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Abstract Rarity, declining populations, and extinctions are natural phenomena. Numerous factors contribute to these phenomena including biotic f~ctors, isolation, and habitat change. Human activity exerts external energy to the environment that accelerates the decline of species and their habitats at rates exceeding natural phenomena. Papers presented in this session provide examples of the effects of human activities on declining species and habitats.
Rarity, declining populations, and extinctions are natural phenomena. Some species are naturally rare because of limited distributions or intrinsic life-histOlY attributes that limit their population size. Limited distributions may occur when a species is endemic to a small isolated area of habitat such as an island.
Islands need not be surrounded by water in the traditional sense, but could exist in terrestrial systems as a patch of habitat, terrestrial or aquatic, surrounded by inhospitable areas. Life history attributes that limit population size can be large territories as evident in many large predators, low rates of fecundity, and limited resources spread across a wide geographic area Further, evolutionruy processes and the inability of species to adapt to changing environments may underlie natural population declines of species leading to extitpations and extinctions (Allendorf and
Leary 1986).
Numerous factors contribute to these phenomena including biotic factors, isolation, and habitat change (Frankel and Soule
1981). With the exception of island situations, biotic factors-namely, competition, predation (exclusive of humans), and disease---are unlikely causal factors that lead to extinctions.
Their primary influence may be to limit populations in size and distribution to the point where additional factors push species towards extinction Ziswiler (1967) noted that 53 of the 77 species of birds or mammals which have gone extinct in recent history occurred in isolated situations. '!\vo underlying reasons
1 Vtlflliam M. Block is Acting Project Leader and Research Wildlife
Biologst, USDA Forest Service, Rocky Mountain Forest and Range
Experiment Station, located at Flagstaff, AZ. Headquarters is in Fort
Collins, in cooperation with Colorado State University. for these extinctions were limited habitat and a deterioration of competitive edge and predatory defense. Habitat alteration is brought about by slow geologic change, climate, catastrophe, and humans. The fIrst three categories of habitat change are largely natural, whereas the fourth is not. Generally, these major factors do not act in isolation, but work simultaneously and result in population and habitat declines.
Human activity exerts external energy to the environment that accelerates the decline of species and their habitats at rates exceeding natural phenomena. Understanding how humans impact ecological systems is essential for developing proactive approaches to conselVe species and habitats, and to allow natural events to act on species' populations and their environments.
Papers presented in this session provide examples of the effects of human activities on declining species and habitats. These papers provide prime examples of anthropogenic processes that lead to species' declines. The central theme of these papers centers on impacts to habitat, resulting in habitat loss and fragmentation.
Simberloff (1993) echoed this theme in his review of the effects of habitat fragmentation, noting that effects of fragmentation vary with size, shape, and juxtaposition of patches. Some effects include increased dispersal distances, increased vulnerability to predation, and disruption of natural processes such as fire. Fragmentation certainly has been a factor in declines of freshwater molluscs (Meblhop and Vaughn 1993), endangered butterflies (Schaeffer and Kiser 1993), and numerous other species (Wilcove et al. 1986).
Declining populations resulting from anthropogenic impacts are evident in numerous taxa ranging from common species such
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as quail (Brennan 1993) to threatened, endangered, and rare species such the black-footed ferret (Mustela nigripes). Land uses that cause population declines vary. For example, combinations of agriculture and silviculture underlie declining northern bobwhite (Colinus virginianus) populations (Brennan
1993). Water and electric power development have impacted populations of mountain quail (Oreortyx pictus) in Idaho and numerous desert fishes in the Southwest (Brennan 1993, Rinne
1993). Grazing has affected populations of numerous native desert fish (Rinne 1993) and assemblages of native grassland species. The negative effects of timber harvest on spotted owls
(Strix occidentalis) are well known, leading to the listing of two subspecies, the Mexican (S. o. lucida) and northern (S. o.
caurina) spotted owls, as federally tJ1reatened (Gutierrez 1993).
Com (1993) noted that reasons for many declining amphibian populations are not clear, and stressed the importance of understanding natural population fluctuations to evaluate whether current trends are indeed caused by human activities.
Obviously, historic patterns of land use, particularly following European settlement of 'North America, have had pronounced, and frequently negative, effects on native flora and fauna. Many cause-effect relationships are understood, far more are not. Further, ramifications of past and present land-use practices on future populations is certainly unknown, but if current practices continue without change, the outlook is bleak for many species.
Papers in this session and throughout the conference shared a common message. That message is that resource conselVation must embark on a new, proactive approach Sustaining current ecosystem conditions will doom numerous additional species to extirpation and possibly extinction The case histories provided in this session certainly verify this as the case. Change cannot be simply a new vocabulary or set of jargon to allow "business as usual" to occur under a newly articulated management vision
It must be a complete change in focus at all levels of resource management. For the conselVation of natural resources to be possible, functional disciplines must break down the barriers that impede communication (Gutierrez 1993). Likely, shifts in social and economic systems will be needed for ecological approaches to resource management to be successful. Reactive management for TES species must move from providing conditions just for those species to considering ecological systems in their entirety.
Certainly, the challenge to resource professionals is great. If we do not meet this challenge, natural systems will continue to erode and the very health of this planet will be in jeopardy.
ACKNOWLEDGMENTS
I thank L.A Brennan, R.I. Gutierrez, and IR. Rinne for reviewing an earlier draft of this summary.
LITERATURE CITED
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Allendorf, F.W.; Leary, R.F. 1986. Heterozygosity and fitness in natural populations of animals. pp.
57-76 in (M.E. Soule, ed.) ConselVation Biology: the Science of Scarcity and
Diversity. Sinauer, Sunderland.
Brennan, L.A. 1993. Broad-scale population declines in four species of North American quail: an examination of possible causes. [these proceedings]
Com, P.S. 1993. What we know and don't know about amphibian declines in the west. [these proceedings]
Frankel, O.H.; Soule, M.E. 1981. Conservation and evolution
Cambridge University Press, Cambridge.
Gutierrez, R.J. 1993. Future conservation planning: lessons from the spotted owl. [these proceedings]
Mehlhop, P.; Vaughn, C.C. 1993. Threats to and sustainability of ecosystems for freshwater molluscs in North America
[these proceedings].
Rinne, J.N. 1993. Declining aquatic habitats and native fishes of the desert southwest; are they sustainable? [these proceedings]
Schaeffer, K.; Kiser, S.L. 1993. Hypotheses concerning population decline and rarity in insects. [these proceedings]
Simberloff, D. 1993. How forest fragmentation hurts species and what to do about it. [these proceedings]
Wilcove, D.S.; McClellan, C.S.; Dobson, A.P. 1986. Habitat fragmentation in the temperate zone. pp.
237-256 in (M.E.
Soule, ed.) Conservation Biology: the Science of Scarcity and
Diversity. Sinauer, Sunderland.
Ziswiler, V. 1967. Extinction and vanishing animals.
Springer-Verlag, New York.
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