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Mutualisms provide benefits to interacting species, but they also involve costs. If costs come to exceed benefits as population density or the frequency of encounters between species increases, the interaction will no longer be mutualistic. Thus curves that represent benefits and costs as functions of interaction frequency are important tools for predicting when a mutualism will tip over into antagonism. Currently, most of what we know about benefit and cost curves in pollination mutualisms comes from highly specialized pollinating seed-consumer mutualisms, such as the yucca moth--yucca interaction. There, benefits to female reproduction saturate as the number of visits to a flower increases (because the amount of pollen needed to fertilize all the flower's ovules is finite), but costs continue to increase (because pollinator offspring consume developing seeds), leading to a peak in seed production at an intermediate number of visits. But for most plant--pollinator mutualisms, costs to the plant are more subtle than consumption of seeds, and how such costs scale with interaction frequency remains largely unknown. Here, we present reasonable benefit and cost curves that are appropriate for typical pollinator--plant interactions, and we show how they can result in a wide diversity of relationships between net benefit (benefit minus cost) and interaction frequency. We then use maximum-likelihood methods to fit net-benefit curves to measures of female reproductive success for three typical pollination mutualisms from two continents, and for each system we chose the most parsimonious model using information-criterion statistics. We discuss the implications of the shape of the net-benefit curve for the ecology and evolution of plant--pollinator mutualisms, as well as the challenges that lie ahead for disentangling the underlying benefit and cost curves for typical pollination mutualisms. Key words: benefits and costs of mutualism; density dependence; interaction frequency; nectar robbery; pollinating seed--consumer mutualism.

Pollinators may be declining globally, a matter of concern because animal pollination is required by most of the world's plant species, including many crop plants. Human land use and the loss of native habitats is thought to be an important driver of decline for wild, native pollinators, yet the findings of published studies on this topic have never been quantitatively synthesized. Here we use meta-analysis to synthesize the literature on how bees, the most important group of pollinators, are affected by human disturbances such as habitat loss, grazing, logging, and agriculture. We obtained 130 effect sizes from 54 published studies recording bee abundance and/or species richness as a function of human disturbance. Both bee abundance and species richness were significantly, negatively affected by disturbance. However, the magnitude of the effects was not large. Furthermore, the only disturbance type showing a significant negative effect, habitat loss and fragmentation, was statistically significant only in systems where very little natural habitat remains. Therefore, it would be premature to draw conclusions about habitat loss having caused global pollinator decline without first assessing the extent to which the existing studies represent the status of global ecosystems. Future pollinator declines seem likely given forecasts of increasing land-use change. Key words: Apis mellifera; bee abundance; bee species richness; Bombus; ecosystem service; global change; habitat loss; land-use change, meta-analysis; pollination; pollinator; pollinator decline.

The structure of mutualistic networks is likely to result from the simultaneous influence of neutrality and the constraints imposed by complementarity in species phenotypes, phenologies, spatial distributions, phylogenetic relationships, and sampling artifacts. We develop a conceptual and methodological framework to evaluate the relative contributions of these potential determinants. Applying this approach to the analysis of a plant--pollinator network, we show that information on relative abundance and phenology suffices to predict several aggregate network properties (connectance, nestedness, interaction evenness, and interaction asymmetry). However, such information falls short of predicting the detailed network structure (the frequency of pairwise interactions), leaving a large amount of variation unexplained. Taken together, our results suggest that both relative species abundance and complementarity in spatiotemporal distribution contribute substantially to generate observed network patters, but that this information is by no means sufficient to predict the occurrence and frequency of pairwise interactions. Future studies could use our methodological framework to evaluate the generality of our findings in a representative sample of study systems with contrasting ecological conditions. Key words: forbidden links; Monte Desert, Mendoza, Argentina; neutrality; phenotypic complementarity; phylogenetic signal; plant--pollinator network; pollination; Villavicencio network.

The structure of ecological interaction networks is often interpreted as a product of meaningful ecological and evolutionary mechanisms that shape the degree of specialization in community associations. However, here we show that both unweighted network metrics (connectance, nestedness, and degree distribution) and weighted network metrics (interaction evenness, interaction strength asymmetry) are strongly constrained and biased by the number of observations. Rarely observed species are inevitably regarded as 'specialists,' irrespective of their actual associations, leading to biased estimates of specialization. Consequently, a skewed distribution of species observation records (such as the lognormal), combined with a relatively low sampling density typical for ecological data, already generates a 'nested' and poorly 'connected' network with 'asymmetric interaction strengths' when interactions are neutral. This is confirmed by null model simulations of bipartite networks, assuming that partners associate randomly in the absence of any specialization and any variation in the correspondence of biological traits between associated species (trait matching). Variation in the skewness of the frequency distribution fundamentally changes the outcome of network metrics. Therefore, interpretation of network metrics in terms of fundamental specialization and trait matching requires an appropriate control for such severe constraints imposed by information deficits. When using an alternative approach that controls for these effects, most natural networks of mutualistic or antagonistic systems show a significantly higher degree of reciprocal specialization (exclusiveness) than expected under neutral conditions. A higher exclusiveness is coherent with a tighter coevolution and suggests a lower ecological redundancy than implied by nested networks. Key words: abundance distribution; biological traits; connectance; degree distribution; ecological networks; interaction diversity; interaction strength; nestedness; null models; specialization.

To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.0030-1299.2008.16737.x Byline: Juan M. Morales, Diego P. Vazquez Abstract: The topology of plant-animal mutualistic networks has the potential to determine the ecological and evolutionary dynamics of interacting species. Many mechanisms have been proposed as explanations of observed network patterns; however, the fact that plant-animal interactions are inherently spatial has so far been ignored. Using a simulation model of frugivorous birds foraging in spatially explicit landscapes we evaluated how plant distribution and the scale of bird movement decisions influenced species interaction probabilities and the resulting network properties. Spatial aggregation and limited animal mobility restricted encounter probabilities, so that the distribution of animal visits per plant deviated strongly from the binomial distribution expected for a well-mixed system. Lack of mixing in turn resulted in a strong decrease in network connectance, a weak decrease in nestedness, stronger interactions, greater strength asymmetry and the unexpected presence/absence of some interactions. Our results suggest that spatial processes may contribute substantially to structure plant-animal mutualistic networks. Article History: Manuscript Accepted 10 April 2008 Article note: J. M. Morales (jmorales05@gmail.com), Laboratorio Ecotono, INIBIOMA, CRUB, Univ. National del Comahue, Quintral 1250, AR-8400 Bariloche, Rio Negro, Argentina. - D. P. Vazquez, Inst. Argentino de Investigaciones de las Zonas Aridas, CONICET, CC 507, AR-5500 Mendoza, Argentina.

To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.2007.0030-1299.15828.x Byline: Diego P. Vazquez, Carlos J. Melian, Neal M. Williams, Nico Bluthgen, Boris R. Krasnov, Robert Poulin Abstract: The strength of interactions among species in a network tends to be highly asymmetric. We evaluate the hypothesis that this asymmetry results from the distribution of abundance among species, so that species interactions occur randomly among individuals. We used a database on mutualistic and antagonistic bipartite quantitative interaction networks. We show that across all types of networks asymmetry was correlated with abundance, so that rare species were asymmetrically affected by their abundant partners, while pairs of interacting abundant species tended to exhibit more symmetric, reciprocally strong effects. A null model shows that abundance provides a sufficient explanation of the asymmetry structure in some networks, but suggests the role of additional factors in others. Although not universal, our hypothesis holds for a substantial fraction of networks analyzed here, and should be considered as a null model in all studies aimed at evaluating the ecological and evolutionary consequences of species interactions. Article History: Manuscript Accepted 2 March 2007 Article note: D. P. Vazquez (dvazquez@lab.cricyt.edu.ar), Inst. Argentino de Investigaciones de las Zonas Aridas, Centro Regional de Investigaciones Cienticas y Tecnologicas, CONICET, CC 507, AR-5500 Mendoza, Argentina. - C. J. Melian, National Center for Ecological Analysis and Synthesis Univ. of California, 735 State Street, Suite 300, Santa Barbara, CA 93101, USA. - N. M. Williams, Dept of Biology, Bryn Mawr College, Park Science Building, 101 N. Merion 10 Ave, Bryn Mawr, PA 19010, USA. - N. Bluthgen, Lehrstuhl Tierokologie & Tropenbiologie, Biozentrum, Univ. Wurzburg, Am Hubland, DE-97074 Wurzburg, Germany. - B. R. Krasnov, Mitrani Dept of Desert Ecology, Jacob Blaustein Inst. for Desert Research, Ben-Gurion Univ. of the Negev, Sede-Boqer Campus, IL-84990 Midreshet Ben-Gurion, Israel and, Ramon Science Center, PO Box 194, IL-80600 Mizpe Ramon, Israel. - R. Poulin, Dept of Zoology, Univ. of Otago, PO Box 56, Dunedin, New Zealand.

Plants engage in multiple, simultaneous interactions with other species; some (enemies) reduce and others (mutualists) enhance plant performance. Moreover, effects of different species may not be independent of one another; for example, enemies may compete, reducing their negative impact on a plant. The magnitudes of positive and negative effects, as well as the frequency of interactive effects and whether they tend to enhance or depress plant performance, have never been comprehensively assessed across the many published studies on plant--enemy and plant--mutualist interactions. We performed a meta-analysis of experiments in which two enemies, two mutualists, or an enemy and a mutualist were manipulated factorially. Specifically, we performed a factorial meta-analysis using the log response ratio. We found that the magnitude of (negative) enemy effects was greater than that of (positive) mutualist effects in isolation, but in the presence of other species, the two effects were of comparable magnitude. Hence studies evaluating single-species effects of mutualists may underestimate the true effects found in natural settings, where multiple interactions are the norm and indirect effects are possible. Enemies did not on average influence the effects on plant performance of other enemies, nor did mutualists influence the effects of mutualists. However, these averages mask significant and large, but positive or negative, interactions in individual studies. In contrast, mutualists ameliorated the negative effects of enemies in a manner that benefited plants; this overall effect was driven by interactions between pathogens and belowground mutualists (bacteria and mycorrhizal fungi). The high frequency of significant interactive effects suggests a widespread potential for diffuse rather than pairwise coevolutionary interactions between plants and their enemies and mutualists. Pollinators and mycorrhizal fungi enhanced plant performance more than did bacterial mutualists. In the greenhouse (but not the field), pathogens reduced plant performance more than did herbivores, pathogens were more damaging to herbaceous than to woody plants, and herbivores were more damaging to crop than to non-crop plants (suggesting evolutionary change in plants or herbivores following crop domestication). We discuss how observed differences in effect size might be confounded with methodological differences among studies. Key words: factorial experiment; Hedges' d; herbivore; interaction effect; log response ratio; meta-analysis; mutualist; natural enemy; pathogen; plant performance.

Herbivores can affect plants not only directly through browsing and trampling, but also indirectly through other species. For example, herbivores could affect the interaction between plants and their pollinators. Because plant population density may affect plant-pollinator interactions and plant reproductive success, we hypothesized that herbivores could affect pollination and plant reproduction indirectly by modifying plant population density. Unlike previous hypotheses, which concerned individual-level effects on vegetative and reproductive traits, our hypothesis focuses on population-level effects and involves a feedback mechanism. To test this hypothesis, we conducted field studies in the temperate forest of the southern Andes, where introduced ungulates are a major source of anthropogenic alteration. For 10 animal-pollinated understory plants, we compared population density, pollinator visitation, pollen deposition in stigmas, and reproduction in four pairs of grazed and ungrazed sites. We found evidence of indirect effects of ungulates on pollination and reproduction only for the herb Alstroemeria aurea (Alstroemeriaceae). The general lack of evidence for indirect effects on most of the species may result from resistance to cattle browsing and trampling, or low statistical power. For A. aurea, we present additional evidence from trampling and hand-pollination experiments, plus path analyses of the effect of density on pollination and reproduction showing that: (l) cattle decrease the absolute and relative population density of this species through trampling; (2) density, particularly relative density, affects pollen deposition on stigmas; and (3) conspecific pollen deposition affects reproduction. Thus, our results indicate that, by directly reducing the population density of A. aurea, cattle are indirectly affecting its reproduction. Key words: cattle; exotic species; grazing; herbivory; indirect effects; introduced ungulates; mutualism; path analysis; plant population density; plant reproduction; pollination; southern Andean temperate forest.

Recent studies have suggested that plant-pollinator interactions may be less specialized than previously thought. We contrasted patterns of specialization observed in five plant-pollinator interaction webs with predictions based on null models. In the five data sets, the observed number of extreme specialists and extreme generalists was significantly higher than the null expectation. This pattern was mostly due to a positive correlation between species frequency of interaction (f) and their estimated degree of generalization (s). After accounting for this association, the expected frequency distribution of degree of specialization generated by the null model closely matched the observed frequency distribution in the five data sets. A second null model, which explicitly incorporated the correlation between f and s, also generated expected frequency distributions of specialization that closely resemble those observed in the data sets. To make progress in understanding the distribution of degree of specialization in pollination systems it will be necessary not only to improve the quality of the data and to refine methods used to quantify specialization, but also tO answer the question of why more frequently interacting species appear to be more generalized. Key words: frequency of interaction; generalization; mutualism; null models; plant-pollinator interactions; pollination; sampling bias; specialization.

The study shows that ecological interaction networks depart from the 'scale-free' topologies observed in many other real world networks. The topology of 17 plant-animal mutualistic networks predicted by a simple null model that assumes that a species degree is a function of its frequency of interaction is compared.