Habitat loss and the disassembly of mutalistic networks

Abstract

Recent studies have described the architecture of plant-animal mutualistic networks, but little is known on how such networks disassemble as a consequence of global change. This is a relevant question because 1) species interactions seem to be very susceptible to habitat loss, and 2) the loss of a critical fraction of interactions can abruptly change the topology of the entire network with potential consequences for its functioning. Here we develop a spatially explicit metacommunity model based on the structure of 30 real mutualistic networks. We find that there is a critical value of habitat destruction beyond which interactions are lost very fast. Second, there is a homogeneous distribution of the number of interactions per patch when the habitat is pristine, while this becomes very skewed at the brink of extinction. This increase in skewness is discussed in the context of potential indicators of network collapse.The consequences of global change have mainly been assessed at the level of individual species. There is now ample evidence that global change affects the distribution, abundance, and physiology of species (Sala et al. 2000, Parmesan 2003. Less attention has been given to the consequences for species interactions, but a recent review has concluded that pairwise interactions are very sensitive to several drivers of global change (Tylianakis et al. 2008). The next step is to scale from such pairwise effects to entire networks of interactions.Recently, several papers have shown that plant-animal mutualistic networks such as those describing pollination and seed dispersal have a well-defined architecture (Bascompte and Jordano 2007). As a preliminary assessment of the implications of network architecture for their robustness to habitat loss, Fortuna and Bascompte (2006) analyzed a spatially implicit meta-community model based on the structure of interactions of real mutualistic networks. This paper found that the number of species collapses after a critical fraction of habitat has been destroyed. However, this implicit approach can not inform on how interactions are lost since two previously interacting species can be found on separate habitat patches and therefore their interaction is lost despite both partners are regionally present. As Janzen (Janzen 1974) already noted it, there is a 'much more insidious kind of extinction: the extinction of ecological interactions'. To tackle the extinction of interactions we need a spatially explicit framework. This spatially explicit approach could ultimately inform us on how to scale from local to regional networks.Previous studies have already considered spatially explicit models of seed dispersal. These papers have emphasized that since the spatial distribution of individuals in a community may influence species interaction probabilities -which in turn will determine network patterns -bird density, landscape structure, and neighborhood effects will affect fruitremoval rates and seed dispersal (Carlo et al. 2007, Morales and Vázquez 2008, Morales et al. 2012. Non...

Effects of habitat loss on the plant–flower visitor network structure of a dune community

“…Other studies have also documented decreases in network nestedness in disturbed habitats (Vanbergen et al , Moreira et al , Revilla et al ) though not always (Spiesman and Inouye ). A reduced nestedness is often associated with lower stability and resilience of plant–pollinator networks to perturbations (Bastolla et al , Fortuna et al ), although there is controversy on this (James et al , Saavedra and Stouffer , Rohr et al ).…”

“…Rare and specialized interactions have shown to be the first to disappear after habitat reduction, and thus an increase in the frequency of generalist plants and/or pollinator species is usually observed (Ashworth et al 2004, Aizen et al 2012, Vanbergen et al 2014. A decrease in network nestedness in disturbed habitats has been reported in several systems (Vanbergen et al 2014, Moreira et al 2015, Revilla et al 2015, which has led authors to predict reductions in the number of coexisting species (Bastolla et al 2009), and in the robustness and resilience of plant-pollinator networks to further perturbations (Bascompte 2009, Fortuna et al 2013). The loss of species and their interactions in a disturbed network can also lead to the formation of isolated compartments within the network (Spiesman and Inouye 2013) which run a higher risk of disappearing after future disturbances than if species are connected in a cohesive network.…”

Positive responses of flower visiting bees to landscape heterogeneity depend on functional connectivity levels

“…Additionally, some authors even suggest that important extinction thresholds for plant-pollinator interaction networks should occur between 50 and 60% of native vegetation loss (Fortuna et al, 2013;Keitt, 2009). For the Atlantic Forest, there is evidence that crucial regime shifts influencing pollinating bee abundance occur in landscapes with about 40% of remaining forest (Ferreira et al, 2015).…”

Decay of interspecific avian flock networks along a disturbance gradient in Amazonia

“…For example, habitat modification affects trophic network structure via changes in species richness and frequency of interactions [12]. Previous work has shown that most ecological networks are resilient to environmental change, but that the threshold at which a community collapses is dependent upon the degree to which species are ecologically redundant and the responses of keystone species to habitat loss [5,10,13]. While trophic networks have been fairly well studied, non-trophic interactions, such as the social mutualisms observed in avian mixed-species flocks, have received considerably less attention.…”