Mário Almeida-Neto

A straightforward computational approach for measuring nestedness using quantitative matrices

Nestedness has been one of the most reported patterns of species distribution in metacommunities as well as of species interactions in bipartite networks. We propose here a straightforward approach for quantifying nestedness using quantitative instead of presence–absence data. We named our estimator WNODF because it is a simple modification of the nestedness index called NODF. We also introduce the NODF-Program that calculates the above described nestedness metrics as well as metrics for idiosyncratic species and sites. Statistical inference is done through a null model approach, in which the user can choose among five null models commonly used for presence–absence matrices as well as three randomization algorithms for matrices that contain quantitative data. The program performs multiple analyses using many matrices. Finally, the NODF-Program provides four sorting options that, together with the null algorithms, cover a range of possibilities to test hypotheses on the possible mechanisms producing nested patterns. By using a set of model matrices, we showed that WNODF differentiates nested matrices with distinct structures and correctly identifies matrices with no nested pattern as having zero degree of nestedness.

A simple stochastic model for complex coextinctions in mutualistic networks: robustness decreases with connectance

Understanding and predicting species extinctions and coextinctions is a major goal of ecological research in the face of a biodiversity crisis. Typically, models based on network topology are used to simulate coextinctions in mutualistic networks. However, such topological models neglect two key biological features of species interactions: variation in the intrinsic dependence of species on the mutualism, and variation in the relative importance of each interacting partner. By incorporating both types of variation, we developed a stochastic coextinction model capable of simulating extinction cascades far more complex than those observed in previous topological models. Using a set of empirical mutualistic networks, we show that the traditional topological model may either underestimate or overestimate the number and likelihood of coextinctions, depending on the intrinsic dependence of species on the mutualism. More importantly, contrary to topological models, our stochastic model predicts extinction cascades to be more likely in highly connected mutualistic communities.

Integrating economic costs and biological traits into global conservation priorities for carnivores

Background Prioritization schemes usually highlight species-rich areas, where many species are at imminent risk of extinction. To be ecologically relevant these schemes should also include species biological traits into area-setting methods. Furthermore, in a world of limited funds for conservation, conservation action is constrained by land acquisition costs. Hence, including economic costs into conservation priorities can substantially improve their conservation cost-effectiveness. Methodology/Principal Findings We examined four global conservation scenarios for carnivores based on the joint mapping of economic costs and species biological traits. These scenarios identify the most cost-effective priority sets of ecoregions, indicating best investment opportunities for safeguarding every carnivore species, and also establish priority sets that can maximize species representation in areas harboring highly vulnerable species. We compared these results with a scenario that minimizes the total number of ecoregions required for conserving all species, irrespective of other factors. We found that cost-effective conservation investments should focus on 41 ecoregions highlighted in the scenario that consider simultaneously both ecoregion vulnerability and economic costs of land acquisition. Ecoregions included in priority sets under these criteria should yield best returns of investments since they harbor species with high extinction risk and have lower mean land cost. Conclusions/Significance Our study highlights ecoregions of particular importance for the conservation of the worlds carnivores defining global conservation priorities in analyses that encompass socioeconomic and life-history factors. We consider the identification of a comprehensive priority-set of areas as a first step towards an in-situ biodiversity maintenance strategy.

Flower-heads, herbivores, and their parasitoids: food web structure along a fertility gradient

Abstract.  1. The ways in which a soil fertility gradient affects three trophic level food webs defined by plants of the family Asteraceae, flower‐head herbivores, and their parasitoids was investigated. It was tested how the fertility gradient alters: (i) the abundance and richness of plants, herbivores, and their parasitoids, (ii) the herbivore–plant ratio, and (iii) the connectance of the plant–herbivore community matrix.

Diversidade e ocorrência de Asteraceae em cerrados de São Paulo

De abril a maio de 2000 a 2002 oito localidades com remanescentes de cerrados sensu stricto no estado de Sao Paulo foram amostradas para o levantamento das especies de asteraceas, uma das familias mais representativas da flora herbaceo-arbustiva nestas formacoes. Foram feitas 23 amostragens e cada area de estudo foi amostrada em media uma vez por ano durante o pico de floracao das plantas. Ao todo foram obtidas 399 amostras, nas quais foram reconhecidas 89 morfoespecies (74 foram identificadas como especies conhecidas). Quarenta por cento das especies foram registradas uma unica vez (unicatas), indicando um grande numero de especies raras. Apenas 10% das especies que ocorreram em mais de uma amostra foram obtidas de uma mesma area (sobreposicao espacial) ou de um mesmo ano de estudo (sobreposicao temporal). A riqueza de especies em cada area foi estimada por meio de transecoes e depois comparada a riqueza total observada em cada area de estudo, sendo esta na maioria das vezes mais alta que a estimada com base nas transecoes. A lista de especies obtida para os cerrados amostrados foi comparada a outras 24 listas publicadas para cerrados no Brasil. Embora a maioria das especies mais comuns tenha coincidido, oito especies (11% das especies identificadas) nao constam das listas publicadas. Concluimos que as areas de cerrado sensu stricto estudadas no estado de Sao Paulo encontram-se isoladas, com uma grande parte da flora herbaceo-arbustiva composta por varias especies raras e exclusivas. Diante deste quadro, sugerimos que a manutencao da biodiversidade de Asteraceae depende da conservacao de todo o conjunto de remanescentes de cerrado do estado de Sao Paulo.

Assemblages of endophagous insects on Asteraceae in São Paulo Cerrados

A survey of the endophagous insects fauna associated to Asteraceae capitula was carried out from 2000 to 2002 in eight cerrado sensu stricto sites located in the Brazilian state of Sdo Paulo. Sixty-four endophagous species of Diptera and Lepidoptera were recorded from 49 asteracean host plants. Approximately half of the species were obtained from a single locality, with a large proportion emerging from a single sample (unicates). Thirty percent of the species were singletons (i.e. only one individual was recorded). The large proportion of rare species suggests a high species turnover among localities. Lepidopteran species were recorded on more host species than dipterans, confirming their more polyphagous food habit, also observed in other Brazilian biomes and in Europe. We conclude that the studied cerrado localities, all within São Paulo State, are isolated with its invertebrate fauna composed of many rare and exclusive species. We suggest that the maintenance of Asteraceae biodiversity and their endophagous insects depend on the conservation of all cerrado remnants in the state.

Phytophagous insect fauna tracks host plant responses to exotic grass invasion

The high dependence of herbivorous insects on their host plants implies that plant invaders can affect these insects directly, by not providing a suitable habitat, or indirectly, by altering host plant availability. In this study, we sampled Asteraceae flower heads in cerrado remnants with varying levels of exotic grass invasion to evaluate whether invasive grasses have a direct effect on herbivore richness independent of the current disturbance level and host plant richness. By classifying herbivores according to the degree of host plant specialization, we also investigated whether invasive grasses reduce the uniqueness of the herbivorous assemblages. Herbivorous insect richness showed a unimodal relationship with invasive grass cover that was significantly explained only by way of the variation in host plant richness. The same result was found for polyphagous and oligophagous insects, but monophages showed a significant negative response to the intensity of the grass invasion that was independent of host plant richness. Our findings lend support to the hypothesis that the aggregate effect of invasive plants on herbivores tends to mirror the effects of invasive plants on host plants. In addition, exotic plants affect specialist insects differently from generalist insects; thus exotic plants affect not only the size but also the structural profile of herbivorous insect assemblages.

Invasive grasses and native Asteraceae in the Brazilian Cerrado

Anthropogenic disturbances frequently modify natural disturbance regimes and foster the invasion and spread of nonindigenous species. However, there is some dispute about whether disturbance events or invasive plants themselves are the major factors promoting the local extinction of native plant species. Here, we used a set of savanna remnants comprising a gradient of invasive grass cover to evaluate whether the species richness of Asteraceae, a major component of the Brazilian Cerrado, is affected by invasive grass cover, or alternatively, whether variation in richness can be directly ascribed to disturbance-related variables. Furthermore, we evaluate whether habitat-specialist Asteraceae differ from habitat generalist species in their responses to grass invasion. Abundance and species richness showed unimodal variation along the invasive grass gradient for both total Asteraceae and habitat-generalists. The cerrado-specialist species, however, showed no clear variation from low-to-intermediate levels of grass cover, but declined monotonically from intermediate-to-higher levels. Through a structural equation model, we found that only invasive grass cover had significant effects on both abundance and species density of Asteraceae. The effect of invasive grass cover was especially high on the cerrado-specialist species, whose proportion declined consistently with increasing invasive dominance. Our results support the prediction that invasive grasses reduce the floristic uniqueness of pristine vegetation physiognomies.

An integrated framework to improve the concept of resource specialisation

Resource specialisation, although a fundamental component of ecological theory, is employed in disparate ways. Most definitions derive from simple counts of resource species. We build on recent advances in ecophylogenetics and null model analysis to propose a concept of specialisation that comprises affinities among resources as well as their co-occurrence with consumers. In the distance-based specialisation index (DSI), specialisation is measured as relatedness (phylogenetic or otherwise) of resources, scaled by the null expectation of random use of locally available resources. Thus, specialists use significantly clustered sets of resources, whereas generalists use over-dispersed resources. Intermediate species are classed as indiscriminate consumers. The effectiveness of this approach was assessed with differentially restricted null models, applied to a data set of 168 herbivorous insect species and their hosts. Incorporation of plant relatedness and relative abundance greatly improved specialisation measures compared to taxon counts or simpler null models, which overestimate the fraction of specialists, a problem compounded by insufficient sampling effort. This framework disambiguates the concept of specialisation with an explicit measure applicable to any mode of affinity among resource classes, and is also linked to ecological and evolutionary processes. This will enable a more rigorous deployment of ecological specialisation in empirical and theoretical studies.

Insects on urban plants: contrasting the flower head feeding assemblages on native and exotic hosts

Exotic plant species very often comprise a large proportion of urban floras. Because herbivorous insects depend on the presence of suitable host plants to maintain their populations, it is imperative to elucidate the relative importance of native and exotic hosts to understand the response of herbivorous guilds to urbanization. By using a plant-herbivore system composed of Asteraceae hosts and flower-head endophagous insects, we investigated whether the diversity and composition of herbivorous insects differs between native and exotic host-plant species in an urban environment. Although we found only seven exotic Asteraceae among the 30 species recorded, the overall abundance of exotics was considerably greater than that of native host plants. Overall, the exotic host species supported a small subset of the herbivore assemblage found on the native ones. The number of herbivore species per host species was significantly higher among the native plants, but we did not find a difference in herbivore abundance. Moreover, the higher taxonomic composition of herbivores on exotic Asteraceae was reduced, being composed of only three genera and two families from a total of 16 genera and six families of herbivores. These results provide support for the idea that plants outside of their original geographic distribution have lower loads of enemies than phylogenetically related native species. Our findings indicate that native host plants in urban areas play a critical role in supporting the native herbivorous insect fauna.