Showing papers in "Ecology in 1991"

Chaos in a Three-Species Food Chain

Abstract: A continuous time model of a food chain incorporating nonlinear functional (and numerical) responses exhibits chaotic dynamics in long-term behavior when biolog- ically reasonable parameter values are chosen. The appearance of chaos in this model suggests that chaotic dynamics may be common in natural food webs.

The Sustainable Biosphere Initiative: An Ecological Research Agenda: A Report from the Ecological Society of America

Abstract: In this document, the Ecological Society of America proposes the Sustainable Biosphere Initiative (SBI), an initiative that focuses on the necessary role of ecological science in the wise management of Earth's resources and the maintenance of Earth's life support systems. This document is intended as a call to arms for all ecologists, but it will also serve as a means to communicate with individuals in other disciplines with whom ecologists must join forces to address a common predicament. This document focuses primarily on the acquisition of ecological knowledge. It identifies the ecological research programs of highest priority and recommends steps required to pursue research objectives. The document also lays the groundwork for improving the communication and application of ecological knowledge. The SBI proposes three research priorities: global change; biological diversity; and sustainable ecological systems.

Effects of temperature and substrate quality on element mineralization in six arctic soils

Abstract: We compared the effects of temperature on rates of microbial respiration, N mineralization, nitrification, and P mineralization in soils from six arctic ecosystems located along a toposequence on Alaska's North Slope. Soils from these ecosystems were incubated aerobically in the laboratory for 13 wk and at temperatures representative of field values during a typical growing season. Rates of C and N mineralization were insen- sitive to temperature between 30 and 90C but increased by factors of 2 or more between 90 and 15?. For both C and N, differences in mineralization rates among soils were greater than differences due to incubation temperature within single soils. This suggests that the quality of soil organic matter varies widely among these ecosystems and is more important than soil temperature differences in controlling rates of these processes in the field. Nitri- fication occurred in all soils, even at 30, but there were large differences among soils in nitrification potentials. Overall differences in P mineralization between soils were small. Rates of P mineralization, however, decreased with increasing temperature in soils from some sites and increased with temperature in others. Carbon respired during the 1 3-wk incubations ranged between 1.5 and 8% of total soil organic C across soil types and incubation temperatures. In contrast to the relatively high C mineralization rates in these soils, net N and P mineralization rates were very low and were likely due to high microbial demands for these nutrients. High microbial demand for mineral nutrients can severely limit plant N and P availability in arctic soils.

Size and Timing of Metamorphosis in Complex Life Cycles: Time Constraints and Variation

Abstract: Complex life cycles are characterized by niche shifts at the time of metamorphosis. Current models predict optimal sizes for metamorphosis based on maximizing growth, minimizing mortality, or some balance of these goals. These models predict optimal sizes that are independent of the time of metamorphosis. Reproduction and other major events in the life history of organisms are often constrained to seasons, and the state (e.g., mass) of the organism at that time is related to fitness. Therefore, an organism's state as well as the time that that state is achieved are central variables in these time—constrained life histories. We extend earlier theory to include explicit time constraints in three, hypothetical, complex life cycles. Dynamic optimization models are constructed to determine optimal time and mass trajectories for niche shifts. First, we consider the habitat shift at emergence in mayflies, where reproduction terminates a growth period in the first habitat and is constrained to a season. Second, we consider the habitat shift at metamorphosis in amphibians, where reproduction terminates a growth phase in the second habitat and reproduction is constrained to a single point in time. Third, we combine the first two effects to allow an extended period of reproduction in amphibians. Here optimal time and mass trajectories are determined for two niche shifts–the shift from aquatic to terrestrial habitat and the shift from a growth phase to a reproductive phase. We present analytical theory that allows both quantitative and qualitative predictions. Problem constructions and solutions are presented graphically to aid intuition in interpreting our results and extending the framework to other parameter values and other life—history examples. The general conclusion is that time constraints on complex life histories lead to optimal sizes for niche shifts that vary with time. In time—constrained life histories, any variation in the state of individuals at some time prior to reproduction will be preserved to some degree at reproduction. Therefore, in time—constrained life histories, we expect optimal switches in habitat use or life history stage to depend not only on state but also on the time that state is achieved.

Estimates of movement and site fidelity using mark-resight data of wintering canada geese'

Abstract: Population ecologists have devoted disproportionate attention to the esti- mation and study of birth and death rates and far less effort to rates of movement. Move- ment and fidelity to wintering areas have important ecological and evolutionary implica- tions for avian populations. Previous inferences about movement among and fidelity to wintering areas have been restricted by limitations of data and methodology. We use multiple observation data from a large-scale capture-resighting study of Canada Geese in the Atlantic flyway to estimate probabilities of returning to previous wintering locations and moving to new locations. Mark-resight data from 28 849 Canada Geese (Branta can- adensis) banded with individually coded neck bands in the mid-Atlantic (New York, Pennsylvania, New Jersey), Chesapeake (Delaware, Maryland, Virginia), and Carolinas (North and South Carolina) were used to estimate movement and site fidelity. Two three- sample mark-resight models were developed and programmed using SURVIV to estimate the probability of moving among or remaining within these three wintering regions. The model (MV2) that incorporated "tradition" or memory of previous wintering regions fit the data better than the model (MV 1) that assumes that a first-order Markov chain described movement among regions. Considerable levels of movement occurred among regions of the Atlantic flyway. The annual probability of remaining in the same region for two suc- cessive winters, used as a measure of site fidelity, was 0.710 ? 0.016 (estimated mean + SE (SE), 0.8 89 ? 0.006, and 0.562 ? 0.025, for the mid-Atlantic, Chesapeake, and Carolinas, respectively. The estimated probability of moving to the Chesapeake from the mid-Atlantic or from the Carolinas was 3 x and 25 x as high, respectively, as the probability of moving in the opposite directions. Changes in estimated probabilities of moving between years corresponded to changes in winter harshness. In warm years, geese moved north and in cold years, they moved south. Geese had a high probability of moving to and remaining in the Chesapeake. Annual changes in the movement probabilities did not correspond to annual changes in the United States Fish and Wildlife Service midwinter survey. Consid- erable numbers of geese from the Carolinas appeared to be wintering in more northerly locations ("short-stopped") in subsequent winters.

Factors Affecting Gerbil Foraging Behavior and Rates of Owl Predation

Abstract: 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.

Zonation of Spartina Patens and Spartina Alterniflora in New England Salt Marsh

Abstract: In New England salt marshes the boundary between frequently flooded low marsh habitats and less frequently flooded high marsh habitats is characterized by striking plant zonation. Spartina alterniflora monocultures dominate low marsh habitats while the seaward border of high marsh habitats is generally dominated by Spartina patens. In this paper I examine the role of interspecific competition in maintaining this zonation pattern. Spartina patens turfs and tillers transplanted into the low marsh were severely stunted with or without S. alterniflora neighbors, and low marsh bare patches bordering S. patens monocultures were not significantly colonized by S. patens in three growing seasons. The limited ability of S. patens to oxygenate its rhizosphere in anoxic soils appears to limit S. patens to high marsh habitats. In contrast, S. alterniflora transplants grew vigorously in the high and low marsh when buffered from neighbors, but were excluded from the high marsh in 2-3 yr when S. patens was present. S. alterniflora also rapidly invaded the high marsh in the absence of S. patens. These results support the hypothesis that S. alterniflora is restricted to low marsh habitats by competitive displacement. S. alterniflora thrives in anoxic low marsh habitats due to its ability to oxygenate its roots and rhizosphere. Rhizosphere oxidation by S. alterniflora, however, is not evident in seedlings and small colonizing patches and both seedlings and small colonizing patches of S. alterniflora are stunted in anoxic low marsh substrates. This suggests that the success of S. alterniflora in anoxic habitats is size dependent and may be driven by group benefits of rhizosphere oxidation. These results suggest that the maintenance of intertidal zonation in rocky beach and marsh plant communities is very similar. In both assemblages, competitive dominants monopolize physically benign habitats and displace competitive subordinates to physically stressful habitats.

Fluxes of Dissolved Organic Nutrients and Humic Substances in a Deciduous Forest

Abstract: We evaluated the importance of dissolved organic matter as a vehicle for the movement of N and P from the canopy and the forest floor into the mineral soil of a deciduous forest. We also examined the origin and nature of dissolved organic matter from the forest floor to see whether it was simply soluble plant material or highly humified matter. The average annual output from the forest floor in the form of dissolved organic matter was 18, 28, and 14% of the input in solid litterfall for C, N, and P, respectively. In throughfall, about half of the dissolved N and P was organic. But, in solution percolating from the forest floor, 94% of the N and 64% of the P was organic. Leaching from the forest floor was not a source of inorganic N and P for the mineral soil. Instead, the forest floor was a sink for the removal of these inorganic nutrients delivered in throughfall. Microbial immobilization was the most likely explanation for much of the inorganic nutrient removal. In contrast, the forest floor was an abundant contributor of N and P to the mineral soil in the form of dissolved, and possibly particulate, organic matter. Much of the dissolved organic matter entering the A horizon originated from the upper (Oa and Oe horizon) forest floor, but it was modified in several respects compared to the original soluble material. The solution percolating from the forest floor over most of the year was much richer in nitrogen, contained a much larger proportion of hydrophilic acids, and contained a much smaller proportion of carbohydrate-rich hydrophilic neutrals, than did the original water- extractable material in autumn litter. However, the fresh autumn litter did contain a similar proportion of soluble hydrophobic acids that resembled dissolved humic substances in several respects. Most of the flux of nitrogen from the forest floor to the A horizon was carried by humic substances and highly colored hydrophilic acids.

Vegetation and climate change in eastern north america since the last glacial maximum

Abstract: 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 inso- lation, 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 land- scapes approach dynamic equilibrium with their changing environment.

Numerical and Functional Responses of Kestrels, Short‐Eared Owls, and Long‐Eared Owls to Vole Densities

Abstract: We studied numerical and functional responses of breeding European Kes- trels (EK) (Falco tinnunculus), Short-eared Owls (SO) (Asio flammeus), and Long-eared Owls (LO) (Asio otus) during 1977-1987 in 47 km2 of farmland in western Finland. The pooled mean yearly breeding density varied from 0.1 to 2.4 pairs/km2. The number of nesting EKs (range 2-46 pairs), SOs (0-49), and LOs (0-19) fluctuated in close accordance with the spring density of Microtus (M. agrestis and M. epiroticus) voles. The mean yearly number of fledglings produced per pair ranged from 0.4 to 3.8 and, for each species, was positively correlated with spring density of Microtus voles. Due to their high degree of mobility, EKs, SOs, and LOs were able to track the population fluctuations of their mi- crotine prey without time lags. An increase in microtine densities caused a rapid immi- gration into the study area and a decrease caused a rapid emigration from the area. Microtus voles were the most important prey group by mass in the diet of each species. Water voles, bank voles, shrews, and small birds were the most frequent alternate prey. The spring density of Microtus spp. was positively correlated with the percentage of these voles in the diet of EK, SO, and LO. The pooled functional response curve of these three raptor species to the fluctuating densities of Microtus spp. was close to linear, indicating that consumption rates are independent of vole densities. Breeding EKs, SOs, and LOs seemed to take a larger proportion of voles available in peak years than in low ones.

Stable Isotope Diagrams of Freshwater Food Webs

Abstract: This formula may be used to calculate 95% confidence intervals around mi values (Fig. 1), aiding the comparison of survivorship patterns in the cohorts (Lee 1980). When this procedure is applied to the data of Pyke and Thompson (1986), the analysis shows that the mortality rates differ significantly in the two cohorts during the intervals between days 0-10, 10-20, 30-40, and 50-60, confirming the conclusions of the analysis of the residuals generated during calculation of the logrank statistic.

Plant Traits and Resource Reduction For Five Grasses Growing on a Nitrogen Gradient

Abstract: Five grass species (Agrostis scabra, Agropyron repens, Poa pratensis, Schiz- achyrium scoparium and Andropogon gerardi) were grown in monoculture for 3 yr on an experimental nitrogen gradient. The species differed significantly in the levels to which they reduced soil solution (0.01 mol/L KCl extractable) nitrate and ammonium concentrations and light penetration to the soil surface. Soil nitrate concentration was an inverse function of root mass, which explained 73% of the observed variance in nitrate. Other species differences explained an additional 9.2%, and total soil N an additional 5% of this variance. Extractable soil ammonium also depended on these variables, but total soil N explained the most variance. Light penetration to the soil surface in these monocultures was a negative exponential function of aboveground biomass (R2 = 0.79). Schizachyrium and Andropogon, the species that reduced soil solution N to the lowest levels on infertile soils, had lower vegetative growth rates, higher root allocation, lower reproductive allocation, and lower tissue N than the other species. Many of these traits are associated with plants of infertile habitats, suggesting a direct link between ecophysiology, resource reduction, and distributional patterns. Because all species survived on even our most nitrogen-poor soil (subsurface sand), differential nutrient reduction, not tolerance, may be the main mechanism favoring these traits in infertile habitats. On infertile soils, the three earlier successional species (Agrostis, Agropyron, and Poa) allocated more to reproduction (rhizome or seed) than the later successional species, but did not reduce soil solution nitrate and ammonium to as low levels. This suggests that our early successional species may be superior colonists but inferior nitrogen competitors compared to the prairie bunchgrasses. Our results can be used to make explicit predictions as to the outcome of nitrogen competition among all possible combinations of these five species.

Component of Plant Competition Along an Experimental Gradient of Nitrogen Availability

Abstract: We tested for variation in the intensity of above— and belowground competition along a 5—yr—old experimental gradient of nitrogen availability in a Minnesota old field. Standing crop increased, species richness decreased, and species composition varied significantly as nitrogen availability increased. Transplants of the three grasses that were dominants at the three levels of the nitrogen gradient (low; Schizachyrium scoparium, intermediate: Poa pratensis, high: Agropyron repens) were grown along the gradient with no neighbors present, with only the roots of neighbors present, or with both the roots and shoots of neighbors present. Resource measurements indicated that the treatment with only neighbor roots present provided at light regime similar to that in which all neighbors had been removed but a nitrogen regime similar to that in which all neighbors were present. At low nitrogen availability, transplants grown with only neighbor roots generally did not differ significantly in biomass or growth rate from those grown with both neighbor roots and shoots, suggesting that the bulk of neighbor effects at low nitrogen were belowground. At high nitrogen availability, plants grown with only neighbor roots generally grew significantly larger than those grown with both roots and shoots of neighbors, but not as large as plants grown with no neighbors, suggesting that both above— and belowground competition occurred. At low nitrogen availability, plants grown with neighbors weighed 3—12% as much as those grown without; at the highest rate of nitrogen addition, plants grown with neighbors weighed 12—58% as much as those grown without, indicating that competition occurred on both poor and rich soils. The intensity of competition, measured as the suppression of transplants by neighbors, did not vary significantly with nitrogen availability. Further, the per—gram effect of neighbors, measured as the slope of the relationship between transplant performance and neighbor biomass, did not vary significantly with nitrogen supply rate. In total, competition was an important force at all points along the experimental productivity gradient, but competition shifted from being mainly belowground in the least productive vegetation to both above— and belowground in fertilized plots.

Temporal Variation in Birds and Fruits Along an Elevational Gradient in Costa Rica

Abstract: We studied temporal fluctuations in fruit production by plants and in pop- ulations of understory fruit-eating birds at three elevations (50, 500, and 1000 m) in Costa Rican wet forests over a 12-16 mo period to investigate effects of resource variation on bird movements and community structure We used mist nets to monitor changes in frugivore abundance, migration patterns, and breeding and molting cycles We sampled understory fruits of each forest concurrent with studies of frugivores Both frugivores and fruit exhibited considerable seasonal variation in abundance Peak frugivore capture rates occurred during peak periods of ripe fruit abundance Altitudinal migrants left lower mon- tane (1000 m) forest during periods of fruit scarcity and were present in lowland (50 m) and foothill (500 m) forest when ripe fruit was abundant Migrants, both altitudinal and temperate, accumulated fat before migration, and perhaps (for altitudinal migrants) in anticipation of breeding Some residents also put on fat before breeding Breeding was seasonal at all forests and occurred when ripe fruit abundance was low Results of this study indicate that birds may track changes in resource abundance Thus, variation in resource abundance influences dynamics of bird communities, both in terms of species composition and abundance Further, results illustrate the importance of viewing com- munities from different scales; dynamics at a local scale (eg, one elevation) can be influ- enced by changes in conditions (eg, fruit abundance) elsewhere That some species regularly move along elevational gradients implies that preservation of many species and of the biotic integrity of entire systems may require conservation of large, connected blocks of suitable habitat

Interspecific Interactions among High Marsh Perennials in a New England Salt Marsh

Abstract: High marsh habitats in New England exhibit conspicuous zonation of vas- cular plants. Spartina patens and Juncus gerardi dominate the seaward and terrestrial borders of the high marsh, respectively, whereas Distichlis spicata is common in disturbed habitats. In this paper I examine the role of interspecific interactions among these marsh perennials in maintaining marsh plant zonation. Removal and transplant experiments were performed to examine species interactions in dense stands. In both types of experiments J. gerardi competitively dominated S. patens and D. spicata, while S. patens competitively dominated D. spicata. Each of these perennials showed maximum growth when transplanted into the J. gerardi zone without neighbors. J. gerardi appears to dominate the terrestrial border of the marsh by competitively excluding S. patens to lower marsh levels, while D. spicata is competitively restricted to disturbed habitats by both J. gerardi and S. patens. Species interactions during the colonization of bare patches were strikingly different than those found in dense vegetation. Colonization of bare patches by both S. patens and D. spicata facilitated the colonization of J. gerardi, the competitive dominant in dense vegetation. Bare patches in the high marsh are typically hypersaline due to increased surface evaporation in the absence of plant cover. Patch colonization by competitive subordinates, which are relatively salt tolerant, appears to reduce substrate salinity by passively shading the substrate and, as a by-product, facilitate the invasion of a superior competitor. While interspecific competition in dense vegetation dictates the zonation of New En- gland perennials in the high marsh, secondary succession in this assemblage may commonly be driven by facilitations. This scenario may not be uncommon among assemblages in harsh physical environments where plants or sessile animals ameliorate their physical environment.

Facilitation and Interference of Quercus Douglasii on Understory Productivity in Central California

Abstract: 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 deter- mine 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 nutrient inputs via litterfall and throughfall under all trees, but that trees with low understory grassland productivities had 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 facili- tating/interfering effects on understory species.

Translating Foraging Movements in Heterogeneous Environments into the Spatial Distribution of Foragers

Abstract: 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 "residenc...

Food and Shelter as Determinants of Food Choice by an Herbivorous Marine Amphipod

Abstract: 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 on 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 abun- dant 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 abun- dance 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 repro- duction, suggests that host-plant use by A. longimana may be strongly constrained by requirements for shelter from predation.

Direct Observations of Owls and Heteromyid Rodents: Can Predation Risk Explain Microhabitat Use?

Abstract: Coexisting heteromyid rodent species of North American deserts differ in habitat use and in locomotory morphology. Quadrupedal species forage primarily in struc- turally 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 micro- habitat 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 Homed 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 het- eromyids, 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 special- izations based on some other factor, such as foraging economics, by restricting quadrupeds more strongly than bipeds to the safety of bushes.

Sapling Survival, Growth, and Recruitment: Relationship to Canopy Height in a Neotropical Forest

Abstract: Treefall gaps are thought to contribute to the diversity of plants in tropical forests by providing opportunities for niche differentiation in modes of regeneration. To examine this hypothesis, we studied the survival, diameter growth, and recruitment of saplings in > 100 species of woody plants in a 50-ha permanent plot of moist tropical forest on Barro Colorado Island, Panama, from 1982 to 1985. The performance of saplings in low-canopy sites ( 10 saplings/ha) species appeared to be generalists. Many rare (<1 sapling/ha) or occasional (1-10 saplings/ha) species survived significantly (P ? .05) less well than the average survivorship of saplings, while many common species survived significantly better than average. Some rare or occasional species grew rapidly, either in low-canopy sites or in both canopy-height categories, while most common species grew slowly in both situations. Rare and occasional species had significantly more recruits per adult than did common species, but often this did not balance their higher mortality. Large differences in survival, growth, and recruitment between canopy-height categories were found only among rare and occasional species.

Seasonal Food Limitation of Detritivores in a Montane Stream: An Experimental Test

Abstract: Many species of detritivorous invertebrates in small streams depend almost entirely on inputs of leaf litter for their nutritional requirements, however the concentration of this resource varies considerably seasonally. An experiment designed to test the hy- pothesis that productivity of coarse particle detritus feeders (shredders) is seasonally food limited was performed using replicate streamside channels that received one of three input rates of whole leaf detritus (rates equal to those naturally falling into montane streams, and two levels of increased inputs). Seven of the nine common shredder species attained significantly higher adult mass, higher densities, or both when food was supplemented. Larval densities of Malenka spp. and Brillia retifinis were significantly higher when additional food was added. Brillia retifinis densities were more than 10 x greater in high food additions than in the "natural" treatment; B. retifinis apparently fills a role as a fugitive in this system and is better able to track shifts in resource abundance by virtue of its short generation time. Zapada cinctipes and Z. haysi were more dense in high and intermediate food input treatments during the last half of the summer. Other taxa had nonsignificant trends to higher densities when extra food was provided. Most of the change in benthic densities can be attributed to decreased rates of emigration with increased food supply. Six of the eight taxa for which adult mass at emergence was measured were significantly more massive (4-46%) when food was added. Both sexes exhibited this increase in mass, but females gained proportionally more in most species. There were no detectable changes in the timing of adult emergence due to food manipulations. The biomass of most taxa increased in proportion to the overall increase in biomass of the common coarse-detritus consumers. The main exceptions to that pattern were a disproportionate increase in the percentage of the biomass represented by Brillia retifinis and a decrease in the representation of Zapada cinctipes. The responses of this community to food supplementation demonstrate food limitation of detritivores. The exponential increase in benthic biomass under food addition shows the scope for productivity if food were not limiting. These coexisting species benefitted from enhanced food supply on a spatial scale that is relevant for population level processes. The large seasonal variation in resource abundance under natural conditions creates bot- tlenecks during periods of low food supply, which constrain subsequent production even during periods when food is abundant.

Large-Scale Patterns of Forest Succession as Determined by Remote Sensing

Abstract: The spatial pattern of and the transition rates between forest ecological states were inferred for °260 000 pixel—sized (3600 m2) landscape units using stallite remote sensing. Transition rates were estimated from 1973 to 1983 Landsat images of the study area, classified into ecological states associated with forest succession. The effects of classification error on transition rate estimates were modeled and error adjustments made. Classification of the 1973 and 1983 Landsat images of the 900 km2 study region required a relatively small set of ground—observed and photo—interpreted plots in 1983, with a total area of just 1.62 km2. An innovative technique for correcting multiyear Landsat images for between—image differences in atmospheric effects and sensor calibration, permitted classification of the 1973 Landsat image using 1983 ground observations. Given current Landsat data, and ground observations in one year, this technique would permit monitoring of forest succession and dynamics for nearly a 20—yr period. Results of applying these techniques to a forest ecosystem showed that during the 10—yr observation period it was patchy and dynamic. For both a wilderness and a nonwilderness area in the study region, sizeable values of transition rates were observed and over half of the landscape units were observed to change state: however, a Markov analysis, using the observed transition probabilities, suggests that at the regional level neither the wilderness nor the nonwilderness areal proportions of ecological states are undergoing rapid change.

The Influence of Island and Mainland Lakeshore Landscapes on Boreal Forest Fire Regimes

Abstract: 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 Quebec. 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 fire scars. Dendroecological techniques were used to deter- mine 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 vege- tation.

The Response of Subalpine Forests to Spruce Beetle Outbreak in Colorado

Abstract: Spruce beetle (Dendroctonus rufipennis Kirby) outbreaks are important dis- turbances affecting subalpine forests of Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), and lodgepole pine (Pinus contorta) in the southern Rocky Mountains. However, little is known about the influences of these outbreaks on overall forest dynamics. We used age-structure analyses and dendrochronological techniques to investigate the effects of a major spruce beetle outbreak on stand composition, dominance, tree age and size structures, radial growth, and succession in subalpine forests in Colorado. This out- break, which occurred in the 1940s, caused a shift in dominance from spruce to fir and a reduction in average and maximum tree diameters, heights, and ages. The outbreak did not favor new seedling establishment of the seral lodgepole pine. Thus, in seral stands spruce beetle outbreak accelerates succession towards the shade-tolerant tree species. The predominant response to the outbreak was the release of previously suppressed small-diameter spruce (not attacked by the beetle) and subalpine fir (a non-host species). Following the 1940s outbreak, growth rates of released trees remained high for >40 yr. The relative increases in growth rates were similar for both species. Both spruce and fir will continue to codominate the affected stands. The predominance of accelerated growth following a spruce beetle outbreak, instead of new seedling establishment, is a major contrast to the pattern of stand development following fire. In some Colorado subalpine forests the effects of disturbance by spruce beetle outbreaks appear to be as great as those due to fire.

Plant litter: light interception and effects on an old-field plant community'

Abstract: We studied the effects of litter ofthe annual grass Setariafaberii, the perennial herb Solidago spp. (mostly S. canadensis), and leaves of the hardwood tree Quercus alba on a successional plant community. We also assessed light interception by these litter types in the laboratory. Light extinction followed the Beer-Lambert exponential law. Solidago litter had the highest transmittance constant and Quercus litter the lowest. The three types of litter produced different light mosaics at the microsite (0.8 cm diameter) scale. In the field, all three litter types affected community structure, but the effect of Quercus was the strongest. Litter reduced the density of the two dominant grasses, Setaria faberii and Panicum dichotomiflorum. Quercus and Setaria litter resulted in biomass compensation (i.e., fewer but larger individuals) by S. faberii but not by P. dichotomiflorum, which probably was always outcompeted by S. faberii. Solanum carolinense, the main dicot in the community, was unaffected by litter addition. Setaria and Solidago litter enhanced the establishment of Erigeron annuus, but Quercus litter reduced it. Litter reduced the number of flowering individuals of S. faberii and the number of seeds per plot; Quercus litter increased the production of seeds per individual. We conclude that species-dependent effects of litter on plant populations may significantly alter interspecific interactions and change plant community structure through direct and indirect effects.

Foraging Cyanobacteria by Copepods: Responses to Chemical Defense and Resource Abundance

Abstract: 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 cyanobac- teria. Our results demonstrate the overriding importance of chemical factors. Copepods exhibited very high clearance rates for some filaments and colonies but rejected morpho- logically 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 ofcopepods 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.

Dynamics of nitrogen competition between successional grasses

Abstract: Pairwise competition experiments were performed for 3 yr on an experi- mental nitrogen gradient at Cedar Creek Natural History Area, Minnesota, where N is the major limiting resource during early succession. Agrostis scabra, an early successional grass, competed against another early (Agropyron repens), a later (Schizachyrium scoparium), and an even later (Andropogon gerardi) successional species. On low N soils, Agrostis was competitively displaced by each of the later successional species, but persisted with Agro- pyron. On high N soils, Agrostis was displaced by all three of the other species. The inferior competitive ability for N of the early successional species refutes the resource ratio hy- pothesis of succession. Rather, the high allocation of Agrostis to seed and its rapid colo- nization of fields support a colonization-competition hypothesis of succession. For two of three cases, the outcome of competition on low nitrogen soils was predicted by R*, the nitrogen concentration to which monocultures of each species reduced extractable soil nitrate and ammonium on N-limited soils. In these cases, the species with the signif- icantly lower R* for nitrogen displaced the other species. In the third case, the species had more similar R* values, and the species with the lower R* had not displaced the other species within 3 yr (but it had done so after 5 yr; see Note added in proof).

Salmon Breeding Behavior and Life History Evolution in Changing Environments

Abstract: Ecologists are increasingly interested in how organisms respond to altered environments. This interest is stimulated in part by the rapid change that human activities are bringing upon the planet. One mechanism for relatively rapid responses may be "conditional strategies" recently documented by behavioral ecologists (Maynard Smith 1982, Parker 1984, Gross 1991). These strategies allow individuals to decide between alternative phenotypic "tactics," depending on the situation. For example, in mate competition a small male may employ a behavioral tactic that involves sneaking, while a larger male employs a behavioral tactic that involves fighting. The decision is based on relative body size, which in turn determines the success likely from either tactic. If the decision is reversible, such as employing sneaking or fighting behavior, the same individual may switch tactics under different circumstances. If the decision is irreversible, as is the case with precocious vs. delayed maturity, the alternatives represent differences in individual life history. In both cases the conditional strategy allows an individual to facultatively adjust its phenotype to maximize its lifetime reproductive success. This paper will illustrate how alternative life-history and behavioral tactics in a conditional strategy may respond to environmental changes caused by humans. Our example will be the conditional strategy of male salmon. Male salmon have two life-history tactics: maturing precociously at a small body size, or delaying maturity until a larger body size is reached. These small and large males in salmon are known by various names, respectively: as precocious parr and adult in Atlantic salmon (Mills 1989), and jack and hooknose in Pacific salmon (Gross 1984). Males also employ two behavioral tactics for gaining access to females on the breeding grounds: small males usually sneak while larger males fight. Before considering how human perturbations to the environment may affect the conditional strategy of male salmon, we first need to review the

Fire and Grazing in the Tallgrass Prairie: Contingent Effects on Nitrogen Budgets

Abstract: 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 enclosures 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.