Invaders of pollination networks in the Galapagos Islands: emergence of novel communities.

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.

Pollination patterns and plant breeding systems in the Galapagos: a review.

BACKGROUND: Despite the importance of the Galápagos Islands for the development of central concepts in ecology and evolution, the understanding of many ecological processes in this archipelago is still very basic. One such process is pollination, which provides an important service to both plants and their pollinators. The rather modest level of knowledge on this subject has so far limited our predictive power on the consequences of the increasing threat of introduced plants and pollinators to this unique archipelago. SCOPE: As a first step toward building a unified view of the state of pollination in the Galápagos, a thorough literature search was conducted on the breeding systems of the archipelago's flora and compiled all documented flower-visitor interactions. Based on 38 studies from the last 100 years, we retrieved 329 unique interactions between 123 flowering plant species (50 endemics, 39 non-endemic natives, 26 introduced and eight of unknown origin) from 41 families and 120 animal species from 13 orders. We discuss the emergent patterns and identify promising research avenues in the field. CONCLUSIONS: Although breeding systems are known for <20 % of the flora, most species in our database were self-compatible. Moreover, the incidence of autogamy among endemics, non-endemic natives and alien species did not differ significantly, being high in all groups, which suggests that a poor pollinator fauna does not represent a constraint to the integration of new plant species into the native communities. Most interactions detected (approx. 90 %) come from a single island (most of them from Santa Cruz). Hymenopterans (mainly the endemic carpenter bee Xylocopa darwinii and ants), followed by lepidopterans, were the most important flower visitors. Dipterans were much more important flower visitors in the humid zone than in the dry zone. Bird and lizard pollination has been occasionally reported in the dry zone. Strong biases were detected in the sampling effort dedicated to different islands, time of day, focal plants and functional groups of visitors. Thus, the existing patterns need to be confronted with new and less biased data. The implementation of a community-level approach could greatly increase our understanding of pollination on the islands and our ability to predict the consequences of plant invasions for the natural ecosystems of the Galápagos.

Origin and evolution of endemic Galápagos Varronia species (Cordiaceae).

Four endemic Varronia species (Cordiaceae) occupy the Galápagos archipelago. Three comprise the V. leucophlyctis complex (V. anderssonii, V. leucophlyctis, V. scouleri), whose species' limits are not well defined but that is morphologically distinct from the fourth endemic species, V. revoluta. Sequence data from the nuclear rDNA ITS region and the cpDNA ndhF gene were gathered from 49 accessions of Varronia from five Galápagos islands in order to test the evolutionary relationships of endemic Varronia species, determine the number of immigration events to the islands and estimate their age of origin. All endemic species nest within the clade of species belonging to Varronia, which is an entirely American genus. We find little evidence of phylogenetic structuring of the V. leucophlyctis complex but divergent phylogenetic signals from nuclear and chloroplast genomes regarding its relationship to V. revoluta. Results are consistent with a hybridization event involving ancestral Galapagean lineages, with chloroplast and nuclear data suggesting one or two dispersal events from the Americas to the Galápagos, respectively. Fossil-based divergence time estimates indicate endemic species diverged from American continental species as early as 4.5 Myr ago and radiated 1.12 Myr, which coincides with ages of exposed and subsided Galápagos islands.