Is there a maximum number of species that can coexist in a given assemblage? And is there a maximum number of species that a lineage can produce? These questions have puzzled biologists for decades because they have important implications for understanding how biological diversity is spatially distributed and evolutionarily regulated. Intuitively, we assume an upper limit to the number of species in a given assemblage, or that a lineage can produce. In effect, ecological and evolutionary theory predicts that an upper limit must exist, but defining and testing for this limit has proven problematic. We reviewed a total of 143 publications associated to the concept of saturation, bounded-unbounded diversity and ecological equilibria.
Results/Conclusions
Our first finding was that almost no study provided a conceptual definition of the term. Furthermore, these studies supported previous comments that our understanding of the topic is hampered by misconceptions, lack of a clear analytical framework, and lack of robust methodological approaches. In this review, we first outline seven general challenges of studies on species saturation, most of which are independent of the actual method used to assess saturation. Among these are the challenge of defining saturation conceptually and operationally, the importance of setting an appropriate referential system, and the need to discriminate between patterns, processes and mechanisms. Second, we argue that assessing species saturation is possible, but that many studies conducted to date have conceptual and methodological flaws that prevent us from currently attaining a good idea of the occurrence of species saturation. Finally, we propose the following definition of saturation: saturation is the dynamic equilibrium of species richness due to niche limitation within a given group of organisms and within an environmental and temporal reference frame. Based on this definition, saturation can only be demonstrated when we have evidence for stability or equilibrium of species numbers as well as evidence for niche limitation.