Disappearance of insectivorous birds from tropical forest fragments

Abstract

Determining the impact of forest disturbance and fragmentation on tropical biotas is a central goal of conservation biology. Among tropical forest birds, understory insectivores are particularly sensitive to habitat disturbance and fragmentation, despite their relatively small sizes and freedom from hunting pressure. Why these birds are especially vulnerable to fragmentation is not known. Our data indicate that the best determinant of the persistence of understory insectivorous birds in small fragments is the ability to disperse through deforested countryside habitats. This finding contradicts our initial hypothesis that the decline of insectivorous birds in forest fragments is caused by impoverished invertebrate prey base in fragments. Although we observed significantly fewer insectivorous birds in smaller fragments, extensive sampling of invertebrate communities (106,082 individuals) and avian diets (of 735 birds) revealed no important differences between large and small fragments. Neither habitat specificity nor drier fragment microclimates seemed critical. Bird species that were less affected by forest fragmentation were, in general, those that used the deforested countryside more, and we suggest that the key to their conservation will be found there.

Figure 1

Comparisons of abundance and species richness of birds captured with mist nets. The asterisks indicate a significant difference (P < 0.05). (A) More insectivorous birds were captured along forest transects (t = 2.67, P = 0.0089), and more omnivorous (t = 2.70, P = 0.0081) and granivorous (t = 2.18, P = 0.0315) birds were captured along fragment transects. Guild assignments were based on ref. . (B) More high forest-dependent species were captured along forest transects (t = 3.63, P = 0.0005) and more low forest-dependent species (t = 2.40, P = 0.019) were captured along fragment transects. High, intermediate, and low forest-dependent classes correspond to forest-dependent, forest generalist, and nonforest species in ref. . (C) Observed and estimated species richness values are based on methods provided in ESTIMATES (ref. , http://viceroy.eeb.uconn.edu/EstimateS). OBS, observed number of species; ACE, abundance-based coverage estimator; ICE, incidence-based coverage estimator; BTS, bootstrap; CH, Chao; JK, Jackknife; MM, Michaelis–Menten. SEs for Chao1, Chao2, and Jackknife1 methods are included.

Figure 2

Comparisons of average number, length, and dry weight of invertebrates captured. For sample sizes, see Table 1. With 1-week pitfall traps and searches, only invertebrates ≥ 5 mm were taken into consideration. The asterisks indicate a significant difference (P < 0.001). (A) Average number of invertebrates captured/sample. (B) Average length of invertebrates captured (mm). (C) Average dry weight of invertebrates/sample (mg). For sticky traps and 1-day pitfall traps, dry weight was estimated by using the appropriate regression equations (19).

Figure 3

Increased species diversity of a bird family in the nonforest matrix correlates with increased species diversity in small fragments (r² = 0.714, P < 0.0001). Families that are not well sampled with mist nets, such as Accipitridae, Strigidae, Trogonidae, and Cotingidae, were not included. The data for all of the species in the study region were provided by James Zook (unpublished data). Note that the four families in the superfamily Furnarioidea, which is particularly sensitive to forest disturbance (2), have the lowest values and are marked with circles.