Population responses of pest birds across a forest cover gradient in the Chaco ecosystem

Abstract

Industrial agriculture can trigger an increase in abundance of some bird species, promoting the emergence of pests, causing damage to crops and economic loss for farmers. Even though there are different strategies to control pest birds (chemicals, hunting, repellents, etc), in general they are inefficient. Moreover, these strategies can threaten non-target species, and toxic substances can be bioaccumulated in the food chain of wildlife animals. An alternative strategy (or at least complementary) could be the design of agricultural landscapes that balance natural habitats and crop production areas in a way that would not trigger the abundance increases of potential pest birds. We assessed population trends of four bird species (potentially agricultural pests) across a forest cover gradient, and consequently, a gradient of agricultural cover expansion, at a landscape level. The study was conducted in the Chaco forest, one of the most threatened ecosystems worldwide due to agricultural expansion. Studied species were Eared Dove (Zenaida auriculata), Spot-winged Pigeon (Patagioenas maculosa), Picazuro Pigeon (Patagioenas picazuro) and Monk Parakeet (Myiopsitta monachus), and their populations are often considered as the major pest birds for agriculture in several areas of Argentina and in other countries of South America. Abundances of birds species were measured in focal sites (forest patches) and the percentage of forest cover was estimated within buffers of 2000 m radius surrounding each site. A total of 12 local landscapes were surveyed. Results show that population responses (i.e. changes in abundance) to the gradient of forest cover were idiosyncratic. The main result shows that relative abundance of Eared Dove, the most abundant species, was triggered when landscapes had less than 16% of forest cover: landscapes with low forest cover (less than 10%) contained six times the relative abundance of Eared Dove than landscapes with a high percentage of forest cover (close to 100%). We propose that as pest birds in agroecosystems are a problem at the landscape or regional scale, solutions should be thought at such scale. In this context, landscapes in the region should conserve more than 16% of forest cover to attempt to control one of the most abundant pest bird species of South America.