Potential consequences of invasive mutualists on plant reproduction
The introduction of alien species into native ecosystems constitutes one of the major threats for biodiversity maintenance and ecosystem integrity. In particular, alien plants can displace native plant species directly though competition for space and abiotic resources (e.g. light, water, nutrients). However, alien and native plants may also compete indirectly through shared biotic resources, most notably pollinators, which can have an immediate effect on plant sexual reproduction. Although evidence for pollinator-mediated competition in general has been mixed, this type of animal-mediated antagonistic interaction has attracted ecologists for more than three decades.
Interest in pollinator-mediated competition is on the rise
In recent years, this topic has regained interest in the context of biological invasions, perhaps because of the potential of many exuberant invasive plants to swamp local floral neighbourhoods. Yet, most studies conducted so far have focused on the effects of one or few alien plant species on the pollination and reproductive success of one or a reduced number of focal native species (reviewed in Morales & Traveset 2009). However, integration of alien species into complex pollination webs may have widespread consequences on the structure and functionality of plant–pollinator mutualisms, which cannot be simply predicted by the study of a limited number of interacting species. This community approach allows a more integrative ecological and evolutionary perspective on the actual and potential consequences of invasive mutualists on the architecture of plant–pollinator networks and mutualism functioning.
Studying complex pollination webs
In their paper 'The tolerance of island plant–pollinator networks to alien plants', the Editor's Choice in this issue of Journal of Ecology, Christopher Kaiser-Bunbury, Terence Valentin, James Mougal, Denis Matatiken and Jaboury Ghazoul evaluated the indirect, pollinator-mediated impact of alien invasive plants on native plant communities using a pollination web approach. Recent work showed that alien mutualists, both plants and animals, can fully integrate into native pollination webs and even become ‘super-generalists’ through the usurpation of ‘native–native’ interaction links (Aizen et al. 2008). However, the chosen study is novel in analysing not only changes in interaction frequency and mutualism structure, but also the consequences of invasive plant assemblages on the reproduction success of native plants in the context of complex networks. To achieve their goals, the authors gathered data on visitation frequency to assemble six well-resolved pollination webs from the ancient steep granitic outcrops, "Inselbergs", of the Mahé Island in the Seychelles. This peculiar habitat bears a highly endemic flora of Gondwanan origin dominated by shrubs and small trees. Despite being islands within an island, some of the Mahé Inselbergs have being heavily invaded by alien plants; thus the studied pollination webs are representative of a broad invasion gradient.
Native plant communities are relatively tolerant to disruption
Results showed that native plant communities from the Inselbergs appeared relatively tolerant to disruption of mutualistic interactions by invasive species. In particular, the authors reported that the interaction frequency with native plants was mostly unaffected by invasion. However, they found that the most common pollinator, the introduced honey bee Apis mellifera, became differentially associated with the alien flora as invasive plants became more abundant. Associated with this shift in the interaction pattern of honey bees were sizeable changes in the structure of pollination webs, which became simpler, and more heterogeneous and temporally variable. Nevertheless, these changes were not strong enough to affect fruit set. Despite a lack of a detectable quantitative effect on reproductive success, Kaiser-Bunbury et al. foresaw a stage of growing interaction vulnerability with increasing invasion as native plants became dependent on fewer pollinator species. Furthermore, simplification and increase heterogeneity of pollination webs could be eventually linked to limited variation in gene flow and increasing reductions in seed quality, which might compromise long-term population persistence. The study of these potential, more subtle effects associated with a decrease in network robustness provides exciting novel venues for research connecting community ecology with plant population genetics and dynamics.
Network structure is altered by high levels of plant invasion
One of the results I found most interesting is the report of an apparent invasion threshold. From the early stages of invasion to the point at which about one third of the flowers belonged to alien plants, the authors failed to detect any sizable effect of invasion on web structure. However, beyond that point they showed that interaction evenness, an indicator of network robustness, declined strongly. This finding has clear conservation implications as it can help to set realistic but effective goals in restoring plant–pollinator webs. This can be particularly important for managing highly invaded sites under a scenario of limited economic or logistic resources, or when complete eradication of the alien plant community is practically unfeasible or even undesirable.
Marcelo A. Aizen
Associate Editor, Journal of Ecology
- Aizen, M.A., Morales, C.L. & Morales, J.M. (2008) Invasive mutualists erode native pollination webs. PLoS Biology, 6, 396–403.
- Kaiser-Bunbury, C.N., Valentin, T., Mougal, J., Matatiken, D. & Ghazoul, J. (2011) The tolerance of island plant–pollinator networks to alien plants. Journal of Ecology, 99, 202-213.
- Morales, C.L. & Traveset, A. (2009) A meta-analysis of impacts of alien vs. native plants on pollinator visitation and reproductive success of co-flowering native plants. Ecology Letters, 12, 716–728.
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