Exploitation competition between seed predators and dispersers introduced to Hawaiian forests

Exploitation competition occurs when one group of organisms reduces the availability of a resource for another group of organisms. For instance, plants produce a certain number of fruits for seed dispersal by fruit-eating animals (hereafter frugivores), and fruit consumption by one group of frugivores can reduce the number of fruits available for other frugivores. However, it is uncertain whether exploitation competition is common among frugivores, particularly in novel ecosystems, where food resources are generally thought to be abundant and invasive species are dietary generalists. In a novel ecosystem in Hawai'i, we used gut passage experiments with captive birds to identify roles of introduced frugivores and found they were either distinctly seed dispersers or predators. We then experimentally tested how frugivory by seed predators influenced frugivory by seed dispersers. Specifically, we used exclosures around fruiting plants that blocked seed predator access, while permitting seed disperser access, and we had two control treatments that allowed for access by all frugivores (n=139 plants). When seed predators were excluded from plants, there was more frugivory by dispersers compared to controls, and results varied by year and plant species. Overall, we show that introduced frugivores occupied distinct ecological roles (seed predator or seed disperser), exploitation competition occurred between these introduced frugivore groups, and seed predators had both direct (via seed destruction) and indirect (via reduction in frugivory by dispersers) effects on seed dispersal. Thus, in this novel ecosystem, multiple frugivory is subtractive, and competition for fruit between introduced seed predators and seed dispersers scales up to affect invasions and the conservation of native flora.

The joint role of coevolutionary selection and network structure in shaping trait matching in mutualisms

Coevolution can sculpt remarkable trait similarity between mutualistic partners. Yet, it remains unclear which network topologies and selection regimes enhance trait matching. To address this, we simulate coevolution in topologically distinct networks under a gradient of mutualistic selection strength. We describe three main insights. First, trait matching is jointly influenced by the strength of mutualistic selection and the structural properties of the network where coevolution is unfolding. Second, the strength of mutualistic selection determines the network descriptors better correlated with higher trait matching. While network modularity enhances trait matching when coevolution is weak, network connectance does so when coevolution is strong. Third, the structural properties of networks outrank those of modules or species in determining the degree of trait matching. Our findings suggest networks can both enhance or constrain trait matching, depending on the strength of mutualistic selection.

The functional roles of species in metacommunities, as revealed by metanetwork analyses of bird-plant frugivory networks

Understanding how biodiversity and interaction networks change across environmental gradients is a major challenge in ecology. We integrated metacommunity and metanetwork perspectives to test species' functional roles in bird-plant frugivory interactions in a fragmented forest landscape in Southwest China, with consequences for seed dispersal. Availability of fruit resources both on and under trees created vertical feeding stratification for frugivorous birds. Bird-plant interactions involving birds feeding only on-the-tree or both on and under-the-tree (shared) had a higher centrality and contributed more to metanetwork organisation than interactions involving birds feeding only under-the-tree. Moreover, bird-plant interactions associated with large-seeded plants disproportionately contributed to metanetwork organisation and centrality. Consequently, on-the-tree and shared birds contributed more to metanetwork organisation whereas under-the-tree birds were more involved in local processes. We would expect that species' roles in the metanetwork will translate into different conservation values for maintaining functioning of seed-dispersal networks.

Reshaping our understanding of species' roles in landscape-scale networks

In network ecology, landscape-scale processes are often overlooked, yet there is increasing evidence that species and interactions spill over between habitats, calling for further study of interhabitat dependencies. Here, we investigate how species connect a mosaic of habitats based on the spatial variation of their mutualistic and antagonistic interactions using two multilayer networks, combining pollination, herbivory and parasitism in the UK and New Zealand. Developing novel methods of network analysis for landscape-scale ecological networks, we discovered that few plant and pollinator species acted as connectors or hubs, both within and among habitats, whereas herbivores and parasitoids typically have more peripheral network roles. Insect species' roles depend on factors other than just the abundance of taxa in the lower trophic level, exemplified by larger Hymenoptera connecting networks of different habitats and insects relying on different resources across different habitats. Our findings provide a broader perspective for landscape-scale management and ecological community conservation.

Native and alien flower visitors differ in partner fidelity and network integration

Globalisation persistently fuels the establishment of non-native species outside their natural ranges. While alien plants have been intensively studied, little is known about alien flower visitors, and especially, how they integrate into natural communities. Here, we focus on mutualistic networks from five Galápagos islands to quantify whether alien and native flower visitors differ consistently in their pairwise interactions. We find that (1) alien flower visitors have more interaction partners and larger species strengths (i.e. plants are more connected to alien insects), (2) native insects tend to have higher partner fidelity as they deviate more from random partner utilisation, and iii) the difference between native and alien flower visitors in network integration intensifies with island degradation. Thus, native and alien flower visitors are not interchangeable, and alien establishment might have yet unforeseen consequences for the pairwise dynamics between plants and flower visitors on the Galápagos - especially on the heavily disturbed islands.

Concomitant predation on parasites is highly variable but constrains the ways in which parasites contribute to food web structure

Previous analyses of empirical food webs (the networks of who eats whom in a community) have revealed that parasites exert a strong influence over observed food web structure and alter many network properties such as connectance and degree distributions. It remains unclear, however, whether these community‐level effects are fully explained by differences in the ways that parasites and free‐living species interact within a food web.

To rigorously quantify the interrelationship between food web structure, the types of species in a web and the distinct types of feeding links between them, we introduce a shared methodology to quantify the structural roles of both species and feeding links. Roles are quantified based on the frequencies with which a species (or link) appears in different food web motifs – the building blocks of networks.

We hypothesized that different types of species (e.g. top predators, basal resources, parasites) and different types of links between species (e.g. classic predation, parasitism, concomitant predation on parasites along with their hosts) will show characteristic differences in their food web roles.

We found that parasites do indeed have unique structural roles in food webs. Moreover, we demonstrate that different types of feeding links (e.g. parasitism, predation or concomitant predation) are distributed differently in a food web context. More than any other interaction type, concomitant predation appears to constrain the roles of parasites. In contrast, concomitant predation links themselves have more variable roles than any other type of interaction.

Together, our results provide a novel perspective on how both species and feeding link composition shape the structure of an ecological community and vice versa.