Conversion of tropical forests to farms is a big driver of wildlife extinctions. But a new study shows that some farming practices have the potential to simultaneously protect natural habitats and boost farm yields

One of the main drivers of extinction is habitat loss that arises when tropical rainforests are converted to farms -- a trend that is escalating at a dramatic rate. But are some farming practices less harmful to local birds and other wild animals? This was the question asked in a study published today in the journal Current Biology. In this study, the authors recorded the overall diversity of bird species living on land-sharing cattle farms -- these appear to be wildlife-friendly because they include isolated trees and forest patches -- and compared it to land-sparing cattle farms that lack trees and forest patches and to neighbouring contiguous forest. The researchers found that land-sharing cattle farms retain less total bird biodiversity than do more intensive land-sparing cattle farms -- but only if the neighbouring contiguous forest is protected from human exploitation. Additionally, they found that overall avian biodiversity drops off faster for land-sharing cattle farms that are located farther away from contiguous forest.

Facebook Twitter Pinterest Colombia’s Chocó-Andean cloud forest. Photograph: James Gilroy/David Edwards et al/Current Biology 2015

Converting forests to farms drives massive losses of avian biodiversity

The human population is projected to reach 11 billion by the end of this century, and the greatest increases will be in tropical developing nations. But at the same time, per-capita consumption of food and other resources will increase even faster due to climate change and to overall increases in global wealth. The vast majority of these demands will be met by a dramatic increase in agriculture, especially in the tropical regions of sub-Saharan Africa and South America. Tropical old-growth forests, woodlands and semi-arid regions are attractive for agricultural development because the local climate allows for year-round production on land that is less expensive than in other parts of the world.

But agriculture is one of the primary drivers of the current global extinction crisis -- a situation that is projected to dramatically increase in the near future (doi:10.1016/j.tree.2013.12.001). According to a 2008 report by the conservation group BirdLife International, agriculture is recognised as posing the greatest extinction risk to birds, contributing to the declines of 87 percent of threatened species. So if we can develop practices that reduce farming’s devastating impact on birds, then widely adopting these methods might lessen the harm done to non-avian species, too.

There are two strategies intended to reduce the impact of agriculture on nature. The land-sharing approach is where farms incorporate “wildlife-friendly” practices, such as retaining some isolated trees and forest fragments, adopting mixed cropping and reducing pesticide use, whereas the land-sparing approach maximises production on already-converted land whilst sparing neighbouring wildlife-rich habitats from human exploitation. Typically, land-sharing farms have lower production than do land-sparing farms, which, it is argued, could lead to increased encroachment of farms into natural areas.

But how can we decide whether land-sharing or land-sparing agriculture is a better strategy? There is a lot of evidence that species-rich communities are more productive, more ecologically stable, and provide more ecosystem services than those with fewer species (doi:10.1073/pnas.1301685110). Species-richness can be assessed by measuring phylogenetic diversity -- the total evolutionary history shared across all species within a community. Additionally, phylogenetic diversity is a good complement to other ways of measuring biodiversity, such as counting the numbers of threatened species.

“A habitat with a finch, a hummingbird and a flycatcher has more phylogenetic diversity than a habitat with three species of finches. This is because you have to look further back in time to find a common ancestor of the finch, hummingbird and flycatcher -- these three species represent many more years of separate evolution than do the three finch species, which have a more recent common ancestor”, explained conservation biologist Ben Phalan, a Zukerman Junior Research Fellow at King’s College, who was not part of this study.

Further, the experimental evidence suggests that communities with more distantly related (older and more evolutionarily distinct) species are the most productive of all (doi:10.1073/pnas.1301685110). Thus, in the absence of any other information, by simply working to protect both evolutionary history and distinctiveness in neighbouring forests, farmlands may also maximise their own productivity.

Facebook Twitter Pinterest The Munchique wood-wren, Henicorhina negreti, is a newly-described tropical member of the wren family (Troglodytidae). Photograph: James Gilroy/David Edwards et al/Current Biology 2015

To gain a clearer understanding of the impacts of cattle farming upon phylogenetic diversity, David Edwards, a Lecturer of Conservation Science at the University of Sheffield, assembled an international team of researchers to test these questions in Colombia’s Chocó-Andean forest.

“The Chocó-Andes are a hotspot of endemism and have been widely impacted by low-intensity farming, making this one of the most threatened faunas on Earth,” said Dr Edwards in email.

“It is vital to consider how best to farm here, but also to use this region as a model for how best to farm in other locations,” explained Dr Edwards.

Dr Edwards’ team sampled three study areas, each comprising contiguous forest abutting cattle pastures. The forests are primarily old-growth cloud forest whereas there were two types of cattle pastures: land-sharing cattle farms which included isolated trees and forest patches, and land-sparing cattle farms that lacked trees and forest patches.

The research team recorded 318 bird species from across the avian tree of life. These bird species represented a variety of lineages, many of which were songbirds. When the researchers mapped where each species was found, they found that numerous Oscine songbirds -- Passeroidea (mostly herbivorous songbirds, such as sparrows and warblers), Sylvoidea (mostly insectivorous songbirds, such as swallows, tits and babblers), and Corvoidea (corvids) -- along with clusters of Tyrannidae (flycatchers), and some Trochilidae (hummingbirds), tended to associate with farmed habitats, and were thus designated as “winners” (pink and lilac in Figure 1):

Facebook Twitter Pinterest Figure 1. Distribution of Chocó-Andean Birds categorized as follows: winners that do best in land sparing (pink); winners that do best in land sharing (lilac); losers that do best in land sparing (red); losers that do best in land sharing (purple). 6 alternatives, from inner to outer colored ring: high productivity (20% concession to conservation) 500m from forest; high productivity, 1000m; high productivity, 1500m; low productivity (80% concession to conservation), 500m from forest; low productivity, 1000m; low productivity, 1500m. Major nodes: Passerines (Pa), Suboscines (Su), Oscines (Os). Illustration: David P. Edwards et al/Current Biology 2015

In contrast, many non-passerine lineages, particularly Psittaciformes (parrots), Piciformes (woodpeckers), and Trogoniformes (trogons and quetzals), were primarily associated with forest and were designated as “losers” (red and blue in Figure 1).

Land-sparing farms save more avian phylogenetic diversity than land-sharing

Dr Edwards and his colleagues then used landscape simulations to examine the outcomes of land-sharing versus land-sparing farming practices as they affect avian phylogenetic diversity. These simulations used sampled data to predict phylogenetic diversity for farmland (yellow; Figure 2) and the neighbouring forest (green; Figure 2) to create a thought experiment. In this thought experiment, a virtual farmland is set up and its phylogenetic diversity is statistically predicted for various distances away from the forest.



Overall, these simulations show that land-sparing (red; Figure 2) retains more phylogenetic diversity than does land-sharing (purple; Figure 2), especially as farmland becomes more distant from the neighbouring intact forest.

When Dr Edwards’ team measured avian phylogenetic diversity for farmland communities, it was lower than expected, unlike forest communities (Figures 2C and 2D). They also found that the average number of years of evolutionary history separating species in a community was greater in forests than farmland (Figures 2E and 2F):

Facebook Twitter Pinterest Figure 2. Phylogenetic Diversity in Forest, in Farmland, and under Land-Sparing and Land-Sharing Simulations. (doi: 10.1016/j.cub.2015.07.063) Illustration: David P. Edwards et al/Current Biology 2015

Dr Edwards and his team estimated that bird communities found on low-intensity farmland experienced a severe loss of phylogenetic diversity relative to neighbouring forest, and can be equated to the loss of over 650 million years of evolutionary history.

Land-sparing farms save more evolutionarily distinct bird species than land-sharing

The team then looked at each species’ evolutionary distinctiveness (ED) and evolutionary distinctiveness rarity (ED rarity), which is a measure that combines distinctiveness with the size of a species’ global range.

“Species with small ranges are proportionately more vulnerable to things like habitat loss, and so are at greater risk of going extinct. ED rarity identifies those species that are high priority both for their evolutionary uniqueness and their threat status”, said James Gilroy, a co-author on the study and a Senior Post Doctoral Research Associate in environmental sciences from the University of East Anglia.

“Of course, all species have their own value and functional importance, regardless of their history. But there’s something particularly tragic about the idea of losing species with unique evolutionary histories that trace back many millions of years”, Dr Gilroy added.

The researchers estimated each species evolutionary distinctiveness and its evolutionary distinctiveness rarity, and found that the more evolutionarily distinctive a species is, the more likely that it will be present in forest (Figure 3B; also Figure 4), whereas the opposite is true for farmland (Figure 3D; also Figure 4):



Facebook Twitter Pinterest Figure 3. Species Occurrences in Forest and Farmland Related to Evolutionary Distinctiveness (A–D) Species occurrence probabilities in contiguous forest (A and B) and farmland (C and D) related to a species’s evolutionary distinctiveness (ED; A and C) or evolutionary distinctiveness rarity (EDR; B and D). Lines indicate slopes and 95% confidence interval (CI) from univariate linear regressions. (doi: 10.1016/j.cub.2015.07.063) Illustration: David P. Edwards et al/Current Biology 2015

Once again, using landscape simulations, Dr Edwards’ team found that an evolutionarily distinctive species that is rare is more likely to be lost from land-sharing landscapes (purple; Figure 4), something that can be avoided with effective land-sparing (red; Figure 4):



Facebook Twitter Pinterest Figure 4. ED in Forest, in Farmland, and under Land-Sparing and Land-Sharing Simulations. (doi: 10.1016/j.cub.2015.07.063) Illustration: David P. Edwards et al/Current Biology 2015

Dr Edwards was surprised by some of his team’s findings.



“I think the most surprising result is that species richness within communities does not explain the loss of phylogenetic diversity under land-sharing,” said Dr Edwards. “So even if farming at low intensity over a larger area retains the number of species present, those species are less evolutionarily distinct and thus preserve less phylogenetic diversity.”

Land-sparing farms must be accompanied by strong environmental protections

“This is a well conceived and executed study”, said tropical conservation biologist, Bill Laurance, a distinguished research professor and Australian Laureate who is the director of the Centre for Tropical Environmental and Sustainability Sciences at James Cook University, who was not part of this study.



“Land-sparing seems especially important for the most evolutionarily distinctive lineages of birds”, said Professor Laurance in email.

“What’s needed next is more work to understand how ‘land sparing’ might work in practice”, said Dr Phalan in email.

“But highly productive farming on its own won’t spare nature — in fact, it could contribute to the destruction of nature”, cautioned Professor Laurance. “Just look at how oil palm, which is highly productive and profitable, is spreading across the tropics like a destructive tsunami.”

“As with most conservation, it will likely be complex and require a lot of different groups of people to work together. Good environmental governance and land-use planning, combined with programmes to involve and reward land-holders, will be needed”, said Dr Phalen.

Dr Edwards’s team points out that “land-sharing policies that promote the integration of small-scale wildlife-friendly habitats might be of limited benefit without the simultaneous protection of larger blocks of natural habitat, which is most likely to be achieved via land-sparing measures.”

“[A]s the authors acknowledge, a critical point about land-sparing is that one absolutely must have effective land-use zoning in order to pull it off”, said Professor Laurance.

“So, land-sparing has the potential to help spare nature, but it won’t achieve it unless it’s paired with effective on-the-ground action — to ensure that some areas are devoted to intensified farming whereas others are protected from human activities”, said Professor Laurance.

“In reality, we can’t talk about land-sparing without focusing also on the dire need for land-use planning, zoning, and enforcement. These are very serious challenges in many developing nations, including the mega-diverse Colombian Andes where this study took place”, said Professor Laurance.

“My feeling is that land-sparing-type approaches -- such as biodiversity offsets, which can protect larger tracts of natural habitat -- are gaining traction, but there is a long way to go for expansion of such policies writ large,” agreed Dr Edwards.

Facebook Twitter Pinterest Male velvet-purple Coronet (Boissonneaua jardini). This hummingbird species is an agriculture loser (land-sparing best). This species has ~6.97 MY of unique evolutionary history. Photograph: James Gilroy/David Edwards et al/Current Biology 2015

You can also look at a photo gallery of tropical birds “winners” and “losers”.

Source:

David P. Edwards, James J. Gilroy, Gavin H. Thomas, Claudia A. Medina Uribe, and Torbjørn Haugaasen (2015). Land-Sparing Agriculture Best Protects Avian Phylogenetic Diversity, Current Biology, published online on 3 September 2015 ahead of print | doi:10.1016/j.cub.2015.07.063 [£]

also cited:

William F. Laurance, Jeffrey Sayer, and Kenneth G. Cassman (2014). Agricultural expansion and its impacts on tropical nature, Trends in Ecology & Evolution 29(2):107-116 | doi:10.1016/j.tree.2013.12.001 [£]

Marc W. Cadotte (2013). Experimental evidence that evolutionarily diverse assemblages result in higher productivity, Proceedings of the National Academy of Sciences 110(22):8996-9000 | doi:10.1073/pnas.1301685110 (OA)

W. Jetz, G. H. Thomas, J. B. Joy, K. Hartmann & A. O. Mooers (2012). The global diversity of birds in space and time, Nature 491, 444-448 | doi:10.1038/nature11631 [£]

BirdLife International (2008). Critically Endangered Birds: a global audit. Cambridge, UK.

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