Environmentally biased fragmentation of oak savanna habitat on southeastern Vancouver Island, Canada
Mark Vellend, Anne D. Bjorkman, Alan McConchie. 2008. Environmentally biased fragmentation of oak savanna habitat on southeastern Vancouver Island, Canada. Biological Conservation 141(2008) 2576-2584.
Selected Excerpts:
Abstract
Quantifying the degree to which natural or protected areas are representative of a specified baseline provides critical information to conservation prioritization schemes. We report results on southeastern Vancouver Island, Canada, where we compared environmental conditions represented across the entire landscape, in oak savanna habitats prior to European settlement (<1850), and in both protected and unprotected oak savannas in the present-day. In this region, oak savannas represent a rare habitat type, harboring many threatened species. Before European settlement, oak savannas occurred in a distinctly different subset of environmental conditions than they do today. Compared to the entire landscape, oak savannas were historically found predominantly in warm, dry, flat, and low-lying areas, but habitat destruction has left oak savannas in largely the exact opposite set of conditions at present. Thus, the range of conditions in both protected and unprotected oak savannas at present are highly unrepresentative of historical conditions. It appears that fire management by indigenous peoples maintained oak savannas historically across large areas of flat low-lying conditions with deep soils, where succession otherwise produces closed coniferous forest. These areas have since been almost entirely converted to agricultural and urban areas, leaving remnant oak savannas largely on steep, rocky hilltops, where the habitat is maintained by shallow soils. Our results provide quantitative guidance for setting conservation priorities for oak savannas in this region, while highlighting the important general issue of the major role traditional land-use practices can play in shaping landscapes, and therefore in influencing the baselines used to set conservation priorities.
Introduction
A central goal of many conservation prioritization schemes is to maximize the “representativeness” of a network of protected areas relative to a specified baseline (World Resources Institute, 1992; Hazen and Anthamatten, 2004). Because ecosystems have been left intact largely in places with limited extractive or commercial value, such as mountainous or arid areas, existing networks of natural or protected areas tend to be highly unrepresentative of their surrounding regions (Pressey, 1994; Margules and Pressey, 2000; Scott et al., 2001; Flinn et al., 2005). Identifying and quantifying such biases provides a critical ingredient to the scientific basis for systematic conservation planning (Margules and Pressey, 2000). …
A critical first step in any analysis of representativeness involves specification of both the focal sites of interest (e.g., natural or protected areas) and the baseline set of conditions to which the focal sites are being compared (Margules et al., 2002). Since ecological communities change over time even in the absence of anthropogenic disturbance, specifying the baseline conditions one would like to represent is not a trivial matter (Foster, 2000; Egan and Howell, 2001). …
In this study we take advantage of an historical reconstruction of the distribution of oak savannas prior to European settlement (Lea, 2006) to establish a pre-settlement baseline of conditions represented in this habitat type. …
Discussion
The most striking result of this study was that both past and present oak savanna distributions, relative to the entire landscape, could be predicted based on environmental conditions, but in exactly opposite directions in the past vs. the present.
On average, oak savannas circa 1850 occurred on shallow slopes at low elevation, and under dry, warm climatic conditions, while present-day oak savannas occupy sites in precisely the opposite set of conditions. As such, present-day oak savannas are highly unrepresentative of historical oak savannas – biased towards cool, wet, steep, and high elevation areas. This pattern is particularly strong at the lower end of slope-elevation gradient, where a large proportion of conditions represented in the pre-settlement landscape are almost completely missing from remnant oak savannas in the present day (Fig. 3F). Oak savannas in protected areas are largely representative of the broader set of oak savannas in the present-day landscape (protected + unprotected), and therefore highly unrepresentative of historical conditions as well.
In southwestern British Columbia, oak savannas represent a relatively rare habitat type (Ward et al., 1998; Lea, 2006). The predominant natural vegetation in the broader coastal region consists of coniferous forests dominated by Douglas fir (Pseudotsuga menziesii), western red cedar (Thuja plicata), and western hemlock (Tsuga heterophylla) (Meidinger and Pojar, 1991). Within this broader region, oak savannas occur only on southeastern Vancouver Island and the adjacent Southern Gulf Islands – the driest and warmest parts of the broader region – such that the wet and cool conditions in present-day oak savannas relative to the Saanich Peninsula as a whole are still relatively dry and warm compared to the broader region. ..
In light of our knowledge of how oak savannas are maintained, our results can be most parsimoniously interpreted as follows. First, the predominance of oak savanna habitats in flat, low elevation, warm and dry areas prior to European settlement, was likely a result of the large indigenous population in the area now occupied by the city of Victoria (Fig. 1; Turner, 1999; Lea, 2006), and traditional management of the landscape in this area using fire. Historically, oak savannas were also present on steep, rocky hilltops at higher elevations, but these comprised a relatively small proportion of the larger extent of oak savanna habitats (Fig. 3). European settlement brought the decimation of indigenous populations, and therefore their traditional land-use practices, as well as active fire suppression, and development of land for agriculture, urban development, and forestry (MacDougall et al., 2004). …
Finally, the role of indigenous cultural practices in shaping the landscape of southeastern Vancouver Island (Turner, 1999; MacDougall et al., 2004), and indeed many landscapes around the world (Denevan, 1992; Gomez-Pompa and Kaus, 1992), needs to be explicitly recognized in prioritization schemes for habitat conservation and restoration. …
