Showing papers in "Ecological Monographs in 1978"

Community Development and Persistence in a Low Rocky Intertidal Zone

Abstract: This paper analyzes the factors controlling the development and persistence of patterns of distribution, abundance, and diversity of space users in the low rocky intertidal zone of New England. The spatial structure of this community changes along a wave exposure gradient. Mussels (Mytilus edulis) dominate at headlands exposed to wave shock, the alga Chondrus crispus (Irish moss) dominates at sites protected from wave shock, and both are abundant at areas intermediate in exposure to waves. Using a combination of experiments (exclosures, enclosures, removals) and observations, we evaluated the effects of several factors on this system, including (1) predation, (2) herbivory, (3) plant—plant competition, (4) plant—animal competition, and (5) physical disturbance from high—energy waves. The interaction having the greatest effect on the structure of this low zone association was predation. At protected sites, the starfish Asterias forbesi, Asterias vulgaris, and the snail Thais lapillus prey heavily on Mytilus, which is the functionally dominant competitor in the low (and mid) zone(s). When secondary succession is initiated by removal of all erect animals and plants, community development in the absence of these predators (predator exclusion) results in competitive elimination of both the barnacle Balanus balanoides and Chondrus by Mytilus. A similar result occurs if predators are excluded from unaltered stands of Chondrus. Controls in these experiments (i.e., with predators present) usually either developed to, or remained as stands of Chondrus. At intermediate sites, patches of Mytilus occassionally escaped from predation, suggesting predation intensity is patchy in space and time. Persistence of Chondrus is thus a by—product of the activities of predators at protected sites. At exposed sites, predators do not control the mussels. As a consequence, Mytilus outcompetes Chondrus and Balanus for space and achieves structural dominance. Periwinkle abundance decreases, and abundance and seasonality of ephemeral algae increase with increasing wave shock. Results of manipulations during both primary and secondary succession indicate that Littorina littorea, the only large, abundant herbivore in the low zone, has no direct effect on perennating (regrowing vegetatively) or established Chondrus, or on its extensive, encrusting holdfast. However, this periwinkle exerts an important indirect effect by consuming seasonally abundant ephemeral algae, which slow the rate of succession by suppressing growth of Chondrus. Once Chondrus is established, L. littorea damps variations in its abundance by cropping epiphytic ephemeral algae. The role of other herbivores seems negligible. Experiments show that limpets and sea urchins potentially could control Chondrus (and its holdfast), but they are normally too scarce to have a detectable effect on the algae. Chondrus thus monopolizes space at protected areas because (1) its competitors (mussels and epiphytic ephemeral algae) are removed by their consumers (predators and herbivores), (2) it has escaped control by herbivores, and (3) it can outcompete other perennial algae by virtue of its ability to perennate, and thus maintain its occupancy of space. The organization of this portion of the New England rocky intertidal region is thus similar in important ways to that of the mid zone (Menge 1975, 1976). Predation intensity, at least partly a function of wave shock, is great at relatively protected sites and determines the observed structural pattern (domination of space by algae). Since consumers are ineffective in controlling prey at exposed sites, mussels outcompete other space users and monopolize space on the shore. Thus, predators apparently determine the "trajectory" followed during succession. The persistence of algae is strongly dependent on the removal of mussels by predators. Herbivores, though ineffective in controlling the structurally dominant perennial algae, control the abundance of ephemeral algae and hence both determine the rate at which a Chondrus bed develops and stablize established beds of Irish moss by reducing variability in its abundance. Thus, herbivores evidently control the rate of community development (succession) and enhance the persistence of this alga. Disturbance from wave shock seems to operate in a fashion similar to both types of consumers by removing mussels and ephemeral algae. However, this sort of removal tends to be more catastrophic, frequently clearing large areas of space and initiating secondary succession, especially at exposed sites. The role of disturbance is thus largely that of inducing, rather than suppressing, variability in this system.

Temporal Variability Patterns and the Distributional Patterns of Stream Fishes

Abstract: The coefficients of variation of daily discharge and the logarithm of daily discharge were calculated from long term records in 15 river systems, including runoff—fed prairie rivers in Illinois and Missouri, groundwater— and spring—fed upland rivers in Southern Ohio, Southern Illinois and the Missouri Ozarks, and the snowmelt—fed Powder River in Wyoming. In almost all cases variability was lower in downstream sections. The patterns of temporal variability were consistent with the hydrology of the basins, with source of flow and size important determinants. Diversity patterns of fishes were tabulated for the same rivers from literature records. In all rivers, diversity increased rapidly from upstream to downstream sections, almost entirely the result of addition of new species with little replacement of the upstream fauna. Maximal species richness was usually in the lowest sections (order VI or greater). Headwater diversity was lowest in the rivers with the most variable headwaters, the increase in diversity downstream steepest for those rivers with the steepest decrease in variability, and the number of species in downstream sections was greater in rivers with more constant downstream sections. Longitudinal replacement rates were very low in all rivers studied, but were generally larger in the most temporally constant rivers. Literal replacement was moderate between small tributaries, but was low between major tributaries. The diversity gradients of individual feeding groups were similar to the gradient for all species, with an increase in richness from headwaters downstream in all groups with intermediate maxima sometimes seen. Piscivores showed consistently steeper diversity increases. These observations were related to hypotheses of factors controlling stream fish distributions. They are most consistent with hypotheses relating control to extermination—recolonization dynamics or the interspecific competition mediated by temporal variability and habitat structure.

The Metapopulation Ecology of the Red-Spotted Newt, Notophthalmus viridescens (Rafinesque)

Abstract: The population dynamics and ecology of the red—spotted newt were studied from 1974 to the present in a series of mountain ponds in the Shenandoah Mountains, Virginia. Adult and juvenile newts were censused using a combination of methods, including drift fences and dip—netting. A technique of individual recognition was developed utilizing the number and pattern of dorsal red spots. Assisted by a simple system of toeclips, designating pond and year of first capture, these methods provided records of age, survival, and movements of >8,500 individuals. The results revealed 2 migratory peaks of adults each year: a breeding migration to ponds in March, and an August—September emigration to terrestrial hibernacula. Breeding population size, ranging from 6 to >2,600 adults per pond was stationary during the study but was significantly correlated with pond age. Annual turnover of breeding populations to new adults exceeded 50%. Sex ratios were consistently 2 ♂ ♂: 1 ♀ in the breeding populations but tended toward 1:1 in the cohorts of new recruits. Annual adult ♀ survival was density dependent and significantly less than that of♂ ♂. The average ♀ expected to breed only 1.3 times, whereas ♂ ♂ had an expectancy of 1.9 breeding seasons. Faithfully homing to the same pond year after year, adults never migrated to new ponds between breeding seasons. Juvenile production at most ponds was poor over the 3 yr. Only 1 pond had a reproductive output consistently above a calculated replacement rate. It is inferred from the combination of poor reproduction and high immigration rates that breeding adults were not replaced by their own progeny but rather by foreign—born individuals. The red—spotted newt is basically a colonizing species responding to pond habitats that rapidly shift in time and space. It is suggested that beaver ponds have the necessary attributes of small size and temporal instability to which the life—history patterns of newts seems adapted. The majority of ponds in this area functioned as reproductive sinks in a larger, metapopulational structure. Adults in such ponds were reproductive failures or most of their lives. Only a small number of adults accounted for the regional reproduction in 1974 and 1975, but I very populous pond produced >91% of the young in 1976. Because reproductive success is pond dependent, interdemal selection may play an important role in newt evolution. It is hypothesized that homing behavior evolved as a consequence of natural selection within metapopulation centers and that the eft stage is the mechanism of dispersibility to new ponds. A new model of metapopulational dynamics is presented.

Ecological interactions between plants and hummingbirds in a successional tropical community

Abstract: At Monteverde, Costa Rica, 10 successional plant species used 14 hummingbird species for pollination. Displacement among flowering seasons suggests that the plants competed for polli- nators. There was no evidence that the flowering of one plant influenced hummingbirds to abandon another. Pollination in simultaneously flowering plants likely suffered nonetheless, since birds tended to move indiscriminately among flowers of different species and could lose much pollen between successive visits to conspecific plants. This may have led to scatter in flowering peaks by favoring the quick establishment of plant colonists with unique flowering seasons over colonists whose flow- ering seasons coincided with those of established species. The continuous supply of nectar provided by staggered flowering peaks maintained a continuous supply of hummingbirds competing for nectar. Even inconspicuous plants with few flowers received sufficient hummingbird visits for moderate to high potential rates of outbreeding. At large, flower- laden trees and shrubs, hummingbirds defending feeding territories evidently effected much inbreed- ing, but movements of intruders between territories kept inbreeding from becoming absolute. Nectar secretion rates varied widely among flowers of each of the 5 plant species in which nectar volume was measured. Many flowers produced little or no nectar, while a few secreted quite copious volumes. This "bonanza" pattern may benefit plants by reducing caloric expenditures on nectar while increasing the duration of hummingbirds' foraging bouts. The latter possibility was tested and verified experimentally with artificial flowers exposed to a free-living hummingbird on Trinidad, West Indies. When pollinators are abundant, plants with "bonanza" patterns can attract consistent visitors and rare, inconspicuous plants can count on consistent service. At Monteverde, the unspecialized, op- portunistic nature of both plants and birds assured abundant hummingbirds and resulted in a well- integrated complex of plants and pollinators despite the transient nature of the successional habitats.

Coral reef fish communities: unstable, high-diversity systems?'

Abstract: We examined the pattern of development of assemblages of coral reef fishes on artificial reefs at One Tree Reef, Great Barrier Reef. Two sets of 8 reefs were built. To investigate the effects of habitat structure on species diversity, each set consisted of replicate pairs of 4 different reef types: plain (no holes), small—holed, medium—holed, and large—holed reefs. To examine seasonal differences in colonization, 1 set was established in summer (October 1971) and the other set in winter (July 1972). Visual censuses of the reefs' fish assemblages were made at monthly intervals over 32 mo (summer set) and 23 mo (winter set). We recorded a total of 105 resident species from the artificial reefs. Although less diverse, the fish assemblages differed little in terms of species composition from those of small natural patch reefs. Artificial reefs showed a high between—reef variability in species composition that was largely unrelated to habitat structure. Competitive interactions between species appeared unimportant in explaining the distribution of species between reefs. Likewise, possible positive associations between species were indicated for only a few species. Almost all settlement of the artificial reefs was by juvenile fishes. Recruitment was markedly seasonal and occurred mainly over summer (September—May). Juvenile recruitment patterns showed little year—to—year constancy. Most species tested showed nonrandom, clumped patterns of settlement on replicate reefs. More than half of species tested showed no significant differences on numbers of individuals which settled on different reefs, but the remainder showed significantly greater settlement on some reefs, suggesting habitat selection. For a few species, there was also greater recruitment to reefs already occupied by conspecifics, suggesting possible social facilitation of settlement. For most species, there were no significant interspecific interactions between adults and newly settling juveniles. Between—census variability in species composition was high and species turnover ranged from °17% of species/mo in winter to °39% in summer. Maximum times of persistence for most species were <12 mo, and for many individuals survivorship was of the order of only a few months. Much of the high turnover appears due to losses through predation. There was no evidence that a persistent species equilibrium was attained during colonization. Because of predation and the seasonally varying, uncertain nature of recruitment, we see the development of any long—term species equilibrium as unlikely. We suggest that nonequilibrium conditions are a characteristic of coral reef fish communities and that because of these conditions, high within—habitat diversities are maintained.

Species Number and Compositional Similarity of the Galapagos Flora and Avifauna

Abstract: The flora and avifauna of the Galapagos Islands are used to reexamine quantitative analyses of species numbers and compositional similarities. Conflicts in the results of previous analyses of the Galapagos flora are reconciled and are shown to be the result of using different species numbers and physiographic parameter measurements. The idea that Galapagos plant species numbers largely reflect conditions in the archipelago during the Pleistocene glaciations (i.e., that relaxation times are very long) is criticized, and the more parsimonious alternative, that they reflect recent conditions, is proposed. Generalizations about factors determining species numbers that are based on multiple regression and correlation are precarious. The number of botanical collecting trips to each of the Galapagos Islands is a better predictor of species numbers than are area, elevation, or isolation. Two null hypotheses concerning the determination of floral and avifaunal compositional similarities among the Galapagos Islands...

Microclimate control of growth rates and habitats of the boreal forest mosses, Tomenthypnum nitens and Hylocomium splendens

Abstract: Tomenthypnum nitens forms tall turfs in fens where the tree and shrub canopy is sparse and the water table is close to the surface. The feather moss species (Hylocomium splendens, Pleurozium schreberi, and Ptilium crista—castrensis) form wefts in shaded and better drained habitats. Growth of T. nitens was measured using small wire stakes, from which net production averaged 190 g/m2 in 1975. Growth rates were correlated with color of the apical region, the result of variation in apical water contents. Seasonal variation in growth rate was correlated with depth from the canopy surface to the ground water table, controlled by the balance between rate of transport of water through the canopy and loss by evaporation. The most important environmental factor appears to be total precipitation, followed by evaporation stress. Growth of the feather mosses was estimated by measuring dry weight changes of segments of Hylocomium splendens. Net production averaged 79 g/m2. Growth was correlated with the length of time that the moss was wet, the result of precipitation frequency. Because growth rates were measurably reduced by removal of a sparse shrub canopy, evaporation stress is probably another important factor. Survival and growth of Tomenthypnum nitens is limited by evaporation stress in habitats where ground water is not available. Feather mosses are limited by radiation damage and evaporation stress in open habitats, by depression of net assimilation and other deleterious effects of saturation in wet habitats, and by insufficient rainfall under dense tree canopies.

Community Structure, Dynamics and Nutrient Cycling in the Okefenokee Cypress Swamp-Forest

Abstract: Aspects of community structure and nutrient circulation are described for the cypress (Taxodium distichum) forest in Okefenokee Swamp, Georgia, USA. This bog environment is char- acterized by low nutrient availability and large peat accumulations. The tree stratum of the forest is dominated by cypress, which is probably due to recurrent under- story fires which eliminate other swamp species. In stands throughout the cypress forest, the density and total basal area of living stems >4 cm diameter vary greatly, but mean values (1,465 stems/ha; 52 m2/ha) are high compared to upland forests. Natural thinning appears to be unimportant. Abundant standing dead trees suggest that differences in density among stands are due to past differences in the frequency and intensity of forest fires during periodic droughts. At an intensive study site, biomass and net primary production were measured using techniques of dimension analysis. Cypress trees compose 98% of the total forest above-water biomass of 307 tonnes/ha. Most of the cypress biomass is in tree boles (96%), little is in foliage (0.8%). Presumably due to the acid, nutrient-poor conditions in the swamp, total above-water net primary productivity is low (692 g m-2 yr-1); the high biomass is the result of the old age (150 yr) of the stand and the high density of trees. Cypress net production is largely channeled into bole wood (41%) and current twigs with needles (41%). It appears that in recent years, above-water biomass has been maintained at a constant level by fire-induced thinning. The nutrient pools in the above-water community are large (666 kg/ha Ca, 111 kg/ha Mg, 230 kg/ ha K, 996 kg/ha N and 46 kg/ha P). These nutrients are largely contained in cypress boles and do not freely circulate. The annual nutrient uptake in the forest is rather small compared to that in upland forests. The nutrient uptake (54 kg ha-' yr-1 Ca, 11 each for Mg and K and 2.3 for P) is largely channeled to the small amount (231 g/m2) of very efficient cypress foliage. Foliage abscission and foliar leaching by rainfall return 73 to 91% of the annual nutrient uptakes each year; therefore, there are only small permanent additions of nutrients to the above-water pools each year. Except for K which is apparently reabsorbed from foliage before abscission, the cypress trees do not appear to conserve foliar nutrients. However, nutrient conservation may be effected by minimizing the amount of foliage. Litterfall from the forest has resulted in a large accumulation of peat. Although some nutrient regeneration apparently occurs, these peat deposits contain large, permanent nutrient losses from the community. Nutrients cycling is dissolved form (e.g., foliar leachates) and nutrients received from atmospheric precipitation are likely to be of special importance to the nutrition of this closed-basin swamp forest community.

Vegetation and Tree Species Patterns near the Northern Terminus of the Southern Floodplain Forest

Abstract: The bottomland forest on Horseshoe Lake Island, located on the Mississippi alluvial plain in Alexander County, Illinois in comprised of 2 stands, one relatively undisturbed and the other which is recovering from disturbance in the late 1800s or early 1900s. In both stands vegetational structure, gradient relationships, diversity and size—class characteristics were studied. In the old—growth stand, 35 soil—site variables were measured or estimated for interpretation of vegetational patterns and species distributions. Both direct and indirect gradient analyses were evaluated for use in floodplain forest studies and ultimately the indirect approach with multiple regression interpretation was used to construct a gradient model of the vegetation. Basal area and density of the old—growth and floristic composition of both stands are typical of much of the original Southern Floodplain Forest. The coenocline was divided into 3 segments, mesic, transition and wet with the first and last corresponding to the ridge and hardwood bottoms described by Braun (1950). The important species on the ridge bottoms are Acer saccharum, Asimina triloba, Liquidambar styraciflua, Ulumus rubra, Quercus rubra and Tilia americana and are associated with well—drained soils and infrequent flooding. The transitional segment is dominated by Asimina triloba, Liquidambar styraciflua, Quercus michauxii, Quercus muhlenbergii and Ulmus americana with Quercus pagodaefolia and Quercus shumardii as associates on moderately heavy and poorly drained soils with intermediate flooding. The hardwood bottom sites are dominated by Acer rubrum, Fraxinus pennsylvanica, Liquidambar styraciflua and Ulmus americana and may be flooded for several months each year and have heavy textured, poorly drained soils. Multiple regression analysis with ridge regression revealed that Fraxinus americana and Acer saccharum have wider tolerances to flooding and poor aeration than previously reported. Importance Values of several species were related to duration and depth of flooding, soil mottling and soil texture indicating that distributions are affected by a site—inundation, soil drainage—aeration complex. Diversity of the tree stratum is comparable with that found in some of the mixed mesophytic forests elsewhere. Due to competition from Asimina triloba and Aesculus discolor and possibly some environmental limitations due to the northern location of the stands, the shrub—sapling stratum is depauperate. Along the coenocline from mesic to wet sites density and equitability decrease while basal area increases. Size—class analysis indicates that both the negative power and negative exponential functions fit the size—class distribution for the 2 stands but that neither is entirely appropriate. The old—growth stand is characterized by 3 groups of individuals each with different mortality rates which give a plateaued deviation from each model. The secondary stand is more balanced and does not exhibit a well developed plateau. A comparison of individual species reveals that various regeneration patterns are present which range from uneven—aged (sized) in the case of Acer saccharum to even—aged for Fagus grandifolia, Quercus rubra and Ulmus rubra. The plateaued pattern in the old—growth stand does not reflect past disturbance but apparently is a characteristic of this forest. The principal mode of regeneration in this stand is gap—phase replacement.

Foraging movements of a frugivorous bat, Carollia perspicillata (Phyllostomatidae)

Abstract: The foraging movements of a Neotropical frugivorous bat (Carollia perspicillata) were studied to better understand its impact on the plants it disperses and the factors influencing its foraging behavior. Radio telemetry was used during 2 wet seasons to monitor the bats' movements in a Costa Rican seasonal tropical forest. We monitored 24 individuals for periods of up to 19 nights. This sample size permitted a statistical analysis of the influence of bats' weight, bat's sex, moonlight, and the distribution of resources on foraging patterns. Carollia perspicillata is a refuging species: individuals dispersed from shared day roosts to feed at night, then returned after foraging. At night there were 3 major activity periods, but there was some activity all night. When foraging, each individual went to 2 to 6 feeding areas that were used consistently through the observation period. Each night it flew back and forth between areas. Search- ing for new feeding areas was uncommon. Average flight distances for individuals were 1.6 kilometres between the day roost and feeding areas, 1.5 km among feeding areas, and 1.6 km to gather fruits within feeding areas. The density at feeding areas decreased with distance from the day roost, as predicted by refuging theory. There was little overlap among different individuals' feeding areas, but the largest amount of overlap was adjacent to 1 day roost. Bats from the 2 major day roosts shared feeding areas only at locations midway between the roosts. Flight patterns were influenced by bats' weights, the patchiness of resources, and the amount of moonlight. From these and other observations, we infer that both predation and energetic consider- Nations affect C. perspicillata foraging. In addition, this species appears to respond to long-term factors as well as night-to-night conditions so that dynamic models will be necessary to describe its foraging.

A Contribution to the Theory of Competing Predators

Abstract: This paper concerns the growth of 2 predator species competing exploitatively for the same prey population. The prey population grows logistically in the absence of predation, and the predators feed on the prey with a saturating functional response to prey density. Specifically, we assume that Michaelis-Menten kinetics or the Holling "disc" model describe how feeding rates and birth rates change with increasing prey density. We focus on the question of which predator species will survive and which will not, given the growth parameters of the prey and the functional response parameters of the 2 predators. Which predator wins or loses depends critically on the relative mag- nitude of the prey carrying capacity, K, and the X parameters of the 2 predators. K, represents the prey density at which the ith predator "breaks even" (equal birth and death rates). This prey density is defined by the product of the predator's half-saturation (Michaelis-Menten) constant times the ratio of the predator's death rate to its intrinsic rate of increase. Coexistence is also possible for a wide range of parameters, but only as a periodic solution. A primary conclusion is that coexistence is possible only if the predator with the smaller half-saturation constant also has the smaller birth-rate- to-death-rate ratio. This necessary constraint is the mechanistic equivalent to requiring that I predator be an "r-strategist" and the other be a "K-strategist." This condition is insufficient to guarantee coexistence, however. If the prey carrying capacity, K, is "too small" the K-strategist wins, and if K is 'too large," the r-strategist wins. The bounded region of intermediate K values permitting coexistence is defined by the functional response parameters of the 2 predator species. The greater the disparity between the half-saturation constants of the 2 predators, the larger the region of K permitting coexistence.

Factors Regulating the Distribution and Population Dynamics of the Amphipod Gammarus palustris in an Intertidal Salt Marsh Community

Abstract: Seasonal changes in the intertidal distribution, abundance and population dynamics of the epifaunal amphipod Gammarus palustris were studied in salt marshes bordering 2 estuarine rivers flowing into the Chesapeake Bay. The amphipod populations inhabiting the Patuxent River study site showed abundance peaks during the spring and autumn, and major declines in density during the summer and winter. Populations at 2 study sites in the Rhode River showed only 1 peak of abundance during the late spring and early summer which was followed by a decline in density throughout the remainder of the year. In both rivers, the low amphipod densities observed during the winter corresponded with a subtidal migration. Migrations did not account for the low numbers observed at other times and an examination of the life cycle of this species as well as an egg—ratio analysis of the populations indicated that low densities during the summer (Patuxent) or late summer and fall (Rhode) were not due solely to life—cycle events. The...

Reproductive Interdependence of Pinon Jays and Pinon Pines

Abstract: Pinon Jays (Gymnorhinus cyanoephalus) and pinon pine trees (Pinus edulis) interact in a mutualistic fashion, in that the jays provide a primary means of seed dissemination for pinon trees in the Southwest, while at irregular intervals the trees provide the jays with an abundant and highly nutritious source of food. Availability of pinon seeds permits both late winter (February) and late summer (August) breeding by the jays. Seeds and photoperiod interact synergistically to accelerate gonadal development in late winter. When seeds are abundant, considerable testis growth takes place in some Pinon Jay ♂ ♂ even before the winter solstice. This relationship was investigated experimentally. Use of a specific food (pinon seeds) as a proximate timer for breeding, the ability of individual jays to breed in spring, molt, and breed again in August without an intervening period of gonadal refractoriness, and testicular development in early December, when day length approaches its minimum, are extraordinary characteristics among north—temperate—zone passerine birds. The synchrony of seed production by pinon pines over large geographic areas is interpreted as an evolved mechanism that (1) overwhelms invertebrate seed and cone predators, and (2) increases the numbers of seeds per tree that will be cached by Pinon Jays and other vertebrates. Caching sites of Pinon Jays are often at locations especially conducive to germination and growth of pinon pine seedlings.

The phenology, growth and ecosystem dynamics of Erythronium americanum in the northern hardwood forest.

Abstract: In the northern hardwood forest, growth of vernal photosynthetic herbs is temporally restricted to the period between spring snowmelt and summer canopy development. This characteristic suggests that several unique adaptations exist which allow the species to complete their life cycles, and that temporal separation of production in the herbaceous layer may add to structural and functional complexity of the ecosystem. Erythronium americanum Ker. (Liliaceae) was examined in central New Hampshire with respect to its natural history, growth characteristics and influence on energy flow and mineral cycling in the deciduous forest ecosystem. Growth leading to the early spring development of photosynthetic tissue begins with fall root growth and continues through a long winter phase during which the shoot elongates from the perennating organ, through the soil and into the snowpack. Following snowmelt, the shoots begin rapid unfurling and maturation of the photosynthetic tissue. The length of the mature leaf phase is controlled by the timing of snowmelt and canopy development, and may be quite variable between successive years. During the short period of production, total biomass increased by 190% in 1972 and 338% in 1973; however, plant weight at the end of the winter period in 1973 had decreased to 28% of the spring 1972 maximum. In the annual energy cycle, biomass losses during the nonphotosynthetic period may amount to more than production during the preceding spring. In comparison with summer green herbs, Erythronium shoot tissue contained significantly higher concentrations of N but lower levels of K, Mg and Ca, suggesting that the spring adaptation may be oriented toward higher N levels of the soil during the spring period as well as higher light levels at the forest floor. Significant correlations of biomass of vernal photosynthetic herbs with summer green species imply that temporally separated species may utilize the same physical site and resources. This adds to the structural complexity and production of the herbaceous layer; however, the vernal photosynthetics account for only 0.5% of total aboveground primary production of the ecosystem. The temporal character of Erythronium's growth and its capacity for rapid biomass accumulation combine to make it a significant factor in nutrient dynamics in the deciduous forest. Uptake of N and K during spring flushing of nutrients from the ecosystem and later release through senescence of shoot tissue appear to reduce gross ecosystem losses of these elements.

Fish species diversity on model and natural reef patches: experimental insular biogeography'

Abstract: Fish communities associated with model and natural reef patches were studied to determine the effects of heterogeneity, area, and isolation on the number of fish species in these systems. The effects of season and succession on the structure of these communities were also examined. Species turnover was estimated to determine if numbers of fish species on small reefs can be viewed as a balance between immigration and local extinction. Interspace size diversity, one facet of reef heterogeneity, was not positively correlated with the number of fish species on either model or natural reefs. In addition, interspace size diversity had no significant effects on species composition. In most of the cases examined reef height, another aspect of reef heterogeneity, was positively correlated with the number of fish species and species diversity, H', on natural reefs. Observations of selected fish species inhabiting model reefs support the hy- pothesis that vertical zonation is a means of resource partitioning in these fish communities. A negative correlation was found between reef isolation and number of fish species on patch reefs. This corre- lation was strongest during periods when fish population sizes were lowest. Season had more of an effect on the structure of fish communities on model reefs than did succession. Immigration and local extinction of fish species occurred on model reefs throughout the study. Patterns of immigration and extinction approximated the predictions of the MacArthur-Wilson equi- librium model of insular zoogeography when species turnover was highest.

Habitat Selection and Interspecific Territoriality among the Sylviid Warblers of England and Sweden

Abstract: The first part of this paper describes the quantitative characteristics of breeding habitats of sylviid warbler species, chiefly Sylvia and Phylloscopus, in Yorkshire, northern England, and in southern Sweden. The second part of the analysis is made up of the factors that permit or preclude the coexistence of various combinations of warbler species–vegetation structure, food density, and competitive regime. Vegetation structure was measured in 346 warbler territories in the 2 locations, including 151 Phylloscopus territories (P. trochilus, collybita and sibilatrix), 171 Sylvia territories (including S. communis, curruca, atricapilla, borin, nisoria and undata and 16 Hippolais icterina territories. From data on the distribution of the vegetation density over height in territories, discriminant function analysis generates new canonical habitat variables U1 and U2 that are independent of each other and reflect the correlation structure of the original habitat variables. Now each species' habitat niche can be shown as a family of "confidence ellipses" in the habitat plane of U1 and U2, and both species habitat preference and interspecific overlaps easily measured, within and between genera and within and between countries. As a result of this analysis, the following conclusions are drawn. The 3 English canopy—foraging Phylloscopus show some interspecific habitat segregation but more noticeably a great deal of overlap in preferred habitat. This overlap is reflected by the accuracy with which the specific occupant of a habitat patch can be predicted on the basis of the vegetation—structural characteristics of the patch: on average 2/3 of such predictions are verified, 1/3 is wrong. Similarly in Sweden, where 1 Phylloscopus drops out and is replaced by an additional sylviid, Hippolais icterina, predictions on specific habitat occupancy are correct just over 2/3 (72%) of the time. But the positions of habitat niches are rearranged between the 2 countries, because in Sweden the habitats of the absent collybita are taken over partially by sibilatrix but mostly by trochilus, and the newly added icterina occupies habitat that in England would be most typical of trochilus. Despite these differences, canopy warblers in both countries use very similar vegetational criteria in habitat selection, and predictions as to the specific occupant of a habitat based on discriminant function analysis in 1 country but tested by warbler data from the other country are nearly as successful as are within—country tests. Five Sylvia species were studied in each country, with 4 species in common. Interspecific habitat segregation is poorer than in Phylloscopus (54% of habitat occupants correctly classifiable). Four species occupy similar habitats in both countries with but minor habitat shifts, and in addition the Swedish nisoria occupies habitats that are structurally more similar to the English undata than to any other Swedish or English warblers. Again, cross—country predictions are just as successful, or as unsuccessful (@50% correct ), as within—country predictions. The second part of the paper describes and analyses the precise disposition of warbler territories within limited number of study sites in a range of habitat from low scrub to tall woodland. Pilot studies in English habitats show that 3 pairs of species: Phylloscopus trochilus and P. collybita, Sylvia atricapilla and S. borin, and Sylvia communis and S. curruca, species pairs with considerable overlap in preferred habitat type, are strongly interactive and show partial to complete interspecific territoriality. More detailed studies were conducted at 3 Swedish locations. In low woodland/tall scrub at Horby, 4 Sylvia species nested. Sylvia communis is shown to leave some potential territories in the field unoccupied, while S. borin occupies practically all habitat that ranks as highly acceptable. However, just 1 pair of atricapilla is present despite the fact that at least 4 additional atricapilla territories might be accommodated in suitable habitat, and likewise curruca fails to occupy all suitable habitat at the site. The presence of other Sylvia species, in particular the dominant borin, accounts for low densities of curruca and atricapilla, and a combination of statistical analysis of the disposition of territories and direct observation of supplanting and replacement through aggressive behavior confirms that interspecific interactions modify and qualify habitat acceptability. In tall woodland at Hoganas borin and atricapilla interact strongly over habitat, and atricapilla attains high occupancy levels only in habitat that is marginally acceptable to borin. The interaction between them is not affected by the presence or absence of Phylloscopus trochilus. At Bejershamn in low scrubland 5 Sylvia species and 1 each of Hoylloscopus and Hippolais breed. The species display a variety of levels of interspecific interaction, from a direct interspecific territoriality (borin—nisoria, communis—nisoria) to more subtle interactions of partial exclusion (borin—curruca). Here diffuse competition is evident (e.g., icterina is not found with borin and curruca together even though the last 2 occur frequently together and even though Hippolais icterina occurs with either separately). With increasing vegetation height in icterina habitat where more warblers coexist with overlapping territories, from 1— to 2— and 3—species combinations. But only 2/7 one—species combinations, 3/21 two—species combinations and 2/35 3—species combinations are common, stable and compatible. The remainder are precluded by differential habitat selection, by simple competition mitigated by direct behavioral interactions, or by diffuse competition with more subtle avoidance behavior. The role of song convergence in the maintenance of the interspecific interactions and the role of food density and distribution in the formation of compatible species combinations is documented, and further questions about the maintenance and evolution of the system are identified and discussed.

A Hierarchical Model of Lotic Ecosystems

Abstract: This paper presents the structure and properties of a total stream model that simulates the dynamics of small, flowing—water ecosystems in the northwestern United States. Conceptually, the model is hierarchically structured, consisting of 7 basic processes: periphyton dynamics, grazing, shredding, collecting, invertebrate predation, vertebrate predation, and detrital conditioning. These processes are subprocesses of 3 echelons of higher level processes: detritivory; herbivory; primary consumption; predation; and the total ecosystem. The model has 14 state variables in the 7 basic processes, and is conceptualized in discrete time with a basic time step corresponding to 1 day. Behavior of the stream model relative to different schedules of energy inputs and to the practice of clear—cut logging was investigated and related to contemporary theory of lotic ecosystems. In general, model behavior indicated that the regulation of biological processes in streams is complex, the mechanisms of which vary seasonally and from process to process. If a process is regulated by food supply, its annual production tends to increase as predation increases, while mean biomass may or may not be affected appreciably. In contrast, an increase in predation tends to decrease both mean biomass and annual production in processes regulated primarily by predation and such life history phenomena as insects emergence. The stream model provided the stimulus that led to a mathematical expression for the rate of production at the level of the entire ecosystem, and model behavior suggests that this rate tends to remain constant along a continuum from small, first—order streams with no tributaries to larger rivers which eventually drain into the sea.

Foraging strategies, diversity, and seasonality in bird communities of Appalachian spruce-fir forests [of North Carolina and in northwestern Maine].

Abstract: Breeding bird communities in spruce—fir forests of the southern Appalachian mountains are considerably less diverse than those of similar northern forests. Censuses also show more widespread coexistence of congeneric and confamilial species in the north. Long—distance migrants dominate the northern forests while resident species dominate in the south. In a 4—yr study of these bird communities, I used observations of foraging strategies and resource partitioning to test hypotheses that might explain this probematic latitudinal gradient in species diversity. I conducted quantitative studies of foraging behavior of arboreal insectivorous birds at sites with very similar plant communities in the Great Smoky Mountains of North Carolina and in northwestern Maine. Six species that breed in both north and south had consistently more generalized foraging in the south, as measured by spatial distribution of foraging, variety of techniques used, directionality in probing and flights, and patchiness of habitat use. T...

Foraging Strategies, Diversity, and Seasonality in Bird Communities of Appalachian Spruce-Fir Forests

Abstract: Breeding bird communities in spruce—fir forests of the southern Appalachian mountains are considerably less diverse than those of similar northern forests. Censuses also show more widespread coexistence of congeneric and confamilial species in the north. Long—distance migrants dominate the northern forests while resident species dominate in the south. In a 4—yr study of these bird communities, I used observations of foraging strategies and resource partitioning to test hypotheses that might explain this probematic latitudinal gradient in species diversity. I conducted quantitative studies of foraging behavior of arboreal insectivorous birds at sites with very similar plant communities in the Great Smoky Mountains of North Carolina and in northwestern Maine. Six species that breed in both north and south had consistently more generalized foraging in the south, as measured by spatial distribution of foraging, variety of techniques used, directionality in probing and flights, and patchiness of habitat use. T...

The Reproductive Cycle of Microtus ochrogaster in Eastern Kansas

Abstract: About 800 Microtus ochrogaster were live—trapped at biweekly intervals from May 1971 through March 1973 in 3 grassland study areas in eastern Kansas, USA. Details of reproduction were determined by autopsy. Population density increased through the first winter, reached a peak in April 1972, then declined sharply during that summer and beyond. Body length, but not body mass, tended to be positively related to density. Both sexes matured at about the same weight; development was somewhat prolonged during the winter months, especially in the winter preceding the population peak. Pregnancy rates were high, approaching maximal iteroparity, throughout the study, with intervals of nonbreeding in both Julys, and in August and December of the population decline. Embryo counts increased significantly during the months of peak density but did not diminish in the period of the population decline. There was no association between either maternal weight or parity and number of embryos. Corpora counts were not higher in...

Ecological shift and release in an isolated population: conus miliaris at easter island'

Abstract: With the exception of a recently discovered rare species, the gastropod genus Conus is represented only by Conus miliaris at Easter Island, an isolated outpost of the Indo-West Pacific (IWP) marine biota probably <2.5 million yr old. Throughout most of the IWP region, C. miliaris co- occurs with 4-23 congeners on marine benches and coral reefs. Compared with these central popu- lations, Easter Island C. miliaris (1) is more abundant in favorable habitats, (2) utilizes different habitat and food resources, and (3) is ecologically released with respect to food and probably depth range, but not microhabitat. Most strikingly, the major prey species (Onuphis sp.; 31% of the diet) belongs to a family of polychaetes not known to be exploited by any Conus species elsewhere. In all, 6 species of polychaetes eaten rarely or not at all by C. iniliaris elsewhere comprise 77% of its diet at Easter Island. Sixteen prey species are eaten at Easter Island (vs. 2-8 in central IWP localities) and prey species diversity measured by the H' statistic is 2.0 vs. 0.2-1.3. Shifts to larger prey species with increasing size of C. miliaris account for some of the high prey diversity, but analysis of multiple feedings revealed that 68% of such individuals had eaten polychaetes of 2 or more species. Several polychaete species that C. miliaris preys on at Easter Island but not elsewhere are important com- ponents of the diets of other Conus species in the central IWP. Some of the ecological expansion of C. iniliaris at Easter Island is thus directed toward resources that absent congeners would be expected to utilize were they present, or if they should colonize the island in the future. It is suggested that the virtual absence of congeneric species and increased intraspecific competition in dense populations at Easter Island have selected for the observed ecological expansion.