CHAPTER FIVE BIOLOGICAL INTEGRITY Patricia N. Manley, Jo Ann A. Fites- Kaufman, Michael G. Barbour, Matthew D. Schlesinger, and David M. Rizzo ©1999 J.T. Ravizé. All rights reserved. CHAPTER FIVE BIOLOGICAL INTEGRITY Patricia N. Manley, Jo Ann A. Fites-Kaufman, Michael G. Barbour, Matthew D. Schlesinger, and David M. Rizzo Introduction elements of biological integrity and ecological “As the wind howled outside my window, sustainability (Angermeier and Karr 1994; Hunsaker heralding the onset of winter, the local band of et al. in preparation). Biological diversity refers to the coyotes sang their evening chorus as they roamed variety of living organisms in an area, encompassing the rich hunting grounds where the wet meadow and a hierarchy of biological organization—genes, forest meet in the backyards of my neighbors. populations, species, communities, ecosystems, and Drifting off into a cozy slumber, pleased with myself biomes (Angermeier and Karr 1994; Noss and for living in a place where I can experience the pulse Cooperrider 1994). We can view biological diversity of the natural world, where coyotes and bears visit as building blocks and natural processes as architects my backyard, I bolted out of bed, realizing I had and engineers determining how the blocks are forgotten to bring my cat inside for the night!” shaped and arranged, together resulting in biological So says a long-time Lake Tahoe resident. integrity. The Tahoe basin offers many natural splendors to its Approaches to the conservation of residents and visitors, several of which appear in the biological integrity focus both on the needs of opening quote. Natural experiences in the basin are individual species and on conserving entire defined not only by characteristic landscape features, ecosystems and their fundamental processes. As the such as Lake Tahoe and the majestic mountain science of conservation matures, it is increasingly ranges surrounding the basin, but also by less recognized that both approaches not only have merit dominant features, such as frequent sightings of but that attention to communities and ecosystems, as wildlife from one’s doorstep, the ability to enjoy well as to species and populations, is important to beautiful forests and meadows, and the sound of the success of large-scale conservation efforts (Noss coyotes howling in the night. Even the fact that 1990), such as the conservation of biological integrity coyotes occasionally prey on domestic cats left in the Lake Tahoe basin. Our assessment addresses outside at night is something that most residents both levels of biological organization. accept as part of living in an ecosystem that still The biological integrity of ecosystems in the supports a diversity of living creatures. Many of the basin has been altered and perhaps permanently natural features that have come to be expected as compromised by human land use over the past 150 part of the “Tahoe experience” are a reflection of years (see Chapter 2). Three facets of biological and depend on the biological integrity of ecosystems integrity have experienced substantial changes over in the basin. the past 150 years, and they form the central topics Biological integrity is “the capability of a of this assessment of biological integrity: community landscape or ecosystem to support and maintain a structure and composition, fire as an ecosystem balanced, integrated, adaptive community of process, and species composition and population organisms comparable to that of natural habitat of characteristics. The topics addressed in this chapter the region” (Karr and Dudley 1981). Biological do not constitute a comprehensive treatment of the diversity and the ecological and evolutionary considerations in conserving and restoring biological processes inherent in natural systems are essential integrity; rather, they are a combination of points of Lake Tahoe Watershed Assessment 403 Chapter 5 greatest concern and factors significantly affecting fire represents a combination of wildfire and biological diversity and integrity in the basin. prescribed burning conducted by the Washoe tribe Communities are composed of species that to manage vegetation, similar to many other Native occur together in space and time, whereas American tribes (Anderson and Moratto 1996). ecosystems include interactions of biological Characteristics of species and populations components (plants, animals, and fungi) with all the are considered primary attributes of biological physical and chemical components of the immediate diversity (Noss 1990). Although it is generally environment (Begon et al. 1990). Although we can accepted that biological diversity in the basin has define communities conceptually, in practice they are been altered in the past 150 years, the degree of difficult to identify and classify. Communities are alteration and its implications for species persistence dynamic collections of species in which each species have not been described previously. Populations responds independently to environmental variation typically are described by their distribution, species (Whittaker 1975; Krebs 1978; Levin 1992). However, frequency of occurrence, and abundance (Noss classification schemes facilitate taking stock of the 1990). Species can be described further in terms of quantity, quality, location, and diversity of their life history characteristics, habitat associations, communities. Common descriptors of communities and exotic or endemic status. include species richness, disturbance regimes, and the composition, age, and physical structure of the Factors Influencing Biological Integrity in the community. The distribution, abundance, and Basin diversity of community types are also informative Factors influencing biological integrity descriptors. Aquatic ecosystems usually are become relevant when attempting to understand its differentiated by both their physical and biological current condition, how this condition came to pass, features and include a wide variety of flowing and and how it may change in the future. The primary standing water systems (Moyle and Ellison 1991). factors influencing biota are divided into physical Physical features of aquatic ecosystems commonly and biological factors and are discussed below. include their size, shape, depth, volume, gradient, shade cover, temperature, water chemistry, and Physical Factors substrates. A myriad of physical factors affect species, Fire is recognized as a keystone process in communities, and ecosystems. Some of the most Sierra Nevada ecosystems. Fire is a natural part of influential physical factors include such major the Sierran environment, and it significantly environmental gradients as elevation, precipitation, influences the distribution and abundance of native and latitude (Schluter and Ricklefs 1993). All of plant and animal species, as well as the physical these gradients affect the productivity of an area and structure of vegetation communities. The arid have been shown in numerous studies to have summers typical of Mediterranean-type climates, significant relationships with biological diversity and such as California’s, cause dead plant material on the the distributions of species and communities ground to dry rapidly. Lightning strikes frequently in (Rosenzweig 1995). In the basin, elevation and late summer and fall. If these events are not followed precipitation vary greatly for such a small geographic by rain, if there is sufficient dry fuel on the ground area and may exert a stronger influence than to carry flame, if the air is dry enough, and if wind expected. Variation in precipitation is largely a conditions are right, then a surface fire results. In the function of basin’s location in a transition zone Lake Tahoe basin, sediment cores from Lake Tahoe between Mediterranean and continental climates. (Davis 1997) and nearby Osgood Swamp (Adams The Sierra Nevada and Carson Range crests, which 1967) show that charcoal was continually deposited flank Lake Tahoe west and east and create its basin, in the Lake Tahoe basin and vicinity in the past, and provide the elevational variation that demarcates indicating frequent fires. This historical pattern of 404 Lake Tahoe Watershed Assessment Chapter 5 the transition between the two very different climatic 100 years (Graber 1996), and the basin is no regimes. exception. Losses of large predators, such as the grizzly bear (Ursus arctos), change the food chain Biological Factors dynamics within communities significantly. The loss Innumerable biological factors influence of top predators may affect many other species along biological diversity and integrity in most systems. It the food chain (e.g., “trophic cascades”; Paine 1980; is difficult, and perhaps inappropriate, to attempt to Power et al. 1985; Power 1990). In addition, many identify a few key factors that shape biological exotic species now occur in the basin, and some are systems. Here, we discuss a limited set of biological aggressive competitors that can outcompete native factors that are of interest in the basin and are species. Finally, as discussed above, fire suppression known to significantly influence biological diversity activities have influenced environmental conditions and integrity: fire and succession, interspecies and shifted the competitive advantage among conifer interactions, and biogeographic dynamics. tree species. Fire and Succession—Paleoecologists such as Biogeographic Dynamics—The basin is in a Axelrod (1986) believe that a fire-prone climate has transition zone between