A VERY BRIEF DISCUSSION OF BIODIVERISTY IN URBAN LANDSCAPES
Biodiversity is a key component of landscapes based on ecological principles. The definition of biodiversity is often elusive and debatable, and includes both quantitative and qualitative measurements. The term originated from the Greek bios, meaning life, and diversity, which has been characterized as 1) the number of different types of items (richness), 2) the number of different types of items and their relative abundance (evenness), and 3) variety (vaguely defined as the class to which the diversity belongs).
Table 1 lists selected definitions of biodiversity from the least inclusive to the most inclusive based on a set of common components and processes referenced from 1976 to 1996 (DeLong 1996). All components listed have been characterized by different authors on the basis of Richness (the number of different types of items - e.g. species, communities - within an area), and Evenness (the relative abundance of different types of items in an area).
Allaby (2010) defines biodiversity as a portmanteau used to describe all aspects of biological diversity, especially species richness, ecosystem complexity, and genetic variation. In this definition the debatable inclusion of abiotic structure and processes is implied by the use of the term ecosystem complexity. Some authors argue the appropriate term when including abiotic components is Ecological Diversity - not Biodiversity. The main point in the argument is that the term Biological (bio/bios) refers only to biotic elements and does not include the non-living portions of a landscape (defined here in a broader sense than merely urban landscapes).
Noss (1990) identified three main attributes of biodiversity that, I think, apply to urban systems: composition, structure, and function. Composition refers to the richness of biotic components and the relative abundance of each in an area. Structure addresses the vertical and horizontal elements of a community or landscape, and the organizational levels (guilds or functional groups) of plant and animal populations. Function includes processes such as herbivory, predation, parasitism, mortality, production, and energy flow through biotic communities. These processes are typically addressed in terms of the identity and number of different types of processes as well as the rate at which each process operates. Noss’ definition applies only to biotic components and does not consider abiotic elements as part of biodiversity, although he later argues the importance of including abiotic components in the definition because of their role supporting biotic communities.
The debate continues. The term biodiversity is often used indiscriminately as a reference to the variety of life in an area. Increasing biodiversity, in this sense, normally refers to an increase in species richness, rarely addressing structural or functional diversity directly. A clear decision on whether or not to include abiotic components or human influences as an element of biodiversity is also commonly avoided.
Dasmann (1991) includes human influences in his definition of biodiversity: “The term biodiversity refers to the totality of species, populations, communities, and ecosystems, both wild and domesticated, that constitute the life of any one area or the entire planet… it specifically includes cultural modifications of the natural world.” I find two interesting points about this definition. First, Dasmann includes ecosystems in his definition of biodiversity, implying inclusion of abiotic components. Secondly, he argues results of human action, while affecting biodiversity, are integral to functional systems (ecosystems). Human actions are inherently a part of urban environments.
As we’ve seen, a universally accepted definition of biodiversity doesn’t seem to exist. Although debates continue about abiotic and biotic components, and whether human influences play a role in defining biodiversity, the practical definition in this field, and the commonly used meaning of the term, refers to richness; simply the number of different types of items. In our case, that normally means the number of different types of plants.
That definition raises another question! Are we simply talking about the number of different species in a constructed landscape, or are we talking about diversity in species, functional groups, and structural characteristics? How about intra-species genetic diversity? What do we really mean when we say that biodiversity is important in created urban landscapes? Why is it important?
Disturbance
A major factor affecting biodiversity in urban settings is disturbance. In ecological theory, disturbance can either permanently change the biotic community, or provide an opportunity for renewal. Extreme disturbance events generally permanently change the community, affecting both above-ground and below-ground processes and communities. Since human activities have affected many natural processes and introduced many non-native species over the past century, severe disturbance increases susceptibility to colonization by invasive species (invasibility) and often permanently alters ecological succession of the site.
Disturbance is common in urban development. This photo is of a development area in Bend Oregon where a natural juniper scrub plant association was destroyed, rock ridges were removed and crushed into gravel, the site is being leveled, and apartment complexes are being built. Ecologically, this qualifies as an extreme disturbance.
Many years ago I was fortunate enough to be able to spend some time in the blast zone at Mount St Helens in Washington state. It was only a few years after the eruption and we were able to see the extent of destruction and disturbance from that major event. The disturbance at the construction site in this photo is on par or exceeds levels of disturbance I witnessed at Mount St. Helens. Both biotic and abiotic components of both landscapes have been highly modified.
St. Helens removed vegetation, destroyed geographic features, altered hydrologic systems, displaced native wildlife, negatively affected soil biota, and completely changed the physical appearance of the area. The disturbance evident in this photo is essentially the same, just on a smaller scale.
If we think of biodiversity and its importance in ecological function, how can we assess effects urban development has on these processes, and at what scale do we make those assessments? How does urban development affect biodiversity on local, regional, statewide, national, global, or cosmic scales? Does development tend to increase species diversity as some people argue, or does development generally lead to homogenization of biological systems, and at what scale?
Part of the work in ecology based landscaping is trying to address those questions in a very practical manner. When working on a project, how does your work affect biodiversity of the individual property and how does that translate to a larger scale? How does your project affect ecological function, including biodiversity, in the neighborhood, municipality, region (thinking ecoregion instead of political boundaries), and physiographic region (e.g. the Northwest, or Southwest, or anywhere else that’s identified physiographically).
Another question from the practical application of these ideas is: Are we really doing the right thing by trying to design replacement ecosystems within urban environments? Are we causing problems for the future (temporal scale)? These are valid questions without a clear answer. Or do we just try to do the best we can and hope we’re smart enough to design something that will actually work?
Now I’m rambling, but these are questions we discuss as a company doing this type of work. I get great pleasure from hearing my crew discuss these questions while eating lunch under a tree. Since this is a fairly new field, these debates are incredibly valuable.
References cited:
Allaby, Michael. 2010. Oxford Dictionary of Ecology; 4th ed. Oxford University Press.
Dasmann, R.F. 1991. The importance of cultural and biological diversity. In: M.L. Oldfied and J.B. Alcorn, eds. Biodiversity, culture, conservation, and ecodevelopment. Westview Press, Boulder, CO.
DeLong, Don Jr. 1996. Defining Biodiversity. Wildlife Society Bulletin 24(4). 738-749.
Noss, Reed F. 1990. Indicators for monitoring biodiversity: A hierarchical approach. Conservation Biology 4(4). 355-364.