Showing papers in "Ecology in 1976"

Behavioral Response of Crayfish to a Fish Predator

Abstract: In laboratory aquaria, smallmouth bass (Micropterus dolomieui) influenced activity, substrate selection, behavior, and food consumption of crayfish (Orconectes propinquus) of different sizes and sexes. In the presence of a predator, crayfish selected substrates affording most protection, significantly modified the frequency of various behavior patterns, and reduced their overall activity. Active behavior patterns such as walking—climbing and feeding were suppressed while defensive patterns, such as burrowing and chelae display increased. Grazing by crayfish on their detrital food supply was also suppressed by bass. Degree of response of crayfish appeared correlated with liability to predation, i.e., small, vulnerable crayfish were most affected, whereas large, less vulnerable crayfish were least affected. Because crayfish ♂ ♂ have larger chelae than ♀ ♀, they apparently were better able to defend themselves against predators; consequently, less vulnerable ♂ ♂ did not modify their behavior as dramatically as ♀ ♀. In field collections crayfish exposed on the substrate were larger than those buried; fewer ♀ ♀ than ♂ ♂ were exposed. Thus, field observations correlated well with laboratory findings, suggesting that predators affect distribution and behavior of crayfish in natural communities. By affecting several trophic levels within aquatic systems, influence of predators goes beyond simple interactions of predator and prey.

Size-Limited Predation: An Observational and Experimental Approach with the Mytilus-Pisaster Interaction

Abstract: In many space-limited systems, for instance forests, coral reefs, and mussel beds, the main biological component of the spatial matrix is provided by large-bodied species. At some point during their growth these matrix species surpass the ability of any single individual of a predator species to consume them, although they still may be overwhelmed by concentrated group attacks (i.e., by aphids, fungi, internal parasites). These species thus surpass through continued growth a size threshold limiting their consumers, and thereby attain a refuge in size. The implications of these refuges are discussed, with special reference to a predatory starfish, Pisaster ochraceus, and its primary prey, the mussel, Mytilus californianus. The mean wet weight of Pisaster varies between localities from 150 g to 2,640 g. Long- term (12-yr) observation indicates little annual variation and generally no trends within areas, yet consistent differences between areas. Five transplant experiments demonstrate that Pisaster is an indeterminate grower, and is capable of adjusting its body size through growth to local conditions. Limited recovery of individually marked starfish suggest that exchange between areas is restricted. Observations on the natural diet of Pisaster show a pronounced maximum body size limit to the mussels that can be consumed by intertidal starfish of a given size. This limit was explored experimentally in two ways. First, Pisaster was allowed to reinvade an area from which it had been manually excluded for 5 yr. In this interval those mussels comprising the matrix of the clump had attained shell lengths > 8-10 cm, and could not be consumed by Pisaster weighing 203 g. Second, artificial clumps of known-size mussels were protected by plastic screening, allowed to attach, and then exposed to predation. Survival was significantly greater in those mussels expected to be too large to be eaten. The significance of size-limited predation is that prey and predator can coexist in very intimate proximity, that the surviving prey often attain large sizes and probably make a re- productive contribution disproportionately greater than their abundance, and that these same prey diversify the spatial structure of the environment, usually by adding a vertical dimension not otherwise present.

Fungi and Bacteria Associated with Leaves During Processing in a Woodland Stream

Abstract: Autumn—shed leaves of white oak (Quercus alba) and pignut hickory (Carya glabra) were fastened together and incubated in a third order hardwater stream in south—western Michigan. At biweekly intervals, leaves were removed and the microbial populations assessed. Direct counts of bacteria increased logarithmically with time, but viable counts were generally lower. Flexibacter, Achromobacter, Flavobacteria, Pseudomonas, and Cytophaga were isolated from leaves. No apparent bacterial succession was detected. The mycoflora was assessed in five ways, including direct examination and incubations ranging from environment simulating to particle platings on nutrient medium. Although this last technique suggested that common soil and litter fungi dominated, direct examination and environment—simulating incubations showed that aquatic hyphomycetes, particularly Alatospora acuminata, Flagellospora curvula, and Tetracladium marchalianum were the dominant members of the mycoflora. The successional pattern of these fungi on both leaf species is discussed. It is concluded that the aquatic hyphomycetes are actively growing and completing their life cycle in this environment and that the terrestrial fungi are normally present mainly in a dormant state, growing and sporulating only in greatly enriched conditions. The highest ATP levels observed corresponded to the periods of highest fungal frequencies .

Spatial Heterogeneity and Bird Species Diversity

Abstract: A heterogeneity index, D, derived from the point—quarter technique was significantly correlated with bird species diversity (BSD) for several shrub and forest areas. It predicted BSD for a series of similar brushlands where other indices had failed. Species richness increased faster than species overlap in a series of increasing complex habitats up to the forest stage. Species overlap was negatively correlated with patchiness. Additional species may be accommodated in preforest habitats primarily by horizontal spatial segregation facilitate by the presence of additional patches. New patches result from the addition of layers of vegetation. In late shrub or forest stages other kinds of segregation such as vertical segregation become important to species packing. Patchiness, as measured here, has a proximate effect on avian diversity. The extent or existence of latitudinal gradients in habitat patchiness and the effect on bird species diversity is unknown. The need remains for a universal simple, yet meaningful, heterogeneity index which incorporates both horizontal and vertical variability of vegetation.

Experimental Zoogeography of Islands: Effects of Island Size

Abstract: A controlled experiment performed on 8 small mangrove islands constituted an exact test of several biogeographic hypotheses which had rested largely on unsatisfying statistical treatments. The islands were censused for arboreal arthropods; each contained a different subset of a species pool of °500. Sections of the islands were then removed and censuses retaken after a waiting period; this procedure was repeated on four islands a second time. The results were consistent with a model which posits the islands as originally in a state of dynamic equilibrium between immigration and extinction, then re—equilibrating quickly when forced into an oversaturated condition. It was specifically demonstrated that: (1) species number increases with island size alone, independent of habitat diversity; (2) the increase with area is faster on separate islands than on increasing subsections of one island; (3) the area effect and the underlying dynamic equilibrium are not due only to an increased sample of transients and vagrants on larger islands, though there are a number of species which are particularly prone to quick extinction and which contribute disproportionately to the observed high turnover rates; and (4) prediction of the effects of decreased area on species composition can be stochastic at best, not deterministic. The equilibrium theory prediction of higher turnover rate on smaller islands could not be conclusively tested because of small sample size. Several species interactions suggested by the distributional data are so subtle that it is apparent that much more intensive work will be required to demonstrate even the existence of interactions, let alone whether they are important. It is clear that much of the dynamic equilibrium and its associated turnover in this system can be ascribed to individual species characteristics and the effects of a rigorous physical environment.

Geographic Variation and Its Climatic Correlates in the Sex Ratio of Eastern-Wintering Dark-Eyed Juncos (Junco Hyemalis Hyemalis)

Abstract: The sex ratio of Dark-eyed Juncos wintering in the eastern and central United States and Canada varies clinally along a latitudinal gradient. The percentage of Y Y among both museum skins and live-caught birds is 70% in the south, 20% in the north. When abundance according to latitude is also considered, an average Y appears to winter farther south than an average 8 and hence probably tends to migrate farther. Latitude alone is an excellent predictor of sex ratio (r2 = 85%), and latitude plus 13 other measures of climate explain virtually all the variation (r2 - 96.6%). Extreme measures of climate, as compared to mean measures, are equally predictive. Principal component analysis indicates that snowfall, temperature, and latitude are the most important climatic variables associated with sex ratio. Because 8 8 average larger than Y y and are concentrated northward, mean wing length increases with latitude and is significantly correlated with climatic measures that vary with latitude. Further, larger birds within each sex may select higher altitudes as wintering sites. Sex ratio does not vary measurably with date in wintering populations. Among possible explanations for clinal variation in sex ratio are sex-associated differences in (1) advantages of early arrival on the breeding or -wintering grounds, (2) impacts of inter- and intrasexual competition, and (3) effects of low temperature and intermittent food avail- ability. Comparison of ; ; and Y Y with respect to potential fasting endurance, a size-related metabolic parameter, indicates that at OC an average 8 should be able to fast 4% longer (1.6 h) than an average Y at standard metabolic rates. An extremely heavy 8 might endure fasting up to 29% (10.7 h) longer than a very light Y. These differences may confer greater survival ability upon the 8 at latitudes where snow cover can often preclude feeding.

Changes in the Chemical Composition of Leaves During Processing in a Woodland Stream

Abstract: The weight loss and changes in the concentrations of major classes of macro—molecules were assessed in white oak (Quercus alba) and pignut hickory (Carya glabra) during processing in a third order hardwater stream in southwestern Michigan. Oak was processed more slowly than hickory. An initial rapid weight loss in hickory was followed by a linear rate of loss, while oak lost weight at a linear rate over the entire processing period. Losses of soluble reducing sugars and polyphenols were rapid, occurring mainly during the initial 2—wk exposure to the stream. Hemicellulose and cellulose losses were similar in both species and nearly paralleled weight loss. An absolute increase in the lignin fraction was observed in both species during the first 6—12 wk of processing. We consider this to be due to an increase of complexed N containing compounds in the lignin fraction. Lipid loss was intermediate between soluble sugars and polyphenols and the structural components. Of the constituents analyzed, only the conce...

Egg size, hatching asynchrony, sex, and brood reduction in the common grackle'

Abstract: In clutches of five, the Common Grackle (Quiscalus quiscula) provisions eggs un- equally and starts incubation before the clutch is complete, thereby insuring initial advantages to some young which are not accrued to others. Field observations and experiments show: (1) egg wt increases with laying sequence within the clutch, (2) intraclutch variation in egg size results in competitive advantages in growth for young from large eggs, (3) asynchronous hatching favors young which hatch early at the expense of those which hatch late, (4) starvation mediates brood reduction of late-hatching young in many nests, and (5) normal brood reduction produces a sex ratio skewed in favor of 9 9. Egg size variation within the clutch and asynchronous hatching are interpreted as antagonistic forms of parental investment. Presence of both adaptations suggests that the parent attempts to ensure the survival of chicks which hatch late as long as possible within an overall pattern of initial investment which ensures starvation of those individual young if food is insufficient to raise the entire brood. This retention is interpreted as a means by which the parent maximizes reproduction in the event of unexpectedly favorable food conditions. Elimination of late-hatching young produces a sex ratio favoring 9 9 because ultimately investment in that sex is dependable if exceptionally healthy, and hence potentially polygynous, d d cannot be produced. Facultative adjustment of sex ratio within the brood may produce a high variance around a population sex ratio of 1: 1 or some kurtosis necessitated by unequal costs of raising individuals of each sex. The overall pattern of brood reduction in this species is interpreted as an evolved system by which a 9 parent reduces risks so as to maximize inclusive fitness.

Apparent Size as the Determinant of Prey Selection by Bluegill Sunfish (Lepomis Macrochirus)

Abstract: Copyright by the Ecological Society of America. This is the publisher's version, also available electronically from http://www.jstor.org/stable/1935055.

Importance of vegetative cover to cycles of microtus populations

Abstract: A hypothesis is presented that a site-dependent threshold level of vegetative cover is necessary for a population of Microtus to increase in numbers sufficiently to undergo a multi-year cycle. The hypothesis is supported by results of studies in grassland habitats in the tallgrass prairie of Oklahoma and Minnesota, the mixed grass prairie of South Dakota, and the shortgrass prairie of Colorado. Levels of cover below the threshold sometimes support resident, breeding populations of voles. Levels above the threshold may influence amplitude, duration, and synchrony of Microtus cycles.

Causes of Juniper Invasion in Southwestern Idaho

Abstract: Invasion of western juniper into vegetation dominated by mountain big sage- brush and perennial bunchgrass on the Owyhee Plateau of southwest Idaho appears to be directly related to cessation of periodic fires Evidence from adjacent climax juniper stands indicates that fires were frequent for at least several hundred years preceding white settlement Fires have been much less frequent during the past century due to active fire control, develop- ment of roads and other fire barriers, and reduced fuel because of heavy grazing and a shift towards decreased precipitation Physical and biotic factors affecting the establishment of juniper, seed dispersal mechanisms, and the fire history of the study area were investigated Results indicated that range condition as such had a negligible effect on juniper establishment Juniper seedlings became established most readily on areas supporting well-developed herba- ceous and shrubby vegetation Seed dispersal was primarily localized, and accomplished by gravity and disturbance by animal trampling Abundant evidence of fire in the form of charred stumps and fire scars on living trees was found throughout the study area Old juniper stands are confined to rocky ridges where understory vegetation is sparse and fires less intense Juniper was apparently kept out of the denser vegetation of deeper soils by more intense fires Most herbaceous and shrubby species survived this treatment due to greater tolerance to fire, or rapid reproduction from seed

Resource Partitioning Among Some Eusocial Insects: Bumblebees

Abstract: Four common species of bumblebees in central Maine each utilized the nectar from a great variety of both native and exotic flowers. The number of individuals that utilized a nectar resource was directly related to supplies available. Seasonally, 92% of the "standing crop" of food energy available per day was utilized in 1972. No interference com- petition was observed, and competition was primarily exploitative. Small wild bees generally foraged at high temperatures from the small food rewards left after Bombus exploitation. Overlap in resource utilization in the guild of bumblebees was avoided by differences in tongue length. Open (short-corolla) flowers were visited primarily by short- and secondarily by long-tongued bumblebees. Deep-corolla flowers were visited primarily by long-tongued bumblebees. Pollen flowers were utilized more by short-tongued than by long-tongued bumblebees. However, the bumblebee forager spectra on given flower species varied from one locality to the next as well as seasonally. Colony polylecty is probably a necessary correlate of social behavior, given short blooming times of the host plants relative to the long durations of the colony cycles. In turn, colony polylecty of eusocial bees may decrease species richness in the bee community due to competition.

Character and Significance of Forest Tree Root Exudates

Abstract: Root exudates from the unsuberized tips of new woody roots were collected from mature Betula alleghaniensis, Fagus grandifolia, and Acer sacclharuml in a northern hard- wood forest. Numbers of new woody root tips were also determined for these three species. Exudates were fractioned into carbohydrates, amino acids/amides, organic acids, and 9 in- organic ions. Considerable species variation characterized the quantitative and qualitative nature of the organic fraction. Organic acids were the most abundant component. Fagus grandifolia released the largest amount of amino and organic acids per hectare, while B. alleglhaniensis exuded the largest amount of carbohydrates. Sodium (possibly anomalous), K, and Ca dominated the cationic fraction, while the anionic fraction was chiefly SO4 and C1. Betula allcghlaniensis and F. grandifolia released considerably greater quantities of inorganic ions than A. saccliarumn, but the inorganic root exudate patterns of the three species were relatively uniform. These results indicate root exudates have some role in intrasystem nutrient cycling.

Underground Niche Separation in Successional Plants

Abstract: Weekly root extension of six successional annuals in glass-faced boxes was measured over a growing season in 42 soil resource states. Niche breadth and overlap were calculated and compared to values obtained from published root diagrams of eight tall grass prairie species. Differences in use of soil resources by the successional species were found and interpreted as primitive niche separation to avoid competition. Temporal displacement in absorption of moisture and nutrients is apparently important only to one species (Ipomoea hederacea). The six species showed marked differences in total composition of N, K, Ca, and Mg, although proportional similarities in nutrient use were high. The dominants of the successional community, Setaria faberii, Polygonum pensylvanicum and Ambrosia artemisiifolia had broader niches than the species with lower biomass and density. Two-dimensional overlap matrices in time and space showed Chenopodium album and Setaria to be highest in mean overall overlap, Polygonum and Ambrosia to be inter- mediate, and Abutilon theophrasti and Ipomoea to be lowest. The use of multidimensional resource states in the construction of an overlap matrix incorporating more than one parameter of the niche hypervolume was shown to be more useful than the multiplication of overlaps along each of the two parameters, space and time. Mean overlap in the use of underground space was considerably lower in the mature community (prairie) than in the successional community (oldfield). This corroborates the hypothesis that competition is less important in the evolution of fugitive species of the oldfield than of equilibrium species of the prairie.

Recolonization of red sea corals affected by natural catastrophes and man-made perturbations'

Abstract: The recovery patterns of hermatypic corals following an unpredicted catastrophic low tide were studied on two reef flats in the northern Gulf of Eilat, Red Sea: (a) the nature reserve of Eilat, Israel which is chronically polluted, and (b) a control reef which is pollution- free. The coral community structure and species diversity in both reefs were studied in 1969 and served as a base line for comparing the extent of mortality during the low tide (1970), and the extent of recovery 3 years later (1973). In 1969 no significant differences were found (p > 0.05) between the coral community structure of the nature reserve and the control reef when the average number of species, number of colonies, living coverage, and diversity (H'.N) per transect were taken into account. During the low tide both reefs suffered mass mortalities of corals (85% at the nature reserve and 81% at the control reef). Although the extent of mortality in both reefs did not differ sig- nificantly (p > 0.05), a marked difference was observed in their recovery 3 years later. The extent of coral recolonization was 23 X greater at the control reef, but no significant difference (p > 0.05) was found in the extent of coral community regeneration in both places (15% at the nature reserve and 19.2% at the control reef). While the number of colonies, number of species, living coverage, and H',v are drastically decreased at the nature reserve 3 years after the catastrophic low tide, the control reef exhibited an outstanding fast recovery (full recovery is expected 5-6 years from the low tide). The commonest species on the control reef in 1969 showed the highest rates of recruitment in 1973, which might indicate the oppor- tunistic life history of these species. I conclude that one of the differences between man-made polluting activities and natural catastrophes on coral reefs, is the possibility that the human- perturbed environment will not return to its former configuration, while reconstitution of reef areas denuded by natural disturbances is mainly a function of time. The higher coral diversity recorded on the control reef in 1973 (H'A = 2.403) as compared to 1969 (H'.v = 2.206) may reflect a succession pattern in which diversity continues to increase in time after a catastrophe, until space becomes limiting and competitive interactions between species cause a decline in diversity. The unpredictable and extremely low tides at the Gulf of Eilat seem to interfere with this scheme and prevent monopolization of the reef flat by competitively superior species. I suggest that catastrophic low tides act as an important diversifying force on the reef flats of Eilat, in a similar way as other biological and physical disturbances affect coral communities (Porter 1974).

Schooling as a mechanism for circumventing the territoriality of competitors

Abstract: The herbivorous striped parrotfish Scarus croicensis exhibits pronounced within- site variability in its social behavior: some individuals hold permanent feeding territories while others form feeding schools. Members of both of these classes are subordinate to the omnivo- rous damselfish Eupomacentrus planifrons which strongly inhibits the feeding of parrotfish in its own feeding territories. To a lesser degree, territorial S. croicensis, whose territories are superimposed on those of E. planifrons, also inhibit the feeding of nonterritorial conspecifics. Data show that nonterritorial S. croicensis in schools feed at higher rates, and are attacked by territory owners less often than nonschooling nonterritorials. This supports the hypothesis that schooling enables those individuals to circumvent the territoriality of their competitors. Non- territorial S. croicensis apparently constitute that proportion of the population that cannot obtain feeding territories, probably primarily as a result of the aggressive activities of E. planifrons, and schooling thus promotes the coexistence of these territorial and nonterritorial forms. A number of other teleost species, benthic-browsing omnivores, and herbivores, that associate with S. croicensis schools and are subject to strong aggression from E. planifrons, derive benefits from this association in the same way as the schooling parrotfish do. Predators that associate with S. croicensis schools, and that are little attacked by E. planifrons, benefit instead by feeding on organisms disturbed by the feeding school.

Plants Bearing Extrafloral Nectaries and the Associated Ant Community: Interhabitat Differences in the Reduction of Herbivore Damage

Abstract: Field experiments, designed to imitate the role of extrafloral nectaries, demonstrate that ants attracted to sugar droplets on plants can reduce the level of herbivore damage. This protection is more effective in disturbed habitats, such as forest clearings and edges, where ant—forager abundance is high. Natural nectary—bearing plants are more abundant in these same habitats. Other forms of disturbance such as annual flooding and fire, reduce ant abundance, reduce protection levels, and alter the frequency of natural nectary plants. See full-text article at JSTOR

Population Consequences of Shell Utilization by Hermit Crabs

Abstract: The nearshore hermit crabs Clibanarius vittatus, Pagurus pollicaris, and Pagurus longicarpus have broadly overlapping shell utilization patterns along the Texas coast. Effects of shell stress resulting from this overlap and from an overall shortage of shells on the re- productive potentials of the crabs were examined. Regression analyses indicate that shell weight and internal volume affect the clutch sizes of C. vittatus and P. pollicaris but not P. longicarpus. Clibanarius vittatus maintained in shells smaller than preferred grew more slowly than crabs maintained in shells of preferred size. Clutch size was highly correlated with crab size in this species. Several commensal animals, including polychaetes, gastropods, a hydroid, and a xanthid crab, consumed hermit crab eggs and/or zoeae in the laboratory. These egg predators were encountered in large shells occupied by e e more frequently than in shells in the size range utilized by ovigerous Y Y. These results indicate that shell size may limit hermit crab clutch size through a variety of mechanisms.

Autumnal Processing of Dissolved Organic Matter in a Small Woodland Stream Ecosystem

Abstract: A bstract . A mass-balance approach was used to construct a dissolved organic matter (DOM) budget for Roaring Brook, a small New England stream, in autumn. Total litter input to the stream ecosystem during the 77-day study period was 345 g/m2, 21 % of which occurred as lateral transport. Nearly 17% of litter input was released to the water as DOM within 3 days of entry. This leachate input represents 42% of total DOM input to the 1260-m study section. The remainder entered by way of inflowing surface and subsurface water. Autumn uptake of DOM was equivalent to 77% of leachate input and 33% of total DOM input. Approximately 88% of DOM removed from the water column is retained within the system and at least 67% is energetically degraded by consumer respiration in autumn. Based upon total annual energy budgets for similar streams, we estimate that DOM utilization in autumn accounts for from 7% to 11% of annual ecosystem respiration in Roaring Brook.

Thermal Biology of Anolis Lizards in a Complex Fauna: The Christatellus Group on Puerto Rico

Abstract: To describe the thermal biology of the three trunk-ground species of the Anolis cristatellus group on Puerto Rico, an island with 10 species of Anolis, we obtained samples of air and body temperatures of A. gundlachli (shady perches, montane forests), A. cristatellus (shady or sunny perches in open or closed forests, lowlands to mid-elevations), and A. cooki (sunny perches in open, xeric lowlands). Average body temperatures parallel altitudinal and habitat association (lowest for A. gundlachi, highest for A. cooki). Within a species, body temperatures are strongly correlated with air temperatures and thus vary with altitude, time of day, habitat, and weather. Observed differences between sympatric species in body tempera- tures and habitats probably reflect physiological requirements, but may be magnified by competition. Relative thermal niche breadth of individuals of these species is approximated and compared with data on species from simple anole faunas to evaluate hypotheses on the evolution of thermal niche breadth. Extent of basking behavior is inversely related to associated costs for these species. In closed forests where costs of raising body temperatures are high, A. gundlachi and A. crista- tellus rarely bask and seemingly are routinely passive to ambient conditions. In open habitats where costs are low, A. cristatellus and A. cooki frequently bask.

Energy Conservation by White-Tailed Deer in the Winter

Abstract: Behavior of white—tailed deer (Odocoileus virginianus) in Itasca Park, north—western Minnesota, USA was analyzed for energy—conservation adaptations during winter. Track records showed a decrease in activity with an increase in perdicted heat loss when activity and heat loss were compared on a sequential basis throughout the winter. Recognition of many seasonal but gradual changes in deer characteristics, such as antler growth, reproductive condition, and molting, suggest that seasonal physiological changes occur and also effect over behavior. Energy may be conserved by reducing the general level of activity, by seeking more level land and lesser snow depths, and by walking more slowly. Such energy—conservation measures may save up to 1,000 kcal/day (= 4,184 kJ/day) for a 60 kg deer, and 0.25—0.50 kg field—weight forage. Deer should remain as undisturbed as possible in the winter; harassment by dogs and snowmobile traffic is counter to their long term physiological and behavioral adaptations.

Plant Selection and Foraging Patterns in Two Species of Leaf‐Cutting Ants (Atta)

Abstract: Three colonies each of two species of leaf—cutting ants (Atta colombica Guer. and Atta cephalotes L.) were studied for 1 yr in the Guanacaste Province of Costa Rica to determine the following: (1) Do colonies of Atta cut leaves from a limited number of species (i.e., are they selective)? (2) What determines which plant species are attacked? (3) Do leaf—cutters optimize foraging in terms of energy expended by attacking palatable plants closest to the nest or do they distribute their efforts more or less evenly throughout their foraging territory? Several line of evidence support the hypothesis that A. cephalotes and A. colombica are selective in terms of plant material attacked. The ants sampled a large majority of the plant species present, but concentrated their foraging on a restricted subset of species. The mature leaves of only 31.4% and 22.0% of the species present were readily acceptable to A. colombica and A. cephalotes, respectively. The new leaves of another 12—16 plant species were acceptable. Different colonies of the same Atta species consistently attacked the same plant species if they were available, cut leaves from the same species at similar rates, and even attacked the same species of plants at the same time of year. The amount of material cut from host plant species was not correlated with host plant abundance in four of the six colonies studied, and was even slightly negatively correlated in two of these four. An analysis of plant distance from the nest versus amount harvested shows that palatable plant species close to the nest have a greater probability of being visited by the ants, but do not necessarily suffer more defoliation than other such plants within 50—60 m of the nest. Amount harvested decreases greatly for plants farther than 60—80 m from the nest. Thus colonies of Atta do not normally concentrate their efforts on plants closest to the nest, but foraging cannot be described as evenly distributed either. The factor responsible for selectivity in colonies of Atta is probably the internal chemistry of the material selected. An Atta colony must provide the fungus garden when a proper balance of nutrients and moisture without overlapping it with secondary compounds from the plants selected. The ants' attempt to solve this problem may explain some of the complexities of leaf—cutter foraging behavior.

Defense of Space and Resources by Chacma (Papio Ursinus) Baboon Troops in an African Desert and Swamp

Abstract: Baboons are not included in the growing body of documentation on primate spatial defense. Instead, most field studies of baboons suggest that troops occupy overlapping undefended home ranges. In southwestern Africa we found defense of space by two troops living in a Namib Desert canyon and by a population of troops on the Okavango Swamp flood—plain in Botswana. Two desert troops encountered one another at contiguous ends of their linear home ranges and established a defended boundary near a waterhole. One troop also dominated a third, a smaller troop at the opposite end of its home range. In the swamp, large troops, living in a more energy and plant species—rich environment, occupied small home ranges relative to troops studied elsewhere in Africa. Here space was defended along well—defined boundaries. Size and configuration of troop space and the arrangement of resources within each space influence the likelihood of intertroop encounters and the expression of spatial defense. See full-text article at JSTOR

Gull Chick Survival: The Significance of Growth Rates, Timing of Breeding and Territory Size

Abstract: Ecology (1976) 57: pp. 62-75 GULL CHICK SURVIVAL: THE SIGNIFICANCE OF GROWTH RATES, TIMING OF BREEDING AND TERRITORY SIZE 1 GEORGE L. HUNT, JR. AND MOLLY WARNER HUNT Department of Ecology and Evolutionary Biology, University of California, Irvine, California 92717 USA Abstract. In a 2-yr study of the survival of Glaucous-winged Gull (Larus glaucescens) chicks, pecking of trespassing chicks by neighboring adults was the major cause of chick mortality. In years of both low and high food availability chick survival was strongly cor- related with growth rates. Chicks that grew slowly were more likely to be killed by neighbors than fast-growing chicks. In the year of low food availability, among slow-growing chicks, those hatched early in the season on large territories bad better survival rates than chicks hatched late in the season on small territories. In the year of high food availability, timing of breeding and territory size had little bearing on chick survival. A model of chick survival in relation to timing of bteeding predicts that when neighbor interference is the major cause of chick mortality, chicks hatche·d early in the season will have the highest probability of surviving. When predators are the major cause of chick mortality, chicks hatching in the middle of the breeding season will have highest survival. A second model relates chick survival to territory size. Chicks raised on large territories will most likely avoid neighbor interference, while predation will select for either small or large territory size, depending upon the effectiveness of group mobbing against the predator. Key words: Colonial nesting; fledging success, gulls; Glaucous-winged Gull; growth rates; Larus glaucescens; model, breeding timing and gull chick survival; model, territory size and gull chick survival; mortality; predation. INTRODUCTION Social systems and spacing patterns of breeding vertebrates have received considerable attention in recent years (see reviews by Brown 1964, Lack 1968, Orians 1969a, Brown and Orians 1970). Colonial breeding has been described as a response to abun- dant or unpredictable food resources (Orians 1961, Crook 1964, Horn 1968, Lack 1968, Emlen 1971, Krebs 1974, Hunt and Hunt 1975), to short breed- ing seasons and reduced food resources (Barash 1974), or to predation pressure (Cullen 1960, Ash- mole 1963a, b, 1971, Kruuk 1964, Patterson 1965, Tinbergen et al. 1967, Parsons 1971, Alexander 1974). These are works dealing with factors ex- ternal to the breeding situation; little information is currently available concerning the internal factors that determine spatial and temporal configurations of breeding colonies. The roles of age relative to position or timing (Coulson 1966, 1968, MacRoberts and MacRoberts 1972) and of mutual stimulation (Collias et al. 1971, MacRoberts and MacRoberts 1972, Victoria and Collias 1973) have been examined. Interference between colony members is another internal factor of possible importance. In some social vertebrates interference between colony members can be as extreme as the killing of yow1g by adults. Crushing and biting by adult northern elephant seals (Mirounga angustirostris) Manuscript received 6 December 1973; accepted 25 August 1975. are major factors in pup mortality (LeBoeuf et al. 1972). In large Larus gulls, killing of chicks by neighbors is an important cause of chick mortality (Paynter 1949, Drury and Smith 1968, Parsons 1971, Hunt 1972). Less extreme interference includes be- haviors such as stealing nest material (Ytreberg 1956, Cullen 1957, Sladen 1958) or food (various gulls which we have observed) from other colony mem- bers. Reproductive patterns that might reduce these forms of interference by neighbors sh@uld exist. In the present study we initially set -out to deter- mine the relative importance of various causes of chick mortality, including killing by neighbors, in the Glaucous-winged Gull (Larus glaucescens), and the characteristics of parents successful in rearing young. Parameters examined included territory size, growth rates and timing of hatching. In the second year we attempted to separate the effects of seasonal changes on reproductive success from effects related to the age or experience of the parents through ex- change of eggs between birds laying at different times. We examined the significance of the internal temporal and spatial structure of a colony in rela- tion to food availability for individual reproductive success. Then, we integrated these factors into an hypothesis that predicts optimal timing and spacing of nesting in relation to conflicting needs for pro- tection against external predators and avoidance of interference by neighbors. This hypothesis may have applicability to other colonial vertebrates.

Empirical Evaluation of the Hypothesis of Food Competition in Salamanders of the Genus Plethodon

Abstract: Two species of salamander, Plethodon cinereus, and Plethodon hoffmani were used in experiments designed to test the hypothesis that food is the object of competition. Plethodon hoffmani occurs in the Appalachian Ridge and Valley Province where it appears to exclude the closely related and contiguously allopatric P. cinereus. Detailed field observa- tions and a laboratory experiment confirm Jaeger's (1972) hypothesis that the food of sala- manders could be limited in availability during dry weather and suggest that the cause of the low availability is the reduced mobility of the salamanders rather than a reduced food supply. However, because stomach content data and behavioral observations indicate that salamanders do not forage when the surface is dry, it was reasoned that food competition could occur under only two restricted conditions: (1) when more than one animal is isolated under damp refugia, such as rocks and logs, during dry weather, and (2) when many animals emerge onto the surface to forage when wet weather follows a prolonged dry period. Empirical evidence suggests that salamanders do not clump in refugia as the habitat dries out and that they do not surface simultaneously to forage after a dry period. Thus, even though food is periodically unavailable, the staggered feeding schedule prevents the density of salamanders from ever reaching competitive levels. Interference mechanisms are not ruled out as mecha- nisms of the observed spacing patterns. Competition for space is suggested as an alternative to the food competition hypothesis.

Plant‐Pollinator Interactions in Hawaii: Pollination Energetics of Metrosideros Collina (Myrtaceae)

Abstract: The most abundant tree species in much of the undisturbed Hawaiian forest was the subject of a 2—yr study on plant—pollinator interactions and energetics. The purposes of the study were (1) to determine the roles of insects and of some endemic Hawaiian birds in the pollination of the tree Metrosideros collina, (2) to test the hypothesis that maximal outbreeding and seed set occur at intermediate levels of nectar availability, (3) to understand the adaptive significance of profuse flowering in this species, and (4) to determine the degree of specialization in the pollination ecology of this species. Endemic Hawaiian birds (Drepanididae) are essential for high levels of fruit—set and outbreeding in M. collina. Fruit—set was much higher in red—flowered individuals when birds were allowed to use inflorescences than when only insects used them. This is apparently caused by partial self—incompatibility, such that maximal fruit—set occurs only with outbreeding, the primary agents of which are the birds. The pred...

Primary Production in a Louisiana Golf Coast Spartina Alterniflora Marsh

Abstract: Live and dead aboveground biomass of Spartina alternitflora Loisel in a Louisiana salt marsh was determined monthly over an annual cycle. Litterbags were used to determine loss rates of particulate vegetation from the marsh surface. Net aboveground primary produc- tion estimates ranged from 750 to 2,600 g m2 yr-1, depending on how data were handled. Analysis of these results suggested that true net production was probably much closer to the highest estimate than to the lowest.

Density-Dependent Aspects of Metamorphosis in Ambystoma and Rana Sylvatica

Abstract: Two species of salamander (Ambystoma) and a frog (Rana sylvatica) were raised at a variety of initial densities in enclosures in a pond to study the effect of population density on survival and body size at metamorphosis. The relationship between the probability of survival and the initial density of the population was fit to two alternative models: a linear model which assumes equal competitive ability among individuals in a population and predicts no survivors from populations with very high densities, and a negative exponential model which assumes unequal competitive abilities and predicts that few individuals will dominate the highest density populations and successfully complete metamorphosis. The negative exponential model fits the data better in four out of five experiments but it is significantly better in only one experiment. The prediction of a few survivors even at very high densities is true in all experiments. Mean body size at metamorphosis decreases exponentially as initial density is increased. These data are interpreted as the result of a functional link between body size and competitive ability. As density increases fewer individuals are able to obtain sufficient resources to to successfully complete metamorphosis. These experiments demonstrate very high variances between simultaneous replicates in the same pond. This variance reduces the ability of field experiments to test general models and demonstrates the importance of environmental heterogeneity and historical effects in the out- come of population interactions.

Two costa rican bat-generated seed shadows of andira inermis (leguminosae)'-

Abstract: Bat-generated seed shadows of two mature Andira inermis (W. Wright) DC. trees in the deciduous forest lowlands of Guanacaste Province, Costa Rica, are highly hetero- geneous; very unequal numbers of seeds are deposited beneath the trees used as feeding roosts. It is calculated that a 45-g Artibeus jamaicensis bat may potentially obtain as many as 1,766 joules (= 422 calories) per Andira fruit per round trip of 270 m between the parent tree and the feeding roost. Seed predation by Cleogonus weevils was found to be highest below the parent tree, intermediate under bat feeding roosts, and lowest among fruits dropped accidentally by bats between the parent tree and the feeding roosts.

Competition and Coexistence with A Guild of Herbivorous Insects

Abstract: Resource exploitation and competition were measured for 13 herbivorous stem—boring insects inhabiting a climax prairie in Illinois. Nine species overlapped > 70% in their exploitation of plant species, stem sizes, and locations within stems, but competition was detected between only two species. In this case the competition resulted from aggression rather than exploitation for limited resources, and exclusion may be delayed or prevented by specific behaviors or refuges. To quantify how closely the guild members were packed upon the available resources, probabilities of coccurrences between species were calculated using resource abundances, population sizes, and overlaps in resource exploitation. The species packing averaged over the entire guild was 30%, but some species had several potential competitors and probabilities of cooccurrences were > 60%. This species structure offers the conditions for diffuse competition, but competition seldom occurred because stems could support several individuals and resources were generally nonlimiting. Although the potential for competition is high and species probably compete occasionally, the high resource overlap and species richness that competition has not been a major organizing force within this guild. When factors known to influence competition are considered, it seems possible that competition has been too infrequent, mild, or nondirectional to cause strong selection for competitive avoidance or displacement. This guild demonstrates that overlap values may not equal competition coefficients and that high overlap may exist because competition is rare. This guild is probably not unique in lacking competitive organization; herbivorous insects may be among the least likely groups to exhibit the patterns predicted by competition theory.