Showing papers in "Ecological Monographs in 2003"

Estimating community stability and ecological interactions from time‐series data

Abstract: Natural ecological communities are continuously buffeted by a varying environment, often making it difficult to measure the stability of communities using concepts requiring the existence of an equilibrium point. Instead of an equilibrium point, the equilibrial state of communities subject to environmental stochasticity is a stationary distribution, which is characterized by means, variances, and other statistical moments. Here, we derive three properties of stochastic multispecies communities that measure different characteristics associated with community stability. These properties can be estimated from multispecies time-series data using first-order multivariate autoregressive (MAR(1)) models. We demonstrate how to estimate the parameters of MAR(1) models and obtain confidence intervals for both parameters and the measures of stability. We also address the problem of estimation when there is observation (measurement) error. To illustrate these methods, we compare the stability of the planktonic commun...

Subalpine meadow flowering phenology responses to climate change: integrating experimental and gradient methods

Abstract: We integrated experimental and natural gradient field methods to investigate effects of climate change and variability on flowering phenology of 11 subalpine meadow shrub, forb, and graminoid species in Gunnison County, Colorado (USA). At a subalpine meadow site, overhead electric radiant heaters advanced snowmelt date by 16 d and warmed and dried soil during the growing season. At three additional sites, a snow removal manipulation advanced snowmelt date by 7 d without altering growing season soil microclimate. We compared phenological responses to experimental climate change with responses to natural microclimate variability across spatial gradients at small and landscape scales, as well as across a temporal gradient from a separate study. Both manipulations significantly advanced timing of flowering for the group of species and for most species individually, closely paralleling responses of timing to natural spatial and temporal variability in snowmelt date. Snowmelt date singularly explained observed shifts in timing only in the earliest flowering species, Claytonia lanceolata. Among all other species except Artemisia tridentata var. vaseyana, the latest flowering species, a consistent combination of temperature-related microclimate factors (earlier snowmelt date, warmer soil temperatures, and decreased soil degree-days) substantially explained earlier timing. Both manipulations also extended flowering duration for the group of species, similar to species' responses to natural snowmelt variability at small spatial scales. However, only early flowering species displayed consistent, significant changes in duration, with extended duration related to earlier snowmelt or warmer spring soil temperatures. Soil moisture was generally not a significant explanatory factor for either timing or duration of flowering. Best-fit microclimate models explained an average of 82% of variation in timing but only 38% of variation in duration across species. Our research demonstrates the value of comparing and synthesizing results of multiple field methods within a single study. This integrated approach makes it easier to identify robust community-wide trends, as well as species-specific responses of phenology to climate change. The predicted short-term flowering phenology responses to temperature-related aspects of climate change may lead to longer term asynchronies in interspecific interactions, potentially altering population and evolutionary dynamics, community structure, and ecosystem functioning. Corresponding Editor: S. C. Pennings

Grassland responses to three years of elevated temperature, CO2, precipitation, and N deposition

Abstract: Global climate and atmospheric changes may interact in their effects on the diversity and composition of natural communities. We followed responses of an annual grassland to three years of all possible combinations of experimentally elevated CO 2 (1300 mL/L), warming (180 W/m 2 , 1;18C), nitrogen deposition (17 g N·m 22 ·yr 21 ), and precip- itation (150%). Responses of the 10 most common plant species to global changes and to interannual variability were weak but sufficiently consistent within functional groups to drive clearer responses at the functional group level. The dominant functional groups (annual grasses and forbs) showed distinct production and abundance responses to individual global changes. After three years, N deposition suppressed plant diversity, forb production, and forb abundance in association with enhanced grass production. Elevated precipitation en- hanced plant diversity, forb production, and forb abundance but affected grasses little. Warming increased forb production and abundance but did not strongly affect diversity or grass response. Elevated CO2 reduced diversity with little effect on relative abundance or production of forbs and grasses. Realistic combinations of global changes had small di- versity effects but more marked effects on the relative dominance of forbs and grasses. The largest change in relative functional group abundance (150% forbs) occurred under the combination of elevated CO2 1 warming 1 precipitation, which will likely affect much of California in the future. Strong interannual variability in diversity, individual species abundances, and functional group abundances indicated that in our system, (1) responses after three years were not constrained by lags in community response, (2) individual species were more sensitive to interannual variability and extremes than to mean changes in en- vironmental and resource conditions, and (3) simulated global changes interacted with interannual variability to produce responses of varying magnitude and even direction among years. Relative abundance of forbs, the most speciose group in the community, ranged after three years from .30% under elevated CO2 1 warming 1 precipitation to ,12% under N deposition. While opposing production responses at the ecosystem level by different func- tional groups may buffer responses such as net primary production (NPP) change, these shifts in relative dominance could influence ecosystem processes such as nutrient cycling and NPP via differences between grasses and forbs in tissue chemistry, allocation, phe- nology, and productivity.

Ecohydrology of a resource‐conserving semiarid woodland: effects of scale and disturbance

Abstract: In semiarid landscapes, the linkage between runoff and vegetation is a par- ticularly close one. In this paper we report on the results of a long-term and multiple-scale study of interactions between runoff, erosion, and vegetation in a pinon-juniper woodland in New Mexico. We use our results to address three knowledge gaps: (1) the temporal scaling relationships between precipitation and runoff; (2) the effects of spatial scale on runoff and erosion, as influenced by vegetation; and (3) the influence of disturbance on these relationships. On the basis of our results, we tested three assumptions that represent current thinking in these areas (as evidenced, for example, by explicit or implicit assump- tions embedded in commonly used models). The first assumption, that aggregated precip- itation can be used as a surrogate for total runoff in semiarid environments, was not verified by our findings. We found that when runoff is generated mainly by overland flow in these systems, aggregated precipitation amounts alone (by year, season, or individual event) are a poor predictor of runoff amounts. The second assumption, that at the hillslope and smaller scales runoff and erosion are independent of spatial scale, was likewise not verified. We found that the redistribution of water and sediment within the hillslope was substantial and that there was a strong and nonlinear reduction in unit-area runoff and erosion with in- creasing scale (our scales were slope lengths ranging fro m1mt o 105 m). Thethird assumption, that disturbance-related increases in runoff and erosion remain constant with time, was partially verified. We found that for low-slope-gradient sites, disturbance led to accelerated runoff and erosion, and these conditions may persist for a decade or longer. On the basis of our findings, we further suggest that (a) disturbance alters the effects of scale on runoff and erosion in a predictable way—scale relationships in degraded areas will be fundamentally different from those in nondegraded areas because more runoff will escape off site and erosion rates will be much higher; and (b) there exists a slope threshold, below which semiarid landscapes will eventually recover following disturbance and above which there will be no recovery without mitigation or remediation.

Assessment of wind dispersal potential in plant species

Abstract: Conventional plant dispersal classification systems use simple binary assignment schemes classifying each species as either being dispersed by means of a certain dispersal vector or not. However, because the dispersal potential ranges continually, this dichotomy appears to be rather artificial, and the existing systems may not be very useful for addressing ecological questions. To quantify gradual differences in the dispersal potential, we developed a system assessing wind dispersal potentials. Wind dispersal potential is defined as the proportion of diaspores exceeding a predefined reference distance under certain weather conditions, to acknowledge that wind dispersal potential is scale and context specific. The system is based on an independently validated simulation model of wind dispersal that was used to compute the proportion of diaspores exceeding predefined reference distances. On an ordinal scale, the proposed system allows one to assess the wind dispersal potential of any plant species with known falling velocity and release height of its diaspores without further computing. The system mainly relies on two traits characterizing the plant species (falling velocity and initial release height of the diaspores) and two context-specific parameters (reference distance and weather conditions). We examined how wind dispersal potential is sensitive to these factors and found that it was most sensitive to weather conditions and falling velocity. The species-specific traits interact with reference distance: the greater the reference distance and the lower the release height in relation, the more relevant a low falling velocity becomes for achieving a high wind dispersal potential. We subsequently applied the system to 335 plant species and found a considerable variation in their wind dispersal potentials. Many species commonly assumed to be wind dispersed exhibit only a low wind dispersal potential. Comparing the wind dispersal potentials to the morphology of the diaspores also reveals a considerable variation of the wind dispersal potential of species classified as the same morphological type. The results show that the conventional assignment of a plant species to a certain mode of dispersal, which is primarily based on the morphology of its diaspores, will often result in misleading conclusions regarding the dispersal potential of the respective species. Corresponding Editor: M. L. Cain.

Relationships at the aboveground-belowground interface: plants, soil biota and soil processes

Abstract: Interactions at the aboveground-below ground interface provide important feedbacks that regulate ecosystem processes. Organisms within soil food webs are involved in processes of decomposition and nutrient mineralization, and their abundance and activity have been linked to plant ecophysiological traits such as species identity and the quality and quantity of plant tissue. We tested aboveground-below ground diversity relationships in a naturally developed plant community of native tallgrass prairie by taking soil samples from beneath naturally established grass tillers of chosen characteristics (e.g., homogeneous vs. heterogeneous plant combinations or C-4 vs. C-3 photosynthetic pathway) without imposing any disturbances to existing plant-soil relationships. The goal of this study was to elucidate the consequences, for soil microbiota (microflora phospholipid fatty acids, protozoa, and nematode functional groups) and for C and N mineralization, of plant community properties such as species richness, resource quality, resource heterogeneity, species identity, and presence of exotics. None of the biotic or abiotic soil variables was related to plant resource heterogeneity. Protozoa were not responsive to any of the plant community traits. Some bacterial and nematode groups were affected by plant characteristics specific to a particular plant species, but no uniform pattern emerged. Invasive and native plants generally were similar with respect to soil variables tested in this study. The lack of clear responses of soil variables to plant community traits indicates that idiosyncratic effects dominate both at the plant and soil biotic level and that generalized plant and soil diversity effects are hard to predict.

Coupling fine root dynamics with ecosystem carbon cycling in black spruce forests of interior Alaska

Abstract: Fine root processes play a prominent role in the carbon and nutrient cycling of boreal ecosystems due to the high proportion of biomass allocated belowground and the rapid decomposition of fine roots relative to aboveground tissues. To examine these issues in detail, major components of ecosystem carbon flux were studied in three mature black spruce forests in interior Alaska, where fine root production, respiration, mortality and decomposition, and aboveground production of trees, shrubs, and mosses were measured relative to soil CO2 fluxes. Fine root production, measured over a two-year period using minirhizotrons, varied from 0.004 ± 0.001 mm·cm–2·d–1 over winter, to 0.051 ± 0.015 mm·cm–2·d–1 during July, with peak growing season values comparable to those reported for many temperate forests using similar methods. On average, 84% of this production occurred within 20 cm of the moss surface, although the proportion occurring in deeper profiles increased as soils gradually warmed throughout the summer. M...

Modeling long‐distance dispersal of plant diaspores by wind

Abstract: We developed PAPPUS, a trajectory model for wind dispersal of plant diaspores under field conditions. The model considers the effects of topography, turbulence (including thermal updrafts), and different weather conditions on the dispersibility of diaspores by wind. In the model, the plant species are characterized by the initial release height and the falling velocity of their diaspores. The common problems in modeling turbulence, which limit the applicability of existing models in predicting long-distance dispersal, are avoided by measuring the high-frequency fluctuations of the wind vector and using these data to simulate the course of the wind vector in the model. Using PAPPUS, we simulated dispersal distance spectra and compared them with those observed during field experiments executed in open habitats. Within a broad range of landscapes and under different weather conditions, the results of the model represent the observed spectra reasonably well. Additionally, we compared the observed dispersal di...

Natural disturbances and directional replacement of species

Abstract: The original concept of succession emphasized directional changes in species composition occurring over time on “blank slates” (new substrates or those that followed catastrophic disturbances). In this paper, we explore relationships between different effects of disturbances on residents and the initiation of directional species replacement on sites in a landscape. We present a conceptual model involving recurrent natural disturbances of sites in a landscape containing two species, one of which arrives at sites early and the other arrives late following disturbance. We predict effects of recurrent catastrophic disturbances (no survivors) and non-catastrophic disturbances (some survivors) on the initiation of directional species replacement, assuming temporal gradients in environmental conditions on sites after disturbance. We predict that directional species replacement will be initiated by catastrophic disturbances, and by non-catastrophic disturbances when the early species does not survive, but reinvad...

Response of eucalyptus‐dominated savanna to frequent fires: lessons from munmarlary, 1973–1996

Abstract: We assess a replicated fire plot experiment undertaken between 1973 and 1996 in two Eucalyptus-dominated savanna vegetation formations (open forest, woodland), at Munmarlary, in monsoonal northern Australia. Four treatments, each with three replicates, were imposed on each vegetation type: annual early dry-season burning; annual late dry- season burning; biennial early dry-season burning; and unburned controls. Treatments were imposed faithfully, with noted exceptions, on 1-ha plots. Fire intensities were typically low (,1000 kW/m) to moderate (1000-2500 kW/m), varied significantly between treatments, and generally were greater in woodland. In both woodland and open forest, pH was sig- nificantly lower and NO 3-N was significantly higher in unburned plots. Organic C was not significantly greater in unburned treatments. Effects of fire regime on other soil chemical properties differed between open forest and woodland sites. Among the grasses, invariant frequent burning led to the dominance of a small number of annual species, notably re- gionally dominant Sorghum. In the absence of burning, annuals declined generally, whereas some perennials increased while most decreased. These responses usually were apparent within the first five years of the experiment. At the relatively small spatial scale of the grass sampling regime, there was high turnover of both annual and perennial grasses. Under low- to moderate-intensity, frequent burning regimes, woody vegetation dominated by ma- ture eucalypts is structurally stable. In the absence of burning for at least five years, there was release of the non-eucalypt, woody component into the midstory; this occurred more rapidly in open forest. Accession of rain forest species occurred on some woodland plots, especially the unburned treatment. In contrast, eucalypts were not released significantly from the understory. Rather, as suggested by other studies, recruitment of eucalypts into the canopy appears to involve significantly reduced root competition through death of dominant eucalypts. Although the Munmarlary experiment provides invaluable quantitative data for exploring relationships between fire regimes and the responses of north Australian savanna systems, it has been less successful in meeting the complex information require- ments of regional fire managers. Replicated experimental fire plot designs, no matter how elegant and rigorously implemented, may substantially fail the test of management rele- vance, given the fundamental requirement for savanna biodiversity managers to experience the integrated effects of fire regimes that vary idiosyncratically over multiple time and spatial scales. We suggest that such information requirements are better met through modest, targeted ''adaptive management'' studies, involving collaborative partnerships between managers and researchers.

Small-mammal density estimation: A field comparison of grid-based vs. web-based density estimators

Abstract: Statistical models for estimating absolute densities of field populations of animals have been widely used over the last century in both scientific studies and wildlife management programs. To date, two general classes of density estimation models have been developed: models that use data sets from capture–recapture or removal sampling techniques (often derived from trapping grids) from which separate estimates of population size (N) and effective sampling area (Â) are used to calculate density (D = N/Â); and models applicable to sampling regimes using distance-sampling theory (typically transect lines or trapping webs) to estimate detection functions and densities directly from the distance data. However, few studies have evaluated these respective models for accuracy, precision, and bias on known field populations, and no studies have been conducted that compare the two approaches under controlled field conditions. In this study, we evaluated both classes of density estimators on known densities of e...

Energetics of a benthic diver: seasonal foraging ecology of the australian sea lion, neophoca cinerea

Abstract: This research examines the foraging energetics and diving behavior of the Australian sea lion, Neophoca cinerea. We examine whether the foraging ecology of the Australian sea lion is typical for an animal that has evolved to exploit benthic habitats. Such a strategy is in marked contrast to those utilized by some seabirds and other pinnipeds that feed in the midwater, where travel and search components of the time at sea become more important. Onshore and at-sea field metabolic rates (FMR) were measured using doubly labeled water in lactating sea lions at Kangaroo Island, South Australia, during the winter of 1988 (early lactation, breeding season 1) and the summer of 1990 (early lactation, breeding season 2). Dive behavior was also measured with dataloggers during these seasons, as well as in the summer of 1991 (late lactation, breeding season 2). The foraging behavior of Neophoca cinerea indicated that it works hard to exploit benthic habitats in the waters around its breeding site. Sea lions maximized ...

Spatiotemporal variability in tree growth in the central pyrenees: climatic and site influences

Abstract: To understand how tree growth has responded to recent climate warming, an understanding of the tree-climate-site complex is necessary. To achieve this, radial growth variability among 204 trees established before 1850 was studied in relation to both climatic and site factors. Seventeen forest stands were sampled in the Spanish Central Pyrenees. Three species were studied: Pinus uncinata, Abies alba, and Pinus sylvestris. For each tree, a ring-width residual chronology was built. All trees cross-dated well, indicating a common influence of the regional climate. For the 1952-1993 period, the radial growth of all species, especially P. uncinata, was positively correlated with warm Novembers during the year before ring formation and warm Mays of the year the annual ring formed. Differences in species-stand elevation modulated the growth-climate associations. Radial growth in P. uncinata at high elevation sites was reduced when May temperatures were colder and May precipitation more abundant. In the 20th century, two contrasting periods in radial growth were observed: one (1900-1949) with low frequency of narrow and wide rings, low mean annual sensitivity, and low common growth variation; and another (1950- 1994) with the reverse characteristics. The increased variability in radial growth since the 1950s was observed for all species and sites, which suggests a climatic cause. The low shared variance among tree chronologies during the first half of the 20th century may result from a ''relaxation'' of the elevation gradient, allowing local site conditions to dominate macroclimatic influence. These temporal trends may be related to the recently reported increase of climatic variability and warmer conditions. This study emphasizes the need to carefully assess the relationships between radial growth and site conditions along ecological gradients to improve dendroclimatic reconstructions.

Community and phylogenetic structure of reproductive traits of woody species in wet tropical forests

Abstract: We compare the distribution of reproductive traits in woody vegetation of 10 wet tropical forests in northeastern Costa Rica. Based on quantitative sampling of seedlings, saplings, and trees, we assess whether particular sexual systems, pollination syndromes, or seed-dispersal modes are associated with successional stage, prior selective logging, woody growth forms, or patterns of abundance or rarity. We further examine the phylogenetic structure of these traits in the regional woody flora, testing explicit hypotheses regarding phylogenetic clustering of reproductive traits and habitat distributions. Animal dispersal and insect pollination predominate across all forest types and size classes. In second-growth trees, relative abundance of species with explosive dispersal, hermaphroditic flowers, and insect pollination is higher, and relative abundance of species with animal dispersal and mammal pollination is lower, compared to old-growth and logged forests. Overall, dioecy and wind dispersal are more frequent than expected in canopy trees, and hermaphroditic flowers are more frequent than expected in shrubs. Reproductive traits, growth-form traits, and relative abundance patterns show significant clustering within the supertree phylogeny. Patterns of trait distribution across forest types are closely linked with patterns of floristic composition at the genus and family level. Species-level associations among reproductive traits and woody growth form can be explained by phylogenetic cor- relations. Wind dispersal and hummingbird pollination are significantly concentrated in clades with hermaphroditic flowers, whereas wind pollination is concentrated in clades with unisexual flowers. Legacies of both phylogenetic history and forest disturbance structure the distribution of reproductive traits within and among tropical wet forest communities.

Nutrient status of tropical rain forests influences soil n dynamics after n additions

Abstract: Soil nitrogen (N) transformations and N-oxide emissions were measured following N additions in three tropical montane rain forests in the Hawaiian Islands that differed in substrate age and nutrient status. Nitrous oxide (N2O) and nitric oxide (NO) emissions were negligible following first-time N additions in a forest where N limits primary production, and they increased significantly following 11 yr of N fertilization. Furthermore, N-oxide fluxes in the N-limited forest were relatively low in response to a range of N additions, and all doses of N except the two highest (125 and 175 kg N/ha) resulted in net negative rates of mineralization and nitrification (i.e., soils showed significant N consumption). Short-term laboratory 15N experiments supported these trends by showing that both 15NH4+ and 15NO3− were strongly consumed in soil from both the control and long-term fertilized plots in the N-limited site. Long-term N fertilization in the N-limited forest significantly increased N availability and turnov...

Prey nutritional quality interacts with chemical defenses to affect consumer feeding and fitness

Abstract: Numerous studies have assessed the individual effects of prey nutritional quality or chemical defenses on consumer feeding behavior. However, little is known about how these traits interact to affect consumer feeding and performance. We tested the separate and interactive effects of prey chemical defenses and nutritional quality on the feeding behavior and fitness of six sympatric crustacean mesograzers. Natural concentrations of diterpene alcohols (dictyols) from the brown alga Dictyota menstrualis were incorporated, or not incorporated, into lower quality and higher quality foods to create artificial diets mimicking prey of variable value and defense. Five amphipods (Ampithoe longimana, A. valida, Cymadusa compta, Gammarus mucronatus, and Elasmopus levis) and one isopod (Paracerceis caudata), representing a continuum of closely to distantly related organisms, were fed intact algae or lower and higher quality diets containing or lacking dictyols. All six mesograzers preferred the green alga Enteromorpha intestinalis to the dictyol producing alga Dictyota menstrualis. In assays allowing consumers to choose between simultaneously available foods, dictyols deterred feeding by all five amphipods, but not the isopod; this occurred for both lower and higher quality foods. In no-choice assays, where consumers were confined with only one of our four treatment diets, effects on feeding became more complex. Nutritional quality alone affected feeding by five of the six species. These grazers compensated for lower quality by increasing consumption. Dictyols suppressed feeding for four of the six species. More interestingly, there were significant dictyol × quality interactions for three species. Dictyols decreased feeding more when placed in lower quality foods than higher quality foods. Two amphipods deterred by dictyols in the choice assays readily consumed dictyol-containing foods in no-choice situations and suffered few negative effects of doing so. Although all amphipods were deterred by dictyols in choice assays, dictyols decreased fitness (survivorship, growth, or reproduction) for only four of the five species. These effects included large and immediate decreases in survivorship, dramatic effects on reproduction, and modest effects on female growth. Dictyols enhanced survivorship of the isopod. Thus, the effects of secondary metabolites on feeding in choice situations vs. fitness in long-term assays were inconsistent. For three amphipods, certain effects of food quality, dictyols, or their interaction were detected only for females. In general, negative effects of dictyols on fitness were greater in lower than in higher quality foods, suggesting that prey nutritional value may counteract the effects of defensive metabolites. For example, when G. mucronatus consumed dictyols in lower quality foods, mortality was >80% by day 5; for dictyols in higher quality foods, 80% mortality took 28 days to develop. Lower quality foods alone significantly decreased growth for the isopod, three of the amphipods, and the females of a fourth amphipod, concomitantly reducing fecundity for four of the five amphipods studied. The effects of both chemical defenses and nutritional quality were unrelated to consumer phylogeny; responses of congeners varied as much, or more, than responses of more distantly related consumers. Understanding mechanisms and consequences of food selection requires that the interactive effects of both chemical defenses and prey nutritional characteristics be considered explicitly. Corresponding Editor: J. D. Witman.

Search strategies of a pursuit-diving marine bird and the persistence of prey patches

Abstract: We explored foraging strategies used by marine animals to search for prey by examining the relative importance of information exchange and memory in a cold ocean environment from 1998 to 2000. Recent technological advances have increased our knowl- edge of the foraging patterns of marine predators, but few of these studies have concurrently measured prey distribution and behavior. We quantified the arrival and departure behavior of a pursuit-diving, colonial seabird, the Common Murre, Uria aalge, at two colonies on the eastern Newfoundland Shelf through observational techniques. We also measured the distribution, abundance, and behavior of the capelin, Mallotus villosus, the main prey species of murres, within foraging ranges of each colony, using hydroacoustic, vessel-based tech- niques. Return and departure flight directions of murres did not match at either colony during the same period. This indicated that murres departing colonies did not use infor- mation on prey distributions outside of visual range of the colony provided by the flight paths of returning flocks of birds to the colony carrying fish. High-abundance aggregations of capelin were reliably found within specific 2.25-km areas ("hot spots") for up to two weeks within the foraging ranges of murres from both colonies (-100 km). This circum- stance suggests that murres could use memory to locate hot spots on the coarse scale (1- 100 km) of foraging ranges from both colonies. Specific commuting routes (regular flight paths) of murres toward and away from hot spots were obvious at sea, and feeding murres consistently marked the location of capelin schools within hot spots. These distributions provided excellent conditions for murres to locate capelin schools on both coarse and fine (1-1000 m) scales by cueing to the activities of conspecifics, known as local enhancement. While central-place foraging from breeding colonies, murres likely use a mixture of memory and local enhancement to locate prey, depending on the spatial and temporal resolution of search and current prey conditions. Uncovering such behavioral mechanisms responsible for predator-prey interactions increases our understanding of linkages among trophic levels and, ultimately, ecosystem dynamics.

Corn pollen dispersal: quasi‐mechanistic models and field experiments

Abstract: To make quantitative predictions about the pollen dispersal of a plant species under different environmental conditions, it is necessary to determine its individual pollen dispersal function, i.e., the two-dimensional density function describing the probability that a pollen grain emitted in (0, 0) fertilizes an ovule in (x, y). This function will depend on biological and climate parameters. We present models for the individual dispersal function of corn. These models are based on Brownian motion with drift and integrate biological (difference of height between male and female flowers) and aerodynamic (settling velocity, wind speed, air turbulence) parameters. The models presented differ in the importance of vegetation in stopping the paths of pollen grains. The models were fitted to data from two large field experiments of corn using the color of kernels as a phenotypic marker for pollen dispersal. The resulting estimations for the parameters of the models and comparisons between models indicate that (1)...

Influence of landscape structure and climate variability on a late holocene plant migration

Abstract: We analyzed and radiocarbon-dated 205 fossil woodrat middens from 14 sites in central and northern Wyoming and adjacent Utah and Montana to document spa- tiotemporal patterns of Holocene invasion by Utah juniper (Juniperus osteosperma). Ho- locene migration into central and northern Wyoming and southern Montana from the south proceeded by a series of long-distance dispersal events, which were paced by climate variability and structured by the geographic distribution and connectivity of suitable habitats on the landscape. The migration of Utah juniper into the region involved multiple long- distance dispersal events, ranging from 30 to 135 km. One of the earliest established populations, on East Pryor Mountain in south central Montana, is currently the northernmost population of the species. Establishment by long-distance dispersal of that population and another in the Bighorn Basin occurred during a period of relatively dry climate between 7500 and 5400 years ago. Further expansion of these initial colonizing populations and backfilling to occupy suitable sites to the south was delayed during a wet period from 5400 to 2800 years ago. Development of dry conditions 2800 years ago led to a rapid expansion in which Utah juniper colonized sites throughout its current range. Landscape structure and climate variability play important roles in governing the pattern and pace of natural in- vasions and deserve close attention in studying and modeling plant invasions, whether exotic or natural.

Successional mechanism varies along a gradient in hydrothermal fluid flux at deep-sea vents

Abstract: Invertebrate communities inhabiting deep-sea hydrothermal vents undergo substantial succession on time scales of months. Manipulative field experiments assessed the relative roles of environmental state and biotic interactions in determining temporal succession along a spatial gradient in vent fluid flux at three vent sites near 9 8509 N on the East Pacific Rise (2500 m water depth). Species colonization patterns on cubic basalt blocks (10 cm on a side) deployed by the submersible Alvin revealed both positive (facilitation) and negative (inhibition) biological interactions, in the context of established succession theory. Over a series of four cruises from 1994 to 1998, blocks were exposed to colonists for consecutive and continuous intervals in short-term (5 1 8 5 13 mo) and longer-term (8 1 29 5 37 mo) experiments. Colonists grouped into a mobile functional group were less abundant in the continuous interval (13 mo) than in the synchronous pooled-consecutive intervals (5 1 8 mo) of the short-term experiment, indicating that early colonists inhibited subsequent recruitment. Colonists grouped into a sessile functional group exhibited the opposite pattern, indicating facilitation. Similar trends, though not statistically significant, were observed in the longer-term experiment. The character of species interactions varied along a gradient in hydrothermal fluid flux (and inferred productivity), with inhibitory interactions more prominent in zones with high temperatures, productivity, and faunal densities, and facilitative interactions appearing where temperatures, productivity, and den- sities were low. Analyses of primary succession on introduced basalt blocks suggest that biological interactions during early vent community development strongly modify initial patterns of settlement, even in the absence of sustained temporal change in the vent fluid flux.

Disturbance and stand dynamics in ponderosa pine forests in rocky mountain national park, usa

Abstract: Fire is thought to be the dominant disturbance agent in pure ponderosa pine (Pinus ponderosa Laws.) forests, but fire severity and disturbances other than fire, and the effects of these events over time, have rarely been analyzed. We first created systematic criteria to (1) identify the causes of tree regeneration and mortality events and (2) classify the severity of these events. These criteria were then applied to understand the effect of events on ponderosa pine forests in Rocky Mountain National Park (RMNP), Colorado. For each of nine intensive study plots sampled in the pure ponderosa pine zone of RMNP, we mapped and dated live and dead trees and the spatial extent of fire and non- fire events using dendrochronology. Events were identified using evidence of disturbance agents, such as blue-stain fungus in the case of mountain pine beetles, fire scars, and climatic reconstructions. Disturbance severity was classified based on amounts and spatial distri- bution of regeneration, mortality, and survivors. We studied the temporal and spatial re- lationships between events and tree age structures to determine historical and contemporary stand dynamics. We identified 103 events among the nine study plots; 97% of these events were fire. High-severity events were 7% of events, 2% of events were mixed-severity, and the other 70% were low-severity. The severity of 20% of events could not be determined. All but one of the high-severity events were fire. In seven plots, at least one crown fire occurred within the last 300 years. In RMNP, pre-EuroAmerican settlement crown fires led to dense, even-aged stands. Subsequent surface fires killed some trees and facilitated the regeneration of others, leading to less dense, uneven-aged stands after ;200-300 years. Climate may be the cause or co-cause of one or two regeneration pulses within the last 300 years, but most regeneration and mortality is associated with fire. The historical occurrence of crown fires, as well as surface fires, in pure ponderosa pine forests in RMNP indicates that the fire regime is more variable than previously thought. Suppression of crown fires, though not completely possible, would move the fire regime outside its historical range of natural variability.

Bridging the gap: influence of seed deposition on seedling recruitment in a primate–tree interaction

Abstract: Frugivores are thought to influence plant recruitment by creating initial seed deposition templates. However, post-deposition processes (e.g., predation) may obscure these initial patterns. Few studies have examined successive life-history stages of animal-dispersed plants to evaluate how initial frugivore seed deposition influences early life stages of plant recruitment. We addressed this issue by quantifying seed removal and seedling recruitment of Monodora myristica (Annonaceae), a tropical, low-fecundity, forest tree in Kibale National Park, Uganda. Fruit morphology suggests that this species is dispersed by only the largest arboreal frugivores. We determined which frugivores removed fruit during focal tree watches and quantified seed fate in experiments designed to mimic natural deposition. We estimated stage-specific transition probabilities of survivorship to seedling establishment and determined expected seedling recruitment from each deposition condition. To evaluate spatial and temporal variation, these methods were conducted at two sites, 15 km apart, over two years. Finally, we compared frugivore abundance between sites to expected seedling and standing seedling, sapling, and pole abundances. We found that large-bodied primates were critical for seed dispersal. They were the only frugivores that opened the hard-husked fruits and were estimated to disperse >85% of mature seeds. Seeds placed away from parents had higher germination and establishment probabilities than those under parents, indicating that seed dispersal is advantageous. Single seeds away from parents (mimicking small-bodied primate seed spitting) had the highest cumulative seedling recruitment probability. However, in three of four site-by-year combinations, 86–94% of estimated recruits came from seeds placed in clumps in dung (mimicking large-bodied primates). High recruitment in the latter condition is due to the large number of seeds estimated to be deposited by large-bodied primates. Despite this concordance, germination and establishment probabilities were highly variable between sites and years. Germination was a limiting step in recruitment, and high seed mortality from beetles and rodents indicates the importance of predation. The site with higher frugivorous primate abundance had higher standing seedling abundance, but lower expected seedling recruitment and lower sapling and pole abundances. Thus, even in a system where frugivores are critical for dispersal, spatial and temporal variation in post-deposition processes reduces predictability of frugivore actions on seedling recruitment. Corresponding Editor: E. S. Menges

Landscape-level patterns of avian diversity in the oregon coast range

Abstract: We used a comparative mensurative landscape-level experiment to quantify the relative importance of mature forest area and fragmentation and differences among watersheds in influencing avian community diversity in the Oregon Coast Range, USA. Our study design included three large hydrological basins, two levels of fragmentation, and six levels of mature forest area. We recorded 82 species of birds in a total of 1046 plots in 30 landscapes. Our four response variables were species richness, species density, species evenness, and total bird density. We used a combination of factorial ANOVA and partial canonical variates analysis to quantify the relative importance of differences in mature forest area, fragmentation, and basin in influencing each response variable and community diversity overall. Bird community diversity was influenced by both the extent and fragmentation of mature forest at the landscape level. Species richness and density responded more strongly to mature forest area than to fragmentation and were significantly lower in landscapes that were completely dominated by mature forest than in landscapes with a mixture of seral stages. This reflected a selective loss of a number of early-seral species when mature forest completely blanketed the landscape. Species evenness was more strongly related to frag- mentation than to area of mature forest at the landscape level. In fragmented landscapes, the most dominant species decreased in abundance, whereas moderately abundant species increased in relative abundance. This resulted in bird communities that were more even in fragmented landscapes. Unlike several other studies, we did not find that the relative strength of fragmentation effects increased as habitat area decreased. Total bird density, in contrast, was not related to either mature forest area or fragmentation, but it varied significantly among basins. In summary, although both area and fragmentation of mature forest at the landscape level were strongly related to bird community diversity, they had qualitatively different effects. These results must be interpreted within the scope of this study, which took place in a forest-dominated landscape. Our results provide an interesting view of the factors that are related to bird diversity in a spatially complex seral mosaic but may not reflect the patterns that would exist in a high-contrast forest-nonforest landscape.

Massive prey recruitment and the control of rocky subtidal communities on large spatial scales

Abstract: What drives community change on large spatial scales? An opportunity to address this fundamental question was provided by a massive subtidal recruitment of the mussel, Mytilus edulis, across the southwest Gulf of Maine (GOM), USA, in 1995. Since M. edulis is consumed by many predator species, we hypothesized that the episodic increase in food resources had a bottom-up effect on populations of mussel consumers. The mussel recruitment event was unprecedented in its large spatial extent, high coverage of the sea floor, and depth of penetration into the rocky subtidal zone. During 1995-1997 we tracked mussels and their consumer populations in the area to document what happened. Surveys across a 120-km marine landscape revealed that juvenile mussels covered 33-91% of the bottom (8-12 m depth) at 13 of 17 sites between October 1995 and June 1996. The cover of mussel prey in June 1996 explained significant variation in the density and biomass of mussel predators, the sea stars Asterias spp. and rock crabs Cancer spp., at 16-17 sites 10, 13, and 23 months after mussel recruitment. These results are consistent with the hypothesis that the extensive mussel recruitment had a bottom-up effect on the subtidal food web. Levels of Asterias spp. recruitment, biomass, and density following mussel recruitment were the highest recorded at two monitored sites in 16-18 years. One year after the large pulse of mussel food resources, Asterias spp. recruitment was positively related to the percent cover of mussels, suggesting a feedback between food supply and consumer re- cruitment on a large spatial scale. Crab size-structure data supported the interpretation that recruitment contributed to the high densities of rock crabs recorded in July 1997. We suggest that the bottom-up effects observed in consumer populations were produced by increased growth, elevated reproduction and recruitment, and possibly by immigration. Coupling between bottom-up and top-down control was observed after a lag of 12-14 months post-mussel recruitment when predation by sea stars, rock crabs, and sea urchins eliminated extensive beds of juvenile mussels across the study region. The elimination of Mytilus prey triggered cannibalism in the sea star Asterias vulgaris, which contributed to density-dependent population declines in sea stars between 1996 and 1997. A key as- sumption of a consumer reproduction and recruitment response—that some of the larvae produced by local adult stocks of consumers could be retained in the study region—was tested by investigating the potential larval dispersal of sea stars and crabs in a simulated three-dimensional flow field of the southwest GOM. The simulation supported the as- sumption as 15-75% of particles released below 1-m depth were retained in the region over 2-5 week periods approximating the larval life-span of the consumers. Furthermore, the degree of retention increased dramatically between the surface (1 m) and 15-m depth, implying that the extent of large-scale connectivity by passive larval dispersal is highly dependent on depth. In this study, massive prey recruitment stimulated coupling between bottom-up and top-down forces, resulting in broad-scale changes in local consumer pop- ulations that persisted after the prey-recruitment signal was removed, underscoring the importance of episodic events in marine community dynamics.

Composition and structure of seasonally deciduous forests in the americas

Abstract: Forest canopies with leaves shed annually (seasonally deciduous forests) produce a different subcanopy environment than canopies with persistent leaves (evergreen forests). Although causes differ with latitude, seasonality might affect structure and composition of forests similarly. We explored changes in species composition and vertical structure in seasonally deciduous forests from 0°–40° N in the Americas. We measured all woody stems ≥1 cm dbh within replicated 0.0625-ha plots (16 at each of 9 sites, N > 31 000). Numbers of species, as well as density and basal area, were analyzed parametrically for canopy trees, subcanopy trees, shrubs, and lianas. Numbers of species, especially shrubs and subcanopy trees, were greater in warm (30° N) than cool (40° N) temperate forests. Much larger numbers occurred in subtropical (20° N) and tropical (10° N and 0°) forests, but did not vary in relation to latitude. Higher than expected numbers in areas <0.25 ha in all forests, but especially subtropical forests, indi...

Coexistence of mutualists and exploiters on spatial landscapes

Abstract: Mutualisms are almost ubiquitously exploited by species that gain the ben- efits that mutualists offer to each other, but that offer nothing in return. This paper inves- tigates the possible dynamical outcomes of a mechanistically formulated model system, involving two obligate mutualists and one exploiter. The model is based conceptually on a mutualism between a plant species and its pollinating seed parasite, in the presence of an obligate, nonpollinating seed parasite. Of particular interest are the conditions under which the exploiter species can invade and coexist with a mutualism that, by itself, possesses an equilibrium stabilized by other density-dependent regulating factors. Two types of models are used in the analyses: a deterministic, nonspatial model described by a set of discrete time equations, and an individual-based simulation incorporating stochastic interactions and spatial structure. Comparing the results of these two models uncovers the temporal dynamics of both well-mixed local systems and spatially distributed populations. Using these two situations, we examine how the predictions of the nonspatial model are affected by habitat structure. In the nonspatial case, and without the exploiters, there are typically two stable equi- libria: one having zero densities for both mutualist species, and the other having nonzero densities. This bi-stability is a characteristic feature of obligate mutualisms. When the exploiter species is included, the system always retains the stability of the zero-density equilibrium, but the dynamics of the upper equilibrium can be more complicated, including limit cycle and extinction dynamics. Simulation results demonstrate that spatial structure is a highly stabilizing influence on the three-species system as long as the exploiter's dispersal distance is large relative to the seed dispersal distance of the plant species. Given this condition, the model predicts spatial distributions that are marked by a patchy distri- bution of plants, with mutualists and exploiters situated about these patches.

Patch structure, oviposition behavior, and the distribution of parasitism risk

Abstract: To date, almost no experimental field studies have attempted to assess the factors that generate heterogeneity in the distribution of parasitism risk, a putative indicator of host–parasitoid stability. In this study, I examined the interaction between a planthopper Prokelisia crocea and its egg parasitoid Anagrus columbi among discrete patches of prairie cordgrass, Spartina pectinata. In particular, I examined how patch geography and host distribution within a patch influenced the distribution of adult parasitoids, parasitoid oviposition behavior, the proportion of hosts parasitized, and the aggregation of parasitism (cv2) and adult parasitoids. Based on six generations of census data, the distribution of parasitism was strongly aggregated within (cv2 = 3.58), but not among (cv2 = 0.58) cordgrass patches. Parasitism was also spatially and temporally density independent. To determine what influences the distribution of parasitism risk, I selected 26 discrete cordgrass patches, removed all sources of A. columbi, and then quantified the immigration and subsequent oviposition behavior of A. columbi colonists. I found that the number of immigrants significantly increased with patch size and decreased with patch isolation. Patch size had no influence on the per capita hosts parasitized per leaf, but there was a significant twofold increase in per capita attacks from the least to the most isolated patches. The isolation effect was likely due to an optimal oviposition response to dispersal distance by A. columbi. For these experimental patches, substantial within-patch aggregation of parasitism (cv2 = 1.63) did not translate into strong among-patch aggregation (cv2 = 0.13). Searching adult parasitoids were randomly distributed within and among patches and thus did not explain the high cv2 within patches. Interestingly, the aggregation of parasitism risk within a patch was significantly negatively correlated with patch size and positively correlated with patch isolation. The distribution of parasitism risk could be divided into two general components. The within-parasitoid component was attributable to individuals engaging in multiple ovipositions within a leaf and the distance-dependent oviposition response. The latter response was likely the cause for the variation in cv2 with respect to patch size and isolation. Within-parasitoid aggregation has no effect on host–parasitoid stability. The among-parasitoid component of aggregation appears to have been due to heterogeneity in the vulnerability of hosts and an edge effect (parasitism risk is 60% more heterogeneous at the edge than interior of a patch) and is in theory stabilizing. Consequently, a change in landscape structure that leads to an increase in cordgrass edge habitat may promote a more stable host–parasitoid interaction. Corresponding Editor: R. F. Denno.

Hydrologically driven hierarchical competition–colonization models: the impact of interannual climate fluctuations

Abstract: A hierarchical competition–colonization model extensively used in ecological dynamics is driven by the stress conditions resulting from the annual fluctuations in rainfall in water-limited ecosystems. The results show that ecosystem composition is very sensitive to the inclusion of realistic amounts of interannual rainfall variability. The evolutionary dynamics of competing trees and grasses exhibit scale-invariance characteristics (i.e., a power law spectra) for the temporal changes in the relative density of the species. This temporal scaling results from the internal dynamics of the competition–colonization process, which is driven by Markovian annual rainfall amounts. The model was also implemented to approximately describe the conditions of two ecosystems characterized by grass–tree competition, one near La Copita, Texas, and one in Nylsvley, South Africa. Long-term simulations run for many different characteristics of the annual rainfall amounts show that the diversity of dynamic states in the tertiary grass/tree/bare soil system (i.e., its information entropy) is at a maximum near historically observed rainfall characteristics. This suggests that, under commonly observed interannual rainfall fluctuations, water-controlled ecosystems tend to self-organize in a manner that maximizes the richness of relative species abundances. Corresponding Editor: W. K. Lauenroth.

Does reproductive site choice in a neotropical frog mirror variable risks facing offspring

Abstract: For organisms whose juvenile stages develop in discrete habitats, the strategy adults use to select patches directly impacts reproductive success. In most environments, the magnitude of juvenile mortality risks varies spatially and temporally among patches, often at several scales. Little is known, however, about how closely adults track such variation in site quality and how the predictability of risks facing juveniles influences the strategy adults adopt. I addressed these questions in a three-year study of Edalorhina perezi, an iteroparous frog that reproduces in forest pools in the southwestern Amazon Basin. Using observations and experiments, I quantified the spatial and temporal variation in direct and indirect mortality risks facing tadpoles at 18–34 pools. Data on larval risks were compared with observations of adult pool choice, including nest distribution and movement trends. Direct risks, pool drying and insect predation, accounted for 95% of estimated tadpole mortality. The indirect effect of tadpole density on mortality was small, slightly increasing tadpole drying risk. When pool risks were combined, persistent pools were predictably lower in risk early and late in the rainy season, while ephemeral pools were slightly advantageous midseason. At the population level, site choice shifted seasonally from persistent to ephemeral pools. Nonetheless, three lines of evidence suggest that adults were sensitive to local variation in larval risks. First, movement patterns were consistent with yearly changes in predation risk. Second, adults favored newly formed pools, which had lower predator densities and were likely to be more productive. Third, adults favored pools without recent reproductive activity. Whether fixed or flexible, the selective advantage for observed site choice was not evident during the last 50 d of the rainy season, when a majority of adults used ephemeral pools at a time when persistent pools appeared favorable for tadpoles. This study suggests that E. perezi responds to variation in juvenile risks at several scales when selecting reproductive sites. Yet adults appeared to be strikingly imperfect at tracking risk. Errors could arise due to cues that differ in their reliability or if adults shift from risk-sensitive to risk-prone strategies. Moreover, the behavior of conspecifics may constrain optimal site choice. Corresponding Editor: D. N. Reznick.

Spatial dispersion and density dependence in a perennial desert shrub (chrysothamnus nauseosus: asteraceae)

Abstract: Dogma asserts that desert shrubs should become uniformly dispersed through time through a process of self-thinning caused by competition for water. We present long-term data (1985–2001) on two nearby populations of the perennial shrub, Chrysothamnus nauseosus ssp. consimilis, in sand-dune ecosystems near Mono Lake, California. Because previous studies established that water limited the growth, survival, and reproduction of this shrub, we searched for population-level patterns of competition. A seven-year history of survival, growth, and flowering of marked individuals delineated five demographic stage classes corresponding roughly to age and plant size: seedlings, nonflowering juveniles, juveniles that later flowered, adults that flowered intermittently, and “immortal” adults that flowered annually. For all stages except nonflowering juveniles, initial size was a strong predictor of life expectancy, and for the reproductive classes, flowering was associated with larger size and higher survival. Survival o...