Habitat Selection and Population Trends in Terrestrial Bird Species of Robinson Crusoe Island: Habitat Generalists Versus Forest Specialists
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Biodivers Conserv DOI 10.1007/s10531-011-0109-x ORIGINAL PAPER Habitat selection and population trends in terrestrial bird species of Robinson Crusoe Island: habitat generalists versus forest specialists Ingo J. Hahn • Pablo M. Vergara • Uwe Ro¨mer Received: 15 October 2010 / Accepted: 2 July 2011 Ó Springer Science+Business Media B.V. 2011 Abstract Habitat loss and degradation on oceanic islands are key processes leading to population decline of endemic birds and facilitating the establishment of invasive bird species. In this study, carried out in the Robinson Crusoe Island, we assessed density and habitat selection of terrestrial bird species, including juan fernandez firecrown and juan fernandez tit-tyrant, two endemics, as well as green-backed firecrown and austral thrush, which apparently originate from the mainland. Results show that perturbed habitats con- tained a low density of the endemic species whereas the mainland species were signifi- cantly more abundant in perturbed scrub habitats. Bird species show different habitat selection patterns, with endemics selecting for native forest and mainland species selecting for perturbed habitats, or using them at random. Bird species experienced temporal trends in their overall population sizes, with the endemic tit-tyrant suffering a significant decline in its population size of about 63% between 1994 and 2009. Only mainland species exhibited temporal changes in habitat use, significantly reducing their densities in the preferred scrub habitats, possibly as a response to decreased habitat quality. Thrushes apparently were able to compensate the population decrease in one non native habitat type by using native forests, a habitat giving them the opportunity of preying on nests of endemic species. We conclude that endemic bird species behave as specialists whereas the mainland species must be treated as invasive generalists on Robinson Crusoe Island. I. J. Hahn (&) Institute of Landscape Ecology, University of Mu¨nster, Robert-Koch-Str. 28, 48149 Mu¨nster, Germany e-mail: [email protected] P. M. Vergara Department of Engineer Geography, Universidad de Santiago de Chile, Av. Lib. B. O’Higgins 3363, 7254758 Santiago, Chile P. M. Vergara CEUS Llanquihue, Universidad de Santiago de Chile, Santiago de Chile, Chile U. Ro¨mer Institute of Biogeography, University of Trier, Am Wissenschaftspark 25–27, 54296 Trier, Germany 123 Biodivers Conserv Keywords Anairetes fernandezianus Á Habitat selection Á Invasive species Á Population declining Á Sephanoides fernandensis Á Juan Ferna´ndez Islands Introduction Habitat loss and degradation on oceanic islands are key processes leading to population decline of endemic birds and facilitating the spread of invasive bird species (Catry et al. 2000; Byers 2002; Soh et al. 2006). These detrimental effects lead to progressive replacement of endemics by exotic species resulting in increased biotic homogenization (Castro and Jaksic 2008). Since oceanic islands are biodiversity hot-spots supporting a large proportion of critically endangered endemic animals (Johnson and Stattersfield 1990; IUCN 2011), it is critical to understand how species select habitats on islands which are exposed to increased habitat disturbances, and how these animal decisions determine population dynamics and persistence. Human induced disturbances on oceanic islands have resulted in the degradation and replacement of the original vegetation by new habitats and land uses, including urban, agricultural and livestock grazing. Island-endemic bird species which are ecologically specialized tend to be disproportionately vulnerable to those habitat changes since their population sizes are typically small due to their naturally restricted distributions (Owens and Bennett 2000; Fordham and Brook 2010). However, endemic and native species may display broad habitat use patterns on islands, resulting in the spread of their populations into the new human-created habitats (Blondel et al. 1988; Palomino and Carrascal 2005; Trainor 2007; Carrascal et al. 2008). On the contrary, the successful establishment of an exotic (i.e. introduced) species usually requires that the species is able to spread into non- perturbed habitats after its introduction (Crawley 1986; Sakai et al. 2001). Therefore, both native and exotic species may become invasive on an island depending on their habitat selection attributes (Colautti and MacIsaac 2004; Vergara et al. 2011). Habitat generalism is an important attribute in determining the ability to persist in heterogeneous and human-perturbed landscapes (Sol et al. 1997; Marvier et al. 2004; Vergara and Armesto 2009). Habitat generalist species have broad habitat amplitudes (occupying several habitat types) and usually they behave as opportunistic species, pre- ferring the habitats offering more resources (Medel and Jaksic 1988; Magura et al. 2003). Unlike specialist species, which are constrained to use a small habitat spectrum, habitat generalists may switch their habitat selection pattern over time as expected from habitat selection theory (Fretwell and Lucas 1970; Rosenzweig 1985; Latta and Faaborg 2002; Chen et al. 2008). Consequently, changes in population size and in the spatial distribution of their resources over time may result in a wide temporal variability in habitat use by generalist species (Diamond 1975; Holt 1993; Mobæk et al. 2009). Robinson Crusoe Island supports two endemic terrestrial bird species, the juan fer- nandez firecrown (Sephanoides fernandensis) and juan fernandez tit-tyrant (Anairetes fernandezianus), which have been reported using remnant native forests (Hahn et al. 2005, 2006). The native forests have experienced a rapid spatial reduction and degradation in the last centuries after human colonization as a result of the land-use change and introduction of exotic herbivores (Cuevas and van Leersum 2001; Ricci 2006). As a consequence of habitat loss and degradation, in addition to the potentially high risk from exotic predators such as rats, cats, and coatis, juan fernandez firecrown and tit-tyrant have population sizes 123 Biodivers Conserv small enough to be considered as critically endangered and near threatened, respectively (comp. IUCN 2011). At present two terrestrial bird species native to the mainland, the green-backed fire- crown (Sephanoides sephaniodes) and the austral thrush (Turdus falcklandii magellanicus) coexist with the endemic firecrown and tit-tyrant in native forest habitats of the Robinson Crusoe Island (Hahn et al. 2006, 2009, 2011). Although there is no conclusive evidence, previous studies suggest that both green-backed firecrown and austral thrush are exotic species in Robinson Crusoe Island (Audouin 1830; Brooke 1987; Stiles 1987; Colwell 1989; Roy et al. 1998, 1999; Hahn et al. 2006, 2009; C. Smith-Ramirez, pers. comm.). Irrespective of their origin, green-backed firecrown and austral thrush in southern South America have a wide geographical distribution, high local abundances and use a wide variety of habitats, including mature forest, degraded forest, forest plantations as well as agricultural and urban areas (e.g. Willson et al. 1994; Estades 1997; Anderson and Rozzi 2000; Jaramillo 2003; Vergara and Simonetti 2004; Grigera and Pavic 2007; Vergara and Armesto 2009; Vergara et al. 2010). The ability to use both native forest and human- modified habitats in the mainland, coupled with the fact that these species pollinate and disperse exotic plant species on the island, may make green-backed firecrowns and austral thrushes invasive species on Robinson Crusoe Island (Duncan et al. 2003; Colautti and MacIsaac 2004). Although previous studies have reported both species using a variety of human-created habitats in Robinson Crusoe Island (e.g. Hahn et al. 2005), their habitat selection patterns and population developments are still unknown. In this study, we assessed the habitat selection pattern and the temporal variation in population size of terrestrial bird species of Robinson Crusoe Island. Specifically, we tested the following hypotheses: (i) The abundance of both green-backed firecrowns and austral thrushes is equal or greater in disturbed habitat than in native forest. These species ran- domly use habitats or select for disturbed habitats (since human created habitats are used by both species in the mainland); (ii) The abundance of the endemic firecrown and tit- tyrant is greater in native forest. These endemic species behave as habitat specialists selecting for native forest or low perturbed habitats; (iii) Endemic species exhibit declining population sizes; (iv) Green-backed firecrowns and austral thrushes exhibit habitat- dependent temporal variation in abundance due to changes in population size and in the spatial distribution of habitat resources over time. In particular, the relative abundance pattern can change when the variability in climatic conditions, the human disturbance regimes and the population size affect differently to the suitability of each habitat. These temporal changes in abundance should result in significant interactions between year and habitat type. Methods Study area Robinson Crusoe Island is the easternmost of the three islands of the Juan Ferna´ndez Archipelago, located ca. 600 km off the Chilean coast in the South-east Pacific Ocean (33°3303800S and 78°5604400W). The island area is 47.11 km2, reaching an altitude of 915 m above sea-level. Juan Ferna´ndez Archipelago belongs to the few places in the Pacific Ocean which remained untouched by humans until the European colonization. The