摘要:

The niche concept is a central organizing aspect of modern ecology. Although its history has often been reviewed, the structure of the concept and its connection to advances in ecological theory has received less recent attention. I review the niche concept using “mechanistic” models of community theory to identify two distinct components. One describes the environmental requirements of organisms and the other describes the per capita impact of organisms on the environment. I argue that these correspond to significant differences between Grinnell's and Elton's concepts distinct from the previously discussed “habitat” vs. “functional” dichotomy. I illustrate the distinction between the requirement and impact components of the niche using models of resource competition and of keystone predators, and I discuss “Gause's axiom” and conventional “niche theory” in the context of these two distinct niche components. I suggest that the niche concept be elucidated by explicit reference to these two distinct components; the “impact” niche (corresponding to Elton's concept) describing instantaneous per—capita effects of species on the environment, and the “requirement” niche describing the responses of species to the environment (corresponding to Hutchinson's definition). This approach connects conventional niche theory with “mechanistic” individual—based ecological models and can help provide a more modern context for the niche concept.

ISSN:0012-9658    

DOI:10.2307/1938141

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

We analyzed changes in the stable C isotope composition (°13C) of bulk tissues and lignin fractions during a 2—yr decomposition study in east—central Minnesota (USA) of aboveground and belowground litter from four perennial grass species: Schizachyrium scoparium (C4), Agropyron repens (C3), Poa Pratensis (C3), and Agrostis scabra (C3). Although lignin concentrations increased for all litter types during decomposition and lignin fractions were consistently depleted in13C compared to bulk tissues (3.6% more negative on average), we found neither convergence of bulk tissue °13C values towards lignin °13C values, nor greater stability of °13C values for lignin fractions. Furthermore, °13C values of C3and C4species shifted in opposite directions during decomposition. Thus, our data do not support the hypothesis that °13C values decrease during decomposition because of the selective preservation of lignin and we instead suggest the isotopic shifts are caused by the incorporation of new C from soil organic matter into litter by microbial decomposers. We estimate that this new C comprised 12—19% of the total litter C, depending on species, at the point of 70% mass loss. In monocultures of these four species plus another C4grass (Andropogon gerardi) growing on initially homogeneous soils with a predominantly C3isotopic signature, soil °13C values increased 1.6—2.2 for the C4species and remained relatively unchanged for the C3species after 4 yr. Averaging across the C4species and the experimental soil organic matter gradient, 14% of the total soil C in these plots must be new C4C to account for this isotopic shift. We estimate that this amount of new soil C equals 30% of NPP summed over 4 yr in these plots.

ISSN:0012-9658    

DOI:10.2307/1938142

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Litter bags with natural mixed litter were incubated until °60—70% mass loss in two oak—hornbeam and two pine—beech forest stands in southern Poland. At the same stands the input of chemical elements with throughfall was followed. Decomposition constants k for the oak—hornbeam litters were —0.57 and —0.55, and for the pine—beech litters —0.30 and —0.27. Chemical elements (except for Cu and Mn) revealed similar relative mobility in the four litters. On average the elements could be ordered by decreasing mobility as follows: K>Mg>Ca>S>Cu>Na>Mn = N>Cd>Pb = Zn>Fe. Instead of the two presupposed factors controlling litter decomposition, biological and chemical, three factors were specified: (1) biological, dominating the decay of organic matter and the dynamics of N, Ca, Mg, Mn, and S; (2) physical, dominated by leaching and atmospheric deposition, and controlling the dynamics of organic matter, K, Na, Pb, Cd, and Zn; and (3) chemical, determining the dynamics of Fe, Zn, Pb, and Cd through the fixation of metal ions to humic substances. Potassium was the only element that decreased in concentration in all litters, while the concentrations of N, Na, Fe, Zn, Pb, and Cd increased in all litters. S. Ca, Mg, and Mn concentrations revealed different patterns in different litters, presumably due to the differences in initial concentrations and soil acidity. No clear trend was found for Cu. In all litter types, Fe, Zn, Pb, and Cd significantly increased in absolute amounts at the end of litter—bag incubation. In all four stands the input with throughfall was high enough to explain the increases in amount of elements, with the exception of Fe in the oak—hornbeam litters.

ISSN:0012-9658    

DOI:10.2307/1938143

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

We tested the Walker and Syers (1976) conceptual model of soil development and its ecological implications by analyzing changes in soil P, vegetation, and other ecosystem properties on a soil chronosequence with six sites ranging in age from 300 yr to 4.1 x 106yr. Climate, dominant vegetation, slope, and parent material of all of the sites were similar. As fractions of total P, the various pools of soil phosphorus behaved very much as predicted by Walker and Syers. HCI—extractable P (presumably primary mineral phosphates) comprised 82% of total P at the 300—yr—old site, and then decreased to 1% at the 20,000—yr—old site. Organic phosphorus increased from the youngest site to a maximum at the 150 000 yr site, and then declined to the 4.1 x 106yr site. Occluded (residual) P increased steadily with soil age. In contrast to the Walker and Syers model, we found the highest total P at the 150 000—yr—old site, rather than at the onset of soil development, and we found that the non—occluded, inorganic P fraction persisted through to the oldest chronosequence site. Total soil N and C increased substantially from early to middle soil development in parallel with organic P, and then declined through to the oldest site. Readily available soil P, NH4+, and NO3—were measured using anion and cation exchange resin bags. P availability increased and decreased unimodally across the chronosequence. NH4+and NO3—pools increased through early soil development, but did not systematically decline late in soil development. In situ decomposition rates of Metrosideros polymorpha litter were highest at two intermediate—aged sites with soil fertility (20 000 yr and 150 000 yr), and lowest at the less—fertile beginning (300 yr) and endpoint (4.1 x 106yr) of the chronosequence. M. polymorpha leaves collected from these same four sites, and decomposed in a common site, suggested that leaves from intermediate—aged sites were inherently more decomposable than those from the youngest and oldest sites. Both litter tissue quality and the soil environment appeared to influence rates of decomposition directly. The highest mean soil N2O emissions (809 mg°m—2°d—1) were measured at the 20 000—yr—old site, which also had the highest soil nitrogen fertility status. Plant communities at all six chronosequence sites were dominated primarily by M. Polymorpha, and to a lesser extent by several other genera of trees and shrubs. There were, however, differences in overall vegetation community composition among the sites. Using a detrended correspondence analysis (DECORANA), we found that a high proportion of species variance among the sites (eigenvalue = 0.71) can be explained by site age and thus soil developmental stage. Overall, long—term soil development across the chronosequence largely coincides with the conceptual model of Walker and Syers (1976). How P is distributed among different organic and inorganic fractions at a given stage of soil development provides a useful context of evaluating contemporary cycling of P and other nutrients, and for determining how changes in P availability might affect diverse ecosystem processes.

ISSN:0012-9658    

DOI:10.2307/1938144

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

In order to better understand the spatial distributions of soil trophic groups and the potential significance of these distributions to ecosystem functioning we initiated a study to describe the within—site variability of nematode feeding groups in a row—crop ecosystem. Soil cores were removed from a 48—ha corn (Zea mays) field in the U.S. Midwest prior to spring planting, and nematodes were identified by phenotypic criteria to four groups: bacterivores, fungivores, omnivores/predators, and plant parasites. Within—site variability was high for all groups; population counts spanned two orders of magnitude, with coefficients of variation ranging from 40—130% (n = 115—138 soil samples). Probability distributions were strongly lognormal. Geostatistical analysis showed that a major part of this variability was spatially dependent; variograms suggest that 70—99% of sample population variance was related to spatial autocorrelation over our geographic range of 6—80 m, except for the parasitic group, for which we detected no autocorrelation to 1200m. Maps of nonparasitic feeding groups across the field showed large multi—hectare areas of low to moderate population densities, with sub—hectare clusters of high—density populations towards one end of the site. Individual feeding groups were only weakly correlated with one another across the field (Kendall's t<0.363, P<0.001). Edaphic factors (bulk density, texture, pH, C availability, N availability) could collectively explain<30% of the variability in the nonparasitic groups across the area sampled. Results suggest that important soil food web components are strongly patterned at sub—hectare scales in this site. That this patterning is maintained in an ecosystem subjected to the homogenizing influences of annual soil tillage and a monoculture plant population is remarkable, and suggests that such patterning may be even more common in less—disturbed sites. Inclusion of these patterns in studies of ecosystem processes and soil community dynamics may significantly improve soil trophic models and our understanding of the relationship between soil populations and ecosystem function.

ISSN:0012-9658    

DOI:10.2307/1938145

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

We examined the influenced of stream water chemistry on relationships between fungal activity and breakdown rates of yellow poplar (Liriodendron tulipifera) leaves in eight streams that varied with respect to pH and nutrient (nitrate and phosphate) concentrations. We also performed a reciprocal exchange experiment of leaves that had been colonized by microoganisms in two streams with contrasting water chemistries. Decomposer activity varied greatly depending on the stream in which the leaves were placed. Variation in breakdown rates of yellow poplar leaves was over 9—fold maximum ATP concentrations associated with leaves varied as mush as 8—fold, and maximum sporulation rates of fungi associated with leaves varied over 80—fold among streams. Among all streams, nitrate, phosphate, and temperature were positively correlated with one another and with decomposer biomass and activity. When hardwater streams were analyzed separately, nitrate concentration was the only variable that was significantly correlated with all measures of microbial activity and leaf breakdown. Consequently, nitrate concentration appeared to explain much of the variation we detected among streams. Responses to the reciprocal exchange experiment were rapid, with significant changes occurring within the first 5 d after the transfer. Leaves transferred from the hardwater stream containing relatively high concentrations of nitrate and phosphate to the softwater stream containing low concentrations of nutrients exhibited by large decreases in both ATP concentrations and sporulation rates, whereas ATP concentrations and sporulation rates increased when leaves received the reciprocal transfer. The fungi associated with decomposing leaves in streams appear to obtain a significant portion of their nutrients (e.g., nitrogen and phosphorus) from the water passing over the leaf structure. These results indicate that the chemistry of the water can be an important regulator of leaf breakdown in streams by affecting the activity of decomposer fungi.

ISSN:0012-9658    

DOI:10.2307/1938146

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Invertebrate diversity patterns were examined in 11 freshwater habitats (10 streams and a windswept lake shore) of similar physicochemical nature but different thermal and hydrologic stability in the Cass—Craigieburn region, New Zealand. Species richness and density were markedly higher at the more stable sites, but species evenness peaked at sites of intermediate stability. Of the 20 environmental variables examined, a multivariate instability index incorporating temporal variation in depth, temporal variation in current speed, substrate stability, the Pfankuch channel stability index, temperature range, and stream reach tractive force was the single best predictor of the number of species, whereas epilithic pigment concentration was the single best predictor of invertebrate density. The pattern in species richness did not support any of three diversity hypotheses considered. In contrast, the pattern in species evenness suggested competitive exclusion may be occurring patchily and that Hutson's dynamic equilibrium model may have some validity, at least at the level of the patch. However, the strong link between productivity and stability apparent in these habitats, and a lack of information on the effects of increased productivity on competition in stream benthic communities makes any firm assessment of the latter model difficult. The observed diversity patterns are, however, consistent with the idea that high diversity is maintained in these habitats by an interaction between low levels of disturbance and habitat patchiness.

ISSN:0012-9658    

DOI:10.2307/1938147

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Stream fish sometimes show mutimodal distributions, with high densities in the tributaries of a river but rarity or absence in the river itself. To assess if predation can produce such a fragmented distributional pattern on a large geographic scale, we determined the density and habitat use of a prey fish in two tropical stream watersheds, each with a barrier waterfall that split the drainage into a region with a strongly piscivorous fish and a region lacking a strong lacking a strong piscivore. In contrast to sites with the strong piscivore, the prey fish in areas above barriers showed a dramatic expansion into the main river, thereby spatially consolidating an otherwise fragmented distribution. Manipulation of piscivores in a third—order stream flanked by a series of first—order experimental streams also showed that the prey distribution expanded into the third—order stream when piscivores were excluded, and that the presence of the piscivores in the third—order stream reduced prey densities both by killing the prey and by inducing the prey to ascend cascades to enter the tributaries. Because the predator created spatial fragmentation of the prey population, we examined prey—fish dispersal in the experimental stream facility to ask whether the predator could reduce prey movement into and beyond the predator—occupied sites. As hypothesized, the experiment revealed that predators could block prey dispersal by killing prey. However, the experiment also suggested that predators may increase prey movement between tributaries by inducing shifts out of river sites. The results suggest that realistic models of dispersal by prey would need to account for effects of both predator consumption and prey behavioral responses to the predator.

ISSN:0012-9658    

DOI:10.2307/1938148

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Females of many insects posses an ovipositor, which in crickets is usually used to place eggs deep into the soil. Ovipositor length in female striped ground crickets (Allonemobius socius) increased at higher latitudes and altitudes in North America, despite an opposing trend in overall body size. Crickets inhabiting regions with short growing seasons and long, cold winters invariably had longer ovipositors than crickets inhabiting warmer regions. The only exception to this observation was for first generation crickets in bivoltine populations where there was no pattern of geographic variation in ovipositor length. However, second generation females from these same populations always had longer ovipositors than their first generation ancestors,and followed a pattern of geographic variation similar to that found for univoltine crickets. Overall, these results support Masaki's (1986) model, which predicts that longer ovipositors will evolve in northern cricket populations as an evolutionary response to the selective advantages of overwintering eggs being oviposited deep in the soil in cold habitats. In order to determine the mechanisms underlying patterns of phenotypic variation in the wild, and to test the hypothesis that interpopulation variation reflects genetic differentiation while intergeneration variation within a population reflects environmentally induced phenotypic plasticity, crickets from eight populations were brought to the laboratory and reared through three generations in a common garden. It was found that laboratory—reared populations varied significantly in ovipositor length, with the pattern of variation mimicking that found for field—collected crickets. This finding supports the hypothesis that geographic variation in the wild has a significant genetic basis. The influence of environment on ovipositor length was examined in 20 full—sib families from a bivoltine population. Crickets reared in "autumn—like" conditions (30°C, 15:9 h, L:D), such as might be experienced by second generation bivoltine crickets in the wild, were generally larger and had significantly longer ovipositors than siblings reared under "spring—like" conditions (27°C 11:13 h, L:D). This pattern corresponds to that observed for bivoltine populations in the wild and suggests the importance of environmental cues for phenotypic plasticity, or polyphenism in ovipositor length in bivoltine populations of this cricket.

ISSN:0012-9658    

DOI:10.2307/1938149

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

To examine the effectiveness of larval antipredator mechanisms and to test the assumption that diet breadth and chemistry are important predictors of predation responses. I offered 70 species of lepidopteran larvae to the predatory ant Paraponera clavata (Hymenoptera: Formicidae). Prey chemistry and diet breadth were significant predictors of rejection, as was plant chemistry but to a lesser degree. Specialist caterpillars were better protected than generalists, and prey with unpalatable extracts were frequently rejected by P. clavata, while prey with palatable extracts were rarely rejected. Specialists with unpalatable host plant extracts were more likely to be rejected than specialists that had host plants with more palatable extracts. Other significant predictors included morphology, behavior, and interactions between developmental stage and prey chemistry and between developmental stage and diet breadth. I concluded that predation could be a substantial selective force in the evolution of narrow diet breadth and that plant chemistry could be the mechanism whereby specialists are better defended.

ISSN:0012-9658    

DOI:10.2307/1938150

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY