Showing papers in "Ecology in 1994"

Competition and Biodiversity in Spatially Structured Habitats

Abstract: All organisms, especially terrestrial plants and other sessile species, interact mainly with their neighbors, but neighborhoods can differ in composition because of dis- persal and mortality. There is increasingly strong evidence that the spatial structure created by these forces profoundly influences the dynamics, composition, and biodiversity of com- munities. Nonspatial models predict that no more consumer species can coexist at equilibrium than there are limiting resources. In contrast, a similar model that includes neighborhood competition and random dispersal among sites predicts stable coexistence of a potentially unlimited number of species on a single resource. Coexistence occurs because species with sufficiently high dispersal rates persist in sites not occupied by superior competitors. Co- existence requires limiting similarity and two-way or three-way interspecific trade-offs among competitive ability, colonization ability, and longevity. This spatial competition hypothesis seems to explain the coexistence of the numerous plant species that compete for a single limiting resource in the grasslands of Cedar Creek Natural History Area. It provides a testable, alternative explanation for other high diversity communities, such as tropical forests. The model can be tested (1) by determining if coexisting species have the requisite trade-offs in colonization, competition, and longevity, (2) by addition of propagules to determine if local species abundances are limited by dispersal, and (3) by comparisons of the effects on biodiversity of high rates of propagule addition for species that differ in competitive ability.

Beyond Global Warming: Ecology and Global Change

Abstract: While ecologists involved in management or policy often are advised to learn to deal with uncertainty, there are a number of components of global environmental change of which we are certain–certain that they are going on, and certain that they are human—caused. Some of these are largely ecological changes, and all have important ecological consequences. Three of the well—documented global changes are: increasing concentrations of carbon dioxide in the atmosphere; alterations in the biogeochemistry of the global nitrogen cycle; and ongoing land use/land cover change. Human activity–now primarily fossil fuel combustion– has increased carbon dioxide concentrations from °280 to 355 mL/L since 1800; the increase is unique, at least in the past 160 000 yr, and several lines of evidence demonstrate unequivocally that it is human—caused. This increase is likely to have climatic consequences–and certainly it has direct effects on biota in all Earth's terrestrial ecosystems. The global nitrogen cycle has been altered by human activity to such an extent that more nitrogen is fixed annually by humanity (primarily for nitrogen fertilizer, also by legume crops and as a by product of fossil fuel combustion) than by all natural pathways combined. This added nitrogen alters the chemistry of the atmosphere and of aquatic ecosystems, contributes to eutrophiction of the biosphere, and has substantial regional effects on biological diversity in the most affected areas. Finally, human land use/land cover change has transformed one—their to one—half of Earth's ice—free surface. This in and of itself probably represents the most important component of global change now and will for some decades to come; it has profound effects on biological diversity on land and on ecosystems downwind and downstream of affected areas. Overall, any clear dichotomy between pristine ecosystems and human—altered areas that may have existed in the past has vanished, and ecological research should account for this reality. These three and other equally certain components of global environmental change are the primary causes of anticipated changes in climate, and of ongoing losses of biological diversity. They are caused in turn by the extraordinary growth in size and resource use of the human population. On a broad scale, there is little uncertainty about any of these components of change or their causes. However, much of the public believes the causes–even the existence–of global change to be uncertain and contentious topics. By speaking out effectively, we can help to shift the focus of public discussion towards what can and should be done about global environmental change.

Evaluating approaches to the conservation of rare and endangered plants

Abstract: Schemske, D.W., B. C. Husband, M.H. Ruckelshaus, C. Goodwillie, I.M. Parker, and J.G. Bishop. 1994. Evaluating approaches to the conservation of rare and endangered plants. Ecology 75: 584-606.

Soil and Stem Water Storage Determine Phenology and Distribution of Tropical Dry Forest Trees

Abstract: Many trees of tropical dry forests flower or form new shoots soon after leaf shedding during the dry season, i.e., during a period when trees are likely to be severely water stressed. To resolve this apparent paradox, phenology and seasonal changes in tree water status were monitored during two consecutive dry seasons in > 150 trees of 37 species growing at different sites in the tropical dry lowland forest of Guanacaste, Costa Rica. Tree development during the dry season varied considerably between species and be- tween sites of different moisture availability. Leaf shedding, flowering, and shoot growth (flushing) were strongly correlated with seasonal changes in tree water status, measured by conventional and newly developed techniques. Tree water status varied with the availability of subsoil water and a variety of biotic factors such as structure and life-span of leaves, time of leaf shedding, wood density and capacity for stem water storage, and depth and density of root systems. Observed tree species differed widely in wood density (from 0.19 to 1.1 g/cm3) and stem water storage capacity (400-20% of dry mass), which was highly correlated with the degree of desiccation during drought. Only hardwood trees at dry upland sites, lacking stem water storage and access to subsoil water, desiccated strongly (stem water potential <-4 MPa) and remained inactive throughout the dry season. In all other trees elimination of transpirational water loss during leaf shedding in conjunction with utilization of residual water enabled rehydration of stem tissues and subsequent flowering or flushing during the dry season. Rate and degree of rehydration varied strongly with the availability of water stored in tree trunks or in the subsoil. Stored water buffers the impact of seasonal drought and enables flowering and flushing during the dry season. Climatic data are thus not sufficient to explain tree phenology in seasonally dry tropical forests. Phenology, seasonal changes in water status, and water storage capacity of tree species are highly correlated. Tree species cluster into a number of distinct functional types ranging from deciduous hardwood trees and water-storing lightwood trees in dry upland forests to evergreen light- and softwood trees confined to moist lowland sites. Seasonal variation in tree water status thus appears to be the principal determinant of both phenology and distribution of tree species in tropical dry forests.

Forest Fragmentation, Pollination, and Plant Reproduction in a Chaco Dry Forest, Argentina

Abstract: In a fragmented, dry subtropical forest in northwestern Argentina, we com- pared pollination levels, fruit set, and seed set among small ( 2 ha) fragments, and continuous forest in 16 plant species representing a wide range of pollination systems, breeding systems, and growth forms. For three species, Prosopis nigra (Mimosoideae), Cercidium australe (Caesalpinoideae), and Atamisquea emarginata (Cap- paraceae), the three treatments were replicated across four sites; we achieved less replication for other species. Because comparisons between forest and fragment populations for dif- ferent species took place in different sites, however, by treating all 16 species as a unit we lessened the potential bias of confounding site effects and could evaluate the overall impact of fragmentation. Significant or marginal (P < .10) fragmentation-related declines in number of pollen tubes per style, fruit set, and seed set occurred in 9 of 16, 5 of 15, and 3 of 14 species, respectively. Overall, significant or nonsignificant declines occurred in 8 1% (pollen tubes), 73% (fruit set), and 79% (seed set) of the species. In all cases these proportions were greater (P ' .06) than the null binomial expectation of a 1: 1 ratio of increases to decreases. Breeding system did not explain sensitivity to fragmentation: the magnitudes of declines in pollen tubes, fruit set, and seed set were virtually indistinguishable between self-compatible and self-incompatible species. At least 4 of the 10 self-incompatible species, however, were heavily visited in small fragments by Africanized honey bees, which may have compensated for a decline in visits by native pollinators. The exact nature of responses varied among plant species. In some, the absolute quantity of pollen grains transferred to stigmas decreased with fragmentation, and sometimes this was reflected in reduced fruit or seed set. In Cercidium, Prosopis, and Atamisquea, the quality of the grains transferred apparently changed: number of pollen tubes produced per pollen grain on the stigma declined with increasing fragmentation, and at least in the latter two species seed production declined as well. Overall, levels of pollination and seed production undoubtedly integrated many idio- syncratic effects of fragmentation on particular plant and animal populations, and indicated that "community health" of fragments suffered in comparison with that of continuous forest. Median decreases in pollination levels and seed output from forest to fragments approached 20%. The impact of these declines on plant recruitment is less clear, however, because cattle grazing and trampling of seedlings and saplings in fragments may constitute a much more serious short-term conservation problem.

Dynamics of Gross Nitrogen Transformations in an Old-Growth Forest: The Carbon Connection

Abstract: We conducted a 456-d laboratory incubation of an old-growth coniferous forest soil to aid in the elucidation of C controls on N cycling processes in forest soils. Gross rates of N mineralization, immobilization, and nitrification were measured by 'IN isotope dilution, and net rates of N mineralization and nitrification were calculated from changes in KCl-extractable inorganic N and NOE-EN pool sizes, respectively. Changes in the availability of C were assessed by monitoring rates of CO, evolution and the sizes of extractable organic C and microbial biomass pools. Net and gross rates of N mineralization (r2 = 0.038, P =.676) and nitrification (r2 = 0.403, P = .125) were not significantly correlated over the course of the incubation, suggesting that the factors controlling N consumptive and productive processes do not equally affect these processes. A significant increase in the NO, pool size (net nitrification) only occurred after 140 d, when the NO3- pool size increased suddenly and massively. However, gross nitrification rates were substantial throughout the entire incubation and were poorly correlated with these changes in NO3 pool sizes. Concurrent decreases in the microbial biomass suggest that large increases in NO3 pool sizes after prolonged incubation of coniferous forest soil may arise from re- ductions in the rate of microbial immobilization of NO3, rather than from one of the mechanisms proposed previously (e.g., sequestering of NH,+ by microbial heterotrophs, the deactivation of allelopathic compounds, or large increases in autotrophic nitrifier pop- ulations). Strong correlations were found between rates of CO2 evolution and gross N mineralization (r2 = 0.974, P < .0001) and immobilization (r2 = 0.980, P < .0001), but not between CO, evolution and net N mineralization rates. Microbial growth efficiency, determined by combining estimates of gross N immobilization, CO2 evolution, and micro- bial biomass C and N pool sizes, declined exponentially over the incubation. These results suggest the utilization of lower quality substrates as C availability declined during incu- bation. Results from this research indicate the measurement of gross rates of N transfor- mations in soil provides a powerful tool for assessing C and N cycling relationships in forests.

Partial Differential Equations in Ecology: Spatial Interactions and Population Dynamics

Abstract: Most of the fundamental elements of ecology, ranging from individual be- havior to species abundance, diversity, and population dynamics, exhibit spatial variation. Partial differential equation models provide a means of melding organism movement with population processes and have been used extensively to elucidate the effects of spatial variation on populations. While there has been an explosion of theoretical advances in partial differential equation models in the past two decades, this work has been generally neglected in mathematical ecology textbooks. Our goal in this paper is to make this literature accessible to experimental ecologists. Partial differential equations are used to model a variety of ecological phenomena; here we discuss dispersal, ecological invasions, critical patch size, dispersal-mediated coexis- tence, and diffusion-driven spatial patterning. These models emphasize that simple organ- ism movement can produce striking large-scale patterns in homogeneous environments, and that in heterogeneous environments, movement of multiple species can change the outcome of competition or predation.

Influences of Trees on Savanna Productivity: Tests of Shade, Nutrients, and Tree-Grass Competition

Abstract: To determine why herbaceous productivity in tropical and subtropical savannas is often significantly higher under crowns of isolated trees than in adjacent grass—lands, experimental plots were established in three concentric zones, crown, tree—root, and grassland, surrounding isolated trees of Acacia tortilis in low—rainfall and high—rainfall savannas in Tsavo National Park, Kenya. Plots were fertilized (to determine the importance of nutrient enrichment by trees), shaded (to determine the importance of crown shade), fertilized and shaded (to identify fertilizer x shade interactions), or trenched (tree roots entering plots were severed to determine the importance of belowground competition between overstory trees and understory herbaceous plants). In addition, vertical root distributions of trees and herbaceous species were determined, and root systems of A. tortilis saplings were excavated. At both sites fertilization significantly increased herbaceous productivity in tree—root and grassland zones, but not in canopy zones; artificial shade had no effect on productivity at the low—rainfall site but increased productivity in the tree—root zone at the high—rainfall site; and severing tree roots had no effect on herbaceous productivity at the low—rainfall site, but increased productivity in the crown and tree—root zones at the high—rainfall site. Roots of herbaceous and woody species co—occurred within the same soil horizons, but tree roots extended farther into grasslands at the low—rainfall site than at the high—rainfall site. These studies suggest that savanna trees completed more intensely with understory plants at wetters sites, where their roots terminated in or near crown zones, than at drier sites, where their roots extended farther into open grassland. Nutrients added by trees to crown zones in the form of tree litter and animal droppings increased understory productivity by fertilizing nutrient—limited soils. Shade contributed more to regrowth after severe defoliation than to growth under more normal conditions.

Feeback between Plants and Their Soil Communities in an Old Field Community

Abstract: The nature of the interaction between plants ant their soil community was investigated by testing for differential responses of four old—field perennial plant species to inocula derived from soil communities that had been grown with ("cultured by") one of these four plant species. The differentiation of the soil communities was evident in measurements of plant survival, phenology, growth, and root—shoot ratios. Effects on survival and growth suggest negative feedbacks between these species and the soil communities that they culture. Survival rates of Krigia dandelion were significantly reduced when grown with their own" soil community. Considered as a whole, the three other species (all grasses) had significantly lower growth and root—shoot ratios when grown with soil communities started with their own inocula compared to soil communities started with the inocula of other species. However, the significance of this effect on growth rate and root—shoot ratios was due primarily to the pairwise comparison of Anthoxanthum odoratum and Danthonia spicata and of Anthoxanthum and Panicum sphaerocarpon, respectively. Pairwise comparisons of Danthonia and Panicum do not suggest differential responses to each other's soils in growth rate or root—shoot ratios nor do soil communities appear to affect the relative competitive ability of these two species. While the components of the soil community responsible for these effects have not been identified, similar although less pronounced patterns were observed in experiments using inocula consisting of washed live root segments as compared to experiments using whole soil as inoculum, suggesting that root pathogens are one important agent.

Population regulation in theory and practice

Abstract: Population regulation is a fundamental process related to most phenomena in ecology, including evolutionary ecology. I review the conceptual basis for defining reg- ulation as bounded fluctuations in abundance, in contrast to the unbounded fluctuations of random-walk populations. Regulation arises as a result of potentially stabilizing density- dependent processes, even when brought about by "non-equilibrium" mechanisms. Al- though the phenomenon is unambiguous in theory, detecting regulation by finding evidence for density dependence in a series of population estimates faces unsolved statistical prob- lems. So, while there is growing evidence for widespread regulation, severe detection prob- lems remain. I illustrate these with data from bird populations. Whether regulation is typically achieved by local stabilizing mechanisms or via metapopulation dynamics remains to be determined. I summarize recent studies on a particularly well-regulated system - red scale (Aonidiella aurantii) and its controlling parasitoid, Aphytis melinus. We tested and failed to find evidence for eight hypotheses that might account for the system's stability, including spatial heterogeneity in attack rates, a refuge, and metapopulation dynamics. We also failed to find evidence for density-dependent parasitism, but such density dependence might be still size-structured~rthu host.eipen

Xylem Embolism in Ring-Porous, Diffuse-Porous, and Coniferous Trees of Northern Utah and Interior Alaska

Abstract: Xylem embolism was measured in nine tree species for one or more years. Species were ring-porous (Quercus sp.), diffuse-porous (Alnus, Betula, Populus spp.) or coniferous (Picea, Larix, Abies spp.). Intraspecific (Populus tremuloides) and intrageneric (Betula, Alnus) comparisons were made between sites in northern Utah and interior Alaska. Most embolism, >90% in some dicot species, occurred in winter. Within sites, dicot trees embolized more than conifers. Between sites, Alaskan dicot trees embolized less than their Utah counterparts. Differences were explained by vulnerability to embolism caused by freeze-thaw cycles. Most conifers were entirely resistant, whereas dicot trees were vulner- able. Less embolism in Alaskan dicot trees was associated with fewer freeze-thaw events in Alaska vs. Utah. Vulnerability was positively correlated with conduit volume and hy- draulic conductance per unit xylem area (ks). Tracheids were superior to vessels in avoiding freeze-thaw-induced embolism, and had lower k,. At the other extreme, ring-porous xylem had the highest k, but lost > 90% of hydraulic conductance after a single freeze-thaw event. Vulnerability to water-stress-induced cavitation was not correlated with conduit volume or k,. Dicot species either reversed winter embolism by refilling vessels with positive root pressures during spring (Betula, Alnus spp.), or tolerated it and relied on new xylem pro- duction to restore hydraulic conductance (Quercus sp.). Conifers reversed embolism by refilling tracheids in the absence of positive pressure. Populus species behaved inconsis- tently, showing some reversal one year but none the next. Even without embolism reversal, Populus species had hydraulic conductances per unit leaf area equal to other diffuse-porous species.

Amino Acid Absorption by Arctic Plants: Implications for Plant Nutrition and Nitrogen Cycling

Abstract: Recent studies of nitrogen (N) cycling in arctic tundra have indicated that inorganic N supplied to plants by mineralization is not sufficient to meet the annual requirement of N by many tundra species. Whereas N mineralization is slow in tundra soils and concentrations of inorganic N are low, these soils have large stocks of both structural and soluble organic N. In light of these observations, kinetics of absorption of three amino acids (glycine, aspartic acid, and glutamic acid) were measured in dominant vascular plant species of the four major ecosystem types in arctic Alaska and compared with concentrations of free amino acids in soils. Absorption rates were measured on roots using '4C-labeled substrates. Concentrations of free amino acids in soil were measured on water-extracted samples by high pressure liquid chromatography. All species had higher capacity (Vmax) for ammonium uptake (measured using methylamine as an ammonium analogue) than for any amino acid. However, at concentrations observed in the field, uptake rates estimated for amino acids were similar to (glycine) or less than (aspartic and glutamic acids) that for ammonium. On the basis of these comparisons, uptake rates of the three amino acids together may account for between 10 and 82% of the total N uptake in the field, depending on species and community. Deciduous shrubs had higher uptake rates than the more slowly growing evergreen shrubs, suggesting that new growth created a sink that strongly influenced capacity for amino acid uptake. In general, ectomycorrhizal species had higher amino acid uptake than did non-mycorrhizal species. In species that were sampled from more than one community, amino acid uptake rates were highest in the community where a given amino acid was most abundant in the soil. The results indicate that, in arctic tundra, plants short-circuit the mineralization step of decomposition by directly absorbing amino acids. This implies that in the organic soils of these tundra systems (1) inorganic nitrogen is an inadequate measure of plant-available soil nitrogen, (2) mineralization rates underestimate nitrogen supply rates to plants, (3) the large differences among species in capacities to absorb different forms of N provide ample basis for niche differentiation of what was previously considered a single resource, and (4) by short-circuiting N mineralization, plants accelerate N turnover and effectively exert greater control over N cycling than has been previously recognized.

Aic model selection in overdispersed capture-recapture data'

Abstract: Selection of a proper model as a basis for statistical inference from capture—recapture data is critical. This is especially so when using open models in the analysis of multiple, interrelated data sets (e.g., males and females, with 2—3 age classes, over 3—5 areas and 10—15 yr.) The most general model considered for such data sets might contain 1000 survival and recapture parameters. This paper presents numerical results on three information—theoretic methods for model selection when the data are overdispersed (i.e., a lack of independence so that extra—binomial variation occurs). Akaike's information criterion (AIC), a second—order adjustment to AIC for bias (AICc), and a dimension—consistent criterion (CAIC) were modified using an empirical estimate of the average overdispersion, based on quasi—likelihood theory. Quality of model selection was evaluated based on the Euclidian distance between standardized ° and ° (parameter ° is vector valued); this quantity (a type of residual sum of squares, hence donated as RSS) is a combination of squared bias and variance. Five results seem to be of general interest for these product—multinomial models. First, when there was overdispersion the most direct estimator of the variance inflation factor was positively biased and the relative bias increased with the amount of overdispersion. Second, AIC and AICc, unadjusted for overdispersion using quasi—likelihood theory, performed poorly in selecting a model with a small RSS value when the data were overdispersed (i.e., overfitted models were selected when compared to the model with the minimum RRS value). Third, the information—theoretic criteria, adjusted for overdispersion, performed well, selected parismonious models, and had a good balance between under— and overfitting the data. Fourth, generally, the dimension—consistent criterion selected models with fewer parameters than the other criteria, had smaller RSS values, but clearly was in error by underfitting when compared with the model with the minimum RSS value. Fifth, even if the true model structure (but not the actual parameter values in the model) is known, that true model, when fitted to the data (by parameter estimation) is a relatively poor basis for statistical inference when that true model includes several, let alone many, estimated parameters that are not significantly different from O.

Importance of stream microfungi in controlling breakdown rates of leaf litter

Abstract: Breakdown of seven leaf species covering a broad range of litter qualities (lignin: 7-31% of leaf dry mass; tannin: 0.0-6.7%; nitrogen: 0.5-2.6%; phosphorus: 0.0 17- 0.094%) and dynamics of fungal biomass and reproductive activity were studied in a softwater mountain stream. Litter breakdown proceeded at exponential rates k ranging from 0.0042 d-l (evergreen oak) to 0.0515 d-l (ash). Fungal colonization of litter was generally rapid, with the fungus-specific indicator molecule ergosterol increasing from initially negligible concentrations to 375-859 pug/g of detrital mass. Using species-specific factors relating ergosterol concentrations to mycelial dry mass, maximum fungal biomass associated with litter was estimated as 61-155 mg/g of total system mass. Minimum estimates of net mycelial production during active growth varied between 0.3 and 3.8 mg g-I d-l, and maximum sporulation rates of aquatic hyphomycetes ranged from 760 to 7500 conidia mg- I d-l. Initially, reproductive activity was largely synchronized with increases in ergosterol concentrations, but it declined dramatically after peak sporulation rates were reached, whereas ergosterol concentrations levelled off or decreased at consid- erably slower rates. Periods of highest fungal productivity were thus limited to an initial breakdown stage of 2-8 wk. Strong correlations were found between the exponential breakdown coefficient and each of three parameters reflecting fungal activity in leaf litter, that is, maximum ergosterol concentration (P = 0.002, r = 0.96), net mycelial production (P = 0.02, r = 0.92), and sporulation rate (P < 0.001, r = 0.99). The initial lignin content of leaves was also significantly correlated with the rate constant k (P = 0.02, r = -0.83), suggesting that lignin was the primary factor determining litter quality and thus breakdown rate. The correlation was even stronger when data were logarithmically transformed (P < 0.01, r = -0.95). Tannin concentration was significantly correlated with k only when two high-lignin species were excluded from the analysis (P = 0.19, r = -0.56 compared with P = 0.05, r = -0.88), while initial concentrations of phosphorus (P = 0.17, r = 0.58) and particularly nitrogen (P = 0.82, r = 0.06) were poor predictors of litter decomposability. These results suggest that the initial lignin content of leaves controlled litter breakdown rate through a kinetic limitation of carbon sources for saprotrophic microfungi. The de- composer activity of these organisms, in turn, would then have governed breakdown rates. In doing this, fungi produced substantial amounts of both mycelial and conidial biomass that was potentially available to higher trophic levels of the food web.

Seedling recruitment in forests: calibrating models to predict patterns of tree seedling dispersion'

Abstract: Recruitment, the addition of new individuals into a community, is an im- portant factor that can substantially affect community composition and dynamics. We present a method for calibrating spatial models of plant recruitment that does not require identifying the specific parent of each recruit. This method calibrates seedling recruitment functions by comparing tree seedling distributions with adult distributions via a maximum likelihood analysis. The models obtained from this method can then be used to predict the spatial distributions of seedlings from adult distributions. We calibrated recruitment functions for 10 tree species characteristic of transition oak- northern hardwood forests. Significant differences were found in recruit abundances and spatial distributions. Predicted seedling recruitment limitation for test stands varied sub- stantially between species, with little recruitment limitation for some species and strong recruitment limitation for others. Recruitment was limited due to low overall recruit production or to restricted recruit dispersion. When these seedling recruitment parameters were incorporated into a spatial, individual-based model of forest dynamics, called SOR- TIE, alterations of recruitment parameters produced substantial changes in species abun- dance, providing additional support for the potential importance of seedling recruitment processes in community structure and dynamics.

Plant species migration in a mixed-history forest landscape in eastern north america'

Abstract: The understory flora of successional forest in the Delaware/Pennsylvania Piedmont zone (USA) is species poor relative to old regrowth stands in the region. Im- poverishment may reflect (a) limitations on dispersal of potential colonists, or (b) the unsuitability of successional stands for establishment. To examine the importance of dis- persal, herb and shrub distributions were surveyed in successional stands of varying age and spatial arrangement. In successional stands contiguous with species-rich old-regrowth forest, understory species richness declined with distance into the successional stands, implying contagion across the old-regrowth ecotone. Individual species showed evidence of contagion in 67% of site x species combinations. Mean rates of migration ranged from no measurable movement (e.g., Carex laxiflora, Cimicifuga racemosa) to >2 m/yr in some species (e.g., Galium aparine, Potentilla canadensis). Significant differences in rate of migration were observed among seed dispersal modes: ingested > adhesive > wind - ants - none. By contrast, there were no significant differences in rate between clonal and exclusively sexual species. In stands disjunct from old regrowth, the understory was species poor relative to old- regrowth forest due to a lack of species dispersed by ants and spores and those with no obvious dispersal vector. Understory species richness was greater in older disjunct stands and in stands closer to potential sources of propagules than in young and isolated stands, trends that were also noted in distributions of individual species. These results suggest that accessibility to colonists plays an important role in deter- mining understory composition of successional stands. Medium- and long-range migration appears to be an issue of seed and spore dispersal rather than vegetative propagation. The extremely low migration rates of some species threaten their continued existence in the second-growth forest landscape.

Carbon dioxide exchange between an undisturbed old-growth temperate forest and the atmosphere

Abstract: We used the eddy-correlation techn~que to investigate the exchange of COZ between an und~sturbed old-growth forest and the atmosphere at a remote Southern Hemi- sphere slte on 15 d between 1989 and 1990. Our goal was to determine how environmental factors regulate ecosystem CO, exchange. and to test whether present knowledge of leaf- level processes was sufficient to understand ecosystem-level exchange. On clear summer days the maxlmum rate of net ecosystem COY uptake exceeded 15 pmo1.m '.s I, about an order of magnitude greater than the maximum values observed on sunny days in the winter. Mean nighttime respiration rates varied between - -2 and -7 pmol.m-'.s-l. Nighttime CO, efflux rate roughly doubled with a 10°C increase in temperature. Daytime variation in net ecosystem C02 exchange rate was primarily associated with changes in total photosynthetically active photon flux density (PPFD). Air temperature. saturation deficit, and the diffuse PPFD were of lesser. but still significant, influence. These results are in broad agreement with expectations based on the biochemistry of leaf gas exchange and penetration of radiation through a canopy. However. at night. the short-term exchange of CO, between the forest and the atmosphere appeared to be regulated principally by atmospheric transport processes. There was a positive linear relationship between noc- turnal CO, exchange rate and downward sensible heat flux density. This new result has implications for the development of models for diurnal ecosystem CO, exchange. The daytime light-use efficiency of the ecosystem (CO, uptakehncident PPFD) was between 1.6 and 7.1 mmol/mol on clear days in the summer but decreased to 0.8 mmol/ mol after frosts on clear winter days. Ecosystem COZ uptake was enhanced by diffuse PPFD, a result of potentially global significance given recent increases in Northern Hemisphere haze.

Ungulate population models with predation: a case study with the north american moose'

Abstract: In this article I examine moose-wolf interactions over a broad spectrum of moose densities with the primary objective to test empirically whether wolf predation can regulate moose numbers. I also present four conceptual models of moose population dy- namics and outline their specific predictions. Based on 27 studies where moose were the dominant prey species, the functional and numerical responses of wolves to changing moose density were derived using an hyperbolic, Michaelis-Menten function. Per capita killing rate was strongly related (P = .01) to moose density, as was the density of wolves (P < .0 1). Killing rate plateaued at 3.36 moose wolf (100 days) I when predators were fully satiated. The asymptotic value for wolf density was 5 8.7 animals/ 1 000 km2. Wolf predation rate, as derived from the total predator response, proved to be density dependent from 0 to 0.65 moose/km2, and inversely density dependent at higher moose densities. Predator: prey ratios reflected wolf predation rate poorly because they did not integrate the functional response. An empirical model based on these results suggests that moose would stabilize at 2.0 moose/km2 in the absence of predators, and at - 1.3 moose/km2 in the presence of a single predator, the wolf. Density-dependent food competition creates these two high- density equilibrium conditions. If moose productivity is diminished through either dete- riorating habitat quality or bear-induced early calf mortality, then a low-density equilibrium (0.2-0.4 moose/km2) is predicted. The model also suggests that when a low equilibrium develops, a "predator pit" is absent or extremely shallow, thus arguing against the appro- priateness of a predation-food two-state model. Further research on the density relationship of bear predation, on the effect of alternate prey on wolf total response, and on the regulatory impact of food competition at high moose densities, is required for a full understanding of moose demography.

Effects of an omnivorous crayfish (Orconectes rusticus) on a freshwater littoral food web

Abstract: Cascading trophic interactions are important in many freshwater pelagic food webs, but their importance in more complex, omnivore-rich littoral-zone food webs is less well known. We tested the existence of a trophic cascade involving omnivorous crayfish (Orconectes rusticus), macroinvertebrates, periphyton, and macrophytes using 9-M2 cages in the littoral zone of Plum Lake, Wisconsin, USA. Treatments in the replicated (N = 4) experiment were crayfish enclosures, crayfish exclosures, and cageless references. During June-September, we measured macrophyte shoot numbers, macroinvertebrate numbers, and periphyton (on plastic strips) chlorophyll a, and dry mass (DM). We expected that crayfish foraging would directly reduce abundance and change species composition of macrophytes and macroinvertebrates and would indirectly enhance periphyton abundance by reducing the abundance of grazing snails. In enclosures, macrophyte and snail (but not nonsnail macroinvertebrate) densities declined significantly throughout the experiment, whereas densities of macrophytes, snails, and nonsnail macroinvertebrates increased in exclosures and cageless references. Some of the reduction in macrophytes resulted from nonconsumptive fragmentation of macrophytes by crayfish. Consistent with the cascading trophic interactions model, periphyton chloro- phyll a per unit surface area increased in enclosures, but declined in exclosures. Periphyton quality (as indexed by chlorophyll a/DM) also increased in enclosures relative to exclosures and cageless references. However, because of large reductions in macrophyte surface area (which periphyton colonizes) in enclosures, total amount of periphyton chlorophyll a in enclosures (relative to exclosures) probably declined while periphyton quantity per unit surface area and periphyton quality increased. Thus, the impacts of crayfish omnivory on periphyton, expressed in two conflicting indirect effects, confirm the possibility that om- nivory can complicate cascading trophic predictions. Overall, results support the existence of strong trophic interactions in the littoral zone, in which omnivorous crayfish control abundance of macrophytes, snails, and periphyton.

Litter Decomposition on the Mauna Loa Environmental Matrix, Hawai'i: Patterns, Mechanisms, and Models

Abstract: We determined controls on litter decomposition and nutrient release for the widespread native tree Metrosideros polymorpha in 11 sites arrayed on gradients of elevation, precipitation, and substrate age on Hawaiian lava flows. The effects of site characteristics were evaluated using three common substrates (Metrosideros leaf litter from one of the sites, wood dowels, and filter paper) decomposed in each of the sites, and the inherent decomposability of tissue (substrate quality) was evaluated using Metrosideros leaf litter from each of the sites decomposed in a common site. Site characteristics were responsible for most of the variation in rates of decomposition in the range of sites and substrates examined. Common substrates decomposed much more rapidly in warm, low elevation sites; apparent Q10 values, calculated on the basis of variation in mean annual temperature with elevation on individual lava flows, ranged from 4 to 11. Litter decomposed slowly in the dry sites, but leaf litter produced in the dry sites decomposed more than twice as rapidly as litter from wet sites when both were measured in the same site. The higher substrate quality of litter from dry sites could be due to trade—offs among nutrient—use efficiency, water—use efficiency, and carbon gain by water—limited Metrosideros. We used these results to test a revision of the CENTURY soil organic matter model that had been designed to simulate the decomposition of surface litter. Simulations accurately matched the pattern but underestimated the magnitude of among—site differences in the decomposition of common substrates in a range of sites. Analyses of both field and simulation results suggested that the decomposition of Metrosideros leaf litter could be limited by nitrogen availability.

Metapopulation structure and migration in the butterfly Melitaea cinxia

Abstract: We describe a spatially realistic metapopulation model and parameterize it for a metapopulation of the butterfly Melitaea cinxia, residing in a network of 50 discrete habitat patches within an area of 15 km2. Parameter values that are difficult to estimate independently are obtained by fitting the model to patterns of patch occupancy and local density. There is no large "mainland" population, and the metapopulation appears to survive at an extinction-colonization equilibrium. Empty patches were smaller than the occupied ones, indicating the extinction proneness of especially small local populations; population turnover was 16% between 2 yr. Density in the occupied patches increased with decreasing isolation and with decreasing patch area, suggesting that migration plays an important role in local dynamics. Mark-recapture results confirmed that migration between local populations was common, in contrast to what is frequently assumed for butterflies with well-defined local populations in discrete habitat patches. The modelling results dem- onstrate that it is possible to have empty habitat patches in a metapopulation in spite of frequent migration, as we observed for M. cinxia. Colonization rate of empty patches may be low for several reasons, including difficulties in mate location at low density (not likely to be important here), conspecific attraction (possibly important), stepping-stone and other forms of nonrandom migration (likely to be important), and weak density dependence (likely to be important). Our results support the assumptions of structured metapopulation models, which demonstrate the possibility of alternative stable equilibria for metapopu- lations in which migration significantly affects local dynamics.

Predicting direct and indirect effects: an integrated approach using experiments and path analysis'

Abstract: Determining the strengths of interactions among species in natural com- munities presents a major challenge to ecology. Using an approach combining experimental perturbations and path analysis, I examined the mechanisms by which birds directly and indirectly affected other members of an intertidal community, evaluated alternative causal hypotheses, and predicted whether interactions among other unmanipulated species would be strong or weak. Comparing treatments with t tests indicated that excluding bird predators with cages caused increases in Pollicipes polymerus, and declines in Nucella spp., Mytilus californianus, and Semibalanus cariosus. However, these conclusions provided no insight into the underlying mechanisms causing the differences. Path analysis permitted insight into the causal mechanisms by making a variety of predictions about the strength of direct interactions: (a) Bird predation negatively affects Pollicipes, but not Nucella, Leptasterias, or Mytilus; (b) Pollicipes reduces Semibalanus and Mytilus abundance because of space competition; (c) Mytilus reduces Semibalanus cover through competition for space; and (d) as prey species, Semibalanus and Pollicipes enhance Nucella density, but Nucella pre- dation does not have important effects on Semibalanus or Pollicipes. Based on the estimated strength of direct interactions, the importance of indirect effects among species could also be predicted. In experiments manipulating Nucella, Pollicipes, Semibalanus, and birds independently of one another, I tested 11 of the interactions predicted by the path analysis; all were supported. Path analysis in conjunction with limited experiments may provide an efficient means to predict important direct and indirect interactions among unmanipulated species within ecological communities.

Synergisms in Plant Defenses against Herbivores: Interactions of Chemistry, Calcification, and Plant Quality

Abstract: Many tropical seaweeds and benthic invertebrates produce both secondary metabolites and calcium carbonate (CaCO3) particles or spicules that serve as possible defenses against consumers. To evaluate the relative defensive value of CaCO3, secondary metabolites, and the potential synergistic or additive effects of the two, we made artificial agar-based "seaweeds" in which we manipulated algal organic content, CaCO3, and the secondary metabolites produced by the calcified green seaweeds Rhipocephalus phoenix, Udotea cyathiformis, and Halimeda goreauii, all of which are relatively resistant to her- bivores. The effects of these manipulations on herbivore feeding were evaluated using three different types of herbivores, the sea urchin Diadema antillarum, the amphipod Cymadusa filosa, and a mixed-species group of small parrotfishes. Addition of finely powdered CaCO3 as 69% of food dry mass had no effect on feeding by parrotfishes, deterred feeding by Cymadusa, and deterred Diadema when food organic content was low but not when it was higher. Although calcification of algal tissues has generally been considered a structural defense that hardens seaweed thalli and makes them more resistant to attack, the decreased feeding on CaCO3-containing foods in our assays occurred without any measurable alter- ation of food toughness. At natural concentrations, semipurified secondary metabolites from Rhipocephalus or Udotea deterred feeding by all three herbivores. In most assays, feeding was depressed more by the addition of metabolites from Rhipocephalus or Udotea than by the addition of CaCO3 even though CaCO3 was added at 1.3-2.2 times the natural concentration for these plants. In contrast, the major metabolite from Halimeda goreauii, when tested alone, did not affect feeding by any of the herbivores. In two of our nine assays, the synchronous combination of CaCO3 and secondary metabolites acted synergistically and deterred feeding significantly more than the sum of the effects of each tested separately. Mechanisms pro- ducing these synergisms are unknown, but it is possible that calcification could also be acting as a chemical defense by altering gut pH in ways that increase the potency of the secondary metabolites. It is common for chemical, structural, morphological, and nutri- tional deterrents to co-occur in individual prey species. For some plant-herbivore inter- actions, the combined effects of these characteristics can be much more than the sum of their separate effects.

The influence of light and nutrients on foliar phenolics and insect herbivory

Abstract: We examined the changes in leaf phenolic chemistry and insect herbivory from saplings of two temperate deciduous species, Liriodendron tulipifera (tulip poplar) and Cornusflorida (dogwood), planted in five microenvironments in Gilmer County, Geor- gia, USA. The experimental design permitted comparisons between saplings grown in an open field, under shade cloth within the field, on the edge between field and forest, in forest understory, and within canopy gaps established within the forest. Half of the trees in each microenvironment were fertilized. Leaves from each tree were sampled at the end of the growing season in 1989 and 1990 and analyzed for toughness, percent dry mass, total phenolics, hydrolyzable tannins, condensed tannins, and insect herbivory (percent leaf area damaged). The shade-tolerant dogwood saplings contained higher levels of total phenolics and hydrolyzable tannins than the shade-intolerant tulip poplar saplings. Dogwood generally had lower levels of herbivory. These results support earlier studies suggesting that slow- growing, shade-tolerant species tend to have higher levels of phenolics and experience lower levels of herbivory than fast growing, shade-intolerant species. However, dogwood leaves contained lower levels of condensed tannins and were as tough as tulip poplar leaves. Sunlight availability had a significant positive influence on levels of phenolics in both species. Leaf phenolics generally increased with greater insolation from forest to field and when sunlight was greater within field or forest habitats. However, the levels of tannins in dogwood saplings only dropped significantly in the deep shade of the forest. The similar levels of dogwood phenolics in most microenvironments are indicative of the relatively high photosynthetic efficiency of this species in reduced light environments. Overall, these results are consistent with carbon/nutrient balance theory that predicts trade-offs in the allocation of photosynthate from defense to growth as light declines. Levels of insect herbivory and total phenolics were inversely related for dogwood. However, the relationship with tannins was less apparent. Herbivory on tulip poplar was unrelated to changes in phenolics, possibly reflecting the greater chemical diversity of that species. Fertilization increased the biomass of both species, but had no apparent influence on levels of leaf phenolics or insect herbivory. The lack of a fertilization effect was unex- pected in light of previous suggestions that fertilization results in reduced phenolics and increased herbivory.

Insectivorous Birds Increase Growth of White Oak through Consumption of Leaf-Chewing Insects

Abstract: We examined the indirect effects of insectivorous birds on plant growth through consumption of leaf-chewing insects in a Missouri Ozark deciduous forest. Over a period of 2 yr, we compared insect numbers, leaf damage, and resultant plant growth for control saplings of white oak (Quercus alba), vs. saplings that we caged to limit access by insectivorous birds but not herbivorous insects. In a third treatment, we sprayed insecticide on young white oak trees to determine the impact of the insect herbivores on plant growth in the presence of birds. The total number of insects encountered on plants inside of cages was twice that on control plants for both years. Insecticide spraying reduced herbivore numbers substantially but did not eliminate them. As a result of the treatments, cage plants suffered 25% leaf area loss, control plants 13%, and spray plants 6% at the end of the first season (34, 24, and 9%, respectively, for the 2nd yr). As a result of the differences in damage, cage plants produced one-third less total aboveground biomass compared to insecticide- treated plants, with control plants producing intermediate values. Differences in biomass production were due mainly to decreased leaf biomass, which in turn was associated with decreased leaf size in subsequent years as a result of high damage during the previous year. This is the first terrestrial ecosystem study to demonstrate a significant impact of insectivorous birds on plant growth. Our results suggest that over the long term observed declines in North American populations of insectivorous birds may reduce forest produc- tivity because of potentially higher numbers of leaf-chewing insects and the concomitant negative effect on plant growth.

Trade-offs in diel vertical migration by zooplankton: The costs of predator avoidance

Abstract: Diel vertical migration (DVM) of zooplankton is a behavioral antipredator defense that is shaped by the trade—off between higher predation risk in surface waters and reduced growth in deeper waters. We conducted two laboratory experiments to quantify the costs connected with DVM. In the first experiment, Daphnia magna were kept individually in thermally stratified flow—through tubes and exposed to seven different concentrations of fish—exuded kairomones. Above a threshold concentration, the strength of migration increased with increases in the concentration of fish exudates. Enhancement of migration resulted in a lower mean ambient temperature experienced by the animals and marked decreases in individual growth and reproduction rates. In order to separate costs due to low hypolimnetic temperatures in a stratified system from costs due to reduced food concentrations in deeper waters, we conducted an experiment with a 2 ° 2 factorial design (fish presence vs. absence and high vs. low food conditions). Differences in mean ambient temperature between Daphnia that performed DVM and nonmigrating animals were found to have a much stronger impact on life history parameters than food effects. A reevaluation of field data on DVM in Daphnia further supports the view that vertical gradients are more important than food gradients.

Plant production and soil microorganisms in late-successional ecosystems: a continental-scale study'

Abstract: Annual C inputs from plant production in terrestrial ecosystems only meet the maintenance energy requirements of soil microorganisms, allowing for little or no net annual increase in their biomass. Because microbial growth within soil is limited by C availability, we reasoned that plant production should, in part, control the biomass of soil microorganisms. We also reasoned that soil texture should further modify the influence of plant production on soil C availability because fine-textured soils typically support more microbial biomass than coarse- textured soils. To test these ideas, we quantified the relationship between aboveground net primary production (ANPP) and soil microbial biomass in late-successi onal ecosystems dis- tributed along a continent-wide gradient in North America. We also measured labile pools of C and N within the soil because they represent potential substrate for microbial activity. Eco- systems ranged from a Douglas-fir forest in the western United States to the grasslands of the mid-continent to the hardwood forests in the eastern U.S. Estimates of ANPP obtained from the literature ranged from 82 to 1460 g-m~ 2 -yr~'. Microbial biomass C and N were estimated by the fumigation-incubation technique. Labile soil pools of C and N and first-order rate constants for microbial respiration and net N mineralization were estimated using a long-term (32 wk) laboratory incubation. Regression analyses were used to relate ANPP and soil texture with microbial biomass and labile soil C and N pools. Microbial biomass carbon ranged from 2 g/m2 in the desert grassland to 134 g/m2 in the tallgrass prairie; microbial N displayed a similar trend among ecosystems. Labile C pools, derived from a first-order rate equation, ranged from 115 g/m2 in the desert grassland to 491 g/m2 in the southern hardwood forest. First-order rate constants for microbial respiration (fc) fell within a narrow range of values (0.180 to 0.357 wk~'), suggesting that labile C pools were chemically similar among this diverse set of ecosystems. Potential net N mineralization rates over the 32- wk incubation were linear in most ecosystems with first-order responses only in the alpine tundra, tallgrass prairie, and forests. Microbial biomass C displayed a positive, linear relationship with ANPP (r2 = 0.51), but was not significantly related to soil texture. Labile C also was linearly related to ANPP (r2 = 0.32) and to soil texture (r2 = 0.33). Results indicate that microbial biomass and labile organic matter pools change predictably across broad gradients of ANPP, supporting the idea that microbial growth in soil is constrained by C availability.

Acorn Production by Oaks in Central Coastal California: Variation within and among Years

Abstract: We measured acorn production by individual oaks of five different species at Hastings Reservation in central coastal California between 1980 and 1991. Variation in acorn production was considerable both within and among years and was generally un- correlated between species. Compared to expected values, variance within years in the size of acorn crops was small, while variance among years was high. Crop failures occurred fairly frequently and large crops in successive years were observed, but no more than expected by chance. Individual trees masted at species-specific intervals, but these patterns did not result in regular masting cycles at the population level. We compared these patterns to predictions of four hypotheses for the evolution of seed production patterns. Observations did not support the hypotheses that production patterns track resource availability (the "resource matching" hypothesis) or that they have evolved to attract seed dispersers (the "seed dispersal" hypothesis). However, they are generally consistent with two additional hypotheses, that masting in these wind-pollinated species evolved because of a proportional increase in fertilization and seed set during mast years (the "wind pollination" hypothesis) and that masting has evolved to maximize the prob- ability of avoiding predation via predator satiation (the "predator satiation" hypothesis).

The Effect of Larval Density on Adult Demographic Traits in Ambystoma Opacum

Abstract: Factors that affect traits of aquatic larvae of amphibians may have long- lasting effects on terrestrial juveniles and adults. I manipulated larval densities of marbled salamanders, Ambystoma opacum, in large-scale field enclosures during 2 yr, released the juveniles that metamorphosed from these enclosures, and tested for effects on adults that returned to the pond during 6-7 subsequent breeding seasons. Individuals from low larval density treatments tended to have greater lipid stores at metamorphosis than those from high densities and survived longer in a laboratory inanition study. In the field, individuals that experienced low larval density returned for their first reproductive bout as larger adults than those from high-density treatments. For 5-yr-old females released in 1986, low larval density was linked to greater clutch size; clutch size in 4-yr-old animals from the 1987 cohort did not differ between larval treatment groups. Larval density also influenced age at first reproduction, as animals reared at low densities returned to breed at younger ages. Averaged across both cohorts, the proportion of animals that returned to breed at least once was 21 % for low-density groups compared to 6% for the high-density groups. The larval environment exerted a strong influence on postmetamorphic traits, and thus larval density likely plays an important role in population regulation in both the aquatic and terrestrial phase of the life cycle.

Edge effects in central Amazonian forest fragments

Abstract: Current descriptive models of edge effects describe little more than the relationship between perimeter and area. A more realistic model, one that incorporates the additive nature of edge effects, is presented and tested. The total edge effect at a location within a patch is modelled as the sum of edge effects at points along the edges of the patch, weighted by the distances from the edge points to the location. At four sites in the central Amazon °80 km north of Manaus, Brazil, I measured vertical stratification of foliage in three primary forest habitats: (1) continuous forest close to linear edges, (2) 10—ha forest fragments, and (3) 1—ha forest fragments. Foliage thickness (foliage density along a vertical line) was scored in six height intervals (0—2, 2—5, 5—10, 10—20, 20—30, and 30—40 m). With decreasing distance from a linear edge, understory (0—5 m) foliage thickness increased and overstory (10—30 m) foliage thickness decreased, and the relationship between foliage thickness and distance to the edge agreed with that predicted by the model. Using parameter estimates obtained from the best “fit” of the model to these data from linear edges, understory and overstory thickness in 1— and 10—ha fragments was predicted. Foliage thickness varied extensively among fragments, but agreed quite closely with that predicted by the model, and the additive model performed better than a curvilinear non—additive one.