The biodiversity numbers game

How many species are there on Earth? Given the effort required to sample habitats and identify species this figure will likely never been known with great certainty. Indeed current rates of discovery may be slower than the current species extinction rate! To make a start ecologists have thus turned to estimates based on extrapolations of well known patterns in the distribution of species, or the rate of taxonomic discovery. Best estimates vary from as few as 3 million up to 100 million species. A recent paper by Mora et al. (2011, PLoS Biology) has come up with a value of ~8.7 million (+/- 1.3 million) eukaryotic species. This is somewhat lower than the previous best estimates of ~15 million species. The authors base their estimate on the predictable increase in the number of taxa with increasing taxonomic resolution (i.e., from phylum, class, order, family, genera and species). Although the number is likely an underestimate (consider the diversity of microorganisms that remain undiscovered) it is the method that has been criticized. Some experts believe that the patterns upon which the estimates are based are more the reflection of the taxonomic enterprise and have little to do with how many species actually inhabit the Earth. Read the piece in the New York Times about this study.

Spare it, Share it, or Both? Balancing agriculture with biodiversity conservation

How can we feed the growing human population while conserving biodiversity?

A recent paper in Science by Phalan et al. makes the case for establishing strict forest preserves and intensifying agricultural practices in the surrounding matrix.The paper compared two biodiversity-management strategies for agricultural landscapes:

1) Land sparing - set aside strict forest preserves while intensively farming the remaining land

2) Land sharing - adopt low-intensity, biodiversity-friendly farming across the landscape

The study correlated crop yields with densities of 341 bird and 260 tree species in India and Ghana across an agricultural intensity gradient (ranging from “diverse low-yielding mosaic agriculture to large-scale high-yielding monocultures”). They showed that, for a given amount of food production, most species (rare and common) would have higher populations under land sparing than land sharing.

There are caveats* to the conclusion that land-sparing is better for biodiversity. The authors point out that the conclusion depends on local context and may not hold for all species in all regions. The study did not discuss "how" to spare land in terms of building a network of forest preserves.

The agricultural landscape surrounding Montreal represents an opportunity to test this question. The image to the left highlights the fragmented forest patches (green) embedded within an agricultural matrix (yellow) and interspersed urban areas (gray). Perhaps this landscape could showcase an intermediate approach whereby biodiversity-friendly farming practices form the matrix around forest preserves and more intensive agriculture. In this model, eco-agricultural methods in the matrix and forest corridors would facilitate species dispersal among forest fragments to enhance biodiversity conservation, ecological functioning and ecosystem services across the forest network.

Certainly we need to apply cross-disciplinary biodiversity science that integrates ecological, economic, humanitarian and social interests to achieve the appropriate balance between land sharing and land sparing.

Reference: Phalan, B., Onial, M., Balmford, A., Green, R. E., Reconciling Food Production and Biodiversity Conservation: Land Sharing and Land Sparing Compared. 2011. Science. 333, 1289-1291.

*I haven't discussed implications of land sparing on food accessibility and security.