John H. Steele

A Simple Plankton Model

Data on plankton ecosystems in large enclosures are used as a basis for consideration of the role of deterministic and random processes in these systems. Using a simple model, it is proposed that the exclusion of random variations in predators can lead to greater extremes in the phytoplankton/herbivore populations in enclosures compared with those outside. These results depend on the relative rates of internal and exogenous changes, and comparisons are made with results for a forest ecosystem.

REGIME SHIFTS IN MARINE ECOSYSTEMS

Time scales, and the trophic relations between these scales, are very different in the sea from those on land. In particular, marine systems are much more responsive to decadal scale alterations in their physical environment but are also much more adaptable. Thus it is difficult, and probably counterproductive, to try to define a baseline state for marine ecosystems. Further, regime shifts in fish communities can have major economic consequences without being ecological disasters. Climatic changes at decadal scales, from natural or anthropogenic causes, are likely to produce or enhance regime shifts. There are different management issues in different sectors. The coastal zone demands our intervention to assure integrated management of the land and sea components. At the other extreme, our understanding of open ocean systems is an essential element of climate prediction and so of eventual management. Between these two environments, our use of resources in continental shelf seas requires an ability to distinguish between human and natural causes of long-term change.

Some Comments on Plankton Patches

The title of this volume mentions only the spatial patterns of plankton, but the populations we are concerned with have other types of variation which cannot be ignored. They have different life spans which must have an effect on their spatial distributions. Also, we must consider the food chain relations between organisms as a factor affecting spatial structure and this involves aspects such as the size distribution of the plants and animals and the numbers of different species within each trophic grouping.

Can ecological theory cross the land-sea boundary?*

Terrestrial and marine ecological research are usually carried out in different institutions, published in different journals and funded from different sources. There are obvious disparities in space and time scales of processes; for example, trees compared with phytoplankton as primary producers; and the structure of their physical and chemical environments are quite distinct. Yet many common problems exist—patch dynamics, foodweb topology, density dependance. Especially there are questions about the nature of the biotic/abiotic relations in the two environments. These comparisons are discussed across sectors particularly in the context of space and time scale interactions between biological processes and the physical milieu. It is proposed that theories developed in one sector can be tested most critically in the other, with potential for greater generality. These extensions are often peripheral to research but the present focus on “global ecosystem dynamics” makes integration of these components a central and pressing issue.

The ocean ‘landscape’

The ocean has a complex physical structure at all scales in space and time, with ‘peaks’ at certain wave numbers and frequencies. Pelagic ecosystems show regular progressions in size of organisms, life cycle, spatial ambit, and trophic status. Thus, physiological and ecological parameters are closely coupled to spatial and temporal physical scales.

Regime shifts in fisheries management

Abstract The collapse of so many fisheries in the waters of “advanced” nations would seem to confirm the opinions of pessimists who decry the possibility of successful management based on existing methodologies. Yet there may be compensations to these closures (or near closures) of fisheries around the North Atlantic and elsewhere. These create new and different circumstances not only for the managers but also for their scientific advisors. There is not merely an opportunity but a necessity to restructure the form in which advice is given and the research on which it is based.

Understanding patterns and processes in models of trophic cascades

Climate fluctuations and human exploitation are causing global changes in nutrient enrichment of terrestrial and aquatic ecosystems and declining abundances of apex predators. The resulting trophic cascades have had profound effects on food webs, leading to significant economic and societal consequences. However, the strength of cascades–that is the extent to which a disturbance is diminished as it propagates through a food web–varies widely between ecosystems, and there is no formal theory as to why this should be so. Some food chain models reproduce cascade effects seen in nature, but to what extent is this dependent on their formulation? We show that inclusion of processes represented mathematically as density-dependent regulation of either consumer uptake or mortality rates is necessary for the generation of realistic ‘top-down’ cascades in simple food chain models. Realistically modelled ‘bottom-up’ cascades, caused by changing nutrient input, are also dependent on the inclusion of density dependence, but especially on mortality regulation as a caricature of, e.g. disease and parasite dynamics or intraguild predation. We show that our conclusions, based on simple food chains, transfer to a more complex marine food web model in which cascades are induced by varying river nutrient inputs or fish harvesting rates.

Marine Functional DiversityOcean and land ecosystems may have different time scales for their responses to change

he term biological diversity has the virtue of containing many different interpretations, scientifically and emotionally. A recent report (OTA 1987) suggested three technical meanings: genetic, species, and ecological diversity. I suggest a fourth category that needs to be considered, which I term functional diversity-the variety of different responses to environmental change, especially the diverse space and time scales with which organisms react to each other and to the envi-

The Significance of Interannual Variability

The attention to climate change has aroused interest in the effects of longer-term physical variability on ecological systems. Can ecological changes be simply related to physical trends; or are the changes so modified by the biological dynamics that simple physical consequences will not be observable?

Ecosystem structure before fishing

Abstract Data from early fisheries in the Northwest Atlantic and elsewhere suggest that catch rates of demersal species were very high despite primitive fishing methods. Circumstantial evidence suggests that earlier stocks may have been an order of magnitude greater than stocks in the last half-century. What was the structure of the ecosystem that supported these stocks? Relative to current ecosystem structure, alternative patterns involve very slow growth rates of the demersal species, very small pelagic stocks, negligible invertebrate predators, and efficient transfer of primary production to fish (no “detritus”). Each of these patterns suggests distinctive dynamics in pristine ecosystems, with different implications for the effects of fishing.