We experimented on how illumination, habitat structure, and three different species of owls affected the foraging behavior of Gerbillus allenbyi and G. pyramidum, two gerbil species that coexist on sand dune habitats in the Negev Desert, Israel. We also tested how illumination and habitat structure affected rates of predation by owls on the two gerbil species. In a large aviary, we manipulated presence and absence of owls, owl species, presence and absence of illumination, and shrub cover. In response to the presence of owls or to increased illumination, gerbils foraged less, shifted foraging activity to the bush microhabitat, and quit patches at a higher giving-up density of resources. In accord with moonlight avoidance, both gerbil species suffered higher predation rates under illumination than in the absence of illumination. In addition, G. pyramidum distinguished among owl species, as indicated by changes in patch use and habitat selection. Habitat structure also affected foraging behavior and rates of predation. Gerbils foraged less in the open than in the bush microhabitat, foraged less when there was no cover present, and foraged less in the bush microhabitat when patches were encumbered by entangling branches. In accord with avoidance of open areas, both gerbil species suffered higher rates of predation when shrub cover was 0% than when shrub cover was 10%. With 0% cover, G. allenbyi suffered higher predation rates than G. pyramidum, but with 10% cover, rates of owl predation did not differ between gerbil species. Rates of owl predation on the two species corresponded to their natural patterns of macro- and microhabitat partitioning; relative to G. allenbyi, G. pyramidum predominates on open sand dunes and biases its behavior toward the open microhabitat. The results suggest that predation interacts with resource competition to determine the distribution and habitat separation of G. allenbyi and G. pyramidum.
Response surfaces describing the empirical dependence of surface pollen percentages of 13 taxa on three standard climatic variables (mean July temperature, mean January temperature, and mean annual precipitation) in eastern North America were used to infer past climates from palynological data. Inferred climates at 3000-yr intervals from 18 000 years ago to the present, based on six taxa (spruce, birch, northern pines, oak, southern pines, and prairie forbs), were used to generate time series of simulated isopoll maps for these taxa and seven others (hickory, fir, beech, hemlock, elm, alder, and sedge). The simulations captured the essential features of the observed isopoll maps for both sets of taxa, including differences in migration patterns during the past 10 000 yr that have previously been attributed to differential migration lag. These results establish that the continental-scale vegetation patterns have responded to continuous changes in climate from the last glacial maximum to the present, with lags @< 1500 yr. The inferred climatic changes include seasonality changes consistent with orbitally controlled changes in insolation, and shifts in temperature and moisture gradients that are consistent with modelled climatic interactions of the insolation changes with the shrinking Laurentide ice sheet. These results pose new ecological questions about the processes by which vegetated landscapes approach dynamic equilibrium with their changing environment.
Coexisting heteromyid rodent species of North American deserts differ in habitat use and in locomotory morphology. Quadrupedal species forage primarily in structurally complex microhabitats, such as under bush canopies, while bipedal species forage in open spaces. A common explanation for this morphology-microhabitat association is that species differing in morphology also differ in vulnerability to predators, that microhabitat structure affects predation risk, and that animals preferentially forage in the safest microhabitats. We tested this for two bipedal and two quadrupedal heteromyid species (matched by body size), and one cricetid species, by quantifying effects of habitat and illumination on activity and on risk of predation by Great Horned Owls. Capture frequencies were lower for all heteromyid species than for the cricetid species, Peromyscus maniculatus. Heteromyid activity was lower in open habitat and under bright illumination. Illumination had no significant effect on risk, perhaps because rodents changed activity patterns under full moon to compensate for a potential increment in risk. Habitat, however, did affect risk: all species were attacked and captured more frequently in the open. Bipedal species were attacked relatively more in the open than were quadrupeds. If these results apply to all predators, they indicate that predation alone cannot account for the divergent microhabitat associations of bipedal and quadrupedal species. Bipedal heteromyids, however, escaped owl attacks more frequently than did quadrupeds of equivalent size. It is therefore conceivable that they experience lower overall risk in nature, where owls may preferentially attack more easily captured prey species when given a choice. Under these circumstances, owl predation could reinforce divergent microhabitat specializations based on some other factor, such as foraging economics, by restricting quadrupeds more strongly than bipeds to the safety of bushes.
Controversy over the effect of Quercus douglasii on the productivity of California grassland has been fueled by conflicting reports. In some studies, understory grassland productivity was < 25% of open grassland productivity, whereas in other studies understory productivity exceeded 200% of that of surrounding grassland. We examined light, temperature, soil nutrients, soil moisture, and fine tree root distributions under selected Q. douglasii trees (12 that appeared to have suppressive effects on understory productivity, and 12 that appeared to enhance understory productivity) in order to determine how variations in these factors were associated with the differences in understory grassland productivities. We found that grassland productivity is likely to be facilitated by nutrients inputs via litterfall and throughfall under all trees, but that trees with low understory grassland productivities has substantially higher amounts of fine roots in the upper 50 cm of soil and much lower predawn xylem pressure potentials than trees with high understory productivities and presumably deeper root systems. Root exclosures reduced the negative effects of these trees on the dominant understory grass species, and further experiments indicated that the negative effects of the tree roots may partially result from allelopathic oak root exudates. Thus, shallow fine tree roots may inhibit understory productivity, and variations in Q. douglasii root morphology may explain the intertree variations in facilitating/interfering effects on understory species.
The relationship between movement and population dynamics in heterogeneous environments has been approached from two directions: theoretical analyses using diffusion models and empirical studies that often employ computer simulations. In this paper I am proposing a unified framework for studying individual movements that brings both of these approaches together. The proposed framework is based on the "diffusion-approximation" procedure developed by Patlak (1953a, b), which translates a probabilistic description of the pattern of individual movements (such as provided by simulation models) into a partial differential model describing the population redistribution in a patchy habitat. My first goal is this paper was to make Patlak's work accessible to ecologists interested in realistic movement models. Secondly, making some simplifying assumptions I solved Patlick's model to obtain the equilibrium distribution of organisms among patches in a heterogeneous environment. This led me to a definition of @'residence index.@' The residence index of a patch is estimated by observing and recording movement trajectories of organisms within the patch. The pattern of variation in the residence index among different kinds of patches specifies the predicted equilibrium densities of organisms in each patch. I assessed the utility of the above approach by using data on movement tracks of Euphydryas anicia females to calculate butterfly residence indices in host-plant patches vs. nonhost environment. The predicted distribution of butterflies among host and nonhost areas differed by only 16% from the actual distribution, as documented by an independent data set. I also applied the model to data from three studies on insect movement and spatial distribution. The accuracy of model predictions varied from adequate to excellent. Finally, I argue that Patlak's model and its residence-index extension provides a general, and at the same time realistic, framework for quantifying individual movements, and for relating movement patterns to spatial population dynamics. Although this approach, as developed here, is explicitly concerned only with population redistribution within an area, birth/death or immigration/emigration terms can be added in a straightforward manner. Thus, this framework can be very useful in studies of consumer-resource (e.g., predator-prey or herbivore-host) spatial dynamics, since movement behaviors of consumers searching for prey, and prey attempting to avoid consumers, are known to be important in affecting the outcome of such population interactions.
In order to characterize the fire regime of the southern boreal forest and to understand the way in which landscape and fire regime interact, a detailed study of fire history was undertaken in two adjacent contrasting landscapes in northwestern Québec. The fire history for the islands of Lake Duparquet was reconstructed and compared to that of the surrounding lakeshore. Fire occurrence was determined by archival search and by collection of information from the fire scars. Dendroecological techniques were used to determine years of stand initiation on the lakeshore and for a subsample of the islands. Stand initiation data were used to estimate fire frequency, fire cycle, and fire intensity (lethal and non-lethal fires). Tree composition and surficial deposits were sampled in order to assess the possible effect of topography and forest cover on fire frequency. Dated fire scars (n = 273) revealed 82 fire years, with the earliest dated to 1593. Islands experienced more fire years (56) than the lakeshore (37), and fire years were uncorrelated. Most 72%) of the lakeshore stands were initiated by a few large fires (eight), whereas the fire size on islands was variable, ranging from <10% to 100% of the total surface of the island. Despite these differences in fire regime, fire occurrence and fire frequency decreased concurrently on the islands and the lakeshore starting @?120 yr ago. The lakeshore fire cycle was 63 yr before 1870 and 99 yr since 1870, whereas the fire cycle on the islands was slightly longer (74 yr and 112 yr) during the same intervals. The long-term decrease in fire frequency of both the lakeshore and the islands was possibly driven by climatic change. Fire frequency was generally similar for all surficial deposits and fuel types on the lakeshore, whereas fires were more abundant in pine woodland growing on bedrock on the islands. Lakeshore fires were generally of lethal intensity, whereas both a long cycle of lethal fires and a shorter cycle of non-lethal fires were observed on 50% of the islands. The morphology and location of islands with both types of fires suggest that a higher frequency of lightning strikes resulted in a regime characterized by more frequent fires and fires of variable intensity. The specific fire regime observed in the island landscape may be the cause of the abundance of pines, whose presence may contribute, in turn, to the continuation of this regime of less intense and more frequent fires. We suggest that fire regime in the boreal forest is controlled by long-term climate change at the regional scale, and by a strong interaction with landscape at the local scale, both components having a great impact on the distribution and the dynamics of boreal vegetation.
Because food and habitat are closely linked for small herbivores that live on plants, food choice in the field may be constrained by the need to choose plants that provide safe living sites. We investigated the importance of food value and refuge value in determining the plant utilization patterns of the herbivorous marine amphipod Ampithoe longimana. When offered a choice of five common seaweeds, this amphipod fed most readily on Dictyota and Hypnea and less readily on Sargassum, Chondria, and Calonitophyllum. Rates of feeding on the different seaweeds were unrelated to seaweed gross morphology, toughness, nitrogen, or protein content. When cultured on each of these seaweeds in the laboratory, amphipod survivorship was high on Dictyota (82%), intermediate (35 and 18%, respectively) on Sargassum and Hypnea, and low (0%) on the other seaweeds. Survivorship on the different diets was strongly correlated (r = 0.930) with algal protein content; however, neither protein content nor amphipod performance of the different diets was significantly related to feeding rates on those diets. Additionally, amphipods from the three seaweed species that produced some survivors did not differ in growth rate, fecundity, egg size, or age at first ovulation. Variance in survivorship, and related measures, among sibling groups of amphipods suggested that this amphipod population possessed heritable variation for performance on the different seaweed species. In the field, abundance of A. longimana on the different species of algae was more clearly related to the preference of omnivorous fishes for these algae than to feeding rates of the amphipods when given those algae in the laboratory.A. longimana was more abundant on Dictyota and Sargassum (both unpalatable to omnivorous fishes), than on Hypnea, Chondria, and Calonitophyllum (all of which are palatable to fishes). During the season when omnivorous fishes were abundant, density of A. longimana increased on Dictyota, which is chemically defended from fishes, but decreased or remained unchanged on the seaweeds that are more palatable to fishes. Competition with other amphipods as a group did not appear to explain the distribution of A. longimana among seaweeds, since there were no negative correlations between A. longimana abundance and total amphipod abundance in any month. The lack of any consistent relationship between host-plant use in the field and either feeding preference or diet value, as measured by survivorship and reproduction, suggests that host-plant use by A. longimana may be strongly constrained by requirements for shelter from predation.
We used laboratory feeding trials to examine the responses of copepods to a variety of cyanobacteria of differing size, morphology, and toxicity. Diaptomus birgei exhibited behaviors ranging from strong preferences for some taxa of cyanobacteria to almost complete rejection of other taxa when allowed to feed in low-concentration mixtures of a high-quality green alga, Chlamydomonas reinhardi, and each of 16 taxa of cyanobacteria. Our results demonstrate the overriding importance of chemical factors. Copepods exhibited very high clearance rates for some filaments and colonies but rejected morphologically similar strains shown to contain potentially toxic compounds. Responses to resource abundance exhibited three patterns, each consistent with the assumptions and predictions of an optimal diet model. First, selectivity for high-ranking cyanobacteria declined in high-concentration mixtures with Chlamydomonas relative to selectivity in low-concentration mixtures. Second, a test of the effects of time without food showed that hunger, rather than food concentration per se, was responsible for these shifts in selectivity. Finally, each toxic strain was consistently ingested at very low rates, even when offered as the sole food resource. Experiments with complex mixtures of natural seston revealed similar patterns of consistently strong discrimination against a presumably toxic cyanobacterium and concentration-dependent selectivity for presumably nontoxic taxa. Our results show how the behavioral flexibility of copepods can be adaptive in lakes with significant populations of cyanobacteria. The ability of copepods to feed size-selectively on filaments and colonies and to discriminate against toxic forms suggests that these grazers may be important in the evolution and continued success of toxic strains.
I examine the nonlethal effects of a predator, an odonate larva (Anax junius), on competitive interactions between two species of anuran larvae (bullfrog, Rana catesbeiana, and green frog, R. clamitans). The anurans coexist in permanent ponds, which sometimes contain high densities of A. junius. Laboratory experiments indicated that in the absence of the predator, the two species were similar in competitive abilities and grew at virtually identical rates. In the presence of caged A. junius, however, the bullfrog grew to be larger and the green frog much smaller than when the predator was absent. Both species reacted to the presence of A. junius by reducing activity and altering space use. I argue that the reductions in activity in the presence of the predator affected the two species differently, which in turn changed competitive interactions. I discuss the relation between activity level and competitive ability, and the potential for predators to alter competitive relations by affecting activity levels of syntopic species (a higher order interaction). Vulnerability to predators also increases with activity, and I discuss the implications of an activity-mediated trade-off between growth rate and predation risk to interactions among anuran larvae. Finally, I present a method for quantitatively evaluating the net balance of (negative) direct mortality effects and (sometimes positive) indirect effects that alter competitive ability.
The abundance and distribution of settlers, recruits, and adults of the barnacle Semibalanus balanoides were monitored in the high, mid, and low intertidal zones at two adjacent, but physically and biologically different sites (A and B) on a rocky intertidal shore in Nova Scotia following a rare occurrence of ice scouring. Availability of planktonic cyprid larvae also was measured at site A. There was a strong concordance between cyprid availability and settler density over the main settlement period among intertidal heights, where each was inversely related to intertidal height. Differences in cyprid availability with intertidal height were related to immersion time as the larvae, in general, were uniformly distributed at the water-substratum interface. Settlement rate was strongly correlated with cyprid availability in the low intertidal zone at site A, where most settlement occurred. In all zones at both sites, recruit density at the end of the main settlement period was a positive function of settler density over this period: on average, 79% of the variation in recruit density was explained by settler density over the main settlement period. Adult density and total settler density also tended to be strongly related: 86% (mid zone at site A) and 73% (high zone at site B) of the variation in adult density was explained by total settler density, except where postrecruitment mortality (i.e., predation) was high (e.g., only 1% of the variation in adult density was explained by total settler density in the low zone at site A). Local hydrodynamics, which influences supply and distribution of planktonic larvae, may dictate the distribution of settlers and, ultimately, adults on this shore.
I examine the nonlethal effects of a predator, an odonate larva (Anax junius), on competitive interactions between two species of anuran larvae (bullfrog, Rana catesbeina, and green frog, R. clamitans). The anurans coexist in permanent ponds, which sometimes contain high densities of A. junius. Laboratory experiments indicated that in the absence of the predator, the two species were similar in competitive abilities and grew at virtually identical rates. In the presence of caged A. junius, however, the bullfrog grew to larger and the green frog much smaller than when the predator was absent. Both species reacted to the presence of A. junius by reducing activity and altering space use. I argue that the reductions in activity in the presence of the predator affected the two species differently, which in turn changed competitive interactions. I discuss the relation between activity level and competitive ability, and the potential for predators to alter competitive relations by affecting activity levels of syntopic species (a higher order interaction). Vulnerability to predators also increases with activity, and I discuss the implications of an activity-mediated trade-off between growth rate and predation risk to interactions among anuran larvae. Finally, I present a method for quantitatively evaluating the net balance of (negative) direct mortality effects and (sometimes positive) indirect effects that alter competitive ability.
In a recent study, pollen deposition and pollen removal were observed in two patches of jewelweed. It was found that pollen-collecting bees remove more pollen but deposit less of it than nectar-collecting bees.
Fire and grazing occur together in many of the world's grasslands, but their effects on nutrient cycling have usually been studied as if they acted separately. We hypothesized that grazing by large herbivores results in conservation of nitrogen that would otherwise be lost from burned grasslands. We tested this hypothesis in a series of experiments on burned and unburned tallgrass prairie grazed by cattle. We manipulated grazing using exclosures and mowing. Combustion losses of N from ungrazed plots (1.8 g@?m^-^2@?yr^-^1) burned in the spring were double those from similarly burned, grazed plots (0.9 g@?m^-^2@?yr^-^1). These losses represented about half of the preburn, aboveground stocks of N. The magnitude of N loss was proportional to the standing crop biomass available for combustion. Fire temperatures and energy release were reduced by grazing. We used mowing to simulate locally heavy grazing in patches. In the absence of burning, mowing patches increased the likelihood that a patch would be regrazed and caused persistent reductions in the residual biomass remaining in a patch at the end of the growing season. Mowing did not influence patch utilization or residual biomass when pastures were burned. Thus, the effects of fire on grassland N budgets were modified by grazing, and the effects of grazing on the patch structure of grasslands were modified by fire. We conclude that accurately predicting volatile losses of nutrients from grassland ecosystems resulting from biomass burning may depend on understanding effects of grazing.
The use of remote sensing data by ecologists and some of the impediments to this use are discussed. The promise that remote sensing technology offers to ecology is to provide data at large and synoptic scales, which will help to discern large-scale patterns in ecological systems.
Water-filled treeholes provide an experimental setting for examining processes within an ecosystem, and influences of external factors on those processes. Using a limnological, experimental approach involving both natural tree holes and laboratory microcosms of the tree hole ecosystem, we identified and studied interacting, biotic processes, including dynamics of bacterial populations and variation in concentration of inorganic nutrients in tree hole water, and density-dependent competition for food among larvae of the mosquito Aedes triseriatus. We characterized the influence of external factors (inputs of leaf detritus and stemflow) on those processes. Analyses of water samples over time showed that tree hole water was rich and dynamic in nutrients (nitrite, nitrate, ammonium, phosphate, and sulfate); ammonium was the dominant form of inorganic nitrogen. Variation in nutrient concentrations in microcosms depended upon exogenous inputs (leaf detritus and stemflow water), dilution of nutrients by stemflow, nutrient cycling processes (nitrification, dentrification, and sulfate reduction), and ammonium excretion by mosquito larvae. The densities of bacteria in tree hole water, obtained using direct counts of DAPI-fluorochrome stained samples and epifluorescence microscopy, ranged from 2.0 × 10^6 to 6.0 × 10^7 cells/mL, and in microcosms from 4.6 × 10^5 to 2.6 × 10^8 cells/mL. Experimentation involving microcosms revealed that bacterial abundance was reduced by mosquito feeding and stemflow flushing. Further experiments showed that stemflow flushing increased mosquito productivity from microcosms several-fold and released mosquitoes from density-dependent competition. This effect was likely related to nutrient input and the simultaneous removal of toxic metabolites owing to inputs of stemflow water. We conclude that disturbance by a physical factor, stemflow, has a major influence on the interactions of nutrient dynamics, bacterial populations, and mosquito productivity in temperate tree-hole ecosystems.
Experiments were conducted to determine if three plant defenses affected the parasitoid Cotesia congregata in a fashion that would influence herbivore-parasitoid interactions. We evaluated the developmental rate, size, and survival of the parasitoid Cotesia congregata, when reared from Manduca sexta fed on diets containing either nicotine, rutin, or hordenine. These results were compared to the effects of the same three chemicals on unparasitized M. sexta. The influences of the three plant defenses differed. In general, however, the effects of these allelochemicals on the parasitoid paralleled those on the unparasitized hornworn. The one major exception was that concentrations of nicotine that had little or no effect on the hornworm caused significant mortality of parasitoids. Neither rutin nor hordenine had large or consistent negative effects, although rutin had a pronounced effect on both hornworm and parasitoid developmental times.