Until recently, large apex consumers were ubiquitous across the globe and had been for millions of years. The loss of these animals may be humankind's most pervasive influence on nature. Although such losses are widely viewed as an ethical and aesthetic problem, recent research reveals extensive cascading effects of their disappearance in marine, terrestrial, and freshwater ecosystems worldwide. This empirical work supports long-standing theory about the role of top-down forcing in ecosystems but also highlights the unanticipated impacts of trophic cascades on processes as diverse as the dynamics of disease, wildfire, carbon sequestration, invasive species, and biogeochemical cycles. These findings emphasize the urgent need for interdisciplinary research to forecast the effects of trophic downgrading on process, function, and resilience in global ecosystems.

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It seems particularly opportune to discuss food webs and evolving views on their structure here for both their genesis and first modern treatment (Elton 1927) and much of their later development (May 1973; Pimm & Lawton 1978) has a decidedly British accent to it. The central significance of webs is derived from the fact that the links between species are often easily identified and the resultant trophic scaffolding provides a tempting descriptor of community structure. If this structure is in any fashion related to the persistence of natural communities or their stability, however defined, then we are dealing with issues of vital ecological importance. Elton's views have admirably withstood the tests of time. They were especially useful to field biologists, and encouraged the assembly and organization of feeding data into networks of trophically bonded species or higher taxa. The early emphasis was on connectedness per se. Perhaps the first significant deviation from this theme was the development of the trophic dynamic viewpoint of Lindeman (1942) and all subsequent efforts to describe energy transfer and material flow through communities. A second departure, and one I believe to be conceptually richer, was the formalization of the view that web structure and community stability were related (MacArthur 1955). May (1973) in another landmark publication questioned this relationship and called attention to four primary web features: the number of species involved, the nature of their interconnections, the number of connections per species, and the intensity of interaction between web members. This focus has stimulated application to agroecosystems (Southwood & Way 1970), new interpretations of the number of trophic levels (Pimm & Lawton 1977), and a resurgence of interest in the significance of mutualism (Vance 1978). It has not been characterized by stunning breakthroughs, ecological stability remains a frustrating issue, and to a field ecologist, the ties between model and reality at times appear remote; All but ignored in these recent developments is an insightful recognition that trophic pathways might contribute little to ecosystem stability, and that the answers lie in the spatial patterning of the environment (Smith 1972). I wish to return to the basic observations on food webs as a naturalist and experimentalist, and employing an approach advocated by Sir Arthur Tansley (Godwin 1977), ask whether we are modelling their correct properties, and if not, what modifications might be made. TERMINOLOGY The assumed importance of predator-prey or consumer-resource relationships, their relative ease of observation, and an attractive, simple graphical format have accelerated the interest of ecologist and mathematician alike on web structure and organization

Abstract. The mussel Mytilus californianus is a competitive dominant on wave-swept rocky intertidal shores. Mussel beds may exist as extensive monocultures; more often they are an everchanging mosaic of many species which inhabit wave-generated patches or gaps. This paper describes observations and experiments designed to measure the critical parameters of a model of patch birth and death, and to use the model to predict the spatial structure of mussel beds. Most measurements were made at Tatoosh Island, Washington, USA, from 1970-1979. Patch size ranged at birth from a single mussel to 38 m 2 ; the distribution of patch sizes approximates the lognormal. Birth rates varied seasonally and regionally. At Tatoosh the rate of patch formation varied during six winters from 0.4-5.4% of the mussels removed per month. The disturbance regime during the summer and at two mainland sites was 5-10 times less. Annual disturbance patterns tended to be synchronous within II sites on one face of Tatoosh over a 10-yr interval, and over larger distances (16 km) along the coastline. The pattern was asynchronous, however, among four Tatoosh localities. Patch birth rate, and mean and maximum size at birth can be used as adequate indices of disturbance.Patch disappearance (death) occurs by three mechanisms. Very small patches disappear almost immediately due to a leaning response of the border mussels (0.2 em/d). Intermediate-sized patches ( <3.0 m 2 ) are eventually obliterated by lateral movement of the peripheral mussels: estimates based on 94 experimental patches yield a mean shrinking rate of 0.05 cm/d from each of two principal dimensions. Depth of the adjacent mussel bed accounts for much of the local variation in closing rate. In very large patches, mussels must recruit as larvae from the plankton. Recovery begins at an average patch age of 26 mo; rate of space occupation, primarily due to individual growth, is 2.0-2.5%/mo. ' Winter birth rates suggest a mean turnover time (rotation period) for mussel beds varying from 8.1-34.7 yr, depending on the location. The minimal value is in close agreement with both observed and calculated minimal recovery times.Projections of total patch area, based on the model, are accurate to within 5% of that observed. Using a method for determining the age of patches, based on a growth curve of the barnacle Balanus cariosus, the model permits predictions of the age-size structure of the patch population. The model predicts with excellent resolution the distribution of patch area in relation to time since last disturbance. The most detailed models which include size structure within age categories are inconclusive due to small sample size. Predictions are good for large patches, the major determinants of environmental patterns, but cannot deal adequately with smaller patches because of stochastic effects.Colonization data are given in relation to patch age, size and intertidal position. We suggest that the reproductive season of certain long-lived, patch-dependent species is moulded by the distu...

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All species have evolved in the presence of disturbance, and thus are in a sense matched to the recurrence pattern of the perturbations. Consequently, disturbances within the typical range, even at the extreme of that range as defined by large, infrequent disturbances (LIDs), usually result in little long-term change to the system's fundamental character. We argue that more serious ecological consequences result from compounded perturbations within the normative recovery time of the community in question. We consider both physically based disturbance (for example, storm, volcanic eruption, and forest fire) and biologically based disturbance of populations, such as overharvesting, invasion, and disease, and their interactions. Dispersal capability and measures of generation time or age to first reproduction of the species of interest seem to be the important metrics for scaling the size and frequency of disturbances among different types of ecosystems. We develop six scenarios that describe communities that have been subjected to multiple perturbations, either simultaneously or at a rate faster than the rate of recovery, and appear to have entered new domains or ''ecological surprises.'' In some cases, three or more disturbances seem to have been required to initiate the changed state. We argue that in a world of ever-more-pervasive anthropogenic impacts on natural communities coupled with the increasing certainty of global change, compounded perturbations and ecological surprises will become more common. Understanding these ecological synergisms will be basic to environmental management decisions of the 21st century.

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