Showing papers in "Ecological Monographs in 1981"

Intertidal Landscapes: Disturbance and the Dynamics of Pattern

Abstract: The mussel Mytilus californianus is a competitive dominant on wave—swept rocky intertidal shores. Mussel beds may exist as extensive monocultures; more often they are an everchanging mosaic of many species which inhabit wave—generated patches or gaps. This paper describes observations and experiments designed to measure the critical parameters of a model of patch birth and death, and to use the model to predict the spatial structure of mussel beds. Most measurements were made at Tatoosh Island, Washington, USA, from 1970—1979. Patch size ranged at birth from a single mussel to 38 m2; the distribution of patch sizes approximates the lognormal. Birth rates varied seasonally and regionally. At Tatoosh the rate of patch formation varied during six winters from 0.4—5.4% of the mussels removed per month. The disturbance regime during the summer and at two mainland sites was 5—10 times less. Annual disturbance patterns tended to be synchronous within 11 sites on one face of Tatoosh over a 10—yr interval, and over larger distances (16 km) along the coastline. The pattern was asynchronous, however, among four Tatoosh localities. Patch birth rate, and mean and maximum size at birth can be used as adequate indices of disturbance. Patch disappearance (death) occurs by three mechanisms. Very small patches disappear almost immediately due to a leaning response of the border mussels (0.2 cm/d). Intermediate—sized patches (<3.0 m2) are eventually obliterated by lateral movement of the peripheral mussels: estimates based on 94 experimental patches yield a mean shrinking rate of 0.05 cm/d from each of two principal dimensions. Depth of the adjacent mussel bed accounts for much of the local variation in closing rate. In very large patches, mussels must recruit as larvae from the plankton. Recovery begins at an average patch age of 26 mo; rate of space occupation, primarily due to individual growth, is 2.0—2.5%/mo. Winter birth rates suggest a mean turnover time (rotation period) for mussel beds varying from 8.1—34.7 yr, depending on the location. The minimal value is in close agreement with both observed and calculated minimal recovery times. Projections of total patch area, based on the model, are accurate to within 5% of the observed. Using a method for determining the age of patches, based on a growth curve of the barnacle Balanus cariosus, the model permits predictions of the age—size structure of the patch population. The model predicts with excellent resolution the distribution of patch area in relation to time since last disturbance. The most detailed models which include size structure within age categories are inconclusive due to small sample size. Predictions are food for large patches, the major determinants of environmental patterns, but cannot deal adequately with smaller patches because of stochastic effects. Colonization data are given in relation to patch age, size and intertidal position. We suggest that the reproductive season of certain long—lived, patch—dependent species is moulded by the disturbance regime. The necessary and vital connection between disturbance which generates spatial pattern and species richness in communities open to invasion is discussed.

Habitat Associations and Community Structure of Birds in Shrubsteppe Environments

Abstract: We studied the relations between bird distribution and abundance and habitat characteristics at a regional scale of investigation, using surveys conducted over three consecutive years on 14 plots at nine locations in the shrubsteppe of the northwestern Great Basin of North America. Bivariate and multivariate analyses revealed no large suites of bird species that were correlated in their distribution and abundance, and few associations existed between pairs of species, suggesting that bird populations in this system vary largely independently of one another. Both bivariate and multivariate correlational analyses between birds and habitat physiognomy indicated that the bird species that were widely distributed in this shrubsteppe system had few significant associations with habitat features, while species with more localized distributions did exhibit habitat affinities, most notably with a suite of characters associated with the occurrence of rocky outcrops. Bird species whose primary distributions and habitat affinities lie in grassland regions to the east demonstrated the greatest degree of correlation with features of habitat physiognomy in this shrubsteppe region, increasing in abundance as vegetation coverage and stature increased and horizontal heterogeneity of habitats decreased. Some bird species, however, exhibited no correlations with the habitat features we measured, and multivariate analyses comparing variation in the bird abundances with variation in features of habitat physiognomy accounted for <17% of the overall variation in the bird abundance matrix. Consideration of coverages of different species of shrubs, however, generally produced more significant correlations with variations in bird abundances, especially for the widespread shrubsteppe species. Attributes of avian community structure varied among the locations surveyed. Variations in the abundance of Brewer's Sparrows, the most abundant species at most sites, accounted for 86% of the variation in total avian density. Species diversity was negatively correlated with total density (presumably because of the overwhelming influence of one species on density) but positively correlated with species richness. Richness variations, in turn, were a consequence of variations in the abundances of several locally distributed shrubsteppe species or grassland species. Richness decreased with increasing horizontal habitat heterogeneity and general vegetation sparseness, but increased with increasing structural diversity of the habitat. Variations in avian community biomass were largely a function of abundances of the widespread or peripheral species; none of the local shrubsteppe forms that contributed so much to species richness was correlated with total biomass variations. We compare the findings of these regional—scale analyses with those of a continental—scale study that included a habitat spectrum ranging from shrubsteppe through tallgrass prairies. Bird species exhibited different patterns of habitat correlations on the two spatial scales. In particular, the characteristic shrubsteppe species showed strong correlations with features of habitat physiognomy in the continental analysis, but in the regional study such associations were generally lacking and these birds instead were correlated with the coverages of various shrub species. This suggests that at a large scale, between—habitat level of analysis these birds may respond to some elements of general habitat configuration, but their within—habitat responses may be more strongly associated with details of habitat floristics. These results complicate studies of avian community/habitat relationships: complete understanding of ecological patterns apparently requires knowledge of vegetational floristics as well as physiognomy; the response of birds to habitat characteristics, and the habitat features that are important, may differ at different scales of spatial resolution. At both spatial scales, however, a substantial portion of the variation in avian abundance remains unexplained after consideration of habitat features. Relatively few significant correlations thus emerge, increasing the probability that those that are revealed may well be spurious, and reinforcing the view that biotic interactions such as competition probably play a minor role in structuring these communities.

Community Organization in Temperate and Tropical Rocky Intertidal Habitats: Prey Refuges in Relation to Consumer Pressure Gradients

Abstract: The structure of a tropical rocky intertidal community on Taboguilla Island on the Pacific coast of Panama is characterized by extremely low abundances of noncrustose algae and sessile animals, indistinct vertical zonation patterns (a result of the low abundances), and the occurrence of most invertebrates (except barnacles) and upright algae in holes and crevices (as opposed to open, smooth surfaces). This contrasts strikingly with two temperate rocky intertidal communities, which have high covers of plants and animals, more obvious zones, and invertebrates and upright algae occurring both on relatively homogeneous surfaces and in holes and crevices. Field experiments were done and observations were made in the Panama community to test the effects of different types of consumers (both predators and herbivores) on their prey and on the types of escapes utilized by the prey. Consumer exclusion experiments suggest that (1) predation and herbivory are severe at all times of the year, (2) consumer pressure is a cumulative function of many types and species of predators and herbivores, (3) the primary effect of larger fishes and crabs is to restrict most prey to three—dimensional refuges (holes and crevices), and (4) the primary effect of smaller consumers, mostly invertebrates, is to keep abundances of the prey low. Thus, in the Panama system, three—dimensional space (holes and crevices) appears to be particularly important as a refuge from consumers, while escapes from consumers in body size, time, or two—dimensional space (e.g., in a higher zone) documented so frequently in temperate areas, assume secondary importance for many prey. This restriction of the types of escapes utilized by prey species appears to be a consequence of two main factors: the presence of fast—moving consumers (i.e., herbivorous and predaceous fishes and herbivorous crabs which are absent or rare in the two temperate communities), and the year—round foraging of all consumers.

Comparative Influences of Light and Temperature on the Growth and Metabolism of Selected Submersed Freshwater Macrophytes

Abstract: Under controlled experimental conditions, the influences of light and temperature on the growth and metabolism of Egeria densa Planch, Hydrilla verticillata Royle, and Myriophyllum spicatum L. were comparatively examined. Light was controlled at six levels ranging between 5 and 75% of full sunlight at solar noon. Water temperature was controlled at five levels ranging between 16? and 32?C. Growth considerations included morphology, biomass, and nutrition. Photosynthesis, respiration, and CO2 compensation points were determined to evaluate physiological differences in plant growth as affected by the experimental ranges of temperature and light. External morphology in these species was significantly affected by the different experimental light and temperature conditions. Both low light and high temperature promoted extensive shoot elongation and associated canopy formation. Biomass production and carbon metabolism in all species were influenced more by temperature than by light. Each of the species demonstrated metabolic acclimation to light over a broad range. Conversely, the macrophyte species considered here were not strictly capable of acclimating to temperature. Growth rate and the seasonal progression of senescence were interrelated in these species. Higher temperatures stimulated growth and promoted a compression of the growth cycle. The relationship between photosynthesis and respiration (P:R) was appreciably reduced by senescence, but the CO2 compensation point did not reflect this condition. In the species examined, CO2 compensation points decreased with increasing temperature, suggesting adaptations to low free CO2 levels in the environ- ment. Latitudinal differences in integral seasonal temperature, in relation to species-specific ranges of thermal tolerance, appear to be important in influencing the geographical distribution of the species considered here. Light may be the primary determinant of their depth distribution, but its importance in this regard could be somewhat diminished by their significant abilities to extend to the water surface under low light conditions.

Prunus mahaleb and Birds: The High‐Efficiency Seed Dispersal System of a Temperate Fruiting Tree

Abstract: Seed dispersal of Prunus mahaleb (Rosaceae), a tree producing large fruit crops, was studied in southeastern Spain to establish the degree of reciprocal dependence between the plant and the birds which disperse it. P. mahaleb drupes contain a relatively large seed (pulp: stone ratio 0.62). The water content of pulp is 82.9% and dry flesh contains 3.2% crude fat and 2.8% crude protein, being largely made up of carbohydrates. Four bird species were the main seed dispersers. Visitation rates, feeding efficiency, and degree of dependence on P. mahaleb fruits for food varied substantially among species. Turdus merula and Sylvia atricapilla showed the highest visitation rates, were behaviorally the most efficient, removed the bulk of seeds and, after feeding, tended to fly preferentially towards the apparently safest sites for growth and survival of saplings. They were also most heavily dependent on P. mahaleb fruits for food, whereas the other disperser species relied largely on insects. Individual plant location and dispersers' habitat preferences produced a differential seeding pattern over the patchy habitat surrounding study trees. The "key" dispersers T. merula and S. atricapilla are specialized frugivores, efficiently dispersing seeds of a plant producing large numbers of extremely low—reward fruits. These results conflict with theoretical expectations and suggest that both the nature of the correlation between bird— and plant—related coevolutionary gradients and their amplitudes, as well as the nature of bird—plant coevolutionary interactions, may differ between tropical and temperate habitats.

A Comparison of the Structure, Primary Productivity, and Transpiration of Cypress Ecosystems in Florida

Abstract: To investigate how inputs of water and nutrients influence the structural and functional characteristics of cypress wetlands, primary productivity, respiration, transpiration, tree growth, biomass, basal area, stem density, standing stocks of phosphorus, and litterfall were measured in several kinds of Florida cypress ecosystems subjected to varying inflows of water and nutrients. A scrub cypress forest, natural and sewage-enriched cypress domes, and a cypress floodplain forest were studied. Floodwaters in these ecosystems ranged from stagnant and phosphorus poor (0.01-0.08 mg P/L) to flowing and phosphorus rich (0.66-5.86 mg P/L). Inputs of total phosphorus transported by water to these forests varied from 0.11 g P m-2 yr-1 in the scrub cypress forest to 1625 g P m-2 yr-1 in the floodplain forest. Phosphorus content of aboveground biomass (ranging from 0.25 to 4.78 g P/M2) appeared to be positively related to phosphorus inputs. Biomass and other structural characteristics of the study sites appeared to be related to stand history rather than to phosphorus inputs. For example, there was little difference in aboveground biomass between natural cypress domes (20.6-26.6 kg/M2), a sewage-enriched dome (21.7 kg/M2), and a floodplain forest (28.4 kg/M2). Aboveground biomass for the scrub cypress forest was low (3.6 kg/ m2). Net daytime photosynthesis (2.1-13.7 g C.m-2 ground surface d-1), plant respiration (0.9-10.9 g C m-2 ground surface .d-1) and an estimate of gross primary productivity increased with increasing inputs of total phosphorus. Aboveground biomass production (sum of wood production, ranging from 44 to 1080 g.m-2 yr-1, and litterfall, ranging from 224 to 941 g m-2 yr-1) increased with increasing total phosphorus inputs in the low range, but biomass production leveled in the high input range. Total water loss from cypress forests increased with increasing P inputs. Mean daily transpiration rates from the scrub cypress forest (1.0 mm) and cypress domes (3.1-3.8 mm) were lower while those from the floodplain forest (5.6 mm) were similar to evaporation rates from open water bodies. Ratios of transpiration to net daytime photosynthesis were also low (156-221 g H20/g organic matter), showing that cypress is efficient in its water use. In cypress domes and scrub cypress forest, where water may be limiting, trees appear to adjust to potential water stress through leaf morphology adaptations and minimum canopy development (leaf area index, LAI = 0.53-3.4 m2/m2). When water is not potentially limiting, as in the floodplain forest, adaptations to conserve water were lacking (e.g., LAI = 8.5 m2/m2). A dome with added sewage effluent had higher leaf biomass, leaf area index, and chlorophyll a content, and more phosphorus stored in leaves than untreated domes. Net primary productivity, litter production, and wood production increased approximately twofold over pre-effluent rates or untreated cypress domes.

Host plant selection by battus philenor butterflies: the roles of predation, nutrition, and plant chemistry'

Abstract: In a previous study it was demonstrated that a major selection pressure responsible for seasonal changes in the searching and oviposition preferences of the pipevine swallowtail butterfly, Battus philenor (Lepidoptera: Papilionidae), is a seasonal increase in the disappearance rate of early- instar larvae from one of their two host plants in east Texas, Aristolochia reticulata (Aristolochi- aceae). The primary objective of this study was to determine the causes, both proximate and ultimate, of that increased disappearance rate. The results may be summarized as follows: 1) The seasonal increase in disappearance rates of larvae feeding on A. reticulata plants is due primarily to a decrease in the size at which larvae disperse from their original host plant. No seasonal increase in the intensity of predation on larvae was detected. 2) Larvae readily consume the young foliage of A. reticulata plants but seldom accept mature foliage. 3) The amount of A. reticulata foliage that is acceptable to larvae decreases between early April and late May. This decrease explains the seasonal decrease in the size at which larvae disperse from their original host plant. 4) Growth rates, corrected for variation in consumption rates, are much lower for larvae fed mature leaves than for larvae fed young leaves. 5) Poor larval growth on mature leaves appears to be due to two effects of sclerophyllization: low nitrogen content and increased toughness. No effects of digestibility-reducing plant secondary com- pounds were detected. 6) For A. reticulata plants in the field, the probability that a leaf will be eaten once it is discovered by a larva is lower for mature leaves than for young leaves. 7) For A. serpentaria plants, which grow sympatrically with A. reticulata, mature leaves and young leaves are equally acceptable to larvae. Mature A. serpentaria leaves are not markedly scle- rophyllous. These results indicate that the causes of the observed seasonal increase in disappearance rates of larvae on A. reticulata can be traced ultimately to changes in the nutritional content of the foliage as it ages. They also suggest that a seasonal deterioration in the nutritional quality of A. reticulata foliage is a major selection pressure leading to the observed seasonal shift in the searching and oviposition preferences of females from A. reticulata in the early spring to A. serpentaria late in the spring. Finally, the results provide evidence supporting the contention that an important ecological function of sclerophylly is to defend plants from herbivores.

Late Quaternary Vegetational History of Illinois

Abstract: Pollen diagrams from the Prairie Peninsula in Illinois record the climatically related vegetation shifts that have occurred since the late Pleistocene They indicate that the major period of prairie development started between 8500 and 7900 BP, apparently occurring first along the southern margin These events mark the beginning of the Hypsithermal interval and are correlated with the final disintegration of the Laurentide ice sheet and the establishment of the postglacial system of atmospheric circulation over North America The late—glacial vegetation was spruce woodland in the north and open spruce woodland/tundra in central Illinois These vegetation communities disappeared from the state in a northward direction between 14 000 and 10 900 BP They were followed in northern Illinois by Pinus, Abies, Betula, and Fraxinus; in central Illinois Fraxinus expanded in the lowlands while the uplands remained relatively treeless These assemblages, which persisted longer in the south than in the north, were subsequently replaced first by a mixture of cool temperate, then warm temperate deciduous trees as climatic amelioration continued By 9000 BP, Illinois was dominated by deciduous forest In response to Hypsithermal climatic stress, prairie vegetation began replacing deciduous forest about 8300 BP on the uplands in central Illinois and by 7900 BP xeric oak—hickory forest became dominant in northern Illinois This is the beginning of the Prairie Peninsula as a discrete floristic area in Illinois About 5000 BP the southern border of the Prairie Peninsula experienced increased moisture and renewed forest development suggesting a western contraction of the prairie margins Central Illinois, however, remained grassland to the present Between 900 and at least 400 yr ago an indicated trend toward cooler temperatures is correlated with late stages of the Neoglacial Pollen evidence from northern Illinois and adjacent Lake Michigan indicates the reappearance or increase in Betula, Pinus, Picea and Larix This short cool episode appears to end prior to the beginning of large—scale historic land clearance, about 140 yr ago, as the evidence for a vegetational readjustment to warmer climate is truncated by the logging of the forests, the plowing of the prairies and the resultant dramatic increase in Ambrosia pollen

Organic Carbon Flow in a Swamp‐Stream Ecosystem

Abstract: An annual organic carbon budget is presented for an 8-km segment of Creeping Swamp, an undisturbed, third-order swamp-stream in the Coastal Plain of North Carolina, USA. Annual input of organic carbon (588 gC/m2) was 96% allochthonous and was dominated by leaf litter inputs (36%) and fluvial, dissolved organic carbon (DOC) inputs (31%). Although the swamp-stream was primarily heterotrophic, autochthonous organic carbon input, primarily from filamentous algae, was important during February and March when primary production/ecosystem respiration (P/R) ratios of the flooded portions were near one. Annual output of organic carbon via fluvial processes (214 gC/m2), 95% as DOC, was 36% of total annual inputs, indicating that the swamp-stream segment ecosystem was 64% efficient at retaining organic carbon. Respiration (327 gC/m2) accounted for most of this retention, or about 57% of annual inputs, and storage accounted for the remaining 7% of inputs. Organic carbon dynamics in the Creeping Swamp segment were compared to those reported for upland stream segments using indices of organic matter processing suggested by Fisher (1977) and a loading potential index suggested here. Creeping Swamp, while loading at a high rate, retains a much larger portion of its organic carbon inputs than two upland streams. Organic carbon loading potential in Creeping Swamp is very large, primarily because of its great width and complete canopy; however, its low gradient and dense vegetation enhance organic carbon retention, especially of coarse partic- ulate organic carbon (CPOC), by maintaining low water velocities with little erosive force, tortuous flow pathways, and debris dams. Most of the retention and oxidation of organic carbon in the flooded Creeping Swamp ecosystem takes place in the floodplain. It is suggested that floodplain exchanges should also be considered in budgets of higher gradient rivers and streams. Despite the high degree of retention and oxidation of organic inputs to Creeping Swamp, there is a net annual fluvial export of 21 gC/m2, mostly in the dissolved form. Watersheds drained by swamp- streams in the southeastern United States are thought to have large organic carbon exports compared to upland forested drainages, because the stream network covers a much greater proportion of the total watershed area.

Fundamental Concepts and Methodology for the Analysis of Animal Population Dynamics, with Particular Reference to Univoltine Species

Abstract: This paper presents some concepts and methodology essential for the analysis of population dynamics of univoltine species. Simple stochastic difference equations, comprised of endogenous and exogenous components, are introduced to provide a basic structure for density—dependent population processes. The endogenous component of a population process is modelled as a function of density in the past p generations, including the most recent. The exogenous component of the process consists of all density—independent components of the ecological factors involved, including enhance variations. The model is called a pᵗ ʰ order density—dependent process. For a successful analysis of a population process by the above model, it is important that the process be in a state of statistical equilibrium, or stationarity. The simplest notion of stationarity is introduced, and the average behavior of the process, under this assumption, discussed. The order of density dependence in the population process of a given species depends on its interaction with other species involved in the food web. Considering certain attributes of the food web, in particular the limited number of trophic levels, the pyrmaid of numbers, the linear linkages between closely interacting species, and niche separation among competing species, it is argued that the order of density dependence is probably not much higher than three. A second—order model is perhaps adequate in many practical cases. The dynamics of some lower order density—dependent processes are compared by simulations, with a view to showing the effect of density—dependent and density—independent components at different orders. Several types of density dependence are discussed. If a given factor influencing the temporal variation in density is by itself influenced by density, it is called "causally density—dependent," which may reveal itself by some degree of correlation with density. A density—independent factor, however, may also show some sort of correlation with density in the recent past. This is called "statistically density—dependent." Such statistical density dependence may be due to: (1) spurious correlation, (2) bias in an estimator of the correlation coefficient, (3) autocorrelations in the density—independent factor, and (4) an intriguing mathematical property of the stochastic process. Particularly because of the last two reasons, it is often difficult to distinguish, by correlation method, between causal and statistical density dependence. Distinction also exists between temporal and spatial density dependence, the latter not necessarily implying the involvement of the former. The importance of the distinction between these types of density dependence is discussed in relation to the data analysis and model building. A Statistical analysis of the effect of ecological factors on population dynamics is attempted. Since it is often difficult to determine, by correlation, the causally density—dependent structure of a population process under the influence of some unknown density—independent factors, it is suggested that we reverse the procedure to determine the effect of the density—independent factors first. To confirm the involvement of some suspected density—independent factors in the species dynamics, I propose several methods of correlation between annual fluctuations in some population parameters, such as density, rate of change in density, and their transforms, and those in suitable indices of the suspected factors. Merits, demerits, and limitations of these methods are also discussed. To simplify the arguments, the correlation models are set up first without stage division, and then are elaborated to those in which the whole generation span is divided into several life—cycle stages, so that life table information can be used effectively for the identification of the density—independent factors involved in each stage. A set of life tables of the spruce budworm, Choristoneura fumiferana (Clem.), is analyzed to provide an example of the application of the above concepts and methods. Concluding remarks include some notes on designing life table studies.

The distribution of an avian guild along a temperate elevational gradient: the importance and expression of competition'

Abstract: A guild of ground-foraging, insectivorous birds which seasonally occupy montane tem- perate forests was investigated. By examining the roles of interspecific competition and specific habitat selection, I have dissected the factors controlling the distribution patterns of the guild members along extensive elevational gradients. The guild was composed of Hylocichla mustelina (Wood Thrush), Catharus fuscescens (Veery), C. guttatus (Hermit Thrush), C. ustulatus (Swainson's Thrush), and C. minimus (Gray-cheeked Thrush). The five thrush species are sympatric on high mountains in the northeastern United States, but as one proceeds southward down the Appalachian Mountain chain, the species drop out one at a time. Anecdotal information indicated that the species showed elevational amplitude expansion as members of the guild dropped out with decreasing latitude. As a consequence of the opportunity for ecological release in southern latitudes, the thrush guild was ideally suited for testing the role of competition in determining distributional limits of the species when sympatric. I studied the behavioral interactions and habitat selection of the five species in sympatry on high mountains in the northeastern United States. The species have distinct distributions along these elevational gradients, but large spatial overlaps between adjacent species were observed. To test for significant differences in habitat selection, the breeding territories of these species were quantified by measuring 55 structural habitat variables. Using stepwise discriminant function analysis, I detected statistically significant differences in the species' breeding habitats. By employing a subset of 9 of the initial 55 variables, over 77% of the Northeastern habitat quantifications were assigned to the correct species group. Additional information extracted from the structural niche analysis indicated the habitat niche breadth of each species and the degree of habitat overlap between adjacent species, and provided insight into the limiting similarity allowed between these coexisting species. Song playback was used to test the contribution of behavioral interactions to the observed distri- bution patterns. All species showed greater response to conspecific song. The results indicated that overt interspecific behavioral interactions contributed little or nothing to the observed distributional and specific habitat selection patterns of these species. To test the contribution of interspecific competition to the habitat selection patterns detected on Northeastern mountains, a subset of the five-member guild was studied on Southeastern mountains. In the Smoky Mountain range only H. mustelina and C. fuscescens breed. By employing identical quantification techniques, I tested for expansion by these two species into habitat types not occupied when sympatric with the other three species. Evidence of extensive ecological release was obtained, implying a strong role for interspecific competition in Northeastern mountains. Thus, my results further implicate interspecific competition as an integral force contributing to the organization of temperate bird communities.

A Predator-Prey Interaction between Two Physically and Biologically Constrained Tropical Rocky Shore Gastropods: Direct, Indirect and Community Effects

Abstract: The archaeogastropod Nerita scabricosta is an abundant and extremely mobile herbivore which occurs from above mean high water through the mid—intertidal zone of exposed rocky areas along the Pacific coast of Panama. It is the principal prey of Purpura pansa, a patchily distributed, high intertidal neogastropod. Both snails exhibit activity patterns and habitat preference which appear to have evolved in response to heavy predation pressure by fish and to harsh physical factors. These behavioral mechanisms restrict movement and limit foraging time for both species, but also result in a cyclic overlap of their distributions. Potential interaction between the two gastropods is thus limited to well defined, discrete periods. Purpura elicits both an escape and an avoidance response in Nerita. The intensity of predation on the nerite by Purpura is lessened by these responses and Purpura appears unable directly to control the abundance of Nerita. However, it is experimentally shown that in a heterogeneous environment, Nerita's distribution is still more restricted and its local abundance further reduced by avoidance responses to temporally stable aggregations of Purpura. These aggregations, centered on specific fish—safe microhabitat, are thus hypothesized to result in the creation of local patches of intertidal substrate which are free from herbivory or disturbance effects by Nerita scabricosta. Evidence from field observations and manipulations is presented to support the hypothesis that in this complex, four trophic level community, a predator—prey interaction between two gastropods can limit potential resource monopolization and affect primary space occupancy.

Ecological abstraction: the consequences of reduced dimensionality in ecological models'

Abstract: Real world ecosystems (as opposed to their mathematical counterparts) are often enor- mously complex associations of species which interact in diverse ways. As a matter of practical necessity, field ecologists can rarely specify, much less quantify, all of the interactions. Consequently, empirically derived equations purporting to describe the dynamics of such systems generally consider fewer than the total number of interacting species. The present paper calls attention to this reduction in dimensionality and explores some of its consequences. In particular, attention is called to what are termed the Abstracted Growth Equations, those of reduced dimensionality, and to the way that these expressions derive from the underlying n-variable equations. The degree to which the Abstracted Equations accurately describe the dynamics of the species of interest is shown to depend on the time scale of these species relative to that of the species which are omitted. A general result relating the product of the eigenvalues of the Abstracted Equations to the corresponding product for the n-variable equations is proved. It is further pointed out that the distinction between Abstracted and n-variable equations suggests experiments which at least in principle should enable the empiricist to estimate the importance of species and interactions which are omitted. The relationship between Abstracted and n-variable equations is also discussed with regard to measuring competition coefficients and related parameters, and also to the problem of determining whether or not higher order interactions are present in laboratory microcosms. The analysis concludes by comparing the stability properties of several simplified models of community interactions with those of the corresponding one-species Abstracted Equations. It is shown, for the case of difference equations, in particular, that analysis of the one-species models may often lead one to conclude that the system is stable, whereas in fact it is unstable due to overdamping. The final Discussion relates the results of the present paper to previous studies that anticipate the view presented here, and comments on the quarrel that has developed between those ecologists who believe in the existence of community-wide patterns of body size and the like and those who reject this view. It is suggested that the resolution of this dispute may depend on our ability to classify subsystems of species (i.e., guilds) with regard to the extent to which their internal organization is influenced by variation in the larger communities in which they are embedded. Finally, it is shown that Roughgarden's principal results for community coevolution can be deduced from the Abstracted Growth Equations of a particular subset of the entire community.

Forest Vegetation of the Big Thicket, Southeast Texas

Abstract: Tree and shrub species composition and environmental characteristics were measured in 56 stands representing the range of natural vegetation types of the lower coastal plain of southeast Texas, and vegetation characteristics were related to the physical environment. Most of the stand—to—stand variation in species composition was expressed by the first axis of a reciprocal averaging ordination, and stand position along the first axis in turn was most highly correlated with percent sand in the surface soil (0—15 cm) (r = —.81, P <.05). The first ordination axis is interpreted as a soil moisture gradient. A more weakly expressed second axis of variation is interpreted as a soil aeration—parent material gradient (from high organic matter, low extractable Ca stands to low organic matter, high Ca stands). Overstory basal area, woody plant species richness of overstory and understory, and relative basal area of broad—leaved evergreens in both strata all varied in a regular way along the first ordination axis, ha...

Structure and Function of Two Antarctic Terrestrial Moss Communities

Abstract: Two bryophyte-dominated communities in the maritime Antarctic are analyzed in terms of the transfer and standing crops of organic matter within them A moss turf dominated by Polytrichum alpestre and Chorisodontium aciphyllum and a moss carpet composed of Calliergon sarmentosum, Calliergidium austro-stramineum, and Drepanocladus uncinatus with the liverwort Cephaloziella varians were investigated at Signy Island, South Orkney Islands Biomass, respiration, and production data for the primary producers (mosses, lichens, liverworts, and algae), for the fauna (Protozoa, Rotifera, Tardigrada, Nematoda, Acari, and Collembola), and for the microflora were synthesized, and annual rates of consumption, egestion, assimilation, and production were derived The two communities showed similar levels of productivity, trophic structure, and efficiencies of organic matter transfer, but different Collembola and Acari standing crops, turnover of mosses, and accumulation of dead organic matter These features are discussed in relation to the role of biotic and abiotic variables in determining community structure and function

Population Dynamics and Bioenergetics of a Fossorial Herbivore, Thomomys talpoides (Rodentia: Geomyidae), in a Spruce-Fir Sere

Abstract: Studies of the bioenergetics of the northern pocket gopher, Thomomys talpoides, are coupled with data on demography, activity budgets, and microclimates to model the energy requirements of individuals and populations in the Wasatch Mountains of Utah during 1976—1979. Metabolic rates during rest increased linearly with decreasing ambient temperature, but burrowing metabolic rates (16.3 mL O₂°h— ¹°g— ⁰ . ⁷ ⁵) were independent of both temperature and physical properties of the soil. Radio—telemetry studies indicated that free—ranging gophers are active °50% of each day. Conservative estimates of true energy consumption were calculated using estimates of habitat—specific minimum daily burrowing requirements. Rates of burrowing measures in the laboratory were either °0.0 or °2.0 cm/min. The low burrowing rate was observed when the soil was frozen or saturated with water, as would occur in the field in early winter and in spring, respectively. Gophers burrowed through soil at the study site at an average rate of °1.5 cm/min. Belowground food energy densities at gopher foraging depth declined from 24.6 to 3.2 J/cm³ along a successional gradient (subalpine forb meadow to Engelmann spruce dominated forest). We conclude that individual gophers are food limited within the climax spruce seral stage. Further, daily energy costs associated with reproduction in females may exceed the belowground energy supply available in intermediate seral stage (aspen and subalpine fir). Reduction of burrowing rates for any reason will affect gophers in the late seral stages proportionately more than those resident in the meadow. The peak gopher densities recorded (from 62 individuals/ha in the meadow to 2 individuals/ha in spruce forest) support these inferences. Detailed demographic information was obtained only in the meadow seral stage. Adult survivorship was lower in winter than in summer and varied greatly between years (0.18—0.70 yr— ¹). Juvenile survivorship from weaning through the first winter was comparable to adult annual rates. The fertility rate was 3.75 young°female— ¹°yr— ¹. The energy supply and demand analyses indicate that the growth of Thomomys talpoides populations in the early seral stages is seldom directly limited by the amount of food present. From our demographic, environmental, and autecological studies we conclude that stochastic events associated with weather affect energy acquisition (burrowing) rates, and thus survivorship. In montane environments, such events may prevent populations from attaining sizes at which territorial behavior would hypothetically limit further increases. The energy flow through the meadow population at moderate to high (1976—1977) densities (at least 1100 MJ°ha— ¹°yr— ¹) indicates that pocket gophers are proficient energy movers relative to nonfossorial small mammals. Subalpine T. talpoides populations appear commonly to attain such densities. More than 30% of the annual primary productivity allocated to belowground parts of meadow forbs may be consumed by gophers.

Late-Pleistocene Vegetational Changes in Northeastern North Carolina

Abstract: Regional and local environmental changes spanning the last 30 000 yr have been reconstructed based on paleoecological studies of the sediments in Rockyhock Bay, a peat—filled Carolina Bay in Chowan County, northeastern North Carolina. During the Plum Point Interstadial (30 000—21 000 BP), temperate forests with both deciduous and coniferous taxa occupied the area. Some boreal elements and constituents of northern hardwoods forests were also present. Climate was somewhat cooler than at present, and Rockyhock was a shallow—water body. During the full—glacial and most of the late—glacial (21 000—10 000 BP), the regional forests were boreal in character and were dominated by northern pines and spruce. Many other boreal taxa were present. The climate was colder, more continental, and drier than at present. Water table in the basin was higher, and there was a dense growth of Isoetes on the bottom sediments. There is indication of slight lowering of water table about 14 400 yr ago. Deciduous forests containing c...

Evaluation of Mortality Factors in Insect Life Table Analysis

Abstract: An individual animal may suffer simultaneously from several different maladies, which lead to its eventual death. Such "overlaps" between contemporaneous mortality factors create some problems in partitioning, in a consistent manner, the animals killed among several specific causes. The aim of this paper is to develop the notion of "killing power" of a mortality factor, so that we can properly evaluate and incorporate in a model the effect of any given factor on the dynamics of the animal population concerned. I first introduce the basic notion of killing power under the assumption that the mortality factors involved are operating independently of each other. The killing power of a given factor is then shown to be equivalent to the marginal probability of an animal being killed by that factor. The assumption of independence of the mortality factors is relaxed, and the more general notion of conditional killing power is introduced. This is equivalent to the conditional probability of an animal being killed by a given mortality factor, when that individual has not been simultaneously affected by other contemporaneous factors. The concept of mutually exclusive mortality factors is discussed to clarify an apparent confusion in some published literature. Typical examples of this type of mortality factor are an extended drought and excessively wet weather, both of which may kill some animals, but which will not occur simultaneously. Therefore, unless the stage division in the life tables so broad that these types of factors can occur in the same stage, we do not need to consider mutually exclusive mortality factors. How to evaluate the killing power of a mortality factor is illustrated with some examples taken from the literature. Also discussed is a more complicated case, in which the annual fluctuation in the proportion of animals killed by a given factor is correlated with those killed by other contemporaneous mortality factors. Concluding remarks include some notes on the division of stages in a life table so as to facilitate the evaluation of the killing powers of as many mortality factors as possible.

Ecological Biogeography of Arthropods on Spartina Islands in Northwest Florida

Abstract: Experimental tests of the equilibrium theory of insular biogeography with the terrestrial arthropod fauna of small islands in Northwest Florida are described. Six islets composed of pure Spartina alterniflora (area: 56 m2 to 1023 m2; distance from mainland: 29 m to 1752 m) were defaunated with methyl bromide gas after intensive, inclusive censuses of their terrestrial arthropod inhabitants. Two additional islands and a large S. alterniflora stand on the mainland were not defaunated and served as controls. The recolonization process was monitored by weekly nonlethal censuses of all the sites. Four large cages were placed over other, similar, islands to determine the persistence of populations trapped inside the cages in the absence of immigration. In addition, six traps located in the study area showed the general movement patterns of arthropods. A distinction was made between transient and resident species colonizing the islands based partly upon 3 previous yr of data on the arthropod communities of the a...

The microbial origin of the allelopathic potential of Adenostoma fasciculatum H & A.

Abstract: The release of phytotoxic compounds has been proposed to be involved in the inhibition of seed germination under the canopy of Adenostoma fasciculatum (McPherson and Muller 1969). McPherson et al. (1971) tentatively identified the phytotoxins as phenolic compounds; however, the soil extraction data presented in this study indicate that insufficient quantities of available phenolics are present in the soil to account for the lack of growth observed in the field. Soil from burned and unburned Adenostoma—dominated sites were exchanged and planted with herb seeds. While toxicity developed in soil from the unburned site, the resulting pattern of germination suggested that the toxicity originated from the soil and not from the shrub canopy. Treatments to disrupt the metabolism of soil microbes, and to isolate selectively the microbial fraction from the soil, as well as other experiments, lead to the conclusion that the soil toxicity was the result of phytotoxins of microbial origin. It is suggested that the role of Adenostoma in the suppression of herb growth may lie in its association with soil microbes that produce and release substances capable of inhibiting the germination and growth of plants.

Phenology of the Drought-Deciduous Shrub Lotus scoparius: Climatic Controls and Adaptive Significance

Abstract: In regions of Mediterranean climate drought deciduousness has been considered an important adaptation for many species. This investigation focuses on the influences of a Mediterranean climate on the phenology of a drought-deciduous shrub Lotus scoparius ssp. scoparius. Two research sites were chosen in the Santa Ynez mountains northwest of Santa Barbara, Cali- fornia for the field investigations. Phenological progressions of leaf production, leaf composition, shoot elongation, lateral branch production, and flowering in even-aged stands of Lotus scoparius were correlated with such factors as temperature, relative humidity, soil moisture, plant water rela- tions, and photoperiod. These correlations were determined during a characteristic and uncharacter- istic growing season for this mediterranean-climate region. Field correlations were the basis for the design of laboratory experiments in which the influences of photoperiod, temperature, water stress, and total daily photon flux on the phenological development of Lotus scoparius were studied. Climatic controls over Lotus scoparius phenology are complex with water relations and photo- period having the greatest influence. Photoperiodic control over leaf abscission during water stress, and leaf production following dormancy, were two important phenological phenomena related to the unpredictably fluctuating Mediterranean climate of southern California. The complex climatic control over phenology in Lotus scoparius constitutes an adaptation to the California mediterranean climate.

Nutritional Ecology of Heliconia Herbivores: Experiments with Plant Fertilization and Alternative Hosts

Abstract: Efficiencies of food use, growth rates, and consumption rates were measured for larvae of eight tropical insect species, reared upon normal and fertilized Heliconia species (Zingiberales: Heliconiaceae) and Musa sp (Zingiberales: Musaceae) in Costa Rica Cephaloleia consanguinea and Chelobasis perplexa (Coleptera: Chrysomelidae, Hispinae) feed only on Heliconia Caligo memnon and Opsiphanes tamarindi (Lepidoptera: Brassolidae) are oligophagous, with known hosts in two families of Zingiberales Sibine apicalis, Sibine sp, and Metraga sp (Lepidoptera: Limacodidae) and Megalopyge sp (Lepidoptera: Megalopygidae) are polyphagous, with dicot and monocot hosts Foliar nitrogen content of natural Heliconia imbricata (° = 176%) was lower than that of H latispatha (° = 301%) and Musa sp (° = 30%) The oligophagous brassolids generally had lower growth rates, lower efficiencies of food use, and lower nitrogen accumulation rates on H imbricata than on Musa sp The hispines had lower relative consumption and growth rates than any of the lepidopterans examined on any host or treatment Growth rates and nitrogen accumulation rates for the hispine species were not affected by host plant fertilization Efficiencies of nitrogen use declined with increasing foliar nitrogen content for the hispines Several parameters of food use efficiency were correlated with larval feeding specialization The specialized hispines and oligophagous brassolids had higher efficiencies of food assimilation and nitrogen use on H imbricata than did the polyphagous limacodids and megalopygid Other efficiency indices, such as net growth efficiency, were not correlated with feeding specialization The hispine Ch perplexa has one of the longest larval developmental times known for a nondiapausing chrysomelid (°200 d) Its slow growth is correlated with a low metabolic rate, which we interpret as a physiological adaptation for starvation resistance Larvae feed only on rolled Heliconia leaves, which are produced infrequently by host plants, and long fasts are a predictable feature of development Ch perplexa's eight larval instars are about equal in duration, and closely match the duration of a rolled leaf's suitability as food for these larvae After a rolled leaf unfurls, the larvae must either wait for another on the same plant, or search haphazardly for one on nearby plants The feeding of Ce consanguinea larvae is apparently moisture limited in this tropical rain forest, and these larvae also have a high resistance to starvation The oligophagous brassolids have become pests of banana, which is introduced into the Americas These species increase growth rates with increased nitrogen content of host tissue They are infrequent on H imbricata, which has few chemical defenses against herbivores Thus, the low nitrogen content of H imbricata may protect it from some herbivory

The indivisible niche of tamiasciurus: an example of nonpartitioning of resources'

Abstract: Tree squirrels in the genus Tamiasciurus are adapted to exploit conifer seeds and fungi in boreal conifer forests. The ranges of the two species in the genus are parapatric, suggesting com- petitive exclusion. Where their ranges are contiguous in the Cascade Mountains of southern British Columbia, Canada, the squirrels differ in at least five characters which adapt each species to be competitively superior in its range. Two of the characters (alarm calls and fur color) adapt the squirrels to avoid predation and the other three (jaw strength, body size, and reproductive rate) adapt them to exploit the resources typical of their ranges. Two or three of these differences also allow each species to be competitively superior on its side of other sections of the species boundary in coastal British Columbia and northeastern Oregon. The two species form rare hybrids, especially where logging has created disturbed habitats. The pattern of carrying food to storage in a central cache is the basis for both the evolution of territoriality in Tamiasciurus and the absence of sympatry for the two species. In most cases where congeneric territorial species are sympatric and carry food to the center of the territory, they spend most of their time searching for and catching food and relatively little time carrying it. For Tamia- sciurus, on the other hand, food is easy to locate and most of the foraging time is spent in carrying food to the cache. Splitting the food niche of Tamiasciurus would involve too large an increase in time and energy for carrying food to be balanced by any gain in feeding and digestive efficiency that could result from specializing on a narrower range of food.

In Situ Foraging Responses of Three Species of Littoral Cladocerans

Abstract: Laboratory experiments have suggested that ingestion rates should increase with food concentration, that these functional responses should be depressed by increases in the availability of alternate foods, and that feeding preference should be shown for the most abundant of available foods. To test these hypotheses, simultaneous measures of the foraging of three species of littoral Cladocera, on two spatially differentiated food sources, were made in situ. The species investigated were Sida crystallina (O. F. Muller), Chydorus sphaericus (O. F. Muller), and Alona affinis (Leydig). The uptake rates of suspension and periphyton were measured with a double isotope technique. In situ foraging responses were determined through regression analysis. Ingestion rates were related to concentration of food, concentration of alternate food, and body size. Functional responses in situ ranged from Holling's type 2 to feeding inhibition, and the increased abundance of alternate food depressed functional responses in some...