Avian conservation in north-central Puerto Rico: Assessing the conservation value of shaded coffee plantations and the influence of nest location and habitat on nest predation rates
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2006-01-23
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Abstract
As size and number of undisturbed forests dwindle due to human encroachment, the importance of disturbed areas, such as secondary forests and shaded coffee plantations, for conservation of avifauna has risen. These 'functional substitutions' are not only recognized and valued at present, but in some cases, they may have played important conservation roles in the past. It is hypothesized that shaded coffee plantations in Puerto Rico played such a role by minimizing extinction rates of resident forest avifauna during periods of widespread deforestation during the 19th and 20th centuries. Implicit in this refugia hypothesis is that shaded coffee plantations harbored successful breeding populations. Data to evaluate this possibility, however, were not available. Likewise, data on breeding productivity and other measures of reproduction were not available for secondary forests, which as of 1992 totaled 41% of the island's forest cover. These measures are intimately related to fitness and, therefore, provide a useful basis to guide habitat conservation initiatives and management.
In this study, I report on the reproductive activity and productivity of resident avifauna in shaded coffee plantations and compare them with similar data from secondary forests in Puerto Rico from 1997 to 1999. During that period, 253 nests were found in coffee plantations (12 ha) and 97 in secondary forests (45 ha). The composition of resident species found nesting in shaded coffee plantations and secondary forests was similar. Some of the most common nesting species were endemics. Predation and nest abandonment each accounted for 38% of nest failures. Nest success did not differ between secondary forest and shaded coffee plantations for vireos, tanagers, and hummingbirds. Similarly, breeding productivity did not differ between habitat types for four species for which sufficient data were available. Although the fate of fledglings (e.g., dispersal, survival) was not followed, documenting successful reproduction in shaded coffee plantations confirmed a fundamental assumption of the refugia hypothesis. This finding, coupled with the fact that shaded coffee plantations were more widespread in the past and were managed as rustic plantations and traditional polycultures (resembling primary forests), lends considerable support to the notion that shaded coffee plantations have served as a refuge for resident avian species during periods of widespread deforestation.
The production capacity of shaded coffee plantations was dependent on nesting substrates provided by the shade vegetation (canopy) layer, not the coffee tree layer. Nesting activity in secondary forests also occurred primarily in the canopy layer. To investigate the basis of this pattern, I tested whether the observed nesting patterns were influenced by differential predation pressure between the understory and canopy layers. I also tested whether predation pressure differed between canopy layers of plantations and secondary forests to further assess the conservation value of shaded plantations. Tests were conducted using a series of carefully designed artificial nest experiments during the breeding season of 2005. I found that predation rates ranged from 0.44 to 0.77 (average = 0.65 ± 0.06 SE) for understory and from 0.45 to 0.80 (average = 0.65 ± 0.05 SE) for canopy nest heights. Predation rates ranged from 0.67 to 1.0 (average = 0.84 ± 0.06 SE) in shaded coffee plantations, and from 0.63 to 0.97 (average = 0.81 ± 0.06 SE) in adjacent secondary forest. Rates for both experiments were not significantly different (P > 0.05). Based on photographic evidence, avian and mammalian nest predators can prey upon nests regardless of height. Experiments and data from natural nests suggest that birds nesting in shaded plantations are not at a disadvantage compared to those nesting in secondary forests. Moreover, the pervasive nature of nest predation reported in this work, and its influence on habitat quality, affirmed the need to identify and manage habitat features associated with nest success.
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avian conservation, artificial nests, predation, Puerto Rico, shaded coffee
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Degree
MS
Discipline
Zoology
