Great Tinamous are surprisingly light-footed for being rather awkwardly shaped. They wander delicately on the rainforest floor and call to one another in haunting tones at dawn and dusk. When approached by people, they disappear nervously into the underbrush.
A Great Tinamou (Tinamus major) in the understory of an aging secondary forest adjacent to old growth forest at Las Cruces Biological Reserve, Costa Rica. Photo by J. Leighton Reid.
This shy, sun-dappled lifestyle works well in endless forest, but it complicates the task of restoring tinamou habitat. You can plant native trees in an old farm field, but a Great Tinamou won’t use the site if it has to cross a kilometer of cow pasture to get to it. Maybe not even one hundred meters. Even if a restoration site is nearby, a Great Tinamou might not use it if the habitat isn’t right. Why would it? And why should anyone care?
Birds both promote and prosper from forest restoration
Birds provide key ecosystem functions that are important for humans. Hummingbirds pollinate, tanagers disperse seeds, and warblers eat leaf-chewing caterpillars, for example. Some of these functions help overcome barriers to forest regeneration, like the barrier of not having enough tree seeds to grow a forest. Birds are also biodiversity indicators – if they’re not doing well, probably other things aren’t doing well either. So understanding how and where to restore forests for birds carries implications for the larger ecosystem.
For the past six years, my colleagues and I have monitored bird biodiversity and behavior in a forest restoration experiment in southern Costa Rica. This experiment, started by Drs. Karen Holl and Zak Zahawi, addresses the question, “How and where can we best restore tropical forests?” Each of the 13 restoration sites on old farmlands (mostly cow pastures) contains three 50 × 50 m plots treated with different interventions. Some areas were left to regenerate naturally – a low intensity intervention – whereas others were established as plantations of native and naturalized tree species – a high intensity intervention. A third treatment is intermediate – small tree islands were planted with the expectation that they would expand and coalesce over time. This set-up not only lets us look at how restoration techniques influence ecosystem regeneration, it also lets us see how this relationship varies across a diverse agricultural landscape.
Visible impacts of forest restoration. The foreground area was a cow pasture that was abandoned to regenerate naturally. The background is a tree plantation, planted in 2005. Photo by Karen D. Holl.
Our observations show that what kinds of birds return to degraded sites depends on both how the site is restored and where it is located. Birds affiliated with mature forests – like Great Tinamous – most often visit intensively-restored plantations that are located in more intact, forested landscapes. This is likely because tree plantations more closely mimic a mature forest understory environment and sites in well-forested landscapes are easier for these shy birds to access. However, even nice tree plantations are rarely visited by forest-affiliated birds when they are in agricultural landscapes far from large forest fragments.
Still, restoring forests in more seriously deforested areas isn’t futile. Tree plantations across the board have greater numbers of fruit-, nectar-, and insect- birds than natural regeneration plots. Tree island plots are intermediate. When we first saw these results, a collaborator, Chase Mendenhall, quipped “I guess birds like trees”. This could be because trees provide refreshing shade, cover from predators, and food – like fruit, nectar, and insects. If planting trees attracts fruit-eating birds, it stands to reason that tree plantations and tree islands should have more seeds dispersed into them; our seed dispersal observations support this conclusion. Seed dispersal is a key ecosystem function that contributes to forest regeneration, so it’s promising that planting trees anywhere in a heterogeneous landscape attracted the birds that do this work.
Accessibility is key
At the policy level, our bird observations suggest that to increase habitat areas for mature forest-affiliated bird species it’s important to put restoration sites in places that those species can access them, like degraded land adjacent to old-growth forest. In contrast, upgrading ecosystem functions might be done more efficiently by planting trees in highly degraded sites, where an influx of local birds could kick-start ecosystem development. These observations highlight that landscape forest restoration strategies will not necessarily optimize biodiversity conservation and ecosystem functioning objectives simultaneously.
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