Do ecosystems exist?
A pond, a meadow and a forest, in Poland. Photo credit Wikipedia
One hundred years ago several people enquiring about how the living world works developed an idea of an ecological system. The system part was analogous to machines with defined flows of materials, energy, and information. These are cybernetic things, amenable to the techniques of systems analysis.
A problem arose when ecosystems were promoted as literal organisms, equivalent with an organism like a worm or a whale. Some researchers strongly disagreed, pointing out that unlike organisms, ecosystems have no distinct boundaries, are not autonomously self-reproducing, and seem to have no mechanism to evolve. These objections went unheeded, and the ecosystem as organism idea penetrated popular knowledge. Many people viewed an ecosystem as a well defined thing where numerous plants and animals lived closely interconnected, all cooperating for their common good. Definitions of ecosystem are ambiguous. "An ecosystem is a system involving the interactions between a community and its non-living environment. A community is a group of interdependent plants and animals inhabiting the same region and interacting with each other through food and other relationships."
These inspire difficult questions. What size is this region and how are its boundaries defined? Is the timescale over which these interactions are measured that of a research project, or since most of the species in the system first evolved? How many of these interdependencies are true mutualisms, or looser non-obligate symbioses, or non-existent? To avoid confusions in this essay, the neutral term assemblage will be used.
These questions lead to the proposition that ecosystems are neither organisms nor any kind of physical entity. They have no mass and no volume. A tree has these properties, a forest also has them; the mass of a tree can be measured and the number of trees in a forest can be counted. An ecosystem has neither mass nor volume because ecosystem is a concept, a method of thinking. The difficulty of improving our understanding of these assemblages of plants and animals is their complexity. If the human brain (along with its human body of course) can be described as the most complex thing in the universe, then how to describe the assemblage that is a forest, including the soil it depends on? In total a vast list of species populations, with the most important hidden underground.
Understanding how animal brains work is one of science's greatest challenges, but much useful knowledge has been gained. Students of behaviour have discovered not only how animals behave but why they behave. Theories of behaviour are testable in the field and laboratory; with difficulty but feasible because many of the behaviours of one or a few species of organisms are easily observed and manipulated. However, the brains within these organisms remain mysterious.
By comparison vast assemblages of organisms are daunting to understand, but gaining pragmatic knowledge makes a start. Despite vague boundaries, and random variability of these assemblages they are greater than the sum of their parts. The plants depend on soil, herbivores depend on plants and predators depend on herbivores, all channelling interconnected flows of energy and materials. The dynamics of birth and death of different populations are interlinked. Evolution of these species responded to competition, herbivory, predation and parasitism. An assemblage needs to be studied at the appropriate hierarchical level where its emergent properties can be measured.
We humans are totally dependent for our reproduction and survival on what these assemblages do for us. We depend on them for oxygen supply, regulation of carbon dioxide, food, fuel, shelter, and even beauty. Within the natural world we are often the dominant keystone species, with much power to disturb the assemblages that we need as natural resources, often for the worse. Even as hunter gatherers we modified our habitats, and now when most of us depend on farms for food there is no effective distinction between wildland and farmland within the ecosystem concept.
Will these assemblages collapse if we disturb them too much? If we hunt the wolves in a large forest so none are left then the deer of the forest will increase in numbers. They in turn will eat so many seedling trees that eventually the forest will disappear, to be replaced by scrubby grassland. In a similar forest there might be a small population of a beautiful species of woodpecker. Rare because it is at the edge of the natural climatic range of the species of tree that it most needs for nesting and feeding. If the woodpecker disappears from that forest would any ecologist detect any change in the rest of the forest?
These are simple examples of a larger problem for ecologists. Is the continuing character of any particular assemblage of plants and animals more likely if the assemblage is highly diverse in terms of the number of species it contains? This is not just of concern to nature conservationists, who hope the answer is yes. Managers of nature reserves and natural resources need to know how much time and money to spend on maintaining diversity of species.
Many observational and experimental studies have been done in recent decades to answer this problem of diversity - stability. Metanalyses have been made, some of them examining more than 100 separate studies. These studies cover the constancy of species composition over time, or the productivity of biomass over time. The consensus now supports the proposition that the more diverse an assemblage is then the more stable or the more productive it is likely to be.
The ecosystem concept explains this positive relationship between diversity and stability as the result of varied levels of interdependencies between populations of species that act as dampers that absorb disturbances. But when it comes to identifying whether a species that is rare and getting rarer is a keystone species, then the complexity of the links overwhelms our ability to predict. So far: but now our understanding steadily increases as theories of ecosystems are put the empirical test. In the meantime most ecologists recommend the precautionary principle. In the lack of sufficient empirical and experimentally tested knowledge we should manage a natural resource or nature reserve to maintain as much diversity of species as possible.
In the book I am currently writing, Trees for People, much of the ecological material will be within the concept of ecosystem as it is currently used by research ecologists and described in textbooks such as that by Charles J. Krebs - Ecology: the Experimental Analysis of Distribution and Abundance.
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Good or bad design of the eye of vertebrates?
Human eye. Credit: Petr Novák, Wikipedia
The eye is the most important sense organ for primates, including us humans. Other vertebrates rely most on other senses, for dogs it is their extraordinay sensitivity to smell. And the eye has been important in the development of ideas about how life evolved. Charles Darwin struggled to explain how the complexity of the eye could have arisen. At that time there was insufficient knowledge of a chain of intermediate stages leading to the evolved design of the eye of vertebrates. Another aspect of studies on the evolution of the eyes of vertebrates is the proposition that the design is poor when compared to the optical specification of a camera made by humans. If the evolved design is poor, then it cannot be the work of an intelligent designer in the form of a deity. Not only is the vertebrate eye compared unfavourably to cameras but also to the eye of squids and octopuses (cephalopod molluscs).
The apparent problem with the vertebrate eye is that the structure that senses the light, the retina, is the wrong way round! The cells sensitive to light, the photoreceptors, face away from the incoming light. Moreover, they are covered with layers of nerve cells and blood vessels. The best studied eye is of course that of humans, with a vast literature written for and by eye surgeons and opticians. A human embryo starts to develop its eyes during the first few weeks of life. The retina in particular forms as a bulbous outgrowth of the brain and will remain a working part of the brain in the adult human. These bulbs each fold in on themselves, they invert by a process of differential cell growth. The inverted bulb comes to have two layers and it is the nature of the embryonic brain that it is the inner surfaces of both these layers that have the capacity to develop into a retina with a layer of photoreceptors pressed against a layer of cells that support the activity of the photoreceptors.
In contrast to vertebrates, the cephalopods have their photoreceptor layer facing towards the lens of the eye, and with the nerve cells going beneath this layer as they lead from the retina to the central brain. That works well - these invertebrate molluscs have good vision. But is it really better than that of humans? Some families of vertebrates show the highest development of vision of all animals. Think of a hawk high in the sky searching for a flicker of movement by a mouse in the grass. We primates evolved in the trees, using our colour vision to search for ripening fruit and our binocular vision to swing with ease amongst the branches.
As for cameras - they are an aid to human vision. Reading glasses are a simple optical aid to the vision of a human who has become, with age, long sighted. Glasses are dead things, tools, without meaning unless used by a human. A camera of powerful design mounted on a robot that lands on a comet in order to beam radio signals back to Earth to be displayed on television screens for astronomers to view is an instrument without meaning unless information reflected from the comet's surface is brought to life and understanding in the retinas and central brains of those ecstatic astronomers. Forget about cameras and similar machines: comparing them with living things like humans with eyes is an error of logic, a category error.
Seemingly the vertebrate eye works so well because of the inverted retina, not despite it. The barrier of the nerve cell layer and retinal blood vessels to high definition vision is solved by the fovea region of the eye where light is naturally focussed. The layer of nerves is as thin as possible and the inner blood vessel layer is absent here. The tiny spot of the fovea works in combination with constant tiny movements of the eyeball called saccades. These are imperceptible to us as they dart around what we concentrate our visual attention on. The immense power of the nerve cell layer of the retina to pre-process visual information helps to create a coherent impression within our central brains of the objective world. The centre of our view appears in sharp focus whilst our peripheral vision, supplied by the rest of the retina, provides the context and warns of hazards approaching.
The biochemical business of photoreception for full colour vision in bright sunlight through to the ability to grope our way by moonlight is enormously demanding of nutrients, oxygen, and the need to dispose of waste carbon dioxide and metabolites. The retina of humans has been measured as having the highest oxygen demand of any tissue in our bodies. Even in the dark it works hard to keep the photoreceptor cells in readiness and good health. To supply the retina there is an extraordinary blood system called the choroid. This is network of small blood vessels that is dense laterally and about three layers deep. It envelopes the outside of the eyeball and presses closely against the outermost retinal layer. The blood vessels have special gaps in their walls, permitted an abnormally high rate of exchange with the retina. This entire adaptation for vision of high information content is physically possible only with an inverted retina.
Using the vertebrate eye to illustrate arguments for intelligent design, or for design by natural selection, seems to be on shaky ground for both sides of the argument. However, life is full of compromises. Evolution throws up many contrary features and problems with design. The contrariness exemplified by the eye, of humans at least, is the ease with which the two layers our retinas separate from each other. There are no anatomical or cellular connections to hold the layers together, only the thin liquid between them acts as a glue, amongst other functions. The repair and prevention of retinal tears and detachments is a big part of the work of eye surgeons to restore and save vision.
The human retina and its problems are the subject of chapter 6 in my book Contrary Life and Technical Fixes. This includes detailed notes and references to the literature. Also follow the papers below to penetrate further into this fascinating field.
Kröger, R.H. & Biehlmaier, O., 2009. Space-saving advantage of an inverted retina. Vision Research, 49: 2318-2321.
Lamb, T.D., 2011. Evolution of the eye. Scientific American, (July issue): 65-69.
Lane, N., 2009. Life ascending: the ten great inventions of evolution. Profile Books, London. [See Chapter 7, 'Sight'.]
Sleggs, G.F., 1926. The functional significance of the inversion of the vertebrate retina. American Naturalist, 60: 560-573.