Showing papers in "Oikos in 1997"
TL;DR: In this paper, the authors analyzed first-year leaf litter decomposition data from 44 locations, ranging from cool temperate sites to humid tropical sites, and found that the control of AET on litter decomposability is partly mediated through an indirect effect of Aet on litter chemistry.
Abstract: Litter decomposition is an important component of the global carbon budget. Due to the strong climatic control of litter decomposition, climate change may significantly affect this pathway. This review quantifies the climatic influences on litter decomposition rates, both directly and indirectly through effects on litter chemistry. To this end, I analysed first-year leaf litter decomposition data from 44 locations, ranging from cool temperate sites to humid tropical sites. Actual evapotranspiration (AET) was used as an index for the climatic control on decomposition. As litter chemistry parameters I included N and P concentrations, C/N and C/P ratios, lignin concentrations, and lignin/N and lignin/P ratios. At a global scale, climate (expressed as AET) is the best predictor for the decomposition constants (k-values) of the litter, whereas litter chemistry parameters have much lower predictive values. Path analysis showed that the control of AET on litter decomposability is partly mediated through an indirect effect of AET on litter chemistry. Thus, the relation between climate, leaf litter chemistry and leaf litter decomposition is a triangular relationship. Mean AET in the humid tropical region is three times as high as in both the temperate and the Mediterranean region and this results in a more than six-fold increase in mean k-values. However, due to the large variability in k-values within each region there is a considerable overlap in k-values between the tropics and the other climatic regions. Within a particular climatic region litter chemistry parameters are the best predictors of k-values, especially in the tropics, whereas the percentage of variance in k-values explained by AET is low or absent. In general, litters from the tropical sites have higher N concentrations and lower lignin/N ratios than litters from other climatic regions. In both the tropics and in the Mediterranean region, the lignin/N ratio is the best chemical predictor of litter decomposability. In the temperate region, however, there is no good chemical predictor of litter decomposability.
1,559 citations
TL;DR: In this paper, two general tactics of resource use that also include foraging decisions are discussed under the concepts of capital and income breeding, defined mainly from the temporal distribution of resource acquisition relative to resource use.
Abstract: In order to compensate for the resource demands of reproduction, organisms usually increase the amount of total food resources available. This may be achieved by different tactics of resource use that also include foraging decisions. Two such general tactics are discussed in this paper under the concepts of capital and income breeding. These are defined mainly from the temporal distribution of resource acquisition relative to resource use. A capital breeder acquires its resources in advance and store them endogenously or exogenously until they are needed to supply aspects of offspring production. An income breeder, on the other hand, adjusts its food intake concurrently with breeding, without reliance on stores. In a perfectly predictable environment without limited resources, income breeding is the best option since capital breeders may have to pay a number of energetic and demographic costs for their stored resources. However, under unpredictable food conditions, food/time limitations, and risky foraging conditions, capital breeding offers many benefits. The costing systems (pre- or postbreeding costs) induced by capital and income breeding will largely influence the opportunity for these energetic tactics to evolve. While capital and income breeders may potentially experience both pre- and postbreeding costs, capital breeders may be more exposed to prebreeding costs due to predation in connection with acquisition and carriage of stores.
747 citations
TL;DR: It is suggested that increased species diversity of plant litter is less important than that of live plants for determining ecosystem properties and perceived relationships between biodiversity and ecosystem function may be of diminished significance when the ecological importance of plant littered is fully appreciated.
Abstract: There has been a rapidly increasing recent interest in the effects of biological diversity on ecosystem properties, and while some studies have recently concluded that biodiversity improves ecosystem function, these views are based almost entirely on experiments in which species richness of live plants has been varied over all the species diversity treatments. However, most net ecosystem primary productivity eventually enters the decomposition subsystem as plant litter where it has important afterlife effects. Weconducted a field experiment in which litter from 32 plant species (i.e. effects. We conducted a field experiment in which litter from 32 plant species (i.e. eight species of each of four plant functional groups with contrasting litter quality) was collected and placed into litter-bags so that each litter-bag contained between one and eight species; the species which were included in the multiple (>2) species litter-bags were randomly selected. This litter diversity gradient was created within each functional group and across some functional groups. We found large non-additive effects of mixing litter from different species on litter decomposition rates, litter nitrogen contents, rates of nitrogen release from litter and the active microbial biomass present on the litter. The patterns and directions of these non-additive effects were dependent upon both plant functional group and time of harvest, and these effects could be predicted in some instances by the initial litter nitrogen content and the degree of variability of nitrogen content of the component species in the litter-bag. There was no relationship between litter-bag species richness and any of the response variables that we considered, at least between two and eight species. Within plant functional groups our results provide some support for the species redundancy and idiosyncratic hypotheses about how biodiversity alters ecosystem function, but no support for the ecosystem rivet hypothesis or the view that species richness of plant litter is important for ecosystem function. We suggest that increased species diversity of plant litter is less important than that of live plants for determining ecosystem properties (and provide possible reasons for this) and conclude that perceived relationships between biodiversity and ecosystem function may be of diminished significance when the ecological importance of plant litter is fully appreciated.
677 citations
TL;DR: Assessment of the independent effects of habitat affinity and habitat carrying capacity on population distributions revealed that the differential interaction of species with landscape structure was the primary determinant of distributional patterns.
Abstract: Landscape connectivity refers to the functional relationship among habitat patches, owing to the spatial contagion of habitat and the movement responses of organisms to landscape structure. Heterogeneous landscapes provide a particular challenge for modelling population-level responses to habitat fragmentation, because individuals may be utilizing multiple habitats to varying degrees across the landscape. We apply neutral landscape models to understand how species' habitat affinities interacted with landscape structure (i.e., habitat abundance, distribution, and quality as measured by carrying capacity) to affect the redistribution of individuals. Two types of neutral models are presented: random maps, in which the distribution of habitat is spatially independent, and fractal maps, in which habitat exhibits an intermediate level of spatial dependence. The neutral landscapes comprised varying proportions of three habitat types, for which species exhibited a preference gradient (high, medium, low). We performed a series of simulation experiments as a factorial design of parameter states to tease apart the underlying factors responsible for population distributional patterns (random vs clumped) in spatially complex mosaics. Landscape connectivity is a threshold phenomenon, in which even a minimal loss of habitat near the critical threshold (p c ) is likely to disconnect the landscape, and which may have consequences for population distributions. The exact value of p c depends upon the spatial arrangement of habitat; fractal landscapes exhibited connectivity across a greater range of habitat abundance (p) than random maps (fractal p c = 0.29-0.50. random p c = 0.59). Although the spatial arrangement of habitat (random vs fractal) was the most important determinant of population distributional patterns, different landscape factors were important in structuring populations in the two types of maps. The relative abundance of habitat had the greatest effect on populations in random landscapes, whereas scale-dependent patterns were evident in fractal landscapes. At fine scales, population dispersion was determined by habitat abundance in both random and fractal maps, although populations were more aggregated (as measured by Morisita's Index, l m ) at this scale in random landscapes. But at coarse scales on fractal maps, population distribution was primarily influenced by species' habitat affinities. Assessment of the independent effects of habitat affinity and habitat carrying capacity on population distributions revealed that the differential interaction of species with landscape structure (i.e., different residence probabilities in each habitat type) was the primary determinant of distributional patterns. Neutral landscape models thus provide a useful tool for determining the relative importance of various components of landscape structure that affect population distributions.
563 citations
TL;DR: There is continued and strengthened support for the general pattern of female biased dispersal established by Greenwood in 1980 and there is now evidence for malebiased dispersal in twenty-two species representing twelve families.
Abstract: We reviewed the avian literature for quantitative studies of sex biases in dispersal and found continued and strengthened support for the general pattern of female biased dispersal established by Greenwood in 1980. We report sex differences in dispersal patterns in natal and breeding dispersal as measured by 1) proportion leaving and 2) distance moved. Greenwood's review found evidence for male biased dispersal only within four species of the Anatidae. We find there is now evidence for male biased dispersal in twenty-two species representing twelve families.
515 citations
TL;DR: A challenge for the future is to assess the adaptive value of particular mechanisms of host-selection, and to relate these to the predictions made in simple adaptive models.
Abstract: Host-plant selection by phytophagous insects is largely determined by adult insects choosing the developmental location of offspring. Knowledge of natural selection leads to theoretical predictions about how adult behaviour might respond to host quality, the abundance of host-plants, adult egg-load, age and available search time, density-dependence, and stochastic effects. Debates about the value of simple adaptive models can only be settled by repeated testing and reformulation. The theoretical basis of adaptive host-selection is quite strong, but several challenges remain. Models are lacking which are both general enough to be applicable to a wide range of species, and easy to test. The role of variability in plant abundance and other stochastic forces requires clarification. Empirically, good field studies of the effect of host-plants on insect fitness are rare, but without them little progress can be made. The assessment of host-preference also requires attention. Quantitative tests of theory are rare, probably because general models do not encompass enough relevant natural history for each particular species. However much anecdotal and qualitative evidence seems to reflect adaptive predictions. A challenge for the future is to assess the adaptive value of particular mechanisms of host-selection, and to relate these to the predictions made in simple adaptive models.
508 citations
439 citations
TL;DR: In this paper, the authors show that the interactions between water infiltration or nutrient retention and plant density potentially give rise to the existence of alternate stable vegetation states and threshold effects in semi-arid grazing systems, even without the effect of a nonlinear herbivore functional response or plant competition.
Abstract: Models that explain the discontinuous behaviour of semi-arid grazing systems usually emphasize herbivore feeding characteristics or plant competition as possible mechanisms. Field studies indicate, however, that plant-soil relations could be more important. We show by means of a graphical model that the interactions between water infiltration or nutrient retention and plant density potentially give rise to the existence of alternate stable vegetation states and threshold effects in semi-arid grazing systems, even without the effect of a non-linear herbivore functional response or plant competition. These interactions may trigger a positive feedback between reduced plant density and reduced resource availability, and lead to a collapse of the system. The model results are in line with well-documented observations of spatial and temporal patterns such as two-phase mosaics and stably degraded grasslands.
333 citations
TL;DR: This work has suggested that microbial activity and diversity are both a part of, and inseparable from, pond ecosystem function, and that concepts such as 'redundancy' of microbial species, and the 'value' of conserving biodiversity at the microbial level have little meaning.
Abstract: The nature and scale of ecosystem functions, such as carbon-fixation and nutrient cycling in a freshwater pond, appear to be governed by complex reciprocal interactions involving physical, chemical and microbiological factors. Moreover, these interactions continuously create new microbial niches that are quickly filled from the resident pool of rare and 'cryptic' (and probably cosmopolitan) microbial species. This could mean that microbial activity and diversity are both a part of, and inseparable from, pond ecosystem function, and that concepts such as 'redundancy' of microbial species, and the 'value' of conserving biodiversity at the microbial level have little meaning.
331 citations
TL;DR: The results suggested that hormonally mediated sex differences in susceptibility to parasitism exist for mammals, even though the tests were extremely conservative, and indicate that differences in parasitism between the sexes are small, and that statistically significant male biases in Parasitism are not a general rule.
Abstract: Sex biases in parasitism may be expected in mammals because estrogens stimulate immunity, whereas androgens depress immunity. Parasites should, therefore, become more readily established in male hosts, leading to higher levels of parasitism for males than for females. We tested this general hypothesis using 145 tests based on mammal hosts taken from 38 published studies. Male biases in parasitism existed overall and for tests restricted to arthropod (but not helminth) parasites. We then controlled for potential dependence by choosing single parasite species based on their likelihood of eliciting immune responses. For these 48 tests, male biases still existed for arthropod, but not for helminth, parasites. As predicted, the average degree of male bias was much higher for studies involving an experimental infection than for field studies. In experimental tests, researchers controlled for differences between the sexes in exposure to parasites. Any biases in parasitism, therefore, should have been due primarily to gender differences in immune response. There also were no male biases in parasitism among juveniles, presumably because juvenile males and females differ less in their hormone profiles or stress levels than do adult males and females. In , our results suggested that hormonally mediated sex differences in susceptibility to parasitism exist for mammals, even though our tests were extremely conservative. Our results also indicate that, on average, differences in parasitism between the sexes are small, and that statistically significant male biases in parasitism are not a general rule.
314 citations
TL;DR: An analytical model of carbohydrate allocation demonstrates that variation in storage allocation can influence survivorship and growth and that the opportunity cost of storage is lower under low light.
Abstract: Total nonstructural carbohydrates (TNC) of roots were sampled in early autumn from saplings of four species that differ in light-dependent growth and survivorship (shade tolerance). The two deciduous species (Acer saccharum and Fraxinus americana) had higher TNC concentrations than the evergreens (Tsuga canadensis and Pinus strobus), presumably because of autumn build-up of reserves for spring refoliation. In separate comparisons of deciduous and coniferous pairs, A. saccharum and T. canadensis had higher low-light TNC concentrations and survivorship than F. americana and P. strobus, respectively. In high light, TNC levels were not significantly different between A. saccharum and F. americana and both species had >98% survivorship. An analytical model of carbohydrate allocation demonstrates that variation in storage allocation can influence survivorship and growth and that the opportunity cost of storage is lower under low light. The model and empirical data are consistent with an observed correlation among species between growth determinancy and shade tolerance and a negative correlation between high-light growth rates and low-light survivorship. Allocation to storage may be an effective strategy of shade tolerance because it is relatively inexpensive under low light and provides a buffer against stresses.
TL;DR: Trade-offs between investments in immunofunctioning and growth and sustained exercise are suggested, that determine the year-round use of particular types of habitat by long-distance migrating shorebirds.
Abstract: On the basis of associations between the characteristics of breeding and wintering habitats, apparent immunocompetence, and chick energetics of shorebirds (Charadrii), trade-offs between investments in immunofunctioning on the one hand and growth and sustained exercise on the other are suggested, that determine the year-round use of particular types of habitat by long-distance migrating shorebirds. Some species appear restricted to parasite-poor habitats (high arctic tundra, exposed seashores) where small investments in immunomachinery may suffice and even allow for high growth rates. However, such habitats are few and far between, necessitate long and demanding migratory flights in the course of an annual cycle and are often energetically costly to live in. Species evolutionarily opting for parasite-poor habitats may be rather susceptible to parasites and pathogens as a result of investments in sustained exercise (including thermoregulation) rather than immunocompetence. Components of this general hypothesis are perfectly testable, and such tests may shed new light on several other biogeographical, energetic and evolutionary riddles.
TL;DR: It is shown that in a spatially structured population system a randomly occurring stochastic perturbation reducing reproduction success is indeed capable of synchronizing subpopulations and that when the Moran effect occurs in most years its synchronizing power wanes, despite the strength of the effect.
Abstract: Our approach is twofold. First, with 1964-1983 data on red squirrel (Sciurus vulgaris) in 11 provinces in Finland we show that population fluctuations in different parts of the country are largely synchronous. Second, using two differing model types for producing the dynamics of populations we set up to examine the synchronizing effect of an extrinsic disturbing factor, the Moran effect. We show that in a spatially structured population system a randomly occurring stochastic perturbation reducing reproduction success is indeed capable of synchronizing subpopulations. The synchronizing effect is achievable with a wide range of probabilities of occurrence and strengths. However, when the Moran effect occurs in most years its synchronizing power wanes, despite the strength of the effect. Allowing regionality in the variance of the strength of the Moran effect reduces the synchronizing capacity of the perturbation. The Moran effect is also capable of producing a wide range of population dynamics from very simple premises.
TL;DR: The results of this experiment indicate that definitions of landscape connectivity depend on both the spatial pattern of the landscape and how individuals move within and among patches, and it appears that movement patterns within a particular patch may be contingent on the characteristics of the surrounding landscape.
Abstract: Simple neutral percolation models of ecological landscapes predict a critical threshold of landscape connectivity at ca 60% coverage of cells that are permeable to movement. Using such models as an inspiration, we conducted a field experiment in which we employed an experimental model system of tenebrionid beetles (Eleodes obsoleta) moving through mosaics of grass and bare ground in which grass coverage was systematically varied from 0% to 80% in a random pattern. Three parameters of beetle movement pathways (mean step length, mean vector length, and net displacement rate) all showed a sharp reduction between 0% and 20% grass cover and no differences over further 20% coverage increments; the mean fractal dimension of pathways (which could not be derived for the 0% coverage treatment) also did not differ with increasing grass coverage from 20% to 80%. The proportion of time spent by beetles in grass patches did not increase with increasing grass coverage, although individuals spent significantly more time steps in grass in the 20% grass-cover treatment than would be expected by chance. The distance moved per time step was greater when beetles moved over bare ground than in grass, but the mean step length while on grass was significantly lower when grass constituted only 20% of the experimental landscape than when more grass was present, perhaps because beetles stopped more frequently on grass when little of it was present. The threshold in beetle movement through the experimental landscapes differed markedly from that predicted by simple neutral percolation models, probably because, in contrast to the models, both cover types were permeable to beetle movements and individuals moved nonrandomly. The results of this experiment indicate that definitions of landscape connectivity depend on both the spatial pattern of the landscape and how individuals move within and among patches. Moreover, it appears that movement patterns within a particular patch may be contingent on the characteristics of the surrounding landscape. As a consequence, a land-cover map alone may not predict whether, to a particular kind of organism, a landscape is fragmented or connected, and the spatial distribution of individuals in a population may not map closely onto the distribution of suitable habitat patches. This finding suggests that assessments of metapopulation structure must consider both overall landscape patterns and the nonlinear responses of organisms to such patterns.
TL;DR: The lower species richness of leaf-mining lineages in comparison to their external-feeding sister groups indicates that the evolution of leaf mining does not represent an innovation that has led to an adaptive radiation.
Abstract: Feeding inside leaves by leaf-mining insects has been viewed as an adaptive innovation for consuming foliage. The leaf mine presumably provides its occupant shelter from the detrimental effects of the physical environment, protection from attack by natural enemies, and potentially a means of avoiding plant defenses concentrated in particular layers of leaf tissue. We examine several hypotheses concerning the adaptive significance of leaf mining using both the published literature and experiments. We also examine the notion that leaf mining is an innovation that has led to an adaptive radiation by examining the phylogeny and species richness of leaf-mining lineages and their sister groups. Both advantages and disadvantages accrue to insects that feed inside leaf mines in comparison to external-feeding folivores. The advantages of feeding within a leaf mine include: a lower incidence of disease infection, a microenvironment with lower evaporative demand and therefore protection from desiccation, protection from the direct and indirect effects of UV radiation by the leaf-mine epidermis, and avoidance of plant defenses resulting in higher feeding efficiencies. The disadvantages of feeding inside a leaf mine include: lower mobility resulting in higher mortality from parasites, higher mortality associated with premature leaf-abscission, and smaller average body size and lower fecundity. Leaf mining is certainly a viable means of feeding upon foliage, and in particular instances it might be the most successful means of feeding on leaves. However, the lower species richness of leaf-mining lineages in comparison to their external-feeding sister groups indicates that the evolution of leaf mining does not represent an innovation that has led to an adaptive radiation. Perhaps the evolution of leaf-feeding per se and the evolution of the ability to feed externally on leaves rather than concealed feeding modes such and mining and galling represent innovations that have resulted in adaptive radiations in the Insecta.
TL;DR: The purpose of NLMs is to provide null models of landscape structure as a baseline for comparison with real landscape patterns, or for evaluating the effects of landscape structures on ecological processes, and their contribution to ecology is reviewed.
Abstract: Neutral landscape models (NLMs) were developed from percolation theory nearly a decade ago. Since then, the original random percolation maps have undergone adaptive radiation and NLMs now include a diverse array of spatially explicit models based on theoretical distributions derived from fractal geometry and spectral synthesis. The purpose of NLMs is to provide null models of landscape structure as a baseline for comparison with real landscape patterns, or for evaluating the effects of landscape structure on ecological processes. As the use of NLMs has expanded beyond the domain of theoretical landscape ecology to applications in other areas of ecology, there is an increased risk that NLMs will be used inappropriately, or that their function will be misunderstood or misinterpreted. NLMs are being subjected to the same general criticisms levied against null models in other areas of ecology. For this reason, we clarify the purpose of NLMs, review the contributions of NLMs to ecology, and evaluate the appropriate use of NLMs in ecological research. NLMs have already made several contributions to ecology: (I) development of spatial indices to describe landscape patterns; (2) prediction of critical thresholds in ecological phenomena; (3) definition of landscape connectivity; (4) development of species' perceptions of landscape structure; (5) provision of a general model of spatial complexity; and (6) determination of the ecological consequences of spatial heterogeneity. In the future, emphasis on NLMs should shift from theoretical development to application and model testing.
TL;DR: Observations support the assumption that in the case of microorganisms everything is everywhere and that their global species diversity is relatively limited.
Abstract: Microbial species diversity. both global and local, is still poorly understood. In this study all species of ciliated protozoa were recorded microscopically from ∼1 cm 2 sediment collected from a small lake and from a marine shallow-water bay. Additional adjacent sediment samples (together representing <50 cm 2 ) then incubated under a variety of culture conditions to reveal cryptic species that are present as resting cysts or are too rare to be found microscopically. About 85 and of the total number of observed species from the limnic and marine sediment, respectively, were such cryptic species. In both cases the number of species found in < 50 cm 2 of sediment represented about 75% of all ciliate species ever recorded from these two previously well-studied habitats, and about 8% of all named free-living ciliates. These observations support the assumption that in the case of microorganisms everything is everywhere and that their global species diversity is relatively limited.
TL;DR: Experimental results suggest that predation is a necessary factor causing these cycles of cyclic fluctuation of hare populations in the southwest Yukon, and delayed numerical responses of both generalist coyotes and specialist lynx were similar, and would contribute to the cyclic dynamics.
Abstract: Coyotes and lynx are the two most important mammalian predators of snowshoe hares throughout much of the boreal forest in North America. Populations of hares cycle in abundance, with peaks in density occurring every 8-11 yr, and experimental results suggest that predation is a necessary factor causing these cycles. We measured the numerical responses of coyotes and lynx during a cyclic fluctuation of hare populations in the southwest Yukon, to determine their effect on the cyclic dynamics. We used snow-tracking, track counts, and radio telemetry to directly examine changes in the numbers, population dynamics, and movements. Numbers of coyotes varied 6-fold and those of lynx 7.5-fold during a 26-44-fold fluctuation in numbers of hares, and the abundances of both predators were maximal a year later than the peak in numbers of snowshoe hares. Cyclic declines in numbers of coyotes were associated with lower reproductive output and high emigration rates. Likewise, few to no kits were produced by lynx after the second winter of declining numbers of hares. High emigration rates were characteristic of lynx during the cyclic peak and decline, and low in situ survival was observed late in the decline. The delayed numerical responses of both generalist coyotes and specialist lynx were therefore similar, and would contribute to the cyclic dynamics.
TL;DR: A class of models where seed mass determines 1) the number of seeds produced via a size-number trade-off and 2) competitive ability - plants arising from large seeds are assumed to have a competitive advantage over those derived from small seeds are explored.
Abstract: Within plant communities seed mass often varies over 3 to 5 orders of magnitude, yet simple evolutionary models predict a single optimum seed mass. Here we explore a class of models where seed mass determines 1) the number of seeds produced via a size-number trade-off and 2) competitive ability - plants arising from large seeds are assumed to have a competitive advantage over those derived from small seeds. In this setting the existence of a single-species global ESS seed mass requires the competitive advantage of large seeds over small ones to be unbounded. If there is a limit on the competitive advantage that large seeds obtain then it is always possible to find a smaller seed mass that will successfully invade. In such circumstances there might be a multi-species coevolutionarily stable coalition of several species each with a different seed mass. In this way a wide range of seed masses could be promoted by evolution. In general the adaptive landscape generated by these models is extremely flat leading to slow evolutionary dynamics. The implications of these results for the interpretation of observational, comparative and experimental studies are discussed.
TL;DR: It is concluded that although optimal foraging theory is a useful paradigm for studying the mechanistic processes of the diet selection problem, its fundamental assumptions are very tenuous and the theory of diet selection by deer must be broader than optimal foraged theory alone.
Abstract: I review recent developments in the theory of diet selection by deer with a focus on the diet selection problem at the level of individual food items. Much progress has been made in quantitatively predicting forage nutritional value, effects of plant tannins and the handling-time costs of forage ingestion and rumen passage. Much progres also has been made in quantifying allometric relations between deer, their nutritional adaptations, and their food requirements. A much greater appreciation has been gained for the complexity of food resources and the heterogeneity of natural environments. More work is needed, however, in all of the above and also in quantitative predictions of the effects of nontannin plant allelochemicals, deer rumen function and passage rate, and interrelations between factors. The diet selection problem is highly complex primarily because of interactions between constraints and objectives and enormous heterogeneity of food resources. The efficiency and variation of learned behavior are additional complications. I conclude that although optimal foraging theory is a useful paradigm for studying the mechanistic processes of the diet selection problem, its fundamental assumptions are very tenuous. Quantitative prediction of a range of potentially good solutions is more reasonable than that of an optimal solution. The theory of diet selection by deer must be broader than optimal foraging theory alone.
TL;DR: In this semiarid locality, it appears that ENSO effects did not cascade down from higher to lower trophic levels, but rather the opposite, and it is predicted that as primary productivity varies with rainfall, so should secondary (mammal prey densities), and tertiary productivity (vertebrate predators).
Abstract: We analyzed the putative effects of the El Nino Southern Oscillation (ENSO) of 1991-92 in a semi-arid locality of northern Chile. We obtained 30 months of pre-ENSO data, followed by 36 months of peak and post-ENSO data (total = 5.5 yr). The rainy winter of 1991 resulted in a three-fold increase in total seed bank (perennial and ephemerals pooled) and in ephemeral (but not perennial) herb cover. Seed and herbage eaters (rodents) irrupted to population levels ca 20 times higher during the breeding season of 1991 than the preceding wintering season. Diurnal carnivorous predators (hawks, owls, and foxes) showed a delayed response to the irruption, increasing from seven individuals sighted during the wintering season of 1991 to 13 during the wintering season of 1992. A seemingly counterclockwise trajectory of predator abundance versus prey levels suggested a pattern of prey-driven dynamics, but confidence intervals were likely broad. In this semiarid locality, it appears that ENSO effects did not cascade down from higher to lower trophic levels, but rather the opposite. In this bottom-up scenario, we predict that as primary productivity varies with rainfall, so should secondary (mammal prey densities), and tertiary productivity (vertebrate predators). Long-term monitoring of this terrestrial ecosystem is needed to test this prediction.
TL;DR: In this article, the effect of mixing tree leaf litter on decomposition rates was investigated by incubating seven tree leaf leaf litter types in all possible two-litter combinations under controlled laboratory conditions for 26 weeks.
Abstract: The effect of mixing litters on decomposition rates was investigated by incubating seven tree leaf litter types in all possible two-litter combinations under controlled laboratory conditions for 26 weeks. Inorganic N and CO 2 release were monitored during the course of the incubation and final litter concentrations of N, P, Ca, Mg and K were determined. Initial Ca content provided the best correlation (r 2 = 0.458, P<0.001) between total respiration of the pure and mixed units and initial litter quality. There was a very poor correlation (r 2 =0.046, P= 0.272) between total respiration and initial N content across all litters, but when alder, and mixtures with alder, were removed from the calculation the remaining litters gave a strong correlation (r 2 = 0.720, P < 0.001). The majority of litter combinations showed interaction effects for CO 2 release at some stage during the incubation, with eight significant positive and only one significant negative interaction for total CO 2 release. All mixtures showed interaction effects for inorganic N, with release from mixtures generally occurring later than expected. Total N loss was significantly lower in four mixtures, and significantly higher in one. It is suggested that the 'mixtures effect' could be a useful management tool for modifying the timing and rate of release of N from decomposing plant residues to improve the synchrony between mineralisation and plant uptake. Increased rates of decomposition appear to have been a result of a 'sharing' of resources between the component litters of a mixture. Elemental translocation by fungal hyphae, along with diffusion, is proposed as a means by which the degradation of one litter was facilitated by the presence of another.
TL;DR: This poster presents a probabilistic procedure for estimating the likelihood that a person’s life expectancy will increase with age, sex, and number of children under the age of five, using a simple, scalable, and scalable method.
Abstract: R. D. Holt, Museum of Natural History, Dept of Systematics and Ecology, Univ. of Kansas, Lawrence, KS 66045, USA. J. H. Lawton, NERC Centre for Population Biology, Imperial College at Silwood Park, Ascot, UK SL5 7PY. K. J. Gaston, Dept of Animal and Plant Sciences, Univ. of Sheffield, Sheffield, UK SlO 2TN. T. M. Blackburn, NERC Centre for Population Biology, Imperial College at Silwood Park, Ascot, UK SL5 7PY.
TL;DR: This paper shows that this entire scenario with the right timing of events comes out naturally from a minimal model of the Daphnia-algae interaction if the parameters of the model are varied sinusoidally over the year to mimic the effect of seasonal variation in temperature, light and predation pressure from planktivorous fish.
Abstract: The classical seasonal sequence of events in lake plankton is an early spring bloom of algae followed by a peak of zooplankton, typically of the genus Daphnia, grazing down algal biomass resulting in the spring clear-water phase around early June. Subsequently, Daphnia numbers usually remain low and algal biomass increases until autumn when a second small Daphnia peak can cause another drop in algal biomass. In this paper we show that this entire scenario with the right timing of events comes out naturally from a minimal model of the Daphnia-algae interaction if we simply vary the parameters of the model sinusoidally over the year to mimic the effect of seasonal variation in temperature, light and predation pressure from planktivorous fish. The model also reproduces the observation that in lakes where fish biomass is low, Daphnia peaks can continue to occur regularly throughout the summer, while in lakes with very high fish biomass Daphnia can be virtually absent throughout the season.
TL;DR: This work simulated the dispersal of forest-dwelling white-footed mice across an agricultural landscape by releasing mice into unfamiliar, hostile agricultural habitat at various distances from fragments of forested habitat, and found that these forest mice have a remarkably low perceptual range with regard to detecting their foresting habitat.
Abstract: We define perceptual range as the distance from which an animal can perceive key landscape elements, such as distant patches of forested habitat. We argue that perceptual range should be a determinant of not only dispersal success in unfamiliar or hostile landscapes, but also of several landscape-level ecological processes influencing population dynamics. To redress the absence of empirical information on perceptual ranges, we simulated the dispersal of forest-dwelling white-footed mice (Peromyscus leucopus) across an agricultural landscape by releasing mice into unfamiliar, hostile agricultural habitat at various distances from fragments of forested habitat. We found that these forest mice have a remarkably low perceptual range with regard to detecting their forested (core) habitat. Mice released into bare fields failed to even orient towards forested habitat as little as 30 m distant, while mice in crop fields appeared unable to locate forest habitat as little as 10 m distant. These mice seemed to locate forested habitat by vision, despite the availability of non-visual cues. Future work will undoubtedly demonstrate vast differences in landscape-level perceptual abilities among animals, and show clearly that the ecological effects of a given landscape configuration will be influenced by the behavioral attributes of the species in question.
TL;DR: In this article, an explicit formula is derived for the dependence of species turnover in space on census plot size, interplot distance, and the SAR exponent z; this formula can be used to determine z over spatial scales that are too large to permit direct estimate of z by censusing of nested patches.
Abstract: Consequences of species-area relationships (SARs) of the form S = cA are derived. One consequence is an endemics-area relationship (EAR); it is of the same power-law form as the SAR but with an exponent z' that is a function only of z and that always exceeds unity. An explicit formula is derived for the dependence of species turnover in space on census plot size, interplot distance, and the SAR exponent z; this formula can be used to determine z over spatial scales that are too large to permit direct estimate of z by censusing of nested patches. The areal dependence of link-species patterns observed in food webs is also examined; SARs are shown to imply the approximate, but not exact, area-independence of link-species relationships of the form L = kSy, where L is the number of trophic links and y < 2. A relationship between the average range of species in a habitat patch and the exponent z is also derived, leading to the result that average range is a decreasing function of both patch area and z.
TL;DR: The results lend support to previous suggestions that the absence of an effect of ectoparasites on the offspring may be due to the fact that parents bear the cost of parasitism.
Abstract: Parental food compensation has been proposed to account for the absence or small negative effects of parasites on offspring in various bird-parasite systems. An increase in the quantity of energy and nutrients provided by the adults would therefore compensate for the offspring's loss of blood to blood-sucking ectoparasites. We studied parental food compensation in a blue tit Parus caeruleus population with experimentally controlled infestations by the bird flea Ceratophyllus gallinae. Parental feeding effort, offspring quality, and parasite reproduction were measured in randomly assigned parasite-free and infested broods. Although the ectoparasites reproduced at a high rate in infested nests, the nestlings did not suffer higher mortality or reduced body size and body condition than nestlings in parasite-free nests. Fleas had a small negative effect on feather development of nestlings. Parent blue tits of infested nests increased rate of food provisioning by 29%. The results support the parental food compensation hypothesis. No short-term costs (i.e. lowered body condition) of parasites on the parents could be detected. Although the long-term costs of parasites on parents were not measured in this study, the results lend support to previous suggestions that the absence of an effect of ectoparasites on the offspring may be due to the fact that parents bear the cost of parasitism.
TL;DR: In this paper, the authors show that if available soil water limits plant growth, the vegetation supported by sandy soils is generally resilient to herbivore impact and rainfall fluctuations, unlike the vegetation on clayey soils.
Abstract: There is an urgent need to develop a mechanistic understanding of how site-specific properties can lead to irreversible vegetation changes. We show, by means of a bifurcation analysis of two mathematical models, how site-specific properties determine the resilience of vegetation changes in semi-arid grazing systems. The models predict that if available soil water limits plant growth, the vegetation supported by sandy soils is generally resilient to herbivore impact and rainfall fluctuations, unlike the vegetation on clayey soils. This depends on the capacity of vegetation communities to improve the structural and water-holding capacities of the soil. In contrast, if plant growth is nutrient limited, vegetation on sandy soils is generally not resilient to herbivore impact and fluctuations in external nutrient input, unlike the vegetation supported by clayey soils. This is affected by the nutrient retention capacity of vegetation communities. We stress the applicability of the general theory provided by this model to the Sahel environment. The model predictions are consistent with field observations documented in the literature.
TL;DR: Cover data from 13 successional seres starting on bare ground in the western part of the Czech Republic were analysed and the majority of traits considered exhibited some trends in the course of succession.
Abstract: Cover data from 13 successional seres starting on bare ground in the western part of the Czech Republic were analysed. The following questions were addressed: (1) whether the participation of species with particular traits exhibits some trends, and (2) whether primary and secondary seres differ from each other in the course of the first 10 yr of succession. Primary seres were distinguished from secondary ones a priori on the basis of absence or presence of (a) organic topsoil, and (b) soil seed bank at the onset of succession. Traits important for the course of succession were considered, their relative importance expressed for each sere and year on the basis of species quantities, and tested for trends (increase, decrease, unimodal response, no evident trend or absence). The majority of traits considered exhibited some trends in the course of succession. Participation of C-strategists, phanerophytes, intensity of lateral spread, presence of VA mycorrhizae, dispersal by animals and wind, and height of plants significantly increased during the first 10 yr of succession, whereas the participation of R-strategists, therophytes, propagule weight, and capability of forming persistent seed bank decreased. Participation of S-strategists, geophytes, hemicryptophytes, and all relevant types of pollination did not exhibit any significant trend. Primary and secondary seres differed from each other neither in trends in life-history characteristics of constituent species nor in their relative importance after the first 10 yr of succession.