Encounter competition partly explains the segregation of the sandy beach amphipods Bathyporeia pilosa and Bathyporeia sarsi. A mesocosm experiment

We propose tests for patterns in meta‐community structure. The tests for clustering and nestedness of the occurrences of species and negative co‐occurrence patterns provide four important innovations. Firstly, they are not restricted to the analysis of communities along one‐dimensional gradients or to the main axis of variation. Secondly, abundance data can also be considered in the null model whereas most previous approaches could consider only presence/absence data. And thirdly, habitat suitability and spatial autocorrelation can be incorporated in the null model so that patterns that might be caused by biotic interactions can be distinguished from patterns which are the result of differences in the suitability or accessibility of sites for the examined organisms. Finally, the test for nestedness is also appropriate if there is more than one set of nested subsets. A re‐analysis of 35 data sets with these tests showed the importance of considering the autocorrelation of the occurrences of species in analyses of meta‐community structure and demonstrated the advantage of abundance data for tests of clustering of species. With abundance data it could be shown that there is a significant clustering of species, i.e. there are positive associations of species in most meta‐communities, even if an environmentally or spatially constrained null model is used for the test. Co‐occurrence patterns that might indicate interspecific competition were found in many of the analysed presence/absence data sets. Surprisingly the analysis of abundance data sets provides less evidence for interspecific competition. A hierarchical organization of communities, i.e. nestedness, turned out to be a rare pattern, if the autocorrelation of the occurrences of species is considered.

In fish, interspecific interactions between nonnative and other sympatric species are considered determinants in shaping species assemblages. Such interactions can also arise between nonnative fish species only, including salmonids such as the brown trout (Salmo trutta, Linnaeus, 1758) and the rainbow trout (Oncorhynchus mykiss, Walbaum, 1792), returning contrasting outcomes. The present manipulative experiment was aimed at exploring the effect of interspecific competition on the body growth and the oxidative status of parr (2 + -year-old individuals) of the brown trout and the rainbow trout. Allopatric (intraspecific competition) and sympatric (interspecific competition) populations of these species were experimentally recreated in two wild streams. At the end of a 2-month-long experiment, changes in specific growth rate (SGR), oxidative status (i.e., levels of reactive oxygen species and activity of antioxidant enzymes such as superoxide dismutase — SOD, catalase — CAT and glutathione peroxidase — GPx) and oxidative damage (i.e., lipid peroxidation) were investigated in brown and rainbow trout individuals maintained in allopatric or sympatric populations. Sympatric interactions between rainbow and brown trout parr resulted in a significant decrease in SGR of brown trout individuals only. Moreover, an overall modulation of the oxidative status, in terms of an increase in ROS levels coupled with the activation of SOD and CAT activity, occurred in brown trout individuals under sympatric conditions. These findings might suggest that, under sympatric conditions, parr of the rainbow trout are more competitive than brown trout for food acquisition. However, this competition affected the antioxidant defenses of the brown trout only, probably because of reduced ingestion of dietary antioxidants or increased physical activity and aggressive behavior. Thus, interspecific interactions can induce physiological and phenotypic effects on parr of nonnative salmonids, with potential consequences on the establishment of populations of these species in freshwater ecosystems.

Intra- and interspecific interactions within communities of species that utilize the same resources are characterized by competition or facilitation. The noctuid stemborers, Busseola fusca and Sesamia calamistis, and the crambid stemborer, Chilo partellus were the most important pests of maize in sub-Saharan Africa before the recent “invasion” of fall armyworm (FAW), Spodopterafrugiperda, which currently seriously limits maize yields in Africa. This new pest is interacting with the stemborer community at the larval stage in the use of maize resources. From previous works on the influence of temperature on the larval intra- and interspecific resources utilization within the community of Lepidoptera stemborers involving B. fusca, S. calamistis, and C. partellus, there is a need to update these studies by adding the new pest, S. frugiperda, in order to understand the effect of temperature on the larval interactions of all these four species under the context of climate change. The influence of temperature on intra- and interspecific larval interactions was studied using artificial stems kept at different constant temperatures (15 °C, 20 °C, 25 °C, and 30 °C) in an incubator and assessing survival and relative growth rates of each species in single and multi-species experiments. After the inclusion of FAW into the experiments, with regard to relative growth rates, both intra- and interspecific competition was observed among all four species. With regard to survival rates, cannibalism can also explain the intra- and interspecific interactions observed among all four species. Interspecific competition was stronger between the stemborers than between the FAW and the stemborers. Similar to lepidopteran stemborers, temperature affected both survival and relative growth rates of the FAW as well. Regardless of the temperature, C. partellus was superior in interspecific interactions shown by higher relative growth and survival rates. The results suggest that the FAW will co-exist with stemborer species along entire temperature gradient, though competition and/or cannibalism with them is weak. In addition, temperature increases caused by climate change is likely to confer an advantage to C. partellus over the fall armyworm and the other noctuids.

Ecological speciation occurs when inherent reproductive barriers to gene flow evolve between populations as a result of divergent natural selection. Frequency dependent effects associated with intraspecific resource competition are thought to be one important source of divergent selection facilitating ecological speciation. Interspecific competition may also play an important role in promoting population divergence. Although evidence for interspecific competition in nature is ubiquitous, there is currently little empirical data supporting its role in the speciation process. Here, we discuss two general models in which interspecific competition among species can promote ecological speciation among populations within a species. In both models, interspecific competition is the source of divergent selection driving adaption to different portions of the resource distribution, generating ecological reproductive isolation from other conspecific populations. We propose that the biology of endoparasitoids that attack phytophagous insects make model systems for studying the role of interspecific competition in ecological speciation. We describe details for one such system, the community of endoparasitic braconid wasps attacking Rhagoletis fruit flies, as a potential model for investigating competitive speciation. We conclude by hypothesizing that a model in which interspecific competition forces an inferior competitor to alternative fly hosts may be a common theme contributing to parasitoid diversification in the Rhagoletis-parasitoid system.

Different responses in leaf-level physiology to competition and facilitation under different soil types and N fertilization

Interspecific interactions between crops influence soil functional groups and networks in a maize/soybean intercropping system

Bream (Abramis brama L.) as zoogeomorphic agents and ecosystem engineers: implications for fine sediment transport in lowland rivers

Effects of exploitative food competition on food niche dynamics — a simulation analysis

"Latitudinal Gradients in Species Diversity": Reflections on Pianka's 1966 Article and a Look Forward.

A fundamental goal of evolutionary ecology is to understand how asymmetric competition influences phenotype expression, yet few studies have quantified the relative effects of intra‐ and interspecific competition on phenotypes. We examined the effects of intra‐ and interspecific larval competition on both discrete and continuous phenotype expression of the facultatively paedomorphic mole salamander (Ambystoma talpoideum). We manipulated the density and frequency of larval A. talpoideum and a common competitor, the spotted salamander (A. maculatum), in experimental mesocosms within a response surface design. The production of discrete ontogenetic morphs (metamorphs, paedomorphs, and overwintering larvae) was affected more by intraspecific than interspecific competition, but the effect varied between morphs. Paedomorph and metamorph production were more strongly affected by intraspecific than interspecific competition, while the production of overwintering larvae was affected by each type of competition approximately equally. Paedomorphs largely occurred only at low conspecific densities, overwintering larvae primarily occurred at high overall densities, and metamorph production peaked at intermediate densities of con‐ and heterospecifics. Density‐dependent intraspecific competition had stronger effects on body size and growth rates than interspecific competition for both metamorphs and overwintering larvae, whereas interspecific competition more strongly affected paedomorph size. Overall, these results support the hypothesis that intraspecific competition influences both continuous and discrete phenotypic expression more strongly than interspecific competition. Fluctuating strengths of each type of competition can affect variation in life history strategies by influencing discrete ontogenetic pathways via individual body sizes, leading to differential morph production and fitness within and among morphs.

ABSTRACTCarnivore assemblages in ecosystems worldwide are shaped by intense interspecific competition for critical resources such as food and habitat. Despite extensive research on coexistence strategies, detailed investigations into interspecific interactions at carcass sites remain limited. This study fills the gap by investigating the interspecific interactions and competition among three carnivore species—spotted hyaenas (Crocuta crocuta), brown hyaenas (Hyaena brunnea) and black‐backed jackals (Canis mesomelas)—at carcass sites in the Madikwe Game Reserve, South Africa. Using motion‐triggered camera traps, we documented the behaviour and interactions of these carnivores around eight carcass sites. Our findings reveal that hyaenas consistently dominate jackals, significantly reducing jackal feeding times and contributing to altered vigilance patterns. Jackals exhibited higher rates of submissive behaviours, whereas hyaenas responded little to their presence. This study highlights the asymmetry in interspecific interactions at carcass sites and underscores the importance of understanding these dynamics for managing scavenger species and their ecological roles. Future research should expand on these findings by incorporating broader spatial scales and environmental variables to better understand the factors shaping competition and coexistence.

In view of its fundamental and pervasive influences and impacts on organism physiology and ecology, body size is recognized as a key component of evolutionary fitness and serves as the cornerstone of a seminal contribution in freshwater zooplankton ecology—the Size Efficiency Hypothesis (SEH) of Brooks & Dodson (Science 150:28–35, 1965). While the roles and implications of body size in predation and competition—central tenets of the SEH—have been widely considered and reviewed, no broader integrated synthesis exists of the collective array of body size determinants and their implications in the ecology in crustacean zooplankton—a numerically and functionally dominant group of aquatic organisms. Focusing on planktonic Cladocera and Copepoda in inland waters, in particular, we provide a wide-ranging overview of the direct and/or indirect effects of environmental conditions, consumable resources and biotic interactions that independently and/or collectively influence the phenotypic expression of body size (particularly as length), both within and between species. Some indirect ultimate evolutionary consequences of body size are considered, and we identify some controversies and unresolved issues related to this biologically crucial trait. While by no means exhaustive, our overview reveals a complex nexus of extrinsic proximate abiotic and biotic factors and interactions that influence body size, the phenotypic expression of which in natural systems commonly reflects contrasting outcomes related to conflicting direct and/or indirect selective pressures. In general, however, body size (both inter- and intra specifically) declines with rising temperature and increases with rising food supply (depending on its quality), although both temperature and food supply exert contrary influences on particular taxa (or life history stages) under certain environmental circumstances. Predation undoubtedly has an overriding influence on body size selection. Depending on its mechanistic basis (visual, tactile or both in tandem), it selectively favours either small or large body size, both within (adults vs. juveniles) and between prey species, which are accordingly often ‘size-trapped’ between contrasting selective pressures, with consequent indirect effects. The bioenergetics of fundamental physiological processes undoubtedly set constraints on body size and serve as the primary determinant. However, within such constraints, the phenotypic expression of body size reflects its adaptive modification in response to the prevailing abiotic and biotic environment. As such, body size represents an emergent ecological property, reflecting the outcome of specific circumstances and conditions, which vary both temporally within and spatially between different ecosystems, and are accordingly context dependent. Nevertheless, underlying physiological advantages of larger size (within and between species) among crustacean zooplankters—lower mass-specific metabolic rates (although recently challenged), higher individual feeding rates (at least among cladocerans), potentially wider food size-ranges, better starvation tolerances, higher potential fecundity, etc.—collectively favour the selection of increased body size, as predicted by the SEH. Although competitive superiority of large size (measured in terms of minimal food requirements) has been confirmed experimentally, this cannot be generalized to natural conditions, where conflicting and temporally variable pressures apply, and contribute to generally mixed, and temporally variable body size compositions. Complex underlying ecological interactions and influences ultimately determine the phenotypic expression of body size in directions consistent with fitness optimization under prevailing circumstances. Certain specific and general deficiencies in information are identified. In particular, the overwhelming emphasis on daphniid cladocerans as model study taxa in freshwater ecosystems has marginalized the acquisition of a comparably broad and penetrating understanding of specific features both of non-daphniid cladoceran and copepod life histories and body size selection. Among daphniid cladocerans, contemporary definitive understanding devolves largely from reductionist laboratory approaches. Holistic re-integration of these mechanistic findings into natural system circumstances presents a difficult challenge that is attracting increasingly attention. With regard to copepods, synthetic integration of the expansive marine knowledge base appears crucial to inform and direct future investigations on freshwater taxa. The question of intrinsic body size regulation in copepods and cladocerans, especially in regard to final phenotypic plasticity in body size expression, awaits resolution. Overall, body size remains a multi-facetted and complex topic, offering promising challenges for further investigation.

-Through natural population changes and experimental field removals, we tested the hypothesis that Least Flycatchers (Empidonax minimus) restrict habitat use by socially subordinate American Redstarts (Setophaga ruticilla). On a 10-ha site 2-yr-and-older (ASY), but not yearling (SY), male redstarts avoided the sector occupied by flycatchers from 1975 to 1980, but preferred this sector from 1981 to 1985 when flycatchers were absent. Vegetation changed subtly on the site but could not account for the sudden shift in redstart settlement pattern. On 6 4-ha sites ASY male redstarts were most abundant in years of Least Flycatcher absence. On the 5 4-ha sites from which Least Flycatchers either disappeared independently or were removed experimentally between 1981 and 1984, redstart abundance increased on four and remained constant on the fifth; on three control areas redstart numbers declined during the same period. Least Flycatchers recolonized one removal site, and ASY redstart abundance subsequently declined. SY male redstart abundance varied inversely with that of ASY male redstarts. We conclude that flycatchers influenced the distribution of ASY male redstarts directly, and that of SY males indirectly, more than either vegetation structure or other habitat characteristics. At no spatial scale examined, however, did total redstart abundance (ASY + SY) vary inversely with that of Least Flycatchers; in fact, their total abundances correlated positively at a regional scale. These findings, combined with a model for asymmetric competition for mutually preferred habitat (Pimm et al. 1985, Rosenzweig 1985), illustrate how a socially dominant competitor could lead to a broadening rather than a narrowing of the habitat breadth of a subordinate species. We show that competitor species abundances need not vary inversely and that age classes may be affected differentially. This species interaction illustrates subtleties and complexities of how competition can modify avian habitat selection. Received 2 September 1987, accepted 22 January 1988. INTERSPECIFIC competition is widely believed to restrict the range of habitats or resources exploited by many species (e.g. Svardson 1949, Connell 1983). If each species reduces the other's abundance under different circumstances, two species should vary inversely in abundance in one habitat over time, or across an array of habitats at a particular time. Avian biologists in particular have frequently interpreted negative correlations in abundance and replacements along habitat gradients as evidence for interspecific competition (Svardson 1949, MacArthur 1972, Cody 1974, Terborgh and Weske 1975, Diamond 1978, Noon 1981, Mountainspring and Scott 1985, Grant 1986). Recent experimental studies, especially those involving the removal of individuals of one or more putatively competing species, have confirmed that interspecific competition for habitat occurs regularly in a variety of bird species (Mewaldt 1964, Davis 1973, Williams and Batzli 1979, Dhondt and Eyckerman 1980, Hogstedt 1980, Reed 1982, Garcia 1983, Alatalo et al. 1985). Other investigators recorded no negative correlations between densities, and concluded that species often respond independently to structural and floristic features of their environment (Wiens 1977, Rotenberry and Wiens 1980, Wiens and Rotenberry 1981, Collins et al. 1982, James and Boecklen 1984, James et al. 1984). These findings have been interpreted as evidence that species are distributed independently and are not strongly influenced by interspecific competition. The contrasting results between these two types of studies have contributed to the recent controversy concerning the relative importance of competition in structuring communities (Strong et al. 1984, Connor and Sim-

Spiders are generalist predators, abundant in terrestrial ecosystems, and hence good candidates for interspecific competitive constraints. Contrary to these premises, evidence of interspecific competition in web‐weaving spiders is ambiguous, and new experiments on natural communities are interesting. This paper reports on an experiment designed to test competitive effects in a natural community of web‐weaving spiders, accomplished by selective removal of each of the three locally dominant species: Araneus marmoreus, Argiope bruennichi, and Agelena labyrinthi‐ca. No statistically significant difference was found among control plots and the plots where one of the three species had been removed, for any of the variables measured, but the small number of replicates limited statistic power. Considering only effect direction, and not statistical significance, these removal experiments supported competitive effects on community structure, habitat overlap, and spider body size. On the other hand, effects were contrary to competitive expectations as regards abundance and habitat breadth. These results are in line with the findings of the other removal experiments conducted so far on spiders, that uncovered small intra‐ and interspecific competitive effects for some variables but not for others. So far, there is no evidence that interspecific competition may affect spider communities, although a low‐level competition remains possible, but difficult to test statistically for its evolutionary consequence.

Tropical forests are vertically stratified ecosystems with distinct abiotic and biotic gradients extending from below ground to the canopy. These vertical gradients are useful systems in which to study species distributions and ecosystem processes due to the large degree of variation found over a small spatial scale. Although our understanding of canopy biodiversity patterns has increased over the last 30 years, canopy access remains challenging and significant knowledge gaps remain. This thesis aims to understand better the abiotic and biotic factors that underpin patterns in ant species distribution and wood decomposition along a vertical gradient within tropical rainforest. First, the role of competition is explored by examining the impacts of suppressing the ground ant assemblage on the distribution of ant species in other strata. Second, patterns in protein preference with height are examined in arboreal ant assemblages. Third, a macroecological approach is taken to test hypotheses explaining patterns in the cuticle colour of ant assemblages along a vertical microclimatic gradient. And finally, the decomposition of dead wood over a vertical gradient is investigated and the relative contributions of biotic decay agents are quantified. These research aims were achieved through novel field experiments in Maliau Basin Conservation Area, Sabah, Malaysia that included: sampling 53,153 ants from four strata across eight experimental plots (four control plots and four ant suppression plots); identifying 318 morphospecies of ants belonging to 60 genera; collecting 5,212 morphological trait measurements; and, quantifying the decomposition of 148 wood blocks. Overall, this work advances our understanding of tropical rainforests as vertically stratified ecosystems. It empirically demonstrates that ant assemblages are stratified both taxonomically and according to functional traits. The findings challenge existing explanations for stratification and suggest that interspecific competition may play an important role in maintaining the vertical stratification of ant assemblages. Results also show that protein preference increases with height for the most abundant ant species and imply that nitrogen is a limiting resource for canopy ants. This thesis demonstrates that patterns in cuticle colour, normally detected along macroclimatic gradients, can also be detected along microclimatic gradients; darker ant assemblages in the canopy and understory can be explained by greater UV-B radiation and lower humidity. Finally, I show how the vertical stratification of communities influences important ecosystem processes; in particular, how dead wood decomposition is dependent on the vertical distribution of termites. This thesis emphasises how community structure and ecosystem processes are determined by complex abiotic and biotic interactions; further studies deciphering the relative influence of each are required if we are to predict or mitigate the impacts of anthropogenic change that continue to threaten tropical rainforests.