The contribution of agroforestry systems to bird conservation in the Andes
Molly E. McDermott, PhD
Advisors: Amanda D. Rodewald and Stephen N. Matthews
In the face of continued loss and fragmentation of Andean forests, shade agroforestry is a promising approach where agricultural production and conservation can co-occur to support environmental and socioeconomic needs. Agroforestry provides habitat for a diverse avifauna, including several migratory species of concern, and thus may significantly contribute to biodiversity conservation in tropical regions. Yet a variety of social and economic pressures have accelerated the conversion of certain agroforestry systems to land uses that are less compatible with conservation. Such is the case with shade-coffee, which continues to be converted to sun coffee and pastures for cattle grazing, which provide fewer ecosystem services, reduce habitat complexity, and support less biodiversity. These changing practices challenge the conservation community to identify alternative practices that can maintain some ability to support biodiversity. For example, silvopastoral systems, which combine grazing pastures and trees, have been increasingly recognized for their ability to restore degraded pasturelands and contribute more to biodiversity than conventional pastures.
Birds are an excellent focal group that can be used to evaluate the contributions of agroforestry systems to conservation. In the Andes, birds often congregate in large foraging flocks comprised of several species of resident and migratory birds. These mixed-species flocks can serve as useful indicators of the relative value of different agroforests because they contain diverse species representing several foraging guilds, are easily studied, and can be sensitive to anthropogenic disturbances. In addition, most Neotropical migratory birds join mixed-species foraging flocks during the overwintering period, with >75% of observed migrants participating in flocks. Thus, studying flocks attended by migratory birds is essential to understanding wintering ecology of these species. Given that agroforestry systems can provide critically important overwintering habitat for flocking migrants where little forest cover remains, my dissertation research aimed to inform management decisions for conservation of Andean flocks.
In my dissertation research, I examined habitat use and behavior of mixed-species foraging flocks and Neotropical migratory bird species among different agroforestry systems to better understand (1) relative conservation value of the systems, (2) physiognomic features that are associated with high avian use of shade-coffee and silvopastures, and (3) sensitivity of birds to characteristics of the surrounding landscape. I studied mixed-species foraging flocks in shade- coffee, shade-cardamom, secondary forest, and silvopasture in the northern Andes of Colombia. The study areas were located in Antioquia Department (5°49'N, 75°49'W) near the towns of Andes, Betania, Ciudad Bolívar, Jericó, Monserrate, Palermo, Salgar, and Támesis in the western chain and Fredonia in the central chain. In January and February of 2011-2013, I recorded species composition and size of flocks along with individual foraging heights for species within 446 flocks. For each flock, I also measured vegetative structure at flock and random locations within each habitat and quantified the composition and configuration of the surrounding landscape.
Results showed that agroforestry systems differed in their ability to support migratory birds and mixed-species flocks. Compared to other shade agroforestry systems, silvopastures were least used by resident forest specialists and Neotropical migratory birds, as indicated by patterns of occurrence and sex ratios. Less structurally-complex silvopastures supported smaller, less diverse flocks with fewer Neotropical migrants and resident forest specialists. Not a single migrant species reached its greatest abundance in silvopasture flocks relative to other habitats, although patterns varied by species. Overall migrant abundances in flocks tended to be highest in shade-cardamom and shade-coffee, although overall flock size and richness were greater in secondary forest. Further, fewer male Blackburnian Warblers flocked in silvopastures compared to other habitats, which is consistent with other studies showing that overwintering male migrants tend to occupy the highest quality habitats. Although forest birds may be better served by silvopastures than conventional treeless pastures, my findings suggest that silvopastoral systems are less suitable for flocking birds than other secondary forest and other shade-agroforestry.
However, agroforestry remains an important complementary approach to other strategies to maintain Andean forests and improve the conservation potential of degraded land.
Despite marked differences in flock use among habitat types, there remains trouble in generalizing the effects of any single habitat type given the high variation in management practices within agroforestry systems. This variability necessitated the generation of habitat- specific recommendations within these systems to improve suitability for flocking birds. My examination of habitat use of Neotropical migrants in diverse mixed-species flocks was restricted to vegetative features within two agroforestry systems: shade-coffee (n=44 plots) and silvopasture (n=40 plots) plots. My results indicate that increasing canopy cover and tree density are important for use of both shade-coffee and silvopastoral systems by foraging flocks. Basal area and structural complexity measures were positively associated with abundance of several migratory bird species within flocks, suggesting that complex agroforests with a mid-range of canopy cover will provide the most suitable habitat for flocks with migrants. This study suggests that suitability of shade-coffee and silvopastoral systems can be improved for overwintering migrants by increasing canopy cover to 25%-40% and incorporating emergent shade trees to reach a basal area of >5 m2/ha, especially if by using tree species known to have multiple benefits.
Although my work showed how habitat features within agroforestry were important to flocking birds, the literature shows that attributes of the surrounding landscape can mediate bird habitat use—something that has been largely unexplored in agroforestry systems. Thus, I aimed to identify how landscape composition and configuration influenced use of agroforests by mixed-species foraging flocks, declining species of Neotropical migrants, and sensitive forest resident specialists. I used an information theoretic approach to determine which landscape metrics, at multiple spatial scales, had most support in explaining bird use of agroforestry habitats.
Landscape composition and configuration mediated avian species richness and abundance within agroforestry systems. Specifically, resident forest specialists and several migrant species increased with woodland cover within 1 km. Forest connectivity also proved to be positively related to abundance of forest resident specialists across all habitats surveyed. The negative relationship between patch density and abundance of two species of concern, Cerulean Warbler and Canada Warbler suggests that migrants may be sensitive to fragmentation, but only in shade- coffee and forest plots, respectively. This study suggests that efforts to improve the quality of the landscape matrix, particularly by increasing tree cover and connectivity (e.g., forested corridors) will improve the suitability of agroforestry habitats for flocking and migratory birds.
My research advances our understanding of the ecology of mixed-species flocks in agroforestry systems, especially little studied silvopastures, and the response of birds to both microhabitat and landscape features in different agroforestry habitats. Overall, I have demonstrated the potential conservation value of silvopastures to mixed-species flocks, although they may be less suitable for some groups of Andean birds. Fine scale vegetation features, especially canopy cover, vegetative complexity, and overstory tree basal area, appear to be important to predict bird use of agroforestry farms, while elements in the surrounding landscape may influence use by resident forest specialists and flocking Neotropical migrant birds. I show the potential for these agroforestry systems to contribute to conservation of Andean birds by enhancing structural heterogeneity within farms and woodland connectivity in the surrounding landscape. This study provides support for agroforestry as an important complementary strategy for conservation of flocking Andean forest birds, and demonstrates the need to consider the landscape context for management of agroforestry habitats.