Showing papers in "Ecological Monographs in 1990"

Structure and Organization of an Amazonian Forest Bird Community

Abstract: To help fill the gap in detailed knowledge of avian community structure in tropical forests, we undertook a census of a 97—ha plot of floodplain forest in Amazonian Peru. The plot was censused over a 3—mo period spanning the 1982 breeding season. The cooperative venture entailed ≈12 person—months of effort. Conventional spot—mapping was the principal method used, but several additional methods were required to estimate the numbers of non—territorial and group—living species: direct counts of the members of mixed flocks, saturation mist—netting of the entire plot, opportunistic visual registrations at fruiting trees, determination of the average size of parrot flocks, color banding of colonial icterids, etc. Two hundred forty—five resident species were found to hold territories on the plot, or to occupy all or part of it. Seventy—four additional species were detected as occasional—to—frequent visitors, wanderers from other habitats, or as migrants from both hemispheres. By superimposing territory maps or t...

Ecomorphology, Performance Capability, and Scaling of West Indian Anolis Lizards: An Evolutionary Analysis

Abstract: Studies of ecomorphology–the relationship among species between morphology and ecology–contain two implicit and rarely tested hypotheses: (1) that morphological differences among species result in differences in performance capability at ecologically relevant tasks, which, in turn, produce differences in behavior and ecology; and (2) that morphology, performance capability, ecology and behavior have evolved synchronously. I tested these hypotheses using the Anolis lizards of Jamaica and Puerto Rico. I measured morphological and performance variables on recently caught lizards. Movement, display rate and microhabitat measurements were made on lizards observed in the field. Body size explained most of the variation in morphology and performance ability, but was not correlated with the ecological or behavioral variables. When the effect of body size is removed from the morphological and performance variables, the ecomorphological hypotheses were confirmed. Species that were similar morphologically were also ...

Spatial and Temporal Variation in Tropical Fish Trophic Networks

Abstract: Observed properties of natural food webs have both important theoretical and important management implications Four lowland aquatic food webs were investigated over the course of two years: a large swamp and a small stream in Costa Rica, and a similar swamp and stream in the Venezuelan llanos Each local ecosystem differed from the three others with respect to environmental changes associated with seasonal rainfall Phylogenetic composition and diversity of biotas also varied among systems Volumetric proportional utilization coefficients from fish gut contents were used as estimates of the intensity of predator—prey interactions An annual and two or more seasonal food webs were constructed for each local community Aquatic communities were defined operationally using common fish species as consumers, and using the sink subweb associated with the top predator of each system A computer calculated a variety of food—web statistics and plotted food—web diagrams containing either (a) all observed trophic links (predator—prey interactions), or (b) subsets with weak links eliminated at prescribed thresholds Individual community food webs contained from 58 (stream, Costa Rica) to 104 (swamp, Venezuela) interactive taxonomic units and from 208 to 1243 total trophic links Food—web parameters were very sensitive to changes in level of link threshold Web connectance and related parameters converged near link threshold 004 (utilization coefficients <004 eliminated) in a variety of inter—web comparisons Despite large differences in assemblage composition and attributes of the physical environment, distributions of trophic levels calculated according to a trophic continuum algorithm were very similar among study systems Herbivores, detritivores, and their direct predators formed the largest proportions of fishes in each assemblage, followed by omnivores and secondary carnivores Fishes that fed at more than one trophic interval were extremely common in all food webs Analysis of covariance was used to compare structural features of different webs across a range of link thresholds Extensive among—site variation in food—web parameters was associated with differences in species richness and environmental differences associated with rainfall patterns, physiography, and gross primary production Seasons generally influenced food—web parameters less than did site differences Relative importance of detritus, aquatic primary production, and terrestrial production in aquatic food webs varied seasonally in each system Detritus, derived primarily from aquatic macrophytes, was an important pathway in both tropical swamp ecosystems Aquatic primary productivity comprised the largest fraction of fish diets during the wet season in the Venezuelan swamp, but it formed the major component of fish diets during the dry season at all other sites Based on comparisons using 13 webs, two—thirds of the pairings among six food—web parameters used (number of nodes, compartmentation, connectance, average number of prey per node, average number of predators per node, ratio of consumer nodes to total nodes) were positively intercorrelated Several food—web relationships previously described as constant (eg, connectance x species richness constancy, species scaling law, link—species scaling law) were not confirmed by my data These earlier food—web trends are extremely sensitive to methodological biases, especially decisions regarding the degree of taxonomic lumping of species into trophic units Although food webs have unique emergent properties and spinoff a number of potentially informative macrodescriptors, empirical studies must achieve greater precision and uniformity before analyses can be performed across different systems Several problems and potential resolutions are discussed

The evolution of flightlessness in insects

Abstract: Many pterygote (winged) insects have secondarily become flightless; why has this occurred? In this paper I test for an association between the frequency of flightlessness and (1) environmental heterogeneity, (2) geographic variables, (3) gender, (4) alternate modes of migration, and (5) taxonomic variation. Various authors have predicted that decreased environmental heterogeneity will favor the evolution of flightlessness. This hypothesis is consistent with a number of studies on the variation of wing dimorphism and flightlessness and with data analyzed in the present paper on the relationship between habitat type and wing morph in the North American Orthoptera. The incidence of flightlessness is also exceptionally high in woodlands, deserts, on the ocean surface, in specific habitats on the seashore (dunes and rock crevices but not the foreshore), in aquatic habitats, in the winter months, in hymenopteran and termite nests, and among ectoparasites of endotherms and parasites of arthropods. It is low in habitats bordering rivers, streams, ponds, etc. and in arboreal habitats. Some of these habitats can be classified as persistent, but others cannot be so designated or are too vaguely defined. The incidence of flightlessness increases with altitude and latitude but, contrary to "conventional" wisdom, it is not exceptionally high on oceanic islands compared to mainland areas. Several authors have hypothesized that the clinal variation in flightlessness is a consequence of clinal variation in habitat persistence. Though the available data are consistent with this hypothesis, other factors such as impairment of flight by low temperatures cannot be rejected, though considered unlikely. Flightlessness is more frequent than expected by chance among parthenogenetic species and more frequent among females than males. Furthermore, there is a significant negative association between female mobility and flightlessness in males. I suggest that loss of flight is favored in females because it permits greater allocation of resources to egg production, but that flight is retained in males because it increases the probability of finding a mate. In some species alternate modes of migration, viz phoresy and ballooning, have evolved. Phoretic transport (i.e., via other animal species) may be important in those species in which it occurs, but it appears to have evolved relatively rarely. Among the Insecta ballooning occurs only in the larvae of Lepidoptera. Aptery (winglessness) is unusually frequent among Lepidoptera that eclose in the fall and winter months and live in woodlands and forests. I hypothesize that larval migration by ballooning, the large—scale spatiotemporal stability of woodlands, and the small—scale unpredictability of spring bud burst are primary factors favoring the evolution of flightlessness in these Lepidoptera. The mode of metamorphosis is correlated with the frequency of flightlessness; hemimetabolous and holometabolous insects are rarely flightless, while it is common among paurometabolous insects. Two factors that may favor such an association are differences in relative mobility of the larvae and niche shifts between the adult and larval stage. In the holometabolous insects taxonomic families with at least one flightless species have more species than those with only winged species. A possible explanation for this is that the occurrence of the appropriate ecological conditions favoring flightlessness in holometabolous insects and the requisite mutations are both very rare events.

Measurement of the realized qualitative niche: environmental niches of five Eucalyptus species

Abstract: A method is described, using Generalized Linear Modelling, for determining the qualitative environmental realized niche (QERN) of plant species. Five species of Eucalyptus growing in south-eastern Australia are used as examples. The frequency of each species occurrence in a data set comprising 6080 observations over an area of -40 000 km2 is modelled as a function of four environmental variables: mean annual rainfall, mean annual temperature, a solar radiation index, and rock type. The position and shape of the response of a species to these four variables is the species' QERN. The effect of rock type on the distribution of each species was found to be highly significant. To reduce model complexity, the data were stratified on rock type, and models for each species were generated for the two common rock types, granites and soft sediments. Whilst the model-fitting procedure generally is the same, each species' response demanded some modifications of the procedure. The continuous variates, mean annual temperature, mean annual rainfall, and solar radiation index, were categorized so that the magnitude of the model coefficients from each category would indicate the shape of each species' response. Continuous functions were then fitted to the categorical response shapes. Of the 10 models (5 species and 2 rock types) 5 revealed a skewed response to one or more of the environmental variables, one was linear, and two were bell shaped. Two more models revealed complex response shapes, suggesting that the environmental variables were inappropriate or insufficient to model those species' realized niches. The results provide evidence for asymmetric responses of species to environmental variables contrary to the bell-shaped symmetrical responses commonly assumed in eco- logical theory. Models of species' realized niches are necessary in community ecology to describe the role of environment, prior to studies on the role of competition in determining community composition. Limitations of the models, both statistical and ecological, are considered, with suggestions for improvements.

Fire and climate change during the last 750 yr in northwestern minnesota

Abstract: Charcoal stratigraphic analysis and fire scars on red pine (Pinus resinosa) trees were used to determine spatial and temporal occurrence of fire in 1 km2 of old-growth mixed conifer/hardwood forests in northwestern Minnesota. Charcoal was analyzed year by year on petrographic thin sections from annually laminated sediments of three small (<5 ha) lakes having adjacent catchments. Dated fire scars (n = 150) from recent treefalls provided an independent record of the spatial patterns of past bums. Sedimentology of the varved sediments, water-balance models that use 150 yr of instrumental temperature and precipitation data, and published data were used to identify climate changes in separate studies, and they were used in this study to examine the possible connection between changing fire regimes and climate change. Fire-history data were used to show the changing probability of fire with time since the last fire and the effects of spatial variance (slope and aspect) on the distribution of fires through time. Over the last 750 yr, fire was most frequent (8.6 ? 2.9-yr intervals) during the warm/ dry 15th and 16th centuries. Intervals were longer (13.2 ? 8.0 yr) during cooler/moister times from AD 1240 to 1440 and since 1600 (the Little Ice Age). The fire regime during the Little Ice Age consisted of periods during the rnid- 18th and mid- 19th centuries char- acterized by longer fire intervals of 24.5 ? 10.4 and 43.6 ? 15.9 yr, respectively, and short-term warm/dry periods from 1770 to 1820 and 1870 to 1920 when intervals were 17.9 ? 10.6 and 12.7 + 10. 1, respectively. The probability of fire increased through time, probably in step with fuel accumulation. South- and west-facing slopes burned more fre- quently than did north and east aspects. Fire suppression began in 1910. During warm periods, probability of fire was sufficiently high that a continuous litter layer was all that was necessary for fire to spread and scar trees. During cool and moist times fire was most likely to occur in years with higher moisture deficits. The combined methods for fire-history analysis provided a more detailed spatial and temporal documentation of fire regimes than has previously been possible from analysis of fire scars or of charcoal counts derived from fossil pollen preparations. Results support predictions of particle-motion physics that thin sections record a local fire history. Because climate varies continuously, the responsiveness of disturbance regime to short- and long- term climatic change suggests caution in the interpretation of fire frequencies that derive from space/time analogies or extrapolation from short-term data.

Organization in Natural Assemblages of Desert Lizards and Tropical Fishes

Abstract: Techniques are developed for the analysis of community organization and bench tested on a set of simple model systems with known structure (i.e., with and without guild structure, with varying degrees of resource partitioning, with and without "core" resources). Proportional utilization coefficients, pi, are positively correlated with the abun- dance of resources, whereas electivities, ei, correlate negatively with resource abundance. The geometric mean of pi and ei, termed gi, is a superior measure of utilization, more nearly independent of biases associated with resource availability than either of its components, and performs better in bench tests. Organization in observed patterns of resource utilization by four desert lizard and four tropical freshwater fish assemblages is critically evaluated via comparisons with results from two randomization algorithms. Randomizations follow a Monte Carlo technique whereby the dimensions of the original m x n resource matrix are preserved during hundreds of independent runs. The first "scrambled zeros" algorithm rearranges observed values for resource utilization by each consumer and retains consumer dietary niche breadths, but destroys guild structure (zero structure) of observed matrices. The second algorithm ("conserved zeros") also retains observed consumer diet breadths, but only rearranges observed resource utilization coefficients among the particular resources actually used by consumers, thus retaining observed guild structure (i.e., matrix zero structure). By plotting average dietary overlap against ordered niche neighbors, we evaluate (1) relative guild structure using the randomization algorithm that scrambles observed matrix zero structure, and (2) consumer resource segregation within guilds using the algorithm that conserves zero structure. Statistically significant guild structure is evident to varying degrees in all but one low- diversity fish assemblage. All four tropical fish assemblages reveal significant partitioning of food resources during both the wet and dry seasons, particularly among intermediate to distant neighbors. The most species-rich assemblage exhibits extremely high levels of resource segregation during the period of desiccation of aquatic habitat and increased fish densities. Diverse Australian desert lizard assemblages show significant partitioning of microhabitats among ecologically similar species, even though only 15 microhabitat cat- egories are recognized. Analysis of dietary resource matrices based on only 19 prey resources shows that Australian lizards are piled up on certain prey types, forming functional dietary guilds of lizards that eat termites, ants, other lizards, etc. However, no dietary segregation is evident when only 19 prey resource states are recognized. In contrast, when the analysis is redone on more refined resource matrices based on more than 200 prey types, guild structure essentially disappears but niche segregation becomes evident. In the less diverse Kalahari desert, lizard assemblages are more variable and do not appear to be as tightly organized. Patterns of resource utilization in several of these natural vertebrate assemblages are, however, sufficiently organized to suggest broad effects of internal biotic factors, such as ecological constraints of functional morphology and physiology (leading to guild struc- ture), and possibly interspecific competition.

The Demographic Benefits of Diel Vertical Migration by Zooplankton

Abstract: Three types of diel vertical migration behavior were expressed within a fjord population of the calanoid copepod Pseudocalanus newmani: (1) reverse migration (nocturnal descent), (2) normal migration (nocturnal ascent), and (3) no detectable migration. Reverse migration by P. newmani occurred in all three study years, but only at a deep (185 m) station at times of year when normally migrating predatory zooplankton (the copepod Euchaeta elongata, the chaetognath Sagitta elegans, and the omnivorous euphausiid Euphausia pacifica) were abundant. A predator—exclusion "natural experiment" tested the hypothesis that the reverse migration is a predator avoidance mechanism: at a shallow (55 m) station within the same fjord, large predatory zooplankton were absent or reduced markedly in abundance. In conditions of reduced nocturnal predation by predatory zooplankton, no reverse migration by P. newmani was detected on any of 11 cruises, consistent with the hypothesis. Instead, at the shallow station, where diurnal predation by planktivorous fish was of increased importance, normal diel vertical migration by P. newmani was observed on several occasions. No migration was detectable at either the deep station or the shallow station at times of year of low feeding activity of predators in surface waters. The predominant migration behavior (reverse, normal, or no migration) varied depending whether the primary source of mortality was predation by nocturnally feeding zooplankton or visually hunting planktivorous fish, and appeared unrelated to resource distributions or temperature gradients. The relative advantage of the three migration behaviors is explored with theoretical life table analyses. Re—examination of the demographic advantage hypothesis proposed by McLaren (1974) illustrates that vertical migration across a thermocline carries a fitness cost, not a benefit. The realized rate of increase of a migrant population of Pseudocalanus is found to be lower than a nonmigrant population for all combinations of surface temperatures, positive thermal stratification, and ratios of mortality for early and late developmental stages. However, avoidance of predators by vertical migrants reverses the outcome of this analysis. Predator avoidance by Pseudocalanus, accompanied by reduced mortality rates, confers a fitness advantage upon a population that migrates across a thermocline. Remarkably small reductions in mortality rates (as little as 12%) confer this advantage, whether migrants avoid nonvisually hunting predatory zooplankton by night or visually hunting planktivorous fish by day. The behavioral variation documented here confirms that diel vertical migration is dynamic rather than a fixed, invariant behavioral trait within a population. In environments where predation pressure and other factors vary through time, studies of limited scope and short duration are unlikely to illuminate the ultimate causes for diel vertical migration. The heritability of the vertical migration trait is unclear. The different phenotypic responses reflected within this single populations of P. newmani may reflect altered behavior of individuals. Alternatively, this behavioral variation may represent a balanced polymorphism maintained by differential mortality of genotypes experiencing selection pressures that vary through time.

Comparative Ecology of Blue Monkeys (Cercopithecus Mitis) in High- and Low-Density Subpopulations

Abstract: Two subpopulations of blue monkeys (Cercopithecus mitis), located 10 km apart, were studied in Kibale Forest, Uganda, for most of a 6—yr period (1978—1984). This study was undertaken (1) to assess and evaluate the major differences in the environment and in the behavioral ecology of blue monkeys living at high and low densities and (2) to explain the differences in blue monkey densities in the two study sites. Methods included the enumeration of trees and primates, and assessments of the availability of fruits and of diets, time budgets, ranging patterns, and demographics of blue monkeys. There was a 10—fold difference in blue monkey densities between the two subpopulations. Major differences were found in the ecology, behavior, demography, and habitats of blue monkeys living at these two densities. Compared to the high—density subpopulation, the low—density subpopulation exhibited shorter tenure lengths for resident males, a greater density of nonresident males, a higher rate of group intrusions by nonresident males, a higher incidence of infanticide by new resident males, hybridization between blue monkeys and redtail monkeys (Cercopithecus ascanius),fewer adult females per adult male, and lower resident male reproductive success. From these observations I conclude that male—male competition for females was more intense where blue monkey densities were low. Predictor variables of food production and food competition indicate that more food was available for the low—density than for the high—density subpopulation of blue monkeys. The habitat containing the low—density subpopulation had a higher tree density and basal cover, greater tree species richness and diversity, and more fruit left uneaten. It also had a lower overall primate density, which, together with assessments of the kinds of foods eaten, dietary overlap and richness, foraging behavior, and home range size, suggests that intra— and interspecific competition for food both were less in the low—density subpopulation. Further evidence that food was more available for the low—density subpopulation is that both the birth rate and the population growth rate of blue monkeys were higher there than in the high—density subpopulations. The low—density subpopulation was apparently below carrying capacity and increasing in a food—rich habitat. In contrast, the high—density subpopulation appeared to be at carrying capacity, stable, and food limited. Therefore, contrary to what was hypothesized, the availability of food does not explain why blue monkey densities differed on these two areas. On this basis I attribute the low density of the one subpopulation to some unknown historical event rather than to current ecological differences between areas. I suggest that, during this study, the low—density subpopulation was recovering from a decline and that the responsible mechanism (e.g., disease) was no longer operating. This paper emphasizes: (1) the considerable variability found in the ecology and behavior of primates–even within one species in the same forest; (2) the need for long—term comparative studies of free—living primates, especially those at low densities; (3) the importance of investigating the density of primate populations relative to the carrying capacities of their environments, and the influence of this relationship on behavior and ecology.

Patterns, Mechanisms, Consequences of Variability in Settlement and Recruitment of an Intertidal Barnacle

Abstract: I examined the settlement or recruitment of the barnacle Chthamalus anisopoma at a variety of scales: temporally, over periods lasting 12 h to 1 mo, and spatially at sites separated by <1 cm to shores separated by 10 km. Both settlement and recruitment were variable at nearly all temporal and spatial scales that I examined. The majority of variability was attributable to differences in settlement or recruitment between sample periods. When only the spatial component was considered, variability decreased with scale. Large, consistent differences in settlement between geographic locations that are at approximately the same tidal height are probably the result of differential larval supply, whereas more subtle but still consistent differences in settlement between adjacent sites may, at least in part, be due to larval behavior. Differences in settlement between close sites were observed only when settlement was relatively high. This observation may be explained as follows. First, Chthamalus settle in pits or small depressions "suitable settlement sites (SSS's)" in the surface of a substrate. Second, within an SSS, space for settlement is restricted. Third, the density of SSS's differ between sites. When settlement is high, settlement differences between locations reflect differences in the number of SSS's, as all are used. However, when settlement is low, settlement will be similar between locations, since SSS's are not limiting. In a series of field experiments using both natural and artificial substrates I found that: (1) Cypris larvae of Chthamalus settled preferentially in the presence of a number of physical and biological factors. (2) Settlement was density dependent but growth and survivorship were not, which suggests exploitative but not interference intraspecific competition. (Although Chthamalus settle gregariously they space themselves out.) (3) Variability in the settlement of Chthamalus affected the ensuing adult population even at the highest settlement densities.

Experimental Studies of Caribbean Seagrass Bed Development

Abstract: Processes important in the development of subtidal seagrass beds composed of Thalassia testudinum, Syringodium filiforme, Halodule wrightii, and many rhizophytic algal species were examined in situ for 52 mo in a coral reef lagoon on St. Croix, United States Virgin Islands. The study emphasized the early stages of development of the seagrass beds and the role played by colonizing rhizophytic algae. I tested the hypotheses that nutrient accumulation in the sediments limits seagrass recolonization, and that rhizophytic algae facilitate sediment nutrient accumulation by stabilizing the sediments and adding organic matter from rapidly decomposing thalli. Vegetation was removed from 0.25- and -iM2 plots in 3 m of water. Plot treatments consisted of: (1) no further manipulation, (2) adding nitrogen plus phosphorus fertilizer to the sediments, (3) removing colonizing rhi- zophytic algae to minimize algal effects (e.g., sediment stabilization, organic input), and (4) removing colonizing algae and adding "plastic algae" to stabilize sediments without organic input. Plant densities, sediment grain size, redox potential, inorganic nitrogen concentrations in porewaters, and ammonium production rates were measured over time in all plots, including undisturbed controls. All recolonization occurred through vegetative propagation. The sequence of plant recolonization was unaffected by the treatments, corresponding instead to life history char- acteristics and nutrient requirements of the species involved. Rhizophytic algae invaded the plots within a few months, followed by the seagrass Syringodium, then Thalassia. The seagrass Halodule was insignificant in the recolonization. Densities of rhizophytic algae and Syringodium declined when the density of Thalassia reached 200 leaf shoots/M2. The rates of increase in seagrass leaf shoot densities and biomass were greatest in the fertilized plots, supporting the nutrient limitation hypothesis. Rhizophytic algae facilitated seagrass recolonization; seagrass densities, biomass, and porewater ammonium concentra- tions were lowest in plots where algae were removed. Sediment ammonium concentrations decreased when Thalassia became dominant. Ammonium production in the sediments increased as the plant community developed. The nitrogen required for Syringodium pro- ductivity was met easily by ammonium production, assuming no competition from Tha- lassia. In contrast, Thalassia accounted for > 93% of the nitrogen required for total seagrass productivity, and ammonium production could supply up to 45% of this requirement. At the end of the experiment (52 mo), Thalassia density and ammonium production rates in the sediments were lower than in the surrounding undisturbed seagrass bed. The experiment provides evidence for a resource-ratio model where the rate of succes- sion is controlled by a sediment nutrient supply that increases over time. The sequence of colonization is determined by relative rates of vegetative propagation by stolons and rhi- zomes across the sediment surface, which are inversely correlated with whole plant pro- ductivity and thus with requirements for nutrients. Algal colonizers tolerate low nutrients by having low productivities. The climax species Thalassia is a competitive species effective at exploiting the sediment nutrient resource. Co-existence, rather than replacement, of species occurs, despite a relatively benign disturbance regime.

Field Tests of Neighborhood Population Dynamic Models of Two Annual Weed Species

Abstract: We report the results of a 4—yr study of the community dynamics of the annual weed species Abutilon theophrasti (velvet leaf) and Amaranthus retroflexus (pigweed). We calibrated neighborhood population dynamic models for communities of these species in the field and then tested the predictions of the calibrated models against census data from independent observations. We also analyzed the calibrated and tested models to predict long—term dynamics and to assess how spatially local interactions, growth, fecundity, survivorship, germination, seed dormancy, and dispersal each contribute to the community's dynamics and structure. We show that calibrated neighborhood models accurately predict dynamics in the field over a 4—yr period. Because the predictive spatial models reduce approximately to simple nonspatial models of competition, the spatial processes that govern the dynamics of velvet leaf and pigweed communities behave as simple nonspatial processes. The models predict that velvet leaf will eventually exclude pigweed because of an asymmetry in the magnitude of inter—individual interference affecting growth. We also show that velvet leaf monocultures would oscillate perpetually in the absence of delayed germination and that demographic stochasticity (May 1971) has little effect on the dynamics of the experimental species.

A simulation model of the population dynamics and evolution of myxomatosis.

Abstract: Myxoma virus was released into Australia to control the introduced Euro- pean rabbit, Oryctolagus cuniculus. Within a few years after introduction, the virulence of the virus had declined to an intermediate level, while the resistance of field rabbits had increased sharply. In the nearly 40 yr since the disease was introduced, host resistance has continued to increase, while viral virulence has only recently begun to show signs of counter- increases in some areas. The two questions of interest are thus: Is this system in a coevo- lutionary arms race (Dawkins and Krebs 1979); that is, will both host and pathogen continue to evolve antagonistically? Will the virus continue to control the rabbit in the future? We present a simulation model based loosely on previous host-pathogen models (An- derson and May 1979), but with detailed accounting of the virus titer in infected hosts, and using realistic estimates of the demographic parameters of the rabbit, including age structure and seasonally varying reproduction. For a single virulence grade, by varying the non-disease (or "natural") mortality of the rabbit, the age at first reproduction of the rabbit, and the virulence grade of the virus, we explored the parameter range for which the rabbit population is controlled. For the most prevalent grades of the virus, grades IIIB and IV, the virus can control the rabbit for most realistic values of natural mortality and age at first reproduction. However, control is dependent on both natural mortality and virus virulence. Since natural mortality varies both geographically and seasonally, the usefulness of the virus may vary geographically and seasonally, and management policies must be sensitive to this variation. When competing against several virus strains that together encompass the complete range of virulence seen in the field, a strain of grade IV virulence competitively excludes strains of all other grades. This competitively dominant grade is close to the most prevalent virulence grades seen in the field. We discuss possible mechanisms of coexistence, including local competitive exclusion with global persistence, variability in host resistance, high mutation rates, and trade-offs between within-host and between-host competitive ability. By examining the effects of flea transmission efficiency, we are able to show that, contrary to commonly held belief, whatever effect fleas have upon the outcome of selection on virulence cannot be due to differences in transmission efficiency between fleas and mos- quitoes. Finally, by including host resistance, we improve our prediction of the most prevalent grade of virulence. We conclude that control of the rabbit by the virus is likely for the near future, but that until we understand the genetics of resistance in the rabbit and the rela- tionship between resistance and virulence for different grades of virulence, we cannot make a useful prediction of the long-term state of this system.

Lunar Reproductive Cycles of Benthic-Brooding Reef Fishes: Reflections of Larval Biology or Adult Biology?

Abstract: Lunar reproductive cycles are common among marine animals. Most hy- potheses concerning the adaptive nature of such cycles consider the effects of the tidal regime or moonlight on the dispersal of planktonic eggs or hatchlings away from adult habitats. We determined the reproductive periodicity of 17 paternal-brooding reef fishes at three neotropical sites with different tidal regimes. We used these and published data to assess the explanatory potential of hatchling-dispersal hypotheses, and of other larval- biology and adult-biology hypotheses. Among the 17 species (15 damselfishes and two blennies), two of which we studied at two sites, we found 14 with lunar and semilunar spawning cycles, one sporadically syn- chronized spawner, two acyclically variable spawners, and one continuous spawner. In 16 species individual nests experienced frequent alternation of short brood-care and rest pe- riods (brood cycling). Simple hatcThling-biology hypotheses are unable to account readily for the range of variation in the types, precision, and lunar timing of spawning cycles among species within and between depth zones at the same site, or for intra- and interspecific variation in spawning patterns in relation to seasonal and geographic differences in tidal regimes. The hypothesis that lunar spawning cycles have evolved to maximize the availability of relatively uniform-age larvae that settle during preferred lunar settlement times is sup- ported by some but not all data. Adult-biology hypotheses can account for the higher frequency of lunar spawning cycles among brooders than nonbrooders, and can accommodate various patterns of reproductive cyclicism and synchronization in brooders. Synchronized activity may be favored in some brooders because colonial spawning provides enhanced defense against egg predators. Fur- ther, the cost of brood care may result in reduced egg survivorship, and both brood cycling and the temporal concentration of spawning may reduce egg losses by allowing males to recuperate and increasing benefits to brood-care for them. Adult-biology constraints may produce variability in spawning patterns. Differences in social systems may determine the ability to form nesting colonies, and to strongly syn- chronize spawning independently of lunar cues. Changes in short-term food availability may influence the regularity of spawning. Adult-biology hypotheses represent generally underemphasized alternatives that have at least as much explanatory potential as larval-biology hypotheses. If hatchling biology is of general importance to paternal brooders, there must be considerably more interspecific variability in hatchling ecology than is generally assumed.

Holocene Paleoecology of the Boreal Forest and Great Lakes-St. Lawrence Forest in Northern Ontario

Abstract: This paper presents pollen and macrofossil stratigraphies derived from sediment cores taken near the deepest parts of Nina Lake (46°36' N, 81°30' W), Jack Lake (47°19' N, 81°46' W), and Lake Six (48°24' N, 81°19' W), which are situated along a transect across the ecotone between the boreal forest and the Great Lakes—St. Lawrence forest in northern Ontario. Paleoecological data from this region of steep climatic gradient and varied landforms provide sensitive records of postglacial vegetational and climatic changes. These data are used to evaluate questions concerning the occurrence of late—glacial vegetation communities without modern analog. Hypsithermal ecotonal movements, and individualistic species response to climatic change and soil development. The early postglacial boreal forest that colonized the Canadian Shield upland after ca. 10 000 BP was dominated by white spruce with little or no black spruce, and probably contained more oak, elm, poplar, and heliophytic herbs and shrubs than its modern counterpart. Its lack of modern analog is probably due to the widespread occurrence of fresh, unleached soil in a newly deglaciated landscape. Spruce declined and was replaced by jack pine after ca. 9000 BP as the climate continued to warm. The boreal forest was enriched floristically by the successive immigration of species such as Myrica. Alnus crispa, and A. rugosa. Boreal forest was transformed into Great Lakes—St. Lawrence forest ca. 7400 yr ago when white pine, beech, and hemlock immigrated to Nina Lake. Species of the Great Lakes—St. Lawrence forest responded individualistically to Hypsithermal climatic changes. White pine populations continued to spread northward during ca. 7000—3000 BP, causing the boreal forest/Great Lakes—St. Lawrence forest ecotone to advance ca. 140 km north of its present position, only constrained by the physiographic boundary between the Clay Belt and the Canadian Shield upland. During 6000—4500 BP northern white cedar proliferated in the Clay Belt lowlands due to a warmer and drier climate. The regional water table was lowered, permitting Thuja populations that were otherwise restricted to the margins of swamps and open peatlands to spread to the center. This vegetation response was landform selective, being more pronounced in the Clay Belt than on the Canadian Shield upland. The predominance of calcareous substrates and wetland habitats in the Clay Belt was favorable to the expansion of Thuja under a suitable climate. Neoglacial cooling decimated the populations of northern white cedar in the Clay Belt and those of white pine in the whole region. Spruce, jack pine, and balsam fir increased over the last 4000 yr. The ecotone retreated from near Lake Six after 3000 BP, reaching Jack Lake at ca. 2600 BP, and was stabilized in its modern position during the last millennium.

Distribution of Long-Lived Radionuclides in an Abandoned Reactor Cooling Reservoir

Abstract: The distribution of '37Cs, 90Sr, 238Pu, 239'240Pu, 24'Am, and 244Cm was studied in the biotic and abiotic components of an abandoned reactor cooling impoundment, Pond B. The impoundment is located at the United States Department of Energy's Savannah River Plant in South Carolina, USA. It received radioactive contaminants via cooling water discharges from R Reactor from September 1961 to June 1964. The radionuclide inven- tories were estimated in water, seston, sediments, and biotic components after 20 yr of equilibration. Chemical, physical, and biological relationships to the radionuclide distri- bution patterns were investigated. Biotic components contained some of the highest ra- dionuclide concentration ratios observed to date. However, most of the radioactivity resides in sediments. The principal mechanisms of loss from the system are radioactive decay and periodic outflow of water and suspended materials; biotic export and seepage appear to be inconsequential. Strontium-90 was much more mobile in the system than the other radio- nuclides. Aquatic macrophytes dominated the biotic component radionuclide inventories and their dynamics exert a strong influence on the spatial distribution and turnover of radioactivity in the ecosystem. Pond B supports a diverse and productive flora and fauna. Cleanup of the system is not indicated. Use of Pond B for recreation is feasible with adequate attention to monitoring and radiological health guidelines.

A 10 000‐Yr History of Natural Ecosystem Acidification

Abstract: This study examines natural, long—term (≈104 yr) acidification processes in New England using paleoecological methods (pollen, diatoms, and the chemistry of three operationally defined sediment fractions) and a paired watershed approach.The primary variable is lithology, and the focus of this study is to illuminate the role of lithology in long—term ecosystem acidification. Unlike most cases of recent (anthropogenic) acidification in which inferred processes of ecosystem acidification occur over short time spans (10—50 yr), natural acidification processes develop over hundreds or thousands of years, providing enough temporal resolution in the stratigraphic record to allow strong inferences about the factors controlling the occurrence, timing, and magnitude of terrestrial and aquatic ecosystem acidification. The two sites in this study are similar in size, lake depth, elevation, aspect, and local climate, but Cone Pond, New Hampshire, is an acidic, clearwater lake in a catchment of thin tills derived from base—poor gneisses and schists, whereas South King Pond, Vermont is a mesotrophic lake in a catchment of thicker tills derived from slates, phyllites, and limestone. Although vegetation history is generally similar between the two sites, the Cone Pond catchment included higher representation of acidophilic vegetation than the South King Pond catchment throughout its 10 000—yr history. There are no known exposures of sulfide minerals in the Cone Pond watershed, nor are the basal inorganic sediments S—rich. Both diatom and geochemical analyses indicated significant long—term acidification of the upland soils and surface waters of the Cone Pond, but not the South King Pond, catchment. At Cone Pond, the uplands exported A1 almost entirely in labile form by 10 000 yr before present (BP). Increases in labile Al 8000—5500 BP, followed by substantial declines in sedimentary MN and in the ratio of Ca to organic matter, indicate soil acidification followed by decreasing lakewater pH. The loss of the Melosira/Cyclotella diatom assemblage ≈7200 BP as hemlock populations began to expand, coupled with the disappearance of the chrysophyte Mallomonas torquata, suggests increasing lakewater transparency possibly accompanied by decreasing lakewater pH. By 5000 BP, all planktonic diatom species has disappeared, and the acidobiontic diatom flora was well established. Diatom—inferred pH reconstructions indicate that the pH of Cone Pond was <5.5 by 6500 BP and may have dropped below 5.0 as long ago as 2000 BP, when spruce recolonized the catchment. A sudden decline in the ratio of acid—labile nonbiogenic Si to acid—labile A1 also occurred ≈2000 BP, and appears to reflect dissolution of an amorphous aluminosilicate complex, providing independent evidence for a precultural pH of ≈5.0. None of these features are found at South King Pond, where the only paleolimnological evidence of acidification is a sharp decline in sedimentary carbonates ≈5000 BP. This decrease, however, appears to be related to climatic factors rather than to lakewater pH. The historical accident of low till deposition in the Cone Pond catchment has been an important predisposing factor for natural, long—term ecosystem acidification, leading to truncated soil profiles, shortened hydrologic flow paths, lower overall supplies of base cations, and an enhanced susceptibility to biogeochemical changes driven by changes in upland vegetation. The potential role of other factors (depletion of base cations, sulfide minerals in the catchment of the aquifer feeding the ephemeral inlet, weathering of peat deposits in the upper reaches of the inlet, and historic A1 acidity) is also discussed.

Optimal body size in lesser antillean anolis lizards -a mechanistic approach

Abstract: For the past twenty years, it has been recognized that body size is convergent among solitary species of Anolis lizards endemic to different island banks of the Lesser Antilles. Community ecologists have assumed that this "solitary size" is an optimal size, yet the basis behind such optimality has not been shown. Our goal in this study was to explore the existence of an energetic basis of an optimal body size in these lizards. A computer model is presented that incorporates quantitative descriptions of lizard metab- olism, locomotion, digestion, and visual acuity, and simulates foraging in a sit-and-wait predator. Quantitative estimates of daily foraging energetics are presented, which are then used together with estimates of resting metabolism to simulate growth. An optimal growth strategy is incorporated to determine adult body sizes that maximize lifetime reproductive output. Such optimal body sizes were determined for different prey densities and activity levels, predator life expectancies, and field metabolic rates. Predicted optimal body sizes are close to the observed body sizes of Lesser Antillean anoles, and are relatively insensitive to both levels of prey activity and up to fourfold differences in prey density, while life expectancy and rates of field metabolism may influence predictions. The insensitivity of predicted optimal body size to prey density lends support to the assumption that the solitary size observed among anoles throughout the Lesser Antilles is an optimal body size. Additional findings were made regarding home-range sizes, growth patterns, and visual constraints to foraging performance.

Simulation of the Diagenesis of Carbon, Sulfur, and Dissolved Oxygen in Salt Marsh Sediments

Abstract: A steady-state numerical model has been developed for simulating vertical profiles of the concentrations of organic matter, pyritic sulfur, dissolved oxygen, and the carbon isotope composition of organic matter in marsh sediments. In the model organic matter enters the sediment via sedimentation, belowground production of roots, and che- moautotrophic fixation of interstitial CO2 associated with pyrite oxidation. Pyrite is formed by sulfate reduction and consumed by oxidation with dissolved oxygen in the interstitial water. Exchanges of organic matter, carbon isotopes, pyrite, and dissolved oxygen between the sediment and surface environment occur via fiddler crab bioturbation. Aeration of the sediment is caused by diffusion of oxygen into the interstitial water from air cavities assumed to be present in roots and in desaturated sediment pores formed in the upper part of the sediment by drainage and/or evapotranspiration. Sensitivity experiments with the model suggest that the accumulation of pyrite and organic matter in marsh sediments is governed in large part by the turnover time of roots and by the mean diameter of roots. The isotopic composition of the sediment was most sensitive to the rate of belowground production and to a lesser extent to the intensity of fiddler crab bioturbation. The model also indicated that fiddler crab burrowing can account for the observed isotope composition of creekbank sediments but not back (mid) marsh sediment. In both back and creekbank marshes, intense aeration of the sediment by roots is required to prevent the buildup of pyrite to unrealistically high concentrations at depth.