Trophic relationships among concentrated populations of small fishes in seagrass meadows

Changes in available food and utilized foods for a densely populated guild of animals can uncover periods of niche displacement among particular consumer species or their separate size classes. Dense populations of small fishes and their foods were çensussed for 13 months and studied experimentally in shoalgrass and turtlegrass meadows of Redfish Bay, Texas. Feeding habits were determined with respect to prey availability, and illustrated the extent to which seasonal partitioning of food corresponded to food depletion among these abundant consumers in seagrass meadow food webs. The darter goby, code goby, pinfish, and Gulf pipefish were the most common species throughout the year. Although the darter goby did not show distinct ontogenetic changes in food habits, the code goby, Gulf pipefish, and pinfish demonstrated major ontogenetic progressions of foods selected.Food availability in the seagrass meadows changed seasonally. When major prey such as amphipods were abundant, during spring, many fish species showed high overlap in food use. Regardless of food availability, the code goby and Gulf pipefish fed mainly on amphipods or copepods. The more common darter goby and pinfish seasonally changed their diets with changes in food availability; the darter goby and pinfish were more carnivorous during spring, but they largely consumed epiphytic algae during summer. Cage experiments were used to monitor foods confined with elevated densities of pinfish and darter gobies, relative to control cages at natural overall densities. Prey items in the former cages decreased sharply, with corresponding dietary shifts by these common fishes. During resource depletion, these changes in resource use by these naturally concentrated consumers appeared as temporary partitioning of available resources. This shift occurred during both natural (seasonal) and experimental depletions of food, and appears to result from increased interspecific and intraspecific competition during periods of depleted preferred foods.     

Effects of species interactions on community organization along a trampling gradient

Abstract. Effects of interspecific interactions on the organization of two trampled communities, Eragrostio ferrugineae - Plantaginetum asiaticae (EP) and Eleusino indicae - Digitarietum violascentis (ED), were examined by 4-yr field experiments. We compared changes in the relative abundance of the main component species of the communities in monoculture and mixed culture along a trampling gradient. At no trampling in mixed culture, Ambrosia artemisiifolia var. elatior and Erigeron annuus (roadside herbs, RH) predominated and excluded the trampled community species. Severe trampling markedly reduced the predominance of these roadside herbs, promoted Eragrostis ferruginea (a perennial grass of EP), but suppressed Eleusine indica (an annual grass of ED). These results suggest that the differentiation of trampled and roadside herb communities under heavy trampling are caused by asymmetric competition between their main species. Several species pairs showed a constant rank order of relative cover at high trampling levels. Pennisetum alopecuroides (a perennial grass of both RH and EP) and Eragrostis coexisted, indicating symmetric competition. Eragrostis and Plantago asiatica (a perennial herb of EP) or Poa annua (a winter-annual grass of both EP and ED) coexisted through separation in phenology. Eragrostis promoted the survival of Plantago by moderating unpredictable drought (commensalistic positive interaction). This suggests that community composition is maintained by several interspecific interactions.     

Foraging behaviour of Nuthatches(Sitta europaea) in relation to the presence of mates and mixed flocks

Summary Nuthatches (Sitta europaea) usually forage in pairs outside the breeding season. I investigated whether foraging site selection in winter (tree species, height, substrate size) differed between sexes and whether this difference was related to the presence of mixed-species flocks. Foraging sites of pair members foraging together were highly correlated. In the rare cases when each used different tree species, males foraged significantly more often on the preferred oak trees than females. Males foraged significantly lower than females in some territories, but not in others. The reasons for this significant variation between pairs remains unexplained. Males also foraged lower than females in the company of mixed flocks. These differences are probably better explained as avoidance of the dominant male by the subordinate female, rather than specialization in resource use to avoid competition.

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Nahrungssuche beim Kleiber (Sitta europaea) im Paarverbund und in gemischten Schwärmen

Zusammenfassung Außerhalb der Brutzeit fouragieren Kleiber gewöhnlich in Paaren. Ich untersuchte, ob sich die Geschlechter in der Wahl des winterlichen Nahrungsortes (Baumart, Höhe, Astdicke) unterscheiden und ob solche Unterschiede durch die Anwesenheit von gemischten Schwärmen beeinflußt ist. Die Ortswahl der gemeinschaftlich auf Nahrungssuche gehenden Paarpartner war hoch korreliert. In den seltenen Fällen, in denen sich die Partner an verschiedenen Bäumen aufhielten, fanden sich die Männchen signifikant mehr an den der Art bevorzugten Eichen als die Weibchen. In einigen Revieren fouragierten die Männchen an signifikant niedrigeren Standorten als die Weibchen, doch war dies nicht in allen Revieren der Fall. Die Ursachen für diese Unterschiede sind nicht bekannt. Auch in gemischten Schwärmen erfolgte die Nahrungssuche der Männchen niedriger als die der Weibchen. Möglicherweise vermeiden die Weibchen so die Konkurrenz durch die dominanten Männchen.

     

Competition for Resources and Its Behavioral Consequences Among Female Primates

Via the current model on the evolutionary ecology of female social relationships, Sterck et al. (1997) argue that ecological conditions determine how competition over food resources affects female fitness. The relative importance of different modes of competition then affects female social relationships and dispersal patterns. I outline the model and review relevant data. There are 3 modes of feeding competition: within-group scramble (WGS), within-group contest (WGC), and between-group contest (BGC), which occur in various combinations in different populations of nonhuman primates. Ecological measures support predictions that limiting resources lead to WGS and clumped resources induce WGC. The ecological basis of BGC remains elusive, but it is probably linked to resource abundance. Tests of the proxies of feeding competition support the idea that short-term search substrates and increasing group size lead to WGS, while high-quality patches of intermediate size relative to group size lead to WGC. However, when tested across populations, independent measures of aggression rates do not always match the actual or presumed competitive regimes. This mismatch might be explained by confounding factors and the predominately indirect measures of feeding competition. Predicted relationships between feeding competition and female social relationships/dispersal are only partly supported. This might be attributed to the fact that few studies have taken ultimate approaches using mechanistic correlates of fitness (net energy gain) or lifetime reproductive success to measure consequences of feeding competition. But to resolve existing inconsistencies, additional factors need to be taken into account as well, for example, male sexual strategies may affect female feeding competition; constraints on group size may enforce female dispersal; and demography may alter rates of alliances. More explicitly, ultimate approaches are needed to test the consistency of the socioecological model.     

Feeding Competition and Agonistic Relationships Among Bwindi <Emphasis Type="Italic">Gorilla beringei</Emphasis>

Testing predictions of socioecological models, specifically that the types of feeding competition and social relationships female primates exhibit are strongly influenced by the distribution, density, and quality of food resources, requires studies of closely related populations of subjects living under different ecological conditions. I examined feeding competition and the resulting female social relationships in mountain gorillas (Gorilla beringei beringei) of Bwindi Impenetrable National Park, Uganda, which has ecological conditions distinctive from those where other gorilla populations live. I observed 1 group of gorillas for 29 mo to examine the proportion of time spent foraging on fruit, the relationship between patch size and occupancy patterns of fruit trees, and agonistic interactions. Patch occupancy time while foraging in fruit trees decreased with increasing number of gorillas in a tree and decreasing tree size, suggesting that fruit trees represent limiting patches and can lead to intragroup scramble competition. Gorillas exhibited higher levels of aggression while feeding on fruit versus other food resources, which indicates intragroup contest competition. I observed a linear dominance hierarchy with no bidirectionality via displacements, and a similar hierarchy via aggression, though a notable proportion of the dyads contained 2-way interactions. However, most aggression was of low intensity (vocalizations) and the recipient typically ignored it. Despite differences in ecological conditions and diet between the Virunga Volcanoes and Bwindi, agonistic relationships among females are largely similar in the 2 populations and are best characterized as dispersal individualistic.     

Resource distribution and soil moisture content can regulate bait control in an ant assemblage in Central Amazonian forest

Resources influence population growth, interspecific interactions, territoriality and, in combination with moisture content, affect terrestrial arthropod distribution and abundance. Ants are usually described as interactive and compete in transitive hierarchies, where the dominants behaviourally exclude subordinate species from food resources. In this study, we evaluated the effects of (i) dominant ants, soil moisture and an artificial resource gradient on the number of ant species attracted to baits; and (ii) how soil moisture and an artificial resource gradient change the number of controlled baits in a Central Amazonian rain forest. We sampled 30 100‐m‐long transects, located at least 200 m apart. The transects were established with six different bait densities varying between six and 41 baits and the soil moisture content was measured at 10 points for each transect. Six ant species were considered dominant, and had negative correlations with the number of species at baits (r² = 0.186; F_1,28 = 6.419; P = 0.017). However, almost half of the transects showed low abundance of dominant species (<30%), and relatively high number of species (mean of 20.1 ± 8.75). Resource availability and soil moisture had negative and positive correlations, with number of controlled baits. These results suggest that, even though the dominance is relatively poorly developed on the floor of this tropical forest, both resource availability and soil moisture affect resource control, and thus, the number of species that use baits.     

Social rank and sex ratio of captive long-tailed macaque females (Macaca fascicularis)

Variation in birth sex ratios in primates can be accounted for by two hypotheses: the local resource competition hypothesis [Silk: American Naturalist 121:56–66, 1983] and the hypothesis of Trivers & Willard [Science 179:90–92, 1973] concerning the maternal effect on the quality of a male. We examined the effects of female dominance rank on aspects of reproduction in three well-established captive groups of long-tailed macaques (Macaca fascicularis). High-ranking females produced a higher proportion of sons than low-ranking females, and factors other than rank did not have significant effects on birth sex ratios. Interbirth intervals following daughters were longer than those following sons, but they were independent of the mother's rank. The sons of high-ranking mothers had better survival prospects than sons of low-ranking mothers in some of the groups; no such difference was found for daughters. Overall, there was no sex difference in survival up to 5 years of age. These results support the Trivers-Willard hypothesis rather than the local resource competition hypothesis. An analysis of interbirth intervals suggested that the deviation in birth sex ratio is already established at conception.     

Female competition in chimpanzees

Female chimpanzees exhibit exceptionally slow rates of reproduction and raise their offspring without direct paternal care. Therefore, their reproductive success depends critically on long-term access to high-quality food resources over a long lifespan. Chimpanzee communities contain multiple adult males, multiple adult females and their offspring. Because males are philopatric and jointly defend the community range while most females transfer to new communities before breeding, adult females are typically surrounded by unrelated competitors. Communities are fission–fusion societies in which individuals spend time alone or in fluid subgroups, whose size depends mostly on the abundance and distribution of food. To varying extents in different populations, females avoid direct competition by foraging alone or in small groups in distinct, but overlapping core areas within the community range to which they show high fidelity. Although rates of aggression are low, females compete for space and access to food. High rank correlates with high reproductive success, and high-ranking females win direct contests for food and gain preferential access to resource-rich sites. Females are aggressive to immigrant females and even kill the newborn infants of community members. The intensity of such aggression correlates with population density. These patterns are compared to those in other species, including humans.