Mariano Devoto

Why nestedness in mutualistic networks

We investigate the relationship between the nested organization of mutualistic systems and their robustness against the extinction of species. We establish that a nested pattern of contacts is the best possible one as far as robustness is concerned, but only when the least linked species have the greater probability of becoming extinct. We introduce a coefficient that provides a quantitative measure of the robustness of a mutualistic system.

Understanding and planning ecological restoration of plant–pollinator networks

Theory developed from studying changes in the structure and function of communities during natural or managed succession can guide the restoration of particular communities. We constructed 30 quantitative plant-flower visitor networks along a managed successional gradient to identify the main drivers of change in network structure. We then applied two alternative restoration strategies in silico (restoring for functional complementarity or redundancy) to data from our early successional plots to examine whether different strategies affected the restoration trajectories. Changes in network structure were explained by a combination of age, tree density and variation in tree diameter, even when variance explained by undergrowth structure was accounted for first. A combination of field data, a network approach and numerical simulations helped to identify which species should be given restoration priority in the context of different restoration targets. This combined approach provides a powerful tool for directing management decisions, particularly when management seeks to restore or conserve ecosystem function.

The ‘night shift’: nocturnal pollen‐transport networks in a boreal pine forest

1. Diurnal plant–visitor networks are well studied, but the community‐level dimension of nocturnal visitation by insects has been largely overlooked.

Analysis and assembling of network structure in mutualistic systems.

It has been observed that mutualistic bipartite networks have a nested structure of interactions. In addition, the degree distributions associated with the two guilds involved in such networks (e.g., plants and pollinators or plants and seed dispersers) approximately follow a truncated power law (TPL). We show that nestedness and TPL distributions are intimately linked, and that any biological reasons for such truncation are superimposed to finite size effects. We further explore the internal organization of bipartite networks by developing a self-organizing network model (SNM) that reproduces empirical observations of pollination systems of widely different sizes. Since the only inputs to the SNM are numbers of plant and animal species, and their interactions (i.e., no data on local abundance of the interacting species are needed), we suggest that the well-known association between species frequency of interaction and species degree is a consequence rather than a cause, of the observed network structure.

Two classes of bipartite networks: nested biological and social systems.

Bipartite graphs have received some attention in the study of social networks and of biological mutualistic systems. A generalization of a previous model is presented, that evolves the topology of the graph in order to optimally account for a given contact preference rule between the two guilds of the network. As a result, social and biological graphs are classified as belonging to two clearly different classes. Projected graphs, linking the agents of only one guild, are obtained from the original bipartite graph. The corresponding evolution of its statistical properties is also studied. An example of a biological mutualistic network is analyzed in detail, and it is found that the model provides a very good fitting of all the main statistical features. The model also provides a proper qualitative description of the same features observed in social webs, suggesting the possible reasons underlying the difference in the organization of these two kinds of bipartite networks.

Reproductive ecology of a perennial outcrosser with a naturally dissected distribution

We surveyed four populations of contrasting size (two isolated and two large stands) of a woody outcrossing species, Discaria trinervis (Rhamnaceae), which has a naturally dissected distribution. Our main goal was to investigate the floral biology, breeding system and pollination mechanism of the species, which in turn may help to explain the mechanisms underlying the survival of the isolated populations. Discaria trinervis is both wind- and insect-pollinated, showing interactions with a large fraction of the available flower visitors at both stand sizes. In the larger populations, stigmatic loads were larger, more pollen tubes reached the ovules, and fruit and seed set were higher; however, the reproductive output in the smaller populations (over 2000 seeds per plant per year) seemed adequate for population persistence. Human disturbance (e.g. destruction of trees for wood extraction) may put these smaller populations at risk through loss of incompatibility alleles. The results suggest that plant species with naturally dissected distributions may provide clues about which reproductive mechanisms ensure survival under fragmentation.

Predicting the consequences of disperser extinction: richness matters the most when abundance is low

Summary 1. The ongoing biodiversity crisis entails the concomitant loss of species and the ecological services they provide. Global defaunation, and particularly the loss of frugivores, may negatively affect the seed dispersal of fleshy-fruited plant species, with predictable stronger impacts in simplified communities such as those on oceanic islands. However, logistical difficulties have hindered the experimental and theoretical need to disentangle the roles of species identity, richness (i.e. number of species) and abundance. Consequently, studies to date have focused exclusively on the loss of species richness leaving us largely ignorant regarding how species identity and abundance affect the loss of ecosystem functions. 2. Here, we applied a network approach to disentangle the effects of disperser abundance, richness and identity on the seed dispersal service provided by frugivores to the Galapagos plant community. 3. We found that both abundance and richness of the dispersers significantly affect the function of seed dispersal and that richness becomes increasingly important as disperser abundance declines. Extinction simulations revealed that the order of species loss has profound implications to the plant community. On one hand, abundant generalist dispersers like the Galapagos lizards, can mitigate the loss of specialized dispersers. On the other hand, specific threats affecting key dispersers can lead to the rapid collapse of the community-level dispersal services. 4. Our results suggest that the identity of the disperser species lost can have a large effect on the number of plant species dispersed, and generalist species are essential to the persistence of the community dispersal service. Both abundance and species richness of seed dispersers are key and synergistic drivers of the number of plant species dispersed. Consequently, the combined effect of their loss in degraded ecosystems can result in positive feedbacks, further accelerating the decline of the dispersal function. This article is protected by copyright. All rights reserved.

Effects of grazing disturbance on the reproduction of a perennial herb, Cypella herbertii (Lindl.) Herb. (Iridaceae)

A recent study in a natural grassland in Argentina revealed that grazing strongly reduced the available pollinators for a population of Cypella herbertii. Comparing grazed and fenced areas (exclosures) we tested the hypothesis that self-incompatibility would be selected against in a pollinator-poor environment leading to changes in breeding system. No differences in breeding system were found between populations from inside and outside exclosures. However, we detected: (i) C. herbertii clearly exhibits a mixed-mating system but has a remarkably low spontaneous fruit formation; (ii) this species though highly self-compatible expresses a significant decay at seed set stage when self-pollinated. We conclude that grazing impact on pollinators together with C. herbertii’s high dependence on pollinators should be considered when assessing the risk of extinction of this species.

Complexity of leaf miner–parasitoid food webs declines with canopy height in Patagonian beech forests

1. Consumer–resource species interactions form complex, dynamic networks, which may exhibit structural heterogeneity at various scales. This study set out to address whether host–parasitoid food web size and topology vary across forest canopy strata, and to what extent foliar resources and species abundances account for vertical patterns in network structure.

Diversity, distribution and floral specificity of tangle-veined flies (Diptera: Nemestrinidae) in north west Patagonia, Argentina

Los nemestrinidos constituyen una familia de Dipteros primitiva y de amplia distribucion. El genero Trichophthalma se encuentra en Australia y Sudamerica y es el unico en la familia con una distribucion disjunta tipicamente gondwanica. La ecologia y distribucion de la mayoria de las especies sudamericanas permanecen virtualmente desconocidas. Estudiamos la diversidad, distribucion y especificidad floral de las especies del genero Trichophthalma de los bosques templados del sur de Sudamerica en diez sitios ubicados a lo largo de un gradiente de precipitacion este-oeste (37-40°S) sobre la vertiente occidental de los Andes. Registramos nueve especies de Trichophthalma, las cuales mostraron una distribucion superpuesta a lo largo del gradiente y diferentes grados de especificidad floral. Tres de las especies fueron registradas por primera vez para Argentina, y tres fueron registradas por primera vez visitando especies de la flora local. Nuestros resultados muestran que mientras en el sur de Africa los nemestrinidos se involucran en interacciones de polinizacion altamente especializadas con especies de tubo floral largo, las especies de Trichophthalma de Patagonia comparten sus flores con una fauna diversa y poco especializada en la cual estan presentes varias especies de moscas, abejas y aves