Abstract: Observations over a 30-yr period revealed a considerable degree of natural variation in the abundance of corals on Heron Island, Great Barrier Reef, Queensland, Australia. Cover ranged from 80%, with a similar large range in colony density, at several temporal and spatial scales. Much of this variation was due to the type, intensity, and spatial scale of disturbances that occurred. Coral assemblages usually recovered from acute disturbances, both on Heron Island and on other Indo-Pacific reefs. In contrast, corals did not recover from chronic disturbances of either natural or human origins, or from gradual declines. Recovery was slower after acute disturbances that altered the physical environment than after disturbances that simply killed or damaged corals. The space and time scales of declines and recoveries in abundance were much smaller on the wave-exposed side of the reef than on the side protected from storms. Recruitment rates were reduced by preemption of space by corals or macroalgae, and by storms that altered the substratum. Thus, the dynamics of abundance in this coral community can be largely understood through the variation in types and scales of disturbances that occurred, and the processes that took place where disturbances were rare.

Abstract: The Yellowstone fires of 1988 affected >250 000 ha, creating a mosaic of burn severities across the landscape and providing an ideal opportunity to study effects of fire size and pattern on postfire succession. We asked whether vegetation responses differed between small and large burned patches within the fire-created mosaic in Yellowstone National Park (YNP) and evaluated the influence of spatial patterning on the postfire veg- etation. Living vegetation in a small (1 ha), moderate (70-200 ha), and large (500-3600 ha) burned patch at each of three geographic locations was sampled annually from 1990 to 1993. Burn severity and patch size had significant effects on most biotic responses. Severely burned areas had higher cover and density of lodgepole pine seedlings, greater abundance of opportunistic species, and lower richness of vascular plant species than less severely burned areas. Larger burned patches had higher cover of tree seedlings and shrubs, greater densities of lodgepole pine seedlings and opportunistic species, and lower species richness than smaller patches. Herbaceous species present before the fires responded in- dividually to burn severity and patch size; some were more abundant in large patches or severely burned areas, while others were more abundant in small patches or lightly burned areas. To date, dispersal into the burned areas from the surrounding unburned forest has not been an important mechanism for reestablishment of forest species. Most plant cover in burned areas consisted of resprouting survivors during the first 3 yr after the fires. A pulse of seedling establishment in 1991 suggested that local dispersal from these survivors was a dominant mechanism for reestablishment of forest herbs. Succession across much of YNP appeared to be moving toward plant communities similar to those that burned in 1988, primarily because extensive biotic residuals persisted even within very large burned areas. However, forest reestablishment remained questionable in areas of old (>400 yr) forests with low prefire serotiny. Despite significant effects of burn severity and patch size, the most important explanatory variable for most biotic responses was geographic location, particularly as related to broad-scale patterns of serotiny in Pinus contorta. We conclude that the effects of fire size and pattern were important and some may be persistent, but that these landscape-scale effects occurred within an overriding context of broader scale gra- dients.

Abstract: Fish assemblages of temperate lakes are structured primarily by an inter- action between piscivory and a small number of environmental variables, but tropical floodplain assemblages have often been viewed as unpredictably structured. We tested the predictability of fish assemblage structure in floodplain lakes of the Orinoco River, Ven- ezuela, in relation to 22 variables describing environmental variation at the microhabitat, habitat, and supra-lake levels. Fish species abundances were estimated through electro- fishing surveys of 20 lakes in three regions for the early and late dry seasons of each of two consecutive years. Canonical correspondence analyses indicated that assemblage struc- ture was predictably related to only four descriptors of lakes: transparency, conductance, depth, and area. Discriminant function analyses revealed that transparency (''clear'': Secchi transparency .20 cm; or ''turbid'': Secchi transparency #20 cm) was tightly associated with the numerical density of six major taxa (82% classification accuracy) and the numerical density of piscivorous species (89% accuracy). Depth and area probably derived their significance from causal relationships to transparency and availability of cover, whereas the influence of conductance arose incidentally through an association with biogeographical zonation. Mantel tests indicated that similarity in structure of assemblages was not strongly related to the distance between lakes. Transparency was a remarkably reliable predictor of species composition. Fish with sensory adaptations to low light were dominant in turbid lakes, whereas visually oriented fishes predominated in clear lakes; seasonal change in- volved decline in the proportion of visually oriented fishes concomitant with a decline in transparency. The effect of transparency on assemblage structure was probably mediated by the relationship of transparency to visibility of prey. The structuring of Orinoco fish assemblages by piscivory, under the influence of transparency as controlled by depth and area, contrasts with previous views emphasizing random assemblage variation in neotropical floodplain lakes and extends the applicability of a conceptual model originally developed for temperate lakes.

Abstract: Predicting the dynamics of natural food webs requires estimates of the strength of interactions among species. The ability to estimate per capita interaction strength from observational data is desirable because of the logistical difficulty of using experimental manipulations to obtain such measures for all species within complex natural communities. In this paper, I derive observational measures of per capita interaction strength having units matching those of dynamic food web models (per capita consumption and assimilation rates). I also highlight the difference between per capita interaction strength (a parameter used in theoretical models) and species impact (empirical measures of total species effect). I then use behavioral observations and population censuses in a rocky intertidal community to estimate both per capita interaction strengths and species impacts on invertebrate prey of Glaucous-winged Gulls ( Larus glaucescens), American Black Oystercatchers (Haema- topus bachmani), and Northwestern Crows (Corvus caurinus). Estimated per capita inter- action strengths exhibited a skewed distribution with many weak interactions and few strong interactions: mean 6 1 SD of log10(interaction strength) 52 1.95 6 1.40 (bird-day/m of shore) 21 . Per capita interaction strength correlated poorly ( r 2 5 0.152-0.157) and nonlinearly with both consumption rates and percentage contribution of a prey species to the diet. Using my observational estimates of per capita interaction strengths, I predicted the species impact of bird predation on different prey taxa. Predictions included strong effects of birds on goose barnacles (Pollicipes polymerus), limpets (Lottia and Tectura spp.), sea urchins (Strongylocentrotus spp.), and large starfish (Pycnopodia helianthoidesand Solaster stimp- soni), but little effect on mussels ( Mytilus californianusand M. trossulus), dogwhelk snails (Nucella spp.), and acorn barnacles (Semibalanus cariosus). I compared nine of the pre- dictions with 126 results of experimental manipulations of birds. The predictions agreed both qualitatively and quantitatively with the experimental results. These findings suggest that observational measures of interaction strength that have units matching those of dy- namical food web models may be reasonable to use in estimating those found in natural communities.

Abstract: Management of ecosystems at large regional or continental scales and de- termination of the vulnerability of ecosystems to large-scale changes in climate or atmo- spheric chemistry require understanding how ecosystem processes are governed at large spatial scales. A collaborative project, the Vegetation and Ecosystem Modeling and Analysis Project (VEMAP), addressed modeling of multiple resource limitation at the scale of the conterminous United States, and the responses of ecosystems to environmental change. In this paper, we evaluate the model-generated patterns of spatial variability within and be- tween ecosystems using Century, TEM, and Biome-BGC, and the relationships between modeled water balance, nutrients, and carbon dynamics. We present evaluations of models against mapped and site-specific data. In this analysis, we compare model-generated patterns of variability in net primary productivity (NPP) and soil organic carbon (SOC) to, respec- tively, a satellite proxy and mapped SOC from the VEMAP soils database (derived from USDA-NRCS (Natural Resources Conservation Service) information) and also compare modeled results to site-specific data from forests and grasslands. The VEMAP models simulated spatial variability in ecosystem processes in substantially different ways, reflect- ing the models' differing implementations of multiple resource limitation of NPP. The models had substantially higher correlations across vegetation types compared to within vegetation types. All three models showed correlation among water use, nitrogen avail- ability, and primary production, indicating that water and nutrient limitations of NPP were equilibrated with each other at steady state. This model result may explain a number of seemingly contradictory observations and provides a series of testable predictions. The VEMAP ecosystem models were implicitly or explicitly sensitive to disturbance in their simulation of NPP and carbon storage. Knowledge of the effects of disturbance (human and natural) and spatial data describing disturbance regimes are needed for spatial modeling of ecosystems. Improved consideration of disturbance is a key ''next step'' for spatial ecosystem models.

Abstract: This study links spatial and seasonal patterns of mortality of the hard clam, Mercenaria mercenaria (L.), in marine soft bottoms with the predation rates and habitat use of its main predator, the blue crab, Callinectes sapidus Rathbun. Patterns of predation on tethered juvenile clams exposed to the natural assemblage of predators were compared among different habitat types in fall and summer. Between-habitat patterns of predation on clams varied with season. In fall, predation on tethered clams was greater in subtidal sand bottoms and just inside the edge of intertidal salt marshes than in intertidal sand flats. In summer, predation on clams was similar in all habitats. Experiments conducted in field enclosures showed that: (a) individual crabs spent more time in the salt marsh habitat than in intertidal sand flats; (b) crab individuals placed in a sand bottom habitat had greater predation rates in high-density prey patches than in low-density patches; (c) individuals had greater predation rates in prey patches located just inside the edge of salt marshes than in intertidal sand flats, when prey density was held constant between the two habitats; (d) at intermediate and high crab densities predation mortality of clams was similar between vegetated and unvegetated habitats; (e) both individual crabs and groups of crabs consumed similar numbers of clams in the two habitat types when large predatory birds (mainly various species of terns, Sterna spp., herring gulls, Larus argentatus Coues, and ring-billed gulls, L. delawarensis Ord) were excluded from enclosures, but the crabs consumed more clams in the salt marsh than in the sand flat habitat in control enclosures where birds were not excluded. In the fall, when Herring and Ring-billed Gulls were abundant in the study area, preference by blue crabs for safer and more profitable habitats may explain the greater predation on clams in salt marshes than in intertidal sand flats. In the summer, when Herring and Ring-billed Gulls were rare and crab densities are 1.5-3 times greater than in the fall, competition with conspecifics may have caused crabs to disperse and feed in intertidal flats and may explain the general lack of differences in predation intensity among habitat types observed in the summer. Thus, patterns of predation and habitat use by blue crabs appeared to explain between-habitat and seasonal differences in predation mortality of clams. Fo- cusing on the variation in the feeding rates of individual predators in response to external conditions can produce the mechanistic understanding of spatial and seasonal patterns of predation needed to understand and better predict the processes that structure benthic marine communities.

Abstract: Many landscapes are characterized by a mosaic of patches in various stages of succession. Whether successional paths dampen, track, or magnify extrinsic variation in initial conditions influences how much historical and site-specific detail is required to explain variation in patch composition. I investigated the patterns and importance of historical effects in a successional marine rocky intertidal community on the central coast of Oregon, USA. Patches in the mid-intertidal mussel bed (M. californianus) were manually cleared in a way that mimicked natural disturbances. In four separate blocks (large patches ∼9 m2), three sets of plots were initiated with their starting dates staggered by one year. Within each set of plots, I manipulated the presence/absence of two groups of early successional sessile species under each of three predator densities. This design allowed me to address the following general questions: (1) What are the separate and interactive effects of successional age, yearly variation, and...

Abstract: Despite the fact that recruitment can significantly influence the population dynamics of benthic marine populations, relatively little is known about the biological and physical processes controlling recruitment. We selected eight closely related coral reef fishes (wrasses in the family Labridae) to examine the temporal and spatial patterns of juvenile recruitment to the Caribbean island of Barbados. We used a comparative approach to study the relationships among patterns of recruitment, early life history traits, and aspects of the physical environment. For 10 wk during each of three peak recruitment (spring) seasons (1990–1992), we used a biweekly census of recently settled juveniles (8–25 mm standard length, SL) to measure the abundance of six congeners, Halichoeres bivittatus, H. radiatus, H. poeyi, H. garnoti, H. pictus, and H. maculipinna, and two confamilial labrids, Thalassoma bifasciatum and Bodianus rufus. Analysis of the otoliths of a sample of collected specimens provided estimates of larval d...

Abstract: This analysis uses information obtained primarily from museum collections to estimate demographic parameters of populations of thrushes (Turdus spp.) from through- out the Western Hemisphere. Adult survival rates were estimated from the proportions of individuals >1 yr old in museum collections, assuming constant population size, discrete reproductive seasons, unbiased collecting, and maturity at 1 yr old. The effects of relaxing each of these assumptions were examined. The resulting biases were found to be small compared to differences between populations, and in several cases, they were opposite to the conventional wisdom that survival rate increases towards the equator. The annual survival rate of adults (S) is estimated without bias by E(S) = A/(A + I), which has a standard error of ((AI)I(A + J)3), where A is the number of adults in a collection and I is the number of immatures. In Turdus, adult and immature birds can be distinguished by the appearance of the secondary coverts, the immature forms of which are retained until the first postnuptial molt at - 1 yr old. The present analysis included 8653 specimens from 30 populations of 19 species from Alaska to southern Patagonia. Additional data tabulated from museum specimens included numbers of juveniles, evidence of breeding, worn plum- age, and molt. Estimated annual survival rates varied from a mean of 0.56 in temperate North American populations, to 0.68 in subtropical South American populations, 0.76 in lowland tropical populations, and 0.80 and 0.85 in tropical montane populations in Central and South Amer- ica, respectively. Samples of adults and immatures restricted to the period immediately prior to the breeding season gave results that were indistinguishable from samples summed over the entire year. Survival rate was most strongly correlated with the difference between maximum and minimum mean monthly temperatures (r = -0.84). Thus, survival is in- versely related to seasonality of temperature. An index to fecundity (J) was estimated by the frequency of juvenile specimens (gen- erally <2 mo old) relative to those of adults in museum collections. This index was inversely related to annual adult survival. A relative index of prereproductive (generally lst-yr) survival (Sa) was calculated as the annual adult mortality (1 - E(S)) divided by J. Estimated Sa was independent of estimated S. Calculations of annual fecundity (F) from field studies on breeding of selected species supported both of these observations. These results, which are consistent with conclusions based upon broader comparisons among birds, suggest several hypotheses: (1) adult survival in Turdus populations is de- termined by physical conditions during the nonbreeding season, particularly various eco- logical effects of low temperature; (2) fecundity is sensitive to density-dependent feedbacks from adult population size and, possibly, to environmental factors correlated with other factors that independently affect adult survival; and (3) lst-yr survival, age at maturity, or both, also are affected by density-dependent factors.

Abstract: A number of hypotheses for hatching asynchrony suggest that the size hi- erarchy among nestlings produced by hatching asynchrony is adaptive and confers benefits to parents. We assessed the costs and benefits of asynchronous hatching in the Green- rumped Parrotlet (Forpus passerinus), a small Neotropical parrot that hatches large clutches very asynchronously. We manipulated eggs to create broods of four, six, or eight young that hatched synchronously or asynchronously. In a second experiment, we tested whether food limits offspring survival by experimentally feeding later hatched young in large asyn- chronous broods. We also examined the premise that food varies unpredictably by sampling seeds throughout several breeding seasons. Experimentally synchronized broods generally fledged as many or more young than asynchronous broods. Synchrony particularly outperformed asynchrony in broods of eight, where food demands should have been greatest. Nestlings had a higher probability of fledging from synchronous broods than from asynchronous broods, from small rather than medium or large broods, and if they were early hatched rather than later hatched. Most mortality in asynchronous broods occurred within 12 d of hatching, and a significantly greater proportion of later hatched chicks died with empty crops than did early hatched chicks. Later hatched chicks grew more slowly than their earlier hatched nestmates, but at fledging they were as heavy or heavier than earlier hatched chicks. Chicks from asynchro- nous broods were slightly heavier at fledging than synchronous chicks, but there was no correlation between fledging mass and the likelihood of being resighted in subsequent years. Cormack-Jolly-Seber model estimates revealed no significant differences in annual survival rates between young fledged from synchronous and asynchronous broods. Female chicks fledged from synchronous broods were recruited into the study population at a lower rate than those from asynchronous broods. Older chicks from reduced broods were less likely to fledge than chicks from broods that fledged all their young. Parents of large synchronous and asynchronous broods provisioned their young at similar rates and did not differ sig- nificantly in their subsequent survival. Females that raised experimentally synchronous and asynchronous broods showed no significant differences in the likelihood, timing, or success of their next breeding attempt. A marginally higher proportion of last-hatched chicks that received supplemental food survived to fledging than last-hatched control chicks, but feed- ing had no effect on penultimate chicks. Seed densities showed a high degree of autocor- relation over spans of 30-50 d. Asynchronous hatching appears to result in the mortality of the smallest young, due in part to the inequitable distribution of food among nestmates, rather than to food limitation, and as a direct result of the size disparities among nestmates. Thus, parrotlet parents appeared to derive no detectable short- or long-term benefits from the staggered hatching of their young through increased nestling growth and survival, reduced parental efforts, or increased parental survival. Although other adaptive benefits from hatching asynchrony are possible that were not tested directly in these experiments (e.g., ''insurance'' that some nestlings will survive), they seem insufficient to account for the extreme hatching asynchrony observed in the parrotlet. Instead, benefits to egg survival derived from the early onset of incubation may offset the costs of asynchronous hatching.

Abstract: We present three models representing the trophic and behavioral dynamics of a simple food chain (primary producers, grazers, and predators) at temporal scales shorter than the scale of consumer rep ...

Abstract: At a subtidal, soft-bottom site in the western Baltic Sea, mussel (Mytilus edulis) patches co-occur with high predator abundances. Sea star (Asterias rubens) biomasses, in particular, exceed reported values considered sufficient for restricting mussels to the intertidal zone. To determine how mussels can persist in the face of intense predation, we decomposed patch space occupancy into the relative contributions of newly arriving individuals (recruitment) and of increases in body size of the individuals already present in the patch over 13 mo. Sea stars, as major predators, were only able to control 77% of the potential per capita recruitment rate of 91 individuals/yr in 2 m depth. The remaining recruitment rate of 21 individuals/yr was sufficient to allow patches to occupy 1.6 times more space per year. Transplantation of patches to 6 m depth, where recruitment is negligible, revealed that sea stars were also ineffective in controlling mussel coverage through consumption of larger mussels (>1 yr, >30 mm shell length). In deeper water, space occupancy of patches through increases in mussel body size was able to balance predation mortality, demonstrating that mussels attained a relative refuge in size at only 33 mm shell length. Based on the measured shell growth rates, mussels attain this size after ≈15 mo. In situ observations of Asterias feeding activity, the ratios between necessary predator sizes to attack prey of a given size, and predator size distributions suggest that sea stars were on average too small to feed effectively on adult (>1 yr) mussels. Probably, Asterias cannot respond to abundant prey and increase its maximal body size at the site because salinities are at its lower tolerable limit (12–18 g/kg). Thus, bottom-up factors such as high prey productivity in concert with subtle size-based ineffectiveness of the predator population allow otherwise unstable predator–prey populations of a generalist predator and its preferred prey to coexist. Although mussel predators were unable to decimate mussels to local extinction, a release of experimental mussel patches from predation with strong recruitment (2 m depth) resulted in an approximately sevenfold yearly areal increase in shallow treatments, which would lead to a 100% mussel cover at the site within 1 yr. Given that mussels can dominate both rocky substratum and soft sediment, we also studied the effect of substratum quality in factorial combination with presence/absence of predation and water depth on mussel abundance. Attachment to stable substratum did not affect recruitment to the patches or patch space occupancy, but it completely prevented patch dislodgment and subsequent drift. In contrast to rocky shores, mussel patch dislodgment may represent the major mode of patch dispersal and new patch formation in soft-bottom environments as demonstrated by a drift collector fence.

Abstract: A model of nutrient uptake was used to determine the factors that control the uptake of ammonium, glycine, and nitrate for an important arctic sedge, Eriophorum vaginatum, because the factors that regulate the nitrogen (N) uptake of plants in natural ecosystems are not well understood and a growing number of studies suggest that organic forms of N, including glycine, are an important source of N for E. vaginatum and other plants. E. vaginatum was selected as an exemplary system to explore nitrogen (N) uptake of a native species in situ, because it appears to be strongly N limited in the field, its N nutrition has been extensively studied, and its root growth habits make it well suited to modeling nutrient uptake. The model accounts for N supply from microbial mineralization and other sources, flux of N through the soil to the root surface, and uptake by the plant root. Based on these simulations, (1) Ammonium, glycine, and nitrate could all potentially make significant contributions to the N nutrition of E. vaginatum. The relative contributions of glycine is difficult to assess, because its behavior in the soil has not been characterized. However, glycine and ammonium contributed in roughly equal proportions formore » most model parameterizations. (2) The importance of factors that regulated modeled ammonium and glycine uptake can be ranked as follows: supply rate > [soil factors (buffer capacity and diffusion coefficient) = root density] > root uptake kinetics. Supply rate was the only factor that regulated nitrate uptake. These results suggest that N uptake kinetics of roots and soil-solution concentrations may not be reliable predictors of whole-plant N uptake in tundra or other natural ecosystems. 48 refs., 9 figs., 3 tabs.« less

Abstract: Noncyclic populations of microtine rodents may be limited within a relatively constant range of densities by generalist predators with a prey base sufficiently diverse to sustain them when rodents are scarce (generalist predator hypothesis). Collared lemmings (Dicrostonyx kilangmiutak) at Pearce Point, Northwest Territories, Canada, are noncyclic and limited to fairly constant low densities in summer by predation, principally by red fox and Rough-legged Hawks. We tested four predictions of the generalist predator hypothesis as a possible explanation for relatively constant lemming densities: (1) predators do not show strong numerical responses to lemming density; (2) the proportion of lemming biomass in predator diets declines with declining lemming abundance, compensated for by increased consumption of alternative prey; (3) predators show a type-III functional response to lem- ming density; and (4) at low densities, predation on lemmings ceases. The first prediction was not satisfied by the principal predators: at very low lemming densities, Rough-legged Hawks did not settle, and breeding success of red foxes and hawks was limited by lemming abundance. However, a number of generalist predators (Golden Eagle, grizzly bear, arctic ground squirrel, Peregrine Falcon, and Gyrfalcon) did not respond numerically to the lowest lemming densities. The second prediction was partly supported: all predators consumed lemmings at a lower rate as lemming densities declined. However, Rough-legged Hawks were not able to compensate fully for the declining consumption by increasing their use of alternative prey, and red foxes were able to do so in one of three years. Regarding the third prediction, foxes showed some evidence of a type-III functional response but hawks did not. As for the fourth prediction, most predators still consumed lemmings at very low densities; lemmings lacked a secure refuge. The Pearce Point system differs from those where microtine dynamics are relatively constant and nonirruptive because of persistent predation by generalists. In terms of biomass, lemmings are the principal prey for their dominant predators. These predators (the semi- generalist red fox and the specialist Rough-legged Hawk) rely on lemmings to breed, but drive them to densities too low to sustain breeding by these same predators in the subsequent spring. In this regard, the system is similar to one driven by specialists. In some winters, however, populations recover because lemmings breed under the snow and most summer predators are absent. As a result, lemming densities in spring are often high enough for specialists and semigeneralists to initiate breeding. When winter breeding and survival fail to allow population growth, hawks and foxes may fail to breed and then leave the system. Even so, summer generalists still persist and continue to consume lemmings, curtailing potential irruptive growth. In this regard, the system is similar to one where prey are relatively constant because of generalists. Community dynamics at Pearce Point can best be understood as a combination of three dominant processes. Summer predation by specialists and semigeneralists results in desta- bilizing declines. Winter breeding, coupled with good survival, can lead to destabilizing growth. However, this growth is curtailed in the following summer by either destabilizing specialist predation or the stabilizing influence of generalist predation. When lemmings are scarce, the semigeneralist red fox and some generalist predators rely on arctic ground squirrels as their primary prey or their principal alternative prey. The ground squirrel appears to be the critical species maintaining this relatively diverse arctic tundra predator community and the relatively constant lemming densities.

Abstract: Strict coevolution requires that interactions among organisms be species- specific. We assessed the relative roles of host- and habitat-specificity in determining the match between a genus of myrmecophytic trees and a guild of obligate plant-ants in the moist tropical forests of Madre de Dios, Peru. Four locally coexisting but habitat-restricted Cecropia species were cultivated in screen tents until all plants had developed myrmeco- phytic traits. Saplings were then placed within replicate blocks of each of two habitat types: riversides and small forest light gaps. Colonization events were recorded every 3 d between June and August of 1992, and queens were later removed from stem internodes for iden- tification and brood censuses. A similar experiment, conducted in September through No- vember of 1993, included just two species of Cecropia hosts. Effects of host species and habitat on queen colonization rates were evaluated by log-likelihood goodness-of-fit tests and contingency table tests. For three ant species, we also conducted queen preference experiments to compare queen behaviors -across a range of host plants. Differences among ants in the extent of habitat-specificity vs. host-specificity provide evidence for multiple evolutionary routes to obligate association with Cecropia. Habitat- specificity exceeded host-specificity in Azteca ovaticeps (Dolichoderinae), for which queen preference experiments revealed no significant discrimination among hosts. This extreme riverside specialist is thought to have descended from generalist live-stem nesters in second- growth habitats. In Azteca australis, host-specificity was strong, and in this species only, directed toward hosts where brood production was most successful. Conflicting habitat associations in the two experiments indicated the weakness or absence of a consistent habitat affiliation in Azteca australis and suggested that colonization frequencies were influenced instead by proximity to foundress sources. Close relatives of A. australis live in exposed carton nests, which may have been positioned ancestrally on key resource plants, e.g., those producing lipid- and amino-acid-rich pearl bodies. Pachycondyla luteola (Po- nerinae) exhibited both strong habitat and host associations and may have undergone pair- wise coevolution with its forest-gap-dwelling primary host. Queens of Camponotus balzani (Formicinae), possibly a recent and secondary associate of Cecropia, were overrepresented in forest gap habitat but were host generalists, underrepresented only on a host with ex- tremely small internodes. Apparently greater host-specificity in C. balzani at later stages of colony establishment may be due to differential post colonization mortality on the various hosts. Attack of ant queens by parasitoid wasps was strongly concentrated in the linear riverside habitat and weak to absent in the patchily distributed forest gap habitat. Due to lower rates of either parasitoid attack or other forms of queen mortality, Camponotus balzani experi- enced greater success in the forest gap habitat, where it was overrepresented in colonization experiments. Historical coincidences and preadaptations appear to have strongly influenced pairings between Cecropia species and their obligate plant-ants and account for much of the "ap- parent" niche partitioning observed in the system. Species-specificity seems to be deter- mined mainly by coincident habitat affiliations of ants and plants ("coordinated dispersal") and by preadapted capacities of ants to distinguish among host-plant species. Multiple mechanisms for species-specificity may be characteristic of relationships in which associates disperse separately from one another (i.e., show horizontal transmission). Our results are consistent with the view that coadaptation and co-cladogenesis are more likely in systems where dispersal of associates is tightly coupled.

Abstract: During a mark–recapture study of Townsend’s ground squirrels (Spermophilus townsendii) on 20 sites in the Snake River Birds of Prey National Conservation Area, Idaho, in 1991 through 1994, 4407 animals were marked in 17639 capture events. This study of differences in population dynamics of Townsend’s ground squirrels among habitats spanned a drought near the extreme of the 130-yr record, followed by prolonged winter conditions. Townsend’s ground squirrels have a short active season (≈4 mo) in which to reproduce and store fat for overwintering. Their food consists largely of succulent grasses and forbs in this dry shrubsteppe and grassland habitat. The drought in the latter half of the 1992 active season produced early drying of Sandberg’s bluegrass (Poa secunda) and was associated with low adult and juvenile body masses prior to immergence into estivation/hibernation. The following prolonged winter was associated with late emergence of females in 1993. Early-season body masses of adults were low in 1993 r...

Abstract: We have studied the net and gross metabolism of Tomales Bay, a temperate climate estuary in northern California. Tomales Bay has proved to be heterotrophic, implying that the bay oxidizes a subsidy of organic carbon from outside the system, in excess of inorganic nutrients supplied to it from outside and in addition to material cycling within it. Net organic oxidation releases dissolved inorganic nutrients, and the system exports these dissolved inorganic products. Dissolved inorganic phosphorus is exported to the ocean via mixing and constitutes the most direct record of net ecosystem production (NEP). Excess dissolved inorganic nitrogen is lost to denitrification. Excess dissolved inorganic carbon largely results in alkalinity elevation and hydrographic export of alkalinity due to sulfate reduction. The negative NEP of this system results in little release of CO2 to the atmosphere, because of this alkalinity elevation. A major purpose of the study was to ascertain the relative importance of various sour...

Abstract: We examined changes in understory vegetation under an intact forest canopy during the first decade following the deposition of tephra (aerially transported volcanic ejecta) during the 1980 eruption of Mount St. Helens, Washington State, USA. Objectives were (1) to document vegetation response to a major disturbance that has received little attention but is widespread and relatively frequent in the northwestern United States, and (2) to analyze vegetation responses in terms of characteristics of the disturbance, responses of growth forms as well as those of species, components of vegetation change, and species autecology. We used permanent plots at four study sites, representing two tephra depths (≈4.5 and 15 cm), to examine understory vegetation change in old-growth, subalpine conifer forests. The two sites at each tephra depth differed in understory vegetation and amount of snowpack at the time of disturbance. At each site, plant cover and density were measured in 100 1-m2 plots with undisturbed tephra c...

Abstract: The foraging location, diving behavior, dietary composition, and feeding rates of female Emperor Penguins (Aptenodytes forsteri) from the Auster and Taylor Glacier colonies in Antarctica were investigated during the 1993 austral winter. The study was conducted between late May and early August, when male emperors remain at the colonies to incubate eggs and females forage at sea for self-maintenance. During winter, two satellite-tracked penguins from Auster foraged ≈100 km northeast of the colony in open water 200–500 m deep, over the outer continental shelf and shelf slope. Ten Auster and four Taylor Glacier penguins that carried time–depth recorders took ≈8 d to reach the ice edge, spent 50–60 d at sea foraging, and took 4 d to return across the fast ice to the colony. The females occasionally huddled together to minimize heat loss while in transit to the ice edge and between foraging days. The penguins foraged on 93.2% of their days at sea and rested for the remainder. On each foraging day, penguins usu...

Abstract: In the southern part of Korup National Park, Cameroon, nine replicate plots (40 × 80 m) in forest with low abundances (5–15% of the basal area of all trees ≥30 cm gbh [girth at breast height]; LEM) and nine with high abundances (45–68%; HEM) of ectomycorrhizal caesalpiniaceous legumes were selected from an earlier enumeration along an east–west 5-km transect. The codominant caesalps in HEM plots were Microberlinia bisulcata, Tetraberlinia bifoliolata, and T. moreliana, and ordination showed the LEM and HEM plots to be floristically distinct. The HEM plots lay in a large well-defined patch of these caesalps, set within surrounding LEM forest. A further contrast to HEM forest was achieved by selecting six LEM and six VLEM plots (very low, ≈0% ectomycorrhizal trees) on a second transect 12 km to the north. Korup has an annual rainfall of 5180 mm with one very distinct 3-mo dry season (December–February). The soils are sandy, acidic, and very phosphorus poor. The aim was to determine whether LEM and HEM plots differed in their litter and soil phosphorus status and the characteristics of their phosphorus cycles. On 13 occasions between August 1988 and September 1990, litter and soils in the surface root layer and the mineral layer 5 cm below were sampled on the main transect and analyzed for phosphorus fractions. This analysis distinguished between inorganic and organic forms and provided various measures of lability. Nitrogen and carbon contents, pH, and moisture contents of each layer, depth of the root layer, and amounts of litter (i.e., litter mass) and soil were the other main variables. In a subset of plots, leaf litterfall was collected over the period and analyzed for phosphorus and nitrogen, and two litter-bag experiments in early wet seasons estimated rates of litter disappearance. Root biomass and change with depth were estimated from pit samples within the plots. HEM plots had slightly higher mean basal area of trees than LEM plots (32.3 and 27.1 m2/ha, respectively) and higher fine root (≤5 mm diameter) biomasses (519 and 364 g/m2, 0–5 cm), but the same mean litter mass (155 g/m2) and annual leaf litterfall (720 g/m2) and very similar disappearance rates (1.82 g·m−2·d−1 on litterfall/mass basis; t1/2 of 53 d from litter bags). Litter mass (and litterfall) reached a clear peak in the mid-dry season in both HEM and LEM plots (slightly earlier in HEM), almost completely disappearing by mid-wet season. The phosphorus concentrations in HEM falling leaf litter was much greater than in LEM (801 and 676 μg/g) leading to greater inputs (1.79 and 1.35 mg·m−2·d−1), while for nitrogen the concentrations and inputs were very similar (means 17.8 mg/g, 35.3 mg·m−2·d−1). An important feature was the marked peak in phosphorus (but not nitrogen) concentration in litter in the HEM plots only in those years that did not follow a mast fruiting. Depth of the root layer was greater in the HEM than LEM plots (8.1 and 4.2 mm), as was carbon content (5.5 and 4.0%), but bulk density was less (0.85 and 1.30 g/cm3). Moisture contents, which tracked the seasons in all three layers, were slightly higher in HEM than LEM, as were clay and silt contents, but pH values were lower (4.19 and 4.59, root layer). The relative differences in the eight soil phosphorus fractions (resin-, bicarbonate- and NaOH-extractable, and chloroform-labile, inorganic, and organic) plus residual and total phosphorus were remarkably consistent between HEM and LEM plots. In the root layer total phosphorus was much higher in HEM than LEM plots (309 and 186 μg/g, respectively) and likewise in the mineral layer (192 and 119 μg/g, respectively): HEM/LEM ratios of ≈1.6 were maintained across nearly all fractions. Covariance analysis indicated that the basal areas of LEM and HEM plots could not account for the differences in phosphorus concentrations. Different fractions followed different trends with time, but these did not match seasons and showed, for the more labile inorganic phosphorus fractions especially, a linear decline over the study period. Interactions between forest type and date (split-plot repeated-measures analysis of variance) were rarely significant even though date itself invariably was. In marked contrast, nitrogen fractions did not decline, and labile organic nitrogen showed clear seasonal peaks. Differences between the LEM forest on the two transects (two occasions in common) were small and inconsistent, supporting the wider contrast with HEM plots. Principal components analysis of the phosphorus and nitrogen fractions showed a strong discrimination between LEM and HEM plots. Calculation with amounts of phosphorus in the top 6 cm of the soil showed very similar patterns to the concentrations. Large ectomycorrhizal trees appeared to have increased the depth of the surface root layer, its phosphorus content, the labile phosphorus fraction (notably its organic component), and to have enhanced phosphorus cycling. The role of ectomycorrhizas in this process is discussed. The decline in labile phosphorus is explained by a phenological and climatic ectomycorrhizal response (PACER) hypothesis that highlights the relationship between phosphorus demand in mast fruiting years and soil phosphorus concentrations. The importance of this in adaptations by ectomycorrhizal caesalps to this strongly seasonal and phosphorus-poor site is considered.

Abstract: A novel dendroecological procedure was developed to elucidate canopy disturbances spanning a >300-yr period for oak (Quercus) forests of central Pennsylvania. Running comparisons of sequential 10-yr ring-width averages may effectively neutralize both short-term (i.e., drought) and long-term growth trends associated with climate while enhancing detection of abrupt and sustained radial-growth increases characteristic of canopy disturbance. Thinning-response studies revealed the conservative tendencies of overstory oak, with substantial basal area reductions (>1/3) required to attain moderate and consistently detectable growth increases. Based on empirical evidence, a minimum growth-response threshold of 25% was established to depict canopy disturbances. This is in contrast to the 50–100% sustained radial-growth release often used to detect disturbance using understory trees in closed forests. Our default threshold was adjusted higher as necessary for those trees highly correlated to climatic trends (as repr...

Abstract: Ecological studies, including those focusing on coastal eutrophication, vary in the emphasis they place on species-level vs. ecosystem-level processes. The degree of variation among interacting species in their response to perturbations to the physical environment is likely to be important in determining when species- or population-level processes will strongly affect attributes measured at higher levels of ecological organization. We conducted mesocosm and small-scale laboratory experiments to determine how low oxygen affects predation rates in a zooplankton–fish larvae–larval predator food web typical of mesohaline areas in the Chesapeake Bay. Dissolved oxygen concentrations in bottom waters of the Chesapeake Bay decline during summer to levels that can be physiologically stressful or lethal to animals dependent on aerobic respiration. Our results indicate that the effects of low oxygen on trophic interactions vary among interacting pairs of species in the food web studied. Low but nonlethal dissolved o...