Urban Natural Areas #4: Conservation of Regional Biological Diversity
Conservation of Regional Biological Diversity
Biological diversity refers to the diversity of all ecological hierarchies including genetics, populations, species, communities and ecosystems. It is important to include the spatial arrangements of each of these elements as well. The concept of a physiographic province allows consideration of a region with a common geologic history and climate. The combination of geology and climate lead to the formation of a related set of soil types. These abiotic factors then delineate the suite of plant communities compatible with site conditions and the assemblage of associated animal populations.
Habitat destruction, fragmentation and degradation associated with urbanization are major threats to the long-term sustainability of a metropolitan region’s biological heritage. Massive alterations of the basic ecological processes undermine the ability of the region to support the productivity and resilience of native plants, animals and ecological and human communities.
The Nature Conservancy (TNC) has suggested the use of a combination of coarse filter and fine filters to capture the biological diversity of a region (Noss 1987). The coarse filter approach suggests that the maintenance of all representative ecosystems will contain the majority of the species. While this concept has not been empirically proven, it is expected to lead to the conservation of regional biological diversity at the 85 to 90 percent levels. This coarse filter approach does not capture the full range of natural variation and so some species would slip through the cracks. The fine filter approach is directed toward individual species that are known to be in direct need of conservation assistance. This management strategy is should be incorporated into urban forest conservation planning.
The forest of Florida’s ‘coastal plain physiographic province’ includes many species of flora that move through the landscape very slowly and disperse over very short distances. Successional trajectories for forest herbs may take decades or centuries (Muller 1982; Perterken and Game 1984). This herbaceous plant layer constitutes the greater portion of plant diversity within these forests.
Modeling efforts have indicated that the spacing of forest plant communities in fragmented forest landscapes have limited species composition, because the more isolated plant communities are more inaccessible to species with low dispersal ability (Hanson et al. 1990; Dunn et al. 1991). While there is a lack of empirically based evidence that demonstrates how migration is affected in heterogeneous landscapes (Pickett et al. 1987), indirect evidence suggests that distance is an obstacle to colonization. The probability of seed arrival declines logarithmically with distance to parent source (Harper 1977; Fenner 1985). Forest plant communities that are disjunct from old regrowth have species poor understories relative to the old regrowth forest. Understory species richness has been found to be higher in old regrowth forests than in younger successional forests and is attributed to the availability of seed sources on site.
The Florida’s ‘coastal plain physiographic region’ experiences high levels of forest land ownership turnover and land use change that has led to a preponderance of second and third growth forest patches. The age of these second and third forest patches is often insufficient to have allowed the recolonization of the herbaceous plants. Compounding the slow migration of forest herbs is the fragmented nature of the region, including severe boundaries that include large urban and suburban zones and their associated infrastructure. This rather static aspect of forest herb distribution suggests that conservation planning concerned with the protection and restoration of forest ecosystems within metropolitan regions should concentrate on ‘old-regrowth’ areas and not rely upon successional forest plant communities.
Species which have already become rare within the region and are unable to recolonize widely scattered forest fragments, are likely to be extirpated from the region. Active conservation of these old regrowth stands adjacent to successional forests represents a conservation opportunity to allow the recolonization of herbaceous and shrub species to other parts of the metropolitan region.
R.F. Noss. 1987. Protecting natural areas in fragments landscapes. Natural Areas Journal 7: 2-13.
R.N. Muller. 1982. Vegetation patterns in the mixed mesophytic forest of eastern Kentucky. Ecology 63: 1901 – 1917.
G.F. Peterken and M. Game. 1984. Historical factors affecting the number and distribution of vascular plant species in the woodlands of central Lincolnshire. Journ of Ecology 72: 155 – 182.
J.S. Hason, G.P Malanson and M.P. Armstrong. 1990. Landscape fragmentation and dispersal in a model of riparian forest dynamics. Ecological Modeling 49: 277 – 296.
C.P. Dunn, D.M. Sharpe, G.R. Gunterspergen, F. Stearns and Z. Young. 1991. Methods for analyzing temporal changes in landscape pattern. In: Quantitative methods in landscape ecology. pp. 173-198.
S.T.A. Pickett, S.L. Collins and J.J. Armesto. 1987. Models, mechanisms and pathways in successeion. The Botanical Review 53: 335-371.
J.L. Harper, 1977. Population biology of plants. Academic Press, N.Y.
M. Fenner. 1985. Seed ecology. Chapman and Hall.