Climate change and birds in the forgotten tropics: the importance of tropical dry forests

In a world with limited financial and human resources, scientists are forced to evaluate research priorities and focus on questions of particular concern (ter Steege 2010). Recently, Harris et al. (2011) argued that more research is needed on the potential effects of climate change in tropical ecosystems (see also Felton et al. 2009). We agree that this topic is poorly researched but are concerned that their article focused exclusively on wet tropical forests, even though dry forests constitute 42% of all tropical forest (Murphy & Lugo 1986). Although large areas of tropical wet forest remain, dry tropical and subtropical forests are endangered everywhere (Olson & Dinerstein 1998, Hoekstra et al. 2005), with < 2% of original forest remaining in some regions (Janzen 1988). Moreover, 97% of the dry tropical forest remaining is considered to be at high risk from one or more threats, including anthropogenic activities and climate change (Miles et al. 2006). Despite this, research has historically focused on wet tropical forests (Sanchez-Azofeifa & Portillo-Quintero 2011). In some regions, dry tropical forests have higher bird species diversity than nearby wet forests (Kepler & Kepler 1970); even when dry forest diversity is lower, many species are endemic to, or have their highest densities in, dry forest ecosystems, and the proportion of species endemic to the habitat is higher (Stotz et al. 1996). The high rate of endemism in dry tropical forests, and the high rate of regional endemism (Stotz et al. 1996) is cause for concern, as specialists are thought to be more vulnerable to climate change than are generalists (Julliard et al. 2004). Nearly 80% of bird species restricted to tropical dry forests are considered to be at risk, a far higher percentage than in other tropical forests (Stotz et al. 1996). The direct effects of climate change vary among tropical regions, with expected temperature increases in all tropical dry forests but region-specific changes in precipitation regimes (Christensen et al. 2007, Meir & Pennington 2011). However, tropical dry forests are among the ecoregions expected to experience the earliest and most severe impacts of climate change (Enquist 2002, Beaumont et al. 2011). In the Neotropics, increased temperatures and decreased precipitation will interact to reduce net available moisture, particularly during the dry season (Singh 1997, Christensen et al. 2007, Meir & Pennington 2011). High temperatures are associated with direct reductions in avian reproductive success and survival, even in wetter or more temperate regions (Williams & Middleton 2008, Santisteban et al. 2012; W. Cox, F. Thompson, J. Reidy & J. Faaborg, unpubl. data). In addition, tropical arthropods and reptiles respond negatively to increasing temperatures (Deutsch et al. 2008, Huey et al. 2009, Bonebrake & Deutsch 2011), altering the food chain and exacerbating the current limitation of food resources during the dry season (Johnson et al. 2006, Toms 2011). Climate change will also interact with other stressors (Brook et al. 2008, Mantyka-Pringle et al. 2012). Dry tropical forests are already heavily impacted by human activities such as agriculture and urban development (Trejo & Dirzo 2000, Fajardo et al. 2005, PortilloQuintero & Sanchez-Azofeifa 2010, Sanchez-Azofeifa & Portillo-Quintero 2011). Conversion of habitat to urban development or agriculture can restrict the ability of species to adjust their range as their climate envelopes shift (Opdam & Wascher 2004). This problem is likely to be most pronounced in ecosystem types where the remaining habitat is limited to highly isolated patches, such as tropical dry forests (Miles et al. 2006, PortilloQuintero & Sanchez-Azofeifa 2010). Importantly, differences in underlying soil properties mean that reductions in net available moisture in moist tropical forests will lead to tropical savannah rather than dry forests in most areas (Quesada et al. 2009). Additionally, climate change can increase the rate of habitat loss by introducing novel stressors such as fire and increasing the prevalence of other stressors such as invasive species, parasites or disease (Vitousek et al. 1997, Harvell et al. 2002, Sanchez-Azofeifa & Portillo-Quintero 2011). In dry regions, climate change will also interact with human activities to increase the risk of desertification (Mabbutt 1989, Leal et al. 2005). The main recommendation of Harris et al. (2011) was to increase biodiversity monitoring throughout wet *Corresponding author. Email: jdthv6@mail.missouri.edu