The view that competition is an important “regulatory” factor in nature is widespread among ecologists. A discussion of the evolutionary significance of interspecific competition is therefore crucial in the context of this book.

Definitions, kinds of competition, historical considerations, factors that bring about competition, and examples of the effects of competition on species and populations were discussed in detail in Rohde (2005). Here we restrict ourselves to a brief outline of the points made in that book, supplemented by evidence presented in the various chapters of the present book.

Definition, limiting factors responsible for competition, and occurrence of competition

Interspecific competition can be defined as an interaction between individuals of different species that arises because of shared requirements for a limiting resource, leading to reduced survival, growth and/or reproduction of at least some of the individuals (adapted from Begon et al., 1996), although Levin (1970) has shown that resource limitation is not the only factor involved. Thus, even two species that feed on different food resources that are not in limited supply cannot indefinitely coexist if they are limited by the same predator. An important criterion for species coexistence is that limiting factors (whether food resource, predation, etc.) differ and are independent.

General background, examples, exceptions

A central question in evolutionary ecology is: what are the reasons for differences in the abundance and diversity of organisms in different habitats and regions? Such differences are universal, i.e., it is highly unlikely that any two habitats will have the same number of species and organisms, and on a larger scale they are apparent between ecosystems and between latitudes, altitudes and different depths (in aquatic systems), as well as between different longitudes. By far the best-documented gradients are latitudinal ones, i.e., a very marked increase in diversity from high to low latitudes. An analysis of these gradients presents us with the opportunity to find a general explanation of the causes that determine diversity.

Latitudinal gradients in species diversity or better biodiversity were first described by Alexander von Humboldt in 1799 during his expedition in South America (Humboldt, 1808), and they have attracted the attention of numerous biologists since (for recent reviews and important papers see Rohde, 1992, 1999; Gaston, 2000; Willig 2001; Willig et al., 2003; Hillebrand, 2004; Mittelbach et al., 2007). They are known for most groups of plants and animals, although there are exceptions. A few examples are given in the following. According to Krebs (1985), the number of ant species at various localities is: Alaska 7 species, Utah 63, Cuba 101, and Brazil 222. Also according to Krebs, there are 22 species of snakes in Canada, 126 in the USA, and 293 in Mexico, and whereas a deciduous forest in Michigan, USA, contains 10–15 species on two hectares, a Malaysian tropical rainforest contains 227 in the same area. Wright (2002, references therein) gives even higher numbers for tropical rainforests: a 0.52 km2 plot in Borneo had 1175 species (with a diameter at breast height of at least 1 cm), and one hectare of Amazonian rainforests can have more than 280 species of trees in one hectare (with a diameter at breast height of at least 10 cm). In the oceans, latitudinal gradients are well documented for surface waters, but there is some evidence that they also occur in the deep sea (e.g., Lambshead et al., 2002).

Introduction

Marine and freshwater fish are hosts to a rich fauna of ectoparasites, living on their gills and skin feeding on blood, mucus and epithelial cells. Fish can easily be obtained and examined in large numbers. Fish ectoparasites represent a highly diverse group including monogeneans, crustaceans, isopods, mollusks and hirudineans. This makes them almost ideal objects for ecological studies. Such studies have been conducted by several researchers, using a range of host species and ecological techniques, with the aim of identifying patterns and processes in parasite communities. Studies have concentrated on different levels of community organization, i.e., those of infra-, component and compound communities, and examined questions of saturation vs. non-saturation of communities, degree of aggregation, temporal and spatial variability of organization, limiting similarity and niche segregation, host specificity, nestedness, and degree of structuring in communities as revealed by null model analyses. All these aspects are of significance in an evaluation of how common equilibrium and nonequilibrium conditions are in ecological communities, the main topic of this book. In this chapter, we provide an up-to-date account of relevant studies.

Parasite communities

Parasite communities have been commonly studied at different levels, i.e., those of infracommunity, component community and compound community (Holmes & Price, 1986). A parasite infracommunity consists of all the infrapopulations (populations of all species) within a host individual. Infracommunities are incapable of self-perpetuation (because most parasites disperse their propagules into the free environment where they usually develop before reaching a host). A parasite component community consists of all infracommunities within a host population. The boundaries for the component community depend on spatial scale (Aho & Bush, 1993). For example, one can consider a component community as (1) all parasites in all individuals of a given host species from a specific collection site of a water body, or (2) all parasites of all individuals throughout the host’s geographical distribution or of a range of host distributions for which the data were obtained.

It is obvious that nature is undergoing rapid changes as a result of human activities such as industry, agriculture, travel, fisheries, urbanization, etc. What effects do these activities have? Are they disturbing equilibria in ecological populations and communities, i.e., are they upsetting the balance of nature, or are they enhancing naturally occurring disequilibria, perhaps with even worse consequences? This book examines these questions, first by providing evidence for equilibrium and nonequilibrium (= disequilibrium) conditions in natural systems, and second by examining human-induced effects, among them those due to climate change, habitat destruction and introduction of alien species. One often hears the argument, not only from non-scientists but also from some scientists, that large-scale fluctuations in climate, sea levels etc. have occurred over and over again in the geological past, long before human activities could possibly have had any impact, that human effects are very small compared to those naturally occurring anyway, and that they cannot significantly affect the environment. Is this indeed so? Or is it possible that naturally occurring fluctuations are being dangerously enhanced by humans?

General background

Freshwater streams and lakes are habitats for complex ecosystems, of which plankton is an important component. Even more extensive are the oceans, which cover about 70% of the Earth’s surface. Marine ecosystems including their plankton have very great ecological and economic significance. Although our knowledge of biodiversity patterns in marine phyto- and zooplankton (compared to terrestrial systems) is still very limited (Irigoien et al., 2004), much work, some of it theoretical, some experimental, has led to important insights.

The study of plankton has played a crucial historical role in our understanding of ecological processes. The famous “paradox of the plankton” formulated by Hutchinson (1961) drew attention to the fact that many more species coexist in the supposedly homogeneous habitat than permitted under the competitive exclusion principle of Gause. Hutchinson suggested that nonequilibrium conditions might lead to the greater than expected diversity, a suggestion shown to be correct by many subsequent studies. Hutchinson himself thought that seasons and weather-induced fluctuations were responsible. But, in addition, as reviewed by Scheffer et al. (2003), homogeneity due to mixing hardly exists, and even in the open ocean meso-scale vortices and fronts result in spatial heterogeneity. Moreover, modeling of plankton communities has shown that even in homogeneous and constant environments plankton may never reach equilibrium, because multi-species interactions may lead to oscillations and chaos. This is supported by laboratory experiments, which have shown highly irregular and unpredictable long-term fluctuations at the species level (Figure 4.1), although total algal biomass and other indicators at higher aggregation levels may show regular patterns.

Why should we conserve biodiversity?

Peter Sale, in his important book Our Dying Planet (2011), presented and critically examined economic and ethical/esthetic arguments for conserving biodiversity. We refer the reader to that book for an overview. An ethical responsibility of humans towards nature is usually denied: man has responsibility towards other humans but not towards animals. There are exceptions, for example in Schopenhauer’s philosophy compassion with fellow humans and animals is the foundation of ethical behavior (Rohde, 2010). In other words, man has the responsibility not to harm any animal needlessly but to safeguard its survival, which implies protection of its habitat, and this may well be an attitude held by many.

The esthetic value of protecting biodiversity is even more controversial. It is almost impossible to define such a value. One is left with pointing out that many of the most important works of art were and are inspired by nature, by a forest, a plant, an animal, and that people enjoy forests and other undisturbed habitats. In the Italian Renaissance, the period when Western modern culture really took off, the development of science and the artistic appreciation of nature’s beauty went hand in hand, sometimes in the same person (e.g., Leonardo da Vinci). Some great physicists (Einstein for example) have used the esthetic beauty of mathematical equations as evidence for their truth. One can argue that esthetics is as defining for humanity as is scientific exploration. It should not be forgotten that humans evolved in environments with rich floras and faunas, and that change to a life surrounded by concrete and in an environment vastly impoverished from its previous condition could have unforeseen consequences for mental and physical health.

Here we return to the question asked in the Introduction to this book: how common are evolutionarily stable strategies and states? These two concepts were developed in the context of games theory.

Background

Games theory was developed by von Neumann and Morgenstern (1944), although the French mathematician Cournot (1838) studied some aspects, further developed by Nash (1950). Its most important contribution to evolutionary biology is the concept of the evolutionarily stable strategy (ESS). It is central to modern evolutionary ecology, and Dawkins (1976) suggests that it may be “one of the most important advances in evolutionary theory since Darwin”. It was introduced into ecology by Maynard Smith and Price (1973), and can be derived from the concept of the Nash Equilibrium (Nash, 1950), according to which none of a number of players in a game can gain by changing her/his strategy unilaterally. Maynard Smith (1982) gave a detailed account of applications of game theory to evolutionary theory, including ESS. However, parts of his book rely heavily on mathematics. Dawkins’s (1976) The Selfish Gene contains a discussion of ESS and many examples, clearly explained without any mathematics. A recent detailed review of applications of game theory and ESS to social behavior was given by McNamara and Weissing (2010).