New mutualism for old: indirect disruption and direct facilitation of seed dispersal following Argentine ant invasion |
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Authors: | Alexei D Rowles Dennis J O’Dowd |
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Institution: | (1) Australian Centre for Biodiversity, School of Biological Sciences, Monash University, Clayton, VIC, 3800, Australia;(2) Present address: Department of Primary Industries, Rutherglen Centre, RMB 1145, Chiltern Valley Road, Rutherglen, VIC, 3685, Australia |
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Abstract: | The indirect effects of biological invasions on native communities are poorly understood. Disruption of native ant communities
following invasion by the Argentine ant (Linepithema humile) is widely reported to lead indirectly to the near complete collapse of seed dispersal services. In coastal scrub in southeastern
Australia, we examined seed dispersal and handling of two native and two invasive alien plant species at Argentine ant-invaded
or -uninvaded sites. The Argentine ant virtually eliminates the native keystone disperser Rhytidoponera victoriae, but seed dispersal did not collapse following invasion. Indeed, Argentine ants directly accounted for 92% of all ant-seed
interactions and sustained overall seed dispersal rates. Nevertheless, dispersal quantity and quality among seed species differed
between Argentine ant-invaded and -uninvaded sites. Argentine ants removed significantly fewer native Acacia retinodes seeds, but significantly more small seeds of invasive Polygala myrtifolia than did native ants at uninvaded sites. They also handled significantly more large seeds of A. sophorae, but rarely moved them >5 cm, instead recruiting en masse, consuming elaiosomes piecemeal and burying seeds in situ. In contrast,
Argentine ants transported and interred P. myrtifolia seeds in their shallow nests. Experiments with artificial diaspores that varied in diaspore and elaiosome masses, but kept
seed morphology and elaiosome quality constant, showed that removal by L. humile depended on the interaction of seed size and percentage elaiosome reward. Small diaspores were frequently taken, independent
of high or low elaiosome reward, but large artificial diaspores with high reward instead elicited mass recruitment by Argentine
ants and were rarely moved. Thus, Argentine ants appear to favour some diaspore types and reject others based largely on diaspore
size and percentage reward. Such variability in response indirectly reduces native seed dispersal and can directly facilitate
the spread of an invasive alien shrub. |
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Keywords: | Ants Biological invasion Facilitation Linepithema humile Seed size |
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