island biogeography - an overview · various theories of island biogeography were developed in the...

BTU Chair General Ecology Island Biogeography 1

ISLAND BIOGEOGRAPHY – AN OVERVIEW

Scriptum

Udo Bröring BTU, Chair General Ecology

Contents

Introduction

Relative Abundance and Relative Distribution of Organisms

Analysis of Species-Area Relations

Analysis of Species-Abundance Relations

The Equilibrium Theory of Island Biogeography

Main Ideas of the Approach

Extensions and Applications

Contradicting Approaches

Habitat-Heterogeneity Hypothesis

Passive Sampling Hypothesis

Examples and Case Studies

Conclusions

Introduction

Various theories of island biogeography were developed in the past fifty years. These theories have been very inspiring for ecology in general, questions as to immigration, colonization of habitats and extinction are of basic interest. Most famous is the “equilibri-um theory of island biogeography”. As it is a rather simple approach the question occurs, what about the significance of the famous equilibrium theory? Is it really a trivial theory? Why is it so famous? Why is it an inspiring approach?

A first but rather preliminary answer can be derived from the history of ecology. The term ecology („Ökologie“) was introduced by Ernst Haeckel in 1866, being an empirical science and a sub discipline of biology. His definition did not contain any research program, this was added by the Danish scientist Eugeniusz Warming in 1895, who was the founder of ecology as a „science“; for animal ecology this was added by Charles Elton („Animal Ecology“, 1929). However, today, there are certain demands of modern philosophers of science: evaluation of theories and hypotheses, consistency of approaches, rigorousity of theories, strong reference to explanation (cause-effect-mechanisms), theory extension etc. Those aspects are met by the equilibrium theory of island biogeography.


History of science shows that scientific approaches were successful, when basic hypothe-ses and theories are expressed in quantitative terms. By this, they could be handled with mathematic techniques and, subsequently, better be confirmed or falsified. According to modern epistemology and philosophy of science it is necessary to express theories in quantitative terms, an explicandum becomes an explicatum only, when it is expressed in quantitative terms, i.e. when measurement is possible. Early ecology was dealing with sampling, ordering, classifying of objects. Theories from other scientific fields (evolution-ary biology, geography) were adopted and transformed. Ecological theories and quantita-tive concepts in a stronger sense were not available. An important reason for the success of the equilibrium theory was the fact that it is an approach working with quantitative concepts allowing confirmation, falsification, explanation, and prediction.

What is ecology dealing with? A modern definition of ecology was given by Begon, Harper & Townsend (1986): Ecology deals with the description, explanation and predic-tion of the distribution of individuals, populations and communities in space and time. By this (1) it is specified that it is a scientific approach (empirism: description, explanation, and prediction), (2) levels of observation (individuals, populations, and communities) are fixed, and (3) a certain matter of interest is given: distribution of objects in space and time.

It was important for the further development that Haeckel introduced ecology as a sub-discipline of biology. The position of ecology within life sciences is contrary to physiology or anatomy an ideographical approach, and contrary to systematics a dynamical approach (see table 1)1. It means that originally in ecology the focus was on explanation and under-standing of singularities and not on detection of general laws, – it was a matter of natural history, and the procedure is historiographic. In the course of time it was often tried to introduce quantitative terms in order to elaborate general scientific laws (and cause-effect-explanations) in ecology as well – with more or less success. Compared to other approach-es the equilibrium theory was most successful in this respect.

Table 1: Nomothetic and idiographic approaches in biology.

Nomothetic approach

Focus: general aspects

Idiographic approach

Focus: special aspects

Morphology, Anatomy Systematics Statical approaches

Physiology Ecology Dynamical approaches

Generalization Historiography

1 See: Rickert, H. 1926. Kulturwissenschaft und Naturwissenschaft. 7. Aufl. Reclam Stuttgart 1986 (Engl.: Science and history: A critique of positivist epistemology). Windelband, W. 1894. Geschichte und Naturwissenschaft. Straßburger Rektoratsrede (http://www.fh-augsburg.de/~harsch/germanica/Chronologie/19Jh/Windelband/win_rede.html ).

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The Equilibrium Theory of Island Biogeography

Main Idea of the Approach

The equilibrium theory of island biogeography was established by the two American scientists Robert H. MacArthur (1930-1972) and Edward O. Wilson (*1929). The basic preconditions MacArthur and Wilson found at the beginning of the 60th can be character-ized by the state of discussion as to the species area and the species abundance relations:

Formalization and quantification of the various approaches Linking models on species area and species abundance relations: The „fourth root

law“ Dynamization of theories by the analysis of immigration and extinction processes Formularization of the area per se – postulate

First, MacArthur and Wilson restricted their approach to islands, by this to isolated areas, because these environments have clear borders which make it easier to be studied. It has been important that Preston (1962) advanced the combination of the species-area and the species-abundance relations: The “fourth root law” stresses the case that a community on an island is canonically lognormal distributed (λ = 1). It is also claimed that the abundance of the smallest population is not related and the total number of individuals is proportional to the area size. Furthermore it is stated that the regression coefficient („slope“, „z“) in log-transformed species-area plots on islands is 0.25 (¼):

ln [ S(A) ] = b + ¼ ln [A] or S(A) = c + A¼

There is an approximation: z = 1/(4 λ) for λ > 1 and z = 1/(1+ λ)2 for λ < 1. The constants c and b are often called „biological“ or „MacArthur constants“. Numerical approximations revealed z = 0.262 (Preston 1962: 191) and z = 0.263 (MacArthur & Wilson 1967)4. – An illustration of the fourth root law is given in figure 6.

Based on these considerations the equilibrium theory was advanced. The process of colonization is analyzed in general. It is stated that the island species number is increasing during the colonization process to a certain extent. However, there is no linear relation, but a logarithmic, and the process stops when the species number of the mainland colonization source is achieved. Subsequently, the development of island species numbers is analyzed in detail by a simple consideration as to the immigration and extinction dynamics in the course of the colonization. Obviously, if species number on the island is small the immi-gration rate I (number of species arriving at the island within a time unit) is comparably higher, because the mean probability is higher that a new species colonize the island form the mainland source, while the extinction rate E (number of species going extinct within a time unit) is comparably small: few species, few extinctions. On the other hand, if species

4 The basic studies: Preston, F.W. 1962. The canonical distribution of commonness and rarity, Part I. and II. Ecology 43 (2,3) 185-215, 410-432. – MacArthur, R.H. & Wilson, E.O. 1963. An equilibrium theory of insular zoogeography. Evolution 17: 373-387. – MacArthur, R.H. & Wilson, E.O. 1967. The theory of island biogeography. New York. – See also: Connor, E.F. & McCoy, E.D. 1979. The statistics and biology of the species-area relationship. Am. Nat. 113: 791-833. – Furthermore important: May, R.M. 1975. Patterns of species abundance and diversity. In: Cody, M.L. & Diamond, J.M. (eds.): Ecology and evolution of communities. Cambridge. – German readers may consult: Wissel, C. 1989. Theoretische Ökologie. Berlin.

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The equilibrium theory is also based on the “area-per-se postulate”. Within this hypothesis it is stated that equilibration process exclusively depends on the island area size: Under the initial condition that the isolation of single islands of an island archipelago are of similar amount the number of species on the island exclusively depends on the island area size. In the case of canonical lognormal distribution the slope is z = 0.25 = ¼ (fourth root law).

There are several limitations of the theory more or less explicitly stated in the original publication:

The equilibrium-theory exclusively refers to the species numbers. Population densi-ties or special species compositions are not considered.

The equilibrium-theory refers to island faunas in total exclusively. Proper results are to be expected only for similar (comparable) groups of organisms.

The theory is purely calculatoric. Historical development of the islands is not taken into account. Especially evolution is not considered.

Immediately after publication of the equilibrium theory (MacArthur & Wilson 1963, 1967) various critiques and objections came up. The most important are:

It is only mathematics, not biology that is reflected. Dispersion ability and possibilities to establish populations are not taken into ac-

count. Competition and further interspecific interactions are not taken into account. Special characteristics of insular habitats are not taken into account. Turnover calculations are dubious due both to „crypto turnover“ (unrealized turno-

ver within „census intervals“) and „pseudo turnover“ (turnover due to „transients“).

On the other hand, the importance and significance of the equilibrium theory for ecological research (practically and theoretically) in general is evident.

Elaboration of different extensions of the theory, which were inspiring for other fields of scientific ecology.

Faculty of derivations of connections with and among other basic approaches and concepts in ecology which were extended themselves subsequently.

Elaboration of interrelations of disparate concepts and theories, especially among biodiversity, species-area and species-abundance theory, which were separated be-fore.

Derivation of connections to practice in the field of applied ecology (nature conser-vation, „SLOSS“, urban ecology).

Extensions and Applications

Since the publication of the equilibrium-theory in 1963, it was an object of violent controversy. On the one hand, several different approaches have been advanced, on the other hand, extensions and modifications of the original theory have been proposed. The literature dealing with problems of the equilibrium theory actually is hardly manageable.

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Relation to succession and disturbance theory. – Simulation models of the variation of landscapes diversity on islands were advanced; it was shown, that succession related to certain disturbance regimes depends on the island area size, larger islands have more constant habitat heterogeneity, and species area slopes depend on the species number of the mainland species pool. By application of Markov-chains turnover process in the course of island colonization were simulated in order to receive reliable predictions.

Applications

Several applications of the equilibrium theory for practical conservation issues have been discussed.9 Two examples are given.

Single Large or Several Small (“SLOSS”)

About 10 years after publication of the equilibrium theory the discussion on „SLOSS“ as to an optimal nature reserve design came up repeatedly. It was discussed, whether nature reserves should have a minimum area size, and whether it is more appropriate to have single large or several small nature reserves under limited conditions.

Proponents of the equilibrium theory admitted in a quite early stage of the discussion, that the MacArthur/Wilson concept does not necessarily lead to an optimal design of nature reserves. However, many ecologists and nature conservationists tried it again and again. – Additionally and subsequently, different concepts as to „minimum viable population“ (MVP, 50/500 rule, etc.) were advanced10.

Urban Ecology

Beside isolated nature reserves (see SLOSS discussion) isolated biotopes in urban areas were investigated as to the applicability of the equilibrium theory. It was assumed that the variation of species number can be analyzed and predicted with the equilibrium theory, for instance in order to argue for the importance of nature conservation in urban areas.

Immediately after popularization of the equilibrium theory the applicability of the concept as to isolated environments of inland areas (e.g. parks in cities) were discussed, although MacArthur & Wilson (1963: 386) wanted to restrict the concept to the avifauna of Indo-Australia exclusively. Soon, evidence could be given that equilibrium theory cannot be applied to inland situations without any additional considerations. The reason might be that the isolation of inland areas is low compared to the migration ability of the organisms to colonize. However, a limited and careful application as to invertebrates may be possible.

Summarizing Some Aspects

The equilibrium theory of island biogeography is a milestone in the history of ecology. This is due to its inspiring power over decades; numerous ecologists discussed, and

9 German readers may check also: Mader, H.-J. 1980. Die Verinselung der Landschaft aus tierökologischer Sicht. Natur u. Landschaft 55: 91-96. 10 See papers in Soulé, M.E. (ed.) 1987. Minimum Viable Populations for Conservation. Cambridge.


elaborated the theory and related it to other scientific approaches in ecology. The theory is a rather simple but effective approach to analyze species-environment relations, and still a valuable approach to provide quantitative terms in order to elaborate scientific theory in ecology. The approach was shown to be rather capable as to distension and extension, and offered multiple connections to different scientific approaches and theories. Significant was the integration and synthetization of different and important approaches and theories dealing with analysis of distribution and abundance of species, which was essentially constitutional for ecology as a science since the beginning of ecological research.

Contradicting Approaches

Habitat-Heterogeneity Theory

The „habitat-heterogeneity theory“ was elaborated based on studies of Williams (1964) (dealing with plants), Lack (1969, 1976) (dealing with birds), and others as to colonization dynamics. It was intended to be a counter draft to the equilibrium theory according to Preston (1962) and MacArthur & Wilson (1963, 1967).

In a first step, it is admitted that there is a basic set of species on islands (theoretical number of species), which is approximated in the course of island colonization. However, it is not the island area size which is important for the amount of island species number, but the habitat heterogeneity of the environment on the island in total. Significant correla-tion between the number of species and the size of the island area are due to the fact, that most probably there are more different biotopes on larger islands and, by this, more ecological niches, which can be used by more different species.

Subsequently, numerous empirical confirmations were found that competition, resource partitioning as well as migration and dispersion abilities of colonizing species, respective-ly, and other effects are most important for the amount of species number on islands. The „habitat-heterogeneity theory“ is an approach that includes biological information in order to explain island colonization process. This is done by referring to environmental proper-ties of the island environment and the ecological properties (niche width, dispersion ability etc.) of the colonizing species.

The number of species and the species composition („species assemblages“) is related to the „between habitat heterogeneity“ approach, however, it was turned out to be extremely difficult to express in quantitative terms (measurement). This is due to the fact that according to the multifunctional niche approach various properties of the environment (the landscape) must be considered. Most often, simply the number of different habitats present on the colonized island is used. Sometimes difficult „neighborhood coefficients“ after raster scanning of habitats are calculated, or calculations of environmental diversity according to Shannon-Wiener or Simpson‘s index. – „Within habitat heterogeneity“ is separated from „between habitat heterogeneity“. Within habitat heterogeneity includes the total heterogeneity of single habitats; frequently, measurement is based on the number of plant species occurring within the habitat.

Of course, adherents of the MacArthur-Wilson theory prefer within habitat heterogeneity in order to explain island species number with the explaining variable „number of plant species“, because island colonization by plant species itself must also be explained by area


size and the equilibrium theory, respectively. According to habitat heterogeneity theory, of course, between habitat heterogeneity is most important.

Many empirical studies have been carried out in order to give evidence for one of the two theories. It is not definitively solved; however, many examples have been published where habitat heterogeneity theory was likely better to explain species numbers of islands – sometimes both approaches have shown to have explaining power in different respects.

Passive Sampling Hypothesis

Subsequently, the famous „passive-sampling hypothesis“ was expressed and developed by Connor & McCoy (1979) after analyzing numerous data sets as to island colonization, relation of species-area significance and variation of species numbers on islands, and after testing equilibrium- and habitat heterogeneity theory. The main idea is that according to this approach island colonization proceeds as passive accumulation of single individuals according to area size: the a priori probability that in the course of accumulation more individuals of more different species arrive is higher on larger islands. This and only this is the reason for the fact that you will find more species on larger islands, and therefore the pure statistical species area relation most often holds.

First, it is stated that the species area relation („area-per-se postulate“) is just a statistical artifact resulting from the evidence of the canonical lognormal distribution of species abundance within an assemblage: actually, there is a high a priori probability for such a species abundance relation, when both comparably large areas and large species assem-blages are included, and many different interactions take place.

Subsequently, it is shown that the slope of ¼ (fourth root law) is just an artifact of the regression system. The intercept occurring within the regression model is an artifact as well, and both the slope and the intercept are biologically senseless.

Additionally, it is stated that in contradiction to the equilibrium theory the immigration rate is not exclusively dependent on the island distance to the mainland source.11 It depends on the area size as well: the larger the island the higher the probability that individuals of a certain species arrive at the island and establish sustainable populations. As to the equilib-rium theory and the species-area relation, respectively, it is concluded that „it is manifested by the degree of correlation between species number and area, not the slope and the intercept parameters“ (Connor & McCoy 1979).

Connor & McCoy (1979) tried to give evidence for a direct proportionality between immigration rate and island area size. Some results have shown that it is true; however, the discussion still proceeds. A further conclusion is that both area-per-se- within the equilib-rium theory and habitat heterogeneityt heory are probably correct and are not in direct contradiction: each of the approaches give answers to different questions.

11 Many papers on this subject, see for example: Lomolino, M.V. 1990. The target area hypothesis: the influence of island area on immigration rates of non-volant mammals. Oikos 57: 297-300.

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By this it is shown that species number of species of true bugs is related to area size to a huge extent: the larger the island the higher the number of species; and as the relation of species number to heterogeneity is significant as well: the higher the heterogeneity of the landscape, the higher the number of ecological niches, the higher the number of bug species. However, the relation of species number to area size is obviously higher, but in order to detect the combination of the most important factors and their relative contribution for higher species richness multivariate statistical analysis became necessary.

By multiple regression analysis a dependent variable is explained by more than one independent variable, here it is tested whether there is a multifunctional relation among different variables (table 2). First, simple correlations can be compared, the factors area size (log-transformed data: lnAREA) and diversity explain species number of true bugs (log-transformed data: lnHET) to a reasonable and significant extent; number of plant species (log-transformed: lnPLANT) is not significant. Interrelation among the explaining variables is high for the combination lnAREA/diversity and lnAREA/lnPLANT. These results can now be compared to the results of the multiple correlations analysis.

Table 2: Results of simple, multiple, and partial correlations among (log-transformed) number of species of true bugs (lnHET, variable to be explained), (log-transformed) area size, (log-transformed) number of plant species, and landscapes diversity on 9 East Frisian Islands (analysis with SPSS; * and ** = significant on p = 0.05 and p = 0.005 level, respectively; data taken from Bröring 1991).

Simple Correlation Coefficient (R) Simple Correlation Coefficient (R)

lnHET-lnAREA: 0.822** lnAREA-diversity: 0.635*

lnHET-lnPLANT: 0.422 lnAREA-lnPLANT: 0.683*

lnHET-diversity: 0.816** lnPLANT-diversity: 0.181

Partial Correlation Multiple Correlation

Impact Constant r(lnS/E,k) multiple R

lnAREA lnPLANT 0.6464 0.966**

lnPLANT lnAREA 0.0717

lnAREA diversity 0.8470* 0.974**

diversity lnAREA 0.7479*

Multiple correlation is extremely high for the combination lnHET/lnAREA/lnPLANT (R = 0.966**), and for lnHET/lnAREA/diversity (R = 0.974**) as well. The best fit for species number of true bugs seems to be the combination lnAREA/diversity as the R-value is slightly higher. In order to detect the relative importance of single values in detail partial correlation coefficients are calculated: Partial correlation lnHET/lnAREA by exclusion of the impact of the number of plant species reduces the correlation coefficient from R =

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Fresh- and brackish water bodies on these islands have also been studied as to species numbers of aquatic heteropteran bugs and the relation to different environmental parame-ters14. The situation of aquatic environments on the islands is different compared to terrestrial habitats, especially, because the isolation effect is twice: On the one hand, there is an isolation of single freshwater habitats from each other on an island. On the other hand, the island itself is isolated from the mainland colonization source. As to the species numbers the two questions arise in particular:

What about the different number of species on different islands (table 3), the larger the island area size the higher the number of species?

What about the different number of species on the level of single freshwater environ-ments on certain islands (table 4), the larger the pond the higher the number of species?

Table 3: Results of regression analysis between numbers of species and area size for Nepomorpha (water bugs) and Gerromorpha (water striders).

Group S – log (A) log (S) – log (A)

Nepomorpha correlation = 0.811** correlation = 0.832** z = 0.224 p

z = 0.005

Gerromorpha not significant not significant not significant

Table 4: Results of regression analysis between water bugs (Nepomorpha) and various environmental factors (N = number of aquatic environments included, log = log-transformed; ** = significant on 0.01 – level).

Nepomorpha (log) Pearson Spearman N Slope Significance

Area size (log) -0.33 0.08 207 -0.012 Not signif.

Salinity (log) -0.353** -0.351** 207 -0.135 0.000

Plant species (log) 0.292** 0.251** 154 0.300 0.000

Oxygen content 0.183** 0.216** 207 0.003 0.008

pH-Value -0.179** -0.219** 207 -0.117 0.010

Plant cover -0.52 -0.040 117 -0.0005 Not signif.

Growing types (no.) 0.385** 0.377** 154 0.071 0.000

14 Data according to Bröring, U. 2001. Analyse der Heteropterengemeinschaften von süßen und brackigen Gewässern auf küstennahen Düneninseln. Habilitationsschrift. Cottbus: 136pp. (available on the internet: http://www.tu-cottbus.de/fakultaet4/de/oekologie/lehrstuhl/mitarbeiterinnen.html).

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island biogeography - an overview · various theories of island biogeography were developed in the...

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