Invaders of pollination networks in the Galápagos Islands: emergence of novel communities

Abstract

The unique biodiversity of most oceanic archipelagos is currently threatened by the introduction of alien species that can displace native biota, disrupt native ecological interactions, and profoundly affect community structure and stability. We investigated the threat of aliens on pollination networks in the species-rich lowlands of five Galápagos Islands. Twenty per cent of all species (60 plants and 220 pollinators) in the pooled network were aliens, being involved in 38 per cent of the interactions. Most aliens were insects, especially dipterans (36%), hymenopterans (30%) and lepidopterans (14%). These alien insects had more links than either endemic pollinators or non-endemic natives, some even acting as island hubs. Aliens linked mostly to generalized species, increasing nestedness and thus network stability. Moreover, they infiltrated all seven connected modules (determined by geographical and phylogenetic constraints) of the overall network, representing around 30 per cent of species in two of them. An astonishingly high proportion (38%) of connectors, which enhance network cohesiveness, was also alien. Results indicate that the structure of these emergent novel communities might become more resistant to certain type of disturbances (e.g. species loss), while being more vulnerable to others (e.g. spread of a disease). Such notable changes in network structure as invasions progress are expected to have important consequences for native biodiversity maintenance.

Bird–flower visitation networks in the Galápagos unveil a widespread interaction release

“…The high network generalization level facilitates the integration of alien plants into the Galápagos bird-flower network, as has also been found in other mutualistic networks 11,36 . Galápagos birds included alien and native plants to the same levels into their diet and may thus enhance plant invasions on this increasingly human-altered archipelago.…”

“…The overall insect species richness of the archipelago is well-known, but insect abundance and biomass are unknown 35 . We did find some evidence of insect poverty in an earlier study from Galápagos 11 , where we scored a connectance C ¼ 5% between native insect species visiting the flowers of native plant species. This figure is lower than we should expect (8.4), if we use the regression model in ref.…”

“…We term this niche expansion process 'interaction release' (as one aspect of the wider concept 'ecological release' 8 ), that is, island species wire up novel mutualists and prey as a response to mutualist and food shortage, and to reduced predation risk. To study the generality of this phenomenon at the community level, we adopted a network approach [9][10][11] . Single species of insect-eating birds and lizards are known to supplement an insect-poor diet with nectar, pollen and fruit [12][13][14][15] .…”

Sampling completeness in seed dispersal networks: When enough is enough

“…The use of ecological networks deepened our understanding of important conservation issues such as environmental degradation (e.g. Traveset et al 2013;Tylianakis, Tscharntke, & Lewis 2007) and ecological restoration (e.g. Heleno, Lacerda, Ramos, & Memmott 2010).…”

First records of Nausigaster flukei (Diptera, Syrphidae) from Argentina

“…However, arid and semi-arid habitats are often overlooked in field expeditions or faunistic studies, and they lack a good knowledge on their fauna and ecology. The fact that these records represent the first citation of Nausigaster in those Argentinean provinces and the first record of this genus from Argentina since 1892, points out the necessity to carry out faunistic studies in arid and semi-arid environs in order to understand the crucial role that invertebrates have in the decomposition and pollination processes, especially syrphid flies Traveset et al 2013, Neil et al 2014. Furthermore, Monte Phytogeographical Province is threatened by habitat conversion, while Chaco Phytogeographical Province is threatened by overgrazing of domestic livestock, seasonal fires and conversion of natural habitats for agriculture (Dinerstein et al 1995).…”