Assessing the impacts of the non-native Black-headed Weaver on nativeAcrocephaluswarblers

Abstract

School of Biological Sciences, University of EastAnglia, Norwich Research Park, Norwich NR4 7TJ, UKBACKGROUND AND IMPLEMENTATIONNon-native species can have severe negative impacts onnative biodiversity (Blackburn et al. 2004, Clavero et al.2009), but not all do. It is desirable to evaluate the risksposed by non-native species during the early stages of aninvasion, when control measures are easiest (Manchester& Bullock 2000, Lodge et al. 2006). However, it is eas-ier to evaluate impacts when an invasion is advanced, asmore data are available, allowing competition to be identi-fied with more confidence (Wiens 1989), so most studiesinvestigating the impacts of non-native species (e.g.Newson et al. 2011) are performed once the species islong established.We investigated the impacts of the introduced Black-headed Weaver Ploceus melanocephalus in the IberianPeninsula during the early stages of its invasion. Black-headed Weavers have been suspected of competing withtwo ecologically similar native species, the Great ReedWarbler Acrocephalus arundinaceus and the EurasianReed Warbler A. scirpaceous (Matias 2002). This couldoccur due to competition for nestling food or for nest-sites. Black-headed Weavers may exclude native ReedWarblers from reedbed through interspecific territorial-ity. If this was happening, we would expect to observeaggression between Black-headed Weavers and nativespecies, possibly a response of Black-headed Weavers toplayback of the songs of native species, low overlapbetween territories of Black-headed Weavers and nativespecies, and a shift in the habitat characteristics of nativespecies’ territories to be less similar to Black-headedWeavers. Alternatively, they may reduce the quality ofreedbed by depleting resources. If this were happening,we would expect to observe an increase in the size ofterritories of native species when Black-headed Weaverswere present and native species to show a preference forareas further away from Black-headed Weaver territo-ries. We conducted fieldwork at four sites in Portugalfrom April to June 2012 to collect data to test thesehypotheses.KEY FINDINGS AND MANAGEMENTIMPLICATIONSThe number of territories of each species at our studysites is shown in Table 1. Despite overlapping resourcerequirements, we did not find evidence for any of thepossible signatures of competition. Therefore, our workdoes not support previous anecdotal suggestions of com-petition. Despite this, we recommend continued moni-toring of the impacts of Black-headed Weavers, asnegative impacts may occur once the species reacheshigher population densities.We are currently preparing a manuscript for submis-sion to Ibis, which will give full details of the results fromthis project. We will also submit a report on the projectto the C^amara Municipal das Caldas da Rainha, allowingthe key findings of the project to be disseminated locally.