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. 2

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.

1. Diurnal plant–visitor networks are well studied, but the community‐level dimension of nocturnal visitation by insects has been largely overlooked. 2. This study focused on the role of moths as pollen vectors in a boreal pine forest in Scotland. Light traps were used to sample moths in 20 plots in two consecutive years. The pollen on moths' bodies was identified and pollen grains counted. This information was used to build a nocturnal pollen‐transport network for each year. These are the first networks to characterise a nocturnal plant–visitor community. 3. A total of 4162 moths belonging to 103 species were captured; 25 moth species were found to carry pollen of 12 plant taxa. Adding nocturnal data to diurnal networks increased number of plant taxa, insect species, and unique interactions in the network. 4. Despite differences in species composition, nocturnal networks exhibit similar properties to diurnal networks: significant nestedness, marked asymmetry of interactions, high dependence on a core of generalists, and high inter‐annual variation in species abundances and occurrence of interactions. 5. Traditional diurnal plant–visitor networks exclude a significant component of the community, i.e. nocturnal visitors. Exploring links across boundaries between networks (such as between diurnal and nocturnal networks) will provide a more accurate picture of ecosystem structure and function.

2005. Patterns of interaction between plants and pollinators along an environmental gradient. Á/ Oikos 109: 461 Á/472.Patterns of variation in plant Á/pollinator (p Á/p) systems in response to environmental variables have been the focus of much recent attention. We analyzed species diversity and generalization of interactions of flower visitors belonging to eight p Á/p networks along a steep rainfall gradient in NW Patagonia, Argentina. To our knowledge, this is the first published study that applies a humidity Á/ gradient approach to p Á/p networks analysis. Throughout the gradient, we recorded 1232 interactions between 413 different animal species and 111 plant species. We found that (a) specialization measures showed no clear pattern of variation throughout the rainfall gradient, (b) the diversity of flower-visiting insects does not consistently respond to rainfall gradients along the gradient, and (c) as we predicted, flies dominated the wetter end of the gradient, while at the drier end bees prevailed. The lack of differences in diversity could be explained by the repeated cycles of species extinctions undergone in the past by the southern temperate forests, which dominate the wetter end of the gradient. A logistic model that related the flies' dominance of the visitor assemblage with latitude was good predictor of the average fly composition of the entire region, although we found major betweensite variations in response to local environmental conditions. The replacement of flies by bees towards the drier end of the gradient seemed to repeat a worldwide pattern where flies dominate humid regions while bees attain their greatest abundance in xeric regions. Regional patterns in the structure of our p Á/p systems (composition of the visitor fauna) were better explained by altitudinal differences, while function (percentage of interactions established by each taxon) seemed to be more influenced by precipitation.

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 & pollinators or plants & seed dispersers) approximately follow a truncated power law. We show that nestedness and truncated power law 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.