Stress and the ecological niche

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Even the most casual observer of natural systems will note that many species tend to occupy a characteristic place in nature. The idea is demonstrated most vividly by gradient studies, for example the distribution of plants on a salt marsh, where each species tends to be limited to a certain zone by a combination of soil texture, redox potential, salt, and lime. Ecologists have invented the concept of an ecological niche to organize their thoughts about the ways in which organisms fit into their environment. The inception of this concept in the ecological literature is attributed to the American ornithologist Joseph Grinnell with his now classical paper on the California thrasher published in 1917, but the most widely used and influential elaboration of the niche concept is due to Hutchinson (1957). Hutchinson defined the niche in terms of any number of conditions and resources that limit the distribution of a species. The niche was pictured as an n-dimensional hypervolume that envelops those values of continuously varying environmental factors that allow long-term survival of the species. As an illustration of the Hutchinsonian niche concept we reproduce a two-dimensional picture of fitness in the collembolan Folsomia Candida as a function of zinc exposure and food density (Fig. 6.1; Noel et al. in press).

Hutchinson (1957) realized that a distinction should be made between the fundamental niche, which comprises all the conditions under which a species potentially may occur, and the realized niche, which is usually more narrow than the fundamental niche due to competition in the field. The fundamental niche is observed in laboratory experiments and in the field when competitors are absent. At the edges of its fundamental niche a species is less well equipped to face competition with others and so when competition is important, it will give away at the edges and occupy a smaller section of the gradient, the realized niche. The formal definitions of the ecological niche by Hutchinson (1957) have spurred a great variety of

Ecological Niche

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Zinc concentration (|ig/g, log scale)

Figure 6.1 Graph illustrating the niche concept in two dimensions. Contours of population growth rate of the soil-dwelling collembolan Folsomia Candida are plotted as a function of dietary zinc concentration and initial population density per microcosm (reflecting food density). Four different ranges of effect are indicated. At high exposure levels zinc becomes toxic and suppresses fitness (T); with increasing population size fitness decreases due to crowding (DD, inverse density dependence; K, carrying capacity); at low exposure levels zinc supports fitness by a stimulatory effect on growth, a phenomenon known as hormesis (H); at low food density fitness decreases due to absence of inter-individual communication stimulating consumption (Allee effect; A). Courtesy of H. Noel and R.M. Sibly, University of Reading.

50 100 200 500 1000 2000 5000

Zinc concentration (|ig/g, log scale)

Figure 6.1 Graph illustrating the niche concept in two dimensions. Contours of population growth rate of the soil-dwelling collembolan Folsomia Candida are plotted as a function of dietary zinc concentration and initial population density per microcosm (reflecting food density). Four different ranges of effect are indicated. At high exposure levels zinc becomes toxic and suppresses fitness (T); with increasing population size fitness decreases due to crowding (DD, inverse density dependence; K, carrying capacity); at low exposure levels zinc supports fitness by a stimulatory effect on growth, a phenomenon known as hormesis (H); at low food density fitness decreases due to absence of inter-individual communication stimulating consumption (Allee effect; A). Courtesy of H. Noel and R.M. Sibly, University of Reading.

studies aiming to explain community structure in the wild from the properties of individual species and their responses to environmental factors.

The ecological niche concept touches the very heart of ecology—the relationship between species and their environment—but this has not prevented the proliferation of a great deal of confusion in the literature. Several reviews have pointed out the historical context of this confusion, which is partly due to independent elaboration by Joseph Grinnell and Charles Elton (Chase and Leibold 2003). On the one hand the concept emphasizes the requirements of species, but on the other hand it includes the species' role or impact on its environment. This Janus-faced property of the niche has caused such confusion that some ecologists in the 1970s have suggested to avoid the term niche altogether. However, Chase and Leibold (2003) revisited the concept and after clearing the decks set the stage for a new synthetic approach, framed by the recent developments of ecology. Their new, synthetic, definition of ecological niche runs as follows.

The joint description of the environmental conditions that allow a species to satisfy its minimum requirements so that the birth rate of a local population is equal to or greater than its death rate, along with the set of per capita effects of that species on these environmental conditions.

This definition joins the two components of the niche mentioned above, and it recognizes that whenever resources and environmental conditions are altered by the organisms themselves, as in the case of predators depleting their prey or ecological engineers altering the physical structure of their environment, this aspect should be included in the niche concept. The definition by Chase and Leibold (2003) also recognizes that the difference between mortality and natality, in other words fitness, is the ultimate measure in which the requirements of the organism are to be expressed.

The niche concept has been highly instrumental in community ecology, as it allows explanations of community structure from the point of view of the placing of species along a gradient of resources or conditions according to their ecological niche. Various competition models have been derived that predict how niche overlap between species is minimized by competition and the maximum overlap between the niches of two adjacent species that will allow coexistence of both. However, less attention has been paid to the question of how the niche itself is shaped by underlying determinants founded in the physiology of the species. The science of environmental physiology, also called ecophysiology or physiological ecology, addresses these aspects. Several physiology textbooks have been written that reach out to ecology, including that by Larcher (2003) on plants, and that by Schmidt-Nielsen (1997) on animals. In this chapter we likewise aim to explore the reductionist path, from niche to genomics.

Environmental physiology has a strong focus on studies of stress. The reason for this bias is that the regulatory mechanisms allowing homeo-stasis of the milieu interne are best seen when these mechanisms are put to the test by pushing the organism to the borders of its ecological niche. This is how we will approach the issue in this chapter; by studying responses at the edges of the ecological niche we aim to reveal the regulatory mechanisms that promote fitness both within and outside the niche.

Like the niche, the concept of stress has a long and confusing semantic history in ecology. Some authors have argued that ecological stress should be defined by analogy to the physical concept of stress, which would imply that it is an external constraining or impelling force applied to an ecological system. Most biologists, however, consider stress as an internal state, brought about by a hostile environment or negative social interaction. Nowadays, there seems to be agreement on the fact that a distinction should be made between a stressor (an external factor), the stress (an internal state brought about by a stressor), and the stress response (a cascade of internal changes triggered by stress). Although the concept of stress can be defined at various levels of ecological integration, stress is most commonly studied in the context of individual organisms, whereas stress responses are studied on the cellular, biochemical, and genomic levels.

We need to realize that the concept of stress is not absolute; it can only be defined with reference to the normal range of ecological function; that is, with reference to the ecological amplitude or ecological niche of the species. What is an extremely stressful condition for one organism (e.g. the absence of free air) is quite normal for another organism (a fish). A definition of ecological stress that incorporates this idea runs as follows (Van Straalen 2003).

Ecological stress is a condition evoked in an organism by one or more environmental factors that bring the organism near to or over the edges of its fundamental ecological niche.

This definition complies with the common physiological usage of the term, which is that stress is an internal condition, not an external factor. In addition, stress has the following properties: (i) it is usually transient, (ii) it involves a syndrome of specific physiological responses, and (iii) it is accompanied by the induction of mechanisms that counteract its consequences. Our niche-based definition of stress is illustrated schematically in Fig. 6.2.

Ecological Niche Graph

Figure 6.2 Graph illustrating a definition of stress based upon the ecological niche of a species. Ecological stress arises when the intensity of an environmental factor increases from 1 to 2 in such a way that in position 2 the organism is placed outside the niche (A). This will evoke stress and stress-response reactions, which fade away when the environmental factor relaxes and the organism returns to its niche (B). Another type of response is to move the border of the niche (C) by genetic adaptation in such a way that position 2 is not experienced as stress anymore. Reproduced from Van Straalen (2003), with permission from the American Chemical Society.

Figure 6.2 Graph illustrating a definition of stress based upon the ecological niche of a species. Ecological stress arises when the intensity of an environmental factor increases from 1 to 2 in such a way that in position 2 the organism is placed outside the niche (A). This will evoke stress and stress-response reactions, which fade away when the environmental factor relaxes and the organism returns to its niche (B). Another type of response is to move the border of the niche (C) by genetic adaptation in such a way that position 2 is not experienced as stress anymore. Reproduced from Van Straalen (2003), with permission from the American Chemical Society.

The stress response can take different forms, depending on the timescale. Calow (1989) distinguished two main types, proximate and ultimate responses. The proximate response implies induction of physiological, biochemical, and genomic mechanisms (physiological adaptation) that allow survival while the stress prevails. Such mechanisms cannot be maintained forever without consequences for normal cell function and so a return to the niche is necessary for long-term maintenance of fitness. The ultimate response implies that genotypes with a greater than average innate capacity to resist the stress are favoured and replace the ones with lower resistance in the next generation. Then, after some generations, the whole population consists of resistant genotypes (genetic adaptation). The boundaries of the niche have been shifted to include the organism's new position, and what was stress before is not stress anymore (Fig. 6.2).

The existence of genetic adaptation makes us realize that the ecological niche is not a property of a species as a whole, but may show variation between populations of a single species. In that case, a species with wide ecological amplitude (a euryoecious species) may consist of several local populations, each with narrow amplitude (stenoeci-ous populations). Consequently, what is experienced as stress for one population is normal for another population of the same species. Such genetically determined polymorphisms in response to stress have been investigated often in evolutionary ecology and provide some fascinating examples of microevolution in real time, such as pesticide resistance in insects and metal tolerance in plants. In addition, stress may lower the threshold for expression of traits and so increase phenotypic variation and accelerate evolution (Hoffmann and Hercus 2000).

In our discussion of stress responses we will emphasize conditions more than resources. Resources are environmental factors that can be consumed and belong to the impact component of the ecological niche. Conditions are factors in the habitat, such as temperature, humidity, osmotic value, and oxygen tension, that are not consumed and can be altered only slightly by the organism. They can be plotted along an axis, as in Fig. 6.2, and the ecological amplitude of the species can be marked by values that depend on the species' physiology. We must add that our niche-based definition of stress does not encompass all stress phenomena. For example, stress may arise in animals upon sight of a predator, or when being chased away by a group member, or when witnessing an overwhelming natural event. These kinds of stress are difficult to relate to the concept of ecological niche, but many aspects of the internal state evoked by social and mental factors are similar to the ones imposed by a harsh environment.

A considerable number of functional genomic studies have been conducted with model organisms under stress. Due to the early availability of a full genomic microarray a lot of work has been done on yeast, which has developed into a classical model for the study of stress responses. There are also a fair number of genomic studies on stress responses in Drosophila and Arabidopsis. As in previous chapters, we will discuss the studies on models even when taken out of the context of their ecology, to illustrate the principles. From there we will try to draw conclusions about the relationship between stress and the ecological niche, which may hold equally for other species.

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  • haile
    What does ecological niche mean?
    2 years ago

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