Journal of Experimental Marine Biology and Ecology 250 2000 51–76
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Experimental ecology of rocky intertidal habitats: what are we learning?
A.J. Underwood
Centre for Research on Ecological Impacts of Coastal Cities , Marine Ecology Laboratories A11,
University of Sydney , Sydney, NSW 2006, Australia
Abstract
Experimental analyses of causes of patterns of distribution and abundance of intertidal animals and plants on rocky shores have been a major activity for many years. In this review, some of the
themes and topics that have emerged from such analyses are briefly discussed to provide an up-date for practitioners and ecologists working in other habitats. Conceptual issues include the
widespread occurrence of transphyletic use of the same resources space and food, theories and experimental analyses of intermediate disturbance in relation to numbers of species, the complex
but pervasive nature of indirect interactions among species, relative importance of ‘top-down’ versus ‘bottom-up’ control of assemblages and the importance to rocky intertidal species of
‘supply-side’ influences on densities and interactions. Methodological advances include ex- perimental designs for complex and patchy, interacting sets of species, the importance of controls
in experimental manipulations and methods for analyses of hierarchical scales of patterns and processes. Finally, some contributions to social issues pollution, biodiversity and some scenarios
for future directions are briefly considered.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Ecology; Experiment; Methods; Rocky intertidal habitats
1. Introduction
1.1. Preamble The ecology of animals and plants on intertidal rocky shores has been a topic of
interest for decades in many parts of the world. There have long been descriptions of
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: ajubio.usyd.edu.au A.J. Underwood. 0022-0981 00 – see front matter
2000 Elsevier Science B.V. All rights reserved.
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.J. Underwood J. Exp. Mar. Biol. Ecol. 250 2000 51 –76
fauna and flora Colman, 1933; Fischer-Piette, 1936 and some manipulative ex- perimentation goes back at least to the 1930s Hatton, 1932.
The ecological study of the organisms has been, in many ways, a source of, or, at least, a major contribution to concepts that have moved out into other areas of ecology.
There have been several syntheses of the field, from the purely descriptive cataloguing of broad patterns of occupancy of shores in different parts of the world e.g. Lewis,
1964; Morton and Miller, 1968; Stephenson and Stephenson, 1972 to those based on experimental analyses of patterns of distribution Connell, 1972; Paine, 1977.
It is often timely to revisit some of the themes and constructs of a particular discipline or sub-discipline in order to take stock of the current frameworks and local geography.
Such an exercise can be self-serving — it is, in fact, intended to be in this case — as an idiosyncratic assessment of where we, the practitioners in or aficionados of a field might
consider ourselves to be. It may, however, also serve to provide an up-date for those in other areas of ecology, to explain what and where current research in intertidal ecology
is at present.
This special edition of Journal of Experimental Marine Biology and Ecology provides an opportunity for such a brief overview. The topic is covered as a voyage of discovery
about the themes intertidal ecologists are learning hence its title and the essay will have been worthwhile if that is all it achieves. On the other hand, if it also helps explain
current activity to scientists elsewhere, it will have achieved other end-points.
1.2. The rise of experimentation One of the key factors of the past 30 years of intertidal ecology has been the rise of
experimental manipulations as a crucial investigative tool. This has been quite phenomenal and represents a major shift of emphasis that has occurred during the same
period as the massive increase in publications of science following the expansion of universities and grant-funding in the 1960s Underwood, 1996a.
It is worth a small amount of space on a reprise of why rocky intertidal habitats have been at the forefront of development of ecological experimentation. Most notably, a
rocky intertidal shore encompasses a gradient of environmental conditions from fully marine below low tidal levels to fully terrestrial where splash and spray reach to the
highest levels above high tide. Mostly, although not always, the gradient occurs over a small metres to tens of metres distance making some patterns of response to
environmental variables relatively easy to see Newell, 1976. Short distances across gradients also allow relatively easy observation and manipulation of environmental
variables e.g. remote sensors in different parts of the range are still quite close to each other.
On such gradients, there is usually a great variety of animals and plants, often having several representatives in functional groups or guilds Menge et al., 1986, e.g. grazing
snails, predatory whelks, crabs, large fleshy, brown seaweeds, etc. Such diversity facilitates comparative studies, often allowing the generality of processes to be assessed
by testing similar or the same hypotheses on several similar species.
At the same time, many of the animals and plants are macroscopic, abundant, slow-moving or sessile as adults Connell, 1972 and interact at small spatial scales
A .J. Underwood J. Exp. Mar. Biol. Ecol. 250 2000 51 –76
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Underwood and Chapman, 1996. Thus, small-scale studies are often at the appropriate scales, and the mechanics and logistics are usually manageable. Animals and plants may
live for extraordinarily long periods, but typically turn over in relatively few years. They therefore complete their life-cycles in the sort of time-scale that is matched by cycles of
grant-funding and scholarships for Ph.D.s. This has the serious down-side that studies have focussed on the conspicuous, abundant, slow-moving and shorter-lived components
of assemblages. This has ignored many of the long-lived plants Slocum, 1980 and the more active, small consumers e.g. Brawley, 1992, although there have been exceptions
e.g. Johnson and Mann, 1988; Duffy and Hay, 1991. It is also the case that the vast majority of studies have been focussed on species characterized by relative immobility.
The immobility has been emphasized by ignoring the potentially great distances travelled Scheltema, 1971 by the dispersive larval phases of life-history, although there
have been exceptions such as Shanks and Wright 1987, Shanks 1995, Eckman 1996 and Shkedy and Roughgarden 1997. Despite such biases, animals and plants on
rocky shores are quite suited by visibility, size, diversity, longevity, abundance and lack of emotional appeal to be subjects of experimental tests of hypotheses about ecological
patterns and processes. So, where are we now and what are we discovering from such experimentation?
2. Some conceptual issues
