Preserving Evolutionary History Alongside Tropical Agriculture

How to feed an ever-growing human population while simultaneously preserving Earth’s biodiversity is a major global challenge. Accomplishing both of these goals requires that we understand the potential for agricultural landscapes to harbor biodiversity. In tropical Costa Rica, where we do the majority of our fieldwork, landscapes are extremely heterogeneous. Agriculture can take the form of sweeping fields of ground-clinging pineapple, with no surrounding tree cover to provide structural complexity, or shade from the noonday sun. Or multiple crop species can be grown together with bananas layered between rows of coffee bushes and live fences and fruit trees planted to demarcate property lines. Or land can be fully protected and set aside in forest reserves. Here, a dense canopy blocks all but a few sun beams from touching the forest floor, and hundreds of unique ecological opportunities exist for species to specialize in the structurally complex understory. This land-use gradient, spanning from intensive monocultures, to diversified agricultural systems, to forest reserves describes landscapes in much of the tropics. But how do these land-use types actually stack up when it comes to preserving biodiversity?

Conservation from a phylogenetic perspective

For those of us who deeply care about conservation, it is critical that we ask ourselves ‘why is it that we care?’ and ‘what exactly is it that we want to conserve?’ For many, the answers to these questions are better represented by gut feelings that cannot easily be transcribed into words, and are often poorly captured by quantitative metrics. Nevertheless, with careful thought, quantitative metrics can be constructed to reflect many of the core reasons we care about conservation and then be used to guide conservation decisions.

Phylogenetic diversity is one such metric. Its formal definition is the sum of all the branch lengths of a phylogenetic tree for all species in a community. More intuitively it is simply a metric that captures the sense that we not only care about the sheer number of species that exist together in an area (species richness), but also the diversity of evolutionary relationships between them. From conservation, aesthetic, and functional standpoints a community containing 10 bird species from across the avian tree of life (high phylogenetic diversity) is often better than a community consisting of 10 highly related species (low phylogenetic diversity).

We can also think about how individual species contribute to the total phylogenetic diversity of life on Earth. Species with many close relatives contribute relatively little to this overall diversity, because they embody relatively little unique evolutionary history. In contrast, single species with few living relatives, sitting out on long isolated branches of the tree of life are highly evolutionarily distinct. They have shared little of their evolutionary history with any other species, and their extinction would constitute not only the loss of yet another species, but also the loss of millions of years of totally unique history.

How do species respond?

By integrating 12 years of bird survey data from across the country of Costa Rica conducted in forest reserves, diversified agricultural systems, and more intensive monocultures, we were able to document how bird species from across the tree of life responded to land conversion. Forests, not surprisingly, contained the most species, and species in forest represented the largest swath of the avian tree of life. Interestingly, diversified agriculture had nearly as many species as forest, but these species were a different subset of the phylogeny, and were dominated by some of the evolutionarily younger branches of the tree—lineages such as blackbirds and seedeaters. Therefore, while diversified agriculture is great for species richness, it is not as good at preserving phylogenetic diversity. Intensive monocultures performed poorly on both fronts, hosting very few species (roughly half the number in diversified agriculture or forest) and strongly favoring a very small subset of the tree of life. Moreover, the most globally unique species were almost twice as likely to go locally extinct in intensive monoculture compared to diversified farms.

Diversified coffee farms in Southern Costa Rica hold significantly more phylogenetic diversity than intensive monocultures. Photo: Daniel Karp

Scope for mixed-use landscapes to preserve evolutionary history

These findings highlight both the potential, and the limits, of diversified agricultural practices for species preservation. Diversifying agriculture greatly increases the number of bird species that can use agricultural habitats. However the bird species that are present tend to come from a smaller portion of the tree of life, and many of the most evolutionarily distinct species are absent. Instead, these unique species are found primarily in forest. Still, diversified agricultural systems are clearly better at conserving evolutionary unique species than monocultures. In order to achieve the best conservation outcomes, a truly integrated approach is needed: preservation of the entirety of avian evolutionary history requires conservation of tropical forests where possible, and support of diversified agriculture where tropical forest conservation is infeasible.

The big question is how general are these results? Are birds unique in having evolutionarily distinct species being the most easily lost when land is converted from forest to agriculture? Or are long branches of the tree of life always the most fragile when it comes to land-use intensification?