Conservation value of silvopastures to Neotropical migrants in Andean forest flocks
Introduction
With ever-growing demands on the environment, society is challenged to find ways that conservation can occur within working landscapes. This is especially true in biodiversity hot spots such as the northern Andes mountains of South America, where deforestation and agricultural development have left little forest intact. Agroforestry is a strategy that can reduce habitat loss for forest species while producing high yields and supporting rural livelihoods (Perfecto and Vandermeer, 2010). However, not all approaches are comparable in terms of ecological value. Market pressures can provoke conversion from more to less environmentally-friendly practices, usually a quick transformation that can take decades to reverse. For example, in Colombia >60% of all shade coffee plantations were converted to structurally simple sun coffee (Perfecto et al., 1996, Rice and Ward, 1996), and rangelands doubled from 1960 to 1995 (Murgueitio et al., 2011). The conversion of agroforestry systems to land uses with less vegetative complexity, such as pasture for cattle grazing, is one example where socioeconomic pressures, including a volatile coffee market and changing demographics, may diminish the ability of land to meet conservation needs (Camargo et al., 2005).
Shade agroforestry is widely known to support diverse bird communities, including migratory birds that are more numerous in shaded crops than large-scale commercial agriculture (Perfecto et al., 1996, Moguel and Toledo, 1999, Rice and Greenberg, 2000, Roberts et al., 2000, Pomara et al., 2003, Komar, 2006, Bhagwat et al., 2008). Although shade plantations may lack resident forest specialists, they usually harbor diverse and abundant populations of migrant species (Greenberg et al., 1997a, Greenberg et al., 2000, Wunderle and Latta, 2000). In fact, abundance of many Neotropical migrants is often greater in shade-coffee compared to primary forest (Greenberg et al., 1997c, Petit et al., 1999, Johnson and Sherry, 2001, Tejeda-Cruz and Sutherland, 2004, Bakermans et al., 2009). Thus, trends in agroforestry conversion may have serious consequences for migratory species such as the Cerulean Warbler (Setophaga cerulea), whose steady population decline may be linked to loss of forest cover in the Andean coffee-growing region.
Among the most striking features of agroforestry systems is their ability to support a diverse group of resident birds, many of which associate closely with Neotropical migrants in mixed-species foraging flocks. This phenomenon is particularly common and widespread in the Andes, where flocks are distinctive because of the participation of diverse foraging guilds using multiple strata, the high representation of tanagers (Thraupidae), and for being among the largest recorded mixed-species flocks, frequently with 10–20 species and upwards of 30 individuals (Moynihan, 1979, Remsen, 1985, Bohórquez, 2003, Arbeláez-Cortés et al., 2011). Mixed-species flocks are important features of Andean forest ecosystems, and in agricultural landscapes as many as 85% of species within the local bird community may participate when migrants are present (pers. obs.). As many migrant species forage almost exclusively in flocks, overwinter survival is thought to be influenced by these flocking systems.
Though shade crops such as coffee and cacao are only grown on a small proportion of global agricultural land, expansive rangelands have great potential to be developed into sustainable systems incorporating trees and shrubs, known as silvopastoral systems. An alternative to conventional cattle grazing, silvopastures have been little studied compared to other agroforestry. Silvopastoral systems are receiving worldwide attention for their economic and conservation potential in agricultural landscapes, including Andean regions (Murgueitio et al., 2011, Cubbage et al., 2012). Planting trees in rangelands may partially mitigate negative environmental impacts of grazing such as deforestation, erosion, and water pollution and provide more ecosystem services such as increased nutrient recycling, soil fertility, and carbon sequestration compared to conventional pastures (Nair, 1989, Murgueitio and Calle, 1999, Chará and Murgueitio, 2005, Ibrahim et al., 2006, Haile et al., 2010). Furthermore, the shade trees in silvopastures provide marketable saw-timber, firewood, fruits, and seeds (Bellefontaine et al., 2002). Although they exhibit less structural complexity compared to many other agroforestry systems, silvopastures provide perches, shelter, foraging, and corridors for a wide variety of resident and migratory birds (Harvey et al., 2005, Fajardo et al., 2009, Rice and Greenberg, 2004). Greenberg et al. (1997b) found that Acacia pennatula groves used for grazing in Mexico had the highest density and diversity of many migratory bird species compared to other habitats. Though not as biodiverse as forests, silvopastures in Colombia support many species and individual birds (Fajardo et al., 2009), but little is known of the relative value of silvopastures to foraging flocks and migrant birds.
In this study we compared relative habitat use of silvopasture, shade-coffee, shade-cardamom, and secondary forest by mixed-species flocks and overwintering migratory birds, with the ultimate aim of informing agroforestry management in the Andes. Specifically, we examined how vegetative structure and flocking bird communities differed among these habitats. Reductions in habitat complexity resulting from land conversion, such as forest lost to pasture or silvopasture, are likely to render habitat less suitable for flocks by impacting resource availability. Simplification of habitat and loss of floristic diversity diminish wintering migrant diversity (Rappole and Morton, 1985, Lynch, 1992) and can reduce functional diversity of the avian community, such as fewer forest understory specialists (Sekercioglu, 2012). Loss of vegetative strata reduces foraging substrates, which could reduce the likelihood of certain species occurring and/or joining flocks. Therefore, we predicted that complex secondary forest would have the largest and most diverse flocks showing greater vertical distribution of individuals compared to structurally simple silvopastures, which were expected to have the least diverse flocks. Because sex segregation can indicate habitat quality, with males occupying the best habitats on the wintering grounds (Sherry and Holmes, 1996, Latta and Faaborg, 2002), we hypothesized that proportion of males of migrant species would be greatest in the most structurally-complex habitats and lowest in silvopastures.
Section snippets
Study areas
Mixed-species foraging flocks were studied in the northern Andes of Colombia, Antioquia Department (5°49′N, 75°49′W). The Colombian Andes are comprised of three mountain chains which divide near the Ecuadorian border: the Cordillera Occidental, or western chain, Cordillera Central in the center, and Cordillera Oriental, the eastern chain. Our study areas were located near the towns of Andes, Betania, Ciudad Bolívar, Jericó, Monserrate (Jardín municipality), Palermo (Támesis municipality),
Results
In three seasons a total of 136 species were recorded in 446 flocks, comprised of 111 resident species and 24 Neotropical migrants. One species, the Red-eyed Vireo (Vireo olivaceus), could not be classified as either because it has both resident and migrant populations in Colombia. Data were collected on 166 flocks in shade-coffee, 135 in silvopastures, 82 in shade-cardamom and 63 in secondary forest, and the total number of species observed in flocks in each habitat type was 121, 115, 105, and
Discussion
This study suggests that silvopastoral systems provide less suitable habitat to flocking birds than other Andean forested habitats. Flocks in silvopastures were smaller, less diverse and comprised of fewer forest-associated residents and Neotropical migrants than those using shade-coffee, shade-cardamom, or secondary forest. In addition, for one common migratory species, the Blackburnian Warbler, fewer males used silvopastures, which is consistent with other studies showing that males tend to
Conclusions
Our findings are consistent with previous studies showing that agroforestry practices, including silvopastures, support diverse and abundant bird communities with species of conservation concern. Further, they do so to a greater extent than reported for conventional pasture and sun coffee plantations (Greenberg et al., 1997a, Petit et al., 1999, Tejeda-Cruz and Sutherland, 2004 Harvey et al., 2006, Sáenz et al., 2006, Borkhataria et al., 2012). That said, our results illustrate that caution
Acknowledgements
We gratefully acknowledge financial support from National Fish and Wildlife Foundation and National Council for Air and Stream Improvement, the United States Fish and Wildlife Service, the Center for Latin American Studies (Tinker Field Research Grant), and the Explorers Club Exploration Fund. The Ohio State University and Terrestrial Wildlife Ecology Lab provided administrative and logistical support. L. Schofield, D. Slager, M. Shumar, J. Muñoz, and S. Chaparro contributed valuable field
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