Low redundancy in seed dispersal within an island frugivore community
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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
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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)