Evidence Of Interspecific Competition Research Articles

Interspecific Competition and Vertical Niche Partitioning in Fiji’s Forest Birds

Charles Darwin proposed his ‘principle of divergence’ to account for changes in traits that could promote speciation and coexistence of diverse forms through occupation of different niches to reduce interspecific competition. We explore interspecific foraging behaviour overlap in Fiji’s forest birds, and address two main questions: (1) Is there vertical stratification of foraging behavior? and (2) Is there evidence of interspecific competition driving the differences in foraging behaviour? We explore these questions across three foraging guilds, nectarivores (three species), insectivores (two species), and omnivores (two species), and find vertical portioning of foraging in each group. To investigate the effect of interspecific competition, we compared foraging heights of the Orange-breasted Myzomela (Myzomela jugularis) honeyeater on Viti Levu Island (where it coexists with two other honeyeater species) and Leleuvia Island (no other honeyeater species). On the main island Viti Levu, we found evidence for vertical niche partitioning within each foraging guild. On Leleuvia, with the ‘one-species only foraging guild’, Orange-breasted Myzomela occupied broader vertical foraging niche than on Viti Levu with two other competitor honeyeater species. This result supports the idea that vertical foraging height can be shaped by interspecific competition. The findings of this study support Darwin’s principle of divergence in Fiji’s forest birds for every foraging guild measured and adds to our understanding of the significance of interspecific competition and niche divergence for patterns of ecological speciation on islands.

Inter-specific competition influences apex predator–prey populations

Abstract ContextTiger (Panthera tigris), leopard (Panthera pardus) and dhole (Cuon alpinus) represent a typical multi-predator system of species of conservation concern. Several studies have addressed this system, with heterogeneous results, and there’s a lack of information on population dynamics of multi-species assemblages. We studied a time series (1998–2009) of abundance indices for three predators and five prey species in Bor Wildlife Sanctuary (BWS), Maharashtra, India, before it was declared as Bor Tiger Reserve (BTR) in 2009. AimsTo analyse the complex relationships within a predator–prey system in a dynamic fashion, to analyse data collected in a stable and undisturbed area and to form a comparison basis for future studies within the sanctuary after its declaration as a Tiger Reserve. MethodsA 24-h effort was made annually to census the BWS. Predators were counted at waterholes from arboreal hideouts. The prey populations were censused along 353-km line-transects. For each species, we analysed the yearly growth rate, testing the effect of inter-species abundance. Key resultsTiger growth rate did not depend on any particular prey, whereas mesopredators seemed to depend on medium-sized prey. A die-out of dholes in 2001 was followed by an increase in tiger populations (from 4 to 11), which, in turn, negatively affected leopard numbers (from 6 to 2).We found no direct evidence of top-down effect, but the density dependence for three of five prey species could be linked to predation pressure. We found some evidence of interspecific competition among prey species, especially among ungulates, potentially being mediated by predation pressure. ConclusionsThe relationships among species in a predator–prey system are very complex and often could be explained only by more-than-two-species interactions. The disappearance of one predator, not necessarily the top predator, could bring multiple effects, for which it could be difficult to detect causal relationships. ImplicationsAll subsequent changes in human activities in the sanctuary, as a consequence of its designation as the BTR in 2009, should be evaluated with respect to the results of the present study. The conservation of large predators should rely on the maintenance of a rich and abundant prey base, in which different-sized prey could lessen interactive-competition among the predators.

Spatial distribution patterns of Symplocos congeners in a subtropical evergreen broad-leaf forest of southern China

Studies on spatial distribution of congeneric species can supplement our understanding of species ecological processes. We analyzed population structure, spatial distribution, intra- and interspecific associations among six Symplocos tree species on a large (50 ha) plot in a subtropical, evergreen broad-leaf forest in southern China using spatial point pattern analysis methods. Our results suggested that the six Symplocos tree species were all clustered at small scales. The aggregation intensity of S. wikstroemiifolia with low abundance but relatively numerous large-diameter trees was much higher than that of the other five Symplocos species that occurred at high abundance but included few large-diameter trees. Spatial associations among the six congeners showed that 12 of 30 pairs were associated positively at small scales, and 13 of 30 pairs were unrelated. For species among different size classes, 79 of 120 pairs were not correlated, and 17 of 120 pairs were associated positively. These results showed insufficient evidence for interspecific competition and congeneric Symplocos species commonly coexist within subtropical plant communities. The spatial patterns of Symplocos species and their correlations changed with size (DBH) class and were simultaneously affected by spatial scales; the intensity of their aggregation and association decreased with increasing area of the sample plot.

Spatiotemporal ecological drivers of an aquatic plant community in a temporary tropical pool

Ecological processes that extend over time and space organize plant communities, producing different spatiotemporal patterns of species aggregation (or non-aggregation). To investigate whether the spatiotemporal distribution patterns of the species are generated by interspecific competition and/or environmental filtering, we investigated the organization of an aquatic plant community in a temporary pool in the semiarid region of NE Brazil. For seven months (during the rainy season), we sampled aquatic plants along transects in the pool and measured limnological parameters. The spatial structure of the community changed significantly over time. The input and permanence of species in the community varied according to the stage of flooding of the pool. This leads to the expectation that the species would have overlapping niches, causing interspecific competition to increase and exclude less efficient competitors. Indeed, we found that the species coexisting in the pool were only slightly phylogenetically related, an evidence of interspecific competition. Therefore, in the temporary pool, the spatiotemporal organization of the aquatic plant community appears to be driven by simultaneous action of ecological abiotic and biotic processes that operate in an antagonistic fashion.

Occupancy of yellow‐billed and Pacific loons: evidence for interspecific competition and habitat mediated co‐occurrence

Interspecific competition is an important process structuring ecological communities, however, it is difficult to observe in nature. We used an occupancy modelling approach to evaluate evidence of competition between yellow‐billed (Gavia adamsii) and Pacific (G. pacifica) loons for nesting lakes on the Arctic Coastal Plain of Alaska. With multiple years of data and survey platforms, we estimated dynamic occupancy states (e.g. rates of colonization or extinction from individual lakes) and controlled for detection differences among aircraft platforms and ground survey crews. Results indicated that yellow‐billed loons were strong competitors and negatively influenced the occupancy of Pacific loons by excluding them from potential breeding lakes. Pacific loon occupancy was conditional on the presence of yellow‐billed loons, with Pacific loons having almost a tenfold decrease in occupancy probability when yellow‐billed loons were present and a threefold decrease in colonization probability when yellow‐billed loons were present in the current or previous year. Yellow‐billed and Pacific loons co‐occurred less than expected by chance except on very large lakes or lakes with convoluted shorelines; variables which may decrease the cost of maintaining a territory in the presence of the other species. These results imply the existence of interspecific competition between yellow‐billed and Pacific loons for nesting lakes; however, habitat characteristics which facilitate visual and spatial separation of territories can reduce competitive interactions and promote species co‐occurrence.

Behavioral and life-history evidence for interspecific competition in the larvae of two heliconian butterflies

Interspecific competition in herbivorous insects remains a controversial issue. To date, many studied systems have not met assumptions of the traditional competition theory, and a new paradigm has been emerging. We examined the behavioral and life-history consequences of common host plant use of Heliconius erato and Dryas iulia (Nymphalidae) in relation to Passiflora suberosa (Passifloraceae). Larvae of the former use the apical portion of this host; the latter is presumably able to explore all plant parts. We assessed host use pattern in relation to leaf age, when reared either alone (D. iulia) or together (D. iulia and H. erato). Larval feeding choice tests with respect to leaf age were performed, and performance was recorded. Observations were made to assess antagonistic behavior of H. erato and D. iulia towards each other, if any. Similarly to H. erato, D. iulia fed on the young leaves significantly more than the mature ones; larvae were not induced to prefer mature leaves. First instars of H. erato used only the apical parts of P. suberosa and displayed aggressive behavior towards D. iulia, which moved to the lower shoot portions. Larval mortality and development time of both species significantly increased when reared together; such performance costs were more pronounced in D. iulia than H. erato. Our study gathers evidences that use of P. suberosa by these heliconian butterflies represent a case of competitive exclusion resulting in niche differentiation, where costs are higher for D. iulia than H. erato.

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

Biotic interactions such as competition and predation are known to play an important role in structuring communities. In this study, the intra- and interspecific competition between two congeneric sandy beach amphipods Bathyporeia pilosa and B. sarsi was examined by means of a two-way factorial mesocosm experiment. The amount of natural food resources as well as population densities of B. pilosa and B. sarsi were manipulated in order to investigate whether firstly, the species compete for food, and secondly, whether crowding effects, such as damaging encounter competition, are present within and between both Bathyporeia species. The absence of food negatively influenced survival and recruitment and enhanced aggressive behavior in the B. sarsi population, while there was no influence of the absence of food in the B. pilosa population. In the field, B. pilosa is occurring high in the intertidal where food supply is low, while B. sarsi is occurring in the mid-intertidal where food supply is higher. Hence, B. pilosa is more adapted to low food conditions than B. sarsi. Although an increased intraspecific density stimulated encounter competition within the B. sarsi population, no evidence of interspecific competition was found in this study. This first report on encounter competition for the deposit-feeding amphipod Bathyporeia sarsi suggests that zonation patterns of the two amphipods may be the result of the combined influence of species-specific physiological restrictions and biotic interactions within the B. sarsi population, indicating that on dissipative beaches, biotic interactions may be more common than generally considered.

Using phylogenetic trees to test for character displacement: a model and an example from a desert mammal community

The distribution of traits within communities is thought to provide information on the evolutionary and ecological forces that structure community composition. Ecological character displacement (evolutionary divergence among populations of interacting species within a community), as well as community‐wide character displacement (even dispersion of species traits within a community via species sorting), have been widely reported and are typically interpreted as evidence for interspecific competition. However, defining an appropriate null distribution with which to assess the observed distribution of traits within a community has proved controversial. Phylogenetic methods provide an alternative approach to evaluating community structure, but such methods also require an appropriate null and have typically overlooked the potential for evolutionary dynamics within communities. Here, we present a novel phylogenetic framework that uses evolutionary expectations to generate a simple null model of the expected distribution of traits among co‐occurring species. Using a stochastic Brownian motion model of trait change, we illustrate that the expected community‐wide dispersion of traits varies with phylogenetic tree shape. We then use data on body mass for mammals to evaluate the accuracy with which phylogeny can predict the empirical distribution of traits and find a strong correlation between predicted and observed trait distributions. We suggest that deviations from phylogenetic expectations may therefore provide a useful tool for evaluating the role of competition in shaping community structure. Finally, we demonstrate the utility of our approach using empirical data on body mass and a phylogeny for a small community of terrestrial mammals in Yotvata, Israel, and reveal evidence consistent with ecological and community‐wide character displacement. Our method unites ecological and evolutionary approaches, and it provides a novel framework for exploring community structure.

Interspecific Competition and Speciation in Endoparasitoids

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.

Small mammal community structure in West Africa: a meta-analysis using null models

Peer-reviewed studies on the community structure of small mammals (Rodentia and Soricomorpha) from the Gulf of Guinea region of West Africa were reviewed. To detect nonrandom patterns in the various assemblages under study, the original datasets were re-analysed using null models [two independent randomization algorithms (RA) for niche overlap] and Monte Carlo simulations. The total species richness in the countries considered in this review was 45 species for soricomorphs and 101 for rodents, and the studies reviewed here reported data for 53.3% of these soricomorph species and for 76.2% of these rodent species. Nonrandom habitat niche partitioning was rarely observed in both rodents and soricomorphs. Instead, aggregated use of habitat resources was frequently detected in both groups. Forest habitat was generally selected as aggregating resource type by small mammal assemblages. Thus, contrary to expectations, our review revealed little evidence for interspecific competition along the habitat niche axis in West African small mammals. However, it is possible that the aggregated use of the forest resource by small mammal species may be apparent, but that the various species partitioned space at a lower scale, for instance selecting different microhabitats. Interspecific competition appeared to be stronger in altered habitats, as predicted by previous studies on other forest organisms in West Africa. Des études révisées par les pairs sur la structure de la communauté de petits mammifères (Rongeurs et Soricomorphes) de la région du Golfe de Guinée, en Afrique de l’Ouest, ont été relues. Afin de déceler des schémas non aléatoires dans les différents assemblages étudiés, les sets de données originaux ont été analysés de nouveau en utilisant des modèles nuls (deux algorithmes de randomisation indépendants pour le recouvrement des niches) et des simulations de Monte-Carlo. La richesse totale en espèces des pays concernés par cette étude était de 45 espèces de soricomorphes et de 101 espèces de rongeurs, et les études révues ici rapportaient des données pour 53,5% de ces espèces de soricomorphes et pour 76,2% des espèces de rongeurs. On a rarement observé un partage aléatoire des niches de l’habitat, aussi bien pour les soricomorphes que pour les rongeurs. On a, au contraire, souvent décelé une utilisation groupée des ressources de l’habitat dans les deux groupes. L’habitat forestier était généralement choisi comme type de ressources rassembleur par des assemblages de petits mammifères. Donc, contrairement aux attentes, notre revue a révélé qu’il y avait peu de preuves d’une compétition interspécifique sur l’axe « niche de l’habitat » chez les petits mammifères d’Afrique de l’Ouest. Il est pourtant possible que l’utilisation groupée des ressources forestières par des espèces de petits mammifères semble évidente, mais que les différentes espèces se répartissent l’espace à plus petite échelle, par exemple en sélectionnant différents micro-habitats. La compétition interspécifique semble plus forte dans des habitats altérés, comme le prédisaient des études antérieures d’autres organismes forestiers en Afrique de l’Ouest.

Competition for foraging resources and coexistence of two syntopic species of Messor harvester ants in Mediterranean grassland

1. Diet composition of two syntopic species of Messor seed-harvester ants (M. wasmanni Krausse and M. minor André) was evaluated during different periods over the year (May, July, October), by analysing food type (plant parts and species) and food size (weight, length, width). Morphological traits of foragers (head width and femur length) considered important features promoting diet partitioning were measured. 2. We used two robust randomisation algorithms (RA2 and RA3), adopted in niche overlap studies, to check for random vs non random utilisation of resources at intra- and interspecific level for the different periods. 3. Analyses showed high levels of overlap in the diet of the two species and no evidence of interspecific competition during most of the activity season. In particular, there was an aggregated use of resources in summer, whilst niche partitioning and evidence of competition when resources decreased in autumn. Intraspecifically, no evidence of competition was found. 4. Results suggest two different mechanisms for minimising competition: when food resources are abundant (summer), ants collected the same plant species but selected different sizes; when food resource is scarce (autumn), ants foraged on different plants. 5. The importance of different factors (morphological, behavioural, ecological) possibly affecting competition and coexistence are discussed.

Competition and habitat suitability: small-scale segregation underpins large-scale coexistence of key species on temperate rocky shores

Identifying the mechanisms that underpin species richness is one of the central issues of community ecology. On rocky shores in north-western Europe, two key limpet species coexist on the mid- and low shore but are segregated at small scales with respect to habitat. Shortterm and long-term experiments were done to test whether habitat suitability or habitat-specific competition drives the small-scale segregation of these species and therefore underpins their coexistence at larger spatial scales. In a controlled short-term experiment, Patella vulgata was transplanted onto open rock and into pools that either contained Patella ulyssiponensis and/or their mucus or from which P. ulyssiponensis and/or their mucus had been removed. After 2 days, P. vulgata remained in all experimental plots in similar numbers irrespective of treatment indicating that there was no negative response to P. ulyssiponensis, mucus or the pool habitat. In a long-term experiment, cage enclosures containing both species were set up in pools and on open rock over a 6-month period. P. vulgata grew equally well on both open rock and in pools but suffered higher mortality in pools. P. ulyssiponensis showed lower growth rate and higher mortality on open rock than in pools. P. ulyssiponensis exhibited increased growth in higher intraspecific densities on open rock and reduced growth in higher intraspecific densities in pools, indicating some degree of intraspecific facilitation on open rock and intraspecific competition in pools. There was no evidence of interspecific competition either in the short term or in the long term. Results revealed that habitat suitability was the mechanism causing segregation of these species at smaller spatial scales enabling them to coexist at larger spatial scales. Conflicting results in the short-term and long-term experiments highlight the importance of considering the correct temporal extent for experimental tests of hypotheses.

Geographic Distribution and Regional Impacts ofOxyops vitiosa(Coleoptera: Curculionidae) andBoreioglycaspis melaleucae(Hemiptera: Psyllidae), Biological Control Agents of the Invasive TreeMelaleuca quinquenervia

The invasive tree Melaleuca quinquenervia (Cav.) Blake is widely distributed throughout peninsular Florida and poses a significant threat to species diversity in the wetland systems of the Everglades. Mitigation of this threat includes the areawide release campaign of the biological control agents Oxyops vitiosa Pascoe and Boreioglycaspis melaleucae Moore. We summarize the results of this release effort and quantify the resulting geographic distribution of the herbivores as well as their regional impact on the target weed. A combined total of 3.3 million individual Melaleuca biological control agents have been redistributed to 407 locations and among 15 Florida counties. Surveys of the invaded area indicate that the geographic distribution of O. vitiosa encompasses 71% of the Melaleuca infestation. Although released 5 yr later, the distribution of B. melaleuca is slightly greater than its predecessor, with a range including 78% of the sampled Melaleuca stands. Melaleuca stands outside both biological control agents' distributions occurred primarily in the northern extremes of the tree's range. Strong positive association between herbivore species was observed, with the same density of both species occurring in 162 stands and no evidence of interspecific competition. Soil type also influenced the incidence of biological control agents and the distribution of their impacts. The odds of encountering O. vitiosa or B. melaleucae in cells dominated by sandy soils were 2.2 and 2.9 times more likely than those predominated by organically rich soils. As a result, a greater level of damage from both herbivores was observed for stands growing on sandy versus organic-rich soils.

Fine-scale habitat selection by white-beaked and common dolphins in the Minch (Scotland, UK): evidence for interspecific competition or coexistence?

A decline in white-beaked dolphins Lagenorhynchus albirostris had been reported in the Minch (Scotland, UK) since the 1990s, coinciding with an increasing occurrence of short-beaked common dolphins Delphinus delphis. This has led to suggestions that rising seawater temperatures are causing D. delphis to out-compete and exclude L. albirostris in this region. A total of 793 km (66.3 hours) of survey data were collected in the Minch during August 2007, to examine fine-scale habitat selection by L. albirostris and D. delphis and investigate whether their ecological overlap is sufficient to result in interspecific competition and/or habitat partitioning. Ten sightings of L. albirostris (70 animals) were recorded in a relatively small spatial area in the northern Minch. In contrast, the eleven sightings of D. delphis (1486 animals) were more widely distributed. The relative abundance per 1/4 ICES rectangle ranged from 0.41 to 0.53 animals/km for L. albirostris and 0.13 to 6.68 animals/km for D. delphis. The mean group size and group body mass were higher for D. delphis than for L. albirostris indicating D. delphis as the dominant delphinid in the Minch during August. Lagenorhynchus albirostris occurred in waters significantly deeper and further from shore than D. delphis, suggesting interspecific differences in preferred habitat. Most dolphin schools were recorded as foraging/feeding. Behaviour and seabird associations indicated that the two species differed in diet and/or foraging strategy, with L. albirostris foraging sub-surface and D. delphis exhibiting surface-feeding with associated gannets Morus bassanus. This is consistent with published information on the stomach contents of Scottish animals. The results suggest that there are subtle differences in habitat selection and diet between these two species, which may enable L. albirostris and D. delphis to coexist in the Minch. Whether these differences result from niche partitioning arising from previous/ongoing interspecific competition or are the result of genuine differences in the habitat preferences of each species, remains unclear.

The invasion of Undaria pinnatifida to a Macrocystis pyrifera kelp in Patagonia (Argentina, south-west Atlantic)

Biological invasions can alter the biodiversity of native communities generating an ecological impact that in many cases is irreversible. In 2001, Undaria pinnatifida invaded Macrocystis pyrifera kelp in Cracker Bay (42°56′S, 64°27′W; Northern Patagonia, Argentina). The possible impact of that invasion was evaluated in March 2004 with a study of population and morphological parameters of both kelp species and their associated holdfast biodiversity. Three nearby areas of spatial distribution of the kelps were differentiated, one dominated by M. pyrifera (M), an intermediate area characterized by M. pyrifera and U. pinnatifida (M+U), and another dominated by U. pinnatifida (U). In each area, sporophytes in nine quadrats of 1 m2 were collected. The density and biomass m−2, the height of the sporophytes and the size of the holdfast of M. pyrifera did not diminish in the presence of U. pinnatifida. The richness, abundance and diversity of the flora associated with the holdfast of both kelps were similar, whereas these parameters were higher in the case of the fauna associated to M. pyrifera than U. pinnatifida Results obtained in this work suggest that, at this invasion stage, there is no evidence of interspecific competition. Nevertheless, since density and diversity of the fauna associated with the holdfast is different, we expect community changes if the U. pinnatifida invasion continues to expand.

The influence of different sediment nutrient contents on growth and competition of Elodea nuttallii and Myriophyllum spicatum in nutrient-poor waters

Elodea nuttallii and Myriophyllum spicatum are highly competitive invaders replacing established vegetation in Europe and North America, respectively. In this study, the effect of different sediment nutrient contents on growth and competition potential of these two macrophyte species in nutrient-poor environments was investigated. When the sediment nutrient content was decreased, both species showed a gradual decrease in vitality, in terms of decreasing nutrient contents in the plant tissue, decreasing length relative growth rate (RGR) and increasing root:shoot ratios. E. nuttallii also revealed a decrease in weight RGR. At lower sediment nutrient contents both species were severely phosphorus-limited. As an evidence for inter-specific competition, we found a decreasing strength of the plant tissue: sediment relationships for nitrogen and phosphorus in M. spicatum if exposed to E. nuttallii.

Diet breadth, coexistence and rarity in bumblebees

Factors that determine the relative abundance of bumblebee species remain poorly understood, rendering management of rare and declining species difficult. Studies of bumblebee communities in the Americas suggest that there are strong competitive interactions between species with similar length tongues, and that this competition determines the relative abundance of species. In contrast, in Europe it is common to observe several short-tongued species coexisting with little or no evidence for competition shaping community structure. In this study we examine patterns of abundance and distribution in one of the most diverse bumblebee communities in Europe, found in the mountains of southern Poland. We quantify forage use when collecting nectar and pollen for 23 bumblebee species, and examine patterns of co-occurrence and niche overlap to determine whether there is evidence for inter-specific competition. We also test whether rarity can be explained by diet breadth. Up to 16 species were found coexisting within single sites, with species richness peaking in mountain pasture at ~1,000 m altitude. Results concur with previous studies indicating that the majority of pollen collected by bumblebees is from Fabaceae, but that some bee species (e.g. B. ruderatus) are much more heavily dependent on Fabaceae than others (e.g. B. lucorum). Those species that forage primarily on Fabaceae tended to have long tongues. In common with studies in the UK, diet breadth was correlated with abundance: rarer species tended to visit fewer flower species, after correcting for differences in sample size. No evidence was found for similarity in tongue length or dietary overlap influencing the likelihood of co-occurrence of species. However, the most abundant species (which co-occurred at most sites) occupied distinct dietary niche space. While species with tongues of similar length tended, overall, to have higher dietary niche overlap, among the group of abundant short-tongued species that commonly co-occurred there was marked dietary differentiation which may explain their coexistence.

Evidence for interspecific competition between feral ass Equus asinus and mountain sheep Ovis canadensis in a desert environment

Abstract Different ungulate species that co-occur over evolutionary time have been hypothesized to develop mechanisms to limit the degree to which they directly compete for the same limited resources (i.e. resource partitioning). In situations where an exotic ungulate has been recently introduced to a system, resource partitioning has not likely developed; this appears to be the situation between introduced feral ass Equus asinus and indigenous mountain sheep Ovis canadensis in the Sonoran Desert of southeastern California, USA. We analysed data from aerial surveys conducted during 1993–2003 to look for evidence of an effect of feral ass abundance on mountain sheep demography. After controlling for the influence of forage availability via rainfall, we found no evidence of relationship between an index of feral ass abundance and indices of reproduction or recruitment in mountain sheep (P≥0.360). However, we found strong evidence for an effect on indices of sheep abundance and population rate of change. The...

Null model tests of clustering of species, negative co‐occurrence patterns and nestedness in meta‐communities

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.

Spatial and temporal foraging overlaps in a Chacoan ground-foraging ant assemblage

In arid habitats ant temporal foraging results from a set of trade-offs between species food discovery and food domination ability and between species thermal tolerance and behavioral dominance. We investigated how this set of trade-offs structures the spatial co-occurrence of ants during foraging, hence focusing on interspecific competition. The consequences of food type on the spatio-temporal foraging structure were also studied because food preferences were expected to optimize the restricted foraging time and space of thermophilic species. The food partitioning between 24 species attracted to protein or carbohydrate baits was studied in a dry forest of the Paraguayan Chaco. Baits were spaced at one meter intervals and inspected every 15 min over 90-min sessions conducted during mornings and afternoons. Null model analyses revealed a temporal food partitioning during afternoon sessions and a spatial partitioning, influenced by both thermal conditions and interspecific competition. Bait type did not influence the spatio-temporal foraging. Unexpectedly, evidence of interspecific competition was scarce and avoidance rather than overt conflicts appeared to be the rule of thumb. Overall, the spatio-temporal food exploitation at a small scale was affected by temperature and stochastic processes. This demonstrates that chance structures ant foraging even in constraining arid environments.