Showing papers in "Ecological Monographs in 1995"

Avian Life History Evolution in Relation to Nest Sites, Nest Predation, and Food

Abstract: Food limitation is generally thought to underlie much of the variation in life history traits of birds. I examined variation and covariation of life history traits of 123 North American Passeriformes and Piciformes in relation to nest sites, nest predation, and foraging sites to examine the possible roles of these ecological factors in life history evolution of birds. Annual fecundity was strongly inversely related to adult survivaI, even when phylogenetic effects were controlled. Only a little of the variation in fecundity and survival was related to foraging sites, whereas these traits varied strongly among nest sites. Interspecific differences in nest predation were correlated with much of the variation in life history traits among nest sites, although energy trade-offs with covarying traits also may account for some variation. For example, increased nest predation is associated with a shortened nestling period and both are associated with more broods per year, but number of broods is inversely correlated with clutch size, possibly due to an energy trade-off. Number of broods was much more strongly correlated with annual fecundity and adult survival among species than was clutch size, suggesting that clutch size may not be the primary fecundity trait on which selection is acting. Ultimately, food limitation may cause trade-offs between annual fecundity and adult survival, but differences among species in tecundity and adult survival may not be explained by differences in food abundance and instead represent differing tactics for partitioning similar levels of food limitation. Variation in fecundity and adult survival is more clearly organized by nest sites and more closely correlated with nest predation; species that use nest sites with greater nest predation have shorter nestling periods and more broods, yielding higher fecundity, which in turn is associated with reduced adult survival. Fecundity also varied with migratory tendencies; short-distance migrants had more broods and greater fecundity than did neotropical migrants and residents using similar nest sites. HowevEr, migratory tendencies and habitat use were confounded, making separation of these two effects difficult. Nonetheless, the conventional view that neotropical migrants have fewer broods than residents was not supported when nest site effects were controlled

Sea Otters and Kelp Forests in Alaska: Generality and Variation in a Community Ecological Paradigm

Abstract: Multiscale patterns of spatial and temporal variation in density and popu- lation structure were used to evaluate the generality of a three-trophic-level cascade among sea otters (Enhydra lutris), invertebrate herbivores, and macroalgae in Alaska. The paradigm holds that where sea otters occur herbivores are rare and plants are abundant, whereas when sea otters are absent herbivores are relatively common and plants are rare. Spatial patterns were based on 20 randomly placed quadrats at 153 randomly selected sites distributed among five locations with and four locations without sea otters. Both sea urchin and kelp abundance differed significantly among locations with vs. without sea otters in the Aleutian Islands and southeast Alaska. There was little (Aleutian Islands) or no (southeast Alaska) overlap between sites with and without sea otters, in plots of kelp density against urchin biomass. Despite intersite variation in the abundance of kelps and herbivores, these analyses demonstrate that sea otter predation has a predictable and broadly generalizable influence on the structure of Alaskan kelp forests. The percent cover of algal turf and suspension feeder assemblages also differed significantly (although less dramatically) between locations with and without sea otters. Temporal variation in community structure was assessed over periods of from 3 to 15 yr at sites in the Aleutian Islands and southeast Alaska where sea otters were 1) continuously present, 2) continuously absent, or 3) becoming reestablished because of natural range expansion. Kelp and sea urchin abundance remained largely unchanged at most sites where sea otters were continuously present or absent, the one exception being at Torch Bay (southeast Alaska), where kelp abundance varied significantly through time and urchin abundance varied significantly among sites because of episodic and patchy disturbances. In contrast, kelp and sea urchin abundances changed significantly, and in the expected directions, at sites that were being recolonized by sea otters. Sea urchin biomass declined by 50% in the Aleutian Islands and by nearly 100% in southeast Alaska following the spread of sea otters into previously unoccupied habitats. In response to these different rates and magnitudes of urchin reduction by sea otter predation, increases in kelp abundance were abrupt and highly significant in southeast Alaska but much smaller and slower over similar time periods in the Aleutian Islands. The different kelp colonization rates between southeast Alaska and the Aleutian Islands appear to be caused by large-scale differences in echinoid recruitment coupled with size- selective predation by sea otters for larger urchins. The length of urchin jaws (correlated with test diameter, r2 = 0.968) in sea otter scats indicates that sea urchins <15-20 mm test diameter are rarely eaten by foraging sea otters. Sea urchin populations in the Aleutian Islands included high densities of small individuals (<20 mm test diameter) at all sites and during all years sampled, whereas in southeast Alaska similarly sized urchins were absent from most populations during most years. Small (<30-35 mm test diameter) tetracycline- marked urchins in the Aleutian Islands grew at a maximum rate of -10 mm/yr; thus the population must have significant recruitment annually, or at least every several years. In contrast, echinoid recruitment in southeast Alaska was more episodic, with many years to perhaps decades separating significant events. Our findings help explain regional differences in recovery rates of kelp forests following recolonization by sea otters.

Relationships Between Landscape Structure and Breeding Birds in the Oregon Coast Range

Abstract: Human-caused fragmentation of forests is increasing, yet the consequences of these landscape changes to vertebrate communities are poorly understood. Although bird community response to forest fragmentation caused by agricultural or urban development has been well studied, we have little understanding of these dynamics in landscapes undergoing intensive forest management, where late-seral forest stands are separated by younger forest stands of varying ages and are part of a spatially and temporally dynamic forest landscape. We investigated the relationship between landscape structure and breeding bird abundance in the central Oregon Coast Range. We sampled vegetation and birds in 30 landscapes (250-300 ha) distributed equally among three basins. Landscapes represented a range in structure based on the proportion of the landscape in a late-seral forest condition and the spatial configuration of that forest condition within the landscape. We computed a variety of landscape metrics from digital vegetation cover maps for each landscape. Using analysis of variance and regression procedures, we quantified the independent effects of habitat area and configuration on 15 bird species associated with late-seral forest. Species varied dramatically in the strength and nature of the relationship between abundance and several gradients in habitat area and configuration at the landscape scale. Landscape structure (composition and configuration) typically explained <50% of the variation in each species' abundance among the landscapes. Species' abundances were generally greater in the more heterogenous landscapes; that is, they were associated with the more fragmented distribution of habitat. Only Winter Wrens showed evidence of association with the least fragmented landscapes. These results must be interpreted within the scope and limitations of our study. In particular, the scale of our analysis was constrained by the lower and upper limits of resolution in our landscapes, as set by minimum patch size and landscape extent, respectively. Thus, our results do not preclude much stronger and different relationships at finer and/or coarser scales. In addition, our community-centered habitat classification scheme and artificially discrete representation of patch boundaries may not have captured the functionally meaningful heterogeneity for each species. Finally, our analysis was limited to relatively common and widespread diurnal breeding bird species. Species sensitive to habitat fragmentation at the scale of our analysis may have been rare already and therefore not subject to the parametrical statistical approach that we employed.

Indirect Effects in Marine Rocky Intertidal Interaction Webs: Patterns and Importance

Abstract: To determine the patterns of occurrence and importance of indirect effects relative to direct effects in natural communities, I analyzed experimentally based studies from 23 rocky intertidal habitats. The vehicle of analysis was the construction of interaction webs, or the subset of species in food webs involved in strong interactions. The analysis focused on indirect effects involving changes in abundance, or interaction chains, since little information was available on other types of indirect effects (behavioral, chemical response, environmental). As expected, number of direct (= strong) interactions, indirect effects, interaction sequences producing indirect effects, and types of indirect effects (e.g., keystone predation, apparent competition, etc.) all increased with web species richness. Less expected, when these measures were adjusted to a per species basis, positive relationships with species richness were still observed for all measures but the number of types. In other words, with increasing web diversity, each species interacted strongly with more species, was involved in more indirect effects, and was part of more interaction pathways. The analysis identified 83 subtypes of indirect effect, including the seven previously identified types. Many of the 76 additional types could be reclassified into the seven types if the original definitions of these "classic" types were expanded to include interactions having similar effects but differing in the specific mechanism (e.g., both interference competition and inhibition of recruitment [preemption] have negative effects involving a spatial resource). Two new types of indirect effect, termed "apparent predation" and "indirect defense" were also identified, producing a total of 9 general types of indirect effect divided among 565 specific indirect effects. Of these, keystone predation (35%) and apparent competition (25%) were most common and exploitation competition (2.8%) was least common in these webs. Two methods of analysis suggested that indirect effects accounted for °40% of the change in community structure resulting from manipulations, with a range of 24—61%. The proportion of change due to indirect effects was constant with web species richness, indicating that strong direct interactions and indirect effects produce roughly the same level of alteration of community structure regardless of the level of web complexity. Several potential artifacts and biases were evaluated. Most importantly, neither variation in level of taxonomic resolution nor intensity of experimentation varied significantly with web size (species richness). Despite a bias toward manipulation of consumers over manipulation of basal species, some predator—initiated indirect effect types were scarce while some basal species—initiated types were common. While the frequency of exploitation competition may have been underestimated, it is unlikely that the frequency of this indirect effect would change dramatically: changes due to this effect should have been detected in many of the studies and reported; and the most intensively studied individual webs did not report frequencies differing much from the average. This analysis suggests investigators effectively identified and first manipulated those species responsible for most indirect effects and that more experiments added decreasing numbers of indirect effects. Moreover, the frequencies and importance of indirect effects may be more predictable than expected on the basis of theory.

Mortality Rates of 205 Neotropical Tree and Shrub Species and the Impact of a Severe Drought

Abstract: Mortality rates of 205 tree and shrub species were estimated during two intervals, 1982-1985 and 1985-1990, in two size classes, 1-10 and ?10 cm in diameter, in a 50-ha census plot in tropical moist forest on Barro Colorado Island in Panama. The severe dry season of 1983 was the focus of the study, since prior observations had dem- onstrated that it caused mortality in the forest. Here we document that forest-wide mortality was z3%/yr during the drought interval but only 2%/yr during the period afterwards, and that excess mortality during the first interval amounted to 2% of stems in the larger size class and 1% in the smaller. Overall, just under 70% of all species had higher mortality during the first census interval, but not all species were equally affected. Canopy trees had significantly higher mean mortality rates during 1982-1985 than during 1985-1990, but treelets and shrubs showed no or slight differences. This was counter to our prediction that species with short root systems would suffer more from a long drought. Shrubs did, however, have higher mortality rates than trees and treelets during both census intervals. We also evaluated mortality rates for subgroups of species that specialized on different microhabitats in the forest. As we predicted, colonist species (those associated with light gaps) had higher mortality rates than generalist species, 7-10%/yr compared to 2-4%/yr, but only in the smaller size class. Unexpectedly, colonizers had similar mortality rates as non-colonizers in the larger size class. Gap colonizers and generalist species were similarly affected by the drought-both had elevated mortality during 1982-1985. Species whose distributions were associated with moister soils (on the slopes around the island's plateau or in a swamp in the midst of the 50-ha plot) also had elevated mortality during the drought period, but no more so than generalist species. This was counter to our prediction that species from moist microhabitats would suffer more during an extended drought than generalists. Understory treelets that were slope specialists had higher mortality than generalists during both census intervals, but not large trees that were slope specialists. Our conclusions emphasize diversity as well as pattern. Every trend we illustrated had well-documented exceptions: large trees with lower mortality during the drought period, for example. Clearly, accurate predictions about how tropical forests will respond to climatic perturbations will require much detailed information from many species.

Mechanisms Associated With Decline of Woody Species in Riparian Ecosystems of the Southwestern U.S.

Abstract: Throughout western North America, riparian ecosystem function has been transformed by anthropogenic influences on riverine environments. Modified flood frequency, duration, or intensity; depressed floodplain water tables; and increased alluvium salinity have contributed to change in riparian forest communities formerly dominated by Populus fremontii and Salix gooddingii. The invasion of the naturalized arborescent shrub, Tamarix ramosissima, potentially alters competitive hierarchies and disturbance regimes in these riparian ecosystems. We evaluated the structure and function of two southwestern riparian communities that differed in the degree of streamflow perturbation to which they had been subjected: the highly regulated lower Colorado River and the less tightly regulated Bill Williams River. Ordination analyses provided evidence that these riparian communities are structured along gradients relating to moisture, salinity, disturbance from fire, and community maturity, with Colorado River sites being more xeric and saline than those on the Bill Williams River. Foliar elemental analyses revealed high sodium concentrations in Tamarix (Na:K ratio = 1.87) and in the native shrub Tessaria sericea (Na:K = 1.56). Evaluation of tissue water relations parameters showed that Tamarix had lower osmotic potentials than sympatric woody taxa, helping to confirm that Tamarix is halophytic and probably capable of greater osmotic adjustment than native species. Carbon isotopic discrimination (△) provided evidence for higher water use efficiency in Tamaris than in Populus, Salix, and Tessaria. Tamarix △ averaged over 1% less than that of the other riparian taxa. Experimental removal of Tamarix from stands where Salix was codominant resulted in growth augmentation, less negative water potentials, and higher leaf conductance in Salix, all providing evidence of interspecific competition. The persistence of Salix, but not Populus, on the Colorado River appears to be due to greater water— and salinity stress tolerance in Salix than in Populus. A preponderance of senescent Populus along the Colorado River is an indication that this formerly dominant species is effectively approaching local extinction in parts of this ecosystem.

Effects of Nutrient Enrichment on Growth and Herbivory of Dwarf Red Mangrove (Rhizophora Mangle)

Abstract: The objectives of this study were to determine responses by red mangrove (Rhizophora mangle) and its primary consumers to nutrient enrichment, to determine if nutrient limitation is responsible for the dwarf plant form of red mangrove, and to evaluate four competing hypotheses for the relation of nutrient status and invertebrate herbivory. In a factorial-designed experiment, I fertilized 48 dwarf trees along gradients of tidal elevation and water depth at Twin Cays, an intertidal mangrove island in Belize, Central America, and measured plant growth responses and herbivory for 2 yr. At the end of year- 2, I compared biomass accumulation and analyzed plant tissue for chemical and structural composition. Dwarf red mangrove trees on this tidal island responded quickly and grew vigorously when treated with P and NPK fertilizers. Leaf number, leaf area, branching, shoot length, and aerial root production increased dramatically over 2 yr. N-fertilized trees grew very slowly and their responses were not different from Control trees. N-fertilized and Control trees changed little over 2 yr. Some responses to phosphorus-containing fer- tilizers vary by water depth and tidal elevation, but physicochemical factors do not explain the differences in growth responses. I conclude that phosphorus availability is a major factor limiting red mangrove growth at my study site in the interior of Twin Cays. Herbivory by two specialized, endophytic insect species (Ecdytolopha sp., which feeds in apical buds, and Marmara sp., which mines stem periderm) increased in P- and NPK-fertilized trees compared to N-fertilized and Control trees. Twice as many apical buds were damaged or destroyed and the frequency of mines increased by 6-8 fold. However, fertilization had no effect on feeding rates and standing damage by a leaf-feeding guild of generalist herbivores or on the frequency of shoots killed by stem borers. Herbivory by the two specialists was not related to C:N ratios, but it was inversely related to concentrations of phenolic com- pounds. Neither nutrient ratios nor concentrations of phenolics affected rates of herbivory by the generalist folivores. Although sclerophylly of red mangrove leaves decreased in P- and NPK-fertilized trees but not in N-fertilized and Control trees, there was no relationship between leaf toughness and herbivory by generalist folivores. These data suggest that sclerophylly in oligotrophic ecosystems may be an adaptive mechanism related to nutrient conservation, and that it is associated with red mangrove survival in phosphorus-deficient soil rather than an adaptation to herbivory.

Effects of fire regime and habitat on tree dynamics in north florida longleaf pine savannas

Abstract: Frequent, low intensity fire was an important component of the natural dis- turbance regime of presettlement savannas and woodlands in the southeastern USA dom- inated by longleaf pine (Pinus palustris), and prescribed burning is now a critical part of the management of these endangered habitats. Fire season, fire frequency, and fire intensity are three potentially important, though still little understood, components of both natural and managed fire regimes. In this long-term (8-yr) study, we experimentally (through the use of prescribed burning) tested for effects of fire season (eight different times throughout the year) and fire frequency (annual vs. biennial burning), on population dynamics (re- cruitment, growth, mortality, change in density, and change in basal area (the total basal area of all stems in a plot)) and species composition of trees in two quite different types of longleaf-pine-dominated habitats (north Florida sandhills and flatwoods). Limited fire temperature and intensity data were also collected during one year to examine the rela- tionship between fire behavior (temperature and intensity) and tree mortality. Contrary to prior hypotheses, our results showed few systematic or predictable effects of season or frequency of burning on dynamics of longleaf pine. Instead, variability in the population dynamics of this species appeared to be related largely to variation in fire behavior, regardless of the season of burning. Consistent with prior hypotheses, we found that deciduous oak species (Quercus laevis, Q. margaretta, and Q. incana) were least vulnerable to dormant-season burning and most vulnerable to burning early in the growing season. This was shown particularly by seasonal trends in the effect of burning on oak mortality (both topkill and complete kill) and, to a lesser extent, on oak recruitment. Oak densities and basal areas also declined in the spring- burned plots, resulting in a shift away from oaks and towards increased dominance by longleaf pine. Detrimental effects of spring burning on oaks were partly explained by fire behavior, but there appeared also to be an important residual effect of burning season, particularly on complete kill. Though longleaf pine population dynamics did not differ markedly as a result of burning season and frequency, we did find important differences in pine dynamics between the two habitats (i.e., sandhills and flatwoods). In general, populations of longleaf pines in the sandhills appeared to be density regulated, while flatwoods pine populations were declining regardless of the level of intraspecific competition. This suggests that long-term persistence of longleaf pine, and perhaps other fire-adapted species in frequently burned longleaf-pine- dominated communities, may be determined by complex interactions between habitat factors and fire regimes.

Phylogeny, ecology, and the richness of parasite communities in vertebrates'

Abstract: The makeup of parasite communities is the result, among other factors, of interactions between the evolutionary history and ecological characteristics of hosts. This study evaluates the relative importance of some ecological factors (host body size, diet, habitat, latitude, and the mean number of parasite individuals per host) as determinants or correlates of parasite community richness in vertebrates, before and after controlling for potential effects of host phylogenetic relationships. Data were obtained from the literature on 596 parasite communities belonging to one of four distinct types: gastrointestinal parasite communities of fish, birds, or mammals, and ectoparasite communities of fish. There were positive correlations between the number of hosts sampled and mean species richness of the parasite community of each genus. In analyses treating host genera as independent statistical observations and using estimates of parasite species richness corrected for host sample size, positive correlations were observed between richness and host body size in gastrointestinal communities of all three groups of vertebrates. The mean number of parasite individuals per host also was correlated positively with species richness. In fish, richness increased with increases in the proportion of animal food in the host diet. Aquatic birds had richer parasite communities than their terrestrial counterparts, whereas marine fish had richer gastrointestinal parasite communities than freshwater fish. The richness of ectopar- asite communities on fish showed no association with any of the ecological variables investigated. Using host genera as independent points in the analyses may lead to biased results since some host lineages are descended from recent common ancestors, and are therefore not truly independent. The comparative analysis was repeated using phylogenetically indepen- dent contrasts derived from the phylogeny of hosts. Once the effects of host phylogeny were removed, somewhat different results were obtained: host body size showed no rela- tionship with parasite species richness in birds, and there was no evidence that habitat transitions resulted in significant changes in parasite species richness in any of the types of communities studied. Of the ecological factors studied, the comparative analyses suggest that only host body size can be an important determinant of parasite community richness in certain host groups. This study illustrates clearly the need to control for phylogeny in investigations of host-parasite interactions.

Spatial Patterns and Succession in a Minnesota Southern-Boreal Forest

Abstract: Succession was studied in a cold-temperate forest in the Boundary Waters Canoe Area Wilderness (BWCAW) of northeastern Minnesota. The 13 X 18 km study area comprises a complex forest mixture of jack (Pinus banksiana) and other pines, quaking aspen (Populus tremuloides), paper birch (Betula papyrifera), black spruce (Picea mariana), balsam fir (Abies balsamea), and white cedar (Thuja occidentalis) on thin soils over the Canadian Shield bedrock. The main objectives of this study were to examine the relationships between spatial patchiness, spatial scale, and canopy succession in the southern-boreal forest of the BWCAW, and to evaluate under what conditions successional direction may remain stable, converge, or diverge. Knowledge of the successional direction of old forests in the BWCAW that are undergoing demographic transition from even­ aged to uneven-aged is important because the landscape now has many old stands as a result of reduced fire frequency. Rotation periods for fires have changed from =50-100 yr in presettlement times to > 1000 yr since 1910. Analyses were conducted at spatial scales ranging from the individual tree (0.01 ha) to the large stand (16 ha). Two permanent mapped plots (of area 0.53 and 0.56 ha) were established in stands of different age. Fine­ scale age structure, successional change, transition from one species to another, and development of small patches (of area <0.25 ha) were studied by means of stand history reconstruction with increment cores, spatial autocorrelation, and analysis of replacement trees in canopy openings. Spatial processes at nested scales of 1, 4, and 16 ha were examined on 15 square 16-ha tracts of upland forest, which are distributed among forests ranging from 15 to 190 yr old. Canopy species composition and patch development over time on these 15 tracts were interpreted on air photos taken in 1934, 1961, and 1991. Thus, the study includes a chronosequence approach, with verification of chronosequence validity by checking patch development processes at more than one time point. This checking was done at small spatial scales by reconstruction of stand history on permanent mapped plots, and at larger spatial scales with sequential air photos of the same locations spanning a 57-year period. Results show that the reduced fire frequency in recent years has changed the dominant successional pathways. When fire frequency was high, jack pine or aspen stands usually burned while still in the even-aged stage of development, and the new trees after the burn were the same species as before. Currently, many stands are undergoing demographic transition from even-aged stands of catastrophic fire origin to uneven-aged stands. This transition parallels a change in canopy composition from jack pine (occasionally red pine (Pinus resinosa)) or aspen to an old-growth multi-aged mixture of black spruce, balsam fir, paper birch, and white cedar. The mechanism that moves this successional path forward is canopy openings, 10-30 m across on average, caused by wind, insect, and disease, that gradually chip away at the relatively uniform canopy of pines and aspen. Successional direction is individualistic in the sense that time and rate of transition from pine/aspen to other species depends on the action of heavy windstorms, insect infestation, and senescence of old pines that create canopy openings. Canopy openings are often filled with one of several species, but if more than one species invades an opening, monodominant patches of each species generally result. Understory-overstory interctctions are very weak; the dominant species within each patch is apparently independent of the overstory species that died when the opening was created or the species dominating surrounding patches.~ring succession, the spatial structure of the stands at the 1-16 ha scales generally changes from a matrix heavily dominated by pine or aspen to a mosaic with relatively large mono-dominant patches that may be remnants of the extensive original matrix, and finally to a mixture with small patches (mean area 35 m2, maximum =0.1 ha) of black spruce, balsam fir, white cedar, and paper birch. Thus, at 1-16 ha spatial scales, succession leads to convergence on a mixture of species. At smaller spatial scales (e.g., 0.01-0. l ha) successional pathways appear to diverge into four community types. The same successional pathways can be reconstructed from historical analysis of individual stands as from a chronosequence of stands; therefore, chronosequences in this area have been stable at least during the lifetime of the current generation of trees.

Spatiotemporal Variation in Demographic Transitions of a Tropical Understory Herb: Projection Matrix Analysis

Abstract: Our goal was to elucidate the population dynamics of the perennial understory herb Calathea ovandensis in a rain forest in southern Mexico using matrix projection model analysis. We emphasize the magnitude and consequences of spatiotemporal variation in (1) basic demographic parameters (growth, survival, and reproduction) (2) asymptotic demographic properties of a given environment (the asymptotic population growth rate and the associated stable—stage distribution and reproductive values) and (3) demographic sensitivities associated with a given environment (sensitivity and elasticity). We obtained 6 yr (1982—1987) of empirical data from four study plots (differing in substrate, light, and density) from which we used the first 5 yr (1982—1986) to construct 16 plot—year and 1 pooled population projection matrices. This stage—structured population was characterized by a long—lived seed bank, temporally variable seedling recruitment (10—fold variability among years), high mortality of seedlings (>90%), very low mortality of reproductives (usually <10%), fertility that increased markedly with plant size, and the ability of large plants to shrink rather than die under adversity. Within these broad outlines, the magnitudes of transitions representing demographic fates exhibited considerable variation through space and time, some parameters varying much more than others (CV from 22 to 400%). Growth and reproduction were positively correlated across environments. The least variable parameters were seed dormancy and stasis of small reproductives. Observed stage distributions were reasonably close to the stable stage distributions (mean = 86.1% similar). In most plot—years, the stable—stage distribution was dominated by seeds, followed by seedlings, and then small reproductives and the reproductive values increased with size class. Population growth rates, given by the dominant eigenvalue of the matrices, ranged from 0.73 to 1.25. Analysis of the mean dynamics gave λ = 0.97 (using a variety of analytical approaches) and our analysis of the overall pooled dynamics gave a λ = 0.99, indicating that the habitat at the study site favored the persistence of Calathea ovandensis. An el Nino even coincided with the year of the highest population growth rate. Survival, growth, and reproduction varied significantly through space and time, and different plot—years were beneficial to different stages. Most interestingly, stage—specific sensitivity parameters (sensitivity and elasticity) also varied through space and time. Spatiotemporal variability of sensitivity structure has important implications. Determination of stages most "critical" to population dynamics will depend upon knowledge of this variation. Population growth rate was significantly positively correlated with elasticity of seed production, seed germination, and seedling growth. These results indicate that the opportunity for selection on plant characters affecting particular life history stages varies through space and time even if the effect on the single—stage transition probability does not vary. Selection on characters affecting juvenile stages may be stronger in populations of higher growth rates.

Measuring resilience in stochastic systems

Abstract: This paper develops mathematical and statistical techniques for measuring the resilience of inherently stochastic ecological systems. To correspond to the deterministic idea of resilience measured by the return time to equilibrium following perturbation, resilience in stochastic systems is defined as the variability in population densities relative to environmentally driven variability in population growth rates. Stochastic resilience defined in this way is calculated from a community matrix describing the average interaction strengths within and among species. The procedure to calculate stochastic resilience is demonstrated on simulated data sets consisting of time series of population densities. The simplicity of the procedure makes it a potential tool for application to a wide range of ecological communities. See full-text article at JSTOR

Predicting Physical Disturbance: Mechanistic Approaches to the Study of Survivorship on Wave‐Swept Shores

Abstract: For the past 30 yr wave-swept shores have served as a model system for experimentation in community ecology. Due in large part to the severity of the physical environment, individual plants and animals are frequently disturbed, turnover in the community is rapid, and experiments can be conducted in months which in other habitats would require years. However, the experimental advantage of rapid turnover must be weighed against our ability to account for its causes. Only if we can predict the rate of turnover can we predict the dynamics of the community. On wave-swept shores where disturbance is dominated by environmental effects, the ultimate ability to predict community structure rests on the proximal ability to predict the physical environment and to understand its consequences. Some important aspects of the wave-swept environment (such as the tides) are well understood, but the effects of wave-induced hydrodynamic forces, perhaps the predominant environmental stress on shoreline organisms, has been thought to be unpredictable. Indeed, the stochastic nature of ocean waves precludes the short-term prediction of wave forces. However, as with the random motion of molecules in a gas, the short-term unpredictability of the ocean's surface can form the basis for a robust statistical approach to the prediction of long-term events. This study employs the statistics of the random sea to predict the largest wave to which a littoral site will be subjected in a year (5.9 times the yearly average significant wave height), and uses hydrodynamic theory to predict the force that this large wave exerts on individual organisms. The result is a quantitative measure of wave exposure, a mechanistic link between local wave climate and species-specific survivorship that can be used as a tool for exploring the relationship between environmental severity and community ecology. The proposed method is tested by predicting the rate at which patches of bare substratum are formed in beds of the mussel Mytilus californianus, a dominant competitor for space on rocky shores in the Pacific Northwest. Predicted rates are very similar to those measured in the field, suggesting that this method can provide useful input into models of intertidal patch dynamics. Data from several sites around the world suggest that the yearly average waviness of the ocean at any particular site can (over the course of decades) vary by as much as 80% of the long-term mean. The methodology proposed here allows this decade-to-decade variation in wave climate to be translated into the resulting variation in survivorship ; predicting, for example, that an increase of 1 m in yearly average significant wave height results in a fourfold increase in the rate of patch formation in a mussel bed. Such a shift would have substantial consequences for community dynamics. M. californianus is unusual in that the expected wave-induced stress is near the species' modal strength, and it will be of interest to determine if this characteristic is common among dominant competitors for space. To ease the application of this method by ecologists, a list of hydrodynamic force coefficients is provided to allow the method to be used with a wide variety of plants and animals, and several sources of information regarding wave heights are suggested. The assumptions, limitations, and procedures for practical application of the approach are discussed.

Nonlinear demographic dynamics: mathematical models, statistical methods, and biological experiments'

Abstract: Our approach to testing nonlinear population theory is to connect rigorously mathematical models with data by means of statistical methods for nonlinear time series. We begin by deriving a biologically based demographic model. The mathematical analysis identifies boundaries in parameter space where stable equilibria bifurcate to periodic 2-cy- cles and aperiodic motion on invariant loops. The statistical analysis, based on a stochastic version of the demographic model, provides procedures for parameter estimation, hypothesis testing, and model evaluation. Experiments using the flour beetle Tribolium yield the time series data. A three-dimensional map of larval, pupal, and adult numbers forecasts four possible population behaviors: extinction, equilibria, periodicities, and aperiodic motion including chaos. This study documents the nonlinear prediction of periodic 2-cycles in laboratory cultures of Tribolium and represents a new interdisciplinary approach to un- derstanding nonlinear ecological dynamics.

Natural Selection and Ecotypic Differentiation in Impatiens Pallida

Abstract: One possible response of plant populations to heterogeneous environments is genetic adaptation resulting in the formation of distinct ecotypes Genetic adaptation to stressful environments may affect both the limits to species boundaries and the potential for response to a changing environment Reciprocal transplant experiments have frequently been used to describe ecotypic differentiation and to infer the role of natural selection when there is evidence for home site advantage The demonstration of a home site advantage, however, does not reveal which plant characters are responsible for conferring increased fitness on populations planted in their native site Here, we combine the classic reciprocal transplant experiment with multivariate regression analysis of selection to ask a series of questions relevant to understanding adaptive genetic differentiation in natural plant pop- ulations Impatiens pallida plants from a mesic floodplain and a dry hillside site were reciprocally transplanted We initially presumed the hillside to be a stressful site for Impatiens given its sparser population of consistently smaller individuals This study describes the two environments from the perspective of the plant to ask whether it is stressful In addition, we investigate genetic differentiation between populations and ask whether the two pop- ulations are distinctly adapted to their home sites To identify traits that may be important for conferring home site advantage, we quantify present-day natural selection in these sites and ask whether the observed selective forces can explain genetic differences Finally, because phenotypic correlations may play an important role in a population's response to its environment, we investigate relationships among traits to determine the extent to which they are genetically and/or environmentally controlled The large reduction in total seed production when plants from both populations were grown on the hillside supported our initial bias that this site was stressful to Impatiens In addition, the higher relative fitness of each population planted in its native site demonstrated that these populations represent distinct ecotypes Genetic differences between populations were observed for several life history and morphological characters In particular, plants from the hillside population were smaller and produced cleistogamous flowers earlier than floodplain plants Selection analysis revealed that, while there is strong selection favoring early flowering on the hillside, there is no advantage to early flowering for plants grown on the floodplain An increased developmental rate, which allows plants to produce seeds before they succumb to drought stress, appears to be the most important mechanism re- sponsible for the greater relative fitness of the hillside population in its native site While greater total plant leaf area is favored by selection on the floodplain, there is no evidence for selection on this trait on the hillside Phenotypic covariances among traits differed between sites and populations, resulting in differences in the action of indirect selection There is evidence that indirect selection on correlated traits is responsible for some of the observed genetic differences

Pre- and post-settlement determinants of estuarine dungeness crab recruitment'

Abstract: Abundance of early juvenile Dungeness crab (Cancer magister) is dramat- ically higher in intertidal shell habitats compared to mud habitats in several coastal estuaries of the Pacific Northwest. To define the mechanisms underlying this habitat-specific pattern in abundance, we concurrently examined four components of recruitment to intertidal shell and mud habitats at two locations within the Grays Harbor estuary (Washington, USA): (1) water column supply of crab megalopae (postlarvae); (2) settlement patterns of crab megalopae 48 h after settlement substrates were deployed; (3) density of first benthic juvenile instars (J1) 48 h after deployment of such substrates; and (4) density of early juvenile crab in shell and mud habitats over a 4-mo period. We also describe the physical processes likely to be influencing postlarval supply within Grays Harbor, and take advantage of natural variation in postlarval supply between two locations, in combination with a predator exclusion experiment, to define the relative importance of postlarval supply vs. post-settlement survival in regulating population size of juvenile crab in certain intertidal habitats. Water column postlarval supply (measured with plankton and neuston nets, and artificial settlement substrates) in terms of both megalopal density (number per cubic metre) and flux (number per hour) was significantly higher in the southern part of the estuary vs. the northern part during a week-long settlement pulse. Our field observations and measurements suggest that spatial variation in postlarval supply was due to local differences in wind- driven surface currents, since tidal current speeds in the two locations were similar. More- over, there was no correlation between current speed and flux of megalopae over the bottom. There was generally no difference in postlarval supply between shell and mud habitats. Our experimental results further indicate that: (1) the abundance of recently settled crab megalopae in 0.25 m2 settlement trays was significantly higher in shell than in mud habitats, irrespective of whether the trays were placed in 3-5 ha of shell vs. mud; (2) there was a positive and significant correlation between postlarval supply and density of megalopae in shell and mud habitats; and (3) there was a positive and significant correlation between postlarval supply and density of J1 instars only in habitats where specific predators were excluded. Once the number of J1 instars at both geographic locations was reduced to similar levels, equivalent but steadily decreasing densities persisted throughout the summer growing season. The decoupling of settlement patterns and density of J1 instars took place within our 48-h sampling interval. Thus, future attempts to examine the correspondence between larval supply and post-settlement abundance of marine benthic species with planktonic larvae should do so at extremely small temporal scales or a critical life history phase may be overlooked. The results from this study demonstrate that substrate selection can affect distribution of juvenile crab, and that predation (including cannibalism) is a key factor regulating local population size of early juvenile crabs in intertidal habitats where postlarval supply is relatively high.

Dispersal Dynamics of the Bivalve Gemma Gemma in a Patchy Environment

Abstract: The purpose of this study was to analyze the dispersal dynamics of the ovo- viviparous bivalve Gemma gemma (hereafter referred to as Gemma) in an environment dis- turbed by the pit-digging activities of horseshoe crabs, Limulus polyphemus. Gemma broods its young and has no planktonic larval stage, so all dispersal is the result of juvenile and adult movement. Animal movement was measured using natural crab pits, hand-dug simulated crab pits, and cylindrical bottom traps in the intertidal zone at Tom's Cove, Virginia, USA. This study demonstrated that horseshoe crabs create localized patches with reduced densities of Gemma, that all sizes and ages of Gemma quickly disperse into these low density patches, and that the mechanism of dispersal is passive bedload and suspended load transport. Freshly excavated natural pits had significantly lower Gemma densities than did undisturbed background sediment, but there were no significant differences in total density of other species, number of species, and species diversity (H'). Equitability (J') was greater in pits than in controls because of the reduced abundance of Gemma, the numerically dominant species. Newly dug simulated crab pits also had significantly lower Gemma densities than controls and returned to control levels by the next day. Density recovery trajectories for individually marked pits showed consistent responses in summer and fall, but not in winter when low Gemma abundance resulted in greater variability among pits. Significant positive correlations between the volume of sediment and the number of Gemma collected per bottom trap support the hypothesis that Gemma dispersal is a passive transport phenomenon. Assuming no active, density-dependent movement, the product of the Gemma density frequency distribution in undisturbed background sediment and the frequency distribution of sediment volume collected per trap created a predicted Gemma frequency distribution in traps that matched the actual distribution. Absolute dispersal rates and relative dispersal rates (absolute dispersal rate divided by background density in undisturbed sediment) into pits and traps were greater in summer than winter. Dispersal rate results suggest that increased horseshoe crab disturbance in summer may cause an increase in Gemma transport. Because Gemma individuals are dispersed by hydrodynamic action, it was expected that small, young individuals would be most easily transported in the bedload. There was, however, little evidence that movement into pits and traps was size- or age-selective. Most recent benthic dispersal research has focused on the large-scale movement and settlement patterns of invertebrate larvae. The results from this study illustrate that dispersal of bottom-dwelling juveniles and adults plays an important role in regulating the local dis- tribution and abundance of Gemma. Previous workers have shown that young Gemma live in dense aggregations and that growth and fecundity are reduced at such high densities, leading to population crashes. This study demonstrated a mechanism by which Gemma dis- perses into low-density patches where intraspecific competition may be mitigated, possibly resulting in enhanced individual reproductive success and population fitness.

Impact of the younger dryas cooling event upon lowland vegetation of Maritime Canada

Abstract: The aim of this research is to determine the response of the vegetation in coastal Maritime Canada to the Younger Dryas cooling event ({approx} 10800-10000 {sup 14}C yr BP) that interrupted the warming trend following the last glaciation. Detailed paleoecological studies were carried out on the organic, pollen, and plant macrofossil content of sediment cores recovered from six small lakes in New Brunswick and Nova Scotia. Results show that there was regional variation in the vegetation response to the cooling. Records from Splan Pond, Mayflower Lake, and Little Lake show a change from boreal forest or woodland to shrub-tundra as a result of the cooling, while Lac a Magie shows floristic changes of shrub-tundra, and Chase Pond and Main-a-Dieu Pond show replacement of shrub-tundra by herb-tundra in response to the Younger Dryas. The climate warming signifying the end of the Younger Dryas caused succession from shrub-tundra to boreal forest, or from herb-tundra to shrub-tundra. Macrofossil evidence of arctic/alpine species such as Dryas integrifolia, Salix herbacea, and Cassiope hypnoides attests to the severity of the Younger Dryas cooling. Vegetation changes in response to the climate cooling and warming, marketing the onset and termination of the Younger Dryas respectively, were very rapid,more » taking only 50-100 yr. However, the response of some taxa, such as dwarf birch, appears to have lagged the onset of the climate change by several decades, although the duration of the lag varies between sites. 53 refs., 15 figs., 2 tabs.« less