Research

Ecologist: Pattern Hunting Is Essential In Ecological Research

Editor's Note: A few months ago, a group from the University of Maryland tried to put a dollar amount on the value of ecosystems (R. Costanza et al., Nature, 387:253-60, 1997). They estimated that the annual average value of the Earth's natural goods and services was about $33 trillion. This is the value of ecosystems writ large, but one somewhat less extensive area of ecological research is understanding the value of biodiversity to the health of the whole ecosystem. Are there redundant species whose loss would pass unnoticed? Do ecosystems function more efficiently when they are rich in species? The Centre for Population Biology at Imperial College, London, is one of several laboratories focusing its research efforts on questions of this kind. The Centre is directed by John H. Lawton. Last spring Peter Moore, ecology correspondent for the newsletter Science Watch, spoke with Lawton about his research. Lawton comes from a biologist/naturalist background and has a wide range of interests and expertise. Although his main research focus has been the population and ecology of invertebrates, he has particular interests in insect pests and hence in the interactions between invertebrate herbivores and plants. His quest for an appropriate insect to control the spread of the noxious fern of grazing lands, the bracken Pteridium aquilinum, continues. At the same time, his passion for birds, which dates back to his youth, has led to his current role as chairman of the Council of the Royal Society for the Protection of Birds, one of Britain's most influential societies for conservation. He also has served as scientific adviser to, and member of, the Royal Commission on Environmental Pollution. If ecologists need to be polymaths, then John Lawton is well qualified for the job. Lawton took his first degree and his Ph.D. in zoology at the University of Durham, earning the latter degree in 1969. Following a short spell at Oxford, he moved in 1971 to the University of York, where he spent the bulk of his research career. In 1989, he was appointed first director of the newly established Natural Environmental Research Council Centre for Population Biology at Imperial College. The Centre is engaged in many aspects of population ecology research, but much of its effort currently is focused on the construction of regulated miniature ecosystems in controlled environment chambers-the Ecotron. Here the species content is determined in advance and the equilibrated systems are manipulated in various ways-for example, by eliminating species to simulate extinctions. In this manner the possible benefits of biodiversity can be studied within an experimental system.  Peter Moore's interview with Lawton, originally published in Science Watch (8[3]:3-4, 1997), is reprinted here with the permission of the publisher, the Philadelphia-based Institute for Scientific Information (ISI). For more information on the citation databases and information discussed in the article, contact Christopher King, editor of Science Watch, ISI, 3501 Market St., Philadelphia, Pa. 19104; (800) 523-1850, Ext. 1341. Fax: (215) 387-1266. E-mail: cking@isinet.com. World Wide Web: http://www.isinet.com.

SW: What is your view of the current state of ecological research?

LAWTON: There is more pessimism about the ecology than there should be. There is a tendency to believe that ecology is all too complex to comprehend and that therefore ecologists must retreat into detailed, specific studies. My main theme is the need to seek the rules and the generalizations that are possible in ecology, and the way in which ecology can become a predictive science. I realize that we do not know, as yet, very much about the individual ecology of most organisms, but that does not mean that we are unable to predict.

SW: One of your very evident beliefs that penetrates many of your papers concerns patterns in nature. Is this the starting point from which to make ecology a predictive science?

LAWTON: I believe passionately that pattern detection is the first stage in ecological work, prior to the development of experimental, manipulative work. We need to detect repeatable patterns, we need to develop mathematical models, and we need to develop manipulative experiments. There is not one true approach to ecological research-we need all three. Experimentation is not the sine qua non of science; if it were so, then astronomy could not be regarded as science.

Take birds, for example. Widespread birds also tend to be common birds, and this can be demonstrated simply by observation-a form of pattern hunting. Experiments are difficult here, but humanity is unwittingly conducting a kind of experiment by fragmenting habitats and reducing many bird ranges. The prediction is then that the abundance of a species will decline within the remaining patches. This idea can be tested by model building and repeated observation rather than experimentation. It often happens that there is a multiplicity of explanations relating the various observations rather than a single one, as is the case with these area/abundance relationships. We should not be asking which explanation is the correct one, but how the different forces operate in concert to produce a strong pattern in nature. In the case of birds, we currently have a wide program of research looking into the question of declining populations of British farmland birds and whether range contraction is accompanied by decrease in population density, or clutch size, [and so forth]. But this is all based on observation and modeling, rather than experimental manipulation.

SW: But here at the Centre, manipulation seems to be the focus of your work.

LAWTON: The focus of our research has been the manipulation of small ecosystems within controlled environment units that we call the "Ecotron." The idea came to a group of us about 10 years ago, when we conceived the aim of putting together simple, self-replicating ecosystems in the laboratory, so that we could test some of the ideas about how ecosystems work. Our system here at [the Centre] has now been in full-scale operation for over six years.

SW: The Ecotron units consist of cubic chambers with a 2-meter dimension. Is there a problem of scale here, extrapolating results from small systems to very large ones?

LAWTON: I tend to turn this type of question around. These chambers have weather, they have dawn and dusk, they don't have seasons or unpredictable events (though we could build these in if we wished), and they don't have immigration and emigration. So they are a simplification of nature. But for the phenomena we are studying, will such factors as immigration and emigration be expected to affect the outcome? Probably not while our questions remain simple and relatively short-term. Questions about successional replacement are clearly beyond the present scale of the current experiments, although we may well begin to understand certain key elements even in this process. For example, clover seems to be selectively favored when earthworms are present, and this could lead to subsequent community changes in the real world. The Ecotron permits the elucidation of such important stages in long-term processes.

In the Ecotron we have been able to conduct experiments that could not reasonably, or economically, have been done in the field, such as the stripping of certain species from the system at various trophic levels in order to observe the impact of reduced biodiversity on ecosystem function. But when we have conducted such experiments we have had to ask ourselves whether the real world, which consists of a series of meter-square patches, behaves as the sum of such patches, or whether something qualitatively different occurs.

SW: You are asking some very big questions of these patches, such as whether species diversity affects ecosystem processes.

LAWTON: Yes. Taking primary production as an example of an ecosystem process, we have shown that over three complete generations of plants the production of the whole system is positively related to the number of plant species present. This supports what Paul Ehrlich has graphically called the "Rivet Hypothesis," where each species that is stripped from the ecosystem weakens its function (as opposed to the "Redundancy Hypothesis," where many species are surplus to ecosystem requirements and whose removal has no effect). In the case of primary production being enhanced by additional plant species, this is not surprising, as one might intuitively expect more plant species to result in better light trapping and hence higher productivity. But pot experiments in greenhouses do not always show this. Being embedded in an ecosystem seems to make a difference. We have 16 species of plants in the Ecotron, but this does not mean that humans need only 16 plants. If we were to bring extreme events and catastrophe into our artificial system, then we would need species that can cope with extremities of environmental conditions. What we can do from the data we have so far is begin to build models that relate diversity and ecosystem function. This is the big challenge that I see over the next five years.

SW: Are you planning to scale up your Ecotron experiments?

LAWTON: We have already done so. We have a field experiment running on comparable sites right across Europe, from the Mediterranean to just short of the Arctic Circle. We have taken the natural vegetation and controlled soil conditions and the numbers of species present so that we can examine the relationship between species richness, or functional groups such as grasses, forbs, [and so forth], and ecosystem processes such as production, nutrient cycling (especially phosphorus, potassium, and nitrogen), and so on.

SW: What about animal manipulations?

LAWTON: We have a field project operating in Cameroon, working on termite diversity in tropical forest. We dig up sections of the forest floor, get rid of the termites, and put it back, while also manipulating habitat microdiversity to see how this affects the process of decomposition in relation to termite diversity.

SW: These are field experiments. What are your current plans for the Ecotron?

LAWTON: The Ecotron is now being used for climate-change experiments where we are varying carbon dioxide levels and temperature in a variety of combinations. It is the first experiment in the world where these two factors have been manipulated separately and in combination. We are operating with complete ecosystems and the most dramatic result so far obtained is the effect of cccccccc on the soil fauna. We inoculate the sterilized soil initially with a suite of fungi, including mycorrhizal ones, bacteria, and the ubiquitous nematodes, then add earthworms, isopods, and collembola to order. We find that enhanced CO₂ ultimately changes soil processes in ways that have big effects on the collembola.

Many experiments are being conducted around the world on the effect of CO₂ on vegetation growth, but they do not combine CO₂ and temperature variation, nor do they generally look at impacts on plant population dynamics over several generations. What they almost all observe is that the enhancement of plant growth that one sees in greenhouse experiments with raised CO₂ is not maintained in the field. Plants may photosynthesize faster per unit of leaf area, but they do not increase in biomass. Partly it's because of nutrient constraints. But what we find is that carbon is entrained into the system and accumulates not in the plant but in the soil.

On a global scale, we know that forests, for example, are an important sink for CO₂, but exactly where the carbon ends up is not clear. Some in the timber, maybe, but certainly some in the soil organic matter. Perhaps there are changes in root exudates in these systems leading to alterations in microbial populations.

SW: How about temperature effects on microbial metabolism and CO₂ release from the soil?

LAWTON: This we don't yet know. We have the results for raised CO₂ conditions but the elevated temperature regime experiment still has five months to run, so we are still awaiting the outcome of this combination.

SW: One other area of work with which you are associated, together with some of your past students, like Stuart Pimm, is food-web complexity. Is this work continuing? 

LAWTON: Very many years ago a field trip to a desert and observations of the low density of vegetation, coupled with the diversity of animal life, brought home to me that food-web complexity and food-chain lengths could not be related simply to primary productivity. Many models, some of them deliberately provocative, were built, but many remain untested, which is an indictment of experimental ecologists. Neo Martinez, currently a visitor to the Centre here, has been gathering data and developing new models to cope with these. This is a good example of how ecology should progress-data generating models, and these models stimulating the gathering of new data and the generation of new models. Here we return to the need for pattern hunting in nature. Perhaps what is required is a world food-chain length program so that teams in different regions can use the same techniques to examine the phenomenon in their particular area. Only then can we ask whether food chains in Costa Rican forests are longer than those of the Atacama desert.

(Ranked by average citations per year)
 
 

Rank	Paper	Total citations through June 1997	Average citations per year (not including 1997 citations)
1	S. Naeem, L.J. Thompson, S.P. Lawler, J.H. Lawton, R.M. Woodfin, "Declining biodiversity can alter the performance of ecosystems," Nature, 368:734-7, 1994.	54	18
2	J.H. Lawton, D.R. Strong, "Community patterns and competition in folivorous insects," Amercan Naturalist, 118:317-38, 1981.	221	15
3	S.L. Pimm, J.H. Lawton, J.E. Cohen,"Food web patterns and their consequences," Nature, 350:669-74, 1991.	86	14
4	J.R. Prendergast, R.M. Quinn, J.H. Lawton, B.C. Eversham, D.W. Gibbons, "Rare species, the coincidence of diversity hotspots and conservation strategies," Nature, 365:335-7, 1993.	56	13
5	M.J. Jeffries, J.H. Lawton, "Enemy-free space and the structure of ecological communities," Biological Journal of the Linnean Society, 23:269-86, 1984.	147	13
Source: ISI's Science Indicators Database, 1981-June 1996