Showing papers in "Ecological Monographs in 1986"

The epizootiology and ecological significance of malaria in hawaiian land birds

Abstract: Laboratory and field experiments were conducted on the island of Hawaii from 1977- 1980 in an effort to determine the impact of avian malaria on the forest birds. At 16 study sites from sea level to tree line in mesic and xeric habitat, birds were captured and bled to determine the host and altitudinal distribution of blood parasites. In the laboratory, six bird species were challenged with malarial parasites to measure host susceptibility. Distributions, activity cycles, and transmission po- tentials of malarial parasite vectors were also analyzed. One species of Plasmodium was present from sea level to tree line, concentrated in the mid- elevational ranges in the ecotonal area where vectors and native birds had the greatest overlap. Native forest birds were: (a) more susceptible to malaria than were introduced species; (b) most likely to have malaria during the nonbreeding, wet season; (c) found ranging lower in xeric than in mesic forests; and (d) found to have a lower prevalence of malaria in xeric forests. Temporal as well as elevational differences in prevalence and parasitemia levels of wild birds were apparent throughout the annual cycle, a result of differing host and parasite responses to biotic and abiotic factors. Avian malaria probably did not reach epizootic proportions on Hawaii until after z 1920. However, since that time it has had a negative impact on the population dynamics of the native forest birds and is today a major limiting factor, restricting both abundance and distribution of these species on the island. In response, a number of native bird species have developed immunogenetic and behavioral responses that reduce the impact of the parasite on host populations.

Partitioning herbivory and its effects on coral reef algal communities

Abstract: Coral reef herbivores were classified into three functional groups according to criteria describing the frequency and intensity of disturbance to the algal community created by their herbivory. These herbivore groups included: microherbivores, which have small foraging ranges ( 1-100 cm2) and a high frequency of grazing any point within their range; the echinoid Diadema antillarum, which has intermediate-size foraging ranges (0.5-1 M2) and intermediate frequencies of grazing any one area within an individual's range; and herbivorous fishes, with large foraging ranges (up to 0.5 ha) and the lowest grazing frequencies at any one site. Algal communities on experimental coral-settling plates were subjected to treatments designed to detect the effects of grazing by representatives of each functional herbivore group on algal biomass, community structure, and primary productivity in a backreef/reefcrest habitat in St. Croix, United States Virgin Islands. Abundances of microherbivores and herbivorous fishes (mainly juvenile scarids) fluctuated over time, while Diadema antillarum abundance remained relatively constant. Grazing intensity (as mea- sured by the rate of algal biomass removal and algal standing crop under different grazing regimes) was greatest for D. antillarum; grazing by D. antillarum resulted in low-biomass, highly productive algal turfs (02 release per unit chlorophyll a per unit time: 1 1.3-22.4 /ug -ug- g h-'). Algal turfs grazed by D. antillarum were dominated by epilithic and endolithic filamentous and crustose species. Grazing by herbivorous fishes led to algal communities with algal turfs of higher biomass than in D. antillarum- grazed treatments; these communities became dominated by a single algal species (Sphacelaria tri- buloides). Fish-grazed algal communities had lower biomass-specific productivity rates than algal turfs grazed by D. antillarum (2.4-8.4 gug -ugg- h'-). Microherbivore-grazed and ungrazed treatments had the lowest biomass-specific productivity rates and eventually became dominated by a few macroalgal species. Although algal biomass in sea urchin-grazed treatments was only 25-50% of the biomass values in treatments not grazed by sea urchins, productivity per unit area was not significantly lower in the former treatments, suggesting a positive effect of sea urchin grazing on algal productivity. This positive effect is a result of the reduction of algal self-shading when grazing is intense and/or the input of regenerated nutrients from D. antillarum excretions. Coral reef plant-herbivore interactions are similar to those in terrestrial grassland-herbivore sys- tems. As in many terrestrial systems, plant-herbivore interactions result in higher overall ecosystem primary productivity and apparently facilitate the flow of energy and materials in this case from the highly productive algal turf component to higher levels in the reef trophic web.

The Influence of Food Habits on the Energetics of Eutherian Mammals

Abstract: The level of energy expenditure in eutherian mammals is correlated with the type of food consumed, especially in medium—sized to large species; some foods (including vertebrates, herbs, and nuts) permit high basal rates, while other foods (including invertebrates, fruit, and the leaves of woody plants) require low basal rates. Various properties of a food, including a low digestibility, a high content of plant secondary compounds, the inability to separate food from non—nutritive particles, and a seasonal reduction in availability, may require a consumer to have a low basal rate of metabolism; all of these factors are exaggerated in importance by the high food intake of large mammals. In small species most foods, except flying insects and seed crops in the desert, permit high basal rates and effective endothermy. Climatic factors modify these relationships to a limited extent, while a specialization in moderately—sized species to arboreal habits leads to reduced activity, smaller muscle mass, and lower basal rate than is found in terrestrial species using the same food resources. Mixed diets generally produce basal rates intermediate to those expected from the component specialist diets. The correlation of basal rate with food habits has the following demonstrable consequences for mammals. (1) The scaling of basal rate usually follows the Kleiber relation, unless food habits require low basal rates, or temperature regulation requires high rates. (2) Mammals that feed on foods associated with high basal rates have a higher reproductive output than mammals of similar size that use foods requiring low basal rates. (3) Predator avoidance is correlated with level of energy expenditure and food habits, terrestrial species with low rates of metabolism relying principally on burrows or passive integumental structures, such as shells, plates, or spines. (4) Most living "conservative" mammals use foods that require low basal rates of metabolism, a condition that protects conservative species from replacement by "advanced" eutherians, which are also required by these foods to have low basal rates.

The Role of Herbivorous Fishes in the Organization of a Caribbean Reef Community

Abstract: Experimental manipulations of grazing intensity were used to examine the role of herbivorous fishes in the families Acanthuridae (surgeonfishes) and Scaridae (parrotfishes) in determining distributions and abundances of benthic species within and among shallow tropical reef habitats. A back reef habitat along the Belizean barrier reef was characterized by a diverse benthic assemblage of algal turfs, coralline algae, and the coral Porites astreoides, but by extremely low macroalgal abundance. In contrast, several nearby shallow habitats were dominated by dense stands of several macroalgal species. Experimental reduction of herbivorous fish grazing in the back reef (achieved by constructing exclosures) rapidly and dramatically altered existing patterns of benthic species composition and species abundances. After 10 wk of reduced herbivory, total macroalgal abundance increased significantly in herbivore exclusion areas relative to unmanipulated controls, and was correlated with decreased percent cover of available space, several algal turf species, crustose coralline algae, and Porites. Some macroalgal species were able to directly overgrow and kill portions of Porites colonies within herbivore exclusion treatments. Successful recruitment and growth of several algal species under experimentally reduced herbivory indicated that macroalgal species distributions may be limited by herbivory rather than by lack of spore availability or unsuitable physical conditions. Algal turfs characteristic of many reef habitats appear to represent herbivore—tolerant assemblages, persisting under high grazing intensity but responding rapidly to reduced herbivory with increased abundances, morphological changes, and altered reproductive status. These results suggest that herbivorous fish grazing profoundly influences benthic species distributions and abundances within some tropical reef habitats. Spatial variation in herbivory appears to be of fundamental importance in determining regional patterns of benthic community structure on tropical reefs. The spatial mosaic of benthic community composition among shallow reef habitats was associated with patterns of grazing intensity by herbivorous fishes. Several reef habitats supporting dense macroalgal stands represented spatial refuges from herbivory, with low herbivorous fish densities and reduced grazing intensities. Transplant experiments revealed that algal species characteristic of these low—herbivory habitats were highly susceptible to grazing by herbivorous fishes. Spatial heterogeneity in grazing intensity may contribute to high regional diversity among tropical reef habitats by maintaining different benthic species assemblages under fundamentally distinct selective regimes.

Larval Settlement and Juvenile Mortality in a Recruitment-Limited Coral Reef Fish Population

Abstract: The temporal and spatial patterns of larval settlement of the-bluehead wrasse, Thalas- soma bifasciatum, were documented in the San Blas Islands of Panama. Daily censuses indicated that larvae settle onto coral reefs in brief episodes that tended to occur around the new moon and peak in intensity between August and December of each year. The magnitude of settlement each day was unrelated to changes in the local population size but was found to be significantly correlated with the nightly catch of planktonic reef fish larvae in the waters over the reef. The spatial pattern of settlement was examined on three scales. On a large geographic scale, 24 reefs within 1000 km2, there was tremendous variability in settlement intensity that was very consistent from one year to the next. The best correlate of recruit density was exposure to the onshore current. It is suggested that some large- scale oceanographic process determines the intensity of settlement of bluehead wrasses on this scale. The densities of adults on these reefs directly reflected the densities of recruits. Counts of daily otolith increments indicated that those larvae that settled onto low-density reefs subsequently grew signifi- cantly faster. On an intermediate scale, sites within 1 km2, there was no consistency among sites or months in settlement intensity, suggesting that variance in settlement intensity on this scale may be the product of random processes. On the smallest scale, habitat selection within a patch reef, it was found that recruits had distinct preferences in their choices of settlement sites. The daily mortality schedule of bluehead wrasses indicated that mortality was extremely high for the first 3 d on the reef. Juvenile mortality was, furthermore, found to be independent of density. As a result, the patterns of larval settlement of bluehead wrasses persisted into the adult population relatively unchanged. It is therefore proposed that the distribution and abundance of bluehead wrasses in this region are more a product of the external processes controlling larval settlement than of any biological interactions within the reef assemblage.

Constraints and Competition in the Evolution of Flowering Phenology

Abstract: We tested whether the flowering times of animal-pollinated plants are influenced by phylogenetic membership and by life form (e.g., annual, perennial, etc.). We analyzed existing data sets on 2298 animal-pollinated angiosperms of North and South Carolina and 1575 animal-pollinated angiosperms of temperate Japan, and also analyzed a null model based upon the Carolina flora. Our analyses of the complete data sets and of subsets including only the largest families showed that: (1) the floras of the Carolinas and temperate Japan have bimodal distributions of flowering times, with peaks in spring and late summer; (2) within each of these floras, families differ very significantly in flowering time; (3) for most families, flowering times in the Carolinas and in Japan are statistically indistinguishable; (4) the sequences of flowering, ordered by family, are also statistically indistinguish- able in the two floras; (5) within each flora, skewness of flowering time differs markedly among families; (6) for a typical family, the skewness of flowering time is the same in the two floras; (7) there is a significant negative correlation between skewness and mean family flowering time; and (8) life forms differ in flowering time, though less markedly than families. These results demonstrate that phylogenetic membership and life form strongly influence a species' flowering time. We argue that seasonal limi- tations of flowering times are caused by phylogenetic constraints, which may not have changed for millions of years. This study does not provide the degree of resolution needed to determine whether or not there is natural selection for alteration of flowering times in these floras, whether by competition or other mechanisms. However, our results do suggest that competition for pollination does not push species' flowering times beyond seasonal boundaries imposed by phylogenetic constraints. The effects of competition on flowering time are probably limited to small and temporary readjustments of the phenological relationships of competitors. Studies of the flowering times of animal-pollinated plants must consider the strong limits to seasonal displacement imposed by phylogeny and life form, and the probable existence of many alternative modes of escape from competition besides seasonal shifts. We consider the implications of recent paleobotanical studies that suggest that temperate plant com- munities may not have been sufficiently stable through time for plants to have achieved competitive equilibria. Phylogenetic and life history constraints are likely to influence the evolution of many community characters besides flowering time. The methods we have used and the conclusions we have drawn are relevant to a wide variety of ecological characters in plants and animals.

Pollination ecology of the spring wildflower community of a temperate deciduous forest

Abstract: I studied the spring wildflower community of mesic deciduous forests in piedmont North Carolina to determine (a) the extent to which fecundity is pollination-limited in the community, (b) the importance of competition for pollination in affecting seed-set, and (c) the characteristics of plants and their floral visitors that most contribute to full pollination. Although inadequate pollination seems likely in the community, supplemental hand-pollination significantly improved fecundity in just 3 of the 12 species I examined. Pollination-limited reproductive success was evident only in a distinctive subset of the community, species pollinated primarily by queen bumble bees. The majority of wildflower species are pollinated by flies and solitary bees. Measurements of visitation rates and pollinator effectiveness on these plants confirmed that they are usually adequately pollinated in spite of a short blooming season, considerable overlap in flowering times, extensive pollinator sharing by concurrently blooming species, and inclement weather that frequently interrupts insect activity. Many of the flies and solitary bees are inconstant foragers, yet competition for pollination among wildflower species through differential pollinator attraction or interspecific pollinator movements usually does not significantly decrease the seed-set of plants with shared visitors. Competition may act with other causes of insufficient pollination, however, as a selective force to maintain a characteristic set of floral biology traits within the community, including autogamy and self-compatibility, extended receptivity, and pollination by a variety of visitor types. That these floral traits contribute significantly to the successful pollination of vernal herbs was demonstrated by observations of visitor behavior, plant caging experiments that excluded visitors or restricted their access to selected flowers, and measure- ments of floral lifetimes and seed-set for individual plants. These traits are effective regardless of the source of pollination-limited fecundity, and it is the prevalence of such traits, rather than floral specialization or character displacement, that distinguishes the forest spring wildflower community from other communities with potentially inadequate pollinator service.

Processes controlling movement, storage and export of phosphorus in a fen peatland

Abstract: Field and laboratory studies were conducted to determine the mechanisms controlling P movement, storage, and export from a minerotrophic peatland (fen) in central Michigan that had demonstrated high P removal from nutrient additions. An annual P budget completed for the fen ecosystem revealed that plant uptake requirements were 7—9 kg · ha—1 · yr—1, but 35% of aboveground P uptake by plants was returned to the peatland surface via litterfall. Permanent storage of organic P in peat ranged between 2 and 5 kg · ha—1 · yr—1 under natural levels of P input. Both microbial uptake and soil exchange capacity controlled the amount of P made available for plant growth. Fertilizer additions of 5.5 kg · ha—1 · yr—1 of P and 17 kg · ha—1 · yr—1 of N in the fen resulted in no significant (P < .05) increase in growth or nutrient uptake by emergent macrophytes as the litter—microorganism compartment (LMC) retained up to 84% of the added P in year 1. A doubling of the P fertilization level resulted in an LMC retention of ...

Bird Community Dynamics in a Temperate Deciduous Forest: Long‐Term Trends at Hubbard Brook

Abstract: Changes in species composition and abundance of birds breeding in an unfragmented temperate deciduous forest in New Hampshire, USA, were studied intensively during 16 consecutive breeding seasons, 1969-1984. The number of species breeding in the 10-ha study area in any one year varied from 17 to 28, and averaged 24. Total numbers of individuals breeding on the 10-ha plot ranged from 214 to 89, with many species (70%) declining during the 16-yr period. Overall, there was significant positive covariation among population trends of all species, suggesting a major, perhaps single, factor affecting population levels, such as weather or food supply. No two species, however, had identical patterns of change across all 16 yr. Thus, populations in this forest fluctuated largely independently of one another, suggesting a different combination of regulatory factors for each species. The major factors pulsing or stressing bird populations in this forest included: (1) changes in food abundance due largely to irruptions of defoliating Lepidoptera (most bird species, but especially vireos and warblers), (2) harsh late spring and summer weather (Scarlet Tanager, American Redstart), (3) changes in habitat structure related to forest succession (Least Flycatcher, Philadelphia Vireo), (4) interspecific interactions, particularly interference competition (Least Flycatcher-American Redstart, Red-eyed Vireo-Philadelphia Vireo), and (5) mortality during winter for both resident and migrant populations (Hermit Thrush, Dark-eyed Junco, permanent resident species such as woodpeckers and nuthatches. Some, but not all, irruptions of defoliating Lepidoptera significantly influenced many bird popu- lations in this forest and contributed to the observed positive covariation in abundances. These irruptions produced pulses of food for breeding birds, but occurred at long and variable time intervals. Between outbreaks, food may regularly limit reproductive output of these forest birds, and low food abundance, along with the effects of predators, weather, and other mortality factors, contributes to high variability in bird reproductive success, which ultimately influences population size. Competition may also beome important during these periods of food scarcity. We therefore propose that birds in these temperate deciduous forests experience periods of prolonged food limitation, interrupted by relatively brief periods of superabundant food, a pattern contrasting with that described for other temperate systems (e.g., by Wiens 1977). From this examination of bird community dynamics on one site over many years, it is apparent that each species responds to its environment in a unique way, as determined by a variety of influences on its populations. Some of these operate on a local scale (e.g., vegetation structure, food abundance, interspecific competitors), while others function at regional (e.g., some weather effects) and global (e.g., winter events) geographic scales. These influences also differ as to their temporal persistence and predictability, and thus are difficult to detect in short-term studies. This pluralistic view of community structure for birds in temperate forests represents a compromise between nonequilibrial and com- petitionist models, and argues against the existence of a tightly organized community at any one spatial or temporal scale.

Proximal Causes of Natal Dispersal in Belding's Ground Squirrels (Spermophilus Beldingi)

Abstract: I studied the proximal factors influencing dispersal from the natal site in two free—living populations of Belding's ground squirrels (Spermophilus beldingi) in the Sierra Nevada of California. I tested 10 hypotheses, each suggesting a different social, ecological, or ontogenetic factor as a proximal cause of natal dispersal in this species. Using discriminant analysis I also examined effects on dispersal of several independent variables concurrently. Data from marking and live—trapping studies over three field seasons (1979—1981) failed to support hypotheses suggesting resource shortage, ectoparasite load, social facilitation, conspecific aggression, or avoidance by conspecifics as proximal causes of natal dispersal in S. beldingi. Furthermore, dispersal is apparently not caused by changes in juveniles' response thresholds to conspecific aggression or by juveniles' attempts to avoid members of their family units, nearest neighbors, or other members of their local populations. My data supported an "ontoge...

Spatial Relationships among Encrusting Marine Organisms in the New England Subtidal Zone

Abstract: Competition for substratum by encrusting marine organisms usually occurs by direct overgrowth at zones of contact. The documented mechanisms of competitive displacement are numerous, but the relative importance of species in a competing assemblage has been difficult to quantify. This study of the encrusting community on subtidal vertical rock walls in northern Massachusetts used 2 yr of photographic sampling of permanent quadrats to examine the competitive role of each common space occupier. The surface area gained or lost by individuals or colonies of each species in interactions with others was measured over intervals of ≥ 30 d. Four quantitative indices were used to describe competitive roles: the first measured the overgrowth activity of each species within a given assemblage (overgrowth index); the second assessed the relative importance of each species as an overgrowth competitor in the assemblage (weighted overgrowth index); the third described each species' ability to resist overgrowth (resistance index); and the fourth measured each species' growth rate when advancing over other organisms. Analysis of percent cover on subtidal rock walls over a 2—yr period showed that the ascidian Aplidium pallidium, the sponge Halichondria panicea, the octocoral Alcyonium siderium, a red fleshy crustose alga, and a mat of amphipod tubes covered most of the rock surface continuously, although the relative abundances of the species varied across sites and seasons. Several of the common species gained space by overgrowing other species directly or by lateral growth pushing the adjacent species away (lateral movement). This group included several colonial and solitary ascidian species, calcareous tubeworms, and encrusting bryozoans. A few species, including the fleshy red crustose alga and the crustose coralline alga Phymatolithon, gained space from others when an overlying organism receded. The various indices of spatial interaction showed that species which were common and frequent overgrowers in the system were usually not good at resisting overgrowth themselves. Species good at resisting overgrowth were generally large and slow—growing, and were not the most active overgrowers of other species. The interactions among species in this study were basically hierarchical, with the larger thicker colonies and individuals being the winners in competition for space. However, there were a number of reversals of competitive success, especially among the thick crustose forms, and there were numerous "standoffs" where borders did not change during the 30—d interval (19—79% of all encounters, depending on the species involved; 41% overall). The indices of spatial interaction (growth, overgrowth, resistance, overgrowth weighted by abundance) were significantly correlated between exposed and protected sites but were not well correlated between warm and cold seasons. Thus, species tended to have the same competitive roles in slightly different communities but their importance varied as their growth rate changed between warm and cold periods. Overgrowth did not always result in death of the organism overgrown. Species such as the fleshy red crustose alga and the coralline alga Phymatolithon were often encountered alive under invertebrates that had been in position for at least several months. Thus, poor competitors in this system were maintained by two processes: some are early recruits that grow rapidly but later get overgrown, while others are able to withstand overgrowth until predation or some other disturbance removes the overgrower.

Disturbance, succession, and maintenance of species diversity in an East Texas forest.

Abstract: Vegetation change following selective logging and major natural disturbance was in- vestigated through the use of stand history reconstruction and population monitoring in an east Texas pine-hardwood forest. High frequencies of release in beech (Fagus grandifolia) and white oak (Quercus alba) occurred after apparently natural disturbances in 1800-1810, 1860-1870, and 1960-1970, and after selective logging beginning 1910. Pulses of recruitment occurred after all disturbances except the most recent one. Age data suggest that recruitment after the 1910 disturbance was asynchronous among species, with loblolly pine (Pinus taeda) regenerating first, followed sequentially by white oak, red maple (Acer rubrum), and beech. At present, only magnolia (Magnolia grandiflora) is showing significant new recruitment. After the 1910 disturbance, regenerating pines grew rapidly and became dominant in the re-forming overstory. Most hardwoods originating after the 19 10 disturbance did not reach the overstory. White oak initially outgrew maple and beech and is presently larger. Over the last 20 yr beech growth has been greater than white oak growth, resulting in a decline in the average size difference between individuals of these species. Presently, mortality rates of understory populations are high for sweetgum, blackgum, and the oaks, and are low for beech and magnolia. These latter two species are also the faster growing. Ifthese trends continue, beech and magnolia will become increasingly predominant in the forest understory and eventually in the overstory as well. Map data show that saplings and small trees are abundant under pine and oak, but not under beech and magnolia. Future disturbance is therefore likely to accelerate succession to more shade-tolerant species in parts of the forest now dominated by pine and oak, but is most likely to re-initiate new regeneration, including pine and oak, in areas now dominated by beech and magnolia. These results suggest a pattern of cyclical replacement driven by disturbance, a pattern which may help preserve species diversity in southern mesic forests.

An experimental study of competition between two species of dasyurid marsupials

Abstract: This study describes an experimental investigation of the hypothesis of interspecific competition between the dasyurid marsupials Antechinus stuartii and A swainsonii, conducted be- tween May 1978 and December 1979 Antechinuses were live-trapped, marked, and released in five natural enclosures (four experimental, one control), each established in dense forest along a valley floor in the Brindabella Range near Canberra, Australia The enclosures were similar in size, shape, vegetation, and climate, and semi-isolated from one another by creeks or by dry, open habitat un- suitable for the marsupials Antechinus stuartii was removed from two enclosures in July 1978 and later reintroduced to one, whereas A swainsonii was removed from two other enclosures and later returned to one Neither species was manipulated in the control enclosure Comparisons of the pop- ulation parameters and resource use were made for each species when it occurred in the presence and in the absence of the other Removal of the larger and more terrestrial species, A swainsonii, produced several effects on A stuartii that were consistent with the interpretation of competition These included increased numbers; enhanced survival of newly weaned young; increases in extent of movements, home range areas, and use of structurally complex terrestrial habitat; and decreased arboreal activity The proportion of large, terrestrial invertebrate prey in the diet also increased The experimental reintroduction of A swainsonii after ,8 mo produced reciprocal shifts in most of these parameters An anomalous finding was that the mean body mass of A stuartii declined when A swainsonii was excluded This was probably due to increased levels of intraspecific interference, since the decline coincided with unusually high population densities in A stuartii Competition with A swainsonii had little evident effect on the time of activity of A stuartii In contrast, manipulations of the numbers of A stuartii produced no changes in the population parameters or the pattern of resource use of A swainsonii; hence it was concluded that the competitive effects of A stuartii on its larger congener are small Predation, the presence of a superfluous third species (a eutherian rat) during the experiments, and genetical differences between the enclosure populations of Antechinus were rejected as possible alter- native explanations of the results; none was as convincing as that of interspecific competition Com- petition probably occurs for food, and is therefore most intense in winter, when surface litter inver- tebrates are scarce This study demonstrates that competition has asymmetrical effects on the two study species, and challenges the notion that competitive interactions are weak or absent in marsupial communities

Nitrogen in West Africa: the regional cycle

Abstract: A nitrogen cycle budget has been calculated for West Africa south of the northern Sahara that quantifies biologically important pools of nitrogen in the region and major fluxes associated with these pools. Major compartments of the model include noncultivated systems broken down by vegetation zone, successional status, and plant components, annual and perennial crops each broken down by crop species and plant and harvest components; wetlands; anthropic systems, and soils and sediments. Base reference year is 1978. Biological nitrogen fixation and precipitation fluxes dominated nitrogen inputs to West Africa in 1978. Approximately 12 x 10/sup 9/ kg were fixed in noncultivated systems; about half of this was fixed in early successional (0-6 yr) rain forests, and much of the rest in grazed or fallow savanna grasslands and woodlands. Less than 0.7 x 10/sup 9/ kg were fixed in cultivated systems; legumes accounted for approx. = 25% of this. Total anthropic sources (fossil fuel combustion, fertilizer production, and agricultural commodity imports) were minor (< 0.3 x 10/sup 9/ kg). Precipitation inputs to the region were approx. = 0.4 x 10/sup 9/ kg. Most nitrogen leaving West Africa did so volatilized by fire (approx. = 8.3 x 10/sup 9/ kg), principally inmore » non-cropped systems. Major losses also occurred via hydrologic export to the Atlantic Ocean (1.5 x 10/sup 9/ kg) and via denitrification (1.1 x 10/sup 9/ kg). Total N losses from West Africa exceeded 11.0 x 10/sup 9/ kg. The overall budget balances within 1%, despite independent calculations of all major fluxes. The balance portrays a nitrogen cycle dominated by pools and fluxes in noncropped systems. Indirect human influences, however, mainly through effects on vegetation cover, appear to be a major determinant of both the rates at which nitrogen is cycled in West Africa and the relative importance of most pools and fluxes.« less

Coastal forest tree populations in a changing environment, southeastern long island, new york'

Abstract: The temporal and spatial dynamics of tree populations in coastal forests of southeastern Long Island, New York, were reconstructed from analyses of (1) historical accounts, maps, and aerial photos, (2) field evidence of forest history, (3) age and height class distributions along the moisture gradient, and (4) fossil-pollen content of bordering salt-marsh deposits. Tide-gauge records of sea level provided independent evidence for long-term change in the physical environment (rise in sea level) that caused populations to migrate up the continental shelf. Field evidence for fire and historically documented land-use change demonstrated the important role of disturbance in the dynamics of tree populations. Spatial and temporal patterns in forests were determined by information on disturbance frequency, dispersal, generation times, and rates of change in the physical environment. Mature cohorts occupied lower elevations than did their progeny because, during the period from germination to seed production, suitable soil-moisture conditions shifted upslope in response to sea-level rise. Establish- ment of new regeneration depended on disturbances that opened the forest canopy and/or prepared seed beds. Frequent fire, land clearance and abandonment, and tree blowdowns insured opportunities for seedling establishment at higher elevations as conditions became unsuitable for regeneration on low-elevation sites. The resultant pattern of range shifts was stepwise, with waves of regeneration following disturbances and subsequent reduced seedling establishment as soil moisture continued to increase. The vegetation dynamics could be explained solely on the basis of life histories of plant species and environmental change; "emergent properties" and "successional stages" in a deterministic sense were not apparent. An understanding of successional change required information on both the spatial and temporal dimensions of vegetation pattern because composition change over time resulted from range shifts in response to environmental transition.

Quantitative Phenology and Water Relations of an Evergreen and a Deciduous Chaparral Shrub

Abstract: Quantitative phenology and pre—dawn xylem pressure potentials (XPP) of an evergreen shrub, Ceanothus megacarpus, and a deciduous shrub, Salvia mellifera, were compared on pairs of plants in shared microsites in southern Californian chaparral for 2 yr. Shoot elongation of both species occurred at essentially the same time during both growing seasons. Leaf maturation also occurred at essentially the same time for both species during the 1977—1978 season, but commenced ≈1 mo earlier on S. mellifera than on C. megacarpus during the 1978—1979 season. No substantial differences in phenology were found among leaves of C. megacarpus canopy, subcanopy, or short shoots. On S. mellifera, however, leaf maturation was more rapid and was completed earlier in the season on flowering canopy shoots than on nonflowering canopy and subcanopy shoots, and although no mature leaves on flowering canopy shoots persisted beyond June during either year, a substantial number of mature leaves on subcanopy shoots remained well into their second growing season. Leaf curling, associated with changes in XPP, was significant in both species, and allowed rapid adjustment of leaf area without the abscission or production of leaves. Flowering and seed production occurred early in the growing season for C. megacarpus and late in the growing season for S. mellifera. This difference in flowering phenology corresponded to the more rapid vegetative development of S. mellifera flowering canopy shoots. In both species subcanopy or short shoots made up a much larger fraction of the total number of shoots per plant, and consequently contributed relatively more to whole—plant leaf phenology and annual shoot production, than did canopy shoots. There were only subtle differences between species in the timing of increases in whole—plant mature leaf area (MLA). Maximum rates of increase of whole—plant MLA occurred in both species when soil temperature (ST) was ≈13.5°C and XPPs were ≈—0.60 MPa, and these rates declined sharply at STs above 15° and XPPs below —1.5 MPa. Ceanothus megacarpus and S. mellifera had nearly indistinguishable XPPs throughout both years. Leaf shedding by S. mellifera was more related to photoperiod and flowering than to low XPP, and was clearly not a successful drought—avoiding mechanism. Like some evergreen species, S. mellifera tolerated long periods of extremely low XPP. Important differences between the species were that over a 12—mo period the integrated total of MLA—days for S. mellifera was 73% of that for C. megacarpus, and leaves produced during the current growing season contributed 71% of the total MLA—days in S. mellifera and only 37% in C. megacarpus. The differences between the results of this study and several accepted generalizations regarding growth and water relations of deciduous and evergreen chaparral shrubs may be due to the observation of these growth forms in different microsites and the lack of truly quantitative phenological measurements in previous studies.

Dynamism in the Barrier‐Beach Vegetation of Great South Beach, New York

Abstract: Pollen data were used to reconstruct the past 340 yr of vegetational changes at Great South Beach, a system of barrier beaches east of Fire Island Inlet, Long Island, New York. The data came from cores taken along 16 transects, 6 of which are described in detail. Dates and stratigraphy from individual cores provided local environmental histories that were combined to reconstruct temporal and spatial patterns of vegetation on local and regional scales. Changes in inlet status and local disturbances accounted for much of the dynamism in vegetation. Salt marshes fringed back-barrier lagoons only when many inlets were open and saline conditions prevailed (1760 to 1835; 1931 to the present). Cyperaceae dominated marshes throughout the re- mainder of postsettlement time. Inlets affected vegetation by altering tidal range and salinity in back- barrier lagoons and provided new substrates for marsh establishment when flood-tide deltas were abandoned by inlet channels. At least once every 100 yr along the six transects, sufficient sand was deposited to destroy all vegetation. The composition of invading assemblages on primary substrates was regulated mainly by physical factors. Annual herbs, shrubs, or high-marsh plants each were "pioneers" at different localities and times. Putative "pioneer" annuals were not limited to the early stages of succession. Fluctuating water tables played an important role in determining composition of freshwater wetlands. Biotically controlled trends in plant composition could not be positively identified. The reconstructions on Great South Beach indicate tight environmental control of plant distribution but emphasize that vegetation does not achieve an equilibrium with the physical environment. Present distributions are largely the product of historical events acting together with existing conditions.

Multiple Regulation States in Populations of the California Vole, Microtus Californicus

Abstract: Two experiments were designed to test which hypothesis of population regulation pro- vided the most consistent explanation of population fluctuations in the California vole. The first experiment tested the effects of spacing behavior and of initial breeding-season density on population regulation. The second experiment tested the effects of resource level, of population history, and of population substructure. A factorial design was used for each experiment. The first experiment was analyzed as two separate factors. The second experiment was analyzed with a 2 x 2 x 2 factorial analysis of variance. The results indicate that naturally occurring populations appear to be regulated by two separate but equally important mechanisms. When habitat occupation is patchy, spacing behavior dominates and densities are regulated below 200 individuals/ha. Individuals from these populations can be characterized as coming from low or early-increase phases. Survival is low due to very high dispersal losses and a moderate mortality rate. Individuals have a greater tendency to be reproductive. Males have lower masses. When habitat occupancy is more uniform, increasing overall density reduces the dispersal rate and, as a result, densities increase further. In this situation, resource limitation dominates the regulation process. Peak densities of these populations range from 500 to 800 individuals/ha. Individuals from these populations can be characterized as coming from the peak phase. Survival is high due to low dispersal losses and a moderate mortality rate. Individuals tend less to be reproductive, and males are heavier.