Editor’s Choice: Volume 110 Issue 1

The editor’s choice for our January issue is “Biodiversity–productivity relationships in a natural grassland community vary under diversity loss scenarios” by Pan et al. Here, Associate Editor Eric Lamb explains the importance of this research.

This paper is a fascinating exploration of the complexity of diversity – productivity relationships. With the recent loss of Philip Grime, these relationships – and the long-standing debates about the shapes of these relationships – are close to my mind.

Pan et al. move this discussion forward in this paper. They first predict that following removal of one or more plant functional groups from a community, there are a wide range of potential community responses. Specifically, the productivity of the remaining functional group(s) within the plant community can respond in a variety of ways, including a negative response (declines in community productivity), no response, or partial to full compensation. Each of these potential responses can shift the shape of the productivity-diversity relationship.

^{Figure 1: A conceptual framework illustrating how abundance and compensation affect the directions and forms of PFG richness–productivity relationships. A natural ecosystem usually contains several plant functional groups (PFGs) with different abundances (a). When PFG(s) are lost, ecosystem function is expected to decline in proportion to the abundance of the lost PFG(s) under three PFG richness levels (bars in b, c and d, represent different diversity loss scenarios of 3, 2 and 1 remaining PFG(s), respectively). Over time, negative, zero, partial, full and overcompensation may occur, leading to differences in observed productivity (e–i). While negative, zero and partial compensation lead to positive biodiversity–productivity relationships (l₁-l₃ in j–m), full and overcompensation result in neutral (l₄ in n) and negative (l₅ in o) relationships, respectively. The combination of full compensation at high richness levels and partial compensation at low levels yields a saturation curve (l₆ in p), while overcompensation at intermediate levels creates a unimodal curve (l₇ in q). Dashed lines indicate the no-compensation baseline}

Pan et al. then present the results of a beautifully designed experiment where they measured productivity following the selective removal of combinations of plant functional groups (perennial rhizomatous grasses, perennial bunchgrasses, perennial forbs, and annuals) from a Mongolian grassland community. They found instances of most of the possible productivity compensation scenarios. The resulting biodiversity – productivity relationships included linear positive, unimodal, and neutral examples. These results demonstrate that the shape of the productivity-diversity relationship depends strongly on the identity of the plants within a community and that the shape of the relationship can shift through time.

To top off the study, the authors re-examine an old functional group removal experiment at Cedar Creek in Minnesota. They find similar patterns at Cedar Creek; testing their hypotheses with a differently defined set of plant functional groups in a very different plant community demonstrates the wide potential generality of the results.

^{Figure 2: General and specific relationships of plant functional group (PFG) richness to productivity in years 2–5 after PFG removal treatment began. ‘All’ lines indicate the general relationships that incorporated all PFG loss patterns (n = 150). The other lines indicate relationships under specific diversity loss patterns in which the labelled PFG is the one remaining (n = 80 for each line). Codes for PFG(s) are as follows: PB, perennial bunchgrasses; PR, perennial rhizomatous grasses; PF, perennial forbs; AN, annuals. Labels (a), (b), (c) and (d) indicate years 2, 3, 4 and 5. Dotted lines indicate none of the three models were significant at p = 0.05.}

In sum, this study demonstrates that complex biodiversity-productivity relationships in natural communities can be driven by the responses of plants to niche spaces left open by the loss of particular functional groups. This paper should prompt research in new directions as we seek to understand the mechanisms underlying the general diversity – productivity relationship.