations above 1,000 μg/L as compared with control sites with Zn concentrations below 30 μg/L. In contrast, at least one site among three polluted sites with Zn concentrations of 80–126 μg/L experienced noncritical effects (i.e., no significant difference was observed between the sample mean at a polluted site and that at the control sites, and the sample mean was within the minimum–maximum range of those at the control sites) for most of the ten community metrics for diversity and abundance. In addition, the abundances of 95% of the dominant taxa were not always significantly lower at more than one polluted site (Zn concentrations, ≥80 μg/L). Our results suggest that Zn concentrations below approximately 100 μg/L are likely to protect most aspects of richness and abundance of the riverine macroinvertebrate communities in the study areas. This study shows a case that the Japanese Zn standard is likely overprotective of the macroinvertebrate populations and communities.     

Stocking of brown trout (Salmo trutta L.) cohorts after liming- effects on survival and growth during five years of reacidification

The chronically acid Hovvatn and the adjoining pond Pollen were limed in March 1981. During the first 4 yr after the liming, a total of 11 437 brown trout were stocked in the two locations. The fish population was monitored by annual testfishing in a five yr period following the liming. The stocking material constituted 6 cohorts and included fish stocked at age 0+, 1+ and 2+. The stocking program assured a 7.9 times higher density of fish ha^?1 in Pollen than in Hovvatn. Consequently, the fish from Hovvatn grew significantly better and had a significantly higher condition factor than fish in Pollen. Monitoring of water chemistry showed that the cohorts were exposed to a gradual reacidification process, abrupted by episodic events of severe acidification. Within 2–3 yr after the liming, the locations had reacidified to conditions which were considered critical for fish. However, the acidinduced increase in mortality appeared 1–2 yr later than what was expected from water chemistry data. This discrepancy could most likely be ascribed to the existence of water chemical refugia which enhanced the survival of the fish. Recaptures were significantly correlated to pH, Ca, labile-Al and ANC. Growth were significantly reduced throughout the reacidification period. Estimated yield showed that fish stocked at age 2+, as opposed to fish stocked at a younger age, managed to turn the high food availability into high growthrates before the reacidification retarded growth and survival.     

Do native species limit survival of reintroduced Atlantic salmon in historic rearing streams?

Reintroduction of extirpated populations creates a unique context that can exacerbate the effects of interactions among species. Thus, reintroduced populations may be particularly vulnerable to predators and competitors, including native species with which they historically coexisted. In this study, we evaluated the effect of native fishes on survival of reintroduced Atlantic salmon (Salmo salar) in the Connecticut River basin, where the native salmon population is extinct. Juvenile salmon are stocked annually in many Connecticut River tributaries. We sampled salmon reintroduction sites across tributaries with different fish communities to determine whether native fish reduce the success of salmon reintroductions (N = 19 site-years). Increased density of slimy sculpin (Cottus cognatus), a native generalist predator, was associated with reduced recruitment of reintroduced salmon. Salmon first-summer survival declined with increased sculpin density across sites, and low first-summer survival led to reduced densities of overyearling salmon the subsequent year. Hierarchical partitioning analysis showed that the negative relationship between sculpin and salmon was independent of potentially confounding variation in other fish community or habitat characteristics. Negative effects of native, historically-sympatric species, particularly generalist predators, can impede restoration of extirpated populations.     

Effects of seasonal grazing and precipitation regime on the soil macroinvertebrates of a Mediterranean old-field

Soil macroinvertebrate communities (SMC) are well known to influence major ecosystem processes, but relatively few investigations have examined the mechanisms and factors involved in SMC regulation. We conducted a factorial experiment with combinations of seasonal grazing by sheep and irrigation (simulating different precipitation regimes) to assess their effects on the SMC of a semiarid Mediterranean old-field. We also analyzed effects on plant species richness, total aboveground biomass, and litter. The data were collected in autumn and spring, the two favorable seasons for SMC and primary production in the region, and season was included as an additional random factor. Main results were: 1) Ungrazed plots accumulated more aboveground plant biomass and litter during spring, providing extra food for soil biota. However, grazing during autumn or spring did not affect SMC characteristics. 2) Reduction of inter-annual precipitation variability in autumn and spring increased the abundance of two decomposer taxa: Oligochaeta and Diplopoda. Additionally, if summer drought was reduced, plant species richness, litter and the abundance of Isopoda were increased. 3) Oligochaeta and Diplopoda increase their abundance in spring, particularly, the most abundant taxon (Oligochaeta). We conclude that inter- and intra-annual variability in precipitation is a key environmental factor for the decomposer soil fauna in Mediterranean ecosystems, modifying the physical characteristics of the soils (humidity, hardness, etc.), as well as affecting the amount or characteristics of plant biomass or litter. The respiration system of the macroinvertebrates (cutaneous, tracheal or branquial) and the capacity to migrate vertically into the soil may determine the decomposers' responses to precipitation.