Urban Regions : Ecology and Planning Beyond the City

Ecosystem, community, and population ecology 87

Figure 4.2Herbivores in heavily browsed woods, white-tailed deer (Odocoileus

virginiana). Note exposed mineral soil with near-absence of leaf litter and humus

cover. Michigan, USA. Photo courtesy of US Department of Agriculture.

Often one or two forest layers appear to be absent because of fire, livestock
grazing/browsing, overpopulation of a vertebrate herbivore, logging, or other
human activity sometime in the past. Removal of a shrub layer, for example, by
deer overbrowsing emphasizes that animals do not simply respond to vegetation,
but affect and in some cases mold the vegetation form we see, especially at edges
(Figure4.2). The loss of a shrub layer is particularly significant because it serves
as cover for so many ground animals. Also it is a primary determinant of light,
temperature, and moisture conditions on the soil, where concentrated seeds
and invertebrates are often important food sources for animals. Urban park
management may need to balance the value of vegetation cover and biodiversity
against shrub-removal security concerns in spots.
Horizontal community structuremay be described as vegetation along a gradi-
ent or inpatches.Agradientrepresents a species assemblage gradually, rather
than abruptly, changing across an area, and is mostly limited to certain natural
conditions such as in a tropical rain forest. However many ecological patterns,
including lichen diversity, soil attributes, and rare species, have been analyzed