Showing papers in "Ecological Monographs in 1985"

Ecology of a Grazing Ecosystem: The Serengeti

Abstract: Primary productivity and herbivory were studied in the Serengeti National Park, Tanzania, and Masai Mara Game Reserve, Kenya, during the annual cycle of 1974—1975, and wet—dry season transitions in 1976—1979. Basic state variables measured were aboveground plant biomass inside permanent and temporary fences, and outside fences. Productivity was calculated as the sum of positive plant biomass increments. Control productivity (cPn) was calculated from biomass dynamics inside permanent fences. Temporary fences were moved in concert with grazing by the region's abundant ungulates to estimate actual aboveground primary productivity (aPn). Primary productivity was highly stochastic with productive periods poorly synchronized even among nearby sites. Short—term productivities could be extremely high, exceeding 30 g°m—2°d—1. Grazing animals adjusted their densities in relation to grassland productivity. The average proportion of annual aPn that was consumed by herbivores was 0.66, with a minimum of 0.15 and a maximum of 0.94. Green forage was available everywhere late in the wet season in May but was available only at high rainfall sites in the northwest late in the dry season in November. By the end of the dry season, the residual plant biomass outside fences averaged only 8% of cPn. Nomadic grazers moved seasonally in response to grassland productivity. The growing season ranged from 76 d in low rainfall areas to virtually continuous in high rainfall areas. Annual cPn was linearly related to rainfall and averaged 357 g°m—2°yr—1 over the year and 1.89 g°m—2°d—1 during the growing season. Actual aPn was substantially greater than cPn at most sites, averaging 664 g°m—2°yr—1. Growing season aPn averaged 3.78 g°m—2°d—1. Grazing stimulated net primary productivity at most locations, with the maximum stimulation at intermediate grazing intensities. Stimulation was dependent upon soil moisture status at the time of grazing. Rain had a diminishing effect on primary productivity as the wet season progressed and plant biomass accumulated. Part of the stimulation of grassland productivity by grazing was due to maintenance of the vegetation in an immature, rapidly growing state similar to that at the beginning of the rainy season. Since grazers overrode rainfall—determined productivity patterns, aPn was more closely related to grazing intensity than to ranfall. Grazing was heavier on grasslands that were intrinsically more productive. Rate of energy flow per unit of plant biomass was much higher in grazed vegetation. Grazers ate green leaves almost exclusively during the wet season, but species composition of the diets of different grazers differed markedly. Diets of nomadic grazers were very different in the wet and dry seasons. Vegetation dried out rapidly at the onset of the dry season and dry plant tissues made up a substantial proportion of ungulate dry season diets. However, green forage commonly was more abundant in diets than in the vegetation. Grazing increased both forage quality and its rate of production. Zebras supplemented a high—bulk diet by eating the seeds of awnless grasses. The foraging patterns of different grazers were differentiated by several vegetation properties, including productivity, structure, and species composition, in a manner suggesting resource partitioning. The relationship between the stability of vegetation functional properties and community species diversity was positive in five of seven tests. Greater species diversity was associated with greater biomass stability through the seasons, greater resistance to grazing by a single species of ungulate in both the wet and dry seasons, and greater resilience after grazing. Species diversity was not associated with greater resistance to grazing by several ungulate species or to plant species extinction. Specific properties of trophic web members were identified that produced greater functional stability in more diverse communities. Fire does not appear to have important effects upon the functional properties of the grasslands except for a weak stimulation of productivity in the wet season immediately following dry season burning. Fire did have an important effect upon structural properties of the vegetation that would tend to regulate ungulate feeding. The ecology of neither the plants nor the animals in the Serengeti ecosystem can be understood in isolation; many traits of both suggest coevolution among trophic web members. The functional dynamics of the trophic web suggest that the acceleration of energy and nutrient flow rates due to intense herbivory has resulted in the development of an entire consumer food web due to additive fluxes rather than mere quasi—parasitic fluxes from plants to animals.

Interactions of Bacteria, Fungi, and their Nematode Grazers: Effects on Nutrient Cycling and Plant Growth

Abstract: The most common system responses attributed to microfloral grazers (protozoa, nema- todes, microarthropods) in the literature are increased plant growth, increased N uptake by plants, decreased or increased bacterial populations, increased CO2 evolution, increased N and P mineral- ization, and increased substrate utilization. Based on this evidence in the literature, a conceptual model was proposed in which microfloral grazers were considered as separate state variables. To help evaluate the model, the effects of microbivorous nematodes on microbial growth, nutrient cycling, plant growth, and nutrient uptake were examined with reference to activities within and outside of the rhizosphere. Blue grama grass (Bouteloua gracilis) was grown in gnotobiotic microcosms containing sandy loam soil low in inorganic N, with or without chitin amendments as a source of organic N. The soil was inoculated with bacteria (Pseudomonas paucimobilis or P. stutzeri) or fungus (Fusarium oxysporum), with half the bacterial microcosms inoculated with bacterial-feeding nematodes (Pelodera sp. or Ac- robeloides sp.) and half the fungal microcosms inoculated with fungal-feeding nematodes (Aphelenchus avenae). Similar results were obtained from both the unamended and the chitin-amended experiments. Bacteria, fungi, and both trophic groups of nematodes were more abundant in the rhizosphere than in nonrhizosphere soil. All treatments containing nematodes and bacteria had higher bacterial densities than similar treatments without nematodes. Plants growing in soil with bacteria and bacterial-feeding nematodes grew faster and initially took up more N than plants in soil with only bacteria, because of increased N mineralization by bacteria, NH4+-N excretion by nematodes, and greater initial exploi- tation of soil by plant roots. Addition of fungal-feeding nematodes did not increase plant growth or N uptake because these nematodes excreted less NH4+-N than did bacterial-feeding nematode pop- ulations and because the N mineralized by the fungus alone was sufficient for plant growth. Total shoot P was significantly greater in treatments with fungus or Pelodera sp. than in the sterile plant control or treatments with plants plus Pseudomonas stutzeri until the end of the experiment. The additional mineralization that occurs due to the activities of microbial grazers may be sig- nificant for increasing plant growth only when mineralization by microflora alone is insufficient to meet the plants' requirements. However, while the advantage of increased N mineralization by mi- crobial grazers may be short-term, it may occur in many ecosystems in those short periods of ideal conditions when plant growth can occur. Thus, these results support other claims in the literature that microbial grazers may perform important regulatory functions at critical times in the growth of plants.

Population Dynamics and Life Histories of Foliaceous Corals

Abstract: The population dynamics of five species of foliaceous corals (Agaricia agaricites forma purpurea, A. lamarcki, Leptoseris cucullata, Montastrea annularis, and Porites astreoides) was followed on Jamaican reefs using annual photographic censuses. Overall, population cover, size frequencies, and number of colonies were stable over the monitored period from 1977 to 1980. However, individual colonies were in turmoil: of the original 883 colonies, 315 were killed outright and 499 suffered partial colony mortality (injury) at least once during the 3 yr. Partial mortality generated an additional 189 colonies by fission, while larval recruitment added another 201, and fusion subtracted 40 colonies. The net result was a decrease of <10% in number of colonies. There was considerable variation among years and sites in measured life history parameters, as well as striking differences between species. The most stable populations were M. annularis and A. lamarcki, followed by P. astreoides, A. agaricites, and L. cucullata. Rates of partial— and whole—colony mortality were strongly dependent on colony size for all species. Typically, small colonies either were unharmed, or were killed outright, while most large colonies survived but were injured each year, often by extensive amounts. The amount of tissue lost from a population through injuries was usually much greater than through the death of whole colonies, even in a year which included a major winter storm. Frequently, large corals were split asunder by partial mortality to produce several daughter colonies, which presumably were of identical genotype. Therefore counts of physically separate colonies exceeded the number of genetically distinct individuals (genets), by at least 20%. Individual genets, measured as the lateral extent of known daughter colonies, were frequently up to 5 m across, and for M. annularis and A. lamarcki were certainly several centuries old. Colony extension rates measured in situ were very weakly dependent on depth from —10 to —55 m, and were independent of colony size. Small colonies showed much faster relative changes in area, although even the largest corals continued to grow if they avoided major injuries. Within a size—class, the fates of colonies were diverse because of differential rates of growth and shrinkage, so that size was a very poor indicator of age. Differences in the life history and "mobility" between species are reflected in the taxonomic and morphological composition of coral communities over the reef. Shallow—water assemblages of foliaceous corals are composed of more dynamic, delicately built species, while many deeper water communities are dominated by slower growing, robust species. Ironically, disturbance on coral reefs often seems to favor the organisms most vulnerable to damage.

Mechanical Limits to Size in Wave-Swept Organisms

Abstract: Plants and animals that inhabit the intertidal zone of wave—swept shores are generally small relative to terrestrial or subtidal organisms. Various biological mechanisms have been proposed to account for this observation (competition, size—specific predation, food—limitation, etc.). However, these biological mechanisms are constrained to operate within the mechanical limitations imposed by the physical environment, and these limitations have never been thoroughly explored. We investigated the possibility that the observed limits to size in wave—swept organisms are due solely or in part to mechanical, rather than biological, factors. The total force imposed on an organism by breaking waves and postbreaking flows is due to both the water's velocity and its acceleration. The force due to velocity (a combined effect of drag and lift) increases in strict proportion to the organism's structural strength as the organism increases in size, and therefore cannot act as a mechanical limit to size. In contrast, the force due to the water's acceleration increases faster than the organism's structural strength as the organism grows, and thus constitutes a potential mechanical limit to its size. We incorporated this fact into a model that predicts the probability that an organism will be destroyed (by breakage or dislodgement) as a function of five parameters that can be measured empirically: (1) the organism's size, (2) the organism's structural strength, (3) the maximum water acceleration in each wave, (4) the maximum water velocity at the time of maximum acceleration in each wave, and (5) the probability of encountering waves with given flow parameters. The model was tested using a variety of organisms. For each, parameters 1—4 were measured or calculated; the probability of destruction, and the size—specific increment in this probability, were then predicted. For the limpets Collisella pelta and Notoacmaea scutum, the urchin Strongylocentrotus purpuratus, the mussel Mytilus californianus (when solitary), and the hydrocoral Millepora complanata, both the probability of destruction and the size—specific increase in the risk of destruction were determined to be substantial. It is conjectured that the size of individuals of these species may be limited as a result of mechanical factors, though the case of M. complanata is complicated by the possibility that breakage may act as a dispersal mechanism. In other cases (the snails Thais canaliculata, T. emarginata, and Littorina scutulata; the barnacle Semibalanus cariosus), the size—specific increment in the risk of destruction is small and the size limits imposed on these organisms are conjectured to be due to biological factors. Our model also provides an approach to examining many potential effects of environmental stress caused by flowing water. For example, these methods may be applied to studies of: (1) life—history parameters (e.g., size at first reproduction, age at first reproduction, timing of reproductive cycles, length of possible reproductive lifetime), (2) the effects of gregarious settlement on the flow encountered, (3) the physical basis for patterns of disturbance, (4) the optimum (as opposed to the maximum) size of organisms, and (5) the energetic cost of maintaining a skeleton with an appropriate safety factor. A definitive answer regarding the possibility of mechanical limits to size depends both upon an accurate measurement of the probability of encountering a wave of specific flow parameters and upon factors that are external to the model considered (e.g., life—history parameters). Further, due to their ability to move with the flow, organisms that are sufficiently flexible can escape the size limits imposed on more rigid organisms. Thus, some macroalgae attain large sizes (2—3 m in maximum dimension). The precise role of these factors awaits further research.

Spatial and temporal variation in settlement and recruitment of intertidal barnacles

Abstract: I examined the spatial and temporal partitioning of variation in settlement and early survival of an intertidal barnacle, Tesseropora rosea. Monthly, for 3 yr, I monitored the density of newly settled barnacles (<30 d old) and new recruits (30-60 d old) on each of three spatial scales referred to as: within sites (i.e., replicate areas within 3 M2), among sites within shores (i.e., six sites arrayed horizontally in the mid-intertidal region, with 20-50 m between adjacent sites), and among shores (seven headlands along the New South Wales coast with 11-555 km between them). Sites and shores were initially chosen to be as similar as possible with respect to height in the intertidal region, density of adult Tesseropora, and wave exposure. All observed variation was, therefore, within-habitat variation and not readily related to any of the factors already known to affect the density of settlers or their subsequent survival, such as exposure or intertidal level. There was significant variation in the densities of settlers and recruits on all spatial scales at any given time. Variation among months and among years was also great at any given spatial scale. There were no simple, consistent trends, however, in the variation within any spatial or temporal scale, i.e., variation at any spatial scale interacted strongly with that for any temporal scale. The shores x years interaction (the interaction of large-scale spatial with large-scale temporal variation) produced dominant year-classes on some shores in some years and on other shores in other years. Proportional survival of new settlers also varied significantly on all spatial and temporal scales mon- itored. Thus, much residual patchiness exists in the spatial and temporal distributions of barnacles <60 d old. Previous authors investigating the structure and maintenance of rocky intertidal communities have played down the fact that settlement and recruitment of the component species vary greatly on different scales. Yet, such variation should be explicitly incorporated in the development of theory for marine populations and communities.

Refuges, Biological Disturbance, and Rocky Subtidal Community Structure in New England

Abstract: The effects of two sources of biological disturbance–predation and sea urchin grazing–on the structure of benthic communities inside and outside beds of the horse mussel, Modiolus modiolus, were examined in the rocky subtidal zone off the Isles of Shoals, New Hampshire, USA. Multivariate analysis revealed three major communities between 8 and 30 m in depth: (1) a Modiolus community; (2) a 30 m community; and (3) an 8—18 m community. At all depths, mussel beds contained significantly higher densities of infauna than did other subtidal habitats. The hypothesis that Modiolus beds provide a refuge from predation for the associated community was tested in five manipulative field experiments. Three members of the mussel bed community, the bivalve Hiatella arctica, the sea urchin Strongylocentrotus droebachiensis, and the ophiuroid Ophiopholis aculeata, were used as experimental prey. Consumption of these prey by a guild of generalist predators was observed outside but not inside mussel beds, which indicates that subtidal Modiolus beds provide a spatial refuge from predation. Deaths from predation were significantly higher at night than during the day for Hiatella but not for Strongylocentrotus. At night, crab (Cancer borealis, Cancer irroratus) and lobster (Homarus americanus) predation accounted for all attacks that were directly witnessed, while fish (Tautoglabrus adspersus, Pseudopleuronectes americanus) predation accounted for 71% of the total prey consumed during the day. Such diel differences in predation corresponded with predator abundance patterns. The sea urchin Strongylocentrotus droebachiensis was the most significant agent of biological disturbance during the 1979—1984 study period. Strongylocentrotus intensively grazed the benthos at 8 m in 1982, causing a 79% reduction (from predisturbance levels) in the mean population density of invertebrates outside the mussel beds. This event served as a natural experiment in which to test the hypothesis that Modiolus beds function as a refuge from severe grazing disturbance. Re—sampling of communities inside and outside mussel beds immediately after the grazing disturbance indicated that the Modiolus community exhibited less change in species composition, dominance, and diversity than communities outside the mussel beds. Such comparisons indicate that Modiolus beds afford protection from severe grazing disturbance for infaunal invertebrates. Long—term photographic monitoring of marked mussel beds at 8, 18, and 30 m depth showed that Modiolus beds at all depths persisted for more than 5 yr. Mortality rates of adult Modiolus were low; however, mortality was highest at the shallow site (8 m). Mussel beds successfully resisted the grazing disturbance which eliminated all other biogenic habitats except those created by crustose coralline algae. Modiolus beds are effective refuges because they persist for many years and resist biotic disturbance. In the subtidal communities examined here, lower levels of disturbance inside mussel beds can account for the abundance and spatial distribution of Modiolus—community species. These results demonstrate the functional significance of mussel beds in cold—temperate subtidal regions where predation and sea urchin grazing are major determinants of community organization.

Ecosystem dynamics of a subtropical floodplain forest

Abstract: The carbon, phosphorus, and water cycles of a subtropical floodplain forest, and related ecosystem characteristics, were studied. Located at 750 m elevation in Puerto Rico (latitude 18? N) the forest had 27 tree species, 3059 stems/ha, a basal area of 42.4 m2/ha, maximum height of 17 m, and a leaf area index of 3.3. Palm (Prestoea montana) dominated the forest, and, with two other species, accounted for 68% of the dominance. Throughfall, stemflow, interception, runoff, transpiration, and evapotranspiration accounted for 81.6, 9.8, 8.6, 77.7, 13.7, and 22.3% of annual rainfall (3725 mm) respectively. The stand carbon storage was 28.77 kg/M2, distributed as follows: aboveground 35.4%, vegetation 44.2%, soil to 1 m depth 55%, palms 10.9%. Vegetation biomass was partitioned as follows: leaves 9.8% (75% are palm leaves), wood 68%, and roots 21.8%. Net aboveground primary carbon productivity (NPP) was 876 g-m-2 yr-'. Average litterfall was 2.4 g.m-2* d-' (palm leaves 32%, other leaves 39%, and wood 10%). Half-lives of decaying material were 188, 306, 462, and 576 d, respectively, for palm leaves still attached to the parent tree, dicotyledonous leaves, palm leaves on the ground, and palm trunks. Total organic carbon concentrations in stream water increased with increasing stream discharge (from 2 g/m3 to 30 g/m3). Watershed export of carbon was 50 g-m-2 yr-' (including 12 g m-2 yr- I in the form of leaf litter). Mean P concentration in palm leaves (1.18 mg/g) was twice that in dicotyledonous leaves (0.64 mg/g). Compared to a rainfall phosphorus input to the watershed of 63 mg m-2 yr-', leaching from the canopy was high (167 mg.m-2 yr-'), as was the loss of P from the watershed (611 mg m-2 yr-'). Phosphorus-conserving mechanisms included a high rate of retranslocation in palms (504 mg m-2 yr-'). In spite of these mechanisms, there was a net P loss from the watershed that ranged from 136 to 544 mg iM-2 yr-'. Periodic flooding, poor soil aeration, intensive year-round rainfall, and low atmospheric saturation vapor pressure deficits are believed to be the main driving forces of the floodplain forest, which exhibits many characteristics typical of lowland rain forests and floodplain wetlands. Rates of NPP, litterfall, and biomass turnover (residence time of 14-17 yr) are faster than expected for the climatic conditions, whereas rates of wood production and storage of organic matter in the vegetation and soil profile are lower than expected for the climate.

The influence of adults of some coral reef fishes on larval recruitment

Abstract: I examined the recruitment of reef fishes to isolated experimental standard-size coral colonies supporting residents of each of four planktivorous fishes: (a) Dascyllus aruanus, (b) D. reticu- latus (both at several densities), (c) Chromis caerulea, or (d) Pomacentrus popei. I removed all recruits at approximately weekly intervals, and summed larval recruitment over each of two settlement seasons. I found two types of interactions between resident fishes and recruits: (1) There was increased settlement of three species in sites where there were resident conspecifics. This appears to be due to habitat selection by settling larvae. More larval D. aruanus, D. reticulatus, and C. caerulea recruited to corals with resident conspecifics than to corals with other congeners or vacant control corals. In addition, corals with the highest densities of resident D. aruanus and D. reticulatus received the most conspecific recruits. (2) The presence of high densities of resident D. aruanus and D. reticulatus reduced recruitment of heterospecific larvae in all cases for which data were sufficient to allow a powerful test. The presence of C. caerulea reduced recruitment of some species but not others. This interspecific effect could be due to habitat selection by settling larvae, and/or emigration and mortality caused by aggressive interactions with residents. Some species were unaffected by the presence of resident P. popei, but the data for most species were inconclusive. Interactions of the first type may be the cause of the clumped distributions of other reef fishes, though this does not seem to hold in some known instances. Interactions of the second type do not seem to be directly related to resource availability. There are biological and statistical reasons why the likelihood of finding such interactions will be positively correlated with the supply of recruits. The general significance of these interactions for the structure of reef fish assemblages will depend on (1) the level of larval supply, and (2) the number and abundance of species that show this kind of behavior.

Feeding preferences of tegula funebralis and chemical defenses of marine brown algae

Abstract: I examined the effects of phenolic compounds in marine brown algae on the feeding behavior of the intertidal gastropod Tegula funebralis and other marine herbivores. The effects of algal nitrogen content and thallus toughness were also investigated. T. funebralis showed consistent feeding preferences in the laboratory and in the field in pairwise tests of 13 species of marine brown algae (Phaeophyta) from central California. Relative levels of phenolic compounds, which are known to deter feeding by herbivorous snails, differed greatly between preferred and nonpreferred species of algae. The total phenolic content of the six most preferred species of algae averaged 0.83% of dry mass; the phenolic content of the seven least preferred species averaged 4.53%. Differences in relative tanning ability were similarly large. Tannic acid, a terrestrial plant tanin, when experimentally added to one kelp species, deterred feeding by T. funebralis. These results and analyses of studies of additional species of brown algae also showed consistent differences in the phenolic content between species in the order Laminariales (typically low) and those in the order Fucales (uniformly high). Feeding preferences of other gastropod, echinoid, and crustacean algivores from the northeast Pacific Ocean are strikingly similar to those of T. funebralis; this supports the contention that high levels of phenolic compounds in brown algae are a generally effective feeding deterrent against many kinds of marine invertebrate herbivores. Organic nitrogen content of the algae was positively correlated with the feeding preferences of T. funebralis. However, this was most likely an indirect effect of the significant negative correlation between algal nitrogen and phenolic contents. Thallus toughness of the algae did not seem important in determining overall feeding preferences of Tegula, but may have affected choices between algae that were low in phenolics. I suggest that feeding preferences of Tegula funebralis for brown algae are primarily due to avoidance of algal defenses; positive aspects of food quality (such as attractants or nutritional content) probably play a lesser role in determining preference.

The Structure and Regulation of Some South American Kelp Communities

Abstract: The main objective of this study was to evaluate how physical stress and herbivores influence the distribution, abundance, size frequency, and mortality of the giant kelp, Macrocystis pyrifera, in the southeast Pacific. These factors were studied for the kelp and the echinoid herbivore Loxechinus albus over a wide latitudinal range (56°—42° S) of South America on RV Hero cruises in 1972 and 1973. There were large kelp forests with many large plants in the southern Isla de los Estados and Tierra del Fuego sites. Sources of kelp mortality in these sites include entanglement with drift plants and heavy encrustation of bivalves. This resulted in a considerable amount of drifting kelp, and the sea urchins appeared to have abundant food and did not attack living kelp plants. Nevertheless, the urchin densities usually were relatively low and the Loxechinus size frequencies skewed to larger size classes, suggesting that their populations may be limited by larval availability in the Westwind Drift. There were also some large kelp forests in the northern Golfo Corcovado and Isla Chiloe areas, but here the kelp forests seemed relatively ephemeral and Loxechinus appeared to be harvested by humans. The intermediate latitudes (54°—46° S) in areas semiexposed to oceanic waves were characterized by situations in which Loxechinus overexploit the kelps and maintain urchin—coralline algae "barren grounds." In these areas kelps occur only where urchin access is restricted by wave exposure, ephemeral clumps of Desmarestia ligulata, and rarely, by a predator, the asteroid Meyenaster gelatinosus. In many areas the shallow distribution of Macrocystis was restricted by competition with the large fucoid Durvillaea antarctica. These relationships were evaluated by removal and addition experiments. Much more important than latitudinal relationships to both kelp and urchin distribution and abundance patterns were exposure gradients, from shores subject to strong oceanic swells to protected fjords. Transect data suggest that Loxechinus disappear from protected waters before Macrocystis. Thus in some semiprotected habitats there are very dense patches of kelps without grazers; kelp removal experiments in these habitats suggest strong density—dependent kelp interactions. In still more protected sites, the Macrocystis growth form changes, the plants appear to have low fertility, and there is very low recruitment. Summarizing, the Loxechinus population seems restricted by larval recruitment in the face of the Westwind Drift in the far south, perhaps by sedimentation or fresh water in protected fjords, and by human fishing in the north. The very low Loxechinus densities in the far south are correlated with large kelp forests in which the main kelp mortality results from drifting Macrocystis plants. The kelp populations appear limited by physical factors in the fjords and by Loxechinus grazing in most other areas.

Dominance and Diversity Maintenance in an Oceanic Ecosystem

Abstract: Disturbance—perturbation, dispersal—reaction, and contemporaneous disequilibrium are similar theories used to explain the maintenance of species diversity in communities. These theories explicitly predict that in patches, on certain time—space scales, there should be substantial shifts in the order of species dominance. There is good evidence that these theories may explain species coexistence in terrestrial and marine systems of sessile organisms. We have tested this set of theories in a mobile pelagic system by examining the order of dominance of copepod species in samples separated in time and space, collected from °30 min to 16 yr apart, and from hundreds of metres to thousands of kilometres apart. We could not detect significant changes among the samples in rank order or in percent similarity of species abundance on any time scale, or on any space scale up to °800 km, either when all 175 species or when only the 30 most abundant were considered. There was small—scale, mesoscale, seasonal, and interannual heterogeneity in physical properties during the time we made our measurements. Although the theories are satisfactory explanations of diversity maintenance in sessile systems, our results fail to validate them in our mobile pelagic system. Because there were episodes of significant physical variability and because of the long—term species equilibrium and constancy of dominance, we believe our highly diverse community to be resilient and robust, rather than fragile. The regulatory forces are strong and almost certainly biological, rather than physical, but we cannot identify them.

Larval Growth and Survivorship of the Black Swallowtail Butterfly in Central New York

Abstract: The objectives of this study were to measure larval growth and mortality rates in a butterfly population and to determine how these rates vary locally in space and time. The survivorship and growth rates of 1353 Papilio polyxenes larvae, placed on natural food plants in typical habitats, were followed during both generations of 1973 and 1974. In each year and generation, cohorts of first- instar larvae were set out on three commonly used host-plant species and at two elevations. Larvae reaching the fifth instar were returned to the laboratory, where the surviving pupae gave rise to 87 adults. Together with estimates of egg mortality in the field (50%) and the sex ratio of emerging adults (50/50), this result indicates that females must lay at least 60 eggs, on average, to replace themselves once in the next generation. Stage-specific mortality rates (qx) averaged 0.59, 0.33, 0.35, and 0.41 for the first four instars, respectively, approximating a Type II survivorship curve (constant mortality rate). Both the shapes of survivorship curves and the probability of reaching the fifth instar were highly variable from one treatment to another, however, providing a heterogeneous pattern of survivorship in space and time. Survivorship was greater in 1973 than in 1974, probably due to different weather conditions; it was greater at high-elevation than at low-elevation sites; and it was greater on wild carrot, Daucus carota, and on parsnip, Pastinaca sativa, than on poison hemlock, Conium maculatum. Survivorship of larvae placed artificially did not differ from that of larvae hatching from wild-laid eggs on the same host species at the same time and place. Arthropod predators attacking larvae included spiders as well as hemipterans of the families Nabidae, Reduviidae, Coreidae, and Pentatomidae. Parasitoids emerging from larvae or pupae be- longed to the families Phoridae, Tachinidae, Ichneumonidae, and Braconidae. Our results, supple- mented by predator-exclusion experiments in 1977, indicate that predation is the major source of larval mortality, small (arthropod) predators having the greatest impact on younger larvae and large (vertebrate) predators the greatest impact on older stages. Larvae grew faster in 1973 than in 1974, probably because of more favorable weather in 1973. Growth was faster at low- than at high-elevation sites and markedly faster on poison hemlock than on wild carrot or parsnip. Unpredictability of larval survivorship in space and time is likely to favor widely spaced oviposition by P. polyxenes females. Such behavior may also be favored by variability in larval growth rates, since adult emergence times may be crucial to the reproductive success of surviving male progeny.

Interspecific Competition Among Hawaiian Forest Birds

Abstract: The object of this study was to determine whether interspecific competition modified local geographic distribution, after taking into account the effect of habitat structure. The tendencies for 14 passerine birds to have positive or negative associations were examined, using 7861 sample points in seven native forests on the islands of Hawaii, Maui, and Kauai. All birds were at least partly insectivorous and were fairly common in forested areas, although some fed chiefly on nectar or fruit. Species-pairs were classified as primary or secondary potential competitors based on general dietary similarity. To evaluate the association between species and to account for the effect of individual species' habitat preferences, partial correlations were computed for each species-pair in a study area from the simple correlations between the species and 26 habitat variables plus two quadratic terms to represent nonlinearity. The partial correlations represented a short-term ("instantaneous") assessment of the strength of competitive interactions, and did not reflect the accumulation of competitive displacement through time. Of 170 partial correlations in the analysis, only 10 indicated significant negative as- sociation. The general pattern was of positive association (76 significantly positive partials), which probably resulted from flocking and from attraction of birds to areas of resource superabundance. Two species-pairs showed consistent patterns of negative partial correlations over several adjacent study areas, the Japanese White-eye/Iiwi in montane Hawaii, and the Japanese White-eye/Elepaio in windward Hawaii; both patterns could be reasonably attributed to direct competition. Species-pairs were grouped by the native or exotic status of the component species. Native/exotic pairs had a significantly greater proportion of negative partial correlations (37%) than either native/ native pairs (8%) or exotic/exotic pairs (0%). This pattern was consistent across the seven study areas and appeared to reflect the occurrence of interspecific competition along a broad and diffuse ecological "front" between a co-evolved native avifauna and recently introduced exotic species. The role of competition in the pattern was corroborated by the significantly higher proportion of negative partial correlations among species-pairs of primary potential competitors than among those of secondary potential competitors. Our results suggested that t47% of the primary potential competitors among native/exotic species-pairs may experience at least small depressions in local population density due to competition. Although the negative correlations were for the most part small (average negative r = 0.06), one species could eventually replace another as spatial displacement accumulated through time. The Japanese White-eye appeared to have a principal role in native/exotic interactions, with 62% of the partial correlations between it and native primary potential competitor species being negative. Noteworthy implications were that (1) it was important to account for the habitat responses of individual species when studying the role of interspecific competition in modifying small-scale geo- graphic distribution; (2) competition was frequently sporadic in its geographic occurrence and in the species affected, thus supporting Wiens' (1977) theory of competition; and (3) as a consequence, the role of interspecific competition in modifying distribution may be difficult to detect statistically with small data sets.

The Characterization of Vole Population Dynamics

Abstract: The population cycles of meadow voles (Microtus pennsylvanicus) were studied in southeastern Manitoba, Canada, from 1968 to 1978 on a 32-ha old field and in surrounding boreal forest. Vole density was monitored on mark-recapture live-trapping grids trapped at 2- to 4-wk intervals throughout the year. This was supplemented by a 3-d spring and a 30-d summer snap-trapping survey. Multiannual and annual fluctuations in density of Microtus were independent of a diversity of patterns seen in other small mammal species. Principal components analysis of demographic statistics was used to characterize changes in seasonal reproduction, survival, and dispersal that accounted for the observed numerical fluctuations. Conditions during a catastrophic winter population decline in 1970-1971 were compared with those in 1974-1975, when numbers recovered quickly after a modest spring decline. Voles were in good condition before and during both declines, as evidenced by normal body composition indices. Large body masses, characteristic of peak populations, preceded both population declines. The main conditions associated with the 1970-1971 decline were unusual patterns in social behavior and dis- persal. Changes in social behavior in the field were reflected in unprecedented numbers of multiple captures in Longworth traps before and during this decline. Field observations were supported by unusually high frequencies of amicable behavior in neutral arena bouts staged between males removed from a control population. Little dispersal was detected with conventional live-trapping during the winter period of chronic mortality in 1971. In contrast, a period of acute mortality, resulting in the near extinction of the population in March 1971, was associated with considerable dispersal. We propose two alternative hypotheses consistent with the unpredictable and episodic nature of vole population declines: (1) an immunological dysfunction exists in voles that makes them susceptible to naturally occurring microparasites or (2) opportunistic, pathogenic microparasites exploit the changes in dispersal and social contacts that occur as a normal part of the vole population cycle.

Water and nutrient outflow from contrasting lodgepole pine forests in wyoming

Abstract: Factors affecting water and nutrient outflow beyond the rooting zone were studied during a 3-yr period, using data from eight contrasting stands of lodgepole pine (Pinus contorta ssp. latifolia) forest in southeastern Wyoming and the output of a hydrologic simulation model (H20TRANS) based on tree physiology. Nutrient outflow during a specific time period was estimated by multiplying simulated water outflow times element concentrations in the soil solution, the latter determined from samples collected periodically near the bottom of the rooting zone. Estimates of actual evapotranspiration (ET) for the period from early spring to late fall ranged from 21 to 53 cm, which was 33-95% (xZ = 73%) of total annual precipitation. For all stands and years, transpiration accounted for 50-61% of ET, and 9-44% of the transpiration occurred during the spring drainage period (vernal transpiration, VT). Estimated VT and outflow varied considerably among the stands, with VT accounting for 4-20% of the snow water. Outflow occurred only during the snow melt period and accounted for 0-80% of the snow water. Snow water equivalent varied annually by 300% or more. Nutrient outflow from the different stands also varied greatly. Ratios between simulated annual outflow and atmospheric inputs (bulk precipitation) were consistently > 1.0 for Ca, Na, and Mg; were consistently < 1.0 for N; and ranged from 0.3 to 2.0 for P and from 0.2 to 3.3 for K. Much of the variability in water and nutrient outflow can be attributed to the degree of biotic control, with water outflow affected by a different combination of factors than nutrient outflow. H20TRANS was used to simulate the effects on outflow of different snow water equivalents and different total leaf areas. One result of the simulations was that nitrogen appears to be retained even at the highest levels of water outflow. Another was that increases in water outflow following reduction in leaf area were proportional to the leaf area removed. The results indicate that stands differing in site or habitat type experience different rates of water and element losses at different times during the snow melt season, and contribute differentially to streamwater quality and hydrograph shape. Factors affecting outflow are discussed in the context of successional trends, common perturbations including timber harvest, and hypotheses pertaining to nutrient conservation in terrestrial ecosystems. Nutrient retention in the snow-dominated lodgepole pine ecosystem appears to be primarily dependent on evergreen leaf area, duration of the VT period, and high carbon/nutrient ratios of the forest floor. Net losses of limiting nutrients probably occur primarily in pulses after abiotic perturbations such as fire.

The Development of a Tidal Marsh: Upland and Oceanic Influences

Abstract: A stratigraphic study was undertaken to determine the historic role of watershed and sea—level changes on the development of Fresh Pond Marsh on Long Island's north shore. Pollen analytic, radiometric, and macrofossil techniques were aimed at differentiating among marsh, watershed, and regional changes over the last 1000 yr. A study of surface pollen distribution was conducted to determine pollen source and tidal influences on pollen deposition and to evaluate comparability of fossil pollen records from various intertidal environments. Regional and local inputs along a transect that included several tidal regimes were identified. Pollen counts from these different environments were comparable and proved useful for marsh reconstructions. Pollen analyses from the coastal marsh and a nearby lake distinguished changes in upland vegetation and cultural patterns within the watershed from those occurring regionally. Records of local pollen, foraminifera, macrofossils, rhizomes, and lithology indicated that the history of Fresh Pond Marsh has included periods of both tidal and freshwater conditions. Close dating control provided by 210Pb measurements and historically documented changes in indicator pollen taxa identified marsh responses to external influences, including dynamics of a baymouth bar, land clearing, agriculture, mosquito ditching, and sea—level fluctuations. Results showed that (1) tidal marsh sections can provide sensitive records of both upland and marsh vegetation histories, (2) pollen and macrofossil records can be closely linked to tide—gauge records and are responsive to short—term changes in sea level with a high degree of temporal resolution, and (3) upland influences can play an important role in determining the course of plant succession in the intertidal environment. Pollen provides the most sensitive record of marsh and upland development, as long as pollen source is accounted for.

Seasonal Reproductive Patterns in Five Coexisting California Desert Rodent Species

Abstract: For 3 yr we studied the reproductive responses of desert rodents in the Owens Valley of eastern California (average annual precipitation 14 cm): four nocturnal heteromyids–the kangaroo rats Dipodomys microps and D. merriami and the pocket mice Perognathus formosus and P. longimembris–and one diurnal sciurid, the antelope ground squirrel, Ammospermophilus leucurus. Reproductive status was assessed by autopsies of adults trapped at approximately monthly intervals. Reproduction differed conspicuously among the five species. Our analysis illustrates effects of body size, phylogenetic association, and adaptation to the desert environment upon reproductive performances and associated life—history parameters. Most breeding occurs in late winter and early spring. Winter rains cause a series of pulses in vegetation growth and an attendant increase in availability of water in food plants, which contribute to rodent reproduction. Among the four heteromyids, onset of breeding is sequential according to body size, wit...

Palynological Evidence for Postglacial Change in the Position of Tree Limit in Labrador

Abstract: The postglacial history of the forest-tundra zone in Labrador is inferred from the pollen stratigraphy of sediment cores from four lakes which differ considerably in the size and relative relief of their hydrologic catchments. Local pollen-assemblage zones are established independently for each pollen sequence by numerical methods. The relative pollen stratigraphies, interpreted through multi- variate statistical comparison with modem pollen spectra from the region, show an overall increase in the tundra component of the forest-tundra vegetation since z3000 BP. Decreasing absolute fre- quencies of Picea pollen in the sediments are interpreted as evidence of the decline in tree numbers within the lake catchments, and therefore as a potential record of lowering of the altitudinal tree limit. The records show diachronous changes between sites because of differences in catchment elevation and relief. Tree limit has apparently fallen at least 40 m between 3000 and 1000 BP. A further fall of 30 m may have taken place by 250 BP. High ground in northern Laborador makes it unlikely that any equivalent southward displacement of latitudinal tree line can be identified.

Avian Community Dynamics in Desert Grasslands: Observational Scale and Hierarchial Structure

Abstract: Avian communities living in a complex environment were studied by censuses of singing males, using several levels of resolution which focussed on different properties of the community. Within each level of resolution, various community parameters in two grassland habitat types were calculated over the entire breeding season, and for census intervals within the breeding season. The community level of resolution focussed on community—wide patterns, such as total density and total biomass, in order to reveal constraints operating on all species in each habitat. An intermediate level of resolution focussed on relative properties of species such as relative abundances, species richness, and dominance concentration. At the individual species level of resolution, densities and identities of species were the properties upon which the analysis focussed. These techniques allowed characterization of responses of the avian communities to complex changes in the system in which they were embedded. Long—term averages in monthly rainfall on the sites used for this study indicated a bimodal distribution of rainfall. The first peak occurred in December and the second in July and August. During the two years of study, summer rainfall was similar, but winter rainfall in 1983 was °2—3 times as great as in 1982. Avian communities in habitats with mesquite trees (mesquite savannah) responded differently than communities in grassland habitats, and these responses could be seen at different levels of resolution. Peak densities and biomass occurred during May—June in mesquite savannah habitats, while grassland communities achieved highest densities and biomass during July and August. Average density per individual showed little seasonal pattern, but was lower for mesquite savannah communities than for grassland communities. Species richness increased in mesquite savannah habitats in 1983, but decreased in grassland habitats that year. Patterns of evenness and dominance concentration suggested that in 1983, rare species of large body size were added to the community in mesquite savannah, while in grassland habitats, species already present during 1982 increased in density. A community ordination revealed that mesquite savannah habitats were dominated numerically by small—bodied insectivorous species during the early part of the breeding season but became similar to grassland communities in July and August. Grassland communities were dominated by large—bodied emberizine finches. These characteristics of the avian communities corresponded closely to the phenology of important primary producers. Mesquite produces leaves high in nutritional value and produces flowers during April—June; thereafter older leaves decline in nutrient content and flowers disappear. These changes in mesquite productivity undoubtedly produce corresponding changes in insect communities exploitable by small—bodied insectivores. Perennial grasses concentrate aboveground productivity to July—August and insect communities associated with grasses increased in biomass during this period, thus increasing resources available for breeding. These results suggest that avian community responses to complex changes in their environments can be identified and related to outside inputs into the environment, such as rainfall. Identification of community responses is facilitated by emphasis on several observational scales. Communities appear individualistic to observers because (1) adaptational units of species are probably much larger than study areas; (2) species ability to adapt to local conditions might be diluted by gene flow among populations; and (3) if most evolutionary change occurs during speciation, then species adaptations will reflect a relatively narrow range of selection pressures that operated briefly on the species during the past.

Nectar-feeding birds on trinidad and tobago: comparison of diverse and depauperate guilds'

Abstract: On Trinidad (4520 kM2), 12 bird species in foothills of the Northern Range (11 hummingbirds and Coerebaflaveola, a passerine) often use floral nectar for carbohydrate food. A subset of 6 (5 hummingbirds and Coereba) occur on Tobago, a smaller (295 kM2), more isolated island. For 13 mo we compared these assemblages at study sites having similar climates and similar sets of bird-visited flowers (34 species at Trinidad sites, 31 on Tobago). Density of nectar resources for birds varied seasonally > 100-fold on each island. During peaks in total flower density, much nectar available to birds went unused. In contrast, during a flower-poor 3-mo period (September-November 1977), nectar available in flowers was held near zero because supplies were consumed as fast as they were secreted. Competition for this limited supply of nectar appeared to be intense on both islands. Diets of coexisting bird populations diverged, especially at the Tobago site, where only 3 nectar-feeding bird species, with disparate morphologies, persisted: 1 short-billed hum- mingbird, 1 long-billed hummingbird, and Coereba. At other seasons, though, a variety of bird species consumed nectar on both islands, and often diets of different species were nearly identical. Despite the twofold difference in species richness, the two bird assemblages used nectar resources similarly, contrary to the theory that species-rich and species-poor guilds should differ in patterns of resource use. There were no statistical differences between the islands in the ratio of demand for nectar to nectar supply (expected by theory to be lower on the average on species-poor Tobago), monthly variation in that ratio (expected to be greater on Tobago), or breadth of the entire guild's diet (expected to be lesser on Tobago). One reason may have been that supplies of nectar at both sites fluctuated so widely that demand for nectar in neither bird assemblage could change quickly enough to keep pace. Second, foraging by the hummingbird Chrysolampis mosquitus, a migrant common on Tobago for the 9 mo of the year outside the period of food shortage, made up in part for nectar use by the 6 hummingbird species found on Trinidad but not Tobago. C. mosquitus on Trinidad rarely entered our primary study site, instead shifting to a type of habitat not available on Tobago and relatively little used by other hummingbird species. We found few significant differences in the intensity of competition on the two islands. One hummingbird species, Amazilia tobaci, won most interspecific aggressive encounters over nectar on both islands. There were no significant differences in the intensity of exploitative competition (measured as mean interspecific crowding) experienced by most species on different islands. C. mosquitus, though, may have left Tobago during the 3-mo nectar shortage due to a combination of interference competition (from A. tobaci) and exploitative competition (from all 3 resident species), whereas at that time the Trinidad C. mosquitus population apparently left the island due to a shortage of appropriate flowers in its preferred habitat. Species composition on Tobago can partly be explained by examining traits of species present on Trinidad, the principal source pool. Although individuals in all 5 Tobago hummingbird populations were significantly larger in one or more morphological dimensions (bill length, wing length, mass, or wing disc loading) than conspecifics on Trinidad, the adaptive significance of these size increases is obscure in all but one case (A. tobaci). In addition to Chrysolampis mosquitus and Coereba flaveola, a passerine that consumes fruits and insects as well as nectar, the other birds that used nectar at the Trinidad study sites were: 2 large-bodied, stout-billed hummingbird species that consume many arthropods and that, like C. mosquitus, may sometimes migrate to the mainland; and two distinct 4-species sets of resident hummingbirds, one set with short bills ( 28 mm), often curved, bills. Only one species from each of these 4-species sets was found on Tobago. We examined Trinidad's 4-species sets closely to see if the Tobago representative from each, short-billed A. tobaci or long-billed Glaucis hirsuta, was a random draw with respect to population traits thought to enhance persistence on small islands. Diets of A. tobaci and G. hirsuta on Trinidad were not particularly broad, nor was either Trinidad population particularly free of exploitative competition, relative to all other species there. At the Trinidad study site, though, A. tobaci maintained the highest and least variable demand for nectar (hence, presumably, the highest and least variable population density) of any short-billed species, and G. hirsuta maintained a higher and less variable demand than 2 of 3 other long-billed species. We argue that the relatively low seasonal variation in demand for nectar (and, presumably, in population density) within the second-growth habitat is the most straightforward ex- planation for persistence on Tobago of A. tobaci and G. hirsuta, rather than other species from their respective groups. The result implies that the other 6 Trinidadian populations wander relatively more than A. tobaci or G. hirsuta, respectively, among different habitat types and thus depend more heavily on a regional habitat mosaic not likely to occur on a small island such as Tobago.

The life history of Lithospermum caroliniense, a long-lived herbaceous sand dune species

Abstract: Pollination, fecundity, dispersal, establishment, and survival of Lithospermum caroliniense (Boraginaceae) were examined over five years on sand dunes of different ages at Miller Dunes, Indiana, USA, to determine the effect of various environmental factors on the relationship between reproduction and other life history stages. Lithospermum caroliniense is a self—incompatible, iteroparous, distylous herb that depends on cross—pollination by insects for fertilization. Level of pollination was highly variable from year to year. For each of the two floral morphs, a high proportion of pollen was derived from the other morph in years of abundant pollination. In years when pollen loads were extremely low, fruit—set was limited by pollinator activity. Even with abundant transfer of compatible pollen, fecundity of L. caroliniense averaged less than one nutlet per flower. Individuals were consistent in level of fruit production from year to year. Emergence of seedlings of L. caroliniense was greatest when propagule...