In this post Victor Rolo discusses his recent paper ‘Taxonomic and functional diversity in Mediterranean pastures: Insights on the biodiversity – productivity trade-off

There is a shepherds’ saying in Spain that goes: “below a shrub, you can raise a lamb”. But, if that were true, why is the presence of semi-natural features perceived as a negative element for herbage productivity in grasslands? The answer is simple, first, not all semi-natural features have a positive effect on herbage productivity; and second, where they have neutral effects, they are a land-use opportunity cost. To complicate things more, the inclusion of semi-natural features in agricultural landscapes may enhance farm biodiversity. Given the loss of biodiversity in agricultural landscapes world-wide, the presence of semi-natural features in grasslands should be promoted. But, how can we balance the need for semi-natural features to enhance biodiversity levels without diminishing herbage productivity?

To answer this question we set up an observational study to assess diversity levels and herbage productivity along a gradient of semi-natural features cover. To fully understand the linkages between both elements, we collected not only taxonomic (i.e. species identity) but also functional (e.g. morphological features) data of herbaceous communities. In total we sampled 50 fields, ranging from 0 to 90 % of woody cover vegetation on nine similarly managed farms in central-western Spain.

Our results confirmed a potential trade-off between herbage production and biodiversity levels. The most productive fields (i.e. with minimum levels of semi-natural features cover) and the least productive fields (i.e. with high levels of semi-natural features cover) showed the lowest number of species, while the number of species peaked at intermediate values of semi-natural features cover. In other words, too open was bad, but too dense vegetation was also bad. This accords with the notion that heterogeneous landscapes can potentially bear a higher number of species than homogeneous landscapes.

When we looked at the morphology of the herbaceous species (i.e. functional traits) and the functional structure of the communities (i.e. the composition and diversity of functional traits of a community) we found a different story. The most productive fields contained a set of species with similar functional traits, such as high values of height, and low leaf dry matter content and seed lengths. However, in the least productive fields, the species present had contrasting morphological features, forming a community with an expanded functional structure. Therefore, in terms of functional diversity, as the cover of semi-natural features increase, so does the functional trait spaces of the herbaceous communities.

The importance of high functional diversity values have been linked to the provision of functions under current or future conditions (e.g. stability). We might think that by promoting communities with high values of height, and low leaf dry matter content and seed lengths, the production of grasslands can be boosted. However, such an approach would result in communities with low diversity levels, which, in systems where disturbances are frequent (e.g. grazing), may not guarantee the provision of a function in the long-term. Our results suggests that maintaining low to intermediate values of woody vegetation could be a feasible tool for enhancing both taxonomic and functional diversity without dramatically diminishing herbage yields. Thus, maybe, the old shepherds’ saying was not wrong. By allowing the presence of semi-natural features, the stability of a herd of sheep can be promoted, which could guarantee the number of lambs over the long-term.