The role of interspecific competition as a key factor in the ecology of natural communities where species exploit limited resources is well established, and the study of competition dynamics in snake communities has received much attention in recent years. Twenty years ago, an acclaimed review (Toft 1985) suggested that snakes were atypical among vertebrates because sympatric species usually partition the food niche. Here, I review the articles published in the last two decades with the aim of finding any general geographical or guild patterns and assessing if Toft's main conclusion is still supported by new evidence. Where appropriate, I use Monte Carlo simulations to establish whether observed patterns of niche overlap are real, or if they have occurred by chance. My study shows clear congruence in the patterns of coexistence exhibited by snake communities in different regions of the world, i.e.: (1) cold regions of the northern hemisphere (high latitudes and altitudes) exhibit low species richness and a very low, or even absent, potential for interspecific competition; (2) aquatic snakes that form communities in temperate regions generally partition the food type available and exhibit a broad similarity in habitat use with subtle differences in microhabitat use; (3) terrestrial snake communities in temperate regions are very variable in terms of their coexistence dynamics and show no evidence of generalised patterns; (4) sympatric viperids in Europe, North America and, most interestingly, tropical Asia partition the available habitat but not the prey resource; (5) competition is much stronger in tropical snake communities, and the intensity of this process fluctuates throughout the year being most intense during periods of low food availability; (6) in general, tropical snakes partition the food resource (prey type and/or prey size), but when this resource is not partitioned competitive exclusion can occur.

Prey resource availability is a fundamental variable for all snake communities; this is clearly documented by studies on terrestrial snakes in Australia where, due to a relative scarcity of prey availability in the field, sympatry among species is much rarer than in other continents. I conclude that, although there are several notable exceptions, Toft's main conclusion is still supported by empirical evidence. However, I disagree with Toft's conclusion that most snakes are food specialists, and I contend that interspecific competition is important in structuring many (if not most) of the snake communities around the world.

https://www.jstor.org/stable/info/40234866

Resource partitioning and interspecific competition in snakes: the search for general geographical and guild patterns

Quantifying diet is essential for understanding the functional role of species with regard to energy processing, transfer, and storage within ecosystems. Recently, variance structure in the stable isotope composition of consumer tissues has been touted as a robust tool for quantifying trophic niche width, a task that has previously proven difficult due to bias in direct dietary analyses and difficulties in integrating diet composition over time. We used carbon and nitrogen stable isotope analyses to examine trophic niche width of two sympatric aquatic snakes, banded watersnakes Nerodia fasciata and black swamp snakes Seminatrix pygaea inhabiting an isolated wetland where seasonal migrations of amphibian prey cause dramatic shifts in resource availability. Specifically, we characterized snake and prey isotope compositions through time, space, and ontogeny and examined isotope values in relation to prey availability and snake diets assessed by gut content analysis. We determined that prey cluster into functional groups based on similarity of isotopic composition and seasonal availability. Isotope variance structure indicated that the trophic niche width of the banded watersnake was broader (more generalist) than that of the black swamp snake. Banded watersnakes also exhibited seasonal variation in isotope composition, suggesting seasonal diet shifts that track amphibian prey availability. Conversely, black swamp snakes exhibited little seasonal variation but displayed strong ontogenetic shifts in carbon and nitrogen isotope composition that closely paralleled ontogenetic shifts in their primary prey, paedomorphic mole salamanders Ambystoma talpoideum. Although niche dimensions are often treated as static, our results demonstrate that seasonal shifts in niche dimensions can lead to changes in niche overlap between sympatric species. Such short-term fluctuations in niche overlap can influence competitive interactions and consequently the composition and dynamics of communities and ecosystems.

Seasonal variation in terrestrial resource subsidies influences trophic niche width and overlap in two aquatic snake species: a stable isotope approach

We present an ethogram for golden-backed uakaris (Cacajao melanocephalus), based on observations in the field and on a captive individual. We also provide additional observations on the ecology of the wild animals. We studied 3 free-living groups (maximum counts of 5, 15, and 26 individuals) during two wet-seasons (March–July 2007 and January–June 2008) in the flooded igapo forest of Jau National Park, Amazonas, Brazil. The groups lived in close proximity but never mixed, because river channels separated them. Groups showed fission-fusion behavior, subgroup sizes varied within groups, and we observed 13 different subgroup compositions. The areas used by the groups were ca. 0.82, 2.35, and 2.45 km2. We defined a total of 9 behavioral categories. In the wild, the amount of time allocated to the behaviors traveling and foraging/feeding differed between months, but we found no difference in the amount of time devoted to behavioral categories across 3 periods of the day, possibly as a result of the patchy and unpredictable distribution or availability of food patches. Further, the activity patterns varied among groups, perhaps as a reflection of the different group sizes and compositions and different range sizes. We recorded 34 feeding items for wild golden-backed uakaris between January and June 2008, mostly fruits and seeds. No significant variation in the number of different food types occurred across months. We recorded 6 primate species in the study areas. The uakaris neither mobbed nor fled from any other primate species, with the exception of white-fronted capuchin monkeys (Cebus albifrons). In addition, golden-backed uakaris sometimes fled when seeing giant otters (Pteroneura brasiliensis), perhaps indicating that the otters are potential predators of the monkeys. The ethogram and behavioral data provided here increase knowledge of the behavioral ecology of the elusive golden-backed uakari, and will facilitate future comparative studies.

Ethogram and Natural History of Golden-backed Uakaris (Cacajao melanocephalus)

Nigeria in Maps

Agricultural landscapes have become increasingly intensively managed resulting in population declines across a broad range of taxa, including insectivores such as the hedgehog (Erinaceus europaeus). Hedgehog declines have also been attributed to an increase in the abundance of badgers (Meles meles), an intra-guild predator. The status of hedgehogs across the rural landscape at large spatial scales is, however, unknown. In this study, we used footprint tracking tunnels to conduct the first national survey of rural hedgehog populations in England and Wales. Single and two-species occupancy modelling was used to quantify hedgehog occupancy in relation to habitat and predator covariates. Hedgehog occupancy was low (22% nationally), and significantly negatively related to badger sett density and positively related to the built environment. Hedgehogs were also absent from 71% of sites that had no badger setts, indicating that large areas of the rural landscape are not occupied by hedgehogs. Our results provide the first field based national survey of hedgehogs, providing a robust baseline for future monitoring. Furthermore, the combined effects of increasing badger abundance and intensive agriculture may have provided a perfect storm for hedgehogs in rural Britain, leading to worryingly low levels of occupancy over large spatial scales.

/pdf/reduced-occupancy-of-hedgehogs-erinaceus-europaeus-in-rural-32du24lgxh.pdf

Reduced occupancy of hedgehogs (Erinaceus europaeus) in rural England and Wales: The influence of habitat and an asymmetric intra-guild predator

Comparisons of sympatric reptile species were used to assess the variation in niche overlap for food between potential competitors at different trophic levels. Omnivorous tortoises and carnivorous vipers inhabiting the rain- forest region of West Africa were used as study models. Food niche overlap between species increased with habitat alteration in both the independent study systems: tortoises (Kinixys homeana and Kinixys erosa) and vipers (Bitis gabonica and Bitis nasicornis) showed lower values of Pianka's niche overlap index in the pristine habitat than in the altered habitat, and these differences in overlap values did not depend on chance after Monte Carlo simulations. There were higher inter-habitat food niche overlaps within-species than between-species. Permutation tests (assessed after 5000 iterations) revealed that, for both study systems, the P-values became significantly smaller with fewer resource states, thus showing the niche overlap between species really increases after habitat alteration. The observed increases in food niche overlap between species accomplished with rain-forest habitat degradation in turn may be predicted to have cumulative effects on reducing the level of forest biodiversity.

Food niche overlap between sympatric potential competitors increases with habitat alteration at different trophic levels in rain-forest reptiles (omnivorous tortoises and carnivorous vipers)

In this paper, the ecological relationships and the resource partitioning patterns in a two-species system of sympatric aquatic snakes (Grayia smythii and Afronatrix anoscopus) from a riverine forest area in southern Nigeria, West Africa, were tested. The monthly availability of their food resources in the field, and the monthly variation in the feeding relationships between these snakes and their preys, were also studied. Food items of 1245 snakes, i.e. 554 Grayia smythii, and 691 Afronatrix anoscopus, were examined. The mean body length of Grayia smythii was significantly larger than that of Afronatrix anoscopus in any interspecific comparison, i.e. males versus males, females versus females, and females versus males; however, in both species the females attained significantly larger body sizes than the males. 676 prey items were obtained from the stomachs of Afronatrix anoscopus, and 390 from those of Grayia smythii. Both species of snake exhibited an increased activity in the open during the wet months, and this increased activity was positively correlated to the higher abundance of prey during the wet season. In both species the diet consisted of a great variety of different amphibian and fish species. Direct interference competition was not observed. The month-by-month dietary patterns exhibited by the two snake species were similar. Mean prey size was significantly larger in the larger species, and the difference in prey size between the two snake species increased during the dry season, i.e. during the period of reduced prey availability. The monthly availability of the three main food types for these snakes varied, and were significantly more abundant during the wet season (April–September). A positive relationship between prey availability and prey use by snakes strongly suggests that the two snake species are predatory generalists, utilizing prey in relation to their abundance in the field.

Interspecific relationships between two species of sympatric Afrotropical water snake in relation to a seasonally fluctuating food resource

The presence and density of two sympatric, large-sized vipers (the Gaboon viper, Bitis gabonica and the nose-horned viper, Bitis nasicornis ) were studied along several transects, during both dry and wet seasons, and at different times of day, in southern Nigeria (West Africa). Three habitat types were found along the various transects (mature rain forest (MF), secondary rain forest (SF), swamp forest (SW)). The detection probabilities for these vipers were modelled with a set of competing models, and the various models were ordered by Akaike Information Criterion procedures. Two classes of models were used: the single-season model, and the multi-species model. The best models (single-season model) suggested that: for the Gaboon viper, habitat types SF and SW were particularly important in detecting this species, especially during the rainy season at 08h00–16h00. For nose-horned vipers, the best models had SW and MF as site-covariates. Application of the multi-species model revealed that there were different detection functions if both species are present at a site, with a ‘negative’ interaction of occupancy between the species. Females and males were similarly detectable in a logistic regression model, but feeding status and pregnancy slightly increased detection probability in a logistic regression model. Viper density was modelled by a DISTANCE sampling procedure. The density of one species tended to be inversely correlated to the density of the other, suggesting that (1) the rain-forest environment does not support abundant populations of both vipers when sympatric, and (2) the two Bitis species subtly partition the habitat resources.

Site occupancy and density of sympatric Gaboon viper ( Bitis gabonica ) and nose-horned viper ( Bitis nasicornis )

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Food niche overlap between sympatric potential competitors increases with habitat alteration at different trophic levels in rain-forest reptiles (omnivorous tortoises and carnivorous vipers)

Interspecific relationships between two species of sympatric Afrotropical water snake in relation to a seasonally fluctuating food resource

Site occupancy and density of sympatric Gaboon viper ( Bitis gabonica ) and nose-horned viper ( Bitis nasicornis )