Oikos

Strauss, Sharon

doi: 10.1111/j.1600-0706.2013.21046.xpmid: N/A

Ulrich, Werner; Gotelli, Nicholas J.

doi: 10.1111/j.1600-0706.2012.20325.xpmid: N/A

Synthesis The identification of distinctive patterns in species x site presence‐absence matrices is important for understanding meta‐community organisation. We compared the performance of a suite of null models and metrics that have been proposed to measure patterns of segregation, aggregation, nestedness, coherence, and species turnover. We found that any matrix with segregated species pairs can be re‐ordered to highlight aggregated pairs, indicating that these seemingly opposite patterns are closely related. Recently proposed classification schemes failed to correctly classify realistic matrices that included multiple co‐occurrence structures. We propose using a combination of metrics and decomposing matrix‐wide patterns into those of individual pairs of species and sites to pinpoint sources of non‐randomness. Null model analysis has been a popular tool for detecting pattern in binary presence–absence matrices, and previous tests have identified algorithms and metrics that have good statistical properties. However, the behavior of different metrics is often correlated, making it difficult to distinguish different patterns. We compared the performance of a suite of null models and metrics that have been proposed to measure patterns of segregation, aggregation, nestedness, coherence, and species turnover. We found that any matrix with segregated species pairs can be re‐ordered to highlight aggregated pairs. As a consequence, the same null model can identify a single matrix as being simultaneously aggregated, segregated or nested. These results cast doubt on previous conclusions of matrix‐wide species segregation based on the C‐score and the fixed‐fixed algorithm. Similarly, we found that recently proposed classification schemes based on patterns of coherence, nestedness, and segregation and aggregation cannot be uniquely distinguished using proposed metrics and null model algorithms. It may be necessary to use a combination of different metrics and to decompose matrix‐wide patterns into those of individual pairs of species or pairs of sites to pinpoint the sources of non‐randomness.

Pujol‐Buxó, Eudald; San Sebastián, Olatz; Garriga, Núria; Llorente, Gustavo A.

doi: 10.1111/j.1600-0706.2012.20617.xpmid: N/A

Synthesis Anti–predator phenotypic plasticity is expected to be one of the major ecological forces driving survival and rapid evolution of prey facing new predators. This implies that biological invasions embody a perfect case for studying the tradeoffs and evolution of phenotypic plasticity. Our manuscript reports on high prey–predator specificity in these reactions and an evolutionary dissociation between behavioral and morphological plasticity in anurans. Each species displayed a particular set of tradeoffs between plastic responses and their costs, but interestingly we also detected mild patterns in combinations using introduced predators. Given the speed at which these evolutionary changes become noticeable and their potential in reducing predation risk, the role of antipredator phenotypic plasticity is expected to be crucial for population dynamics during biological invasions. Although the purely ecological impacts of biological invasions have been well studied, a less thorough effort has been made in terms of their evolutionary ecology. Previous studies show that anti‐predator phenotypic plasticity may be one of the major ecological forces driving survival and rapid evolution of prey facing new predators. In turn, this means that biological invasions embody a perfect case for studying the tradeoffs and evolution of phenotypic plasticity per se. Here, we studied the plastic responses of native (Pelodytes punctatus) and invasive (Discoglossus pictus) anurans facing a native (dragonfly Anax sp.) and two invasive (fish Gambusia holbrooki and crayfish Procambarus clarkii) predators. Marked responses were reported against the native predator from both the native and the invasive anuran, but they both responded mildly to the exotic predators as well. Native P. punctatus displayed a morphological reaction to invasive P. clarkii after scarcely 30 years of coexistence with this predatory crayfish and responded behaviorally to the invasive fish G. holbrooki. Invasive D. pictus reacted behaviorally to all predators, but unexpectedly only reacted morphologically to native Anax sp. All these results support high prey–predator specificity in these reactions and an evolutionary dissociation between behavioral and morphological plasticity in anurans. Each species displayed a particular set of tradeoffs between plastic responses and their costs, which is probably due to differences in ecological niche and evolutionary history, but interestingly we usually detected unexpected patterns in combinations using introduced predators. This suggests that perhaps singular plastic shifts usually occur when tadpoles face recently introduced species. Given the speed in which these evolutionary changes become noticeable and their potential in avoiding predation risk, this study supports that phenotypic plasticity might play an important role in population dynamics during biological invasions.

Soininen, Eeva M.; Bråthen, Kari Anne; Jusdado, Juan German Herranz; Reidinger, Stefan; Hartley, Susan E.

doi: 10.1111/j.1600-0706.2012.20689.xpmid: N/A

Silica defences in grasses have recently been suggested to be a potential driver of vole population dynamics. However, the ability of grasses to induce silica in response to herbivory has not been tested in northern ecosystems where small rodents are important herbivores. We conducted a large‐scale field experiment in subarctic tundra using three river catchments differing in herbivore densities, and examined the effects of small rodent and/or reindeer exclusion on leaf silica levels in five grass species (Avenella flexuosa, Anthoxanthum nipponicum, Calamagrostis phragmitoides, Deschampsia cespitosa and Phleum alpinum). We also conducted a greenhouse experiment using three of these species (A. flexuosa, A. nipponicum and D. cespitosa) and Festuca ovina to determine whether intraspecific genotypic variation affects baseline silica concentrations and the capacity to induce silica in response to simulated grazing. Baseline leaf silica concentrations and silica induction varied with plant species in both experiments, with catchment in the field experiment and with genotype in the greenhouse experiment. These findings show that the allocation to silica defences in grasses is highly variable, and suggest that the combined effects of grazing pressure, plant species and intraspecific genotypic differences are likely to determine the circumstances under which silica induction may be an optimal defence strategy. A better understanding of the interplay between grazing and other factors influencing silica induction is necessary to interpret the role of silica in plant–herbivore interactions.

Fonderflick, Jocelyn; Besnard, Aurélien; Martin, Jean‐Louis

doi: 10.1111/j.1600-0706.2012.20308.xpmid: N/A

Human driven changes in land‐use have increased the need to understand how landscape structure affects species distribution. We studied how forest edges affected the distribution of birds in grasslands recently encroached by forest patches. We investigated how species’ biological traits influenced their response to vegetation change near forest edges. We censured birds along 300‐m line transects run into the open habitat perpendicularly to forest edges. We recorded habitat variables and landscape context along each transect and characterized edges and forest patches. We recorded 33 bird species in 153 transects for a total of 654 individuals. We analyzed species response to edges with generalized linear mixed models. Habitat preference was prevalent to explain species response to forest edges. The abundance of open‐habitat birds such as skylark Alauda arvensis decreased significantly in the vicinity of edges. This negative response extended within 150 m from the edge. The effect was disproportionately higher in open‐habitat species with high conservation concern. The abundance of species feeding or/and breeding in both forest and open habitat, such as woodlarks Lullula arborea, sharply increased near edges (positive edge response). Abundance of shrub and non‐shrub dependent species increased with distance to edge. The two species groups did no differ in abundance/distance to edge relationship. Intensity of species response to forest edges varied among transects in relation to transect vegetation characteristics. Edge length or aspect, diet and nest height had no direct effect. We discuss the possible role of variation in resources and nest predation risk to explain observed patterns.

Werner, Franziska Julie; Matthiessen, Birte

doi: 10.1111/j.1600-0706.2012.19952.xpmid: N/A

Understanding the ecological mechanisms that underlie species diversity decline in response to environmental change has become an urgent objective in current ecological research. Not only direct (lethal) effects on single species but also indirect effects altering biotic interactions between species within and across trophic levels comprise the driving force of ecosystem change. In an experimental marine benthic microalgae–grazer system we tested for indirect effects of moderate temperature change on algal diversity by manipulation of temperature, nutrient supply and grazer density. In our model system warming did not exert indirect effects on microalgal diversity via effects on resource competition. However, moderate warming strengthened consumer control and thereby indirectly affected algal community structure which ultimately resulted in decreased diversity. Only in low temperature and low nutrient regimes did the antagonizing mechanisms of bottom–up and top–down regulation establish a balancing effect on algal diversity within 29 days (corresponding to 15–29 algae generations). Effects of thermal habitat change did not appear before 9–18 algae generations, which points to the relevance of longer‐term experiments and ecological monitoring in order to separate transient biotic responses and subtle changes of community dynamics in consequence to global change.

Aragón, Pedro; Sánchez‐Fernández, David

doi: 10.1111/j.1600-0706.2012.20348.xpmid: N/A

There is a debate on whether an influence of biotic interactions on species distributions can be reflected at macro‐scale levels. Whereas the influence of biotic interactions on spatial arrangements is beginning to be studied at local scales, similar studies at macro‐scale levels are scarce. There is no example disentangling, from other similarities with related species, the influence of predator–prey interactions on species distributions at macro‐scale levels. In this study we aimed to disentangle predator–prey interactions from species distribution data following an experimental approach including a factorial design. As a case of study we selected the short‐toed eagle because of its known specialization on certain prey reptiles. We used presence–absence data at a 100 km2 spatial resolution to extract the explanatory capacity of different environmental predictors (five abiotic and two biotic predictors) on the short‐toed eagle species distribution in peninsular Spain. Abiotic predictors were relevant climatic and topographic variables, and relevant biotic predictors were prey richness and forest density. In addition to the short‐toed eagle, we also obtained the predictor's explanatory capacities for 1) species of the same family Accipitridae (as a reference), 2) for other birds of different families (as controls) and 3) artificial species with randomly selected presences (as null models). We run 650 models to test for similarities of the short‐toed eagle, controls and null models with reference species, assessed by regressions of explanatory capacities. We found higher similarities between the short‐toed eagle and other species of the family Accipitridae than for the other two groups. Once corrected by the family effect, our analyses revealed a signal of predator–prey interaction embedded in species distribution data. This result was corroborated with additional analyses testing for differences in the concordance between the distributions of different bird categories and the distributions of either prey or non‐prey species of the short‐toed eagle. Our analyses were useful to disentangle a signal of predator–prey interactions from species distribution data at a macro‐scale. This study highlights the importance of disentangling specific features from the variation shared with a given taxonomic level.

Locke, Sean A.; McLaughlin, J. Daniel; Marcogliese, David J.

doi: 10.1111/j.1600-0706.2012.20211.xpmid: N/A

Parasite communities tend to be dissimilar in hosts that are geographically, phylogenetically, ecologically and developmentally distant from one another. The decay of community similarity is a powerful and increasingly common method of studying parasite beta diversity, but most studies have examined only a single type of distance. Here, we evaluate distances based on the phylogeny, ecology, spatial proximity and size of hosts, as predictors of the similarity of parasite communities in individual hosts, host populations and host species. We surveyed parasites in six species of fish collected simultaneously from six localities in the St. Lawrence River, Canada, and species in a common group of larval parasites were discriminated using DNA sequences from barcode region of cytochrome c oxidase I. Distances based on the habitat use patterns of host species were good predictors of short‐term, ecological similarity of parasite communities, such as that operating at the scale of the individual host. The genetic distance between host species was associated with almost all types of similarity at all scales, particularly qualitative and phylogenetic similarity of parasite communities at the level of populations and meta‐populations of hosts. The trophic level, diet, spatial proximity and size of hosts were poor predictors of parasite community similarity. The increased taxonomic resolution provided by molecular data increased the explanatory power of regression models, and different factors were implicated when parasite species were distinguished with DNA barcodes than when larval parasites were lumped into morphospecies, as is commonly practiced.

Kardol, Paul; Souza, Lara; Classen, Aimée T.

doi: 10.1111/j.1600-0706.2012.20546.xpmid: N/A

Assembly history, including the order in which species arrive into a community, can influence long‐term community structure; however we know less about how timing of species arrival may alter assembly especially under varying resource conditions. To explore how the timing of species arrival interacts with resource availability to alter community assembly, we constructed experimental plant communities and manipulated the interval between plantings of groups of seedlings (0, 5, 10, 15 or 20 days) at low and high levels of soil nutrient supply. To see if community changes influenced ecosystem‐scale processes, we measured parameters across the plant–soil continuum (e.g. plant biomass and net ecosystem carbon dioxide exchange). We found that the timing of species arrival had a large impact on community assembly, but the size of the effect depended on soil fertility. As planting interval increased, plant communities diverged further from the control, but the divergence was stronger at high than at low nutrient supply. Our data suggest that at high nutrient supply, early‐planted species preempted light resources more quickly, thus preventing the successful establishment of later arriving species even at short planting intervals. Finally, we found that assembly related divergence in plant communities scaled to impact ecosystem‐level characteristics such as green leaf chemistry, but had little effect on total community biomass and net ecosystem exchange of CO2 and water vapor. Our data indicate that the effect of a stochastic factor, here the timing of species arrival on community composition, depends on the resource level under which the community assembles.

Sasal, Yamila; Morales, Juan Manuel

doi: 10.1111/j.1600-0706.2012.20669.xpmid: N/A

Despite the acknowledged importance of frugivores as seed dispersal agents we still lack a general understanding of the mechanisms by which these animals could shape plant populations and communities. We used a spatially explicit stochastic simulation to explore how frugivore movement decisions interact with landscape properties, thus affecting plant population dynamics through dispersal. The model simulated bird movement, foraging, seed deposition and plant recruitment. We assumed that plants lived only for one season and that recruitment was a function of local seed density. We also considered the effect of perches as non‐food landscape features. Our simulation experiments consisted in varying the parameters governing bird foraging decisions in relation to 1) how fruit abundance biased their movement, and 2) how the willingness to visit a plant or perch decreased with distance to current location. Simulated plant population dynamics was strongly influenced by bird behavior. The scale of foraging decisions had a much stronger effect on plant dynamics than biases due to fruit abundance. Birds tended to concentrate their activities in the center of the landscape where plants became more abundant, increasing local competition. The presence of perches reduced this tendency resulting in larger population sizes. The importance of perches highlights the fact that behaviors other than foraging can have a strong impact on the patterns of seed deposition and hence on plant population dynamics. Several recent studies have combined animal movement data with seed retention time in order to predict seed dispersal kernels. These studies usually emphasize the ecological implications of the scale and shape of such kernels. However, our simulation results reveal that movement directionality and the fact that birds moved mostly among plants and perches can have a major impact on plant population dynamics.