Ecosystem Models DANIEL PAULY AND VILLY CHRISTENSEN

10 Ecosystem Models DANIEL PAULY AND VILLY CHRISTENSEN

10.1 INTRODUCTION:

when fishing that it destroys the flowers of

ECOSYSTEM MODELS

the land below water there, and also spat of oysters, mussels and other fish upon which

Historical sources indicate that the fishers and the great fish are accustomed to be fed and scholars of centuries past were well aware that the

nourished. (Alward 1932) fish and invertebrates then exploited by fisheries were embedded within what we now call ‘ecosys-

Incorporating the commonsense knowledge tems’ (see e.g. Thompson 1910; Cotte 1944). As expressed here into a rigorous ecosystem-based well, this knowledge was used to draw inferences understanding of fisheries took, however, an ex- structurally similar to contemporary hypotheses. tremely long time, and the path taken was particu- Thus, for example, Bernard de Palissy (1510–90) in- larly tortuous, with various blind alleys and terpreted fossil fish as the remnants of populations enticing traps (see also Smith, Chapter 4, this that had been ‘trop pêchées’ (i.e. overfished to volume). Indeed, many fisheries scientists, accus- extinction; Tort 1996, p. 3345), thus admitting to tomed to ‘single-species stock assessments’, are the possibility of humans having such impacts – still uncomfortable with the broad ecological prin- possibly based on observations at the time. This ciples required for such understanding, not to inference that fishing had ecosystem effects is speak of regulatory agencies, whose mandate often illustrated by a petition from the year 1376 to precludes them from even beginning to address Edward III, King of England:

ecosystem-related issues (NRC 1999). This chapter therefore introduces a number of The commons petition the King, complain-

ecological principles which formalize and render ing that where in creeks and havens of the

mutually compatible earlier ideas on how the dif- sea there used to be plenteous fishing, to the

ferent elements of aquatic ecosystems interact. profit of the Kingdom, certain fishermen for

Moreover, it illustrates how applying these princi- several years past have subtly contrived an

ples can improve one’s understanding of a given instrument called ‘wondyrechaun’ made in

aquatic ecosystem, and of such systems in general. the manner of an oyster dredge, but which is

The link with fisheries is then established by considerably longer, upon which instrument

documenting how fish and invertebrate resource is attached a net so close meshed that no fish,

species are embedded in ecosystems and how they

be it ever so small, that enters therein can es- must (and do) follow the same principles as non- cape, but must stay and be taken. And that

exploited species. Our goal, thus, is to marry key the great and long iron of the wondyrechaun

parts of Ecology with the best parts of Fisheries runs so heavily and hardly over the ground

Science. In doing so, we build on the foundations of

Chapter 10

the two chapters in Volume 1 that examine trophic interactions in freshwater (Persson, Chapter 15) and marine communities (Polunin and Pinnegar, Chapter 14). Our chapter, in turn, provides a theo- retical and methodological underpinning for the ecosystem impacts of fishing documented by Kaiser and Jennings (Chapter 16, this volume).

Both ecology and fisheries science share the same ancestry, having been spun off Natural History in the late 1800s. However, their subse- quent fates differed. Ecology went to University, theorized, and usually failed to get its hands dirty. Fisheries science, on the other hand, decided to work for the Government, and became very ap- plied, hence a prime suspect every time some fish stock collapses. The union we advocate may be thus likened to a shotgun marriage: the partners do not necessarily match, but strong external con- straints force them together. The shotgun itself is the crisis of fisheries, and the ensuing threats to the systems studied by both (aquatic) ecologists and fisheries scientists, and as well, to the funding of these often feuding disciplines. A similar con- vergence of protected area management and fish- eries has been identified in the context of marine protected areas by Polunin (Chapter 14, Volume 1).

Before all, however, we must define the con- cepts in our title, ‘ecosystems’ and ‘models.’ Ecosystems, in this chapter are sites occupied by elements such as individual members of different plant and/or animal species, which interact such that the sum of their mutual interactions is much greater than the sum of their interactions with the elements of adjacent sites. The interactions within the ecosystems thus defined may lead to distinct structures for these ecosystems. However, our definition does not require adjacent ecosystems to

be structurally different from each other. Lakes are excellent examples of ecosystems de- fined here (Thienemann 1925; Golley 1993). Thus, if consumed at all, the primary production of a lake from phytoplankton and macrophytes is grazed within that lake, as is its secondary production which is composed of zooplankton, and the pro- duction by fish and other organisms is supported by feeding on the secondary producers. Indeed, in lakes, it is usually only that part of primary produc-

tion taken up by the few aquatic insects that make it to the adult stage, and by birds, which is available for transfer to adjacent, terrestrial ecosystems.

This definition, however, also fits more open systems, such as coral reefs, seamounts or estu- aries, whose different components have co-evolved such that most of their trophic fluxes are ‘inter- nalized’, if at the cost of large supporting energy fluxes, in the form of light for coral reefs (Grigg 1982), zooplankton for seamounts (Koslow 1997), or detritus for estuaries (Odum and Heald 1975). On the other hand, this definition of ecosystems may exclude beaches and other shallow-water areas whose inhabitants often consist of the juvenile form of adults living in adjacent, deeper waters. The concept of ‘subsystems’ is appropriate for such highly connected parts. As will be shown below, stratification by subsystems is crucial when con- structing models of ecosystems, our next topic.

Models are coherent representations of systems and/or of the processes therein, and may consist of words (‘word models’), graphs or equations. Words alone can often describe complex systems or processes adequately, as in the case of natural selection (Darwin 1859). Graphs can also make compelling cases, as did, for example, the trophic pyramid of Lindeman (1942). However, equations that capture essential aspects of systems or pro- cesses always outperform word or graph models, if only because the application of standard algebraic or other mathematical rules to these equations often leads to the discovery of unknown properties of the systems or processes in question. This non-intuitive, and in fact wondrous property of mathematical descriptions (Wigner 1960) has, moreover, the distinct advantage of allowing the testing of hypotheses about future states, or previ- ously unobserved features of ecosystems, besides allowing for testing the adequacy of the initial description.

Our models will thus be mainly equations, though not necessarily complex ones. The criteri- on for assessing the value of a model is not the ex- tent to which it reproduces the complexity of the real world. Rather, it is the ratio of insight gained versus effort extended. As we shall see, ‘simple’ models that capture key features of an ecosystem Our models will thus be mainly equations, though not necessarily complex ones. The criteri- on for assessing the value of a model is not the ex- tent to which it reproduces the complexity of the real world. Rather, it is the ratio of insight gained versus effort extended. As we shall see, ‘simple’ models that capture key features of an ecosystem

13, this volume) review both philosophical and practical aspects of using models.