Seed dispersal networks in the Galápagos and the consequences of alien plant invasions

Abstract

Alien plants are a growing threat to the Galápagos unique biota. We evaluated the impact of alien plants on eight seed dispersal networks from two islands of the archipelago. Nearly 10 000 intact seeds from 58 species were recovered from the droppings of 18 bird and reptile dispersers. The most dispersed invaders were Lantana camara, Rubus niveus and Psidium guajava, the latter two likely benefiting from an asynchronous fruit production with most native plants, which facilitate their consumption and spread. Lava lizards dispersed the seeds of 27 species, being the most important dispersers, followed by small ground finch, two mockingbirds, the giant tortoise and two insectivorous birds. Most animals dispersed alien seeds, but these formed a relatively small proportion of the interactions. Nevertheless, the integration of aliens was higher in the island that has been invaded for longest, suggesting a time-lag between alien plant introductions and their impacts on seed dispersal networks. Alien plants become more specialized with advancing invasion, favouring more simplified plant and disperser communities. However, only habitat type significantly affected the overall network structure. Alien plants were dispersed via two pathways: dryfruited plants were preferentially dispersed by finches, while fleshy fruited species were mostly dispersed by other birds and reptiles.

Connecting Resource Tracking by Frugivores to Temporal Variation in Seed Dispersal Networks

“…Evidence for resource tracking has been found in both temperate (García and Ortiz-Pulido, 2004;Gleditsch and Carlo, 2010) and tropical systems (Johnson and Sherry, 2001;Moegenberg and Levey, 2003;McConkey and Drake, 2007;Blendinger et al, 2012;Heleno et al, 2013). The ubiquity of resource tracking across diverse habitats is likely due to the patchiness of resources in most ecosystems, irrespective of geographical and climatic zone.…”

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

“…The arrival of new species to islands may notably alter the structure and dynamics of their networks [9,[16][17][18][19]. However, the generality of this is still under debate, and our knowledge of the mechanisms underlying the integration of alien species into native networks and their subsequent impact is still limited.…”

“…Likewise, interaction evenness, which measures the uniformity in the distribution of interaction frequencies and is inversely related to network stability [24], may decrease with higher invasion intensity [18]. Level of invasion may also reduce species specialization (d 0 ) of native species [19], implying that they become less selective in their choice of mutualists by being compelled to interact with the most abundant aliens as invasion progresses. Invaders can also induce changes in levels of network nestedness [16,25] and modularity [25]; both link patterns are also diagnostic signs of network stability [26,27].…”

Sampling completeness in seed dispersal networks: When enough is enough

“…Heleno, Olesen, Nogales, Vargas, & Traveset 2013;Carlo & Yang 2011;Donatti et al 2011;Mello et al 2011). The dispersal of seeds away from the mother plant is a key process, allowing plants to avoid competition, find suitable conditions for germination, and expand their distribution range (Traveset, Heleno, & Nogales 2014).…”