Read an Excerpt
ANIMALS AS NEIGHBORS
The Past and Present of Commensal Species
By Terry O'Connor Michigan State University Press
Copyright © 2013 Terry O'Connor
All rights reserved.
ISBN: 978-1-61186-095-5
CHAPTER 1
The Human Environment
To speak of the "human environment" may seem redundant. There are few terrestrial environments where some human influence or modification cannot be discerned. We are making small but significant changes to the composition of the atmosphere, major changes to the ecology of all but the deepest oceans, and we have radically altered the composition and distribution of plant and animal communities all across the Earth's land surfaces. Human activities are now a major factor in soil and sediment erosion in many parts of the world. In the more densely occupied regions, sediments directly resulting from human activity are a significant facies of the superficial geology, attracting the attention of us archaeologists.
So widespread and distinctive is this impact that some scientists have proposed the term "Anthropocene" to define the current period of the Earth's history, in which the human "fingerprint" can be detected in most geological and atmospheric processes. The Anthropocene is certainly useful as a concept (and a better term than some of the alternatives that come to mind—the Tarmaciferous, the Machinian?) and is becoming widely used before its formal adoption as a geostratigraphic unit. There is little question that we are in the Anthropocene, but when did it begin? Different authorities have proposed the eighteenth century A.D., linking it with the Industrial period, while others have pointed out that human environmental impact began much earlier, perhaps as farming spread around the world. Either may be an appropriate definition, depending on the scale at which we examine those environmental changes. From the perspective of human-animal associations, what mattered was the point at which human impact on the local environment disrupted existing ecologies and created new opportunities. The timing of that point will have been locally contingent, a key element being the construction of permanent human settlements.
In the modern world, the majority of people live in towns and cities: huge, complex artifacts that retain little of the original landscape other than as modified fragments that constitute isolates and corridors within the human construction. To humans, towns are a tool by which we shape our surroundings and facilitate our social and economic activities. To other species, our towns and other settlements are both a challenge to their adaptability and an opportunity. The nature of those challenges and opportunities will be, to some degree, locally contingent: every settlement will have its own particular attributes. However, we humans have enough needs and cross-cultural behaviors in common that we can, perhaps, generalize usefully about the challenges that will face most commensal species.
From the outset, we should be careful not to regard environmental modification as a uniquely human trait. Most species alter the environment around themselves to some extent. As parallels to our own towns and cities, we are familiar with the complex nest systems and fabricated "mounds" of ants and termites, which not only consist of the nest construction but also often show a surrounding environment heavily modified by the feeding and cropping activities of the insects. In marine ecosystems, colonies of invertebrate hydroids secrete for themselves elaborate calcareous structures that cover many square kilometers with coral reefs, and which will provide a living for numerous commensal species. More spectacular still is the environmental change wrought by green plants, which have permanently altered the Earth's atmosphere by dumping into it their metabolic waste product, that highly reactive and dangerous gas oxygen. We humans cannot match that achievement, despite our best (or worst) efforts, though we have constructed numerous patches of wholly fabricated habitat—our houses, towns, roads, and so on—and substantially altered the environment around those constructions. It is easy to regard a road or a new town as environmentally destructive, but what such constructions do is to replace one environment, one set of habitat patches, with another. That new environment may not be "natural" in any sense, but that does not mean that it is not attractive to, and a viable home for, quite a range of species. We may not be the only species that actively modifies its environment, but we are certainly the only one that worries whether or not that environment is "natural."
A pedantic digression is required here to consider what we mean by "environment." Like so many of the terms that have found their way into the discourse of contemporary politics, environment has acquired a number of subtly differentiated meanings. Perhaps the simplest, and the most useful for us here, is to say that the environment of a population of people or other animals is the physical setting in which that population exists, defined in space by the range of the population concerned, and subsuming a diversity of descriptive parameters. In that sense, "environment" always has to be qualified: the environment of who or what? The journalistic tendency to refer somewhat hand-wavingly to "the environment" is not helpful here. So as I write this chapter, my environment consists of a breeze-block constructed garage-cum-office that is situated within a small patch of highly diverse herbaceous and shrubby vegetation that is mostly alien to the geographical region. Beyond that small patch is an extensive landscape of undulating grassland maintained by grazing farm livestock, interspersed with clumps and lines of trees and several small streams. The climate is cool-temperate, with year-round precipitation averaging around 800mm per annum and prevailing westerly winds. I could go on, adding detail such as the cats and numerous frogs that share the same patch, the seasonal influx of butterflies, the significance of neighbors' offspring as an environmental noise factor. The point is that "my environment" consists of several different, nested, but somewhat overlapping zones. At the core is a built environment, a structure within which subsistence and other activities take place, which consists of a mix of manufactured (breeze-block, concrete) and "natural" (wood) materials, but which would not exist without human deliberation and manufacture. Within that built environment there are patches of different habitats (dusty crannies where the woodlice—Isopoda or pill-bugs—live, well-lit window corners favored by spiders). The building is a distinctive environment, but quite heterogeneous at a small scale. Beyond and around the buildings is space in which daily activities take place, and which has been heavily modified to meet a priori needs. The garden could be regarded as manufactured, part of the built environment. It is true that the human population has little to do with life at the bottom of the pond, but the pond itself is a construction. Scaled up, when we discuss the built environment of towns and cities, we conventionally include intervening green spaces. Beyond the garden lie fields and hills—the product of geology, climate, biota, and time, and so not originally a human construction. However, the flora and fauna is appreciably modified by our choices and decisions. The land is farmed, the range of herbaceous and woody plants greatly reduced and homogenized through grazing by the sheep and cattle deliberately placed in those fields, but also by the numerous rabbits that have invited themselves. In this year, the neighboring farmer has chosen to cut his meadows late in the summer to make hay, in contrast to his usual practice of cutting early to make silage. If that change of behavior is maintained for a few more years, we would expect the flora of those fields, and hence their fauna, to change. On the hills that overlook those fields, the August sunshine is opening the flower spikes of heather (Calluna vulgaris), which covers many square kilometers of sandstone hills in this part of northern England. This landscape is frequently described as "wild" or even "natural," but it is not. First, this environment is maintained by active human management. Light grazing by sheep removes plant species that might otherwise crowd out the heather, and occasional superficial burning removes old growth and rejuvenates the heather plants. Second, heather is by no means the "natural" vegetation of those hills. There is substantial archaeological evidence of prehistoric settlement and agriculture under the heather, and clear evidence that the soils were formerly deeper and more fertile, supporting a more diverse shrub and herbaceous flora. It was prehistoric agriculture, possibly with a nudge from minor climate change, that pushed the environment of those hills over to the low-diversity flora of heather and bracken fern (Pteridium aquilinum), and it is today's management that maintains the heather communities. In short, the purple heather-covered hills are as much a modified environment as the pasture and meadows below them.
In terms of generic structure, then, we need to consider two zones of the human environment: the built environment and the modified environment. Apart from their spatial relationship, there will obviously be some movement of species between them, and the precise boundary may sometimes be hard to define. However, for the purposes of this book, it is a useful distinction to make, separating an environment that is essentially artificial, one that may arise quite suddenly, de novo, from one that, however much it may be amended by human activity, has time depth and perhaps some spatial articulation with environments that are barely modified by people, if at all.
Time is an important parameter in the modified environment. One of the strengths of an archaeologist's perspective is that we see the landscapes around us as a palimpsest of traces of different processes and events in the past. We are accustomed to the idea that we inherit structures such as field systems, burial mounds, or ancient buildings from the past into our present-day landscape, but we must remember that a non-constructed habitat patch—the grassy field within the field system, or the vegetation overgrowing the abbey ruins—also has a time depth that is inherited into the modern landscape. And when we consider communities of animals, such as the birds and mammals that live in the overgrown ruins, they too have a time depth. The attributes of that community today may reflect contingent events in the past as much as they reflect the habitat that we see today. Our present-day observations cannot be other than a slice through a complex bundle of environment and community processes, acting at different rates and with varying degrees of interaction and synergy across habitats that were influenced or modified by, or wholly constructed by, people.
What are the main generic impacts that people make as they build and modify their environment? Obviously, any detailed answer to this question must be highly case-specific. Construction of the Great Pyramid had a quite different environmental impact than the building of the Hanging Gardens of Babylon, which in turn had quite a different impact than laying out New York's Central Park. However, there are certain consequences of human activity, certain repeated generic impacts and outcomes, that can be identified, and that allow us to generalize about the human environment as a place of challenge and opportunity for commensal species. At the risk of oversimplifying the matter, I suggest six major characteristics of the human environment that are particularly significant for other species.
The first, and most obvious, is disturbance. People are enthusiastically active builders and modifiers, and few habitats within the built or modified environment are likely to persist for considerable periods of time without significant disturbance. We are accustomed to the idea of human environments being frequently disturbed, by observing the world in which we live. Urban brownland turns over on a timescale of months to years, seldom decades to centuries. Despite the false impression of stability that the time-averaged archaeological record sometimes conveys, human environments of the ancient past were probably just as subject to disturbance. From an ecological perspective, disturbance (or perturbation, to use the ecologists' rather appealing term) has two aspects: frequency and intensity. Frequency is obviously important, and the timescale of disturbance in human environments varies considerably. Refuse may be cleared daily, roadside verges may be mown fortnightly, a public park pruned and raked annually, a flat roof regraveled every five years. The point is that some disturbance will be on a shorter timescale than the generational span of many of the species that might otherwise colonize that environment, and some will be on a shorter timescale than the seasonal fluctuations of temperature and humidity that will be essential to some species. Small variations in the frequency of disturbance may therefore have substantial outcomes in terms of allowing successful colonization and establishment of stable commensal populations. Intensity of disturbance can also encompass quite a range, from a minor perturbation that temporarily alters the physical structure of a habitat patch to the complete destruction of that patch and all that lived in it. Perturbations that are not catastrophic may temporarily disturb food webs and cause a shuffling of living space just sufficient to facilitate further colonization, giving another species or two a foothold. It is not surprising, therefore, to find that environments that are occasionally and nonintensively disturbed often show a higher biotic richness and species diversity than those that have long-term stability with little perturbation, a phenomenon sometimes packaged as the Intermediate Disturbance Hypothesis. Disturbance, then, is something that characterizes the human environment, and in terms of biodiversity its effects range from beneficial to catastrophic.
A more specific consequence of human environmental modification is the redirection and concentration of surface water. At one end of the scale, this is reflected in the emergence of the great riverine civilizations of Mesopotamia and Egypt. Little could be done about the narrowness of the belt of cultivable land along the Nile, nor could much be done to control the regular inundation of that land. However, the construction of fields and water channels could at least ensure a thorough and consistent separation of land and water between inundations. The same is true even in the quite different setting of the British Isles, where one of the distinguishing features of "natural" landscapes is that they are wet underfoot. People need and use water, but also prefer to stay dry. Surface water, whether in pools or flowing, needs to be managed, moved around, kept in certain places and out of others. We have a rather odd relationship with standing water: appreciating its reflective qualities and incorporating it into our built landscapes, yet aware of it as an inconvenience to movement from place to place and as a possible health hazard. As human modification of the environment intensifies, streams disappear underground into culverts, ponds and damp patches are drained, and bodies of standing water (often with fountains) are constructed where no pond or lake would naturally occur. Those water bodies are then, usually, carefully managed to ensure that only a limited, tolerated range of other species makes use of them: ducks are allowed, leeches are not.
That question of which species are tolerated introduces the third significant impact, namely, the replacement of any "undesirable" biota by a "useful/desirable" biota. The subject of human transportation of animals around the world will recur a number of times throughout this book. For now, the crucial point is that colonization of the human environment is not necessarily subject to the many factors that control colonization of habitat patches in general. People decide to remove this species or to introduce that species, and do so with an enthusiasm that can be quite breathtaking. Sometimes this biotic turnover is obviously utilitarian, as when introducing domestic livestock to a newly cleared area of farmland, or extirpating a potential pest species. Ridding a landscape of undesirable animals becomes a worthy act in some cultures, hence the highly doubtful claim that St. Patrick, a holy Welsh expatriate, drove all the snakes out of Ireland. Sometimes the turnover is for aesthetic reasons, as when landed gentry introduced fallow deer (Dama dama) to their parks in medieval England, or when someone introduced the attractive but noisy, fecund, and highly gregarious ring-necked parakeet (Psittacula krameri) to southern England (chapter 6). Utility aside, the decision that a particular species is or is not desirable will be essentially a cultural one, and it may be susceptible therefore to changes of attitude. One of the most obvious, to which we return in a later chapter, is the cultural rise and fall of the street pigeon. Concerning pigeons, Jerolmack observes that animals become a problem when they transgress spaces designated for human habitation. I think this is too simple. The historical and archaeological records are, as we shall see, full of examples of such transgression being either tolerated or actively initiated. Problematizing the transgression comes later, if at all, and may be due as much to changed perceptions of what is "right" for a place of habitation as to the abundance or attributes of the species concerned. Attitudes to animal neighbors are likely to be replicated across large geographical areas, especially if transmitted by a culturally vigorous and expanding people. The consequences of European expansion across the Old and New Worlds can be seen in the similarity of farm and urban faunas across current or former colonial areas. In some instances this has been relatively harmless (European starlings Sturnus vulgaris in the United States), and in some it has been an ecological disaster (rabbits Oryctolagus cuniculus in Australia). The term biotic homogenization has been applied to this tendency to see the same species recurring in heavily modified environments, in part because certain species have adapted so well to the opportunities that we have offered them, but in part because people have deliberately added and subtracted species according to some culturally informed preconceptions.
(Continues...)
Excerpted from ANIMALS AS NEIGHBORS by Terry O'Connor. Copyright © 2013 Terry O'Connor. Excerpted by permission of Michigan State University Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
