There is now substantial evidence for global warming and its biological consequences (Hughes, 2000; Walther et al., 2002; Parmesan & Yohe, 2003; Root et al., 2003; Karoly & Wu, 2005; Hegerl et al., 2007; Rosenzweig et al., 2007). These biological responses include predictions of future species extinctions (Peterson et al., 2002; Williams et al., 2003; Thomas et al., 2004; Thuiller et al., 2005) and empirical observations that link warming to extinctions (Pounds et al., 1999, 2006). However, to the best of our knowledge, globally only one field study has so far reported species extinctions that are associated with upslope distribution displacement, where species are pushed up and off the tops of mountains (Pounds et al., 1999). Upslope distribution shifts represent one of the biological fingerprints of global warming, when distributions move either in direct response to increasing temperature (Parmesan & Yohe, 2003; Root et al., 2003), or in combination with other changes such as a lifting-cloud base (Pounds et al., 1999; Still et al., 1999) or the availability of new habitat resulting from deglaciation (Seimon et al., 2007). Upslope displacement has been recently reported by multiple temperate studies (e.g. Grabherr et al., 1994; Parmesan, 1996; Pauli et al., 1996; Kullman, 2001; Erasmus et al., 2002; Epps et al., 2003; Konvicka et al., 2003; Pauli et al., 2007), but there is currently very little information available for tropical regions (IPCC, 2007b).
Tropical montane regions typically exhibit high levels of local endemism, which may also include species confined to narrow elevational zones close to summits (Ricketts et al., 2005). Yet, to date, the vulnerability of most tropical montane assemblages to upslope extinction has not been well documented (Rull & Vegas-Vilarrúbia, 2006). Concerning tropical herpetological assemblages, a frog population census in Ecuador (1967–2003) found increases in the upper elevation limit in six of 76 surveyed species (Bustamante et al., 2005), and more generally for montane tropical frogs, a high incidence of ‘enigmatic’ declines (declines or disappearances in apparently intact primary habitats) has been suggested as a potential consequence of climate change or disease by the Global Amphibian Assessment (Stuart et al., 2004). More recently, Pounds et al. (2006) have reported evidence linking Atelopus frog extinctions to warming, and have suggested that a pathogenic chytrid fungus may be promoted by warming. Other studies have found strong evidence supporting emerging infectious diseases driving enigmatic amphibian declines in some tropical regions (e.g. Lips et al., 2005, 2006), or else gradual declines in herpetological assemblages that might possibly be linked to local climate change (Whitfield et al., 2007).
The biodiversity of Madagascar has long been recognized as one of the world's most threatened biotas, due to both the high levels of diversity and endemism on the island, and the decline of natural habitats (e.g. Jenkins, 1987; Myers et al., 2000; Ricketts et al., 2005). The montane assemblages of Madagascar have been subject to periods of intense research (see Andriamialisoa & Langrand, 2003) and are remarkable for their high degree of regional endemism that is often specific to individual massifs. Although the species richness of assemblages is depressed at high elevations, a significant component of Madagascar's endemic species richness is confined to high montane environments (Jenkins, 1987). For example six of 35 Calumma chameleon species are endemic to zones within 600 m elevation of the highest summits (Raxworthy, in press; Raxworthy & Nussbaum, 2006). Many of Madagascar's montane species might thus be potentially vulnerable to upslope distribution shifts from climate warming.
Although the evidence for global warming is well established (Hegerl et al., 2007; Trenberth et al., 2007), regional warming trends in Madagascar have not been widely explored and the potential vulnerability of these endemic montane assemblages has not been discussed or studied. No observations of changes in physical or biological systems were noted for Madagascar in the latest Intergovernmental Panel on Climate Change report (Rosenzweig et al., 2007). The only specific studies for Madagascar known to us are Jury (2003) who reported an increase in coastal sea surface temperatures around Madagascar of ∼1 °C over the past century (see also Karoly & Wu, 2005) and Heiss et al. (1998) who detected a general warming trend (with oscillations) from 1930 to present in a core taken from a 350-year-old coral off the coast of southwest Madagascar. In addition, the widespread bleaching of coral reefs in Madagascar during 1988 has been linked to the exceptionally high (33 °C) ocean temperatures of this period (Wilkinson et al., 1999) and it has been predicted that the vegetation cover of Madagascar will show negative responses to a possible increasing frequency of El Niño Southern Oscillation (ENSO) phenomenon that is associated with global climate change (Ingram & Dawson, 2005).
For this study, we selected the Tsaratanana Massif in northern Madagascar (sometimes referred to as the Northern Highlands) as the region of investigation (Fig. 1). We chose this massif for the following reasons: (1) the massif includes the highest summit in Madagascar (Maromokotro, 2876 m elevation, 14°01′S, 48°58′E); (2) the massif is rich in locally endemic species (e.g. Jenkins, 1987; Raxworthy & Nussbaum, 1997; Fig. 2); (3) the massif includes an intact transect of primary habitat between 1400 and 2876 m, and (4) during 1993 and 2003 we conducted distributional surveys of the herpetofauna (reptiles and amphibians) of the Tsaratanana Massif using the same elevational sampling transect and research camps. To the best of our knowledge, these surveys represent the only repeat biological surveys of any montane elevational transect in Madagascar over the past two decades.
The objectives of this study are fivefold: (1) determine if there is evidence for temperature warming trends for northern Madagascar (including the Tsaratanana Massif) based on meteorological observations and gridded monthly mean observations constrained to the period of herpetological surveys; (2) compare these meteorological observations with historical climate simulations; (3) apply a simple elevational range displacement analysis (based on lapse rates) to estimate upslope displacement at Tsaratanana; (4) determine whether there is supporting evidence for current upslope shifts of reptiles and amphibians at Tsaratanana; and (5) assess potential extinction vulnerability from upslope displacement for this and other montane assemblages in Madagascar.