Biological diversity is the product of the evolutionary process and it is in decline because of a relatively recent change in the hands that are on the controls. For millions of years the allocation of resources between competing life forms was accomplished by the evolutionary process in accord with the metric of relative fitness. Now the allocation of the resources necessary for survival is determined not by nature but by human societies. For whatever reason, humans have been able to usurp this evolutionary role for their own use.
Why would humans choose to re-make the diversity on earth? In particular, why would they choose to re-fashion it in a way that would vastly reduce the diversity generated by evolution? Given that the previous metric implied a relatively well-developed capacity to make use of a given niche, it is not straightforward to understand or to explain why it is that human societies have found it desirable to remove so many of these proven producers.
This section shows that human societies have chosen this route precisely on account of the benefits of homogeneity, that is, development has occurred in part because human societies have been able to benefit from making their natural and biological environments more uniform, and thus more suited to their tools and technologies. Diversity is in decline because development has been based in part on uniformity.
The nature of the extinction process The process of extinction threatening diversity is wholly distinct from the extinction process that has contributed to the generation of evolved diversity. The evolutionary process generated the naturally-existing pattern of life forms through a process of competitive allocation of base resources to the best-adapted forms. Extinction played a role in this process as species are removed for better adapted competitors in a process of niche refinement. It is a human choice process that is generating the decline of this naturally determined diversity, and this process is appropriating niches rather than refining them. It is important to point out the distinctions, and the similarities, between the two processes.
The extinction process has been a natural process and the ultimate fate of every species (Futuyma, 1986; Cox and Moore, 1985). Studies of the fossil record show that the average longevity of a given species lies in the range of one to ten million years (Raup, 1988). Over the four and a half billion years of evolutionary history, the mosaic of life forms on earth has been completely overhauled many times over.
This process is necessary in order to preserve life on earth. The continuous and contemporaneous processes of speciation and extinction form the two necessary prongs of the evolutionary process. It is this constant re-shaping of the biological diversity on earth that allows life to continue in the context of a changing physical environment. Ecologists conceptualise life on earth as a single body. 'Life' is the conduit through which the energy of the sun flows on this planet. It is constantly shifting and re-shaping itself, via the evolutionary process, in order to better adapt to prevailing conditions. The various parts (life forms) come and go but maintain the integrity of the whole. Any given life form maintains its place within the system solely by virtue of its current capacity to capture some part of the flow of solar energy.
Some life forms ('plants') are able to capture solar energy directly, simply by virtue of being alloted a 'place in the sun' on earth. The fundamental constraint that they have in their competition for existence is territorial. If a plant life form receives an allocation of land, then this is often the only base resource that it requires for survival. Plants capture the energy of the sun through the process of photosynthesis, and in so doing generate the 'Net Primary Product' (NPP), that is, the product generated from energy captured directly by life forms on earth (Leith and Whittaker, 1975). It is estimated that the NPP on earth is about 225 billion metric tons of organic matter per annum (Ehrlich, 1988). All non-plant life forms must be sustained from this base resource.
It is the allocation of one of these two forms of 'base resource' (land or NPP) that has determined whether a given life form will exist, or continue in existence. In the course of evolution, this allocation process was a wholly natural one for billions of years. The competition between plant forms for land allocations and the competition between other life forms for NPP allocations was then determined by which life form was best-adapted to the available resources. Therefore, it was the role of the evolutionary process to allocate base resources between competing life forms.
The evolutionary process has allocated base resources in such a fashion as to render solar energy appropriable (by life on earth) over a wide range of physical conditions. This has implied both a wide range of life forms on earth (pertaining to the wide range of physical conditions existing here) and a changing variety of life forms (in accord with changing physical conditions). Extinctions were a necessary and constructive by-product of this natural process of 'niche refinement'.
From the ecological perspective, the particular shapes that life takes derive from the relative advantages of these life forms in appropriating some part of the base resource. A 'species' is the name given to a particular form that life takes for this purpose (Barton, 1988). Ecologists conceptualise energy flows to earth as diffusing over uneven gradients, for example uneven latitudes and uneven topographies. Given this uneven distribution, base resources will be similarly concentrated; that is, there will be various peaks and troughs across this gradient. A species is defined as the form of life that has established itself as the appropriator of the energy flow across one of these 'resource peaks' or niches. Therefore, the species and its 'niche' are coincident concepts; a species is the particular form that life takes to fit a particular pattern of the energy throughput on earth (the niche).
There is an in-built rate of turnover for each niche. First, niches are in a state of constant competition through the process of genetic recombination, mutation and dispersion. At any time a new form of life may arise that is better 'adapted' to the existing niche; that is, there is a better fit to the distribution of the base resource. Then the existing life form may be supplanted.
There is another sense in which this process of adaptation must always remain a dynamic one. This is the result of a continuously changing physical environment. For example, the geography of the planet is dynamic. The tectonic plates of the continents have always moved at an average pace of approximately 5-10 cm per year. These changes create gradual shifts in continental climates. In addition, the earth's overall climate is also in a perpetual state of change. The mean summer temperature in 'Europe' has cycled about 12 times over a range of 10-15 degrees centigrade during the past million years (West, 1977). The Yugoslav physicist Milutin Milankovich hypothesised that this cycle resulted from the superimposition of three periodic cycles: a 100 000 year cycle resulting from the eliptical shape of the earth's orbit; a 40000 year cycle resulting from the earth's tilt on its axis; and a 21000 year cycle resulting from the earth's wobble of the axis of rotation (Hays et at., 1976).
These in-built mechanisms for environmental variability have generated a system in which the shape and location of any given niche is dynamic. In essence, a niche may be thought of as a peak in the uneven distribution of the base resource, and a species is not itself a static concept, the process of species definition must also be dynamic.
It is the static result of this dynamic process that can be seen in the distribution of species prevailing at any given time, and 'extinction' is the term applied to the changes which occur between static states. The existence of genetic mutation maintains a pool of omnipresent potential competitors. With the shifting of the underlying base resource, these invaders may be provided with an opportunity at any time. If the invader is successful, and so genetically dissimilar as to not interbreed, then the previous occupant of the niche is superseded, and its range is restricted. If this same result occurs across the entirety of its range, then the totality of its niche is appropriated, that is, it becomes 'extinct'.
This is the nature of the natural extinction process, as demonstrated by four billion years of evolutionary history. It is not so much a process resulting in the removal of a species, in the sense of niche abandonment, as a process of species resolution, in the sense of niche refinement. It is a natural process for life forms to change, over time and over space. With changing environments, former inhabitants have lost their 'best-adapted status' to invaders, and been replaced. This is recognised as extinction if it occurs across the entire geographical range of the life form; however, in the natural process, it is more accurately conceptualised as necessary turnover deriving from the better adaptation of 'life' of the current state of the niche.
Obviously, this description of the natural extinction process does not accord well with the prospect of losing half of all life forms in the coming 100 years; that is, the problem of biodiversity losses. Adaptation is more of a gradual process in the aggregate sense, even though it represents millions of starts and stops for individual species. Species resolution is a process that occurs over periods of hundreds of millions of years. This is not intended to imply that all mass extinctions must necessarily be human-induced; they result from any large-scale shock to the life system. There have been several occasions prior to human existence when the rate of extinction of species far exceeded the rate of speciation.
There are at least five occasions indicated in the fossil record during which over 50% of the then-existing animal species were rendered extinct (Raup, 1988). These mass extinctions have always been the result of a sudden and dramatic change in the physical nature of the system, for example hypothesised sunspots, asteroids, geothermal activity, etc. The dramatic shift of the physical system places a stress on the life system for immediate adaptations. The result of this stress is the loss of many species without their immediate replacement. In essence, this manner of extinction occurs when the niche has been so severely dislocated by a physical event that the species find themselves without the capability to make a claim on the base resource. The 'peaks' in the base resource have shifted out from underneath them.
The current mass extinction is not being initiated by one of these exogenous physical phenomena creating a shock to the system. There is indeed a tremendous stress being placed on the life system on earth, but it is arising from within the system. This is unique in evolutionary history, and it is not a part of the natural process of extinction. It is a part of the human choice process.
In essence, within the natural evolutionary process, the allocation of a portion of 'base resource' (land or NPP) was determined in a natural competition between various life forms. In the past ten thousand years, this allocation decision has been usurped by the human species. Base resources (land and NPP) are now allocated by humans to the various species that continue in existence. The land use conversions outlined in the previous section are indicators of the scale and rate of diffusion of this technological change. It is a process which commenced with the initial idea of cultivation and domestication, namely, the selection of species for use rather than the use of the prevailing species. It is an idea that has diffused across the whole of the globe. Now, human choice rather than competitive adaptation determines the range of life forms that exist on the face of this planet. Recognising that it was possible to select a species to which to allocate the base resource, and then use that species, human objectives rather than evolutionary adaptation became the driving force in determining extinction. From that point on, the competition for base resource allocations was a social process.
Human usurpation of the evolutionary function of allocating base resources is not a sufficient explanation for diversity decline. It remains necessary to explain why humans would exercise this power in such a fashion as would cause diversity to decline. The key to this explanation probably lies in a technological change that occurred originally about ten thousand years ago; this was the realisation of 'agriculture' by human societies.
Agriculture has consisted of the selection of a few prey species, and the expansion of their ranges. Prior to the occurrence of this idea, human societies preyed on the species over the ranges that the evolutionary process had allocated them (hunting and gathering); afterwards, human societies transported the species they used with them, displacing the naturally selected varieties. The discovery of this strategy (domestication and cultivation) and its implementation constituted a very important part of a technological shift that occurred in the late Pleistocene (about ten thousand years ago). This was a process that was important to the advancement and development of human society and civilisation as we know it, but it is also a process that has generated the potential for a decline in biodiversity as a by-product.
Human advancement through agriculture has not been built directly on diversity decline, in the sense of the overuse and/or mining of biomass. Rather, human advancement has come through reliance on a small set of species and the expansion of their ranges (at the expense of other species). The expansion of their ranges (with the simultaneous constriction of the range of other prey species), and the consequent expansion of the human niche (with the simultaneous constriction of the ranges of other predator species), has resulted in the global homogenisation of the biosphere. It is this homogenisation which, on the one hand, has generated human development and, on the other, has generated the decline of diversity.
The earliest archaeological evidence of agriculture dates back only about to 6000 or 10 000 years ago. This consists of the first signs in the fossil records that human societies were selecting individual species and translocating them with their culture. It is now the case that the biological production 'menu' for the bulk of all human society has converged on a relative handful of species. Of the thousands of species of plants which are deemed edible and adequate substitutes for human consumption, there are now only 20 species which produce the vast majority of the world's food (Vietmeyer, 1986). In fact, the four big carbohydrate crops (wheat, rice, maize and potatoes) feed more people than the next 26 crops combined (Witt, 1985). There are now less than two dozen species which figure prominently in international trade. In short, humans have come to rely on a minute proportion of the world's species for their sustenance; these species are termed here the 'specialised species'.
Human choice resulted in biosphere homogenisation at the lower trophic levels, by reason of human selection of prey species. It also resulted in homogenisation at higher levels, by reason of human population expansion (and by reason of the elimination of other predators' prey species). It was at this same time (about ten thousand years ago) that the population of the human species began to record unprecedented growth. The development of human technologies (cultivation and domestication) in the neolithic period enormously expanded the human niche from the capacity to support perhaps ten million individuals to a capacity of hundreds of millions in a relatively short time period (Boulding, 1981). Most paleoarchaeologists date a substantial increase in human populations to this period (Biraben, 1979).
It is this same process of homogenisation that is at the basis of the land use conversions within the developing world today. These conversions are still occurring for the purpose of replacing the diverse with the specialised, namely the replacement of forests with cattle ranches and croplands. The biodiversity problem is the result of the diffusion of a homogeneous process of development on to the last unmodified habitat on earth. Diversity decline is the by-product of this scoping-in process by which the global biosphere is being homogenised for purposes of human development.
Diversity decline from uniformity in development Once species are perceived as 'assets', that is, as competing 'means of producing biological product', it is possible to identify general forces that would threaten all life forms with the prospect of conversion. In short, these assets will be converted if they are inferior assets (when compared with competing methods of production) in terms of productive capacity by themselves or in combination with their ancillary resources (land or management).
Conversion as a concept is able to explain the prospect of replacement with regard to every species in existence on earth; however, it is not in itself sufficient to explain the potential for a mass extinction. For this, a force must be identified that will generate not only an expected re-shaping of the global portfolio of natural assets, but also a narrowing of that portfolio. Conversion as an economic force explains only why it is the case that the natural slate of biological resources might be replaced by another human-selected slate on any given parcel of land, depending on relative productivities, but it does not explain why a small number of species would replace millions across the whole of the earth; that is, this force implies conversion but not necessarily homogenisation. In order to explain the global losses of biodiversity, that is, a narrowing of the global portfolio, it is necessary to identify the nature of the force that would generate this homogenisation of the global biosphere.
Specifically, it is unlikely that a wholly natural process would drive the world toward less diversity. This would require the evolution of both biological generalists (species with superior productivity across many niches)
and uniform human tastes (across the globe). In fact, the current drive toward uniformity is contrary to the very idea of evolutionary fitness. Fitness implies competitive adaptation to the specific contours of a certain niche. The evolutionary process generates species that are well-adapted to their own specific niches through a process of niche refinement; that is, a surviving species represents a 'good fit' to its own niche (Eltringham, 1984).
It is equally unlikely that human tastes are so uniform as to demand the homogenisation of biological resources. Communities 'coevolve' in order to better fit with the system in which they participate. It would be expected that the preferences of predators would be determined generally by their available prey species. In fact, there is ample evidence to support this expectation that human communities would prefer to consume the resources they depended on traditionally. This confirms that the depletion of diversity is not a natural phenomenon; rather, it is a socioeconomic one. The process of the selection of assets for society's portfolio is an economic decision, determined by forces that shape the perceived relative advantage-ousness of different assets. There is no reason to expect that nature would have evolved these biological 'generalists' that are now monopolising global production, because competitive adaptation and coevolution militate against that conclusion. If it is not the naturally-given characteristics of the domesticated and cultivated species that is determining their universality, then it must be some characteristic related to the economic production system of which they are a part. In this sense, the process of homogenisation of the biosphere is a wholly economic process, and not a natural one.
The technological change known as agriculture was of this nature. The idea that originated about ten thousand years ago was centred on the idea of creating species-specific tools and technologies, and translocating species as 'methods of production'; that is, the idea concentrated on the development of the technologies for efficient agricultural production that were focused on a single set of species. The result was the development of two new important factors of production in the production of biological goods: species-specific learning. It is the combination of these factors, together with the specialised species, that generates the force for biosphere homogenisation. As a single method of production (species-tools-experience), the originally selected species are able to outcompete the naturally-existing varieties at any given location on account of the tools and experience that come with them. Agriculture becomes embedded in certain pre-selected species.
The accumulation of agriculture-related capital goods goes hand-in-hand with the adoption of the specialised species. This may be seen wherever the
'conversion frontier' exists. For example, the number of tractors in Africa increased by 29% over the past ten years; they increased by 82% in South America and by 128% in Asia. During the same period the number of tractors decreased by 4% in North America (World Resources Institute, 1990). Societies that are introducing the specialised species do so in part because these species are tailored to the tools that are used with them. It is the combination of species and species-specific tools that constitutes a 'method of production'. When a conversion decision is being made, a country will consider all of the possible methods of production (species/capital goods combinations) in a search for the most efficient method. Most species have no set of tools that is known to apply well to them.
The other important factor introduced into the production of biological goods was species-specific learning. With more experience with a particular species, it was possible to become even more efficient in its production (by reason of increased understanding of its biological nature, as well as intervention to determine the same). This information became another crucial factor for agricultural production, but it existed only in one form: embedded in the received forms of the domesticated and cultivated varieties.
It is the nature of this final factor that generates the forces for the convergence of the biosphere on a small set of specialised species. It is the dynamic externality inherent within accumulated knowledge and learning that generates the non-convexity within the system, so that human choice falls again and again on the same small set of life forms. Specifically, accumulated knowledge in this context is a non-rival good in the sense of Romer (1987, 1990a, b); that is, it is of the nature of a 'design or list of instructions' that is distinct from the medium on which it is stored, and thus (as pure information) it may be used simultaneously by arbitrarily many agents without added cost. The accumulated experience in regard to the specialised species is inherent within the capital goods and species as they stand, and is available at no added cost to the marginal user (Romer, 1990ft).
The general argument of this section is that global environmental problems can be caused by uniformity and universality of societal developmental paths, as much so as the pure scale of development. When each human society on earth pursues development in an identical fashion, relying on the same small set of resources, this sameness in development strategies can in itself generate global consequences when it diffuses across the face of the earth.
Biodiversity decline has been portrayed here as the outcome of develop mental uniformity when applied to the biosphere. Human societies realised the possibility of developing the biosphere with the advent of agriculture. Since that time societies have chosen the portfolio of living assets on which they will rely, rather than using that which nature had allocated to that territory. The chosen species have become a part of the overall 'method of production' that humans use in biomass production. As this same development strategy has diffused across the earth, it has resulted in the homogenisation of the biosphere, and the decline of diversity.
Biodiversity decline, therefore, is a by-product of the development process, on account of the uniform and universal manner in which it has been pursued over the past ten thousand years. The disinvestment in diverse resources occurs through a multitude of distinct routes: land conversions, non-management, mining, etc.; however, these are all proximate causes, not fundamental ones. The fundamental cause of diversity decline has been the human pursuit of'development', that is, the pursuit of human objectives at the expense of the biosphere. Thus the base forces for diversity decline are the fundamental human drives for resources (economic) and fitness (biological) in the context of a technological shift that rendered the pursuit of these objectives most easily achievable in the context of the homogenisation of the biosphere.
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