THE INFLUENCE OF HABITAT HETEROGENEITY ON BIRD DIVERSITY IN IRISH FARMLAND

Abstract

ABSTRACTThe intensification and specialisation of agricultural practices have led to a widespread decline offarmland bird diversity across much of Europe. Increasing habitat heterogeneity at multiple spatialscales has been suggested as one possible strategy to reverse these declines. This study examined therelationship between habitat heterogeneity, as measured by a number of indices, and bird diversity atfarm scale using study sites in the east and south-east of Ireland. Generalised Linear Models indicatedthat there was a positive relationship between the documented heterogeneity of farmland habitatsand the numbers of bird species observed during standardised farm surveys. However, thisrelationship was not evident when bird diversity was quantified using both the Shannon and theSimpson’s diversity indices. It is concluded that agri-environmental policy that aims to influence theheterogeneity of habitats at the farm level can provide a useful means to maintain farmlandbiodiversity. Future research should aim to develop an improved system of habitat classification and adeeper understanding of the ecological value of different habitat types in Irish farmland.INTRODUCTIONMany authors have stressed the importance oflandscape structure in understanding the causes ofdecline in farmland bird populations (McGarigalet al. 2002; Sua´rez-Seoane et al. 2002; Benton et al.2003; Verhulst et al. 2004). Landscape ecology islargely based on the presumption that changes in therange and diversity of landscape elements have astrong influence on ecological characteristics,including bird populations, over time (McGarigalet al. 2002). One of the first to note that wildlifediversity was greater in more diverse landscapes wasLeopold (1933). In Sweden, the structure of thesurrounding landscape explained at least some of thevariation documented in both the species richness(number of species)and the diversity of a wide rangeof taxa (vascular plants, butterflies, bumblebees,ground beetles, dung beetles and birds) found insemi-natural grasslands (So ¨derstrom et al. 2001).Different landscape features have been found toinfluence the species richness and diversityof different taxa. For example, landscapeheterogeneity has been highlighted as an importantdeterminant of carabid beetle species diversity(Weibull and O¨ stman 2003). Bird species richnessand abundance are influenced by local resourceavailability and vegetation composition, in additionto the size of habitat patches (MacArthur andMacArthur 1961; Cody 1985). It has been arguedthat agricultural intensification has resulted in a lossof ecological heterogeneity at multiple spatial andtemporal scales and that biodiversity policy shouldaim to reverse this trend in Europeanagroecosystems (Benton et al. 2003).The Common Agricultural Policy (CAP)substantially influenced the intensification ofagriculture within the European Union (Donaldet al. 2001). The CAP has also resulted in theabandonment of agricultural practices withinproductively marginal parts of Western Europe(Pain and Pienkowski 1997), which is alsobelieved to have had a negative effect onbiodiversity (Sua´rez-Seoane et al. 2002). Studieshave shown that extensively used farmland can holdthe highest diversity and abundance of farmlandbirds (Verhulst et al. 2004). This view is reinforcedby the work of Sua´rez-Seoane et al. (2002), whofound that agricultural mosaics of low-intensitycultivation maintain the highest diversity ofendangered bird species.Different elements of landscape structure,including the density of hedgerows and woodlands,have been highlighted as likely predictors of overallbird species abundance. For example, cereal cropsare positively associated with open-land and arablespecialists, such as skylark (Alauda arvensis) andyellowhammer (Emberiza citronella), respectively(Fuller et al. 1997). An index of the ecologicalcharacteristics of field boundaries*the fieldboundary evaluation and grading system (FBEGS)developed by Collier and Feehan (2003)*has beenshown to be a useful predictor of bird speciesrichness and diversity within field boundary habitats