Seed dispersal of alien and native plants by vertebrate herbivores |
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Authors: | Maria Calvino-Cancela |
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Institution: | (1) Centre for Ecosystem Diversity and Dynamics, School of Agriculture and Environment, Curtin University of Technology, PO Box U1987, Perth, WA 6845, Australia;(2) Department Ecology and Animal Biology, University of Vigo, EUET Forestal, Campus Universitario, 36005 Pontevedra, Spain |
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Abstract: | Seed dispersal is crucial for the success and spread of alien plants. Herbivores often establish a dual relationship with
plants: antagonist, through herbivory, and mutualist, through seed dispersal. By consuming plants, herbivores may disperse
large amounts of seeds, and can facilitate the spread of alien plants. However, seed dispersal of alien plants by herbivores
has been largely uninvestigated. I studied factors associated with dispersal of alien and native seeds by the three most important
vertebrate herbivores in SW Australia: emus (Dromaius novaehollandia), western grey kangaroos (Macropus fuliginosus) and European rabbits (Oryctolagus cuniculus). Overall frequencies of alien and native seeds dispersed by these herbivores were determined by differences among them in
(1) the plant groups they predominantly disperse, that differed in frequencies of aliens versus natives, and (2) the predominant
dispersal of aliens or natives within those plant groups. Emus and kangaroos (natives) tended to disperse predominantly alien
seeds within plant groups (defined by life forms, dispersal syndromes, and diaspore size), whereas rabbits (alien) tended
to disperse predominantly natives. This agrees with the hypothesis that herbivores will use predominantly plants that have
evolved in different areas, because of less effective defences against new enemies. Overall frequencies were consistent with
this pattern in kangaroos and rabbits, but not in emus. Kangaroos dispersed mostly plant groups that were mainly aliens (herbaceous
species and small and medium sized dispersal units and seeds), which together with their predominant use of aliens over natives
within groups resulted in the highest overall frequency of alien seeds (73%). Rabbits were similar to kangaroos in the type
of plants dispersed, but their predominant use of natives over aliens within groups contributed to an overall predominance
of native seeds in their pellets (88%). Emus dispersed mostly plant groups that were mainly natives (e.g. woody species with
big diaspores), resulting in low overall frequency of alien seeds (11%), despite their predominant use of aliens over natives
within plant groups. Thus, the within-groups trend pointed to a facilitative role of native herbivores of plant invasions
through seed dispersal, but was obscured by the different use by herbivores of plant groups with different frequency of aliens. |
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