Are bird communities full of species, or can they accept more? Will new species compete or coexist with existing species?
An anthropogenic habitat facilitates the establishment of non-native birds by providing underexploited resources.
Sullivan, M.J.P., Davies, R.G., Mossman, H.L., Franco, A.M.A. 2015. PLOS One 10: e0135833. DOI: 10.1371/journal.pone.0135833
The transport and introduction of non-native birds provides an (un)natural experiment allowing ornithologists to examine whether bird communities can accept more species. From a management perspective, it is also important to identify whether these non-native species will compete with native species.
In the Iberian Peninsula, a large number of cage birds originating from sub-Saharan Africa or Southeast Asia have escaped or been released. Some of these have established self-sustaining populations. The most widespread, the Common Waxbill, has spread rapidly since its introduction in the 1960s and is now found in over 10% of 10km squares in mainland Iberia (Sullivan et al. 2012). Other passerine species with self-sustaining populations include the Yellow-crowned Bishop, Black-headed Weaver and Red Avadavat. All these species are small seed-eating birds, so could potentially compete with native finches, sparrows and buntings.
All these non-native birds appear to be associated with rice fields. We set out to test whether this was the case, and to try and understand why that might be. We hypothesized that recent human-modified habitats with few local analogues, like rice fields, provide novel combinations of resources that are not fully exploited by native species. If non-native species can access these resources then they could experience less competition in rice fields than in other habitats. To test this, we surveyed bird communities in rice fields and adjacent open habitats throughout the western half of the Iberian Peninsula, and examined resource use by native and non-native species.
We found that non-native species were indeed associated with rice fields. The functional diversity of native species was lower in rice fields than in other habitats, but non-native species plugged this gap. Native species used trees and bushes for shelter, which were rare in rice fields, while non-native species used the reeds that lined irrigation ditches crossing rice fields. There were also differences in the selection of resources for feeding; non-native species were lighter and could feed on the flimsy grasses that dominated rice field margins, while heavier and less agile native species preferred feeding on sturdier forbs that were predominantly found outside rice fields. These differences in resource use allow non-native birds to exploit the resources found in rice fields (Sullivan et al. 2015).
Our work suggests that the underexploited resources in rice fields can provide a ‘welcome mat’ that helps non-native species establish. However, all four species are found outside rice fields to some extent. How do they coexist with superficially similar seed-eating birds there? In addition to our work, several differences in resource use have been documented. Helena Batalha and colleagues collated data on morphology and diet of Common Waxbills and native species, and found that common waxbills occupied peripheral niche space (Batalha et al. 2013). Recent work by Ana Sanz-Aguilar and colleagues has found that these non-native birds have longer breeding seasons than native species, so benefit from reduced competition later in the breeding season (Sanz-Aguilar et al. 2015).
While these non-native birds seem to be able to coexist with native seed eating birds, could we be missing interactions with other species? Anecdotal reports suggest that Black-headed Weavers interact aggressively with native Reed and Great Reed Warblers, and could potentially compete with them for nest sites in reedbeds. Does this aggression mean that Black-headed Weavers are having negative population level impacts? In a separate study, Grundy et al. (2014), we investigated whether Black-headed Weavers had a negative impact on native species.
It is much easier to assess the impacts of non-native species if they are widespread because we have a larger sample size and so can use elegant methods to disentangle to effect of non-native species from other factors that impact native species (see Newson et al. 2011, Roy et al. 2012 for nice examples). However, from a management perspective it is necessary to assess impacts while the non-native species still has a restricted distribution and eradication is still feasible both practically and economically.
Black-headed Weavers are still restricted to a few sites, meaning that there is not enough data to directly assess whether they are impacting the populations of native species. Instead, we have considered the pathways by which competition from Black-headed Weavers could lead to population level impacts on native species, and came up with a series of testable hypotheses along these pathways (Grundy et al. 2014). For example, negative impacts from Black-headed Weavers could come from interspecific territoriality, where they exclude native species from areas of reedbed. We tested if this is happening by performing playback experiments to see if they reacted aggressively to native species, and by assessing the extent to which territories of Black-headed Weavers overlap with those of native species.
We did not find any pathway to competition to be supported, and concluded that at current population densities Black-headed Weavers were unlikely to be having a negative impact on native species (Grundy et al. 2014). We may have missed some subtle impacts, as statistical power was often low, but using a framework such as this means that scientists can start to provide information to inform management policy when management actions, such as eradication are still available.
References and further reading
Batalha, H., Ramos, J.A., Cardoso, G. (2013) A successful avian invasion occupies a marginal ecological niche. Acta Oecologica 49: 92-98. View
Grundy, J.P.B., Franco, A.M.A., Sullivan, M.J.P. (2014) Testing multiple pathways for impacts of the non-native Black-headed Weaver Ploceus melanocephalus on native birds in Iberia in the early phase of invasion. Ibis 156: 355-365. View
Newson, S.E., Johnston, A., Parrott, D., Leech, D.I. (2011) Evaluating the population-level impact of an invasive species, Ring-necked Parakeet Psittacula krameri, on native avifauna. Ibis 153: 509-516. View
Roy, H.E., Adriaens, T., Isaac, N.J.B., et al. (2012) Invasive alien predator causes rapid declines of native European ladybirds. Diversity and Distributions 18: 717-725. View
Sanz-Aguilar, A., Carrete, M., Edelaar, P., Potti, J., Tella, J.L. (2015) The empty temporal niche: breeding phenology differs between coexisting native and invasive birds. Biological Invasions 10.1007/s10530-015-0952-x. View
Sullivan, M.J.P., Davies, R.G., Reino, L., Franco, A.M.A. (2012) Using dispersal information to model the species-environment relationship of spreading non-native species. Methods in Ecology and Evolution 3: 870-879. View
Sullivan, M.J.P., Davies, R.G., Mossman, H.L., Franco, A.M.A. (2015) An anthropogenic habitat facilitates the establishment of non-native birds by providing underexploited resources. PLOS One 10: e0135833. View
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