The productivity–diversity relationship: varying aims and approaches

Abstract

The productivity-diversity relationship is an impor tant and heavily debated issue in ecology. A major advancement in exploring this relationship has been the analyses of large numbers of case-studies (Mittelbach et al. 2001, Gillman and Wright 2006). These works have shown that the relationship between productivity and diversity varies, and that a single general relationship does not exist. We investigated the productivity diversity relationship with respect to the species pool concept (P?rtel et al. 2007), which postulates that more species are expected to evolve in conditions (ecosystems) that have been historically more common. Tropical humid ecosystems have been relatively more productive: during the past hundreds of millions of years, whereas productivity in temperate ecosystems has been limited by low temperatures and repeated glaciations. We found that positive productivity-diversity relationships are more common in the tropics, where the species pool of productive habitats is large, whereas declines in species richness at high productivities is more common in temperate regions, since species pools in high produc tivity ecosystems are likely to be relatively small. Similarly, we showed that positive productivity-diversi ty relationship can be common even in temperate regions, if only woody species diversity is considered (Laanisto et al. 2008). This finding might reflect evolutionary history: most temperate woody species originate from lineages and exhibit tropical patterns due to niche conservatism. Robert J. Whittaker (2010, hereafter RJW) criticizes our above-mentioned studies, claiming we were too liberal in selecting case studies. RJW's argument relies on the assumption that the seven conditions for case studies he describes as reasonable and necessary criteria, are always valid. We argue that different research aims require a priori criteria to be defined. Demanding implementation of criteria as hindsight criticism of existing studies can easily lead to paradoxes (e.g., richness rather than diversity should be used for diversity relationships). Our approach in selecting case studies for analysis was straightforward: a case study must reveal a suitable habitat-productivity-plant-diver