Showing papers in "Ecology in 1978"

Macroclimate and Lignin Control of Litter Decomposition Rates

Abstract: In order to develop a general model of litter decomposition rates suitable for the prediction of regional variations in decay rates, and to determine the relative control by macroclimate and litter quality on decomposition rates, data were selected from 5 locations ranging in climate from subpolar to warm temperate. Actual evapotranspiration (AET) was selected as an index of the climatic (energy and moisture) forcing function of the specialized decomposers which is superior to temper- ature and precipitation. Lignin concentration was selected as an index of litter quality and may be treated as a mediator of climatically (AET) regulated decay rates. In a stepwise, multiple linear correlation-regression, using AET, lignin concentration (%) and AET/lignin concentration (interac- tion), AET alone accounted for 51% of the variance in observed decay rates, AET/lignin concentration (interaction) added l9Wo and lignin concentration added 2% of the total (72%) variance accounted. Simple correlation of the five locations between lignin concentration and decomposition rate ranged from r = .32 to r = .95, however, the regression lines for each of the 5 locations indicated that these slopes progressively declined with AET. Moreover, the slope decline was not parallel, indicating a climatically variable control by lignin concentration on decay rates. In low-AET (not arid) climates, litter with high and low lignin, will decay at more nearly similar rates, but as the AET environment increases, the difference in decay rates becomes progressively greater than the increase in AET alone would seem to warrant. A general model of the interaction control by AET and lignin concentration on decomposition rates was formulated which overcomes the restraints of the multiple regression model. At the scale of subpolar to warm-temperate climates, the climate as indicated by AET is several orders of magnitude more important as a predictor of decay rate than is litter quality. This importance is evident in spite of the fact that the data on lignin concentration used in this analysis had a 12-fold range while the AET values had a 2.3-fold range.

The Measurement of Niche Overlap and Some Relatives

Abstract: Existing overlap indices are examined and are judged to be inadequate on the grounds (1) that they lack simple and appropriate biological interpretations, and (2) that they ignore possible variation among resource states in abundance or availability (a_i). Two indices interpretable in terms of encounters are proposed. Niche overlap (L) is measured as the degree to which frequency of interspecific encounter is higher or lower than it would be if each species utilized each resource state in proportion to its abundance (a_i). Directional overlap (Z_(xy)) is measured as the density of species Y encountered, on the average, by an individual of species X. When resource states are equal in size, L is equivalent to Lloyd's `interspecies patchiness,' and Z_(x(y)) is equivalent to his `mean crowding on species 1 by species 2.' Indices which in corporate variation in resource state abundance are also developed for mean crowding, patchiness and niche breadth. See full-text article at JSTOR

Habitat Structure and Stream Fish Communities

Abstract: Stream habitat complexity is correlated with fish species diversity in selected Indiana and Panama streams. Habitat diversity was measured along 3 dimensions judged important to a wide range of fish groups and applicable to many stream conditions: stream depth, bottom type, and current. Increasing community and habitat diversity followed stream-order gradients. Natural streams supported fish communities of high species diversity which were seasonally more stable than the lower-diversity communities of modified streams. After disturbances such as channelization, seasonal peaks in species diversity attain levels typical of undisturbed streams. Because seasonal changes in stream quality are high, the stability of the fish community is lower in modified than in natural streams. The general correlation between habitat characteristics and presence and absence of fish species sug- gests that most fishes of small streams are habitat specialists.

Avian Nest Dispersion and Fledging Success in Field-Forest Ecotones

Abstract: Observations of 21 species of open-nesting passerines breeding in contiguous field and forest habitats at Rose Lake Wildlife Research Area, Michigan, were made during 1974 and 1975. Data were collected on nest dispersion, clutch-size, and fledging success in relation to the field-forest edge. Losses of eggs or nestlings were attributed to predation, inclement weather, Brown-headed Cowbird (Molothrus ater) parasitism, nest desertion, hatching failure, and adult death. Each bird species seemed to have a preferred distance from the habitat discontinuity that was used as a nest site. Furthermore, nests were not uniformly distributed on the area. Over one-half of the nests were found within ?15 m of the habitat discontinuity. Seventy-five percent of the nests belonged to birds characteristic of mixed breeding habitats, i.e., birds requiring an open overstory canopy with elevated singing and observation perches and dense cover near the ground for nesting and feeding. These mixed-habitat species also accounted for the increase in avian species nesting near edges. Based on Kendall rank and partial rank correlation tests, increasing numbers of nests and the percentage of total clutches ^3 eggs were found to be negatively correlated with increasing distance from the habitat discontinuity. Correlation between fledging success and increasing distance from the edge was positive and highly significant. Of the several mortality factors investigated, predation and cowbird parasitism were found to be the most important. The increased predation rate with decreased distance from the edge was attributed primarily to a functional response to higher numbers of nests and a greater activity of potential nest predators in the vicinity of the habitat discontinuities. Our results indicate that habitat suitability decreases with increasing numbers of nests toward the narrow field-forest edges. Although such abrupt habitat discontinuities did attract a variety and abundance of birds characteristic of habitats with mixed life-form, these discontinuities seemed to function as "ecological traps" by concentrating nests and thereby increasing density-dependent mor? tality. Ironically, the cowbird was also a victim of the increased predation rate. As these man-made forest edges are of recent origin, they are perhaps unrepresentative of the ecological niche in which these species evolved, and thus they may be poorly adapted to cope with the increased nest predation.

Measuring preference in selective predation

Abstract: Selective predation occurs when the relative frequencies of prey types in a predator's diet differ from the relative frequencies in the environment A measure of preference is proposed which is derived from a simple stochastic model involving probability of encounter and probability of capture upon encounter The measure is applicable to any number of prey types and methods of estimation are given for both constant and changing prey numbers Because the measure is based on a biological model, it can be manipulated and interpreted in a meaningful way

The Role of Seed Banks in the Vegetation Dynamics of Prairie Glacial Marshes

Abstract: The presence of viable seed in 24 substrate samples from Eagle Lake, a marsh in north- central Iowa, was tested by placing subsamples of each sample under 2 environmental conditions. One set of subsamples was placed underwater (submersed treatment). Seeds of 20 species germinated and grew in this treatment. On the average, there were 8.3 species/sample. The 2nd set of subsamples was kept moist, simulating conditions on an exposed mud flat (drawdown treatment). In the drawdown treatment, on the average, seeds of 12.9 species germinated/sample. Altogether seeds of 40 species germinated in this treatment of which only 24% were also found in the submersed treatment. By combining the results from the 2 treatments, the seed banks in the 6 vegetation types studied were estimated to range from 21,445 to 42,615 seeds/m2 on the average in the upper 5 cm of soil. Field studies at Eagle Lake (1974) and Goose Lake (1976), when these marshes had no standing water, revealed that the most abundant species whose seeds germinated on exposed mud flats were the same as the most abundant species in the experimental drawdown samples from Eagle Lake. In 1975, when Eagle Lake had standing water again, the submersed and floating species that were found were the same as those found in the experimental submersed samples from Eagle Lake. The seed-bank results and vegetation sampling reveal that there are 3 types of species present in prairie marsh seed banks: emergent species (Typha, Scirpus, Sparganium, Sagittaria) germinate on exposed mud flats or in very shallow water; submersed and free-floating species (Lemna, Spirodela, Ceratophyllum, Naias, Potamogeton) whose dormant seeds or turions can survive on exposed mud flats for a year and which germinate when there is standing water; and mud-flat species (Bidens, Cyperus, Polygonum and Rumex) which are ephemerals whose seeds can only germinate on exposed mud flats during periods when no standing water exists in the marsh because of drought or water level manipulation. When the marsh refloods, these species are eliminated from the visible marsh flora. Primarily because of fluctuating water levels and muskrat damage, prairie marshes have cyclical changes in their vegetation during which mud-flat, emergent, or submersed and free-floating species replace each other as the dominant type of species in a marsh.

Fungal-small mammal interrelationships with emphasis on oregon coniferous forests"

Abstract: Most higher plants have evolved with an obligatory symbiotic relationship with my- corrhizal fungi. Epigeous mycorrhiza formers have their spores dispersed by air currents, but hy- pogeous mycorrhizal fungi are dependent upon small mammals as primary vectors of spore dissem- ination. Mammalian mycophagists defecate within the coniferous forest ecosystem, spreading the viable spores necessary for survival and health of the conifers. As one unravels and begins to un- derstand the interrelationships between small-mammal mycophagists and mycorrhizal fungi, it be- comes apparent that the various roles of small mammals in the coniferous forest ecosystem need to be reevaluated. One can no longer accept such simplistic solutions to timber management as poisoning forest rodents to kkenhance,, tree survival. One must consider the direct as well as the indirect costs and benefits of timber management decisions if one is to maintain balanced, healthy coniferous forests.

Competition for Hummingbird Pollination and Sequential Flowering in Two Colorado Wildflowers

Abstract: The simultaneous flowering of co—occurring plant species with similar pollinator affinities may result in interspecific pollen transfer and consequent fecundity reductions due to wastage of pollen, stigma surfaces, and effective pollinator visits. In such cases competition for pollination occurs and may lead to or maintain sequential flowering. Two common perennials in the mountains of west—central Colorado, Delphinium nelsoni and Ipomopsis aggregata, flower sequentially in the same meadows and are visited commonly by Broad—tailed Hummingbirds (Selasphorus platycercus). Hummingbirds carry pollen of both species and their exclusion from flowers leads to significant seed set reductions. During the brief period of flowering overlap between D. nelsoni and I. aggregata in natural meadows, hummingbirds visit both species, carry mixtures of their pollen, and appear to cause interspecific pollen transfer. Flowers of both species receptive during this period suffer significant seed set reductions relative to those receptive during nonoverlap periods. Interspecific pollinator flights and pollen transfer also occur in mixtures of potted plants, and seed set reductions consistently occur for both D. nelsoni and I. aggregata in such mixtures relative to single—species controls. Finally, seed set reductions occur for both species following interspecific hand pollination of potted plants. Fecundity reductions in natural and synthetic mixtures of D. nelsoni and I. aggregata indicate that the 2 species compete for hummingbird pollination and suggest that the competitive interaction involves interspecific pollen transfer. The observed reproductive effects represent a selective force sufficient to maintain divergent flowering times of D. nelsoni and I. aggregata in nature.

Flowering Ecology of Some Spring Woodland Herbs

Abstract: Fecundity characteristics, phenology, and behavior of insect flower-visitors were stud- ied for 7 early flowering woodland herbs: Claytonia virginica, Dentaria laciniata, Dicentra canaden- sis, Dicentra cucullaria, Erythronium albidum, Isopyrum biternatum, and Sanguinaria canadensis. Sanguinaria canadensis is facultatively autogamous, the Dicentras are obligate outcrossers, and the remainder are self-compatible, at least within a stem. All are insect pollinated except sometimes S. canadensis. The numbers of ovules per flower and flowers per stem tended to be inversely correlated, and large-seeded species (S. canadensis, E. albidum, I. biternatum) had lower numbers of potential seeds per stem than did small-seeded species. Flowering of all species typically occurred during the first prolonged period of weather suitable for pollinator activity and ceased by the time the canopy closed. Annual differences in flowering times were associated with differences in average temperatures (i.e., early blooming in a warm, early spring), but cumulative degree-hours or degree-days of air or soil temperatures were not well corre- lated with flowering times. Other constraints on flowering phenology are discussed, including the predictability of suitable conditions, a proposed "fail-safe" mechanism that may assure flowering before canopy closure even if temperatures are abnormally low, and the importance of nontemperature factors in defining suitable conditions. Flowering time was not very finely tuned to the temperature regime and pollinator activity; flowers blooming during the flowering peak often had low seed pro- duction and the fertilization rate of most species was low. Evidence that seed production may have been pollinator limited for several species was obtained by comparing the success of hand pollination and of natural pollination, rarity of certain specialized pollinators, and estimates of the abortion rates of fertilized ovules. We suggest that flowering in early spring is a high-risk option in terms of insect- mediated sexual reproduction. Certain flower-visiting insects favored D. laciniata out of proportion to its abundance, but no effect on seed set of other species was detectable. Honeybees were abundant and active flower visitors with the potential for disrupting ecological/evolutionary relationshps between native insects and flowers.

Microhabitats in a Forest-Floor Small Mammal Fauna

Abstract: This study examines microhabitat configurations for 3 small mammal species occupying second—growth mesic forest on Walker Branch Watershed in eastern Tennessee. Small mammal populations were sampled in oak—hickory, chestnut oak and pine forest types, with three 0.36—ha live—trapping grids per forest type. There were 255 total captures for 4 forest—floor species during 9,696 trap nights between 1 May and 16 August 1973. These captures included 2% Blarina brevicauda, 66% Peromyscus leucopus, 5% Ochrotomys nuttalli and 27% Tamias striatus. Blarina was captured most frequently in the pine forest type, Peromyscus and Tamias most frequently in the oak—hickory, and Ochrotomys exclusively in the pine. Twenty—nine variables describing proximate habitat structure were measured at each sample mammal capture site. There were significant overall species differences on 18 of these variables. A subset of variables was identified for each of the 3 most frequently observed species, consisting of only the variables of particular significance for distinguishing the capture sites of that species from those of the other species collectively. Discriminant analysis of these few variables both described the average microhabitat configuration, for a species and indicated the relative importance of each variable for characterizing or describing that configuration, subject to the assumptions of the analysis. The observed differences between species microhabitat configuration are consistent with both the composition by forest type of each species sample and the literature information for each species. Microhabitat segregation among these species is discussed with reference to recent theories of habitat selection, ecological, specialization and resource partitioning in equilibrium faunas.

Refuges, Disturbance, and Community Structure: A Marine Soft‐Bottom Example

Abstract: Disturbance is a significant mortality source in many assemblages. The susceptibility of organisms to this mortality source is, in part, a function of the availability of substrate hetero- geneities that act as refuges from the disturbance process. There are at least 5 major categories of temporal and spatial refuges from disturbance: (1) temporal periods outside the activity range of the disturbance process; (2) temporal periods within the activity range of the disturbance process; (3) spatial zones beyond the activity range of the disturbance process; (4) physical heterogeneities within the activity range of the disturbance process; and (5) biologically generated refuges within the activity range of the disturbance process. The last category is particularly interesting because it involves an organism's utilization of a refuge which is the product of another organism or organisms. Data from a marine system are used to demonstrate the effectiveness of several types of refuges, particularly biologically generated refuges. The refuge-forming species is Diopatra cuprea, an onuphid polychaete which inhabits shallow water, medium-grained sand flats from Cape Cod to Florida. The abundance and species richness of other members of the infauna are shown to be positively associated with the presence of the tubes of Diopatra. This effect is confined to the area immediately surrounding the tubes of Diopatra. I dem- onstrated experimentally that a tube-like structure, such as a plastic straw, has the same effect on the infauna as does the tube of Diopatra. Thus, as predicted, the physical and biological refuges affect infaunal abundances similarly. They should not show similar patterns of distribution in space and time however and this is discussed.

The effect of fire on nutrients in a chaparral ecosystem

Abstract: The distribution of nutrients in plants, litter, and soil were studied before and after a prescribed burn in southern California chaparral. Total N, P, K, Na, Mg, and Ca were measured. The various nutrients were distributed differently in the plant parts and the litter before fire and this affected their redistribution by fire. Measurable losses of only 2 nutrients occurred–N (146 kg/ha) and K (49 kg/ha). Erosional losses of nutrients during the first rainy season after fire are also reported.

A General Explanation for Insular Body Size Trends in Terrestrial Vertebrates

Abstract: The insular body size trends for different vertebrate families are compared. Certain groups such as lagomorphs, bats, artiodactyls, elephants, foxes, raccoons, snakes, and teiid and lacertid lizards are habitually represented by relatively smaller forms on islands. On the other hand, cricetid rodents, iguanid lizards, tortoises, and bears often have races with larger body sizes on islands. Contrary to conventional niche theoretic concepts, in many instances knowledge of the body sizes of some of these animals' insular and mainland competitors does not help explain the difference in that species body size in the 2 places. To account for these divergent size changes I examine optimum body size models that use as the optimization criterion the net energy gained by an organism over a given time period. These models predict that increases in the mean amount of available food should lead to evolutionary increases in body size, but only if body size is not tightly constrained by additional physical or biotic fac...

Resource Partitioning in Bumblebees: Experimental Studies of Foraging Behavior

Abstract: A system comprising 2 species of bumblebess (Bombus appositus and Bombus flavifrons) and 2 species of flowers (Delphinium barbeyi and Aconitum columbianum) in Gothic, Colorado, USA, was manipulated to determine whether resource utilization by each bumblebee species was influenced by the presence of the other species of bumblebee. Each bumblebee species concentrated its foraging efforts on a different flower species, apparently choosing the species whose corolla tube length matched its proboscis length most closely. When each bumblebee species was temporarily removed from its preferred flower species, the remaining bumblebee species increased visitation to the other, previously less—utilized, flower species. The remaining bumblebees visited more flowers per stay in the patch, suggesting that they were finding greater amounts of nectar in the absence of other bumblebee species. These removal experiments demonstrated that the bumblebees were sampling flowers frequently enough and were flexible enough in thei...

Sea Otter Predation and Community Organization in the Western Aleutian Islands, Alaska

Abstract: Predation by the sea otter (Enhydra lutris) limits epibenthic invertebrates, especially sea urchins (Strongylocentrotus polyacanthus), in turn allowing a luxuriant development of the macroalgal canopy. Where sea otters are abundant, sea urchins are small and scarce in shallow water, and the association of fleshy macroalgae apparently is regulated by competition. Sea urchins are larger and more abundant in deeper water, where they are less accessible to sea otters. Macroalgae are most abundant, and competition in the plant association is severest, near the sublittoral fringe where sea otters can remove sea urchins most efficiently. In deep water, competition among maccroalgae is reduced because the light intensity is lower and grazing by sea urchins increases. On islands where sea otters are absent, sea urchins are abundant, large, and are probably limited by intraspecific competition; and they have eliminated fleshly macroalgae. Available data suggest that the association of Laminaria spp. and Agarum cribrosum contributes most to primary production in nearshore areas of the western Aleutian Islands. Where sea otters are absent and sea urchins have eliminated this plant association, some higher trophic forms also are absent or less abundant than where sea otters are common and the plant association is well developed. Earlier studies of sea otter food suggested that low—density populations of sea otters consume primarily sea urchins and mollusks in the western Aleutian Islands. Later studies of high—density populations showed a wider variety of foods consumed, with fish an important component of the diet. These studies support our observations on the differences in availability of these foods between islands with and without sea otters.

Evolution of Reproductive Characteristics in Impatiens (Balsaminaceae): The Significance of Cleistogamy and Chasmogamy

Abstract: The reproductive biology of Impatiens pallida Nuttal and Impatiens biflora Walt (Bal- saminaceae) was studied in relation to the evolutionary significance of cleistogamy and chasmogamy. Plant survivorship varied markedly between sites and was affected by abiotic agents such as drought and flooding, and occasional heavy damage from host-specific herbivores. Cleistogamy was the dom- inant mode of reproduction at all sites, but absolute and relative chasmogam production generally increased with total bud output. Extreme variability in seed production and reproductive mode was observed within and between sites. Cleistogams produced fewer seeds per ovary, matured faster and had a greater probability of surviving to produce seeds than chasmogams in all populations. The energetic investment in fertilization costs (i.e., sepals, petals, pollen and nectar) of chasmogams was > 100 times that of cleistogams. Calculation of energetic expenditure per seed based on survivorship curves for each flower type and energy investment for major developmental stages indicated chas- mogams were 2-3 times as costly as cleistogams. Greater chasmogam cost was also suggested by the earlier onset of senescence in greenhouse plants with high relative chasmogam output. Plants grown in the greenhouse under different light regimes displayed extreme plasticity in re- productive response as measured by absolute and relative output of cleistogams and chasmogams. The proportion of chasmogams produced per plant generally increased with light intensity. Plants collected from I. pallida populations where chasmogamy was rarely observed had significant chas- mogam production when grown in the greenhouse. The tremendous differences observed in plant survivorship and reproductive output demonstrate a high degree of temporal and spatial variation in quality of Impatiens habitats. It is suggested that these conditions have exerted strong selective pressure for reproductive plasticity. Multipurpose genotypes capable of producing cleistogams and chasmogams, depending on habitat suitability, are highly adaptive in environments where genetic tracking is inefficient. The evolution of a dual repro- ductive mode in Impatiens provides seed production at minimal expense via cleistogamy and the opportunity for outcrossing and pollen donation through chasmogamy. The outcrossing potential of Impatiens chasmogams is enhanced by (I) functional monoecy and (2) a fivefold difference in relative lifespans (J phase > Y), which results in strongly skewed intraplant sex ratios.

Influence of Economics, Interspecific Competition, and Sexual Dimorphism on Territoriality of Migrant Rufous Hummingbirds

Abstract: Migrant Rufous Hummingbirds (Selasphorus rufus) arrive in eastern Arizona in late summer and establish feeding territories from which other hummingbirds are excluded. Territories vary 100-fold in area and 5-fold in number of flowers. A simple cost-benefit model accounts for observed variation in territory size and number of flowers defended. Both sexes defend territories, but d d utilize denser flowers than Y Y. These differences appear to be related to sexual dimorphism in wing disc loading. Selasphorus rufus appears to have sacrificed efficient flight for aggressive ability as a strategy for competing with resident hummingbird species during its migration. Comparison of feeding territories of S. rufus and other nectarivorous birds indicate similarities which suggest that these systems may be subject to similar economic constraints.

The Role of Microhabitat in Structuring Desert Rodent Communities

Abstract: Interspecific competition is thought to be important in determining patterns of resource use and species abundances in natural communities. However, there have been few field tests of competition-based models of community structure. In this study, experiments were conducted with 4 coexisting desert rodent species to see whether competition is a sufficient explanation for their resource use and abundance patterns. Results were consistent with 3 predictions from competition theory. (1) The 4 species differed in their use of a resource, foraging microhabitat, which is potentially limiting to their populations. (2) Each species shifted its use of microhabitats in predicted directions when competitors were removed from or added to outdoor enclosures. (3) Each species was most dense where its preferred microhabitat was abundant, and augmentation of 1 microhabitat led to an increase in the density of the appropriate microhabitat specialist. These results suggest that compe? tition maintains interspecific differences in foraging microhabitat, and that the availability of appro? priate microhabitats determines species abundances on a local scale.

Food Availability as a Proximate Factor Influencing Individual Growth Rates in the Iguanid Lizard Sceloporus Merriami

Abstract: Von Bertalanffy, logistic-by-length, and logistic-by-weight models of individual growth for 2 populations of Sceloporus merriami were estimated from 3 yr (1974-1976) of mark-recapture data using nonlinear regression. The logistic-by-weight model was found to provide the best fit to observed growth rates. Abundance of arthropod prey was estimated by 2 techniques and found to be serially correlated with seasonal precipitation. Prey abundance was also correlated with annual precipitation. Individual foraging success was found to be correlated with estimated prey abundance. Parameters of logistic-by-weight growth models were compared for the 3 yr of the study and revealed that variation in food availability resulted in predictable variation in individual growth rates. Individual growth rates were positively correlated with active-season and annual precipitation and with estimates of food availability. The limitations of using stationary growth models to describe a potentially nonstationary process such as individual growth are discussed.

A Model for Seed Scatterhoarding: Coevolution of Fox Squirrels and Black Walnuts

Abstract: Granivores are likely to store food in numerous, widely scattered, small caches if they are unable to defend concentrated large caches against interspecific competitors. This scatterhoarding of seeds makes it impossible for individuals to defend all their scattered caches against intraspecific competitors as well. Optimal spacing of scattered caches should result from a balance between decreasing loss of caches to naive competitors with decreasing density and increasing cost of storage with decreasing density. A mathematical model predicting optimal density is presented. One prediction of the model is that as the (θ) of habitat suitable for seed burial surrounding a seed source is decreased, the average distance (D) a cache is taken from the source by scatterhoarders should increase. Another prediction is that increasing the number of seeds (N) at a source, either by an increase in the size of a single seed crop or by the presence of 2 or more conspecific seed producers in close proximity to one another, should increase D. One trait of trees which increases single crop sizes and average seed dispersal distance (D) is the pattern of withholding energy from reproduction some years to allow unusually large crops during mast years. Three field tests of the model and its predictions were conducted: (1) Juglans nigra seeds were buried at 3 densities and their survival in time from predation by Sciurus niger was found to increase with decreasing density; (2) average distances that J. nigra saplings occurred from 16 parent walnut trees were found to have a statistically significant correlation with the D calculated from the model and the θ of suitable habitat surrounding each tree; (3) Sciurus niger individuals were observed to scatterhoard Juglans nigra seeds in a pattern that maintained a mean cache density that gives low rates of loss to naive competitors as indicated from field test (1). The results strongly suggest maintenance of optimum cache density by S. niger. The coevolution of trees which have mast years and the animals that scatterhoard their seeds is discussed.

The Weibull Distribution: A New Method of Summarizing Survivorship Data

Abstract: Survivorship data can be effectively summarized using the shape and scale parameters of the Weibull frequency distribution. The shape parameter controls the rate of change of the age- specific mortality rate and, therefore, the general form of the survivorship curve. Estimates of shape and scale parameters and their confidence intervals can be easily calculated and used to compare survivorship curves of different populations.

Nutrient Retention by the Root Mat of an Amazonian Rain Forest

Abstract: Surface root mats on oxisols and spodosols near San Carlos de Rio Negro, Venezuela, were sprayed with (45)Ca and (32)P to simulate the addition of nutrients to the soil surface through leaching of decomposing litter, and through precipitation and throughfall. Collections from lysimeters placed below the root mat and humus layer showed that in all but 1 case, <0.1% of the radiotracers leached past the root–organic mat, and leaching stopped completely after 1 to 2 months. Through analysis of root mat samples, the radioisotopes were found to have been taken up and translocated by living roots. This is the first evidence supporting the ability of the root map on the surface of poor, depleted tropical soils to efficiently take up dissolved nutrients before they percolate down to mineral soil. Other studies have shown up that one of the uptake mechanisms is mycorrhizal fungi. See full-text article at JSTOR

Habitat Selection by Pemphigus Aphids in Response to Response Limitation and Competition

Abstract: The leaf galling aphid, Pemphigus betae can be highly resource and habitat limited and has evolved to reduce these limitations. Over a 3—day period in spring as much as 83% of the overwintering population migrates to immature leaves of Populus angustifolia where individuals are rapidly entombed by expanding leaf tissue. The size of the mature leaf is critical to ultimate fitness of the individual colonizing stem mother. Probability of stem mother failure, body weight of stem mother and progeny, number of progeny, development rate of progeny to maturity, and number of embryos in mature progeny are all correlated with leaf size. Due to these selection pressures, stem mothers search out and colonize large leaves. Thirty—two percent of the tree's leaves were so small as to result in an 80% probability of total aphid failure. Nearly all of these leaves were avoided. In comparison, only 1.6% of the tree's leaves were so large as to result in a 0% probability of failure, and 100% of these leaves were colonized. Even though most stem mothers were forced to colonize. Even though most stem mothers were forced to colonize suboptimal leaves because competitor density (35/100 leaves) was much > the availability of optimal leaves, the average stem mother colonized a leaf 60% > the mean leaf size of the tree. Resulting from this degree of selectivity, the minimum increase in fitness was 2 times > expected if leaves had been selected at random. At low competitor densities reproductive output would still be limited by energy intake such that if there were only 1 gall on a tree, the colonizing stem mother and her progeny would still be resource limited. Apparently, no leaf produced by the tree is so large that a further increase in leaf size would not result in a further increase in fitness. Thus, even at low competitor densities, resources would still be important in the habitat selection process and in determining the parasite distribution.

Differential Predation on Metamorphic Anurans by Garter Snakes (Thamnophis): Social Behavior as a Possible Defense

Abstract: The diets of garter snakes (Thamnophis) collected at several localities were examined and the developmental stage of anurans in the diets were recorded. During periods of anuran metamorphosis, garter snakes converge on the shoreline and their diets are biased towards transforming stages. This bias appears to reflect the locomotor ineptitude of metamophosing anurans;a transforming anurans can neither swim nor hop as effectively as premetamorphic tadpoles or postmetamorphic frogs, respectively. Metamorphic synchrony in certain anurans (e.g., Bufo) may have evolved as a defense against predation during metamorphosis. This synchrony could satiate predators. Certain anurans (e.g., Bufo, Scaphiophus) form postmetamorphic aggregations which may represent `selfish herds' in which individuals gain protection by association with more vulnerable conspecifics. See full-text article at JSTOR

Primary Productivity and Water Use in Native Forest, Grassland, and Desert Ecosystems

Abstract: The relationship between abovegound net primary production (ANPP) and water use varies significantly among ecosystem types. For both hot deserts and shortgrass prairie—cold deserts which are water limited, ANPP is linearly related to annual water use above minimum amount of water, estimated at 38 and 170 mm, respectively, needed annually to sustain each system. Once the minimum water too sustain ANPP is reached, ANPP increases an estimated 0.38 g and 1.09 g per 1000 g of additional water in the hot desert and the shortgrass prairie—cold desert. In forest systems not water stressed, ANPP was not related to water use. For grasslands representing a gradient from water stressed toward not water stressed, ANPP correspondingly declined per unit of water used. Classically evaluating water—use efficiency as annual ANPP divided by annual evapotranspiration, forests are the most efficient, 0.9 to 1.8 g ANPP/1000 g water, followed by shortgrass prairie, 0.2 to 0.7, then hot deserts, 0.1 to 0.3. See full-text article...

Foraging Ecology and Energetics of the Frugivorous Bat Artibeus Jamaicensis

Abstract: In Panama that bat Artibeus jamaicensis feeds primarily on figs (fruits of the genus Ficus). One hundred forty—two Ficus trees in 25 ha were mapped and their fruit production monitored for 2yr. Radio—tracking observations of foraging A. jamaicensis revealed that each bat carries away 9 ± 2 figs per night, enough to provide 40% more than the 43.9 kJ needed daily for basal metabolism and minimum maintenance flying. The estimated 6 ± 2 A. jamaicensis per ha carried away °7% of the 200, 000 figs (650kg) produced annually per hectare. Artibeus jamaicensis appears to be the most important dispersal agent for fig seeds in this area. See full-text article at JSTOR

Responses of Annual Vegetation to Temperature and Rainfall Patterns in Northern California

Abstract: Weather patterns significantly influence annual vegetation, both within and between years. The impact of these weather patterns on standing crop, cover, and botanical composition were investigated during the years 1955—1973 inclusive. Standing crop increased as the growing season progressed from March to June, while cover declined. Erodium spp. Carduus pycnocephalus, Geranium spp. and Hypochoeris glabra all declined between these 2 calendar dates. Other annual species increased in botanical composition as the growing season progressed. Weather patterns primarily influenced total standing crop during the initial period of plant growth. Following germination, temperatures were typically warm and conducive to plant growth; drought at this time suppressed total standing crop. Consistent precipitation following fall germination, and again during the period of rapid plant growth in spring, contributes to a "grass year" in the annual type. The sequence of freezing temperatures in relation to phenology of develop...

The Influence of Male-Defended Oviposition Sites on Early Embryo Mortality in Bullfrogs

Abstract: Field studies were conducted on factors affecting embryo mortality in bullfrogs, Rana catesbeiana, in 1975 and 1976 at the E. S. George Reserve of the University of Michigan. Larger 9 9 produced significantly larger clutches than smaller 9 9 (6000 to >20,000). Older 9 9 produced 2 clutches each yr with 2nd clutches containing significantly fewer eggs than 1st clutches. Egg size appeared to be unrelated to 9 size; however, 2nd clutches contained significantly smaller eggs than initial clutches for all 9 9. Embryo mortality depended on 9 choice of oviposition sites. Such sites were controlled by territorial 6 6. Larger 66 controlled oviposition sites that had significantly lower embryo mortality than the sites of smaller 6 6. Sources of embryo mortality included developmental abnormalities and predation. Choice of oviposition sites included: (a) avoidance of areas with high water temperatures (>32?C) that result in increased developmental abnormalities and (b) preference for areas that increase embryo survival by increasing developmental rate and/or decreasing efficiency of predation on embryos by the leech, Macrobdella decora.

Population ecology of alberta red squirrels

Abstract: Temporal changes and spatial differences in population structure and density of red squirrels (Tamiasciurus hudsonicus) were investigated in 3 habitats near Rochester, Alberta. Between 1966 and 1969, red squirrels were most abundant in spruce (Picea mariana and Picea alba) stands (161-684/100 ha), less abundant in jack pine (Pinus banksiana) stands (86-264/100 ha), and least abundant in aspen (Populus tremuloides) woods (0-99/100 ha). Summer populations fluctuated be- tween 67 (1966) and 151 (1968) per 100 ha of mixed habitat, but spring populations were more sta- tionary (32-49/100 ha of mixed habitat). Ovulation rates of adult and yearling squirrels and the percentage of yearling 9 9 which bred varied annually. Variation in annual reproductive rate (2.4-4.4 young/ 9) was correlated with the date of onset of breeding, and the number of days of snow cover subsequent to 1 January. Juvenile sex ratios were 1:1 but there was a trend toward preponderance of 6 s in older age classes. Although sex ratios on the study areas were close to 1:1 during early summer, late-summer mortality of adult 9 9 resulted in a preponderance of d d (60W) during the remainder of the year. Equality of sex ratio was restored during the breeding season when d d experienced high mortality rates. After fall dispersal, the percentage of adults was higher in the spruce (78-85%) than in pine (30- 50%o) but virtually 100% of the squirrels occupying aspen habitat were juveniles. Changes in age ratios suggested that juveniles suffered higher overwinter losses than did adults. Spring age ratios were not related to fall age ratios because of differential annual survival of juvenile squirrels. Dispersal movements occurred in spring and fall; squirrels were relatively sedentary the remainder of the year. The spring shuffle involved breeding d d visiting territories of 9 9 and redistribution of squirrels from deciduous and pine forest into spruce forest. Almost all fall dispersants were young of the year. High adult densities in spruce stands prompted emigration of most juveniles from these areas. Fewer juveniles emigrated from jack pine where adult densities were lower and territories were available. Juveniles apparently dispersed into the large tracts of aspen-dominated deciduous forest. Survival of squirrels varied seasonally and annually, among habitats and between sex and age cohorts. Heavy mortality of adult 9 9 and nestlings occurred between parturition and weaning (July- September). The overwinter period (October-February) was one of high juvenile losses. During the 3-mo breeding season (March-May), d d experienced higher mortality rates than 9 9. Annual survival was highest in the spruce and lowest in the aspen. Survival rates were highest during 1966-1967, --309o lower in 1967-1968, and '-44% lower in 1968-1969. Survival of juveniles was lower in all years than survival of adults. In the wild, an average of 639 meristematic buds, and the seeds from 35 pine cones were consumed by a single squirrel each day. Captive feeding trials indicated that an average of 176 black spruce cones or 123 white spruce cones per day were necessary to sustain 1 squirrel when offered no supplementary foods. Territorial boundaries in spruce and pine did not change detectably between 1966 and 1969. Within monotypic habitats, territories were similar in size. Territories in white spruce were smallest (xt = 0.24 ha), those in mixed spruce larger (xt = 0.35 ha), and those in jack pine much larger (t = 0.66 ha). Territory size may be related to cone supply in poor mast years. In 1967, a year of poor cone mast, sufficient numbers of cones were produced per territory to provide one-half of the annual energy requirement of 1 squirrel. The stability of spring populations, despite the change in mast crop from very poor (1966 and 1967) to medium-heavy (1968), implies regulation of squirrel densities at levels which preclude critical food shortage. We hypothesize that territorial behavior is the mechanism which regulates densities about a mean determined by food supply in years of poor cone mast, and that it operates through density-dependent mortality by excluding nonterritorial squirrels from winter cone supplies. Similar- ities in red squirrel densities (or territory sizes) in similar forest types recorded by numerous authors in different areas and different years imply consistency of territory size within particular habitats. We believe that variation in numbers of squirrels harvested primarily reflects variation in annual production.

Cherries, Ants and Tent Caterpillars: Timing of Nectar Production in Relation in Relation to Susceptibility of Catepillars to Ant Predation

Abstract: The North American black cherry, Prunus serotina Ehrh., has extrafloral nectaries which are most active during the first 3 wk after budbreak. These nectaries attract large numbers of the ant Formica obscuripes Forel, especially to trees within °20 m of an ant colony. The average number of ant visits per bud is highest just after budbreak, decreasing as the number of active extrafloral nectaries decreases. These ants are predacious on many species of insects found on P. serotina, including the eastern tent caterpillar, Malacosoma americanum Fabricus, the major defoliator of black cherry. Formica obscuripes is only able to prey upon M. americanuum within °3 wk of budbreak, after which time, tent caterpillar larvae are large enough that most escape ant predation. Tent caterpillar survivorship was found to be positively related to distance from colonies of F. obscuripes. It is suggested that the ant—cherry relationship is a faculative mutualism and that nectar production is timed so as to maximize the chance of successful ant predation on tent—caterpillar colonies.