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Still hope for tropical biodiversity in human modified landscapes

As primary forests become increasingly rare and expensive to protect, many ecologists are looking to better management of Human Modified Landscapes (HMLs) to shepherd and shield biodiversity in the tropics. Secondary forests, selectively logged forests and lands devoted to sustainable agriculture already play an important role in conservation efforts. However, the idea that HMLs will serve as a “Noah’s Ark” for biodiversity, is controversial.

In the 2013 paper, “On the hope for biodiversity friendly tropical landscapes” published in Trends in Ecology and Evolution, scientists venture into the complex topic of HMLs, setting up a framework that connects disturbance, land use, ecosystem services and biodiversity. The paper also offers a model showing potential outcomes for forests in response to different styles of forest management.

“The most important controversy is that the HML is sometimes assumed by default as viable conservation landscapes, mostly based on the number of species they can harbor at the present,” lead author Felipe Melo of Universidade Federal de Pernambuco, Brazil, told “However, we argue that biodiversity persistence in HML is completely context-dependent.”

Context does seem to be key. Along with human use and disturbance, forest size, type, and the surrounding landscape matrix all affect the biodiversity friendliness of an HML.

The landscape matrix is the background cover type in a landscape, the stuff between and around the HMLs. An HML is limited in biodiversity conservation potential by this very matrix. For instance, a forest fragment embedded in a harsh matrix, such as a large clear-cut area, does not fare well. Yet even small patches of tropical forest can support long-term biodiversity if they are embedded in a large-scale forested landscape.

Land sparing versus land sharing? Here Amazon rainforest abuts against industrial soy fields. Photo by: Rhett A. Butler.

Land management practices vary, and researchers are at odds as to which best support biodiversity. Beyond this management-style disagreement, a debate exists known as “land sparing versus land sharing.” Land sparing argues that setting aside intact ecosystems, for example in large parks, while dedicating remaining lands to intensive agriculture is the best approach to species conservation. Land sharing, however, argues against industrial agriculture urging mixed land use, for examples supporting farming practices such as agroforestry, which combines forest components or tree planting with the growth of products for people.

Research supports both schools of thought. In several instances, “land sharing” agroforestry practices have proven effective in retaining biodiversity. In the Western Ghats of India, areca nut plantations still retain 90 percent of the bird diversity of native forests in the region, after two thousand years of cultivation in the area. Due to costs, scientists often measure bird and tree diversity as biodiversity indicators, and many data sets support the idea that shade-grown crops support bird diversity. This has left many questioning the effects on other species. In the Brazilian Atlantic forests where less than 6 percent of primary forest remains, one study found that in addition to birds and trees, 70 percent of bat, butterfly, fern, frog, and mammal species (to name a few) flourished in shade grown cacao crops.

A 2011 study conducted in Ghana and published in Science came out in favor of “land sparing” when researchers concluded that setting aside land for protection and using other land for intensive agriculture was the best way to both feed people and conserve species. In later comments about the study on, the lead researcher made it clear that they were not making a case against agroforestry, but that they encourage both “land sparing” and a move away from harmful industrial agricultural practices such as monocultures.

When asked for some successful cases of HMLs as havens for biodiversity, Melo replied, “There are no safe havens for biodiversity, at all. Even protected areas are suffering from local extinctions of forest dependent species. Amongst the HML, however, we believe that low-intensity agriculture practices such as agroforestry and some shaded cultures such as cacao and coffee should be friendlier to biodiversity. Thus, the main challenge is making such wildlife-friendly farming more profitable and competitive.”

The study by Melo and his team provides a conceptual model showing the results of different styles of forest management for forests. Given the overall complexity of the topic, this model represents a major contribution to the field.

“The model aims to show that some key landscape attributes such as the amount of old-growth forest habitat and connectivity among forest patches will determine the potential for natural regeneration and the probability of long-term persistence in the HML,” Melo says. “The take-home message being that lack of management and forest protection will drive HML to degraded state that is likely to be irreversible without human intervention such as forest restoration.”

Many questions remain unanswered in this emerging field, which Melo calls “the ecology of disturbance,” especially in regards to how ecosystems will perform in a human matrix. Given this, the study ends with a call for more research: “Classical ecological theory was developed to explain the world under ‘normal’ circumstances and to understand the ecology of the disturbance should be a good chance to refine our understanding of the real world.”

CITATION: Melo, F., Arroyo-Rodriguéz, V., Fahrig, L., Martínez-Ramos, M., Tabarelli, M. On the hope for biodiversity friendly tropical landscapes. Trends in Ecology and Evolution, 2013.

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