September 19, 2012

Big Farms, Small Farms, and Biodiversity

With the IUCN World Conservation Congress complete and the Convention on Biological Diversity still ahead, the Landscapes Blog turns toward focusing on issues of agriculture and biodiversity. A new study in the journal Ecology Letters describes the bird diversity within Costa Rican agricultural landscapes. Lead author Daniel Karp, a doctoral candidate at Stanford University, shares the significance of the research for people, food, and nature.

Trees snake through the Coto Brus Valley of southern Costa Rica, diverging into long fencerows beside coffee farms, and reuniting in small forest patches. This scene was once typical of much of the tropics, where multiple crops were grown together, amidst a network of native trees and forest patches. Now, tropical landscapes are changing as smallholder farms are replaced by large-scale high-intensity, commercial agricultural operations. Rather than a mosaic of trees and crops, vast fields of agriculture stretch for miles with almost no remaining native vegetation.

For several decades, our team of biologists at Stanford University’s Center for Conservation Biology has studied the consequences of this agricultural intensification for biodiversity. Most of Earth’s biodiversity exists because different habitats host different sets of species. The bird communities that inhabit the dry forests of northern Costa Rica are very different from those that live in the rainforests of southern Costa Rica. But tropical forests are being cleared rapidly for agriculture. Will farms located many miles away from each other continue to maintain different sets of bird species or always host a similar set of species? We set out to answer this question by examining bird communities in Costa Rican farmland.

Since 1999, Jim Zook, an expert tropical ornithologist, has surveyed birds across Costa Rica along transects, or 200m stretches of roads and paths that wind through forest, low-intensity agriculture, and high-intensity agriculture. So far, Jim has identified over 130,000 birds of 487 species. Using this incredibly comprehensive dataset, we found that similar species tend to be found in high-intensity agriculture, even for sites hundreds of miles apart. In contrast, low-intensity farms in different regions had many different species. By replacing native trees with a single crop variety, farmers transformed distinct habitats into uniform cropland, and the limited sets of bird species that thrived in these conditions spread over great distances. In contrast, low-intensity family farms maintained distinct vegetation types; therefore, different bird communities were found in farms in different regions.

These results indicate that if agricultural intensification continues unabated, biodiversity will decline beyond current dire predictions. Agricultural land grabs in Africa number in the hundreds of thousands of hectares, and agriculture may replace forty percent of the Amazon by 2050. While biologists typically account for losses in diversity on the farm, the extra diversity decline that results from the spread of a characteristic set of farmland species across great distances often goes unnoticed.

But there is hope. Local, low-intensity farming practices improve habitat for many species. Moreover, individual landowners can sustain biodiversity not only on their own farms but also at much larger scales because low-intensity agricultural practices retain distinct bird communities in different regions. The farmers themselves may even benefit from conservation, as an increasing body of research shows that native wildlife can pay farmers back. Maintaining small patches of forest supports bees that pollinate coffee. Birds that rely on small patches of forest often venture into agricultural fields to consume insect pests. Boosts in farm yields from these species may increase farm revenues by tens of thousands of dollars annually.

With a growing human population and per capita consumption, pressure to expand and intensify agriculture has never been higher. However, we find that lowering the intensity of agricultural practices in some dimensions may result in great conservation gains and can even benefit the farmers themselves.

Read a news article and access the full paper online for more details.

Photo credits: Daniel Karp

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