Role Of Intraspecific Competition Research Articles

Quantifying shrub-shrub competition in drylands using aerial imagery and a novel landscape competition index.

The Jornada Basin Long-Term Ecological Research Site (JRN-LTER, or JRN) is a semiarid grassland-shrubland in southern New Mexico, USA. The role of intraspecific competition in constraining shrub growth and establishment at the JRN and in arid systems, in general, is an important question in dryland studies. Using information on shrub distributions and growth habits at the JRN, we present a novel landscape-scale (c. 1 ha) metric (the 'competition index', CI), which quantifies the potential intensity of competitive interactions. We map and compare the intensity of honey mesquite (Prosopis glandulosa, Torr.) competition spatially and temporally across the JRN-LTER, investigating associations of CI with shrub distribution, density, and soil types. The CI metric shows strong correlation with values of percent cover. Mapping CI across the Jornada Basin shows that high-intensity intraspecific competition is not prevalent, with few locations where intense competition is likely to be limiting further honey mesquite expansion. Comparison of CI among physiographic provinces shows differences in average CI values associated with geomorphology, topography, and soil type, suggesting that edaphic conditions may impose important constraints on honey mesquite and growth. However, declining and negative growth rates with increasing CI suggest that intraspecific competition constrains growth rates when CI increases above c. 0.5.

Dynamics of an unstirred chemostat model with Beddington–DeAngelis functional response

This paper deals with an unstirred competitive chemostat model with the Beddington–DeAngelis functional response. With the help of the linear eigenvalue theory and the monotone dynamical system theory, we establish a relatively clear dynamic classification of this system in terms of the growth rates of two species. The results indicate that there exist several critical curves, which may classify the dynamics of this system into three scenarios: 1) extinction; 2) competitive exclusion; and 3) coexistence. Comparing with the classical chemostat model [26], our theoretical results reveal that under the weak–strong competition cases, the role of intraspecific competition can lead to species coexistence. Moreover, the simulations suggest that under different competitive cases, coexistence can occur for suitably small diffusion rates and some intermediate diffusion rates. These new phenomena indicate that the intraspecific competition and diffusion have a great influence on the dynamics of the unstirred chemostat model of two species competing with the Beddington–DeAngelis functional response.

Dynamical behaviour of a predator–prey system encapsulating the fear affecting death rate of prey and intra-specific competition: Revisited in a fluctuating environment

This paper revisits a population model incorporating a fear factor and intra-specific competition. An extension of the model from a deterministic framework to a stochastic one is also taken into account as an additional degree of realism. The prey–predator system in the absence of environmental noise is investigated in terms of equilibria, stability and positivity of the model. For this purpose, single parameter numerical continuation is performed to analyse the impact of fear factor in the reproduction and death rate of prey species, and the role of intra-specific competition in predator population. Then, the environmental noise is incorporated in the death rates of both species, leading to stochastic differential equations. It is demonstrated that the model has a global positive solution in the presence of white Gaussian noise and the extinction conditions of populations are analytically presented. Lastly, a deterministic approach to explore the statistical properties of the stochastic model is performed. The theoretical results of this paper are complemented with detailed numerical simulations.

The role of intraspecific competition between plants in a nursery pollination system-Comments on Villacañas de Castro and Hoffmeister 2020.

We present comments on an article published by Villacañas de Castro and Hoffmeister (Ecology and Evolution, 10, 4220; 2020). The authors studied a tritrophic system composed of a plant, its pollinating seed predator, and a parasitoid of the latter. Their concern was whether the parasitoid modifies the interaction between the plant and its pollinator–herbivore along the mutualism–antagonism gradient, but they reduced their question to how the parasitoid impacts plant fitness. After showing that the parasitoid increases seed output of the plant by decreasing the amount of seeds consumed by the pollinating seed predator, they tested whether seed output is a good proxy for plant fitness. They argue that it is not by showing that the increased seed density has a negative impact on survival probability and flower production, likely due to plant intraspecific competition. The work presented shows careful experimentation and interesting results, but we do not share some of their conclusions. Most importantly, we believe that the net effect of the parasitoid on the plant–herbivore interaction cannot be adequately investigated by focusing on individual plant fitness. Thus, we first suggest considering the number of surviving plants up to adulthood as a proxy for population performance to address this question. Using this proxy, we show that the increase in seed output due to the parasitoid is beneficial to the plant population until its carrying capacity is achieved. Next, using a population dynamics model, we show under which particular conditions the negative effect of intraspecific competition outweighs the positive effect of seed density increase (due to parasitoid's defense). When these conditions do not hold, the role of plant intraspecific competition is basically limited to the prevention of unbounded population growth, while the parasitoid increases the plant's equilibrium density above its carrying capacity as measured when interacting only with the pollinating seed predator, thus making the system more stable.

Role of intraspecific competition in intrinsic growth rate regulation in an Oikopleura dioica (Tunicata) population

Planktonic Oikopleura dioica respond almost instantly to opportunistic algal blooms, but it is unknown whether the population increase can change from an exponential to a logistic model to avoid a final sudden collapse. To test the hypothesis that intraspecific competition regulates the intrinsic rate of natural increase (r), density-dependent effects on growth and reproduction performance were investigated in O. dioica via laboratory incubations. Over a large food concentration range, batch maturation was observed above the per capita food supply (PFS) of 8.1 µgC/ind. in 4.5 d. Somatic growth was saturated beyond this PFS value, whereas gonad length increased continuously. Below this threshold, individuals reached small body and gonad lengths, and maturation was rarely observed during the incubation period. The gonad/body volume and maturation ratios also increased with the PFS. Instead of the food concentration, the r values were regulated by competition pressure via variability in maturation duration and the proportion of mature individuals in the cohorts. When the minimum food demand was satisfied in the designated generation time, the r value tended to be regulated by the spawning proportion in the population. Otherwise, prolonged development duration and decreased r values were expected.

The genetics of evolutionary radiations.

With the realization that much of the biological diversity on Earth has been generated by discrete evolutionary radiations, there has been a rapid increase in research into the biotic (key innovations) and abiotic (key environments) circumstances in which such radiations took place. Here we focus on the potential importance of population genetic structure and trait genetic architecture in explaining radiations. We propose a verbal model describing the stages of an evolutionary radiation: first invading a suitable adaptive zone and expanding both spatially and ecologically through this zone; secondly, diverging genetically into numerous distinct populations; and, finally, speciating. There are numerous examples of the first stage; the difficulty, however, is explaining how genetic diversification can take place from the establishment of a, presumably, genetically depauperate population in a new adaptive zone. We explore the potential roles of epigenetics and transposable elements (TEs), of neutral process such as genetic drift in combination with trait genetic architecture, of gene flow limitation through isolation by distance (IBD), isolation by ecology and isolation by colonization, the possible role of intra-specific competition, and that of admixture and hybridization in increasing the genetic diversity of the founding populations. We show that many of the predictions of this model are corroborated. Most radiations occur in complex adaptive zones, which facilitate the establishment of many small populations exposed to genetic drift and divergent selection. We also show that many radiations (especially those resulting from long-distance dispersal) were established by polyploid lineages, and that many radiating lineages have small genome sizes. However, there are several other predictions which are not (yet) possible to test: that epigenetics has played a role in radiations, that radiations occur more frequently in clades with small gene flow distances, or that the ancestors of radiations had large fundamental niches. At least some of these may be testable in the future as more genome and epigenome data become available. The implication of this model is that many radiations may be hard polytomies because the genetic divergence leading to speciation happens within a very short time, and that the divergence history may be further obscured by hybridization. Furthermore, it suggests that only lineages with the appropriate genetic architecture will be able to radiate, and that such a radiation will happen in a meta-population environment. Understanding the genetic architecture of a lineage may be an essential part of accounting for why some lineages radiate, and some do not.

Where to spend the winter? The role of intraspecific competition and climate in determining the selection of wintering areas by migratory caribou

Depicted as predictable movements, migrations can, however, show important interannual variations, making the conservation of migratory species particularly challenging. Plasticity in migratory behaviour allows individuals to adjust their migratory tactics to maximize their fitness. Destination of migration, and therefore migration patterns, may vary according to climatic and environmental conditions encountered during migration or at the arrival site but also according to competition. In northern‐Québec and Labrador, Canada, fall migration patterns of caribou from the Rivière‐George (RGH) and the Rivière‐aux‐Feuilles (RFH) herds have varied greatly during the last decades. Meanwhile, both herds have shown large fluctuations in abundance. We assessed the influence of environmental factors and changes in population size on wintering area selection. Based on 649 fall migrations of 284 females equipped with ARGOS collars, we used a machine‐learning algorithm, the random forests, to assess how climate, resources and population size affected the selection of four different wintering areas. Individuals followed over several years switched to a different wintering area 45% of the time between consecutive years, and this probability increased at high population size. The main determinant of wintering area selection was the population size for both herds, suggesting intra‐ and inter‐herd competition for wintering areas. The long migrations of RGH toward the western wintering areas, also used by RFH, were favoured when the herd was abundant and when the availability of resources was low at the departure. The migrations of RFH toward the south‐western area increased as RGH declined, possibly because the past presence of RGH in this area reduced access for caribou from RFH. These results highlight the flexibility in the migratory behaviour of caribou in response to variation in competition. Our study is the first to suggest that wintering area selection can be determined by competition between populations of the same ungulate species.

The role of intraspecific competition in the dispersal of an invasive fish

Abstract Ponto‐Caspian gobies are among the most successful fish invaders in inland waters of Europe and in the Great Lakes of North America. Their invasions appear to comprise a combination of passive and active dispersal mechanisms, both natural and human‐mediated. Despite the significance of Ponto‐Caspian gobies as invasive species, there is little information about the mechanisms underpinning their dispersal. They are relatively small benthic fish, with high site affinity. Thus, actively dispersing individuals must express a behavioural motivation to engage in persistent directional movement. Several recent studies have suggested that inter‐individual behavioural differences in boldness, activity level and tendency to explore might underpin dispersal. In addition, because males are highly territorial, intraspecific competition may lead to density‐dependent dispersal of subordinate individuals. To date, studies on this subject have focused mainly on comparisons between individuals from the core of established populations with those at the margins and, thus, address the outcome of dispersal rather than the mechanism itself. We conducted a series of experiments on the racer goby Babka gymnotrachelus to address the question of what behavioural and physiological traits predict the tendency of an individual to perform dispersal behaviour, specifically considering the role of conspecifics in influencing the tendency to disperse. We used an artificial channel to measure dispersal tendency in this species in combination with behavioural trials. Our results showed that fish with a greater tendency to disperse in an experimental channel grew more slowly and were bolder; that is displayed a greater propensity to emerge from a cryptic background onto a white background and performed worse in prey capture trials. As predicted, intraspecific competition played a primary role in the dispersal of the racer goby. Dominant males showed a tendency to monopolise limited shelters with an outcome that subordinates were forced to disperse upstream. The specific growth rate of individuals appeared to be a good indicator of social position. Subordinate males expressed the lowest rates of growth, probably as a result of long‐term deprivation of food in the presence of dominant conspecifics. They were also quicker to explore novel environments, possibly to search for food. Contrary to expectations, subordinate individuals performed relatively poorly in feeding trials when tested individually. Our findings suggest that intraspecific competition in racer goby males is an important mechanism for active dispersal. It can also influence inter‐individual variation in traits such as boldness and tendency to explore novel environments. Similar responses to competitive interactions may have encouraged the invasive expansion of other Ponto‐Caspian gobies following establishment in new environments, as well as other fish species that exhibit territorial behaviour.

Venom in Furs: Facial Masks as Aposematic Signals in a Venomous Mammal.

The function of colouration in animals includes concealment, communication and signaling, such as the use of aposematism as a warning signal. Aposematism is unusual in mammals, and exceptions help us to understand its ecology and evolution. The Javan slow loris is a highly territorial venomous mammal that has a distinctive facial mask and monochromatic vision. To help understand if they use aposematism to advertise their venom to conspecifics or predators with different visual systems, we studied a population in Java, Indonesia. Using ImageJ, we selected colours from the facial masks of 58 individuals, converted RBG colours into monochromatic, dichromatic and trichromatic modes, and created a contrast index. During 290 captures, we recorded venom secretion and aggressiveness. Using Non-metric Multidimensional Scaling and generalised additive models for location, scale and shape, we found that young slow lorises differ significantly from adults, being both more contrasting and more aggressive, with aggressive animals showing fewer wounds. We suggest aposematic facial masks serve multiple purposes in slow lorises based on age. Change in colouration through development may play a role in intraspecific competition, and advertise toxicity or aggressiveness to competitors and/or predators in juveniles. Aposematic signals combined with intraspecific competition may provide clues to new venomous taxa among mammals.

Stabilizing effect of intra-specific competition on prey-predator dynamics with intraguild predation

Multi-species population models with various types of interactions are important to understand the stable coexistence of prey and predator species and the role of various factors leading to complex dynamics and extinction of one or more species. Models of two predators sharing one prey population are analyzed by some researchers and the role of nonlinear functional responses and interaction strengths are reported to be responsible for the stable coexistence of three species. However, one-prey-two-predator models with intraguild predation have received relatively less attention from researchers. Recent literature on this topic mainly focused on the study of local and global stability properties of possible equilibria and appearance of various types of bifurcations. Intra-specific competition among predator species is an important factor for coexistence of more than one predator species competing for one shared prey. Hence it is quite essential to understand the role of intra-specific competition on the stability and bifurcation behavior. In this work we wish to demonstrate the stabilizing role of intra-specific competition among intermediate and top predators when the growth rate of prey species is adequate to support both the predator species. With the help of stability and bifurcation analysis we show the stabilizing role of intra-specific competition among predators. Moreover, strong self-regulation in intraguild predator prevents extinction of intraguild prey at high productivity and resolve the contradiction between empirical and previous theoretical results on the fate of intraguild prey at high productivity. To establish our claim we present, apart from analytical results, extensive numerical simulation results and couple of bifurcation diagrams.

A RNA-seq approach to identify putative toxins from acrorhagi in aggressive and non-aggressive Anthopleura elegantissima polyps.

BackgroundThe use of venom in intraspecific aggression is uncommon and venom-transmitting structures specifically used for intraspecific competition are found in few lineages of venomous taxa. Next-generation transcriptome sequencing allows robust characterization of venom diversity and exploration of functionally unique tissues. Using a tissue-specific RNA-seq approach, we investigate the venom composition and gene ontology diversity of acrorhagi, specialized structures used in intraspecific competition, in aggressive and non-aggressive polyps of the aggregating sea anemone Anthopleura elegantissima (Cnidaria: Anthozoa: Hexacorallia: Actiniaria: Actiniidae).ResultsCollectively, we generated approximately 450,000 transcripts from acrorhagi of aggressive and non-aggressive polyps. For both transcriptomes we identified 65 candidate sea anemone toxin genes, representing phospholipase A2s, cytolysins, neurotoxins, and acrorhagins. When compared to previously characterized sea anemone toxin assemblages, each transcriptome revealed greater within-species sequence divergence across all toxin types. The transcriptome of the aggressive polyp had a higher abundance of type II voltage gated potassium channel toxins/Kunitz-type protease inhibitors and type II acrorhagins. Using toxin-like proteins from other venomous taxa, we also identified 612 candidate toxin-like transcripts with signaling regions, potentially unidentified secretory toxin-like proteins. Among these, metallopeptidases and cysteine rich (CRISP) candidate transcripts were in high abundance. Furthermore, our gene ontology analyses identified a high prevalence of genes associated with “blood coagulation” and “positive regulation of apoptosis”, as well as “nucleoside: sodium symporter activity” and “ion channel binding”. The resulting assemblage of expressed genes may represent synergistic proteins associated with toxins or proteins related to the morphology and behavior exhibited by the aggressive polyp.ConclusionWe implement a multifaceted approach to investigate the assemblage of expressed genes specifically within acrorhagi, specialized structures used only for intraspecific competition. By combining differential expression, phylogenetic, and gene ontology analyses, we identify several candidate toxins and other potentially important proteins in acrorhagi of A. elegantissima. Although not all of the toxins identified are used in intraspecific competition, our analysis highlights some candidates that may play a vital role in intraspecific competition. Our findings provide a framework for further investigation into components of venom used exclusively for intraspecific competition in acrorhagi-bearing sea anemones and potentially other venomous animals.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-1417-4) contains supplementary material, which is available to authorized users.

Bacteriocin-mediated interactions within and between coexisting species.

Bacteriocins are bacteriocidal toxins released by almost all bacteria. They are thought to have a narrow range of killing, but as bacteriocin-mediated interactions have been rarely studied at biologically relevant scales, whether this narrow range of action falls mostly within or mostly between coexisting species in natural communities is an open question with important ecological and evolutionary implications. In a previous study, we systematically sampled Xenorhabdus bacteria along a hillside and found evidence for genotypic variability and bacteriocin-mediated interactions within Xenorhabdus bovienii and X. koppenhoeferi colonies that were collected only a few meters apart. In contrast, colonies that were isolated from the same soil sample were always genetically similar and showed no inhibitions. Here, we conducted pairwise growth-inhibition assays within and between seven X. bovienii and five X. koppenhoeferi colonies that were isolated from different soil samples; all seven X. bovienii colonies and at least three of the X. koppenhoeferi have been distinguished as distinct genotypes based on coarse-grain genomic markers. We found signatures for both conspecific and heterospecific bacteriocin inhibitions in this natural community of Xenorhabdus bacteria, but intraspecific inhibitions were significantly more common than interspecific inhibitions. These results suggest that bacteriocins have a major role in intraspecific competition in nature, but also suggest that bacterocins are important in mediating interspecific interactions among coexisting species in natural communities.

Biosurfactants in plant–Pseudomonas interactions and their importance to biocontrol

Production of biosurfactants is a common feature in bacteria, and in particular in plant-associated species. These bacteria include many plant beneficial and plant pathogenic Pseudomonas spp., which produce primarily cyclic lipopeptide and rhamnolipid type biosurfactants. Pseudomonas-derived biosurfactants are involved in many important bacterial functions. By modifying surface properties, biosurfactants can influence common traits such as surface motility, biofilm formation and colonization. Biosurfactants can alter the bio-availability of exogenous compounds, such as nutrients, to promote their uptake, and of endogenous metabolites, including phenazine antibiotics, resulting in an enhanced biological activity. Antibiotic activity of biosurfactants towards microbes could play a role in intraspecific competition, self-defence and pathogenesis. In addition, bacterial surfactants can affect plants in different ways, either protecting them from disease, or acting as a toxin in a plant-pathogen interaction. Biosurfactants are involved in the biocontrol activity of an increasing number of Pseudomonas strains. Consequently, further insight into the roles and activities of surfactants produced by bacteria could provide means to optimize the use of biological control as an alternative crop protection strategy.

Invertebrate Diet of Breeding and Nonbreeding Crested Caracaras (Caracara cheriway) in Florida

ABSTRACT We compared the invertebrate component of the diet of breeding and nonbreeding northern Crested Caracaras (Caracara cheriway) during the breeding season, January through April, in Florida, using pellet analysis. Pellets from breeding adults were collected at active nests and pellets of nonbreeding caracaras were collected from beneath a communal roost. During the breeding season, breeding and nonbreeding caracaras consumed invertebrate prey from a minimum of 61 genera and a total of 33 families from eight orders. Nonbreeding caracaras consumed greater total numbers of invertebrates, more different prey types, and a greater diversity of invertebrates than did breeding caracaras. Pellets of nonbreeding caracaras contained more invertebrates per pellet, and carrion insects occurred more frequently in pellets of nonbreeding caracaras. Our findings suggest hypotheses regarding the possible role of intraspecific competition in the use of particular habitats and food resources by breeding and nonbreedin...

Resurrecting the Ghost of Competition Past with Dormant Zooplankton Eggs

A common prediction of evolutionary theory is that the strength of interspecific competition should decline over time among sympatric populations of competing species. Here we provide experimental evidence of historical declines in competition effects among competing zooplankton populations. Using diapausing eggs, we resurrected clones of three species of zooplankton obtained from different periods of community assembly in a single lake. We show that clones of Daphnia ambigua obtained from early in assembly when D. ambigua was dominant became extinct in competition with clones of Daphnia pulicaria and Daphnia dentifera (the current lake dominants). In contrast, D. ambigua clones obtained from later in the lake's history experienced weaker competition effects and persisted with D. dentifera. While we cannot rule out the role of intraspecific competition within D. ambigua, our results are in line with the view that natural selection favors reduced interaction strength among co-occurring species, facilitating coexistence and population persistence.

Resurrecting the Ghost of Competition past with Dormant Zooplankton Eggs

A common prediction of evolutionary theory is that the strength of interspecific competition should decline over time among sympatric populations of competing species. Here we provide experimental evidence of historical declines in competition effects among competing zooplankton populations. Using diapausing eggs, we resurrected clones of three species of zooplankton obtained from different periods of community assembly in a single lake. We show that clones of Daphnia ambigua obtained from early in assembly when D. ambigua was dominant became extinct in competition with clones of Daphnia pulicaria and Daphnia dentifera (the current lake dominants). In contrast, D. ambigua clones obtained from later in the lake’s history experienced weaker competition effects and persisted with D. dentifera. While we cannot rule out the role of intraspecific competition within D. ambigua, our results are in line with the view that natural selection favors reduced interaction strength among co‐occurring species, facilitating coexistence and population persistence.

Population regulation in Little Penguins (Eudyptula minor): the role of intraspecific competition for nesting sites and food during breeding

Although studies of seabirds have had a fundamental influence on development of theories of population regulation, the role and importance of potential factors regulating seabird populations remain unresolved. Food supply is commonly thought to be a significant limiting factor regulating seabird populations but difficulty in measuring availability of prey has meant that direct evidence is usually lacking. Here we examine the role that intraspecific competition for breeding sites and food during the breeding season plays in determining the size of populations of Little Penguins (Eudyptula minor) in south-eastern Australia. The relation between population sizes and available breeding area suggest that numbers of Little Penguins may be limited on smaller islands by area but clearly are not on larger islands. Little Penguins have very short foraging ranges and thus are likely to experience density-dependent food shortages when breeding. We found support for Ashmole's ‘halo effect’ in that there was an inverse relationship between colony size and mean mass of chicks near fledging. However, more recent variations of this model (e.g. the ‘hungry horde’, or ‘hinterland’) do not explain the observed distribution and numbers of penguins breeding in Victoria. We conclude that both factors may be involved in the regulation of Little Penguin populations locally, and propose a model in which the area available for nesting is combined with intraspecific competition for food during breeding as co-determinants of population size.

POSTGLACIAL POPULATION EXPANSION DRIVES THE EVOLUTION OF LONG-DISTANCE MIGRATION IN A SONGBIRD

The evolution of long-distance migratory behavior from sedentary populations is a central problem in studies of animal migration. Three crucial issues that remain unresolved are: (1) the biotic and abiotic factors promoting evolution of migratory behavior, (2) the geographic origin of ancestral sedentary populations, and (3) the time scale over which migration evolves. We test the role of postglacial population expansions during the Quaternary in driving the evolution of songbird migration against prevailing views favoring the role of intraspecific competition. In contrast to previous attempts to investigate these questions using interspecific phylogenies, we adopt an intraspecific approach and examine the phylogeography of a North American songbird, the chipping sparrow (Spizella passerina), which exhibits both long-distance migratory behavior in temperate North America and sedentary behavior in Mexico and Central America. We show that migratory populations descend from sedentary populations in southern Mexico and that migration has evolved as a result of a northward population expansion into temperate North America since the last glacial maximum 18,000 years ago. Migration appears to have evolved rapidly in some species as populations colonized areas of high seasonality in the temperate zone. The phylogeography of the yellow-eyed junco (Junco phaeonotus), a strictly sedentary species, provides a null model supporting the view that northward range expansions were driven solely by environmental factors and not by a predisposition to evolve migratory behavior. These results provide the strongest evidence to date that historical climate patterns can drive the rapid evolution of avian migration in natural populations, and they suggest a general mechanism for the repeated evolution of migration within and across bird lineages.

Large size in an island-dwelling bird: intraspecific competition and the Dominance Hypothesis.

Differences between island- and mainland-dwelling forms provide several classic ecological puzzles. Why, for instance, are island-dwelling passerine birds consistently larger than their mainland counterparts? We examine the 'Dominance hypothesis', based on intraspecific competition, which states that large size in island passerines evolves through selection for success in agonistic encounters. We use the Heron Island population of Capricorn silvereyes (Zosterops lateralis chlorocephalus), a large-bodied island-dwelling race of white-eye (Zosteropidae), to test three assumptions of this hypothesis; that (i) large size is positively associated with high fitness, (ii) large size is associated with dominance, and (iii) the relationship between size and dominance is particularly pronounced under extreme intraspecific competition. Our results supported the first two of these assumptions, but provided mixed evidence on the third. On balance, we suggest that the Dominance Hypothesis is a plausible mechanism for the evolution of large size of island passerines, but urge further empirical tests on the role of intraspecific competition on oceanic islands versus that on mainlands.

Do snakes exhibit shifts in feeding ecology associated with the presence or absence of potential competitors? A case study from tropical Africa

Two species of rain-forest snakes, the colubrids Natriciteres fuliginoides and Natriciteres variegata, are potential competitors, because they attain, on average, the same body size (20–30 cm long), have similar habits and ecological requirements (they are both diurnal and nocturnal, often semi-aquatic), and have similar dietary spectra ( including both small vertebrates and invertebrates). I studied prey type and predator size – prey size relationships of these snakes in four swamp-forest areas of southeastern Nigeria (West Africa): only N. fuliginoides was found in two of these areas, whereas both species were common in the other two areas. The feeding ecology of N. fuliginoides shifted greatly in the presence of its potential competitor, N. variegata, in terms of prey-type preferences and predatory behaviour as well as in terms of predator size – prey size relationships. In particular, the effects of the potential competitor on the feeding ecology of N. fuliginoides were (i) a shift toward eating many invertebrates from a diet based on many small vertebrates, (ii) a shift toward eating many terrestrial organisms from a diet based on many aquatic organisms, and (iii) a shift in mean prey size for females toward relatively larger prey from a diet based on smaller prey. The resulting effect was a clear partitioning of food resources between the two predator species in both the areas where they are sympatric, whereas N. fuliginoides had a wider dietary spectrum (covering that of the two species together) in the areas where its competitor is not found. The role of intraspecific competition seems less strong, as no apparent resource partitioning in prey type or prey size was observed between males and females of either species or in any study area. However, mean prey size was significantly greater for females than for males of both species and in all study areas, but depended on the significant reverse sexual size dimorphism observed in both species.