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Klaus Rohde: Nonequilibrium Ecology

Wednesday, January 31st, 2007

A central problem in ecology is the relative importance of equilibrium and nonequilibrium conditions in populations, communities and ecosystems. The prevailing view was for a long time that systems are generally at or near equilibrium, although deviations from equilibrium due to environmental disturbances are possible and even common. In such near-equilibrial systems, consisting of densely packed populations, competition between individuals of the same or different species is believed to be of paramount importance. However, much evidence contradicts this prevailing paradigm. A recent book examines the evidence for equilibrium and nonequilibrium in ecological systems (Klaus Rohde 2005. Nonequilibrium Ecology. Cambridge University Press, Cambridge). The book contains a brief discussion of the theoretical background and history (in greater detail discussed in G.C.Cooper, The Science of the Struggle for Existence, Cambridge Studies in Philosophy and Biology, Cambridge University Press, Cambridge 2003), detailed discussions of competition (and the often faulty evidence given for it), a discussion of non-competitive mechanisms responsible for niche restriction and segregation, and detailed examples of equilibrium and nonequilibrium over evolutionary time, in populations and metapopulations, in communties, and in ecosystems. An attempt is made to explain why different communities and ecosystems differ in the degree of equilibrium/nonequilibrium. Finally, “prospects for an ecology of the future” are discussed. Particular attention is paid to latitudinal gradients in species diversity, and to nonequilibrium caused by climate change. Details of the book are available at:

http://www.cambridge.org/9780521674553

From the only review available to date:

…..this is a useful book that should be read by any ecologist and particularly by any graduate student interested in a refreshingly different perspective of our science than the one dished up too frequently in survey courses and the conservation press.

Peter F. Sale, Conservation Biology 21, 282, 2007.

Klaus Rohde editor: Marine Parasitology

Wednesday, January 31st, 2007

Marine parasites are among the most common and most diverse animals on earth, and most species have not yet been described. Many species are important as agents of disease in the oceans and in aquaculture. The study of marine parasites should be an essential component of any course in marine biology. This post draws your attention to a new book on marine parasites, the standard work on the subject. It was published by CSIRO Publishing, Melbourne and CABI Wallingford, Oxford, in November 2005. 75 experts in their fields contributed. There are sections on the various parasite groups (from protistans to vertebrates), on behavioural, ecological, evolutionary and zoogeographical aspects, and on aspects of economic, environmental and medical importance. Details of the book are available on the website of CSIRO Publishing:

http://www.publish.csiro.au/pid/5045.htm

In the following I show extracts from the three reviews available to date.

1.) “……..It offers a very thorough review of the present knowledge of virtually all marine parasites and most of what concerns their biology and ecology…….The scope of the book reaches from tree of life based on molecular analysis, fossil parasites or mites parasitic on walruses to medical importance of marine parasites. … We can suppose that the book will become a standard text, stimulating work not only by students recently attracted to marine parasite research but also of established scientists. At present, when most scientists studying marine parasites are specialised in their field of research and cannot be equally proficient in other directions, even for them the book will be an invaluable source of information. However, the book will find its place on the shelf of every biologist who likes the intricacies and charms of organisms adapted to symbiotic existence in the immense and rich realm of marine life.” Jiri Lom, Folia Parasitologica 53, 77-78 (2006).
2.) “One of the world’s foremost experts on the ecology of host–parasite relationships in marine systems is Klaus Rohde. In 1993, the second edition of his Ecology of Marine Parasites was printed (CABI International).
Twelve years later, he expanded the approach from the ecology of marine parasites to just about everything that has anything to do with marine parasitology. ……. he is the editor of an 11-chapter volume, collectively written by some 75 authors. Each of the chapters is divided into a various number of sections, with each section written by an authority.

I think this is a very important piece of work. I would strongly urge anyone with an interest in marine parasites, and marine biologists in general, to purchase it for your own bookshelf. If you are interested in marine biology, or if you have any kind of marine program in your institution, then I also would strongly recommend that your library have a copy. It is well worth the money.”
Gerald W. Esch, Department of Biology, Wake Forest University, Winston-Salem, North Carolina 27109. American Journal of Parasitology 92(2), 2006, p. 312.

3.)”This is a beautiful book, and I recommend it for everybody with an interest in marine ecology.”
Peter Heuch, National Veterinary Institute Oslo, Norway. Published in “Monoculus”, the newsletter of the World Association of Copepodologists.

Vacant niches

Wednesday, January 31st, 2007

This post is based on my earlier entry in Wikipedia. This seems necessary because Wikipedia permits changes made by anybody, whether with the necessary knowledge or not.

The concept of a vacant or empty niche has been controversial in ecology. It is at the basis of any discussion of whether equilibrium or nonequilibrium conditions prevail in ecological systems. A vacant niche can be defined as the possibility that in ecosystems or habitats more species can exist than are present at a particular point in time, because many possibilities are not used by potentially existing species (K. Rohde 2005 Nonequilibrium Ecology, Cambridge University Press, Cambridge, http://www.cambridge.org/9780521674553).

1. History of the concept

Hutchinson, G. E. (1957) Concluding remarks. Cold Spring Harbour Symposium on Quantitative Biology 22, 415-427 was apparently the first who considered the possibility of vacant niches. He writes: (p.424): “The question raised by cases like this is whether the three Nilghiri Corixinae fill all the available niches……….. or whether there are really empty niches.”…….“The rapid spread of introduced species often gives evidence of empty niches, but such rapid spread in many instances has taken place in disturbed areas.”

Since then, the concept “vacant niche” or “empty niche” has been used regularly in the scientific literature. Some of the many examples are Elton, C. (1958) The ecology of invasions by animals and plants. Chapman and Hall, London, UK. 181 pp.; Rohde, K. (1977). A non-competitive mechanism responsible for restricting niches. Zoologischer Anzeiger 199, 164-172; Rohde, K. (1979). A critical evaluation of intrinsic and extrinsic factord responsible for restricting niches. American Naturalist 114, 648-671; Rohde, K. (1980 ). Warum sind ökologische Nischen begrenzt? Zwischenartlicher Antagonismus oder innerartlicher Zusammenhalt?. Naturwissenschaftliche Rundschau, 33, 98-102;
Lawton, J.H. (1984). Non-competitive populations, non-convergent communities, and vacant niches: the herbivores of bracken. In: Strong, D.R. Jr., Simberloff, D., Abele, L.G. and Thistle, A.B. eds. Ecological communities: conceptual issues and the evidence. Princeton University Press, Princeton, N.J., pp. 67-101; Price, P.W. (1984). Alternative paradigms in community ecology. In: Price, P.W., Slobodchikoff, C.N. and Gaud, W.S. eds. (1984). A new ecology. Novel approaches to interactive systems. John Wiley & Sons, New York, Chichester, Brisbane, Toronto, Singapore, pp.353-383; Compton, S.G., Lawton, J.H. and Rashbrook, V.K. (1989). Regional diversity, local community structure and vacant niches: the herbivorous arthropods of bracken in South Africa. Ecological Entomology 14, 365-373; Begon, M.J., Harper, L. and Townsend, C.R. (1990). Ecology. Individuals, populations and communities, second edition. Blackwell Scientific, Boston; and Cornell, H.V. (1999). Unsaturation and regional influences on species richness in ecological communities: a review of the evidence. Ecoscience 6, 303-315.
Further examples, some of them in great detail, are discussed in K. Rohde (2005) Nonequilibrium Ecology, Cambridge University Press, Cambridge.

2. Causes of vacant niches

Vacant niches can have several causes.

One cause is radical disturbances in a habitat (biotop). For example, draughts or forest fires can destroy a flora and fauna partially or completely. However, in such cases species suitable for the habitat usually survive in the neighbourhood and colonize the vacated niches, leading to a relatively fast reestablishment of the original conditions.

Further causes of vacant niches are radical and long-lasting changes in the environment, such as ice ages.

Vacant niches can also be due to evolutionary contingencies: suitable species did not evolve for usually unknown reasons.

3. Demonstration of vacant niches

Vacant niches can best be demonstrated by comparing the spatial component of niches in simple habitats. For example, Lawton and collaborators compared the insect fauna of the bracken , Pteridium aquilinum, a widely distributed species, in different habitats and geographical regions and found vastly differing numbers of insect species. They concluded that many niches remain vacant (e.g., Lawton, J.H. (1984). Non-competitive populations, non-convergent communities, and vacant niches: the herbivores of bracken. In: Strong, D.R. Jr., Simberloff, D., Abele, L.G. and Thistle, A.B. eds. Ecological communities: conceptual issues and the evidence. Princeton University Press, Princeton, N.J., pp. 67-101.

Rohde and collaborators have shown that the number of ectoparasitic species on the gills of different species of marine fishes varies from 0 to about 30, even when fish of similar size and from similar habitats are compared. Assuming that the host species with the largest number of parasite species has the largest possible number of parasite species, only about 16% of all niches are occupied. However, the maximum may well be greater, since the possibility cannot be excluded that even on fish with a rich parasite fauna, more species could be accommodated (recent review in K. Rohde (2005) Nonequilibrium Ecology, Cambridge University Press, Cambridge.

Using a similar way of reasoning, Walker, T.D. und Valentine, J.W. (1984). Equilibrium models of evolutionary diversity and the number of empty niches. American Naturalist 124, 887-899. estimated that 12-54% of niches for marine invertebrates are empty.
The ground breaking theoretical investigations of Kauffman, S.A. (1993). The origins of order. Self-organization and selection in evolution. Oxford University Press, New York Oxford and Wolfram, S. (2002). A new kind of science. Wolfram Media Inc. Champaign, Il. also suggest the existence of a vast number of vacant niches. Using different approaches, both have shown that species rarely if ever reach global adaptive optima. Rather, they get trapped in local optima from which they cannot escape, i.e., they are not perfectly adapted. As the number of potential local optima is almost infinite, the niche space is largely unsaturated and species have little opportunity for interspecific competition.

The packing rules of Ritchie, M. und Olff, H. (1999). Spatial scaling laws yield a synthetic theory of biodiversity. Nature 400, 557-562 can be used as a measure of the filling of niche space. They apply to savanna plants and large herbivorous mammals, but not to any of the parasite species examined so far. It seems likely that they do not apply to most animal groups. In other words, most species are not densely packed: many niches remain empty (Rohde, K. (2001). Spatial scaling laws may not apply to most animal species. Oikos 93, 499-503).

That niche space may be nonsaturated, is also shown by introduced pest species. Such species lose almost without exception all or many of their parasites (Torchin, M.E. and Kuris, A.M. (2005). Introduced parasites. In: Rohde, K. (Ed.) Marine Parasitology. CSIRO Publishing Melbourne und CABI Wallingford, Oxon., pp. 358-366). Species that could occupy the vacant niches either do not exist or, if they exist, have not had the chance to invade these niches.

Diversity of marine benthos, interrupted by some collapses and plateaus, has increased from the Cambrian to the Recent, and there is no evidence that saturation has been reached (Jablonski, D. (1999) The future of the fossil record, Science 284, 2114-2116).

Simulations of latitudinal gradients in species diversity using the Chowdhury ecosystem model, have shown that results are closest to reality when many niches are kept empty (Rohde, K. and Stauffer, D . 2005 Simulation of geographical trends in the Chowdhury ecosystem model. Advances in Complex Systems 8, 451-464).

4. Consequences of the nonsaturation of niche space

The view that niche space is largely or completely saturated with species is widespread. It is thought that new species are accommodated mainly by subdivision of niches occupied by previously existing species, although an increase in diversity by colonization of large empty living spaces (such as land in the geologic past) or by the formation of new bauplans also occurs. It is also recognized that many populations never completely reach a climax state (i.e., they may come close to an equilibrium but never quite reach it). However, altogether the view prevails that individuals and species are densely packed and that interspecific competition is of paramount significance. According to this view, nonequilibria are generally caused by environmental disturbances.

Many recent studies (above and Rohde, K. (2005) Eine neue Ökologie. Aktuelle Probleme der evolutionären Ökologie. Naturwissenschaftliche Rundschau, 58, 420-426; Rohde, K. 2005: Nonequilibrium Ecology, Cambridge University Press, Cambridge support the view that niche space is largely unsaturated, i.e. that numerous vacant niches exist. As a consequence, competition between species is not as important as usually assumed. Nonequilibria are caused not only by environmental disturbances, but are widespread because of nonsaturation of niche space. Newly evolved species are absorbed into empty niche space, that is, niches occupied by existing species do not necessarily have to shrink.

5. Relative frequency of vacant niches in various groups of animals and plants

Available evidence suggests that vacant niches are more common in some groups than in others. Using SES values (standardized effect sizes) for various groups, which can be used as approximate predictors of the filling of niche space, Gotelli, N.J. and Rohde, K. (2002). Co-occurrence of ectoparasites of marine fishes: null-model analysis. Ecology Letters 5, 86-94 have shown that SES values are high for animal populations which occur in large population densities and/or are of large body size and are vagile, they are low for animal groups which occur in small population densities and/or are of small body size and have little vagility. In other words, more vacant niches can be expected for the latter.

6. Criticisms of the concept

The concept of vacant niche is not accepted by all. The reason given is that a niche is a property of a species and does therefore not exist if no species is present. In other words, the term is thought to be “illogical”. However, some authors who have contributed most to the formulation of the modern niche concept (Hutchinson, Elton) apparently saw no difficulties in using the term. If a niche is defined as the interrelationship of a species with all the biotic and abiotic factors affecting it, there is no reason not to admit the possibility of additional potential interrelationships. So, it seems logical to refer to vacant niches.

Furthermore, it seems that authors most critical of the concept vacant niche really are critical of the view that niche space is largely empty and can easily absorb additional species. They instead adhere to the view that everything is much of the time in equilibrium (or at least close to it), resulting in a continual strong competition for resources. This view, indeed, is the basis of Darwinian natural selection. Many recent studies, some empirical , some theoretical, have provided support for the alternate view that nonequilibrium conditions are widespread (see above and the recent review in Rohde K. Rohde 2005) Nonequilibrium Ecology, Cambridge University Press, Cambridge.

In the German literature, an alternate term for vacant niches has found some acceptance. It is that of “freie ökologische Lizens” (free ecological license) (Sudhaus,W. und Rehfeld, K. 1992. Einführung in die Phylogenetik und Systematik. Gustav Fischer Verlag Jena. It has the disadvantage that it does not convey immediately and easily what is meant, and it indeed does not correspond exactly to the term vacant niche. The usefulness of a term should be measured on the basis of its pregnancy and easy understandability, and on how fertile it is in promoting future research. The term vacant niche appears to fulfill these requirements.

Fuzzy chaos modelling in ecology and economics

Saturday, January 27th, 2007

The aim of this post is to draw the attention of economists to some results obtained for ecological systems, because they may provide insights into how economic systems work. In a paper published a few years ago (Klaus Rohde and Peter P. Rohde 2001. Fuzzy chaos: reduced chaos in the combined dynamics of several independently chaotic populations. American Naturalist 158, 553-556) we have shown that chaos in populations is reduced in metapopulations consisting of several largely independent subpopulations with different reproductive rates. Examples are given in figures 1 and 2. Population sizes x are plotted as fractions of carrying capacities (0-1) at different reproductive rates r of the population. Figure 1 shows a bifurcation diagram for a single population; the insets show population sizes plotted against time for a few selected reproductive rates. Note that chaotic fluctuations in population size begin at r=3.57. Figure 2 shows a bifurcation diagram for a metapopulation consisting of 5000 subpopulations, illustrated only for reproductive rates of r=3.50 and larger. Note that there still are chaotic fluctuations, but the width of the fluctuations is significantly reduced.

fig1.jpg

Figure 1: Bifurcation diagram for a single population.
Fig.2. Bifurcation diagram for 5000 subpopulations.

Figure 2: Bifurcation diagram for a metapopulation consisting of 5000 subpopulations.

This may suggest that chaotic fluctuations are much stronger in single large economies, for example due to globalisation, than in the world economy consisting of national economies that are largely separated.

I invite comments to point out any errors in the argument.

recent papers

Thursday, January 25th, 2007

This is a test run, i.e., my first blog. I am using it to draw your attention to some recent papers on ecological/evolutionary modelling done jointly with Dietrich Stauffer of the Institute of Theoretical Physics, Universität Köln, Germany. Dietrich Stauffer is one of the leading computational physicists in the world. His “Introduction to Percolation Theory” has been cited around 4000 times. His many papers include applications of models to physics, chemistry, genetics, immunology, language evolution, geology and biology. His most recent book, published jointly with some colleagues, is on “Biology, Sociology, Geology by Computational physicists” (Elsevier 2006).

Our papers are:

Rohde, K. and Stauffer, D. 2005. Simulation of geographical trends in the Chowdhury ecosystem model. Advances in Complex Systems 8, 451-464. http://arxiv.org/q-bio/0505016

Stauffer, D and. Rohde, K. 2006. Simulation of Rapoport’s rule for latitudinal species spread. Theory in Biosciences 125, 55-65. http://arxiv.org/q-bio/0507033

Stauffer, D., Schulze C., Rohde K. submitted. Habitat width along a latitudinal gradient. View et Milieu http://arxiv.org/q-bio/0612012

In the first of these papers, we use the Chowdhury ecosystem model to analyse latitudinal gradients in species diversity. We found that complexity of foodwebs increases with time and at a higher rate at low latitudes. Keeping many niches empty makes the results correspond more closely to natural gradients.

In the second paper, we use the Chowdhury ecosystem model to test Rapoport’s rule, according to which latitudinal ranges of species are greater at high latitudes. We did not find support for the rule, in agreement with empirical studies.

In the third paper, we use the Chowdhury ecosystem model to test the latitude-niche breadth hypothesis, which explains the higher species diversity in the tropics by narrower niches there. We did not find support for the hypothesis, in agreement with some empirical studies.

I have examined the same ecological/evolutionary problems in a number of earlier papers using empirical data. References can be found at http://www-personal.une.edu.au/~krohde/