Bird Conservation: More than Just Chasing the Tails.
The Shape of the Natural Bird Environment
As anyone who watches birds knows, some species are rare, some are fairly common, and some can be found on almost any trip to the field. These differences in species' abundances occur naturally. If one were to plot these natural abundance patterns on a graph, with relative abundance on the horizontal axis and the number of species on the vertical axis, it would look similar to the classic bell-shaped curve (Figure 1a). The top of the curve represents the vast majority of species, birds that are of average abundance. As one proceeds outward on the curve from the peak, species become either far more (to the right) or far less (to the left) abundant. In a natural, balanced environment, a small set of species occupy these "tails" of the distribution. Natural processes, like weather, predation, disease, and food and habitat availability, have shaped these patterns of species abundance for millennia. In recent years, however, human activities have disrupted many of those natural processes, resulting in a change in the "shape" of the bird environment.
[Figure 1 ILLUSTRATION OMITTED]
What is happening to bird habitat?
The past few decades have been witness to dramatic changes in the quality and quantity of bird habitat. Throughout the Western Hemisphere, wetlands continue to be drained and filled, forests cut and fragmented, and grasslands paved over. Many of these changes are not what they appear. For example, while forest cover in some areas has actually increased, the quality of those habitats compared to the original forests may not be similar at all because of changes in vegetation composition and artificially abundant predator populations.
Other seemingly less obtrusive land use practices have upset the natural balance as well. For example, waste grain left in fields in the southern United States has increased overwinter survival of some species of geese. While this can dramatically increase the survival of overwintering individuals, sometimes it means that many more birds fly north in the spring than can be supported by the breeding habitat at the other end of their migration route. The result in one case has been severe damage to the tundra ecosystem around Hudson Bay. One thing is certain: while bird habitats have been changing rapidly during the 20th century, our approach to wild bird conservation has not evolved at a pace necessary to prevent the occurrence of many serious problems.
Chasing the Tails
Recent years have seen an increasingly skewed pattern of avian abundance. The traditional approaches to bird conservation have not been inclusive enough to prevent this reshaping of the bird environment. Management agencies and conservation organizations have chased the tails of the distribution--both rare or declining species and overabundant species--and largely assumed that the vast majority of species occupying the middle ground would "take care" of themselves.
Traditional bird conservation efforts have not prevented the slide of more and more species from the peak of the curve to the tails. After more than a century of rapid environmental degradation (especially habitat loss and fragmentation), without proactive conservation measures in place, the bell-shaped curve has flattened for birds (Figure 1b). With this change has come a massive, expensive, and reactive workload for bird conservationists.
The Left Tail: Storks, Sparrows, Warblers, and Woodpeckers
The Endangered Species Act of 1973 provided a significant safeguard in which species sliding down the left side of the curve (i.e., less abundant) are kept from the grasp of extinction. The wood stork (Mycteria americana), golden-cheeked warbler (Dendroica chryoparia), red-cockaded woodpecker (Picoides borealis), and many other species of migratory birds owe their current existence in the United States to the determined, last-ditch efforts carried out under this legislative milestone. But attempting to pull species back from the brink of extinction can be an expensive and contentious proposition.
Even today, despite considerable conservation gains in the past few years, many challenges still threaten to drive species down the left side of the curve, away from healthy populations, and onto the endangered species list.
The Right Tail: Cormorants, Cowbirds, Crows, and Geese
Just the opposite phenomenon occurs on the right tail of the curve. Here, birds that thrive in highly altered or artificial environments propagate to the point that they exceed their natural carrying capacity, and thus impose significant economic losses on local human communities, inflict severe damage to natural habitats, and cause unnatural population crashes in other species. If you've visited a golf course in the east lately, you've probably seen one of the problems. Some populations of Canada geese (Branta canadensis), whose natural migratory habits have disappeared in the past few decades, now remain year-round on their once southern wintering grounds. Open, grassy, artificial habitats and abundant food--lawn grasses and bread from park visitors, for example--provide the geese with no incentive to leave these areas for more northerly climes during the breeding season.
Superabundant and artificial sources of winter food (e.g., aquaculture, grains), which created winter habitat that maintains more birds than the corresponding breeding habitat can support, has increased overwinter survival of double-crested cormorants (Phalacrocorax auritus) and snow geese (Chen caerulescens) to the point that these species have become significant problems in many areas.
Songbirds are not exempt from these habitat management problems. Brown-headed cowbirds (Molothrus ater) and American crows (Corvus brachyrhynchos) have exploded in numbers with the opening of eastern forests. Now, in many forested habitats, crows represent the most significant predator of songbird eggs and nestlings. Brown-headed cowbirds, once restricted to the Great Plains, can be found in nearly every State and province of North America and now parasitize more than 200 species of birds. It is clear that formerly natural systems have been altered and now foster overabundant, problematic species that are on the rise.
A Vicious Circle: When the Tails Meet
One of the most distressing scenarios occurs when the right tail meets the left. For example, after gaining access to the highly fragmented habitats of both Kirtland's and golden-cheeked warblers, the overabundant cowbirds have propelled these endangered species even closer to extinction. Mass destruction of tundra and other wetlands along Hudson Bay--a by-product of the exponential growth of snow goose populations in the 1980's and 1990's--is threatening the phalarope, shorebird, and songbird populations in that region. The European starling (Sturnus vulgaris), an introduced species, displaces native woodpeckers, flycatchers, bluebirds, and other cavity-nesting species. Expanding double-crested cormorant populations have overtaken night-heron rookeries along the Great Lakes, and that situation is only likely to get worse. Even overbrowsing of vegetation by white-tailed deer, one of the principal species for which many wildlife agencies create habitat -- highly fragmented habitat -- can significantly impact shrub-nesting songbirds and other wildlife that depend upon forest understory. All of these increasingly abundant species drive other bird species towards the left-hand tail of the abundance curve and towards extirpation.
The Need to Also Manage for the Middle
Reacting to the status of species at the tails of the distribution is the traditional approach of natural resources agencies and organizations, but it creates a situation where one is always left in a "chasing" mode. Always playing catch-up is not an effective approach to bird conservation. Clearly, a new paradigm for conserving all wild birds--one that includes those species in the middle of the distribution--is sorely needed.
Fortunately, some science-based conservation initiatives do exist, and can serve as models. Principles of conservation biology serve as the foundation for these approaches. For example, whenever possible: (1) management actions should address the needs of entire species suites or communities through conservation or restoration of intact, natural ecosystems; (2) local management objectives and priorities should directly contribute to conservation objectives and priorities at larger, regional levels; and (3) conservation plans should attempt to integrate the social and economic, as well as biological concerns of stakeholders. Representative James Scheuer, retired Chairman of the Subcommittee on the Environment, U.S. House of Representatives, summed it up best: "We need to become proactive and holistic in our policies and move towards an integrated, multi-species and ecosystem approach to land use and conservation. The issue isn't endangered species, but endangered ecosystems. Our goal should become the management of ecosystems for the sustainable use of biological resources and the conservation of biodiversity" (Conservation Biology 7:206-207, 1993).
The above strategy would ensure that regardless of where a species falls on the abundance curve--at the tails or somewhere in-between--it would stand a good chance of continued survival. To accomplish this goal, biologically based plans that include clearly articulated conservation priorities must be developed and supported by land management agencies, conservation organizations, private landowners, and the public in general. The soon-to-be-completed conservation plans of Partners in Flight and the vision laid out in the 1998 "Update of the North American Waterfowl Management Plan" are two examples of proactive road maps that will lead to more effective bird conservation in the 21st century. While the future conservation of wild birds is certain to contain some steep and rugged topography, a holistic, ecosystem-based approach will guarantee that at least we have covered all the species in both the valleys and the peak.
Paul R. Schmidt is Chief of the FWS Office of Migratory Bird Management. Daniel R. Petit is a Wildlife Biologist in the Office of Migratory Bird Management and National Coordinator of Partners in Flight for the FWS.
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|Author:||Schmidt, Paul R.; Petit, Daniel R.|
|Publication:||Endangered Species Bulletin|
|Date:||Jul 1, 1998|
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