Academic journal article Estonian Journal of Ecology

Fifty-Year Dynamics in a Temperate Raptor assemblage/Roovlinnukooshise Dunaamika Sauel Viiekumne Aasta Jooksul

Academic journal article Estonian Journal of Ecology

Fifty-Year Dynamics in a Temperate Raptor assemblage/Roovlinnukooshise Dunaamika Sauel Viiekumne Aasta Jooksul

Article excerpt

INTRODUCTION

Raptorial birds of the orders Accipitriformes, Falconiformes, and Strigiformes constitute a well-known charismatic functional group of terrestrial animals, which have been popular monitoring objects for decades (Newton, 1979; Kovacs et al., 2008). Because most raptors are top predators, their abundance and performance can be used for screening biotic effects of environmental contaminants, landscape change and, perhaps, general level of biodiversity (Sergio et al., 2008). The group also includes many rare and threatened species that depend on adequate conservation action (Burfield, 2008). Yet, when designing avian indicators of land-use effects on bird communities (e.g. Angelstam et al., 2004; Gregory et al., 2005; Billeter et al., 2008), raptors are frequently excluded because their low abundance, large home-ranges, and use of land mosaics instead of distinct landcover types present special challenges for fieldwork and analysis. Therefore, most raptor monitoring has remained voluntary work of amateur ornithologists, which also means that long-term data on raptor assemblages are both invaluable and rare worldwide and even those results are seldom published in primary scientific literature. At the same time, long time-series are valuable because the longevity of individuals causes time lags and various cumulative effects in raptor population responses (Newton, 1979).

Apart from large-scale inventories on single rare species, the main general method of raptor monitoring in Europe has been a plot-based survey of nesting territories (Kovacs et al., 2008; Saurola, 2008). In Estonia, such local raptor surveys have been integrated to a national monitoring programme since the late 1980s (Lohmus, 1994), but earlier data are very scarce. The first local census results originate from short-term surveys in the 1960s (Randla, 1976), while the single published long-term census only started in 1978 (Lelov, 1991). Because of the lack of earlier data, long-term trends in Estonian raptor populations and their relationships with environmental change have been difficult to detect (note that perhaps the most abrupt re-organization of land use only took place in the early 1990s; Palang et al., 1998).

This paper summarizes a 50-year (1961-2010) raptor survey in the surroundings of Saue, North Estonia. This is the longest continuous time-series on nesting raptor assemblages in the Baltic States and remarkable in a global perspective as well. It covers a period after decades of heavy raptor persecution (see Randla, 1976; Lohmus, 2011) and during land-use transitions from traditional to extensive agriculture and suburbanization. Compared with the earlier overviews of raptor abundance and nesting ecology in this area (Tuule et al., 2001, 2007), we have expanded the time period and focus on the assemblage characteristics rather than individual species. We check for trends in total density, diversity, and general species composition. We then compare the trends in species with similar habitat requirements to explore whether an assemblage change reflects landscape change. Thus, although lacking spatial replication, this study complements the chronosequence approaches where temporal changes in raptor habitats have been addressed by spatial analogues (e.g. Bosakowski & Smith, 1997; Berry et al., 1998; Sanchez-Zapata & Calvo, 1999).

MATERIAL AND METHODS

Study area and field methods

The study area covers 60 [km.sup.2] in Harju County, northern Estonia (59[degrees]18' N, 24[degrees]34' E; see Tuule et al., 2007, for a map). The terrain is flat (on average 22 m a.s.l.); it is intersected by the Vaana and Keila rivers. The mosaic landscape is dominated by forests and agricultural lands, but the land cover has significantly changed during the 50 years (Table 1). The main negative trend (see also Kana et al., 2008) has been an over 10-fold loss of meadows. Open seminatural meadows (notably on floodplains) were completely lost during the 20th century; they were mostly replaced by cultivated areas and often artificially drained. …

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