Low redundancy in seed dispersal within an island frugivore community

AoB Plants, Jan 2015

The low species diversity that often characterizes island ecosystems could result in low functional redundancy within communities. Flying foxes (large fruit bats) are important seed dispersers of large-seeded species, but their redundancy within island communities has never been explicitly tested. In a Pacific archipelago, we found that flying foxes were the sole effective disperser of 57 % of the plant species whose fruits they consume. They were essential for the dispersal of these species either because they handled >90 % of consumed fruit, or were the only animal depositing seeds away from the parent canopy, or both. Flying foxes were especially important for larger-seeded fruit (>13 mm wide), with 76 % of consumed species dependent on them for dispersal, compared with 31 % of small-seeded species. As flying foxes decrease in abundance, they cease to function as dispersers long before they become rare. We compared the seed dispersal effectiveness (measured as the proportion of diaspores dispersed beyond parent crowns) of all frugivores for four plant species in sites where flying foxes were, and were not, functionally extinct. At both low and high abundance, flying foxes consumed most available fruit of these species, but the proportion of handled diaspores dispersed away from parent crowns (quality) was significantly reduced at low abundance. Since alternative consumers (birds, rodents and land crabs) were unable to compensate as dispersers when flying foxes were functionally extinct, we conclude that there is almost no redundancy in the seed dispersal function of flying foxes in this island system, and potentially on other islands where they occur. Given that oceanic island communities are often simpler than continental communities, evaluating the extent of redundancy across different ecological functions on islands is extremely important.

A PDF file should load here. If you do not see its contents the file may be temporarily unavailable at the journal website or you do not have a PDF plug-in installed and enabled in your browser.

Alternatively, you can download the file locally and open with any standalone PDF reader:

https://aobpla.oxfordjournals.org/content/7/plv088.full.pdf

Low redundancy in seed dispersal within an island frugivore community

AoB PLANTS www.aobplants.oxfordjournals.org Low redundancy in seed dispersal within an island frugivore community Kim R. McConkey 0 2 Donald R. Drake 1 Associate Editor: Anna Traveset 0 Present address: School of Natural Sciences and Engineering, National Institute of Advanced Studies, Indian Institute of Science Campus , Bangalore , India 1 Department of Botany, University of Hawai'i at Manoa , 3190 Maile Way, Honolulu, HI 96822 , USA 2 School of Biological Sciences, Victoria University of Wellington , PO Box 600, Wellington , New Zealand The low species diversity that often characterizes island ecosystems could result in low functional redundancy within communities. Flying foxes (large fruit bats) are important seed dispersers of large-seeded species, but their redundancy within island communities has never been explicitly tested. In a Pacific archipelago, we found that flying foxes were the sole effective disperser of 57 % of the plant species whose fruits they consume. They were essential for the dispersal of these species either because they handled .90 % of consumed fruit, or were the only animal depositing seeds away from the parent canopy, or both. Flying foxes were especially important for larger-seeded fruit (.13 mm wide), with 76 % of consumed species dependent on them for dispersal, compared with 31 % of smallseeded species. As flying foxes decrease in abundance, they cease to function as dispersers long before they become rare. We compared the seed dispersal effectiveness (measured as the proportion of diaspores dispersed beyond parent crowns) of all frugivores for four plant species in sites where flying foxes were, and were not, functionally extinct. At both low and high abundance, flying foxes consumed most available fruit of these species, but the proportion of handled diaspores dispersed away from parent crowns (quality) was significantly reduced at low abundance. Since alternative consumers (birds, rodents and land crabs) were unable to compensate as dispersers when flying foxes were functionally extinct, we conclude that there is almost no redundancy in the seed dispersal function of flying foxes in this island system, and potentially on other islands where they occur. Given that oceanic island communities are often simpler than continental communities, evaluating the extent of redundancy across different ecological functions on islands is extremely important. Ecological redundancy; flying foxes; frugivore; fruit bats; functional extinction; Pacific islands; Pteropus; seed dispersal - Introduction Resilience to disturbance is greatest in ecosystems that have high species diversity because of the buffering effect diversity can have on function (Mayfield et al. 2010; Dalerum et al. 2012; Reich et al. 2012). When multiple species perform a given ecosystem function, there is redundancy within the system, and the function may be fully or partially maintained following perturbations in species populations (Dalerum et al. 2012). As ecosystems lose species, however, associated declines in functional redundancy increase the vulnerability of these ecosystems to further change (Reich et al. 2012). Islands are characterized by inherently low species diversity compared with continents (MacArthur 1965; Whittaker and Ferna´ ndezPalacios 2007), and they have been disproportionately further depleted by human-mediated extinctions (e.g. Olson and James 1982; Steadman et al. 1991; Steadman 2006). Hence, current island ecosystems might exhibit especially low functional redundancy, which makes the ongoing human-mediated disturbances to them (Brooks et al. 2002; Whittaker and Ferna´ ndez-Palacios 2007) a serious threat to their stability (Cox et al. 1991; Traveset et al. 2012). An alternative view is that island systems may be somewhat buffered against low functional redundancy because island species are often generalists, or even super-generalists, in their diet and habitat use (Banack 1998; Olesen et al. 2002). Hence, understanding the vulnerability of island species to a lack of functional redundancy is complicated, but important, to ensure that functional ecosystems are maintained. Fruit bats in the family Pteropodidae are effective seed dispersers throughout the Old World tropics (Richards 1990; Rainey et al. 1995; Banack 1998; Hodgkison et al. 2003; Bollen et al. 2004; Nyhagen et al. 2005). Flying foxes (Pteropus spp.) are predominantly found on islands, with a distribution stretching from the coast of East Africa, through tropical Asia, to Polynesia. Simplified frugivore communities exist on many of these islands, with especially low diversity in the tropical Pacific (Steadman 2006). Here, flying foxes, many species of which declined following human discovery of the islands (Steadman 2006), have generalist diets (Banack 1998) and are often regarded as ‘keystone’ seed dispersers, particularly for large-seeded plants, because of a relative lack of other large frugivores (Cox et al. 1991; Rainey et (...truncated)


This is a preview of a remote PDF: https://aobpla.oxfordjournals.org/content/7/plv088.full.pdf

Kim R. McConkey, Donald R. Drake. Low redundancy in seed dispersal within an island frugivore community, AoB Plants, 2015, 7, DOI: 10.1093/aobpla/plv088