Okay, if you can look past my anthropomorphic statement that wildlife make decisions, the topic I would like to address deals with the adoption and use of ecological principles by the design community. Patch size, landscape connectivity, edge effects, corridor ecology, landscape ecology, and metapopulation theory are just a few terms and ideas put forward by researchers to address the biological integrity of wildlife populations. Often, planners, landscape architects, engineers, architects and other built environment professionals adopt these ecological principles into their designs of regions, cities and individual developments. But do these designs function as originally intended?
Figure 1 Red-shouldered Hawk in San Francisco Photo Credit Walter Kitundu
Habitat Patches and Design
Figure 2 New York City Central Park, Photo Credit by Sergey Semenov
A common application of ecological principles into urban/rural design is the establishment of natural to semi-natural patches (or remnants) of areas that would serve as habitat for wildlife. This design application begs the question, "For which wildlife species?" In many (of my) dealings with design firms and city/county departments, this rarely is addressed. For the most part, people look at a land use/cover map and try to conserve as many of patches as possible, without much thought about wildlife species in the area or those migrating through. While conserving any remnant patches is a laudable goal, in many instances, the amount of patches, in terms of actual area, that a developer will conserve is limited. Thus, it is critical to select the patches that "give the most bang for the buck." An ecologist would select those patches that benefit local species or improve species richness, depending on the original goal and what the site can offer. Selecting the "best" patches can benefit a variety of species, but it depends on the scale of the design and those species that respond to the geometry of the landscape at that scale.
Animals locate themselves based (for the most part) on the spatial geometry of landscape structure across a region. However, smaller animals have a very different view of the landscape than larger animals. Imagine a Carolina Wren (Thryothorus ludovicianus) and a Red-tailed Hawk (Buteo jamaicensis) flying over a neighborhood (Figures 3 and 4). Both birds are responding to landscape structure within cities that attracts them to one area versus another. The smaller Carolina Wren and the larger Red-tailed Hawk respond to landscape structure across a range of scales, but the range of scales are different between the two species.
Figure 3 Carolina Wren (photo credit Dan Pancamo, Wikipedia)
Figure 4 Red-tailed Hawk (photo credit Dan Sudia)
Scale essentially means the size of an area (e.g., 1 hectare, 5 hectare, 20 hectare, etc.). When a bird "responds to" an area, it is attracted to that area based on spatial objects within it. "Spatial objects" are the actual structures (such as trees, bushes, fields) within a given area that an animal uses to fulfill daily food, cover and water needs. The scales at which wildlife respond to spatial objects are an important part of habitat selection.
Let's take a theoretical representation of a Carolina Wren and a Red-tailed Hawk responding to spatial objects as they search for habitat (Figure 5). The wren searches a tract of land to establish a home range. At the next scale, the wren searches its home range for suitable habitat patches for nesting or foraging for food. Then, within these habitat patches, the wren locates food patches where food items (e.g., insects) are abundant. This is the smallest scale in which the wren searches for food. The hawk has a similar set of decisions, but it selects much larger areas and objects at each comparable scale. Notice that the only overlap in scales is at the food patch level for the hawk and at the tract level for the wren.
Figure 5. Scale-dependent decisions of a Red-tailed hawk and a Carolina Wren. Illustration by Rebekah McClean.
Different wildlife species respond to different objects within a landscape. The type of object a species prefers is dependent on its natural history: what it eats, what it needs for nesting, etc. For example, one wildlife species could prefer tree patches. Another species prefers flowering plants. Others prefer woods along streams (riparian habitat). Some prefer natural, open fields. Some even prefer the actual homes (buildings) and others prefer lawns. In addition, the size of these spatial objects is important too. Different species may respond to different sizes of a particular object in the landscape. Let's say two species like open areas (e.g., lawn). One species, such as a robin, may be attracted to a front yard. Another species, say a hawk, may prefer large expanses of lawn (e.g., golf courses). They both respond to lawn. However, the area of lawn is much bigger on a golf course than a front yard.
In addition, during different periods of an animal's life, it may have different requirements for food, water, cover and space. For example, birds may have vastly different requirements when breeding than when they are migrating or wintering in an area. Some bird species only nest in large expanses of wooded areas to keep their nests hidden from predators while primarily catching insects to feed their young. However, outside the breeding season, these same birds can be found in small patches of forest feeding on a variety of food items including fruits and seeds (e.g., Ovenbird, Seiurus aurocapillus).
Figure 6 Ovenbird, Photo Credit AJ Hand, Connecticut Ornithological Association
After the breeding season, many young animals disperse from their natal site looking for new areas that provide food and shelter. For animals that are dispersing, many urban sites that may not be appropriate for breeding could serve as "dispersal sites" where animals can feed and rest when searching for new habitat. These dispersal sites can serve as corridors that help animals move from one habitat to the next. In addition, urban sites can serve as "stopover sites" for birds that are looking for food and shelter along their migration route. Radars around cities have detected massive amounts of birds flying at night in and around cities. Urban areas can also serve as wintering sites for animals that normally breed outside of urban areas.
In summary, a particular patch can serve as habitat for animals during different times of the year. A patch of woods, for example, may be a breeding area for some animals while at other times of the year it may serve as a stopover site or wintering site. In many cases, the property may serve primarily as a 'connector' between natural areas -- an important role to permit the movement of animals.
Part two coming next. This blog originally appeared in the Nature of Cities.