Reply to Horizons Article ‘Plankton functional type modelling: running before we can walk’ Anderson (2005): II. Putting trophic functionality into plankton functional types

Abstract

While Anderson (Anderson, 2005) considers the pros and cons of plankton functional type models, I question whether we are missing something far more basic— trophic functionality, the ways in which organisms interact with each other. What is a functional type? Does the grouping accord with physiology (e.g. ‘Si-requiring’; mainly diatoms) or with respect to ecological function (‘dominant primary producer in an immature ecosystem’; diatoms need not always fulfil that role)? There is some level of acknowledgement (e.g. ERSEM; Blackford et al., 2004) that plankton functional types (PFTs) describe ‘ecological functionality’, but their description is still based on simple physiology. Even on a physiological basis, PFT design is fraught with problems; summer diatoms will not have the same physiology as their winter, spring or autumnal counterparts. That is not to say that physiology is unimportant; it is indeed vital because without it we cannot appreciate how the performance of an individual organism or group may be affected by the presence or absence of others. But is such gross physiological detail sufficient? Surely, competitive advantage for phytoplankton must be a function of more than just maximum growth rates, the value of alpha (slope of the Chl-specific photosynthesis–irradiance curve) or substrate affinity constants? Even if it were described in these terms, the simple sad fact is that not for a single clone of a plankton species can we construct a model describing its activity under a realistic range of environmental conditions. This is before we consider the impact of genetic diversity. I suspect that the vast majority of PFT models used in ecological simulations would fail to describe adequately a culture flask experiment in a manner representative of the organisms they purport to describe, even if one could find such a data set for verification. We need data, and lots of them. But we do not need them just for the growth of individual organisms; vitally we also need data for combinations of organisms. What is becoming increasingly clear is that there are a whole host of interactions between members of the plankton that the vast majority of models do not even hint at. Although it may appear that PFT approaches are developing such that ‘complexity in nature is mirrored by complexity in models’ (Anderson, 2005), I fear that the description will have to become more complex yet before we make real progress, even if it is to appreciate better how we must then simplify models back down again. Most models of planktonic systems take a collection of very crude models, at best purporting to have a physiological basis, parameterized using rather few, and sometimes unreliable, values taken from the literature or by tuning between parameter values from the same literature. Complexity is more than using an increasing number of arrayed boxes describing different groups; we must recognize not only that interactions between groups exist (Anderson, 2005) but also how these change during population growth and trophic dynamics. Currently, for example, even in the more comprehensive models, predator–prey interactions are usually described using a fixed set of rules based on JOURNAL OF PLANKTON RESEARCH j VOLUME 28 j NUMBER 9 j PAGES 873–875 j 2006