Mean Prey Size Research Articles

Observations on the natural history of Lacerta lepida in Cádiz Province, Spain

Abstract Cádiz Province, Spain, is the southernmost extent of the range of L. lepida. Within this area, L. lepida demonstrates no distinct preference for any habitat and inhabits man-made and natural shelters with equal frequency. L. lepida is diurnal and active at the surface at ambient temperatures between 15.6°C and 42°C from February through November. Males are 4% to 8% heavier than females of equivalent body length. The rate of parasitism from nematodes is low (6%). The Cádiz Province population has the most diverse diet of any population studied to date. The taxonomic composition of the diet of males and females is identical, and consists mostly of invertebrate prey. Mean prey size and prey diversity are not correlated with lizard body size. Lacerta lepida is cannibalistic. Spermatogenesis and follicular development occur from April through July, egg laying occurs between early June and early July, and clutch size may vary from 4 to 18. Lacerta lepida reproduces only once annually.

Experimental predation by sticklebacks on larval mackerel and protection of fish larvae by Zooplankton alternative prey

Three-spined sticklebacks Gasterosteus aculeatus L. strongly prefer larvae of Atlantic mackerel Scomber scombrus L. over smaller wild invertebrate Zooplankton in experiments where the sticklebacks were allowed to feed ad libitum and to voluntary cessation. The relative preference for fish larvae increased as their proportion declined, a “counter-switching” behavior. In spite of this increasing preference, the presence of Zooplankton as alternative prey strongly protected the fish larvae from predation. This occurred because at high plankton abundance (or low ratio of fish larvae to zooplankton) the total number of fish larvae (and total biomass) ingested by the sticklebacks during their voluntarily limited foraging periods decreased, despite lack of satiation. The mechanism for this interaction appears to be interference by the zooplankton (nonpreferred prey) in the stickleback's search for fish larvae. We interpret the stickleback's response (cessation of feeding before satiation) as equivalent to control of total ingestion by mean prey size, which would have the effect of maximizing the rate of food intake while minimizing foraging time. This result and the counter-switching are consistent with adaptations for minimizing risk from its own predators while foraging.

Biology and Growth of the Black-naped TernSterna Sumatrana:An Hypothesis to Explain the Relative Growth Rates of Inshore, Offshore and Pelagic Feeders

The Black-naped Tern breeds on tropical and sub-tropical islands throughout the Indian and Western Pacific Oceans. In Australian waters they are non-sedentary. They usually breed during spring and summer. The mean clutch size is about 1.5 and it is incubated for 21 to 23 days. Most Black-naped Terns hunt within two kilometres of their colonies. Young are fed often; the modal interval between feedings was four to five minutes and the mean was 49.6 minutes. They ate fish from at least 15 families and up to 10 cm long. The mean size of prey fed to chicks increased during the course of the breeding season. Nestlings were fed a greater number of small fish than were chicks (> 7 days old) and mates. The size of fish fed to chicks and mates did not differ significantly but the ones eaten by foraging adults differed significantly from those that they fed to their mates. Breeding success of Black-naped Terns was higher at Eagle Island than at One Tree Island. The major causes of mortality of eggs and chicks were predation by gulls and flooding of nesting areas. Growth rates of Black- naped chicks (inshore feeder) were faster than those of offshore and pelagic feeders. We hypothesise that the ability of parents to transport food to their young (Ricklef's power requirements model) in combination with the nutrient composition of their diets explain the non-genetic differences in the growth rates of inshore, offshore and pelagic feeders. We postulate that inshore feeders provision their young with more of the limiting nutrient per unit time by supplying their young with more food than is reauired to meet their enerev needs. Ricklefs model also accounts for the changes in.the use of foraging zones during the cburse of the breeding season.

A comparative analysis of food-niche relationships and trophic guild structure in two assemblages of vertebrate predators differing in species richness: causes, correlations, and consequences.

We conducted a comparative analysis of foodniche relationships and trophic guild structure in two assemblages of vertebrate predators (including hawks, owls, snakes, and carnivores) living in similar habitats of Chile and Spain, which differed in species richness (11 and 25 predator species, respectively), to explore how the structure of predator assemblages reflects an increase (or decrease) in the number of coexisting species. Our results indicate that the Spanish assemblage appears enriched by the symmetrical "addition" of species with disparately large and small values of body weight, diet breadth, and mean prey size, around the same median value found in Chile. Or, alternatively, the Chilean assemblage appears impoverished by the symmetrical "loss" of peripheral species - those at both tails of the frequency distribution of the above trophic estimators. Spanish predators overlap less extensively among themselves, and with smaller variance, than Chilean predators. Consequently, the higher predator richness in Spain is not ascribable to their narrower diet breadths or more extensive diet overlaps as predicted by some theoretical models. Instead, it is associated with a more spread-out use of the available resources. Trophic guilds can be recognized in the two countries, but in Spain guilds are less tightly packed. Some taxomically unrelated species are close diet analogues between Spain and Chile, whereas some closely related species cannot be matched well between countries. Circumstantial evidence suggests that the presence of some trophic guilds and of some specialist predators in either country is based on the high abundance attained by some particular prey types.

Size-related selection of two intertidal gastropods by the reef crab Ozius truncatus

Reef crabs, Ozius truncatus H. Milne-Edwards 1834, and gastropods, Bembicium nanum (Lamarck, 1882) and Nerita atramentosa Reeve, 1855, were collected from a South Australian rocky intertidal platform in April 1981. Laboratory experiments were conducted to determine (1) the size of prey chosen, and (2) the energy gained by the reef crabs feeding on B. nanum and N. atramentosa. The time taken to break open and consume prey increased exponentially with prey shell height. Crabs with larger master claws had shorter handling times for a given prey size. When offered a range of prey sizes, crabs selected prey of a certain size, well below the maximum prey size that they could handle. The mean prey size selected by crabs did not concide with the size predicted to yield the maximum rate of energy gain. For example, one reef crab preferred B. nanum 5 to 8 mm in size, but the size class predicted to yield the greatest energy gain was 4 to 4.9 mm. The results of the laboratory prey-choice experiments are discussed in relation to the shore-level size gradients of B. nanum and N. atramentosa.

Random Prey Capture or Active Choice? An Experimental Study on Prey Size Selection in Three Marine Fish Species

Prey size selection was studied in the marine fishes Pomatoschistus microps, P. minutus, and Platichtys flesus. Three size classes of the amphipod Corophium volutator were used as prey in laboratory experiments, and field samples were taken to compare Corophium size in the fish stomachs and in the surrounding environment. When there were equal numbers of the different prey sizes in the aquaria, P. minutus and P. flesus ate an increasing proportion of large Corophium as prey density increased. P. microps were offered the Corophium size-classes in different proportions. When there were equal numbers of the size-classes, P. microps showed a strong preference for large prey, but as small prey increased in abundance, selectivity decreased and prey choice became random. In the field, small Corophium were more abundant than the other size-classes. P. microps and P. minutus ate this prey most frequently, while P. fiesus ate more medium-sized Corophium. For P. minutus and P. flesus mean prey size increased with increasing fish size in the laboratory experiments. Prey size selection was thus influenced by absolute and relative densities of prey and by predator size. Differences in visibility and evasion capability of different sizes of prey also affected the food choice of the fishes in this study.

Ecological comparisons of robber fly species (Diptera: Asilidae) coexisting in a neotropical forest.

Data regarding seasonal abundance, microhabitat preference, and diet were collected over 3 field seasons for adults of 15 robber fly species on Barro Colorado Island, Panama. The species comprised 2 distinct "thermal guilds"; light-seeking (hereafter LS) species foraged in sunlit areas, while shade-seeking (SS) species foraged only in deep shade. All species were rare during the dry season. During the rainy months, most SS species had flight periods of 2-3 months, and no temporal segregation was apparent. In contrast, most LS species had flight periods of only 4-6 weeks, and a distinct sequence of occurrence was evident during 2 years of censusing. Most SS and LS species displayed a pronounced specificity for perches of a particular substrate type. However, the ranges of perching heights utilized varied considerably among species. Dietary comparisons revealed that mean and maximum prey sizes increased with increasing robber fly size, while minimum prey sizes were constant. Robber fly species <20 mg fed primarily upon nematocerous Diptera, whereas larger species generally fed upon a wide variety of prey types. For each thermal guild, the actual mean overlap for a particular niche dimension was compared to mean overlaps generated by randomly assigning species to thermal guilds. No significant differences from the random null hypothesis were found for the SS guild. However, niche complementarity between dietary and spatial overlaps and dietary overlap was apparent among the 5 large LS species.

Population Experiments with the Anolis Lizards of St. Maarten and St. Eustatius

During 1980 and 1981 population experiments were performed with two two—species guilds of Anolis lizards: A. gingivinus and A. wattsi pogus on the island of St. Maarten (Netherlands Antilles) and A. bimaculatus and A. wattsi schwartzi on the island of St. Eustatius (Netherlands Antilles). These experiments were designed to test the hypothesis that the strength of interspecific competition is inversely related to the degree of interspecific resource partitioning. The experimental design consisted of four field—enclosures (12 x 12 m) on St. Maarten and four on St. Eustatius. On each island two enclosures contained only the larger native species (A. gingivinus or A. bimaculatus), and two contained both native species. Resource partitioning was measured inside each of the two—species enclosures. There was more interspecific partitioning of space and prey (arthropods) on St. Eustatius than on St. Maarten. A comparison of the one— and two—species enclosures on St. Maarten revealed five competitive effects of A. wattsi pogus on A. gingivinus. In the presence of A. wattsi pogus, A. gingivinus individuals had lower growth rates and increased perch heights. Also, dissected A. gingivinus from the two—species enclosures contained smaller volumes of developing eggs per female, smaller volumes of insect prey per lizard, and smaller mean prey sizes than A. gingivinus from the single—species enclosures. In contrast, no consistent and significant competitive effects of A. wattsi schwartzi on A. bimaculatus were observed on St. Eustatius. These results support a central assumption of competition theory: the strength of interspecific competition increases as the amount of interspecific resource partitioning decreases.

Predator–prey size relationships in North American dabbling ducks

Diets of dabbling ducks (Anas: Anatidae) usually have been recorded only with respect to taxonomic composition and not prey size. Interspecific diet overlap thus has been termed large. However, using published taxonomic diet lists for seven species of ducks, measurements of bill morphology from museum specimens, and "handbook" data on the sizes and caloric density of invertebrates, prey size distributions were found to differ among species (P &lt; 0.001). Also, the sizes of prey in each species' diet differed from that found in the environment. The number of lamellae per centimetre of bill negatively correlated with mean prey size (P &lt; 0.001); body size and bill length did not. Based on the similarity of environmental and dietary prey size distributions, species were classified as specialist or generalist foragers. This classification corresponded well with one generated from analyses of foraging behaviour alone. An explanation for the diminuitive sizes of ducks on islands was advanced: if interspecific competition for food selects for niche divergence among coexisting dabbling ducks along a prey size niche dimension, then in the absence of competition from small species on islands, large solitary species have evolved to a smaller size which may be optimal for the distribution of energy among prey size classes in the environment.

Partitioning of Food and Niche Characteristics of Coexisting Accipiter During Breeding

AbstractThe abundance of resources and their use by Accipiter in two areas within Oregon from 1969 through 1974 are presented. In the coastal mountains of northwestern Oregon only Sharp-shinned Hawks (Accipiter striatus) and Cooper's Hawks (A. cooperii) nested. In the remainder of the state, A. striatus, A. cooperii, and Northern Goshawks (A. gentilis) were syntopic during the nesting season. The number of species and the total density of birds in eastern Oregon were about one-half those in northwestern Oregon. Mean prey size of A. striatus in northwestern Oregon (12.8 g) was significantly smaller than that of the same species in eastern Oregon (28.4 g). Accipiter striatus diets in both areas were composed almost entirely of birds (&lt;5% mammals). Mean prey size of A. cooperii in both areas was nearly the same (134.7 g vs. 136.3 g) and was significantly larger than mean prey size of A. striatus. The frequency of occurrence and the size of prey taxa in A. cooperii diets, however, varied between areas: 74% birds (x̄ = 79.2 g) and 25% mammals (x̄ = 296.4 g) in northwestern Oregon versus 47% birds (x̄ = 123.7 g) and 53% mammals (x̄ = 147.5 g) in eastern Oregon. Mean prey size of A. gentilis in eastern Oregon (306.6 g) was significantly larger than that of A. cooperii and consisted of 55% birds (x̄ = 195.5 g) and 45% mammals (x̄ = 445.2 g). An analysis of prey sizes, prey taxa, and foraging heights indicated that, in general, food was partitioned in both areas primarily according to prey size. In eastern Oregon, however, where A. cooperii and A. gentilis overlapped broadly in prey size, these species tended to take different prey taxa. Both A. cooperii and A. gentilis foraged primarily in the lower zones (ground-shrub and shrub-canopy), whereas A. striatus foraged in the upper canopy.

A New Estimate of Zooplankton Retention by Gill Rakers and Its Ecological Significance

Abstract The probability that prey will be retained by gill rakers of white crappies Pomoxis annularis (9–15 cm total length) was determined by comparisons of size-frequency distributions of prey in a laboratory pool and in stomach contents. White crappies trained to feed on large-bodied Daphnia magna were released individually into a pool containing that species and an assemblage of small-bodied species. Most attacks were directed at Daphnia magna, but several prey were ingested for every attack, and nonselective capture was assumed for the small-bodied among them. Estimated retention probabilities for species of Ceriodaphnia, Bosmina, and Cyclops were less than 10% for mean prey sizes smaller than 0.35 mm and increased linearly with mean prey size to 100% for prey larger than 0.55 mm. In contrast, retention probabilities calculated by traditional means from distances between gill rakers predicted that all prey larger than 0.23 mm would be retained. The new retention estimates, based on actual results of...

Nonrandom Provisioning by the Digger Wasp, Palmodes laeviventris (Hymenoptera: Sphecidae)

Predation strategies of Palmodes laeviventris (Cresson), a predator of Mormon crickets, are described. Prey taken are compared with prey available. Although mean prey sizes are no larger than the population means, a positive correlation between wasp size and prey size indicates that females do take large prey but are limited by their own body size. Size selection by wasps may explain the strong bias toward female prey. An alternative hypothesis is that the sex bias is a result of female Mormon crickets being more vulnerable to predation because they are more exposed than males due to their reproductive behavior.

The Trophic Structure of Sympatric Assemblages of Diurnal and Nocturnal Birds of Prey

Patterns of food-niche breadth, food-niche overlap and mean prey weight in the diet are used to characterize and compare the trophic structure of five assemblages of diurnal and nocturnal birds of prey. The localities surveyed are in Michigan, Wisconsin, Utah, Spain and Chile. With few exceptions, no significant differences are demonstrated between sympatric assemblages of hawks and owls in the statistical distributions of raptor weights or of the three trophic estimators above-mentioned. Within each hawk and owl assemblage positive correlations are found between raptor weights and mean prey weights but not between raptor weights and food-niche breadths. Parallel geographic variation of diurnal and nocturnal raptor assemblages is observed in species richness (which is always higher for hawks), sizes and trophic statistics. Mean food-niche breadths within assemblages vary inversely with number of prey categories available per raptor species at a given locality. The trophic structure of sympatric assemblages of hawks and owls is very similar and covaries geographically because they are opportunistic predators that vary their trophic relationships according to the available food resources. INTRODUCTION Local assemblages of hawks and owls have been considered as ecological counterparts that hunt at different times of the day (Grossman and Hamlet, 1964; Craighead and Craighead, 1969). Thus far, this claim has been largely based on qualitative or semiquantitative assessments of feeding patterns of these raptors. In fact, few authors have reported adequate data on the food habits of entire raptorial assemblages (Errington, 1932, 1933; Valverde, 1967; Craighead and Craighead,, 1969; Smith and Murphy, 1973; Jaksic et al., 1981), and they have not usually characterized the trophic relations by means of standard statistics widely used in other dietary studies of birds (e.g., Hespenheide, 1973, 1975; Baltz and Morejohn, 1977; Wiens and Rotenberry, 1979). Standard quantitative estimators of the trophic niche are food-niche breadth (Levins, 1968; Petraitis, 1979), food-niche overlap (Colwell and Futuyma, 1971; Linton et al., 1981) and mean prey size (Storer, 1966; Hespenheide, 1973). Although many researchers have used these trophic estimators to test predictions about species diversity and packing, generalization vs. specialization, character displacement, guild structure, niche partitioning and the like (MacArthur, 1972; Cody, 1974; Pianka, 1974; Cody and Diamond, 1975; May, 1976), only rarely have these statistics been used to describe the trophic ecology of hawk and owl populations (Marti, 1974; Herrera and Hiraldo, 1976; Jaksic et al., 1981). Consequently, almost nothing quantitative is known about the food-niche relations of raptorial assemblages or about their spatial variation. To characterize the trophic structure of hawk and owl assemblages, sympatric diurnal and nocturnal raptors are compared and their geographic variation analyzed. Trophic structure is a shorthand term for the patterns of food-niche breadths, overlaps and prey sizes of sympatric raptor populations. I address the following questions: (a) To what extent is the trophic structure of hawk and owl assemblages similar? (b) Are the trophic statistics reasonably constant properties of given raptorial species or do they vary among localities? I hope that in answering these basic questions background information will be provided for more sophisticated analyses of the modes of predation of hawk and owl assemblages.

Habitat Structural Complexity and the Interaction Between Bluegills and Their Prey

Structural complexity of the habitat often reduces predatory efficiency by reducing prey capture rates. Prey density is often positively correlated with habitat structure because it pro- vides food and substrate to the prey as well as a relative refuge from predators. Dense structure inhibits foraging, allowing abundant, highly profitable prey to coexist with predators. Sparse structure allows efficient foraging and generally contains few highly profitable prey. This suggests that feeding rates of predators may be maximized at intermediate structure. If this is true, we might also expect predator growth rates to be higher in intermediate structure habitats. Since diet breadth is thought to be related to rates of encounter with profitable prey, we also expect diets of predators to be narrower at intermediate structure than in either sparsely or densely structured habitats. Bluegill sunfish (Lepomis macrochirus) restricted to experimental ponds varying in vegetation density grew better and consumed more prey at intermediate macrophyte density than fish held at either low or high macrophyte densities. Fish at low macrophyte density had narrower diets than expected due to high initial prey availability relative to prey available at intermediate and high macro- phyte density. Fish at high macrophyte density ate fewer, but larger, prey and thus had a narrower diet than expected. Fish predation reduced total prey biomass as well as mean prey size and altered the prey community structure by removing large active invertebrate predators and herbivores with subsequent release of smaller invertebrate predators and herbivores. These changes in prey com- munity structure were also mediated by habitat structure. Habitat structure-food density interactions may be added to temperature and presence of predators as variables that influence the use of resources by fishes.

Prey Selection and Handling in Two Neotropical Hover‐Gleaning Birds

Prey handling and prey selection were studied by releasing over 150 live insects to wild White—fronted Nunbirds (Monasa morphoeus, Bucconidae) and a caged Bright—rumped Attila (Attila spadiceus, Tyrannidae). Handling time was determined for each insect taken and the relationship between handling time and several prey characteristics was examined. Log—transformed x— and y—axes (power function) better linearized the relationship between handling time and prey size than log—transformed y—axis (exponential function) or untransformed axes did for acridid—tettigoniid Orthoptera, the insect group used to compare handling abilities of the two species. The species power function slopes were statistically indistinguishable, but y—intercepts differed such that the smaller attila (40 g) required significantly longer than nunbirds (108 g) to handle prey of a given mass or width. An energetic cost—benefit analysis using these handling time regressions showed that handling costs wer negligible compared with benefits and could not explain the upper limit to prey size (2—6 g) for nunbirds. The cost/benefit functions of the two species largely coincided, and the energetics of prey handling would not likely predispose these two species to take different mean prey sizes. The most important difference between the two species should be the range of sizes effectively utilized; the larger predator can consume much larger prey then the smaller predator can. For all insects successfully handled, prey body width was the best predictor of handling time in both species. We argue that the primary function of prey handling in these birds is to render prey swallowable. Thus gape size sets a limit to the largest noncreshable prey, and determines how much handling is required for malleable prey. A variety of insects were rejected by nunbirds because of the insect size and/or shape in combination with hardness and distastefulness. Knowledge of prey—handling behavior in these birds predicts effective insect morphologies to impede avian handling abilities.

Growth of Young-of-the-Year and Yearling Centrarchids in Relation to Zooplankton in the Littoral Zone of Lakes

Correlations were determined between growth/body condition of centrarchids age 0+ to 1+ and abundance of specific taxa of zooplankton in the littoral zone of three lakes in piedmont North Carolina over a period of eight months. Stomach analyses were conducted, and electivity indices calculated to provide comparative data. Significant differences were found in body condition, growth rate, and zooplankton abundance between lakes (P < 0.05, ANOVA). First year growth and changes in body condition of age 0+ Lepomis, Pomoxis and Micropterus species were positively correlated with the abundance of copepod nauplii and copepodids in each lake (mean rs = 0.898, P < 0.01). The abundance of adult copepods was also frequently correlated with growth and body condition, but at a lower level of probability (mean rs = 0.713, P < 0.05). The number of significant correlations dropped sharply as fish age increased to 1+ years. Significant associations could not be generated using data for Rotifera, Protozoa or Ostracoda, and were quite infrequent for Cladocera. The correlations suggest that copepod nauplii and copepodids are primary food during the critical period when larval centrarchids switch from endogenous to exogenous nutrition. The correlation data were supported by stomach analyses and electivity indices which indicated 0+ fishes to be mainly utilizing copepods, supplemented with cladocerans, insect larvae, and fishes in the more piscivorous species. The mean prey size for larval bluegill and largemouth bass (15 mm total length) was found to be 0.48 mouth gape and 0.60 mouth gape respectively, which translates to prey sizes of 0.11 mm to 0.35 mm during the larval stage. Due to mouth size during the period of first feeding, most adult copepods and cladocerans are too large to be ingested even if prey could be handled with dimensions approaching 0.99 mouth gape. The relative insignificance of Cladocera in the diet of post-larval fishes appears to be due to negative selection rather than to influences of seasonal abundance or encounter probability. It is evident that in these North Carolina lakes, cladocerans are not so universally important in the diet of age 0+ centrarchids as studies on northern and midwestern lakes would indicate. Correlations, stomach analyses, and electivity indices can be integrated into a multiple factor approach to diet studies, allowing consideration of several environmental and population variables and yielding more meaningful results than by using one method alone. Potential impacts of aquatic pollution on food organisms required during the first weeks of feeding should place additional priority on diet studies and ecological relationships between centrarchids and the littoral zone.

Changes in the Food Niche during Postmetamorphic Ontogeny of the Frog Pseudacris triseriata

The food niche and foraging strategies of the chorus frog Pseudacris triseriata were investigated by examining the prey/predator size relationship as it relates to prey availability, and they were found to be dynamic from metamorphosis to adulthood. Snout to vent length of the frogs proved to be a good predictor of the mean size of prey taken. Relative availability of the different sized prey items in the environment was estimated from samples of potential prey obtained by suction sampling. This availability information was used in addition to the data on stomach contents to describe the foraging strategies of the different size classes of frogs. It was found that small frogs (- 17.0 mm) ate prey as they were encountered in the environment, and that large frogs (>23.1 mm) selected larger prey which were rarer in the community. Intermediate sized frogs (between 17.1 and 23.0 mm) were characterized by an intermediate pattern. Intermediate frogs consumed large numbers of prey per unit time, and the possibility that this facilitated rapid growth is discussed. The mean maximum prey size increased with increasing frog size, but leveled off for the largest size classes, whereas mean minimum prey size continued to increase with increasing frog size. The large frogs were selective in their diet as they apparently chose the more optimal (in terms of benefits and costs of pursuit, capture and consumption) sized prey while they discriminated against the smaller prey. This ontogenetic pattern of foraging strategies is somewhat unique among predators for which there exists information in the literature, and it is suggested that the small body size (<30 mm) of P. triseriata relative to the size of their prey (terrestrial arthropods) allows this pattern.

The guild structure of a community of predatory vertebrates in central Chile.

The trophic ecology of eleven predator species (Falconiforms: Buteo polyosoma, Elanus leucurus, Falco sparverius, Geranoaetus melanoleucus, Parabuteo unicinctus; Strigiforms: Athene cunicularia, Bubo virginianus, Tyto alba; Carnivores: Dusicyon culpaeus; Snakes: Philodryas chamissonis, Tachymenis peruviana) in two nearby localities of central Chile is analyzed. The localities exhibit the typical climate (hot-dry summers, coldrainy winters), and vegetation (chaparral), of mediterranean ecosystems. Densities of the staple prey (small mammals) were estimated by seasonal trapping during two years in both open and dense patches of chaparral.The trophic parameters examined are: 1) proportion of diurnal, crepuscular, or nocturnal prey found in the predators' diet; 2) relationship between abundance of different mammalian prey in the predators' diet, and in both open and densely vegetated habitat patches; 3) mean weight and variance of weight of small mammal prey consumed; 4) average weight of the predators; 5) food-niche breadth of the predators; 6) relationship between average weight of predators and mean weight of mammalian prey taken, its variance, and food-niche breadth; 7) overlap in food-niche between all the predator species; 8) guild packing of the predators. Parameters 1) and 2) are used to assess the importance of temporal and habitat segregation of the predators, respectively; parameters 3), 4), 5), and 6) provide information on the possibilities of partitioning the prey resources among the predators; parameters 1), 2), 7) and 8) are used to investigate the organization of the community in terms of guilds.Three niche dimensions seem to be important in determining the structure of the predator community: 1) hunting activity period (diurno-crepuscular, nocturno-crepuscular), 2) hunting habitat (open, or both open and dense patches), and 3) mean prey size taken. Segregation along these three axes results in generally low food niche overlaps (<54% in 47 of the 55 pairwise comparisons) among the predators, but it is not possible to determine whether this was produced by competitive interactions or stochastic differences. Three guilds (niche overlap >90% in pair-wise comparisons) can be recognized: a) the carnivorous-insectivorous guild formed by the diurnal raptors A. cunicularia and F. sparverius, which tend to hunt in open habitat patches; b) the herpetophagous guild formed by the diurnal snakes P. chamissonis and T. peruviana, which presumably hunt in open habitat patches; c) the carnivorous guild (highly specialized in the capture of two rodent species) formed by the diurnal raptors B. polyosoma, G. melanoleucus, P. unicinctus, and the carnivore D. culpaeus, which hunt in open habitat patches. The diurnal raptor E. leucurus is not clearly associated with any guild, and the only two nocturnal raptors in the community (B. virginianus and T. alba) exhibit marked differences in their trophic ecology.

Ecological Relationships of the Tree Snakes Uromacer catesbyi and U. oxyrhynchus (Colubridae) on Isla Saona, República Dominicana

Abstract Uromacer catesbyi and U. oxyrhynchus are diurnal, arboreal colubrids endemic to Hispaniola. Uromacer oxyrhynchus is an attenuate-snouted, slender ambush predator that is morphologically and behaviorally adapted to prey on active lizards (primarily Anolis, but also Ameiva), and it feeds exlusively on them. U. catesbyi is blunt-snouted and heavier-bodied than U. oxyrhynchus and exploits branches of wider diameter. The diet of U. catesbyi is composed of diurnally sedentary hylid frogs (ca. 96% by vol. and 71 % by frequency) and active lizards (Anolis). Trophic niche overlap between catesbyi and oxyrhynchus on Isla Saona is low (0.37). U. catesbyi was encountered with food in its stomach less frequently than U. oxyrhynchus (22.4% vs. 42.8%), but mean prey size (vol.) was 2.8 times larger for U. catesbyi.

Resource Allocation, Coexistence, and the Niche Structure of a Streambank Salamander Community

A streambank salamander community has proved to be an ideal system for investigating resource allocation and the evolution of community organization. Four aspects of microhabitat were investigated: substrate, substrate moisture, proximity to surface water, and size/type of cover utilized. Selection of substrate particle size was strongly positively correlated intra— and interspecifically (particularly the latter) with salamander body size, appearing to be a highly adaptive feature. The breadth or diversity of utilization of microhabitat components was interspecifically inversely related to body size. Definite shifts in microhabitat utilization and narrowing of niche (resource) breadths were observed in the presence of larger congeners. The mechanism of the shifts as well as the selective force that molded the organization of this community appear to be interspecific interference competition. Therefore, body sizes and species compositions in streambank salamander communities are determined by substrate size, its patchiness, and the presence of interference competitors. Observed macrohabitat distribution patterns have been analytically described and can be predicted by a knowledge of microhabitat preferences and niche breadths. The ratios of snout—vent lengths or head widths between adjacent species on a body size gradient for both adults and juveniles were remarkably constant, generally between 1.2 and 1.3. Mean prey size (width, length, and volume) also formed a geometric spacing which paralleled body sizes. The coefficient of variation (standard deviation/mean) was constant within a given measure of prey size. Morphological ratios for sympatric competitors in the range of 1.2 to 1.3 have been interpreted by investigators as character displacement to allocate prey sizes and therefore minimize competition. Such interpretations must be made carefully because in this study observed body size differences primarily reflect microhabitat allocation (the principal niche dimension of this study) and only secondarily allocation of prey sizes. Also, species packing (d/w) along the prey size resource gradient is much greater than anticipated by the theoretical May—MacArthur model and is demonstrated to be high in a community of opportunistic generalist insectivores. Species packing also requires careful interpretation because in reality spacings and widths of utilization functions are not uniform and separation of community members along a single niche dimension is rarely if ever encountered. Total niche overlap among community members (species and age classes) can accurately be determined because the relationships among the four microhabitat resources and the prey size dimension are known. The use of prey taxa as a niche dimension for opportunistic generalistic insectivores either overestimates or underestimates total niche overlap, since the utilization of prey items is a function of their size, the microhabitat preference of the predator, and stochastic events of prey occurrences. The community appears to be well coadapted since the following have been demonstrated: (1) total niche overlap is low, therefore minimizing competitive interactions, (2) interspecific predation could not be documented and must be minimal, and (3) demographic strategies are consistent in all macrohabitats regardless of congeneric composition.