Showing papers in "Ecological Monographs in 1991"

Artifacts or Attributes? Effects of Resolution on the Little Rock Lake Food Web

Abstract: A detailed and relatively evenly resolved food web of Little Rock Lake, Wisconsin, was constructed to evaluate the sensitivity of food-web patterns to the level of detail (degree of resolution) in food-web data. This study presents definitions (e.g., eco- system food webs) and methods for constructing and reducing the resolution of food webs to provide relatively pragmatic and rigorous touchstones for consistency in future food- web studies. This analysis suggests that food-web patterns such as the scale-invariant links- per-species ratio, short chain lengths, and limited number of trophic levels are constrained by the resolution of food-web data rather than by ecological factors. Patterns less sensitive to changes in resolution such as directed connectance (the proportion of observed directed links to all possible directed links) may be robust food-web attributes. The food web of Little Rock Lake appears to be the first highly and evenly resolved food web of a large natural ecosystem originally documented for the purpose of examining quantitative food-web patterns. This ecosystem food web contains roughly twice as many species as the largest web to date. It also may provide the most credible portrait available of the detailed trophic structure of a whole ecosystem. The 93-trophic-species web of Little Rock Lake differs from previously published trophic-species webs by having more links per species (L/S = 1 1), longer chain lengths (average: - 10, maximum: - 16), species at higher trophic levels (maximum: = 12), higher fractions of intermediate species, and smaller fractions of top species and links to top species. The sensitivity of quantitative food-web patterns to changes in resolution was examined in several series of trophically aggregated Little Rock Lake webs. Each of the series starts with a highly and relatively evenly resolved web with 182 consumer, producer, and de- composer taxa and ends with low-resolution webs with 9 aggregates of taxa. Taxa were aggregated based on the proportion of predators and prey shared by the taxa. Different series of webs were generated using different criteria for linking aggregates to evaluate the sensitivity of food-web patterns to linkage criteria. The sensitivity analysis revealed that several, but not all, quantitative food-web patterns are very sensitive to systematic aggregation of the web. Sensitive patterns include number of links per species, linkage complexity, the distributions of chain lengths and species among trophic levels, and the proportions of top species and links to top species. Less- sensitive patterns include connectance, the ratio of predators to prey, the proportions of intermediate and basal species, and the proportions of links that are between intermediate and basal species. Directed connectance is the only pattern examined that is both very robust to trophic aggregation and generally comparable to other community webs. Quan- titative food-web patterns in published community webs are generally similar to highly aggregated Little Rock Lake webs (versions with 9-40 aggregates). These findings suggest that previously described community food webs are severely aggregated versions of more elaborate webs similar to that of Little Rock Lake.

Estimation of Growth and Extinction Parameters for Endangered Species

Abstract: Survival or extinction of an endangered species is inherently stochastic. We develop statistical methods for estimating quantities related to growth rates and extinction probabilities from time series data on the abundance of a single population. The statistical methods are based on a stochastic model of exponential growth arising from the biological theory of age- or stage-structured populations. The model incorporates the so-called en- vironmental type of stochastic fluctuations and yields a lognormal probability distribution of population abundance. Calculation of maximum likelihood estimates of the two un- known parameters in this model reduces to performing a simple linear regression. We describe techniques for rigorously testing and evaluating whether the model fits a given data set. Various growth- and extinction-related quantities are functions of the two param- eters, including the continuous rate of increase, the finite rate of increase, the geometric finite rate of increase, the probability of reaching a lower threshold population size, the mean, median, and most likely time of attaining the threshold, and the projected population size. Maximum likelihood estimates and minimum variance unbiased estimates of these quantities are described in detail. We provide example analyses of data on the Whooping Crane (Grus americana), grizzly bear (Ursus arctos horribilis) in Yellowstone, Kirtland's Warbler (Dendroica kirtlandii), California Condor (Gymnogyps californianus), Puerto Rican Parrot (Amazona vittata), Palila (Loxioides balleui), and Laysan Finch (Telespyza cantans). The model results indicate a favorable outlook for the Whooping Crane, but long-term unfavorable prospects for the Yellowstone grizzly bear population and for Kirtland's Warbler. Results for the California Condor, in a retrospective analysis, indicate a virtual emergency existed in 1980. The analyses suggest that the Puerto Rican Parrot faces little risk of extinction from ordinary environmental fluctuations, provided intensive management efforts continue. However, the model does not account for the possibility of freak catastrophic events (hurricanes, fires, etc.), which are likely the most severe source of risk to the Puerto Rican Parrot, as shown by the recent decimation of this population by Hurricane Hugo. Model parameter estimates for the Palila and the Laysan Finch have wide uncertainty due to the extreme fluctuations in the population sizes of these species. In general, the model fits the example data sets well. We conclude that the model, and the associated statistical methods, can be useful for investigating various scientific and management questions concerning species preservation.

Production: Biomass Relationships and Element Cycling in Contrasting Arctic Vegetation Types

Abstract: Primary production, plant biomass, plant element content, and various mea- sures of turnover and element cycling were compared among four contrasting tundra vegetation types near Toolik Lake, Alaska. The study sites were selected to represent extreme examples of the wide variation in plant growth form composition that is typical of northern ecosystems. The aim of the research was to determine whether vegetation types that differ in their dominant plant growth form also differ in their production: biomass relationships and overall patterns of element use. The four sites included tussock tundra, a deciduous shrub-dominated riparian tundra, an evergreen heath tundra, and wet sedge tundra. Biomass and element content (N, P, K, Ca, and Mg) were determined for both vascular and nonvascular plants, and production estimates were obtained for vascular plants. Production and biomass of most tissues were determined by quadrat harvest methods, with additional, separate determinations of stem secondary growth and belowground rhizome growth as components of net primary pro- duction (NPP). Production, biomass, and element content of roots were not determined. Vascular plant biomass (excluding roots) varied by nearly 9 x among sites, from 217 to 1877 g/m2. At least 50% of the vascular biomass at all four sites was belowground stems, but the relative allocation to leaves vs. aboveground stems varied greatly. When mosses and lichens were included, total biomass varied by only 5 x among sites, and lichens were in fact the most abundant plant form at the evergreen heath site. The element content in vascular biomass of these communities varied by 8-21 x, depending upon the element; including mosses and lichens, element content varied by 6-12 x. Primary productivity of vascular plants also varied sharply among sites, from 32 to 305 g m-2 yr-I (not including root production). Leaves were the largest single component of NPP at all sites, but the relative importance of above- and belowground stem growth varied considerably. The element requirements of vascular NPP varied by 9-17 x, de- pending upon the element. Despite these order-of-magnitude differences among sites in biomass, production, and element requirements, and the dramatic variation in allocation patterns, there was re- markably little difference among sites in the overall production: biomass relationships and element turnover. These overall similarities were due to the dominant effect of stems and rhizomes in the whole-plant and whole-vegetation biomass and element budgets. The wide range of leaf turnover rates and leaf production efficiencies was compensated at the whole- plant level by stem processes, especially element storage. Thus, we conclude that plant growth form composition is not a good indicator of whole-vegetation biomass turnover rates and patterns of element use, although growth form composition is related to total production and biomass.

Designing Environmental Field Studies

Abstract: The present paper provides a classification and description of methods for designing environmental studies, with emphasis on techniques as yet little used in ecology. Eight categories of techniques for field studies are defined in terms of the nature of control exerted by the observer, by the presence or absence of a perturbation, and by the domain of study. The first two categories include classical experimental approaches, replicated an unreplicated. Sampling for modeling provides efficient designs for estimating parameters in a specified model. Intervention analysis measures the effect of a known perturbation in a time series. Observational studies contrast selected groups from a population, while analytical sampling provides comparisons over the entire population. Descriptive survey sampling estimates means or totals over an entire population, while sampling for pattern deals with spatial patterns over a selected region. The authors propose that the statistical concept of a superpopulation may be useful in ecology, and that it may be desirable to approach community and ecosystem studies in a sampling framework, with experimentation used for a fairly narrow range of subsidiary investigations. Much more attention to processes for drawing inferences about cause and effect is needed, in any case.

Natural Disturbance Regimes in Hemlock-Hardwood Forests of the Upper Great Lakes Region

Abstract: The frequency of natural disturbances and their influence on the forest landscape mosaic were investigated on three large tracts of primary forest in Upper Michigan. Seventy 0.5—ha plots were randomly distributed in a total forest area of 23 000 ha dominated by sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis). Radial increment patterns were used to estimate canopy accession dates for each of a number of randomly selected overstory trees on each plot. From these data a disturbance chronology, representing the percentage of stand area occupied by cohorts originating during each decade over the last 130 yr, was compiled for each plot. Average rates of disturbance or canopy mortality are estimated at 5.7 to 6.9% per decade. The corresponding average canopy residence time of a tree is 145—175 yr. No significant differences were detected in average disturbance rates among the three study areas, between plots near the coast of Lake Superior and inland plots, among several different aspects, and among several different slope positions. Natural rotation periods increase exponentially with increasing disturbance intensity, which is defined as the approximate percentage of the plot area converted to gaps during a disturbance episode. Estimates of rotation periods range from 69 yr for ≥10% canopy removal to 1920 yr for ≥60% canopy removal. Spatial autocorrelation analysis indicated that plots with light and medium disturbances (<40%) are randomly distributed over the landscape. Plots wit heavy disturbances (≥40%) are clustered with a patch radius of ≈2 km, consistent with the sizes of thunderstorm downbursts. The data indicate that light and medium disturbances dominate the disturbance regime. The majority of stands on the landscape are composed of several major and many minor age classes. Even—aged stands with one predominant age class are uncommon. The age distribution of individual patches or cohorts in the two larger study areas (14 500 and 6073 ha) follows a nearly uniform distribution. None of the three study areas had more than 15% of the forest area converted to gaps in a single decade. The two larger areas meet most of the criteria that have been proposed for equilibrium landscapes.

Ecomorphological Diversification in Lowland Freshwater Fish Assemblages from Five Biotic Regions

Abstract: This study investigates the relationships among species diversity, community structure, and convergent evolution among divergent fish faunas. The author examined 30 morphological features related to the ecology of the dominant fish species from lowland stream and backwater habitats in five widely separated geographic regions: nearctic Alaska, temperate North America, Central America, South America, and tropical Africa. The analysis supports the thesis that species interactions are important determinants of community morphological features, particularly in species-rich tropical regions. Relative to assemblages in the other four regions, the two Alaskan fish assemblages were hyperdispersed within a comparatively small morphological space. The Alaskan fish assemblages probably formed via geologically recent, and perhaps repeated, colonizations of polar freshwaters by species with evolutionary histories in more diverse southern and coastal fish faunas. The author interprets evidence of greater niche diversification at lower latitudes within a habitat type as being derived primarily from the influence of competition and predation, whereas differences between habitats within regions seems to be associated with the combined effects of biotic interactions and differences in habitat volume and heterogeneity. Even though different regions within ecomorphological space were dominated by different fish orders, numerous ecomorphological convergences and several one-for-one ecological equivalents were identified withinmore » different biotic regions.« less

Biogeochemical Diversity Along a Riverside Toposequence in Arctic Alaska

Abstract: Nitrogen and phosphorus pool sizes, distribution, and cycling rates were described and compared for six different ecosystem types occurring along a single topose- quence in northern Alaska. The toposequence was located on a series of old floodplains of the Sagavanirktok River, in the northern foothills of the Brooks Range. From tussock tundra in the uplands, the toposequence passed through a relatively dry hilltop heath zone, a hillslope shrub/lupine/Cassiope zone, a footslope Equisetum zone, a wet sedge tundra, and a riparian shrub zone. A late-melting snowbank covered the hillslope site in early June of each year, and the sites consistently varied in soil temperature, soil moisture, thaw depth, and the seasonal pattern of soil thaw. The standing stocks of N, P, and C in soils of these six ecosystem types varied dra- matically but not monotonically along the toposequence, as did the turnover rates of these elements. Several measures were used in comparisons of N and P availability, including soil solution concentrations, in situ accumulation on ion-exchange resins, and levels of KCl-extractable N and P. Annual rates of net N mineralization were assayed using a buried bag method, and ecosystem respiration was measured by trapping CO2 in soda lime (NaOH + Ca (OH)2). Soil P pools were characterized by sequential extraction methods into four major pools, including loosely bound P, Al- and Fe-bound P, primary mineral P, and organic P. Both N and P availability were low in all six ecosystems when compared with temperate forests or wetlands. Among ecosystems, however, there was considerable variation in the relative availability of N vs. P, and in the apparent relative importance of nitrate as a nitrogen source.

Comparative demography of three species of scleractinian corals using age- and size-dependent classifications'

Abstract: Three species of massive reef-building coral, Goniastrea aspera, G. favulus, and Platygyra sinensis, were studied on two fringing reef flats in the central Great Barrier Reef from 1982 to 1984. Total population sizes ranged from 25 to 292 colonies, and remained relatively constant. Population structures and dynamics based on both age and size were described. Differences between these two classifications were primarily due to tissue fission or shrinkage of colonies. In populations dominated by small size classes, young corals were not necessarily the most abundant. Similarly, populations dominated by the largest size classes were not always dominated by the oldest corals. In several populations, mean colony size decreased slightly with increasing age, though variability in size also increased. Mortality patterns were similar in all three species, with the youngest genets and smallest ramets suffering the highest death rates. While mortality in older or larger corals was low, the incidence of partial mortality and fission was higher in these groups. Age-specific fecundity increased rapidly once reproductive age was reached at - 5 yr, but fecundity decreased slightly in the older age classes due to the decrease in mean colony size. Estimated mean generation times ranged from 33 to 37 yr.

Tree and shrub seedling colonization of old fields in central new york

Abstract: The main objective of this study was to determine the important mechanisms regulating woody plant establishment in recently abandoned agricultural fields in central New York. The study focused on the early life stages, from the dispersal of seeds to the fate of seedlings at the end of the second growing season, of common old-field invaders, primarily Acer rubrum, Pinus strobus, Cornus racemosa, and Rhamnus cathartica. We investigated patterns of survivorship and causes of mortality during each of these life stages in microsites dominated by previously established herbs and in experimental openings in the herb canopy. Establishment of a tree or shrub seedling was much less likely from seeds landing among herbs than in openings. During the fall, seed predation by mice, especially under the cover of herbs (median of averages across all species = 6% in open, 85% under herbs), was an important process regulating the relative abundance and spatial distribution of seeds after dispersal. Seedling emergence rates were generally quite low, but for most species tended to be greater in the presence of perennial herbs (12%) and were lower under annuals and biennials (7%) and in bare soil (6%). For cohorts of seedlings followed for two growing seasons (Experiment 6A), some mortality occurred due to frost heaving (17% in open, 4% under herbs) and apparent heat stress or desiccation during the summer (5% in open, 0% under herbs); however, the largest source of mortality, especially under the herb canopy, was predation by rodents, especially meadow voles (6% in the open, 62% under herbs). Competition with herbs, especially aboveground competition, significantly reduced growth of woody seedlings. In spite of growth reduction in the presence of herbs, there was no significant relationship between seedling size and survivorship within any treatment, in- dicating that mortality could not be directly attributed to competition. For different species, and at different life stages, support was found for all three of the succession models of Connell and Slatyer (1977). The tolerance model was supported in some cases of seed predation (Experiment 2), seedling emergence (Experiments 4 and 5, in part), and seedling predation (Experiment 7). The facilitation model was supported in some cases by increased seedling emergence (Experiments 4 and 5, in part) and increased seedling survivorship in the presence of herbs during periods of environmental stress (Experiments 6A and 8). The inhibition model was also supported in some cases of de- creased growth (Experiment 6B) and decreased survival of seeds and seedlings (Experiments 1, 3, 6A, and 8) in the presence of herbs. The lower survivorship under herbs was largely the result of seed and seedling predation, which were the two most important processes regulating tree and shrub colonization of old fields in this study. Reduced growth of later colonists due to competition with earlier colonists is considered direct inhibition, while increased mortality due to predation on later colonists caused by animals whose activity is necessarily associated with the presence of earlier colonists is considered indirect inhi- bition. Seed and seedling predation are potentially important mechanisms regulating suc- cession and should be explicitly included in studies of succession.

Models and mechanisms of succession: an example from a rocky intertidal community'

Abstract: An investigation of the processes that cause succession was performed in an intertidal community on the central Oregon coast. The community was dominated by barnacles and several species of macroalgae. The successional sequence was determined at three different sites by clearing sets of plots in a way that mimicked natural disturbance. Succession at each of these sites followed the same general sequence. A barnacle, Chtham- alus da/li, first colonized the plots and was later replaced by a second barnacle, Balanus glandula. The macroalgae Pelvetiopsis limitata, Fucus distichus, and Endoc/adia muricata colonized the plots only after Balanus was established. While the order of species arrival and departure was consistent, the rate of succession varied greatly among sites. The sequence of community development that was observed at one site over a 36-mo period occurred in < 12 mo in a nearby area. Differences in the rate of succession appeared to result from variation in the timing of successful Balanus recruitment. The mechanisms of succession were investigated in a series of field experiments. An experiment with Balanus-removal, Chthamalus-removal, and control plots was used to assess the interactions between barnacles. A direct interaction, competition for space with Balanus, caused Chthamalus to decrease in abundance as succession proceeded. Chtham- alus did not affect the establishment of Balanus. Successful Balanus recruitment depended on occasional periods of larval settlement followed by periods of favorable weather. At all three sites, algal colonization was dependent on the presence of barnacles. Balanus greatly facilitated algal colonization, while Chthamalus only weakly facilitated algal colonization. Facilitation of algal colonization by epoxy-filled barnacle tests indicated that facilitation resulted from barnacle tests altering the substrate, rather than the activities of the living animals. A factorial experiment involving manipulations of barnacle and consumer (limpet) abundances demonstrated that the facilitation of algae by barnacles is an indirect interaction that is mediated by limpets. Barnacles decreased limpet foraging activity and thereby increased algal abundance. Succession in this community is complicated by several processes that are not included in traditional views of succession. (1) Not all early successional species have the same effect on the establishment of later successional species. This results in spatial variation in the rate of succession. (2) The model of succession is different in each step in the successional sequence. The Chthamalus-Balanus interaction is an example of the tolerance model, while the barnacle-algae interaction is an example of the facilitation model. (3) Both direct and indirect interactions between species determine the course of succession. The results of this study support a general model that predicts the effect of consumers on the rate of succession. Consumers slowed succession in this community in two ways. First, limpets delayed the establishment of Balanus and the competitive exclusion of Chthamalus. Second, limpets delayed the establishment of macroalgae. Previous studies in marine and terrestrial habitats have found that consumers may slow, accelerate, or have no effect on the rate of succession and these observations are consistent with the predictions of this general model.

Aggregation and Coexistence in a Carrion Fly Community

Abstract: In this paper I combine experiment and theory to demonstrate that aggregation of carrion flies ovipositing among carcasses may facilitate coexistence in nature. The paper is divided into an experimental part and a theoretical part. In the experiment part, I present field experiments that show the aggregative pattern of carrion flies ovipositing among carcasses, as well as associated experiments that explore the behavior leading to these patterns. I also use experiments to quantify the strength of larval competition between Phaenicia coeruleiviridis (Calliphoridae) and Sarcophaga bullata (Sarcophagidae). In the theoretical part, I develop two methods that use these data to assess the effect of aggregation on competition in the field. From the first method, I predict that intraspecific aggregation of P. coeruleiviridis females causes a 26% decrease in the expected recruitment of P. coeruleiviridis and a 74% increase in the expected recruitment of S. bullata. As a result, within a single generation, intraspecific aggregation increases intraspecific competition and reduces interspecific competition. The second method for assessing aggregation involves a new mathematical formula that gives conditions under which species may coexist indefinitely. This formula requires estimates of only the degrees of intra— and interspecific aggregation among species; there is no need to estimate adult survival and larval competition. I use this second method to show that the minimum effect of aggregation on coexistence for five carrion fly species is equivalent to reducing the amount of larval competition between pairs of species by an average of 57%. Aggregation may therefore play a major role in explaining the coexistence of carrion fly competitors.

Dynamics of Canopy Structure and Light Interception in Pinus Elliottii Stands, North Florida

Abstract: In order to develop a model of the carbon cycle for mature slash pine (Pinus elliottii) stands in north Florida, we studied seasonal variation in leaf area index (LAI, all- sided), aboveground biomass increment and litterfall, and light penetration through the forest canopy, over a 3-yr period. The primary approach to establishing monthly LAI included annual destructive analyses and monthly measurements of needle fall and elon- gation. Imagery from the Landsat Thematic Mapper (TM) and patterns of light penetration were also used in attempts to derive less arduous estimates; the TM imagery was most promising. LAIs ranged from 3.0 to 6.5 on control plots over the 3 yr, with repeated fertilization increasing maximum LAI by >40%. Seasonal variation was high (40%), as was variation from year to year. An average of 3 1% of the incident photosynthetically active radiation (PAR) penetrated the canopies annually, ranging from 18 to 42% seasonally. Seasonal light penetration could not be described using a simple application of the Beer-Lambert law, perhaps due to the highly aggregated nature of the canopies. Models incorporating more information on canopy structure are necessary to predict light penetration through slash pine stands accurately. A model of needle litterfall was derived that could account for much of the seasonal and annual variation using stand basal area and climate conditions from the spring of the previous year; this model may be useful for developing climate-driven predictions of LAI. Efficiencies of use of incoming and intercepted PAR were low compared to other forest types. Low light interception and high nutrient-use efficiencies (demonstrated in earlier studies) are important adaptive characteristics of slash pine stands to these relatively warm and nutrient-poor sites.

Temporal Variation in Food Web Structure: 16 Empirical Cases

Abstract: Analysis of food web structure and temporal dynamics is essential to understanding energy flow and population dynamics of species, and may contribute to conservation, wildlife management, and disease and pest control. This report synthesizes all the observational studies of food web dynamics to which we have access. Most published food webs are static and cumulative: they depict information gathered over many occasions. A web observed over a single, relatively short time period is time specific. Here we analyze the relation between cumulative and time—specific versions of webs in 16 published cases. Fourteen of the 16 webs are from detritus—based habitats that harbor large fractions of arthropod species: carcasses, tree fluxes, felled logs, treeholes, dung pads, and an acidic pond. The other two webs describe soybean fields and the arctic tundra. These webs are presented here in a consistent format and are analyzed in four ways. First, we quantified temporal trends and levels of variation in nine web properties: the percentages of species in the web that are top predators (%T), intermediate species (%I), and basal species (%B); the ratio of number of prey species to number of predator species (P); the mean chain length (µ); the product of species richness and connectance (S × C); and the numbers of total species, newly arriving species, and local extinctions. In most webs %I and %T fluctuated widely; the latter generally increased in time or remained constant, while the former correspondingly decreased or remained constant. Since the number of basal species usually varied little, changes in %B were inversely associated with changes in species richness over successional and seasonal time scales. Predictable changes in P, µ and S x C accompanied the changes in %B, %I, and %T. The numbers of total species, new arrivals, and local extinctions displayed no consistent increasing or decreasing trends. Second, we compared cumulative and time—specific webs from the same habitat to determine which properties, if any, of time—specific webs might be predicted from cumulative webs. In cumulative webs, P, µ, and %T came closest to the median of the values from time—specific webs, followed by %I, S x C, and %B Cumulative webs, which usually appear in general ecology textbooks, overestimate S x C and underestimate %B relative to time—specific versions. In five studies cumulative webs were completed when the last or next—to—last samples were taken; additional sampling in these cases would probably have uncovered more species. Third, opportunistic species were removed from four time—specific webs to determine how these species influenced web structure. Removing one top—feeding opportunistic species from each web caused a dramatic rise in %T, small reductions in %I, S × C, µ, and P, and a negligible rise in %B. A single opportunistic species, even though it makes only rare and brief visits to a habitat, can dramatically reshape web structure. Fourth, properties of cumulative and time—specific versions of the 16 food webs were compared to properties of cumulative webs in two published web catalogs. The cumulative versions of the 16 webs grossly resemble the cumulative webs in both prior catalogs, but the median S × C is greater and the median %B is lower in the 16 cumulative webs than in either prior catalog. Even for these two statistics, the median value for the 16 cumulative webs falls well within the range of variation of both prior catalogs. The time—specific webs in the 16 cases differ from those of the two prior catalogs somewhat more than do the cumulative webs. Comparisons between time—specific and cumulative versions of a web, one system at a time, are more sensitive than rough comparisons between collections of webs because the methods used to define species and links are (usually) consistent within a study.

Loess Ecosystems of Northern Alaska: Regional Gradient and Toposequence at Prudhoe Bay

Abstract: Loess-dominated ecosystems cover {approx} 14% (11,000 km{sup 2}) of the Arctic Coastal Plain and much of the northern portion of the Arctic Foothills. Knowledge of this poorly known ecosystem is important for sound land-use planning of the expanding developments in the region and for understanding the paleoecological dynamics of eolian systems that once dominated much of northern Alaska. A conceptual alkaline-tundra toposequence includes eight common vegetation types and associated soils and vegetation downwind of the Sagavanirktok River. Properties of loess tundra important for land-use planning include: (1) its high ice content, which contributes to its susceptibility to thermokarst; (2) high salinities, which hamper revegetation efforts; and (3) presence of certain plant species such as Dryas intergrifolia, which are particularly sensitive to disturbance. The loess gradient provides a natural analogue for road dust, and extensive disturbance associated with oil-field development.

Late quaternary vegetational history of grays lake, Idaho

Abstract: Pollen contained in sediment cores, from Grays Lake, Idaho, indicate cold, dry conditions from ≈70 000 to 30 000 yr BP when an Artemisia steppe surrounded Grays Lake and extended into the nearby mountains. From ≈30 000 until 11 500 yr BP, somewhat moister conditions supported a conifer woodland. Pinus and other conifers grew in the foothills and among Artemisia on the open plains. Increases in Picea, Abies, and Artemisia signaled the onset of a cool transitional climate ≈11 500 yr BP. Juniperus and members of the Chenopodiaceae/Amaranthaceae and Compositae families all became more abundant as the climate became warmer and drier. Xeric conditions persisted from ≈10 000 to 7100 yr BP, reaching a maximum ≈8200 yr BP. Subsequently, the climate of the Grays Lake Basin has been cooler and moister, with increases in conifers and decreases in Juniperus and steppe plants. See full-text article at JSTOR

Latitudinal Variation in Activity Season Mortality Rates of the Lizard Uta Stansburiana

Abstract: It is often claimed that predation pressure on plants and animals increases toward the equator. Here I address the specific hypothesis that predation pressure is greater on lower latitude populations of the lizard Uta stansburiana. Standard mark-recapture techniques (10 997 captures of 5371 individuals) were used to estimate mortality rates for seven populations that lie along a transect of nearly 150 latitude (central Washington to extreme southeastern California, USA). Mortality was estimated for both the early (i.e., spring to summer) and late (i.e., summer to late summer or fall) portions of the activity season. As in previous studies of this species, I found that emigration was negligible for adult lizards. Thus, disappearance rates serve as reliable indices of mortality rates. Although predation is known to be a major source of mortality for populations of U. stansburiana, direct measurement (i.e., observation) of the demographic impact of pred- ators is impractical. My strategy was to examine the potential demographic importance of mortality sources other than predation (e.g., senescence, physiological stress). If alternate sources of mortality can be shown to be of trivial importance, then overall mortality rates can be used to infer variation in predation pressure. To assess the importance of these mortality factors I examined variation in mortality within sites (e.g., among seasons, years, and individuals) in relation to data about the lizards (e.g., age, physical condition) and about their environment (e.g., recent precipitation). My results suggest that predation is the only source of activity season mortality that is of demographic importance for these populations. For example, I found no evidence that physiological stress was ever an important source of mortality. Lizards in relatively poor physical condition (i.e., those with small length-corrected mass) did not experience a greater risk of mortality, and survival actually increased during the presumably more stressful late activity season. Mortality rates were also depressed during a drought at one site, despite the poor physical condition of the lizards. Low mortality rates during the late activity season and during drought conditions probably reflect reduced activity of U. stansburiana and their predators. Reduced activity levels discourage predator-prey encounters. Because sources of mortality other than predation appear to be of trivial demographic importance, overall mortality rates can be used to infer relative predation rates. Accord- ingly, I examined latitudinal variation in mortality rates to address the hypothesis that predation pressure is greater on lower latitude populations of U. stansburiana. Daily mor- tality rates were not higher among southern populations. Even annual losses to predators do not appear to be greater among lower latitude populations. These results were surprising because U. stansburiana is one of the species for which predation pressure was thought to increase with decreasing latitude. Higher than expected mortality rates in the north may in part be due to longer daily activity periods among northern Uta and their predators. Interestingly, several indirect measures of predation intensity (e.g., tail break frequencies, measures of predator species richness) did increase significantly from north to south. Thus, these less direct measures of predation pressure yielded a misleading impression of lati- tudinal variation in predation pressure.

Long-Term Experiments on Calanoid-Cyclopoid Interactions

Abstract: Zooplankton species composition was manipulated in an 8-mo long series of experiments in outdoor 450-L tanks. The first experimental stage evaluated the impact of cyclopoids (Mesocyclops, Microcyclops, Cyclops) and calanoids (Diaptomus) on each other. The second and third stages evaluated the effects of Daphnia and mosquitofish (Gambusia affinis) on the calanoid-cyclopoid interaction. Cyclopoid predation initially caused a 50-70% reduction in calanoid abundance. After a few months, however, cyclopoids caused >300% increases in calanoid abundance, ap- parently by preventing calanoid overexploitation of the phytoplankton. The presence of calanoids depressed cyclopoid abundance by up to 70-90%, presumably by diminishing the availability of edible phytoplankton to cyclopoid nauplii and of prey, such as rotifers, to cyclopoid adults. The presence of Daphnia depressed calanoid abundance, probably by reducing the phytoplankton abundance, but did not affect cyclopoid abundance, perhaps because cyclopoid adults could feed on items other than phytoplankters. Coequality (i.e., similarity in abundance) of closely related taxa was in 4 of 5 cases favored by the addition of a more distantly related taxon. Thus the addition of cyclopoids favored the coequality of two Diaptomus species (D. clavipes, D. siciloides), the addition of these Diaptomus species favored the coequality of two cyclopoid genera (Mesocyclops, Microcyclops), and the ad- dition of Daphnia favored coequality of cyclopoids and calanoids and coequality of the two Diaptomus species, but the dominance of Microcyclops over Mesocyclops. Effects of Gambusia included a drastic reduction of Daphnia populations, moderate reduction of calanoid populations, and negligible effects on cyclopoid populations.

Species distribution controls across a forest-steppe transition: a causal model and experimental test'

Abstract: Steppe communities of interior Alaska are restricted to steep, generally southfacing bluffs. Two competing hypotheses explain this distribution: (1) steppe taxa are restricted to bluffs by their requirements for the abiotic conditions of these sites, and (2) steppe taxa have broader physiological ranges than expressed in nature, but are restricted to bluff sites by competition with other plants. We addressed these hypotheses using correlative and experimental methods to identify controls over the distribution of steppe and forest understory species on Eagle Bluff, Alaska. We developed a causal model and used path analysis, a correlative method, to estimate strengths of hypothesized causal relationships. Based on the results of path analysis we hypothesized that light intensity and soil moisture were important controls over the composition of the vegetation and species distributions. We tested this hypothesis by examining the responses of two species native to steppe (Linum lewisii and Potentilla hookeriana) and two species native to forest (Moehringia lateriflora and Pyrola secunda) to manipulation of light intensity, soil moisture, and soil type. The forest species had reduced survival and biomass when grown in the high light and dry soil moisture characteristic of steppe environments. Conversely, survival of steppe taxa was independent of treatments, and the response of survivors to treatments differed between species. Linum had higher seed production with higher soil moisture than it normally experiences and grew less under lower light intensity. Potentilla responded positively to higher moisture and had weakly negative (but not significant) response to reduced light levels. Our experiments confirmed the importance of light and soil moisture to vegetation on Eagle Bluff, but revealed the individualistic nature of species response to environment.

Arthropod dynamics on sagebrush (Artemisia tridentata): effects of plant chemistry and avian predation

Abstract: We conducted a field experiment to assess interrelationships between leaf- tissue secondary chemistry, avian predation, and the abundance and diversity of arthropods occurring on sagebrush (Artemisia tridentata) in central Oregon. Arthropods were removed from individual shrubs, some of which were then caged to exclude birds. Secondary chem- istry and arthropods were sampled at intervals up to 56 wk following the defaunation/ caging treatment. Recovery rates differed among arthropod taxa and functional groups. Several sap- sucking homopterans and hemipterans reached control levels within 2-4 wk of the treat- ment, whereas abundances ofparasitoids and predators recovered to match control numbers only 6 wk after defaunation. Abundances of several herbivorous leaf-chewing taxa (pri- marily lepidopteran larvae) remained significantly depressed even 56 wk after the treatment. Fungivores (oribatid mites) reached greater abundances on defaunated than on control shrubs by the end of the experiment. There were also significant changes in the concen- trations or frequency of occurrence of several chemical compounds following the defauna- tion treatment. Several hydrocarbons, sesquiterpenes, and monoterpenes that were present in most shrubs exhibited sharply reduced concentrations in leaf tissues within 4 wk of the treatment, whereas some alcohols and ketones (linalool, borneol, thujone), which occurred at relatively low frequencies among control plants, increased dramatically in their fre- quencies of occurrence following arthropod removal. Both changes persisted for the du- ration of the experiment. We found several significant associations between the abundance or occurrence of arthropod taxa or groups and concentrations or frequencies of secondary compounds, but these were most prevalent among the leaf-chewing lepidopterans. We suggest that the shrubs responded to the removal of herbivorous, leaf-chewing arthropods by altering chemical allocation patterns; the changes in chemistry persisted for over a year because recolonization of the defaunated plants by these herbivores was slow. Effects of the caging treatment were much less obvious. The recovery of the diversity of arthropods known to constitute prey for birds in this system was slower on shrubs

Inputs, outputs, and accumulation of nitrogen in an early successional moss (polytrichum) ecosystem'

Abstract: Measurement of total ecosystem nitrogen accumulation in a moss (Poly- trichum) ecosystem after 13 yr of primary succession upon exposed sands of glacial origin in New Hampshire, USA, revealed an N accumulation rate of 10.1 kg ha-' yr-'. Com- parison of the measured accumulation with a mass balance analysis of measured inputs (bulk precipitation inorganic N plus biological N2 fixation plus windblown coarse particulate matter) minus outputs (hydrologic losses plus N20 and N2 emissions) revealed that 35% of the inputs required to satisfy the observed rate of N accumulation had not been measured. These unmeasured sources include bulk precipitation organic N, dry deposition, and dew inputs. Total N inputs, including the unmeasured sources, were 10.5 kg* ha-' yr-1. Bulk precipitation was the dominant N input, comprising 58% of the annual input. Dinitro- gen fixation and coarse particulate organic matter together represented only 7%; unmea- sured sources accounted for the remainder (35 %). The predominance of bulk precipitation as an input source to the moss ecosystem is in contrast to many other primary succes- sional systems where N2 fixation is the dominant source of N. Nitrogen outputs from the ecosystem were small (0.40 kg ha-1 yr-'), with most coming from hydrologic export (0.29 kg ha-l yr-') and a smaller fraction (0.10 kg ha- yr-') from N20 effluxes. No N2 losses were detected. The moss ecosystem was extremely efficient at retaining bulk precipitation N inputs. Additions of 15N-labelled simulated rainfall showed that soil and belowground live moss biomass retained most of the input. The amount of N retained by each component of the ecosystem was related directly to the amount of biomass (living or dead) in the component. Soil organic matter was also important in retaining nitrogen inputs. In a short-term mi- crocosm experiment, soil with a high organic matter content (2.71%) retained 47% of inorganic N in simulated rainfall, compared to only 27% retained in soil with a low organic matter content (0. 5 1%). The moss vegetation exerted control over the N retention efficiency of the ecosystem. Removal of mosses resulted in a short-term retention of N inputs, soon followed by losses that exceeded inputs. Nitrogen retention again resumed following re- colonization of the soil by mosses. Development of the moss ecosystem appears to have been enhanced by two positive feedback mechanisms. First, establishment of the mosses led to biomass and soil organic matter accumulation, enhancing N capture and retention, and thus increasing biomass accumulation. Second, development of aboveground biomass further enhanced collection of N in dry deposition and dew, increasing total inputs.