Showing papers in "Ecology in 1977"

Turnover Rates in Insular Biogeography: Effect of Immigration on Extinction

Abstract: Demographic and genetic contributions from nonspecific immigrants tend to reduce ex- tinction rates of insular populations. The MacArthur-Wilson model of island biogeography is modified to provide for this effect of immigration on extinction, which we call the rescue effect. This new model predicts that when immigration rates are high relative to extinction rates, turnover rate is directly related to the distance between an island and the source of colonizing species. A field study of the distribution of arthropods among isolated plants supports the model.

Resource Competition between Plankton Algae: An Experimental and Theoretical Approach

Abstract: The results of 76 long—term competition experiments between two species of freshwater algae (Asterionella formosa and Cyclotella meneghiniana) grown along a resource gradient agree with the predictions of two different models of resource competition Both models are based on the functional resource—utilization response of each species to limiting resources The Monod and the Variable Internal Stores model of competition made similar predictions Asterionella was observed to be competitively dominant when both species were phosphate limited; Cyclotella was dominant when both species were silicate limited; and both species stably coexisted when each species was growth—rate limited by a different resource Almost 75% of the variance in the relative abundances of these two species along a natural silicate—phosphate gradient in Lake Michigan is explained by the Monod model

Bird Species Diversity on an Andean Elevational Gradient

Abstract: This paper analyzes patterns of bird species diversity on an elevational transect of the Cordillera Vilcabamba, Peru. Major changes in climate and vegetation are encompassed by the tran- sect which extended from the Apurimac Valley floor at 500 m to the summit ridge of the range at > 3,500 m. Four vegetation zones are easily discerned-lowland rain forest, montane rain forest, cloud forest, and elfin forest. In progressing upwards there is a monotonic trend toward decreasing canopy stature and reduced number of plant strata. The vegetation gradient provided the opportunity to examine the relation between bird species diversity and habitat complexity in an entirely natural setting. The decrease in forest stature with elevation was closely paralleled by decreasing avian syntopy (the total number of bird species cohabit- ing the forest at a given elevation). Bird species diversity was shown to be highly correlated with foliage height diversity, using either four or five layers in the foliage height diversity calculation (r = .97), and less well correlated using three layers, as defined previously by MacArthur (r = .84). At this superficial level the trend in bird species diversity seemed to be adequately explained as a response to the vegetation gradient. This preliminary conclusion was found to be illusory when the elevational trend in syntopy was reexamined separately for three major trophic subdivisions of the fauna. The number of insectivores decreased 5.2-fold from the bottom to the top of the gradient, frugivores decreased by a factor of 2.3, and nectarivores showed no change. It was now clear that the diversity in each of these trophic categories was responsive to environmental influences other than, or in addition to, the gradient in habitat structure. Additional factors implicated by the available evidence are competitive interactions with other taxa at the same trophic level, changing composition of the resource base as a function of elevation, and declining productivity at high elevations. Analysis of netted bird samples revealed an unexpected diversity maximum in the lower cloud forest zone. The immediate cause of this was a relaxation of the vertical stratification of foraging zones, such that an anomalously large fraction of the species present entered the nets. The excess diversity was found to consist almost entirely of insectivores. Several factors appear to contribute to the ultimate causes of the diversity maximum: greater patchiness of the montane forest due to the rugged topogra- phy, a higher density of foliage near the ground, and possibly increased resource productivity. A correlation between diversity and density in the netting results suggested a causal connection mediated via resource levels. The conclusion is that diversity is a complex community property that is responsive to many types of influences beyond simply the structure of the habitat.

The Importance of Predation by Crabs and Fishes on Benthic Infauna in Chesapeake Bay

Abstract: The significance of large motile predators in controlling the distribution and abundance of the macrobenthic invertebrates within the sediments (the infauna) in a shallow subtidal sand com- munity was tested using manipulative field experiments. The blue crab (Callinectes sapidus) and 2 species of bottom-feeding fishes, spot (Leiostomus xanthurus) and hogchoker (Trinectes maculatus), were either excluded from or confined to small areas using wire mesh cages. Callinectes and Leios- tomus effectively reduced infaunal densities; Trinectes did not. The infauna responded to decreased predation with a large increase in density and diversity within 2 mo. The largest population increases were exhibited by opportunistic species, which are considered to be most subject to predation. Species whose populations were least affected by predation were those species which either live deep in or quickly retract into the sediment. These species thus avoid predation and were generally the dominant species in the natural community. For species whose density increased greatly in exclosures, recruitment was by planktonic larvae and growth was very rapid, individuals growing to maturity in only a few months. Densities of all infaunal species increased in exclosures, suggesting that their population densities under natural conditions are not controlled by competitive interactions. In this community, infaunal population sizes are limited by predation and not by food or space. Severe predation pressure and physical disturbances, particularly sediment instability, keep population levels far below the carrying capacity of the environment. Severe predation, and the rapid growth, short generation times, and rapid turnover rates of constituent populations suggest that such infaunal communities, despite a low stand- ing crop, are an important food source for predator species important to man.

Leaf Area of Mature Northwestern Coniferous Forests: Relation to Site Water Balance

Abstract: Leaf area of mature coniferous forest communities of western Oregon appears to be related primarily to site H20 balance rather than characteristics of tree species composing the commu- nity. Leaf areas were determined for stands in communities ranked along measured gradients of precipi- tation and evaporative potential. Nine coniferous and 1 deciduous tree species were found in the various stands along these gradients. Leaf areas of these stands were linearly correlated with a simple site H20 balance index computed from measurements of growing season precipitation, open pan evaporation, and estimates of soil H20 storage. Species composition had no apparent influence on the relation between community leaf area and site H20 balance.

Feeding Strategies of the Little Brown Bat, Myotis Lucifugus, in Southern New Hampshire

Abstract: Feeding strategies of the little brown bat, Myotis lucifugus, were investigated in southern New Hampshire USA from early May through late 1974. Nightly food consumption was estimated by comparing mean prefeeding body weights with postfeeding weights taken as individuals returned to the roost from their first feeding period (at 2200 to 2400h) and from a subsequent foraging period (at 0330 to 0500 h). Pregnant bats consumed an average of 2.5g of insects (2.72kJ/g prefeeding body weight) nightly. lactating females ate 3.7 g (4.23 kJ/g), and juveniles ingested 1.8 g (2.47 kJ/g). Increased food consumption in lactating bats accommodated reproductive energy demands and was facilitated by rising food availability. Increasing levels of independent food consumption in juveniles accompanied weaning. Fecal analysis revealed that diets of individual bats were diverse. All available insects 3 to 10 mm in body length were accepted as food items. Nematoceral Diptera were by far the most common insects taken in light—trap samples, and constituted a major portion of the diet throughout the summer. Coleoptera, Trichoptera, Lepidopter, Ephemeroptera, and Neuroptera were also consumed in appreciably numbers. Comparison of dietary composition with prey availability indicates that pregnant bats consumed 3—10 mm prey in approximate proportions encountered during June, when insect availability was low and unpredictable. However, lactating, postlactating, and nonproductive ♂ ♂ exhibited more selective feeding in July, when insects were more abundant. This increase in selectivity reflected exploitation of beetles and mayflies, which were uncommon in trap samples. In August, juveniles approximated random feeding patterns, as they learned to forage. We suggest that increased resource availability allowed selective feeding in adult bats during July, as predicted by presy selection models. However, reduced discriminatory abilities may prevent similar levels of prey selection in juveniles.

Selective Predation, Optimal Foraging, and the Predator-Prey Interaction Between Fish and Crayfish

Abstract: Through laboratory experimentation, I quantified the behavioral interaction between an inshore fish predator, the smallmouth bass (Micropterus dolomieui) and an active, defensive prey organism, the crayfish (Orconectes propinquus). Experiments, run in aquaria or large wading pools, involved placing different sizes and life stages of crayfish with smallmouth bass to document selective predation. To determine whether bass, in preying selectively on crayfish maximized net energy intake, a predictive foraging model, in which handling and pursuit times are balanced against prey digestible organic ((Ht + Pt)/O), was developed and tested. Fieldwork, designed to yield electivity indices such that laboratory findings could be confirmed or denied, involved simultaneously sampling inshore fish predators and crayfish from 3 lakes in northern Wisconsin. In laboratory experiments, when offered a choice of crayfish sizes on sand, smallmouth bass (25 cm, total length) chose the smallest crayfish (4 mm, carapace length) first, and then consumed animals in ascending order of size. When offered the same choices on pebble (16-32 mm), intermediate-sized (16-20 mm) crayfish were eaten first. On large substrates, small size classes are relatively less exposed than large size classes. Therefore, the increased search (i.e., waiting) time to obtain these small size classes appears to decrease their value relative to more available intermediate size classes. Both pre- dicted size from (Ht + Pt)/O and selection sequence from the pebble substrate agreed quite closely with field electivity indices, suggesting that bass are optimal foragers. Within any size class of adults, life stages ordered from low to high susceptibility to predation by smallmouth bass were: ovigerous Y Y, Form I (FI, capable of breeding) d s, Y Y Form II (FII, incapable of breeding) d 6, and recent molts. Determined empirically via laboratory experimentation, this order was validated by electivity indices from the field and predicted by the (Ht + Pt)/O optimal foraging construct. As a result of their differential susceptibility to predators, some sizes and life stages (juveniles, Y Y, and recent molts) appeared to modify their field microdistribution to minimize risk to predators. In general, crayfish densities on sand were inversely related to relative densities of inshore fishes; degree of behavioral response of individual life stages appeared correlated with vulnerability. Fish predators not only prey selectively on crayfish but also cause shifts in their microdistribution and behavior. In fish-crayfish interactions, sizes and life stages with the greatest vulnerability due to morpholog- ical and physiological traits also possessed the most significant behavioral attributes for reducing risk. In general, selection of particular life stages from crayfish populations makes ecological sense; fish- crayfish interactions as predator-prey systems persist. Their persistence results from the subtle in- teraction between predator and prey, ultimately producing a stable system in which "important" life stages (ovigerous Y Y or Fl d d) are more or less exempt from predatory mortality.

Population Growth Rates and Age Versus Stage-Distribution Models for Teasel (Dipsacus Sylvestris Huds.)

Abstract: Mathematical models are developed to examine the population—level response of an herbaceous plant species (teasel, Dipsacus sylvestris Huds.) which was experimentally introduced into several habitats and monitored for 5 yr. Models based on morphological stages (size) rather than chronological age give more satisfactory results. Populations growth rates (λm) range from 0.63 to 2.60, which are likely typical for fugitive plants. Values are interpreted as responses to both external and internal factors. Grass litter, and the presence of other dicotyledonous species, and the overall primary productivity of the rest of the community are important factors determining the success or failure of an attempted colonization by teasel. Individual plant and population—level growth rates seem to be determined independently.

The presettlement forest and natural disturbance cycle of northeastern maine

Abstract: Land survey records of 1793-1827 containing forest data for 1.65 X 1Off ha of northern Maine were analyzed for species composition, successional status, and frequency of large-scale disturbance. Quantitative data consists of 1,448 sample trees spaced 1.6 km apart along a 9.7- X 9.7-km grid. Species which each comprised > 10% of the total were Picea spp., Fagus grandifolia, Abies balsamea, Thuja occidentalis, and Betula lutea. These forests appeared to be largely in a climax state as indicated by the dominance of shade-tolerant species and the small percentage (8% ) of intolerant or early successional species. However, 9.3% of the tract was burned land and birch-aspen forest at the time of the survey, mostly the result of large fires in 1803 and 1825. Windfalls occurred along 2.6% of the surveyed distance. If the amount of disturbed forest at this time was typical of the natural disturbance regime, then the average recurrence interval of fire and large-scale windthrow for a given site would be 800 and 1,150 years, respectively. Data on the structure of remnant virgin stands in the region likewise suggest that the time interval between severe disturbances was much longer than that needed to attain a climax, all-aged structure.

Reproductive Strategies: Food Availability as a Source of Proximal Variation in a Lizard

Abstract: Reduced food availability in 1974 significantly altered the reproductive characteristics of a population of Urosaurus near Animas, Hidalgo County, New Mexico compared to 1973. Relative food availability, which was monitored using sticky traps, was attributed to reduced precipitation levels. Fat storage prior to reproduction was greatly reduced in 1974. As a result, size of the first clutch was reduced from 10.9 (1973) to 6.8 eggs, and clutch frequency was reduced from two clutches by most females (90%) in 1973 to only one clutch in 1974 (8.7% produced two). The reproductive strategy in this population of Urosaurus compared to that previously presented for a Texas population differs primarily by a restriction of the reproductive season and reduced clutch frequency but an increase in clutch size. The limited reproduction in a low resource year may represent an adaptation to reduce risk and effort when current reproduction is less profitable than survivorship and future reproduction See full-text article at ...

Competition and Habitat Shift in Two Sunfishes (Centrarchidae)

Abstract: The bluegill sunfish (Lepomis macrochirus) in small ponds feeds on relatively large prey associated with the vegetation. However, in the presence of the green sunfish (L. cyanellus) it shifts to feeding on smaller, less preferred prey in the open water column. The mechanisms responsible for this habitat shift were examined by experimentally confining both species, alone and together, in homogeneous patches of the preferred habitat (vegetation). When confined together in the vegetation the green sunfish exhibited higher survivorship, growth rates, and amount of food in the stomachs than the bluegill. The bluegill fed on smaller items and consumed more benthic prey than did the green sunfish. The presence of the congener did not alter the food habits of either species or the growth rates of the green sunfish in relation to species stocked alone. Presence of the congener did affect the growth rates of the bluegill. Overlap in the diet was 70o when these species were confined to the vegetation as compared to 44% in an earlier study where habitat separation was permitted. The green sunfi'sh is more of a sit-and-wait predator and is able to utilize a wider food size spectrum than the bluegill. This results in a strong asymmetry in the competition function favoring the green sunfish in the vegetation. However, in the open water column the distribution of food sizes is truncated and this provides a competitive refuge for the bluegill which handles small foods more efficiently. The bluegill appears to be more flexible in its habitat use while the green sunfish is more aggressive and limited in the habitats utilized. Comparisons with studies of habitat shifts in salmonids suggest that the competitive mechanisms outlined are of general relevance in fish communities. Consideration of the relation between habitat structure, the correlated distribution of food sizes, and species morphology provides a framework for specifying the occurrence of habitat shifts and which species of the interactive set will shift.

Interactions of Nutrients, Plant Growth and Herbivory in a Mangrove Ecosystem

Abstract: The effect of nutrient enrichment of red mangroves (Rhizophora mangle) was studied by comparing two mangrove-covered islands in the Indian River at Ft. Pierce, Florida, USA, one (high nutrient) with and one (low nutrient) without a breeding colony of pelicans and egrets. Repeated measurements taken on > 100 tagged branches in each area revealed higher growth rates at the high nutrient site. Trees at the high nutrient site showed greater additions of (1) leaves; (2) reproductive parts; (3) new lateral branches; and (4) larger increments to existing stems. Growth in the fertilized stand also began earlier in the year and had a second maximum not shared by the low nutrient area. Both leaves and fruits at the high nutrient site were richer in nitrogen. More striking than the effects on plant growth, however, was the proportionately much greater stimulation of herbivory by insects in response to nutrient enrichment. Larvae of the five lepidopteran species that we observed feeding on buds or leaves were either more abundant or only present in the high nutrient area, as was the scolytid beetle that infested seedlings before they dropped from the parent tree. This difference in herbivory between sites disappeared when the birds seasonally migrated away from their nesting areas at the high nutrient site. This observation and the demonstration that the mangrove skipper Phocides pigmalion attains a higher growth efficiency on high nutrient leaves are both consistent with the hypothesis that increased nutritive value of vegetation (correlated with the increased concentration of nitrogen) is responsible for the 4x greater losses to herbivores in the high nutrient. Implications for environmen- tal management and in more complex communities are discussed.

Species Diversity and Community Organization in Desert Seed-Eating Ants

Abstract: Patterns of species diversity and community organization in desert seed-eating ants were studied in 10 habitats on a longitudinal gradient of increasing rainfall extending from southeastern California, through southern Arizona, and into southwestern New Mexico. Local communities of harvester ants include 2-7 common species, and at least 15 species from five genera of Myrmecines compose the total species pool in these deserts. Ant species diversity is highly correlated with mean annual precipitation, an index of productivity in arid regions. Communities are structured on the basis of competition for food, and interspecific differences in worker body sizes and colony foraging strate- gies represent important mechanisms of resource allocation. Seed size preferences, measured for native seeds and in food choice experiments with seeds of different size but uniform nutritional quality, are highly correlated with worker body sizes. Species of similar body size can coexist within local habitats if they differ in foraging strategy. Interspecific aggression and territorial defense and mi- crohabitat partitioning all appear to be relatively unimportant in these ant communities. Patterns of species diversity and community organization in harvester ants are strikingly similar to those reported for communities of seed-eating rodents that occupy many of the same desert habitats. Separate regressions of within-habitat species diversity against the precipitation index of productivity for the two groups correspond closely in slope, intercept, and proportion of explained variation. Resource allocation on the basis of seed size characterizes local communities of both ants and rodents. Parallels between these two groups suggest that limits to specialization and overlap may be specified by parameters such as resource abundance and predictability that affect unrelated taxa similarly.

Bird activity and seed dispersal of a tropical wet forest tree

Abstract: Feeding assemblages of birds were observed throughout a fruiting season at a Costa Rican population of the rain forest tree Casearia corymbosa in order to distinguish effective dispersal agents from visitors which used arils for food without dispersing seeds. This system is of special interest because the tree fruits during an annual period of fruit scarcity in a forest characterized by high proportions of animal-dispersed plants and obligate frugivores. Results show that the Masked Tityra (Tityra semifasciata) is an effective dispersal agent of seeds of this tree because it: (1) regurgitates viable seeds, (2) is a common and regular visitor throughout the season, (3) has high feeding rates, (4) removes seeds from the vicinity of the parent tree before processing them, and (5) depends on this rather than other plants fruiting at the same time. Twenty-one other visitors were deficient dispersers, although some depended heavily on fruit of this tree for food. Two parrots (Amazona autumnalis and Amazona farinosa) preclude dispersal by strip- ping arils with their bills and dropping all seeds in situ. Fourteen species are occasional visitors. Five visitors (Ramphastos sulfuratus, Ramphastos swainsonii, Pteroglossus torquatus, Myiozetetes similis, and Myiozetetes granadensis) process seeds undamaged and are sometimes numerically abun- dant, but are absent part of the season and tend to regurgitate seeds in situ. Because fruiting activity of this tree occurs during annual scarcity, it represents a pivotal species in the community. Three obligate frugivores, including one effective disperser and two fruit thieves, depend almost entirely on it for food for periods of 2-6 wk. I hypothesize that extinction of this plant at this site would lead to disappearance of these birds, and would almost certainly depress recruitment of other species of trees for which they serve as dispersal agents at other times of the year.

Seasonal Variation in Thermoregulatory Behavior and Body Temperature of Diurnal Kalahari Lizards

Abstract: We discuss seasonal variation in thermoregulatory behavior and its consequences on body temperature for 12 species of diurnal lizards in the southern Kalahari semidesert of Africa and also evaluate several methods of attempting to document thermoregulatory behavior using a descrip- tive data base. Lizards vary time of activity among seasons, which limits the variation in ambient conditions actually experienced. Ground-dwelling lizards and probably arboreal lizards move nonrandomly with respect to sun and shade; thus the percentage of lizards in sun is inversely proportional to air tempera- ture. Arboreal lizards shift to higher perches at midday in summer and to logs or ground in winter thus decreasing and increasing incident heat loads, respectively. Both juveniles and adults of 3 species, only juveniles of 2 species, and only adults in I species are active in winter: both adults and juveniles of 6 species brumate (= hibernate). Mean body temperature (Tb) varies within days and among months and is positively correlated with corresponding mean air temperature (Ta) in almost all species. Nonetheless, correlation and regression analysis suggests that thermoregulatory behaviors reduce the impact of variations in ambient condi- tions on Kalahari lizards. The mean Tb of different species reflect evolutionary relationships. In summer, mean Tb is propor- tional to the percentage of lizards in sun and with the tendency of lizards to be active only in summer. Thus, lizards with inferred low optimal temperatures are active during more months of the year.

A Complex Community in a Simple Habitat: An Experimental Study with Bacteria and Phage

Abstract: Continuous culture populations of the bacterium especially coli and its virulent virus T7 have been studied as a model of a predator—prey in a simple habitat. These organisms maintain apparently stable states of coexistence in: (1) a phage—limited situation where all of the bacteria are sensitive to the coexisting virus and the sole, and potentially limiting carbon source, glucose, is present in excess; and (2) a resource—limited situation where the majority of the bacteria are resistant to these phage and in which there is little free glucose. The composition of these interacting populations is examined in detail and evidence indicating that this simple experimental culture system can support relatively complex communities is presented. In the predator—limited situation, two populations at each of two trophic levels can be maintained; the wild—type bacterial and phage strains, denoted B0 and T70, a mutant bacterial clone which is resistant to T70, denote B1 and a host range mutant phage, T71 which is capable of growth on both B0 and B1. In the resource—limited situation, three populations of bacteria and two populations of phage can coexist. The include the above described clones and a third bacterial strain, B2, which is resistant to both T70 and T71. In phage—free competition, the wild—type B0 bacterial clone has a marked advantage relative to both B1 and B2 while no difference is detected between B1 and B2. When competing for a B0 host, the wild—type T70 phage clone has a marked advantage over T71. The fit of these observations to some previously developed theory of resource—limited growth, competition and predation is discussed and a mechanism to account for the persistence of these communities is presented. The latter assumes that their stability can be attributed solely to intrinsic factors, i.e., the population growth and interaction properties of the organisms in this continuous culture habitat.

A Simulation Model for Decomposition in Grasslands

Abstract: A model has been developed to simulate the dynamics of decomposers and substrates in grasslands. Substrates represented are humic material, feces, and dead plant and animal remains. Except for humic material, substrates are further divided into a rapidly and a slowly decomposing fraction. The proportion of rapidly decomposing material in a substrate is predicted from its initial nitrogen content. The belowground portion of the system is divided into layers because temperature and soil water, the most important driving variables for the model, vary with depth. Decomposition rates are predicted from temperature, water tension, and inorganic nitrogen concentration.

Grazing Effects of Four Marine Intertidal Herbivores on the Microflora

Abstract: The impact of 4 species of intertidal gastropods (3 limpets and 1 periwinkle) on microalgal species composition and biomass has been evaluated and the degree of overlap of grazer diets has been examined. A series of clay pipes set in a mud flat became artificial substrates for diatom assemblages; grazers were added to the outside of these pipes after an initial period during which the flora developed. The mud proved to be an effective barrier to both immigration and emigration. All 4 gastropods selectively removed 3 diatom species from the pipes: Melosira moniliformis, Melosira nummuloides, and Fragilaria striatula var. californica. These species consist of long chains of moderate—sized cells that form an overstory within the diatom mat; a total of 130 additional species may be found within the diatom community. Selective removal may be facilitated by the chain morphology, by the location of these diatoms in the outermost portion of the mat, and by loose adhesion of these species to the rock surface. Diatoms with potentially tighter adhesion such as the stalked Achnanthes spp. are less affected by grazers. The abundance of 3 species of blue—green algae was little changed by grazing. All 4 grazers had remarkably similar diets, apparently using the same resource in about the same way. Data on the amount of chlorophyll a per m2 on natural rock surface indicate that grazers greatly reduced microalgal standing crops. Differences in grazer zonation and use of macroalgae, coupled with increased food availability during reproductive seasons (winter and spring), allow continued coexistence of these very similar herbivores.

Reconstruction of a Mixed‐Species Forest in Central New England

Abstract: A 0.36—ha area in the Harvard Forest, Petersham, Massachusetts, was intensively analyzed to determine its history. Natural and man—caused disturbances of varying magnitudes occurred periodically in the central New England mixed—species stand. Evidence of two hurricanes and a fire prior to 1803 were found. Between 1803 and 1952, 14 natural or man—caused disturbances of various magnitudes occurred in the area. Large disturbances created new age classes, but small disturbances did not. Species arising together after large disturbances formed a distinct vertical stratification, with northern red oak (Quercus rubra L.) arising after several decades to the dominant canopy. Smaller disturbances to the overstory allowed understory trees such as black birch (Betula lenta L.), red maple (Acer rebrum L.), and eastern hemlock (Tsuga canadensis L. Carr.) to emerge to the dominant canopy. The composition of this forest was more the result of allogenic influences rather than autogenic development. See full-text article ...

Density‐Dependent Aspects of Growth and Metamorphosis in Bufo Americanus

Abstract: Larvae of Bufo americanus (Anura: Bufonidae) were reared at controlled densities and food levels in laboratory populations and at controlled densities in enclosures in a farm pond in one— and two—species populations. Survival during the larval period was independent of population density however, the proportion of the population that successfully metamorphosed was a negative, exponential function of density. This outcome is interpreted as the result of the effect of density on the growth rate of larvae. In high density populations, a few individuals grow at the expense of smaller members of the cohort, which then have a lowered probability of metamorphosis. In spite of the strong effect of environmental heterogeneity in field experiments, the mean size at metamorphosis was significantly decreased as the initial density of either Bufo (—0.00031 g—body wt per unit increase in density) or Rana palustris (—0.00020 g—body wt per unit increase) was increased. Rana palustris frequently breeds at the same time an...

The Nature of Prey Selection by Planktivorous Fish

Abstract: The nature of prey selection by planktivorous fish is presented as a model. The model conceptualization is based on the predation cycle, and the effects of prey distribution, encounter rate, handling time, capture success, and optimal foraging on food selection are explicit. The model as presented assumes a multiple species prey assemblage admitting within-species size variation. Conditions are given so that the optimal strategy of prey utilization can be expressed as a breadth of diet, and the specific components of the predator-prey interaction of fish and zoo plankton are discussed in detail. A reduced form of model is applied to two experimental situations. Specific factors which influence prey selection are also discussed.

Territoriality and male mating success in the green frog (Rana clamitans)

Abstract: Territorial behavior of Rana clamitans was studied in an enclosed experimental pond. Males maintained territories from June through August. Most 6 6 occupied more than one site during the breeding season. Most sites were occupied for < 1 wk before changing ownership, but some were occupied by a single 6 for up to 7 wk. Large 6 6 remained at individual sites longer and spent more total time in territories than small 6 6. The smallest 6 6 usually acquired territories only after other 6 6 had abandoned them, while larger 66 sometimes ousted residents with aggressive behavior. Males sometimes adopted a satellite role in the territories of other 6 6 and waited for sites to become available. Individual territory sites were ranked by relative quality based on the physical structure of the sites. Female choice of mates appeared to be related to territory quality, and especially to density of vegetation in the water. Males that spent the most time in high quality territories acquired the largest number of mates. The social system of the green frog probably evolved as a result of intense 6-6 competition for mates. This in turn is probably related to its prolonged summer breeding season. tition in animals were first stated clearly by Darwin (1871) in his theory of sexual selection. Darwin and subsequent workers recognized several forms of sex- ual selection. In some species, males fight among themselves for possession of females, with the largest or most aggressive animals obtaining the largest number of mates. In others, females choose among

The Relationships of Locomotion to Differential Predation on Pseudacris Triseriata (Anura: Hylidae)

Abstract: Laboratory experiments, using the garter snake Thamnopphis sirtalis as predator and the chorus frog Pseudacris triseriata as prey, show that metamorphosing anurans are captured more frequently by T. sirtalis than either premetamorphic tadpoles frogs. Stomach contents of wild—caught Thamnophis have similarly revealed more anuran prey in transformation than at earlier or later stages. The high susceptibility of transforming Pseudacris to snake predation appears to be a direct consequence of a body form not conductive to effective locomotion. The sustained swimming ability of premetamorphic and metamorphic P. triseriata was examined in the laboratoy. It was found that until the eruption of the forelimbs, the larger a tadpole is, the longer it can maintain its position in a steady current. The length of the tail appears to be a positive factor in sustained swimming. Large hindlimbs do not appear to assist swimming and may increase drag. Tadpoles that have begun to metamorphose (forelimbs visible) demonstrate ...

Energy Costs of Reproduction in the Cotton Rat, Sigmodon Hispidus

Abstract: Individual organism costs of reproduction were investigated by a series of observations and experiments with the cotton rat, Sigmodon hispidus. Energy budget techniques, including respirometry, showed that ingestion in pregnant rats was increased 25%, while that in lactating rats was increased 66%, over nonreproductive rats. Digestive efficiency of rats fed a commercial standard diet was 84% and did not change as a result of pregnancy or lactation. Respiration per gram of reproductive rats was not significantly different from nonreproductive rats. Twenty—six days of pregnancy required an average additional ingestion of 221.8 kcal [=928.0 kJ]; 12 days of lactation required an extra 270.7 kcal[=1,132.6 kJ]. Total cost per offspring 12 days of age was 100.5 kcal[=420.5 kJ]. The total cost of producing a litter was a linear function of litter size. Two—thirds of the energy accumulated by a pregnant rat was not in the neonates but deposited as storage and mobilized during lactation when the precocial young gre...

Decomposition and nutrient exchange of litter in an alluvial swamp forest

Abstract: Weight loss from cellulose sheets was measured monthly at three sites (river, swamp floor, natural levee) of a North Carolina swamp forest dominated by Nyssa aquatica. Rates of loss were significantly different seasonally and between sites. Both temperature and moisture appeared to be important in controlling decomposition rates. For Nyssa leaves, dry weight decreased to 25% of original after 48 wk while twigs fell to 80Wo of original after 56 wk. Three modes of nutrient exchange were found: (1) accumulation of N, Ca, and Fe by both leaves and twigs, with stronger leaf accumula- tion of these elements; (2) strong leaf accumulation with rapid twig loss of P; and (3) losses by both leaves and twigs of K and Mg. Final atomic ratios of C:N and C:P were about 15:1 and 500:1, respectively, suggesting that P may be in short supply. The significance of nutrient accumulation is that there was little or no net release of P, N, Ca, and Fe from autumn leaf fall until tree growth in the spring. Conservation and recycling of nutrients appear to be tight even in swamps open to flooding during tree dormancy.

Carbon Cycling in a Mixed Deciduous Forest Floor

Abstract: A study of carbon cycling within the floor of a mixed deciduous forest included esti- mates of litterfall, litter standing crop, root turnover, lateral root standing crop, and measurements of gaseous carbon losses from the various components. Annual carbon inputs to the system were 161 g C m-2 as litterfall and 733 g C m-2 as small roots (<0.5 cm diam). Mean annual standing pools were: 01 litter 126 g C i-2, 02 litter 111 g C m2, small roots 340 g C in2, large roots 395 g C m-2, and soil detritus 12,850 g C m-2. Annual total carbon efflux, measured in situ with an open airflow infrared gas analysis system was 1,065 g C m-2. Of this total, 35% was accounted for by respiration of live roots, 42.2% from root decay, 10.9% from 01 litter, and 9.7% from 02 litter. The remaining 2.2% is within the range likely associated with catabolism of root exudates and soil organic detritus. These component estimates were based on respiration rates of the individual components using a differential respirometer.

Seed Production, Seed Populations in Soil, and Seedling Production After Fire for Two Congeneric Pairs of Sprouting and Nonsprouting Chaparal Shrubs

Abstract: A study of seed production, seed storage in the soil, and seedling production after fire was undertaken for a sprouting and a nonsprouting congeneric pair of species of Ceanothus and Arctostaphylos. All species exhibited large fluctuations in annual seed production. There was a signifi- cant correlation between fruit production and precipitation in the previous year. It is hypothesized that high carbon gain in years of high precipitation results in high numbers of floral primordia which, in these species, remain dormant until the following year. It was also noted that high fruit production was not dependent upon high precipitation the same year; suggesting that the fruits were utilizing carbon stored from the previous year. All 4 species were capable of producing more seeds in a single season than were stored in the soil. Apparently the soil seed pools do not represent a steady accumulation of seeds in the soil but rather are the result of dynamic fluctuations in seed inputs and outputs. Each species also had more seeds in the soil, by several orders of magnitude, than seedlings after fire in an adjacent burned stand. The sprouting and seeding reproductive strategies are quite different in the two genera. The infor- mation from this study coupled with that from other studies indicate 4 reproductive modes: sprouting and seedling production (C. leucodermis), abundant seedling production (C. greggii), low seedling production but better "equipped" seedlings (A. glauca), and predominantly sprouting (A. glandulosa).

Foraging ecology and community organization in desert seed-eating ants'

Abstract: Granivorous ants in the southwestern deserts of the United States are characterized by species-specific colony foraging behaviors that determine their efficiencies at utilizing seeds from different density distributions. Workers search for food either in groups or as individuals, and these feeding strategies represent specializations for high and low density resources respectively. While gathering experimental seeds, simultaneously supplied in clumped and dispersed distributions, feeding by group foragers focused on the high density resource patches, while individual foragers harvested seeds predominantly from the dispersed distribution, which required that prey be independently dis- covered. Collecting native seeds from unmanipulated environments, individual foragers tended to spend proportionately more time searching and experience lower foraging success than did group foragers in the same local habitat. During a period of high seed abundance, group foragers collected a much narrower range of prey types than did individual foragers. Colonies of Pogonomyrmex rugosus exhibited a mixed foraging strategy, with the most distinct feeding columns occurring during a period of peak seed abundance. Among group foragers, greatest activity coincided with periods of relatively high seed densities and low climatic stress, and these species utilized tactics such as seed storage, hibernation, and estivation to weather less favorable periods in a resting state. Individual foragers were active at intermediate levels during less favorable periods. Although high and low density seed resources are not renewed independently of one another, density specialization appears to promote coexistence between group and individual foragers. Their capacity to stably partition resources in this way should depend on (1) the degree of difference in their efficiencies at exploiting different density distributions of seeds and (2) the availability of seeds in the habitat as a function of seed density. In more mesic habitats, where seeds are renewed more frequently and probably also in larger pulses, group foraging species are proportionately more abundant.

Selection of Seed Distribution Types by Dipodomys Merriami and Perognathus Amplus

Abstract: Laboratory experiments were conducted to investigate the foraging responses of kangaroo rats (Dipodomys merriami) and pocket mice (Perognathus amplus) to aggregates of seeds which differed in size, depth, and degree of clumping. Kangaroo rats effectively used clumps of seeds, whereas the pocket mice did not differentially select clumps of seeds over a scattered distribution of seeds. In addition to distribution, clump size and depth were important criteria for both species in determining foraging efforts. These data, coupled with recent information which suggests wide arrays of seed distribution types in the desert, lead us to propose that rodents select seeds on the basis of spatial distribution of the resource. This is an attractive hypothesis because it can accommodate both of the primary mechanism previously suggested for desert rodent coexistence, seed size selection and habitat selection.

Nonrandom foraging by sunbirds in a patchy environment

Abstract: Sunbirds (Nectarinia spp.) feeding at the East African mint Leonotis nepetifolia en- counter great variations in the nectar contents of flowers blooming in dense fields. The dispersion patterns of nectar are attributable to nectar removal from some flowers by the sunbirds themselves in earlier foraging and also to intrinsic floral variations. The problem facing the foraging sunbird is to increase its foraging efficiency (net energy gain per unit time) by avoiding recently visited, empty flowers and by visiting flowers with greater than average nectar volumes. Sunbirds patterned their foraging in 3 major ways. First, they used initial flowers probed on an inflorescence as an assay of what the rest of the flowers in that inflorescence contained and they rejected inflorescences with little nectar. Such rejection increased nectar intake per flower by as much as 15%. Second, territorial sunbirds preferentially fed at unvisited inflorescences, increasing nectar intake 25% relative to random foraging. This was accomplished at least in part by foraging at different heights on successive foraging bouts. Third, flight distances to the next flower changed in response to immediate reward levels in some species but not in others. In general, sunbirds feeding at Leonotis responded less to reward levels by differential turning and movement than some other organisms, possibly reflecting different prey distributions or boundary constraints on their foraging.