Showing papers in "Oikos in 2006"
TL;DR: The standard similarity measure based on untransformed indices is shown to give misleading results, but transforming the indices or entropies to effective numbers of species produces a stable, easily interpreted, sensitive general similarity measure.
Abstract: Entropies such as the Shannon–Wiener and Gini–Simpson indices are not themselves diversities. Conversion of these to effective number of species is the key to a unified and intuitive interpretation of diversity. Effective numbers of species derived from standard diversity indices share a common set of intuitive mathematical properties and behave as one would expect of a diversity, while raw indices do not. Contrary to Keylock, the lack of concavity of effective numbers of species is irrelevant as long as they are used as transformations of concave alpha, beta, and gamma entropies. The practical importance of this transformation is demonstrated by applying it to a popular community similarity measure based on raw diversity indices or entropies. The standard similarity measure based on untransformed indices is shown to give misleading results, but transforming the indices or entropies to effective numbers of species produces a stable, easily interpreted, sensitive general similarity measure. General overlap measures derived from this transformed similarity measure yield the Jaccard index, Sorensen index, Horn index of overlap, and the Morisita–Horn index as special cases.
3,677 citations
TL;DR: The data suggested no relationship, or perhaps a weak negative relationship, between seed size and survival from seedling emergence through to adulthood, and it seems more likely that seed size evolves as part of a spectrum of life history traits, including plant size, plant longevity, juvenile survival rate and time to reproduction.
Abstract: We compiled information from the international literature to quantify the relationships between seed mass and survival through each of the hazards plants face between seed production and maturity. We found that small-seeded species were more abundant in the seed rain than large-seeded species. However, this numerical advantage was lost by seedling emergence. The disadvantage of small-seeded species probably results from size-selective post-dispersal seed predation, or the longer time small-seeded species spend in the soil before germination. Seedlings from large-seeded species have higher survival through a given amount of time as seedlings. However, this advantage seems to be countered by the greater time taken for large-seeded species to reach reproductive maturity: our data suggested no relationship, or perhaps a weak negative relationship, between seed size and survival from seedling emergence through to adulthood. A previous compilation showed that the inverse relationship between seed mass and the number of seeds produced per unit canopy area per year is countered by positive relationships between seed mass, plant size and plant longevity. Taken together, these data show that our old understanding of a species' seed mass as the result of a trade-off between producing a few large offspring, each with high survival probability, versus producing many small offspring each with a lower chance of successfully establishing was incomplete. It seems more likely that seed size evolves as part of a spectrum of life history traits, including plant size, plant longevity, juvenile survival rate and time to reproduction.
535 citations
TL;DR: The concept of habitat remains associated with descriptive/correlative analyses of the environments of organisms, while the niche concept is reserved for mechanistic analyses as mentioned in this paper, and it is necessary to understand the way an organism's morphology, physiology, and especially behaviour, determine the kinds of environment it experiences when living in a particular habitat, and also how those environmental conditions affect fitness (growth, survival and reproduction).
Abstract: The terms 'habitat', 'environment' and 'niche' are used inconsistently, and with some confusion, within the ecological literature on species distribution and abundance modelling. Here I suggest interrelated working definitions of these terms whereby the concept of habitat remains associated with descriptive/correlative analyses of the environments of organisms, while the niche concept is reserved for mechanistic analyses. To model the niche mechanistically, it is necessary to understand the way an organism's morphology, physiology, and especially behaviour, determine the kinds of environment it experiences when living in a particular habitat, and it is also necessary to understand how those environmental conditions affect fitness (growth, survival and reproduction). While distributions can potentially be predicted by modelling descriptions or correlations between organisms and habitat components, we must model an organism's niche mechanistically if we are to fully explain distribution limits. A mechanistic understanding of the niche is also critical when we want to predict an organism's distribution under novel circumstances such as a species introduction or climate change. © OIKOS.
482 citations
TL;DR: It is argued that differences among patterns represent outcomes of distinct evolutionary and ecological processes in these highly diversified assemblages of plant and animal species linked by interactions such as pollination, frugivory or herbivory.
Abstract: We present a comprehensive approach to detect pattern in assemblages of plant and animal species linked by interactions such as pollination, frugivory or herbivory. Simple structural models produce gradient, compartmented or nested patterns of interaction; intermediate patterns between a gradient and compartments are possible, and nesting within compartments produces a combined model. Interaction patterns can be visualized and analyzed either as matrices, as bipartite networks or as multivariate sets through correspondence analysis. We argue that differences among patterns represent outcomes of distinct evolutionary and ecological processes in these highly diversified assemblages. Instead of choosing one model a priori, assemblages should be probed for a suite of patterns. A plant-pollinator assemblage exemplifies a simple nested pattern, whereas a plant-herbivore assemblage illustrates a compound pattern with nested structures within compartments. Compartmentation should reflect coevolutionary histories and constraints, whereas differences in species abundance or dispersal may generate nestedness.
400 citations
TL;DR: It is suggested that connection of isolated habitat fragments may in some cases reduce, rather than enhance, landscape-level biodiversity, and implies that biodiversity at the regional level will be maximized if the local habitat patches vary widely in size and degree of connectivity.
Abstract: Contemporary ecological landscape planning is often based on the assumption that small isolated habitat patches sustain relatively few species. Here, we suggest that for shallow lakes and ponds, the opposite can be true for some groups of organisms. Fish communities tend to be poor or even absent in small isolated lakes. However, submerged vegetation is often more abundant in such waterbodies. As a consequence of low fish biomass and high vegetation abundance, the richness of aquatic birds, plants, amphibians and invertebrates is often relatively high in small, shallow, isolated lakes. Although the rarity of fish is in line with expectations from the ruling paradigms about effects of habitat fragmentation in landscape ecology, the relative richness of various other groups of organisms in small ponds is opposite to these expectations. The case of shallow lakes illustrates that incorporating ecological interactions is essential to understanding the potential effects of habitat fragmentation. Single-species meta-population approaches may be misleading if ecological interactions are strong. A meta-community approach that explicitly incorporates biotic interactions, also those involving different trophic levels, is needed. Our diagnosis suggests that connection of isolated habitat fragments may in some cases reduce, rather than enhance, landscape-level biodiversity, and implies that biodiversity at the regional level will be maximized if the local habitat patches vary widely in size and degree of connectivity.
385 citations
TL;DR: This study tested whether flower size parameters set constraints on the morphology of the potential nectar feeding visitors and thus determine the number of visitor species, and hypothesized that the stronger size constraints and the smaller flower abundance, the smaller the numberof visitor species will be.
Abstract: The number of interactions with flower visitor species differs considerably among insect pollinated plants. Knowing the causes for this variation is central to the conservation of single species as well as whole plant � /flower visitor communities. Species specific constraints on flower visitor numbers are seldom investigated at the community level. In this study we tested whether flower size parameters set constraints on the morphology of the potential nectar feeding visitors and thus determine the number of visitor species. We studied three possible constraints: the depth and width of tubular structures hiding the nectar (nectar holder depth and width) and the size of flower parts that visitors can land on (size of the alighting place). In addition we assess the role of flower abundance on this relationship. We hypothesized that the stronger size constraints and the smaller flower abundance, the smaller the number of visitor species will be. Our study of a Mediterranean plant � /flower visitor community revealed that nectar holder depth, nectar holder width and number of flowers explained 71% of the variation in the number of visitor species. The size of the alighting place did not restrict the body length of the visitors and was not related to visitor species number. In a second step of the analyses we calculated for each plant species the potential number of visitors by determining for each insect species of the local visitor pool whether it passed the morphological limits set by the plant. These potential numbers were highly correlated with the observed numbers (r 2 � /0.5, p B /0.001). For each plant species we tested whether the observed visitors were a random selection out of these potential visitors by comparing the mean of the observed and expected proboscis length distributions. For most plant species the observed mean was not significantly different from the random means. Our findings shed light on the way plant � /flower visitor networks are structured. Knowing the constraints on interaction patterns will be an important prerequisite to formulate realistic null models and understand patterns of resource partitioning as well as coevolutionary processes.
276 citations
TL;DR: The goal of this paper is to discuss "ecological fitting" as an important process in structuring plant -insect associations and to consider its implications for one widely studied aspect of these associations, host plant selection by herbivores.
Abstract: are generally highly host specific which leads to numerous questions regarding the factors that generate, maintain, and constrain these associations (Ehrlich and Raven 1964, Fox and Morrow 1981, Jermy 1984, Strong et al. 1984, Janzen 1988, 2003, Jaenike 1990, Farrell and Mitter 1993, Futuyma et al. 1993, Bernays and Chapman 1994, Menken and Roessingh 1998, Janz and Nylin 1998, Janz et al. 2001, 2006, Wiengartner et al. 2006). The goal of this paper is to discuss "ecological fitting" as an important process in structuring plant -insect associations and to consider its implications for one widely studied aspect of these associations, host plant selection by herbivores. Ecological fitting is a concept that was proposed by Janzen (1985) as the process whereby organisms colonize and persist in novel environments, use novel resources, and/or form novel associations with other species as a result of the suites of traits that they carry at the time they encounter the novel condition (e.g. a novel host plant). Ecological fitting represents an additional mechanism to cospeciation, coevolution and in situ evolution behind the generation and persistence of species associations, although it differs from the latter processes in that evolution by either member of an association is not a prerequisite for its formation or persistence. My motivation for this discussion is three-fold. First, ecological fitting has great explanatory power for the study of species associations and community assembly (Gill 1987, White and Stiles 1992, Chenuil and McKey 1996, Flowers and Janzen 1997, Yu and Davidson 1997, Brooks and McLennan 2002, Janzen 2003, Wilkinson 2004, Agosta and Janzen 2005, Brooks and Ferrao 2005), but in general is rarely cited or discussed. Second, the traditional view that plantinsect associations are examples of tightly coevolved systems (Ehrlich and Raven 1964) continues to be under close scrutiny in light of mounting phylogenetic, ecological, and macroecological data that suggest otherwise. Rather, the accumulated data demand a more pluralistic view of
264 citations
TL;DR: It is argued that focusing future research on the evolution of partner choice by myrmecochores and its effects on the overall plant fitness will be more fruitful than putting an emphasis on classifying the selective advantage to plants into distinct categories and test for their existence separately.
Abstract: Myrmecochory, or seed dispersal by ants, is a dispersal syndrome found among several thousand plant species occupying different ecosystems and geographical regions. Typically, ants benefit from consuming a lipid-rich appendage on the seed and in return provide seed dispersal service to the plant. Several hypotheses have been proposed to explain the selective advantage for plants resulting from myrmecochory, including directed dispersal, dispersal for distance and escape from seed predators. I contrast the evidence available in the literature for these hypotheses and distinguish the studies on the basis of ecosystem and plant growth forms. The predator-avoidance and the distance dispersal hypotheses were supported in most studies that addressed them, and the directed dispersal hypothesis was supported in about half of the studies that tested it. Multiple hypotheses were supported in most studies that tested more than one hypothesis, suggesting that the various selective advantages conferred from myrmecochory are seldom exclusive. I also review evidence for the hypothesis that plants have evolved adaptations both for selecting seed dispersers and for manipulating the behavior of those dispersers. Based on this evidence, I argue that focusing future research on the evolution of partner choice by myrmecochores and its effects on the overall plant fitness will be more fruitful than putting an emphasis on classifying the selective advantage to plants into distinct categories and test for their existence separately.
255 citations
TL;DR: It is proposed that the 'accumulation of local pathogens' hypothesis could serve as an alternative explanation for the enemy release hypothesis to be considered in further experimental studies on invasive plant species.
Abstract: Recent studies have concluded that release from native soil pathogens may explain invasion of exotic plant species. However, release from soil enemies does not explain all plant invasions. The invasion of Ammophila arenaria (marram grass or European beach grass) in California provides an illustrative example for which the enemy release hypothesis has been refuted. To explore the possible role of plant–soil community interactions in this invasion, we developed a mathematical model. First, we analyzed the role of plant–soil community interactions in the succession of A. arenaria in its native range (north-western Europe). Then, we used our model to explore for California how alternative plant–soil community interactions may generate the same effect as if A. arenaria were released from soil enemies. This analysis was carried out by construction of a 'recovery plane' that discriminates between plant competition and plant–soil community interactions. Our model shows that in California, the accumulation of local pathogens by A. arenaria could result in exclusion of native plant species. Moreover, this mechanism could trigger the rate and spatial pattern of invasive spread generally observed in nature. We propose that our 'accumulation of local pathogens' hypothesis could serve as an alternative explanation for the enemy release hypothesis to be considered in further experimental studies on invasive plant species.
242 citations
TL;DR: In this paper, the authors propose a process-based conceptual landscape model, which is derived from continuum theory and recognises the importance of space-related ecological variables alongside other factors, such as the availability of suitable food, shelter, and climatic conditions.
Abstract: Animal distribution patterns in human-modified landscapes are often examined from the basis of the “fragmentation model”, which recognises habitat patches located within an inhospitable matrix. The fragmentation model can establish correlations between landscape pattern and animal distribution patterns. However, it is limited in its ability to generate a process-based understanding of species distribution patterns. Here we propose a process-based conceptual landscape model. The “continuum model” is derived from continuum theory, and recognises the importance of space-related ecological variables alongside other factors, such as the availability of suitable food, shelter, and climatic conditions. The continuum model allows for gradual changes in these variables through space, and assumes species respond individualistically to their environment. We contrast the continuum model with the fragmentation model, and outline how it can be used to interpret and design empirical studies. While the fragmentation model may provide a satisfactory description of ecological patterns where many species are confined to human-defined “patches”, the continuum model can help to establish links between fundamental ecological processes and individualistic species distribution patterns. Conservation guidelines arising from the fragmentation model will emphasise the importance of large and well-connected pre-defined “habitat” patches. Conversely, the continuum model recognises potentially large discrepancies between different species’ ecological requirements. Conservation guidelines arising from the continuum model therefore will focus on habitat heterogeneity at multiple spatial scales to enhance the number of niches available to different species.
239 citations
TL;DR: Evidence is provided that large parts of the tundra biome can be in either of three relatively discrete vegetation states and that changes in reindeer/caribou density are responsible for sudden, predictable but often reversible state transitions.
Abstract: Range expansion and increasing densities of large herbivores are held responsible for large-scale habitat degradation in a wide range of natural and semi-natural ecosystems. Herbivore-driven ecosystem changes frequently represent predictable transitions from one vegetation state to another. Whether such predictable changes justify the value judgement 'habitat degradation' may be debatable as this strongly depends on individual perspective.
To further the debate on herbivore-driven habitat degradation, I apply the concept of alternative stable states to arctic tundra as a framework to capture predictable stepwise vegetation transitions in which the productivity and hence herbivore-carrying capacity increases with grazing pressure. Specifically, evidence is provided that large parts of the tundra biome can be in either of three relatively discrete vegetation states and that changes in reindeer/caribou density are responsible for sudden, predictable but often reversible state transitions. From this, it appears that the relatively rapidly emerging vegetation changes do not necessarily equate to habitat degradation, but in many cases reflect predictable vegetation change. Acknowledgement of the existence of predictable state transitions in tundra ecosystems may help to evaluate the observed radical vegetation changes occurring throughout the reindeer/caribou range.
TL;DR: Gower's formula and UPGMA clustering are suggested in this paper as a standard combination of techniques for calculating functional diversity (FD), and the effect of individual species on FD is best evaluated by species removals and subsequent comparisons of tree length values.
Abstract: Euclidean distance is commonly involved in calculating functional diversity (FD), for example, in measures based on dendrogram branch lengths. We point out that this function is inappropriate in many cases and that the choice of clustering method is more crucial than earlier thought. Gower's formula and UPGMA clustering are suggested here as a standard combination of techniques for calculating FD. The advantage of Gower's measure is its suitability to a mixture of scale types and its tolerance to missing values. Examples demonstrate that UPGMA clustering is more robust and has a better goodness of fit to dissimilarities than complete and single linkage classifications. In addition, we propose that the effect of individual species on FD is best evaluated by species removals and subsequent comparisons of tree length values. The influence of each functional trait is optimally judged by considering both dendrogram length and topological changes.
TL;DR: The mechanism that best explains these results is associational avoidance of saplings that grow near to unpalatable shrubs, which is necessary to introduce this neighbour effect in theoretical models and food-web approaches that analyse the plant-herbivore relationships.
Abstract: Spatial distribution of palatable and unpalatable plants can influence the foraging behaviour of herbivores, thereby changing plant-damage probabilities. Moreover, the immediate proximity to certain plants can benefit other plants that grow below them, where toxicity or spines act as a physical barrier or concealment against herbivores. This paper presents the results of a multi-scale experiment performed to test the effect of shrubs as protectors of tree saplings against herbivores and the mechanism involved in Mediterranean ecosystems. We performed a factorial design in two mountain ranges, similar in physiognomy and vegetation, planting saplings of a palatable tree, the maple (Acer opalus subsp. granatense), and an unpalatable tree, the black pine (Pinus nigra), under three different types of shrubs. We considered four experimental microhabitats: highly palatable shrub (Amelanchier ovalis), palatable but spiny shrub (Crataegus monogyna or Prunus ramburii), unpalatable spiny shrub (Berberis vulgaris subsp. australis) and control (gaps of bare soil without shrubs). Three main factors were found to determine the probability of sapling attack: sapling palatability, experimental microhabitat and plot. Palatable saplings (maples) were browsed much more than unpalatable ones (pines). The degree of protection provided by the shrub proved greater as its palatability decreased with respect to sapling palatability, the unpalatable spiny shrub being the safest microhabitat for palatable saplings and bare soil for unpalatable ones. The differences found in number of attacked saplings between plots may be attributable to differences in herbivore pressure. The community context in which interaction takes place, namely the characteristics of the neighbours and the intensity of herbivore pressure, are determining factors for understanding and predicting the damage undergone by a target plant species. The mechanism that best explains these results is associational avoidance of saplings that grow near to unpalatable shrubs. It is necessary to introduce this neighbour effect in theoretical models and food-web approaches that analyse the plant-herbivore relationships, since it can strongly determine not only the intensity of the interaction, but also the spatial distribution and diversity of the plant community.
TL;DR: This paper argues that the assumption that animals may have "prior opinions" is reasonable, and predicts how animals may combine previous experience with sampling information to make optimal decisions.
Abstract: Bayesian decision theory can be used to model animal behaviour. In this paper we give an overview of the theoretical concepts in such models. We also review the biological contexts in which Bayesian models have been applied, and outline some directions where future studies would be useful. Bayesian decision theory, when applied to animal behaviour, is based on the assumption that the individual has some sort of "prior opinion" of the possible states of the world. This may, for example, be a previously experienced distribution of qualities of food patches, or qualities of potential mates. The animal is then assumed to be able use sampling information to arrive at a "posterior opinion", concerning e.g. the quality of a given food patch, or the average qualities of mates in a year. A correctly formulated Bayesian model predicts how animals may combine previous experience with sampling information to make optimal decisions. We argue that the assumption that animals may have "prior opinions" is reasonable. Their priors may come from one or both of two sources: either from their own individual experience, gained while sampling the environment, or from an adaptation to the environment experienced by previous generations. This means that we should often expect to see "Bayesian-like" decision-making in nature.
TL;DR: The results of this analysis indicate that conditions under which a negative association between resistance and tolerance occurs and the evolution of multiple defensive strategies in plants is constrained, are much more restrictive than previously assumed.
Abstract: Resistance and tolerance represent two general strategies of plant defence against herbivores. Since resources available for allocation to defence are limited and resistance and tolerance are likely to serve the same functions for plants, the occurrence of trade offs between these two strategies has been assumed. We review the empirical evidence for tolerance-resistance tradeoffs by means of meta-analysis of genetic correlations between resistance and tolerance obtained from 31 ecological and agricultural studies published during 1980-2003 and conducted on 17 different plant species. The sign of the relationship between tolerance and resistance differed depending on the type of plants examined. Tolerance and resistance tended to be positively correlated in crops and negatively correlated in wild plants, but the mean correlation coefficients in both plant types were not significantly different from zero. The magnitude of correlations was affected neither by the tolerance measure (reduction in growth or in fitness in damaged plants) nor by the resistance measure used (inverse of damage, antibiosis, antixenosis, or specific resistance trait). In wild plants correlations between resistance and tolerance were significantly negative (r = -0.069) only in studies where resistance was assessed as a specific chemical or mechanical resistance trait, but this correlation is based only on two studies. No difference in the mean resistance-tolerance correlations was found between studies conducted in the field and in the greenhouse; in both cases mean correlations tended to be positive. The results of our analysis indicate that conditions under which a negative association between resistance and tolerance occurs and, thus, the evolution of multiple defensive strategies in plants is constrained, are much more restrictive than previously assumed. However, the currently available studies are still scarce and taxonomically skewed to allow a thorough analysis of sources of variation in resistance-tolerance relationship. Specifically, we need more studies examining the relationship between specific resistance and tolerance traits, studies on perennial plants and under different environmental conditions.
TL;DR: The results suggest that predation risk and prey availability influence dolphin habitat use at multiple spatial scales, but intrinsic habitat risk, and not just predator encounter rate, is important in shaping dolphin space use decisions.
Abstract: Prey availability and predation risk are important determinants of habitat use, but their importance may vary across spatial scales. In many marine systems, predator and prey distributions covary at large spatial scales, but do no coincide at small spatial scales. We investigated the influences of prey abundance and tiger shark (Galeocerdo cuvier) predation risk on Indian Ocean bottlenose dolphin (Tursiops aduncus) habitat use across multiple spatial scales, in Shark Bay, Western Australia. Dolphins were distributed between deep and shallow habitats and across microhabitats within patches approximately proportional to prey density when shark abundance was low. When shark abundance was high, foraging dolphins greatly reduced their use of dangerous, but productive, shallow patches relative to safer deep ones. Also, dolphins reduced their use of interior portions of shallow patches relative to their edges, which have higher predator density but lower intrinsic risk (i.e. a higher probability of escape in an encounter situation). These results suggest that predation risk and prey availability influence dolphin habitat use at multiple spatial scales, but intrinsic habitat risk, and not just predator encounter rate, is important in shaping dolphin space use decisions. Therefore, studies of habitat use at multiple spatial scales can benefit from integrating data on prey availability and the subcomponents of predation risk.
TL;DR: Signs of a tradeoff between fungal growth and tolerance towards bacteria are found, and it is shown that the antagonism between bacteria and fungi is connected to competition for substrate, but that this competition can be drastically altered if fungi are given an opportunity to establish before inoculation of bacteria.
Abstract: Bacteria and fungi often share a common substrate, and their spatial proximity in many environments has lead to either synergistic or antagonistic interactions. In this paper, the interaction of bacterial and fungal decomposers from an aquatic environment was studied. We found indications of a tradeoff between fungal growth and tolerance towards bacteria. Fungal strains growing best in absence of bacteria were most severely affected by bacterial presence, while those less suppressed during co-existence with bacteria had lower maximal growth rates in bacterial absence. Additionally, we show that the antagonism between bacteria and fungi is connected to competition for substrate, but that this competition can be drastically altered if fungi are given an opportunity to establish before inoculation of bacteria. Established fungi out-competed bacteria, and gained higher biomass than in simultaneously inoculated treatments with higher substrate concentrations.
TL;DR: It is argued that interspecific competition is important in structuring many (if not most) of the snake communities around the world, and Toft's main conclusion that most snakes are food specialists is disagreed with.
Abstract: 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.
TL;DR: In this article, a vegetation primary succession on a glacier foreland (Rutor glacier, Aosta, Italy) was used as a 'test bed' for the hypothesis that CSR classification can distinguish functional shifts during this vegetation process.
Abstract: CSR classification aims to apply CSR theory to large numbers of plants in situ, thereby allowing the investigation of communities within a functional context. However, it has only ever been applied to British vegetation, during the development of the technique, and has not yet been used to investigate specific vegetation processes. Here, a vegetation primary succession on a glacier foreland (Rutor glacier, Aosta, Italy) was used as a ‘test bed’ for the hypothesis that CSR classification can distinguish functional shifts during this vegetation process. Morpho-functional traits were used to calculate CSR coordinates for 45 species throughout the glacier foreland. General functional similarities between species were verified using principal components analysis (PCA). CSR classification demonstrated a functional shift from broadly ruderal pioneers towards stress-tolerance in late succession. PCA 1 correlated with S and R strategies, confirming this gradient. Till deposited at the retreating glacier terminus provides a substrate that can support faster growing species (with high foliar N contents), but is only tenable to those that can avoid physical disturbance via rapid phenological development (i.e. ruderals). Stress-tolerance and lower N contents in late succession suggest selection for efficient nutrient use. CSR classification demonstrated that competitive traits were ubiquitous but of much lesser importance than stress-tolerance or ruderalism (also correlating with PCA 2 and 3). The detailed visualization provided by CSR classification, combined with its mechanistic explanation of community change, demonstrate the promise of this methodology as a quantitative tool for comparative community ecology.
TL;DR: The hypothesis that aggressive ant bodyguards of plants deter pollinators is tested, and mechanisms by which Ferocactus wislizeni, an extrafloral nectary bearing cactus, limits conflicts between its pollinators and bodyguards are explored.
Abstract: Plant defenses against herbivores may be costly if they exclude mutualists. Here, I test the hypothesis that aggressive ant bodyguards of plants deter pollinators, and explore mechanisms by which Ferocactus wislizeni, an extrafloral nectary bearing cactus, limits conflicts between its pollinators and bodyguards. Flower visitation by ants and pollinating bees differed among plants tended by four different ant species. The ant species most rarely found in flowers showed the strongest aversion to F. wislizeni flower petals in laboratory assays, suggesting that those structures may include an ant-deterrent. Species-specific estimates of mean ant abundance within flowers and aggressiveness towards other arthropods were used to distinguish the relative threat of ant attack in flowers on plants tended by each ant species. Pollinator surveys in 2003 and 2004 demonstrated that bee visitation rates and the duration of flower occupation differed among plants with different ant associates, decreasing as the threat of ant attack increased. Flowers on plants tended by Solenopsis xyloni, the best ant bodyguard, were more dangerous than those on plants tended by three milder species, due to that ants' greater aggressiveness and abundance within flowers. These flowers were visited by pollinators least frequently and for less time per visit, and produced fruits with significantly lower total seed mass, fewer seeds, and lighter individual seeds, relative to fruits from similarly-sized plants tended by three other ant species. As a result, the best bodyguard may indirectly constrain plant reproduction in some settings. Conflicts between mutualistic guilds may be particularly common in generalized systems, where there is variation in partner quality and in the relative importance of the protection and pollination mutualisms.
TL;DR: The effects of plant secondary metabolites and nutrients dictate herbivore growth rates, which in turn influence their susceptibility to the third trophic level and the importance of defenses.
Abstract: A survey of 85 species of Lepidoptera feeding on 40 hosts on Barro Colorado Island, Panama showed that growth and defensive traits of caterpillars were correlated with the nutritional and defensive traits of their hosts. Growth rates were faster on young than mature leaves, reflecting the higher nitrogen and water content of the former. Growth was also positively correlated with leaf expansion rate, partially because of higher nitrogen and water contents of fast-expanding young leaves. Specialists grew faster than generalists, but both responded positively to nutritional quality. There was no effect of lepidopteran family on growth. In analyses where the effects of nitrogen and water were removed, the residuals for growth rate were greater for young than for mature leaves and were positively correlated with expansion rates of young leaves. This suggests that traits other than nutrition were also important. As young, expanding leaves cannot use toughness as a defense, one possible explanation for the differences in growth is differences in chemical defenses. Growth rate residuals for both specialists and generalists were higher for the more poorly defended fast-expanders, but the effect was greatest for generalists, perhaps because generalists were more sensitive to secondary metabolites. We predicted that slow growth for caterpillars would increase their risk to natural enemies and would select for higher defenses. Generalists had more defensive traits than specialists and were less preferred in feeding trials with ants. Similarly, species feeding on mature leaves were the most defended and those feeding on fast-expanding young leaves were the least defended and most preferred by ants. Thus the effects of plant secondary metabolites and nutrients dictate herbivore growth rates, which in turn influence their susceptibility to the third trophic level and the importance of defenses.
TL;DR: Using satellite data (NDVI), it is determined how plant productivity from birth to fall influences the following winter body mass of roe deer fawns and found contrasting results between sites, with a strong effect of plant productivity in spring (April-May), but no effect in the Trois Fontaines population.
Abstract: Animals in seasonal environments are affected by climate in very different ways depending on season and part of the climatic effects operates indirectly through the plants. Vegetation conditions in spring and summer are regarded as decisive for the reproductive success and the offspring's condition of large herbivores, but objective ways to determine key periods during the growing season have not been done often due to limitations in plant data. Using satellite data (NDVI), we determined how plant productivity from birth to fall influences the following winter body mass of roe deer fawns. We do this in two populations, the first inhabiting the low productive Chize´ reserve in south western France with an oceanic climate and the second from Trois Fontaines, a highly productive forest with continental climate in east France. The effect of plant productivity was similar for male and female fawn mass, as expected from the weak intensity of sexual selection in roe deer life history traits. We found contrasting results between sites, with a strong effect of plant productivity in spring (April-May) in the Chize´ population, but no effect in the Trois Fontaines population. The relatively low variability in winter fawn body mass could account for the absence of NDVI effects at Trois Fontaines. However, such results might also point to a limitation in the use of the NDVI, since the relationship between the canopy and the plant productivity at the ground level might be weak in the highly productive forest of Trois Fontaines.
TL;DR: The results suggest that the STR is as fundamental an ecological pattern as the SAR, and raise questions about the general processes underlying this pattern, and highlight the dynamic nature of most species assemblages.
Abstract: The species-time relationship (STR) describes how the species richness of a community increases with the time span over which the community is observed. This pattern has numerous implications for both theory and conservation in much the same way as the species-area relationship (SAR). However, the STR has received much less attention and to date only a handful of papers have been published on the pattern. Here we gather together 984 community time-series, representing 15 study areas and nine taxonomic groups, and evaluate their STRs in order to assess the generality of the STR, its consistency across ecosystems and taxonomic groups, its functional form, and its relationship to local species richness. In general, STRs were surprisingly similar across major taxonomic groups and ecosystem types. STRs tended to be well fit by both power and logarithmic functions, and power function exponents typically ranged between 0.2 and 0.4. Communities with high richness tended to have lower STR exponents, suggesting that factors increasing richness may simultaneously decrease turnover in ecological systems. Our results suggest that the STR is as fundamental an ecological pattern as the SAR, and raise questions about the general processes underlying this pattern. They also highlight the dynamic nature of most species assemblages, and the need to incorporate time scale in both basic and applied research on species richness patterns.
TL;DR: Testing the densities of juvenile Atlantic salmon at two life-history stages in the wild demonstrates that the way density-dependent effects are partitioned amongst survival, growth and dispersal changes throughout ontogeny, and suggests a general paradigm for the way in which juvenile density-dependence is manifest in spatially structured populations of highly fecund organisms.
Abstract: The way in which density-dependent effects are partitioned amongst survival, growth and dispersal are key in determining the temporal and spatial dynamics of populations. Here we propose a mechanistic approach to understanding how the relative importance of these sources of density dependence can change over ontogeny through changes in dispersal abilities, energy stores and mortality risks. Whereas the potential for active dispersal typically increases over ontogeny as a function of body size, susceptibility to starvation and predation decreases. The joint effect of these mechanisms suggests a general model for the ontogenetic sequence of how density dependence is manifested, with density dependence early in ontogeny being primarily expressed as mortality on local spatial scales, whereas later stages respond to local density in terms of dispersal and potentially growth. Here we test this model by manipulating the densities of juvenile Atlantic salmon (Saimo salar L.) at two life-history stages in the wild. Density-dependent mortality during the early juvenile stage (i.e. fry at onset of exogenous feeding) was accompanied by no effects on body size and weak effects on dispersal. In contrast, dispersal of older juveniles (i.e. parr 2-3 months after onset of feeding) was strongly density-dependent, with more individuals emigrating from high-density release sites, and with no effect of initial density on mortality. This dispersal, however. appeared insufficient to produce an ideal free distribution within the study stream, as indicated by the effect of spatial variation in density on body size by the end of the first growth season. These results demonstrate that the way density-dependent effects are partitioned amongst survival, growth and dispersal changes throughout ontogeny. Furthermore, these changes occur in correlation with changes in individual mortality risks and dispersal abilities, and suggest a general paradigm for the way in which juvenile density-dependence is manifest in spatially structured populations of highly fecund organisms.
TL;DR: This study contributes to the quantification of relative species importance and provides an exact and a priori determination of a class of candidate keystone species that can inform applied and conservation ecology as well as theoretical concerns.
Abstract: The local extinction or large fluctuation in abundance of a species may seriously affect other species in the community. The effects spread through the community by direct and indirect interactions. The network perspective on ecology can help map the pathways of these effects, for food webs, the pathways of indirect trophic interactions. Indirect interactions typically decay in intensity as they spread. Therefore, there is a conceptual maximum range in topological space beyond which interactions have no effects, even though all species remain connected. Neither the local characteristics of species, nor the global characteristics of entire webs, suitably quantify this range. We therefore apply intermediate scale indices that reflect the limitations imposed by effect damping in networks. We present a complex analysis of the topological positional importance of species in the Chesapeake Bay web. This web is a carbon-flow network that represents trophic interactions. We present several different indices reflecting different properties and discuss which questions the different indices best answer. We look for the best indices for identifying the key players in ecosystem functioning. Our study contributes to the quantification of relative species importance and provides an exact and a priori determination of a class of candidate keystone species that can inform applied and conservation ecology as well as theoretical concerns.
TL;DR: According to their specific functional and numerical responses, each predator plays a key role at some point of the lemming cycle, but only the stoat has the potential to drive the leMming cycle.
Abstract: Gilg, O., Sittler, B., Sabard, B., Hurstel, A., Sane, R., Delattre, P. and Hanski, I. 2006. Functional and numerical responses of four lemming predators in high arctic Greenland. - Oikos 113: 193-216. The high-arctic tundra ecosystem has the world's simplest vertebrate predator-prey community, with only four predators preying upon one rodent species, the collared lemming {Dicrostonyx groenlandicus). We document the functional and numerical responses of all the four predators in NE Greenland. Using these data, we assess the impact of predation on the dynamics of the collared lemming with a 4 yr cycle and > 100-fold difference between maximum and minimum densities. All predator species feed mostly ( >90%) on lemmings when lemming density is > 1 ha~ \ but the shapes of the predators' responses vary greatly. The snowy owl (Nyctea scandiaca) is present and breeds only when lemming densities at snowmelt are >2 ha~ \ giving rise to a step-like numerical response. The long- tailed skua {Stercorarius longicaudus) has a type III functional response and shifts from alternate food (mainly berries and insects) to lemmings with increasing lemming density. The skua surpasses all the other predators in summer by its total response. The type III functional response of the Arctic fox (Alopex lagopus) starts to increase at much lower lemming densities than the responses of the avian predators, but it has only a weak numerical response. Finally, the stoat (Mustela erminea) is the most specialized predator and the only one with a clearly delayed numerical response. According to their specific functional and numerical responses, each predator plays a key role at some point of the lemming cycle, but only the stoat has the potential to drive the lemming cycle. Stoat predation is greatly reduced in the winter preceding the lemming peak, and it reaches a maximum in the winter preceding the lowest lemming summer density. Stoat predation appears to maintain low lemming densities for at least two successive years. Our study provides empirical support for the specialist predator hypothesis about small mammal population cycles
TL;DR: The results showed that species richness of butterflies and moths peaked in taller vegetation compared with vascular plants, corresponding to a lower disturbance intensity and increasing time since abandonment, which supports the application of regionally and temporally varying grazing intensities in grassland conservation management.
Abstract: Empirical studies have suggested that species richness of grassland insects usually decreases under grazing management. By contrast, grazing has been shown to increase the species density and richness of vascular plants, especially on productive soils. In order to test the suggested differences in response to management between plants and insects, we simultaneously studied species richness of vascular plants and their insect herbivores, butterflies and moths, in 68 semi-natural grasslands with varying grazing intensity and frequency in SW Finland. Species richness of plants and insects was for the first time related to a quantitative measure of disturbance intensity and successional age, mean vegetation height, by using generalized additive models (GAM). The effects of soil nutrients on vegetation height were accounted for by using phosphorus concentration as a productivity surrogate. The results showed that species richness of butterflies and moths peaked in taller vegetation compared with vascular plants, corresponding to a lower disturbance intensity and increasing time since abandonment. These patterns are discussed in the light of two hypotheses, the ‘‘structural diversity hypothesis’’ and ‘‘dynamic equilibrium model’’ of Huston, both suggesting a weaker disturbance tolerance of insects compared with plants. Butterflies and moths which are specialists in their larval host-plant use (monophagous and oligophagous species) preferred lower vegetation (higher disturbances) compared with generalists (polyphagous species), as predicted by Huston’s model. This difference indicates a stronger relationship with plant species richness for specialist than for generalist butterflies and moths. Our results support the application of regionally and temporally varying grazing intensities in grassland conservation management.
TL;DR: Increased feeding of gastropods at low P and/or N availability leads to depletion of periphyton biomass; hence compensatory feeding would shift the benthic herbivore community from a P or N limitation to a C limitation and thus have whole-ecosystem effects.
Abstract: Nitrogen (N) and phosphorus (P) are considered to be essential nutrients that control secondary production in various ecosystems; insufficient availability of N and P can limit herbivore growth. Here, data are presented from field samplings and from a laboratory experiment on the potential of primary producers low in P, N, or P and N to constrain growth of the freshwater gastropod Radix ovata. The filamentous green alga Ulothrix fimbriata was cultured under different nutrient regimes, resulting in algae with different C:N:P ratios. The pure algae were fed in high and low quantities to juvenile R. ovata. Low availability of N and especially P in the algae strongly constrained the biomass accrual of the herbivore. In accordance with theoretical predictions, these food quality differences were highly dependent on the food quantity. The snails’ growth rate was significantly related to their body C:P ratio, thereby supporting the growth rate hypothesis. R. ovata displayed a pronounced compensatory feeding response to low-nutrient food that could partly dampen but not fully compensate the food quality effects on snail growth. Increased feeding of gastropods at low P and/or N availability leads to depletion of periphyton biomass; hence compensatory feeding would shift the benthic herbivore community from a P or N limitation to a C limitation and thus have whole-ecosystem effects.
TL;DR: The most parsimonious explanation for such long-term changes in body weight is global warming, as predicted by Bergmann's rule, but greater site and species-specific effects on wing length suggest a less straightforward conclusion concerning Allen's rule.
Abstract: We tested the prediction that global warming has caused recent decreases in body weight (Bergmann's rule) and increases in wing length (Allen's rule) in 14 species of passerine birds at two localities in England: Wicken Fen (1968-2003) and Treswell Wood (1973-2003). Predicted long-term linear decreases in residual body weight occurred in four species: dunnocks (Wicken Fen), and great tits, blue tits and bullfinches (Treswell Wood). Non-linear decreases also occurred in reed warblers and blackcaps at Wicken Fen, which also had a surprising linear increase in residual body weight in blackbirds. Residual wing lengths increased linearly, as predicted, in six of seven species at Wicken Fen. Whereas there were non-linear long-term increases in wrens, dunnocks and blackbirds in Treswell Wood. Unexpected linear decreases also occurred in residual wing lengths in willow warblers (Wicken Fen), and blue tits, great tits and chaffinches (Treswell Wood). The most parsimonious explanation for such long-term changes in body weight is global warming, as predicted by Bergmann's rule. Greater site and species-specific effects on wing length (e.g. non-linear changes plus shorter wings in the woodland habitat) suggest a less straightforward conclusion concerning Allen's rule, probably because wing length involved variation in both bone and feather growth. Changes in residual body weights and wing lengths often differed between species and were sometimes non-linear, perhaps reflecting short-term modifications in selection pressures. Human-induced influences are discussed, such as avian predator population densities and land-use change. Short-term variation in temperature had little effect, but rainfall did explain the unusual increase in blackbird body weight, possibly as a result of improving food (earthworm) availability.
TL;DR: The high proportion of females that did not breed during two consecutive years, and the lower probability of being a successful breeder after a greater reproductive effort confirmed the existence of reproductive costs, especially during the second half of the lactation.
Abstract: Life history theory hypothesises that breeding events induce reproductive costs that may vary among individuals. However, the growing number of studies addressing this question are taxonomically biased, therefore impeding the generalisation of this hypothesis, especially with regard to marine top predators. This study investigated age-related survival and breeding performances in subantarctic fur seal (Arctocephalus tropicalis) females from Amsterdam Island, southern Indian Ocean. Using multistate capture–recapture models on data obtained from known-age tagged females over eight consecutive years, we tested for evidence of senescence, individual quality, and reproductive costs in terms of future survival and fecundity. Adult female yearly survival appeared high and constant throughout time. While a two age-class model was preferred in non-breeders, breeding females exhibited three age classes with a maximum survival for the prime-age class (7–12 years). Survival and reproductive probabilities decreased from 13 years onward, suggesting senescence in this population. Survival was lower for non-breeders than for breeders, among both prime-aged (0.938 vs 0.982) and older (0.676 vs 0.855) females. Furthermore, non-breeders exhibited higher probabilities of being non-breeders the following year than did breeders (0.555 vs 0.414). Such results suggest consistency in female breeding performance over years, supporting the hypothesis that non-breeding tend to occur among lower quality individuals rather than representing an alternative strategy to enhance residual reproductive value. However, the high proportion of females that did not breed during two consecutive years, and the lower probability of being a successful breeder after a greater reproductive effort confirmed the existence of reproductive costs, especially during the second half of the lactation. These results also suggest that younger age-classes included a higher proportion of lower quality individuals, which are likely to face higher costs of reproduction. Such hypotheses lead to consider the first breeding event as a filter generating a within-cohort selection process in females.