Article

Different season, different strategies: Feeding ecology of two syntopic forest-dwelling salamanders

Authors:
  • CESBIN Srl
  • Agenzia Regionale per la Protezione dell'Ambiente Ligure
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Abstract

Abstract: Information on individual trophic specialization may be relevant to better understand the ecological adaptation of populations to their environment and the evolution of their realized trophic niche. In this study, we analysed the trophic specialization at the individual level in a population of the plethodontid Northwest Italian Cave Salamander (Speleomantes strinatii (Aellen, 1958)), a terrestrial generalist predator. Salamanders were sampled in northwestern Italy on the forest floor in autumn (n = 49) and spring (n = 47) along with their available prey. In autumn, when trophic resources showed a twofold reduction in abundance, the population trophic niche width (TNW = 2.58) was significantly broader than during spring (TNW = 2.25), and in both seasons, individual specialization (IS) was significantly higher than expected by chance (P = 0.001). There were no sexual or ontogenetic differences in IS within each season, but IS in autumn was significantly higher than in spring (IS = 0.34 and IS = 0.41, respectively; P = 0.01). These findings are in accordance with the niche variation hypothesis, which predicts a positive relationship between TNW and IS. Therefore, while the population became more generalist, individual salamanders shifted towards a more specialized diet by adapting their feeding behaviour to changes in prey availability.

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... Prior to this study, Salamandrina was known to behave as a specialist predator on Collembola, especially in Autumn 10,11 , while during the transition from Autumn to Spring a shift towards a more generalist foraging strategy was described 10 . However in the present study, based on a larger sample size, Salamandrina behaves as a specialist predator in both seasons. ...
... The slightly contrasting results we obtained from available data 10 may only be apparent. In the other studied site 10 Salamandrina shares the environment with another terrestrial salamander, the plethodontid Speleomantes strinatii. As reported by those authors 10 the dietary pattern showed in their study population, contrasting the Optimal Foraging Theory, may be strongly influenced by the syntopy of two (or more) species that have similar ecological and trophic requirements and niche partitioning may play a key role in their long-term coexistence [13][14][15] . ...
... In order to investigate prey use and selection we collected samples of potential invertebrate prey at the study site in two seasons: Autumn (25-29 October 2012) and Spring (8)(9)(10)(11)(12) May 2013, at the end of spawning). To obtain representative data of prey availability we used three different sampling methods. ...
Article
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Specialization is typically inferred at population and species level but in the last decade many authors highlighted this trait at the individual level, finding that generalist populations can be composed by both generalist and specialist individual. Despite hundreds of reported cases of individual specialization there is a complete lack of information on inter-individual diet variation in specialist species. We studied the diet of the Italian endemic Spectacled Salamander (Salamandrina perspicillata), in a temperate forest ecosystem, to disclose the realised trophic niche, prey selection strategy in function of phenotypic variation and inter-individual diet variation. Our results showed that Salamandrina is highly specialized on Collembola and the more specialized individuals are the better performing ones. Analyses of inter-individual diet variation showed that a subset of animals exhibited a broader trophic niche, adopting different foraging strategies. Our findings reflects the optimal foraging theory both at population and individual level, since animals in better physiological conditions are able to exploit the most profitable prey, suggesting that the two coexisting strategies are not equivalent. At last this species, feeding on decomposers of litter detritus, could play a key role determining litter retention rate, nutrient cycle and carbon sequestration.
... Past works have highlighted ecological insights about how ontogenetic shifts and seasonal patterns in diet can promote coexistence among syntopic species in amphibian assemblages (Fasola and Canova, 1992;Schellekens et al., 2010, Kopecký et al., 2012Salvidio et al., 2012). Recently, several authors have taken into account ontogenetic dietary shifts and individual dietary variation to improve the awareness of food resource partitioning among amphibians (Schriever and William, 2013;Salvidio et al., 2015). ...
... Diets of salamanders usually vary across the seasons (Guerrero et al., 1990;Fasola and Canova, 1992;Salvidio et al., 2012;Vignoli et al., 2007a). We show that the primary dietary components (Diptera and Plecoptera) and individual dietary specialization tend to remain across seasons, but a distinct prey size use over the seasons was found. ...
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Trophic interactions are important factors structuring animal communities. Although food resource partitioning among salamanders living in syntopy has often been explored in freshwater systems, the potential influence of ontogenetic shifts and seasonal patterns on resource partitioning is little known. The aim of this study was to explore the seasonal patterns in food resource partitioning among developmental stages of two Urodela species (Calotriton asper and Salamandra salamandra) in a fishless stream. We tested the hypothesis that seasonal patterns in intra- and interspecific food resource partitioning may occur at two different levels: (i) diet composition and (ii) prey size. For the purpose of this study, food resource partitioning was approached at three scales: (i) ontogenetic (larva, immature and adult), (ii) seasonal (winter, spring, summer and autumn) and (iii) species. Interand intraspecific competition for food was measured as dietary overlap according to Schoener’s index. The overlap in diet among developmental stages and species was high. However, prey size and the contribution of terrestrial invertebrates to the diets changed among developmental stages of species. This suggests that competition for Rusfood may be alleviated by ontogenetic dietary shifts in prey size and terrestrial invertebrates use.
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"The merits of this work are many. A rigorous integration of phylogenetic hypotheses into studies of adaptation, adaptive radiation, and coevolution is absolutely necessary and can change dramatically our collective 'gestalt' about much in evolutionary biology. The authors advance and illustrate this thesis beautifully. The writing is often lucid, the examples are plentiful and diverse, and the juxtaposition of examples from different biological systems argues forcefully for the validity of the thesis. Many new insights are offered here, and the work is usually accessible to both the practiced phylogeneticist and the naive ecologist."—Joseph Travis, Florida State University "[Phylogeny, Ecology, and Behavior] presents its arguments forcefully and cogently, with ample . . .support. Brooks and McLennan conclude as they began, with the comment that evolution is a result, not a process, and that it is the result of an interaction of a variety of processes, environmental and historical. Evolutionary explanations must consider all these components, else they are incomplete. As Darwin's explanations of descent with modification integrated genealogical and ecological information, so must workers now incorporate historical and nonhistorical, and biological and nonbiological, processes in their evolutionary perspective."—Marvalee H. Wake, Bioscience
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"Measuring Biological Diversity assumes no specialist mathematical knowledge and includes worked examples and links to web-based software. It will be essential reading for all students, researchers, and managers who need to measure biological diversity."--BOOK JACKET.
Article
There were about 2950 salamanders per ha (1770 g/ha wet wt) in the Hubbard Brook Experimental Forest in New Hampshire. The terrestrial species, Plethodon cinereus, accounted for about 93.5% of the total biomass while the streamside species, Desmognathus fuscus, Eurycea bislineata and Gyrinophilus porphyriticus, accounted for the remaining 6.5%. Notophthalmus viridescens was present, but was rare and insignificant in the biomass calculations. The population size of salamanders at Hubbard Brook appears to be stable. The biomass of salamanders is about twice that of birds during the bird's peak (breeding) season and is about equal to the biomass of small mammals.
Article
The salamander Plethodon richmondi shenandoah is restricted to areas of talus on Hawksbill Mountain, Shenandoah National Park, Virginia, while P. cinereus inhabits the soil outside the talus. To test the hypothesis that the former species is excluded from the soil by the latter species, three enclosures were constructed in each of four habitats: (i) in talus consisting only of bare rocks, (ii) in talus where soil has infiltrated between but not under rocks, (iii) in isolated pockets of shallow soil within the rocky confines of the talus, and (iv) in deep soil outside the talus. In each habitat one enclosure contained an isolated sample of shenandoah, another an isolated sample of cinereus, and the third a mixture of the two species. The above testes indicated that: (i) neither species survived the extremely dry conditions of the bard rocks for more than a week, (ii) shenandoah expressed a higher survivorship than did cinereus in this relatively dry habitat, (iii) in the shallow soil shenandoah had lower survivorship in the presence of cinereus than it did in isolation, and (iv) in the deep soil shenandoah had poorer survivorship than cinereus in the mixed species enclosure and poorer survivorship than its isolated control. P. cinereus apparently inhibits the presence of shenandoah in areas of soil, while shenandoah survives better under rocky conditions. However, the talus is a suboptimal habitat for shenandoah, since its survivorship in isolated enclosures is significantly better in the presence of soil. It seems unlikely that salamanders other than cinereus could exclude shenandoah from the soil or that other animals exert a strong influence. Predation, diseases, and parasites also do not appear to exclude shenandoah from areas of deep soil. Competitive exclusion by cinereus has probably restricted the distribution of shenandoah to the suboptimal talus refugium.
Article
Many organisms exhibit ontogenetic shifts in their diet and habitat use, which often exert a large influence on the structure and expected dynamics of food webs and ecological communities. Nevertheless, reliable methods for detecting these niche shifts from consumption data are limited. In this study we present a new approach for the detection and analysis of ontogenetic diet shifts, based on complex network theory. As a case study, we apply these methods to the endangered, small fish Aphanius iberus. The stage-structured consumer population and its set of consumed prey are represented as an unweighted bipartite network. A statistical evaluation of the resulting network structure permits to uncover empirical patterns of ontogenetic diet shifts. We test for changes in niche breadth, as well as nestedness and diet modularity along ontogeny. These tests were carried out on the subnetworks describing consumption, positive electivity, and negative electivity on prey items. The statistical significance was established by means of null model analyses. Our analyses reveal a nested diet, along with a gradual decrease in diet breadth and a modular structure (i.e. abrupt changes) of elected preys along the ontogeny of A. iberus. The detection of network structure by means of the use of tools from complex network theory is shown to be a promising method for studying ontogenetic niche shifts.
Article
High-speed cinematography was used to compare the kinematics of terrestrial prey capture among postmetamorphic individuals of the family Salamandridae that have a) terrestrial (Salamandra, Salamandrina, and Tylototriton), b) amphibious (Pleurodeles, Cynops, and Paramesotriton), and c) aquatic (Pachytriton) lifestyles. Tongue projection was common during prey capture attempts in Salamandra, Salamandrina, and Tylototriton, occurred frequently in Cynops and Paramesotriton, but was absent during terrestrial feeding by Pachytriton. The gape cycle consisted of four distinct phases that were associated with discrete movements of the hyolingual apparatus. In all salamandrids that projected the tongue, the gape 1) increased rapidly while the tongue was elevated from the floor of the mouth in preparation for protraction, 2) was stabilized while the tongue was protracted, 3) increased to maximum during retraction and 4) decreased rapidly while the tongue transported the prey into the oral cavity. The absence of tongue projection during some (Cynops, Paramesotriton), or all prey capture attempts (Pachytriton) resulted in gape profiles distinctly different from those described above. Most notably, these profiles lacked the gape stabilization component characteristic of tongue projection. The maximum distance that the tongue was projected beyond the mandible approached 6% snout-vent length (SVL) in most salamandrids, but reached 20% SVL in Salamandrina. Several elements of the feeding system in Salamandrina are functionally related to the evolution of its specialized tongue projection apparatus. These include a gape cycle of relatively long duration (200 ms) and the absence of a head-body surge and of eye retraction during the strike.
Article
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.
Article
Complex interactions such as interference competition and predation, including intraguild predation, are now recognized as important components in animal community structure. At the lower end of a guild, weasels may be highly affected by other guild members due to small body size in relation to other predators. In 2000 and 2001, we radio-collared 24 ermines Mustela erminea and 25 long-tailed weasels M. frenata in 2 areas that differed in abundance of guild members. We tested the hypothesis that when faced with an increased density of other guild members, weasels would modify space and habitat use to reduce the risk of predation associated with encounters involving guild members. We predicted that weasels would increase use of specific habitats (such as refuges) to reduce encounter rates in the presence of a greater number of guild members. Because M. erminea is smaller than M. frenata and thus better able to take advantage of small rodent burrows as refuges from predators and as feeding grounds, we also predicted that M. frenata would show a stronger response to a higher abundance of guild members than M. erminea. Results were consistent with our predictions. Faced with an increased abundance of guild members, M. frenata showed increased habitat selectivity and reduced activity levels, which resulted in increased daily travel distances and increased home ranges. Mustela erminea responded to an increased abundance of guild members through reduced use of preferred habitat which M. frenata already occupied. The contrasting pattern of habitat selection observed between the 2 mustelid species suggested cascading effects, whereby large-predator pressure on M. frenata relaxed pressure of M. frenata on M. erminea. Our results draw attention to the likelihood that competitive intraguild interactions play a facilitating role in M. erminea–M. frenata coexistence.
Article
Lizards have been widely used as model organisms in community ecology over the past 30 years. I have reviewed more than 50 studies from the literature on the community ecology of lizards worldwide, with a focus on studies from 1990–2007. I determined if there is support for the hypotheses that many lizard communities 1) are non-randomly organized along the trophic niche dimension, and 2) partition the available food resource to minimize interspecific competition. I used data on number of prey items and percentage of prey volume, to calculate dietary overlap among species. I compared true datasets to randomly generate new datasets produced by 3×104 Monte Carlo permutations using two algorithms (RA2 and RA3). The great majority (more than 80%) of the communities were randomly organized along the trophic niche dimension using RA2 or RA3; the frequency of occurrence of detecting a non-random structure in lizard community dietary studies did not differ significantly between datasets based on either number of prey items or prey volume. Thus, lizard communities usually do not partition the trophic niche axis. Concerning the number of prey items, logistic regression models showed that the presence of a structure in the dataset did not depend on number of species, method employed to obtain dietary data, matrix size, or location (continent), but instead significantly depended on whether a community was tropical. Concerning prey volume, presence of a structure in the dataset also did not depend on number of species, method employed to obtain dietary data, and tropical vs non-tropical origin, but was marginally dependent on continent (South America was favoured for identifying a structure in the dataset) and matrix size. In general, a structure in the dataset was more often uncovered by using RA2 than RA3. Overall, I conclude that 1) lizard communities are unlikely to partition available food resources, and 2) as the method for identifying dietary items does not allow for accurate prey identification (at least at the genus level), there is a risk of obtaining false null results about community structure. This is because structure along the trophic niche axis is genuinely rare in lizards and also because inappropriate methods may erroneously lead to this conclusion. Therefore, conclusions from studies utilizing stomach dissection as a source of diet data (still common in the literature) should be cautiously considered for ethical reasons and because it is difficult to find any difference between stomach-dissection and fecal-pellet data when assessing the presence of a structure in datasets. My data also showed there are minor differences in the probability of detecting a structure in datasets using prey item number or prey volume data for lizards. The biases, both statistical and biological, associated with this meta-analysis are discussed.
Article
A modification of the graphical Costello method is proposed for the analysis of stomach contents data. The new method allows prey importance, feeding strategy and the interand intra-individual components of niche width to be explored using graphical presentation. The analysis is based on a two-dimensional representation of prey-specific abundance and frequency of occurrence of the different prey types in the diet. The paper describes the new method and the parameters therein, and also present some examples of the utilization of the method. The method may be particularly well-suited for the examination of predictions made from optimal foraging, competition and niche theories.
Article
Terrestrial salamanders have been hypothesized to play an important role in the regulation of invertebrate communities on the forest floor. Previous studies testing this hypothesis have used field enclosure or laboratory microcosm methods that may bias outcomes, and no studies have addressed the extent to which environmental heterogeneity affects the strength of salamander effects. In the present study, the effects of the red-backed salamander, Plethodon cinereus, on forest-floor invertebrates were examined in open field plots in which neither the movements of salamanders nor invertebrates were impeded. This 2-year study assessed the influence of the bottom-up effect of leaf-litter mass by comparing salamander effects in the spring when litter was abundant to autumn, just prior to leaf fall, when litter thickness was low. Plethodon cinereus was found to exert top-down effects on several mesofaunal taxa, but no effects on macrofauna were detected. Salamander effects varied among sampling periods. Leaf-litter mass and, to a lesser extent, litter moisture were the determinants of invertebrate density in spring and fall 2003, whereas salamanders were the single significant factor influencing mesofaunal density in spring 2004, when both leaf-litter mass and litter moisture were especially high. Leaf-litter mass and salamanders in combination influenced mesofaunal densities in fall 2004. The results suggest that the top-down effects of salamanders are strongest in spring when hydric conditions and availability of moisture-retaining cover enhance salamander foraging success, and when salamanders affect initial, rapid stages of mesofaunal population growth.
Article
Interpretations of habitat use in tropical frog assemblages have centred on resource partitioning and stressed the influence of interspecific interaction and climatic fluctuation on numbers of species using various habitats. We used audio strip transects and visual methods to determine the species composition, reproductive modes, and habitat occupancy patterns of the entire assemblage of frog species in 1900 hectares of primary forest north of Manaus in the central Amazon. We then compared taxon, reproductive mode, and habitat of species at six analogous lowland forest sites of similar species richness (five in the Amazon and one in Southeast Asia) to determine similarity of habitat use among sites and whether habitat is strongly associated with species» systematic positions. In all lowland Amazonian faunas, most species with aquatic development use pools, many species undergo some degree of terrestrial development, and few species are riparian or develop in streams. In contrast, about half the species in Southeast Asian assemblages are riparian and develop in streams, and few species develop terrestrially. Because reproductive mode and habitat associate strongly with taxon, patterns of habitat use observed at this regional scale are better explained by historical biogeography and differential rates of speciation than by proximal selection generated by contemporary environmental conditions. This study presents an inventory of frog species in a central Amazonian terre-firme forest and measurements of habitat availability and use by an entire assemblage of frogs throughout a large area (other portions of this study were published by Gascon, 1990, 1991; Zimmerman & Rodrigues, 1990; Zimmerman, 1991). We asked whether this local pattern of habitat occupancy differed from the regional Amazonian pattern and whether local species composition could be predicted from (sub)habitat composition. Viewing the assemblage at the local level did reveal species-(sub)habitat relationships masked at the broader regional level. About half the pool-breeders at the Manaus forest study sites would not use pools that could be flooded by a permanent stream; several species distinguished between permanent and temporary ponds; and some species occupied all available breeding habitat, whereas others occurred patchily. This pattern was maintained over four breeding seasons, and species composition could be predicted from (sub)habitat composition. Phylogeny was not a predictor of subhabitat occupancy. Perhaps species are phylogenetically constrained to develop in pool, stream, riparian, or terrestrial habitats, but contemporary selection governs their narrow distribution within these major habitat types. Finally, we asked whether anuran species richness in the central Amazon differs from that of the upper or lower Amazon. One genus, Eleutherodactylus, accounts for elevated species richness at upper Amazonian sites. Dry seasons in the central and lower Amazon are unlikely to restrict the spread of eleutherodactylines, which reproduce terrestrially. There are as many non-eleutherodactylines with terrestrial development at seasonal sites as there are at continually wet sites. Colonization history and the topography of central and lower Amazonia are more likely to limit eleutherodactyline richness.