Biological diversity : status and trends in the United States

34 Pages·1994·2 MB·English

by  Langner, Linda L

Historic, Archive Document
Do not assume content reflects current
scientific knowledge, policies, or practices.

United States Biological Diversity: Status and
Department of
Agriculture
Trends in the United States
Forest Service
Rocky Mountain
Forest and Range Linda L. Langner and Curtis H. Flather
ExperimentStation
o
c
d
Fort Collins ;mcp
Colorado 80526
General Technical :o
Report RM-244
3>
£0 p"' -<
o
n £
in
Acknowledgments
Special thanks go to Laura Copelin for screening, reviewing,
and summarizing hundreds of publications on biological diver-
sity. Stanley Temple, University of Wisconsin-Madison; David
Wilcove,EnvironmentalDefenseFund;LindaJoyce;AnneHoover;
Thomas Crow; and Stanley Krugman provided valuable review
comments.
This publication was printed on recycled paper.
USDA Forest Service April 1994
General Technical Report RM-244
Biological Diversity: Status and Trends in the
United States
Linda Langner, Economist
L.
Resources Program and Assessment Staff1
Curtis H. Flather, Research Wildlife Biologist
Rocky Mountain Forest and Range Experiment Station2
'Headquartersisin Washington, DC.
2Headquartersisin FortCollins, in cooperation with ColoradoState University.
Contents
Page
INTRODUCTION
1
DEFINITION OFBIOLOGICALDIVERSITY 1
Genetic Diversity 1
Species Diversity 2
Community and Ecosystem Diversity 2
THE GLOBAL CONTEXTFORBIOLOGICAL DIVERSITYINTHE UNITED STATES 3
STATUS AND TRENDS IN BIOLOGICAL DIVERSITY 4
Status and Trends in Genetic Diversity 4
Genetic Structure 4
Population Viability 5
Status and Trends in Species Diversity 6
Trends in Extinction 6
Threatened and Endangered Species 7
Status of Other Species Groups 8
Status and Trends in Community and Ecosystem Diversity 13
Ecosystem Composition 13
Ecosystem Configuration 16
FEDERAL ROLE IN BIOLOGICAL DIVERSITY 16
SUMMARYAND CONCLUSIONS 17
REFERENCES 18
Biological Diversity: Status and Trends
in the United States
Linda L. Langner and Curtis H. Flather
INTRODUCTION Genetic Diversity
TheNatureConservancy (1975) issuedareporton Geneticdiversityis thevariabilityofgenes, calcu-
naturaldiversity,almost20yearsago,thatindicated latedfromthenumberand frequenciesofallelesand
a rapid loss of ecosystems and communities in the their combinations, among individuals in a species
United States. Thereportemphasized therelianceof or population. A species' evolutionary ability de-
societyonecologicalsystems,andtheneedtoprotect pends on sufficient genetic diversity to maintain
natural areas that support species and natural com- immediate fitness and adaptability (Soule 1980),
munities. Today the same concerns are being ex- which, in turn, are a function of population size,
pressed in terms of biological diversity. subpopulationstructure,interpopulationgeneticvaria-
Thisreportsummarizesinformationavailablethat tion, matingstructure, generation time, and geneflow
can provide evidence about the status and trends in (Frankel 1974; Franklin 1980; Namkoong 1983).
biological diversity inthe United States. A literature Concernaboutgeneticdiversityismostseriousfor
review of the topic has been handled elsewhere populations that are either naturally small and iso-
(Myers 1983, Wilson 1988a, Wilson 1992). lated,orpopulationsthathavebecomesmallbecause
ofchangesintheirenvironment.Consequently,popu-
DEFINITION OF BIOLOGICAL DIVERSITY lation size (Lande and Barrowclough 1987) and the
distribution ofinteractingpopulations (Gilpin 1987)
Diversity is a complex quality that includes both are critical attributes used to evaluate genetic diver-
thenumberandrelativefrequencyofbiologicalenti- sity. If populations decline to critical levels and be-
ties. Measures of diversity should include both of come isolated, inbreeding (mating among relatives)
thesefeatures. Peet (1974) argued that,becausethere can reduce genetic variation and increase the fre-
was no consensusonthebestway tomeasure diver- quencyoflethalalleles,evenoveraperiodofseveral
sity,itwasdifficulttoclearlystateideasandhypoth- generations. The probability of losing rare alleles
eses about diversity. Patil and Taillie (1982) sug- increases during such episodes. These rare alleles
gested that a consensus will be difficult, despite maybeimportantforthespeciestosurviveinspecial
extensiveliterature. Decisivedefinitionsofdiversity environmental circumstances. Loss ofgenetic diver-
that would guide the development of operational sityalsoisassociated withincreased mortality rates,
approachestoconservationofdiversitystillarelack- reduced fecundity, increased incidence of birth de-
ing (Samson and Knopf 1994). Despite the problems fects, and reduced resistance to disease, a pattern
ofsemantics, there is no disagreement that reducing referredtocollectivelyasinbreedingdepression(Lacy
thenumberofbiologicalentities ina systemormak- 1992). Inbreeding ultimately threatens the persis-
ing some of them less abundant reduces diversity. tenceoftheoriginalpopulation.Theprocesswhereby
This report adopts the definition from the Key- allelefrequencieschangebychance(i.e., therandom
stone report (1991:6): "the variety of life, and its sampling of gametes during mating) is called "ge-
processes; including the varietyoflivingorganisms, netic drift." In small, isolated populations, genetic
thegeneticdifferencesamongthem,andthecommu- drift gradually reduces genetic diversity.
nities and ecosystems in which they occur." This The relationship between genetic diversity and
definition incorporates the concept of interactive population persistencecanbeanalyzed using popu-
"levels" of diversity. The three levels commonly lation viability analysis. Population viability analy-
identified are genetic diversity, species diversity, sis examines the influence of random processes on
and community or ecosystem diversity. the persistence of small populations, rather than
1
systematic pressures (e.g., habitat loss, exploitation) Within this hierarchy, several diversity indices
thatnormallyreceiveresourcemanagementempha- have been developed, each representing an attempt
sis (Brussard 1991). Shaffer (1981) proposed four to quantitatively distinguish among species collec-
random—processesthatcanaffectviability: (1) demo- tions. Althoughtheprofusionofmetricshascontrib-
graphic intrinsic random va—riation in birth and uted to the criticism of diversity indices, there are
death rates; (2) environmental variation in birth valid concerns about the interpretability of a single
and death rates attributed to changes in habita—t, number that represents the occurrence and abun-
interspecific interactions, and disease; (3) genetic dance among all species in a collection (Hurlbert
random events that are associated with changes in 1971). Comprehensive discussions ofdiversity indi-
allele frequency caused by—inbreeding or genetic ces are in Pielou (1975) and Magurran (1988).
drift; and (4) catastrophic random incidence of
devastating environmental events, such as fire or
drought. Within the context of these random pro- Community and Ecosystem Diversity
cesses,populationviabilityanalysispredictstheprob-
ability that a population ofa certain size will persist Community and ecosystem diversity reflects the
for a specified period of time. Because populations kinds and amount of community/ecosystem types
require space (habitat), it is important to identify acrossageographicarea.Communitiesgenerallyare
those habitat characteristics (quantity and quality) described as a group of interacting plants and ani-
that will support the specified viable population mals in an area, and frequently are defined by their
(Gilpin and Soule 1986). floristic and faunal similarity. Often, communities
are delineated as plant associations, such as decidu-
ous forests, tall-grass prairies, or sedge meadows.
Species Diversity Ecosystems encompass these communities, but in-
clude both biotic and abiotic factors that are united
Species diversity is a function ofboth the number by the exchange of energy or matter (MacMahon et
of species (referred to as richness) and the propor- al. 1981). Whilecommunitydefinitionsfocusonspe-
tional number of individuals within each species cies composition, ecosystem classifications include
(referred to as abundance or evenness) in a defined structuraland functionalattributes, suchas theabil-
area orgroup oforganisms. Species collections tend ity to capture and transfer energy and nutrients
to be characterized by many species that are rela- (Solbrig 1991).
tively rare, and few species that are very common The two terms often are used somewhat inter-
(Hairston 1959; Hughes 1986). This qualitative gen- changeably in discussions of biological diversity,
eralization notwithstanding, species assemblages andoften inthecontextofmappableentities distrib-
from different ecological communities tend to have uted across alandscape. This latter,restricted aspect
varyingpatternsofcommonnessandrarity(Kilburn ofcommunity and ecosystem is measured to evalu-
1966). ate community/ecosystem diversity. Community/
Whittaker (1960, 1977) developed a hierarchical ecosystem diversity can be characterized according
typology of diversity measures that was keyed pri- to the kinds and relative areas of community or
marily to the geographic scale of measurement. Al- ecosystem types within a region. Consequently,
though seven measures were eventually proposed simplemeasuresoflandtypecomposition,andmore
(Whittaker 1977), three are commonly referenced: complex attributes reflective of the configuration
alpha,beta, andgammadiversity. Alpha diversityis and spatial arrangement ofland types, all qualifyas
within-habitatdiversity, orthe numberofspecies in measures ofcommunity or ecosystem diversity. Ex-
a particular habitat. Beta diversity is the between- amples of spatial attributes include degree of frag-
habitat diversity, or species that are added when mentation, connectedness, patch shape, edge com-
additionalhabitatsaresampled. Gammadiversityis plexity, and average patch size, attributes that are
thenumberofspeciesinalargelandscapecontaining associated with measures of landscape structure
many different habitats (Peet 1974, Cody 1975, (O'Neill et al. 1988; Turner 1990b; LaGro 1991).
Whittaker 1972).
2
0
—
THE GLOBAL CONTEXT FOR BIOLOGICAL Table 2. Comparison of degree of threat to mammal and bird
andplantspeciesamongselectedtemperateandtropicalcoun-
DIVERSITY IN THE UNITED STATES
tries.
One of the most commonly recognized
macroecological patterns of biological diversity is Country Mammal & Percent Plant Percent
bird species threatened species threatened
theincreased richnessofthebiotaasonemovesfrom
the poles to the tropics. Wilson (1988b:8) estimated
that, although tropical habitats only comprise 7% of Temperate
United States1 1556 4% 20,000 12%
the global land base, they support more than 50% of Canada 623 2% 3,220 <1%
the species. This pattern appears to be consistent Spain 452 6% 8.937 11%
amongvarioustaxa.Tropicalcountriessupportmore Japan 856 4% 4,022 17%
Argentina 1182 7% 9,000 <1%
speciesinalmostalltaxonomicgroups. Forexample,
Tropical
BrazilandColombiabothhaveover50% moreavian Brazil 1961 7% 55,000 <1%
species than the United States, and more than twice Colombia 2023 5% 45,000 <1%
Madagascar 355 23% 12,000 2%
as many plant species (table 1). The difference in Indonesia 1979 9% 32,600 <1%
speciesrichnessisevenmorestrikingincomparison Zaire 1495 3% 11,000 <1%
to other temperate countries (table 1), because the
United States has considerably more species diver- Source: World Resources Institute (1992)
sity than most other temperate countries. 'Thenumberofthreatenedspecieslistedforthe U.S. isless
than currentlyon the federal threatenedandendangered
Varyingareaamongcountriescanconfoundinter- species list.
pretation of simple measures of species richness.
However, Reid and Miller (1989) found the latitudi- ened by extinction. The World Resources Institute
nal gradient in diversity is maintained even after (1992) compiled data on the number of threatened
controllingfordifferencesintotalarea. Forexample, species, by taxonomic group, in all countries. The
theUnited States supports approximately2,000vas- percentofthreatened species in a sampleoftemper-
cular plant species/104 km2, compared to 6,000 spe- ateand tropicalcountriesaredisplayedintable 2for
cies/104km2inBrazil.MexicoandZairesupport108 mammals and birds and plants. Except for the high
and96mammalspecies/104km2 respectively,com- percentage of threatened mammals and birds in
,
paredto60species/1 4km2intheUnitedStates.The Madagascar, the percent of threatened mammals
species richness of the tropics is the primary reason and birds is notgreatly differentbetween temperate
why those areas dominate discussions of biological and tropical countries. The threat to plant species
diversity. However, biotic integrity is independent appears to be greater in the temperate countries.
of the absolute species richness of any given geo- However,itisimportanttobecarefulininterpreting
graphic region. these data, because of the differences among coun-
Bioticintegrityisnoteasilyquantified (Karr1992); tries in efforts made to identifythreatened flora and
yet, asimple indicatorofintegritycanbeinferred by fauna. Until standard criteria for identifying species
lookingatthepercentageofspeciesconsideredthreat- vulnerable to extinction canbe implemented world-
wide, this will continue to hamper assessments of
species rarity.
Table 1.—Species diversity bytaxonomic groupand country. Protection of natural areas is a key component of
protectingbiological diversity. Aninventoryofpro-
Country Mammals Birds Reptiles Plants tected areas in 1990, compiled by World Resources
Institute (1992), indicated that the United States had
United States 466 1090 368 20.000 themostacressetaside innaturalareas, about twice
Canada 197 426 42 3.220 aslargeanareaasthecountrywiththesecondlargest
Spain 108 344 64 4,900
Mexico 439 961 717 20,000 protected acreage. Twenty countries had a higher
India 341 1178 400 15,000 percentage of their land base in protected areas.
Colombia 358 1665 383 45,000 Ecuador had the highest percentage (38%), com-
Brazil 394 1567 467 55,000
paredto10.5%intheUnitedStates.Worldwide,4.8%
Source: World Resources Institute (1992) of the land base was classified as protected areas.
3
However,sizeis notasimportantaswhichelements Genetic Structure
of diversity are being protected. Also, the degree of
protection afforded to these "protected areas" can Studies of the genetic structure of individuals or
vary greatly. populations of species are relatively scarce, because
techniques fordetectingvariationare newand diffi-
cult. Our understanding of speciation, adaptation,
STATUS AND TRENDS IN BIOLOGICAL DIVERSITY and evolution will increase as more information on
the genetic structure of species becomes available.
Ideally, data on biological diversity should de- Loveless and Hamrick (1984) found such information
scribe both the distribution and abundance of com- scarceinplants.Thereappearstobeevenlessinforma-
ponents of diversity over time. However, informa- tion on animal species, particularlywild species.
tion about the status and trends ofbiological diver- Initial concern about genetic diversity focused
sityisgenerallyoftwotypes.Thefirstinvolvesdirect primarily on the genetic erosion of major crop spe-
measures, suchas population trends orspecies rich- ciesintheUnitedStates.Questionswereraisedabout
ness. The second type involves surrogate indicators the ability of existing gene pools to meet future
thatprobablyarecorrelatedwithtrendsinbiological human needs for food and fiber (National Research
diversity. This report uses a combination of direct Council 1978, Namkoong 1981). Detailed examina-
and correlated trend information to describe diver- tionofthe statusofgeneticresources forcropbreed-
sityforeachofthethreelevelsdescribedpreviously. ingisinOldfield(1984).Geneticresourcesforcontin-
Much of the data are of limited scale and ued improvements in agricultural crops continue to
generalizability.However,theaccumulationofsmall- be important. The wild relatives and progenitors of
scale evidence, from different geographic regions, almost all major U.S. crops are found outside the
can provide indications of large-scale patterns. United States; but, the continued existence of these
Other than simple, descriptive statistics reflective genetically diverse wild relatives is very important.
of trends in biological diversity, determining the Native food crops, such as cranberry (Vactinium
causes of observed trends is more difficult. Two macrocarpon Ait. and V. oxycoccus L.), and sunflower
broad categories ofcausation are important: natural (Helianthus annus), are of minor commercial value
causes (e.g. succession, natural catastrophe) and relative to introduced species, such as corn (Zea
human-induced causes (e.g. habitat alteration). In mays) and wheat(Triticum aestivum) (Oldfield 1984).
many cases, there are multiple, often interactive, Wildspecies,particularlyplants,arealsothesource
factors that affect diversity. Although the relative ofmanymedicinalcompounds.Systematiceffortsto
contributions ofnatural and human-induced factors screen species for medicinal properties are now un-
are not easily separable, evidence indicates that hu- derway in the U.S. and abroad. An example in the
man-induced factors are the primary cause of con- U.S. is the Pacific yew (Taxus brevifolia), a species
cern. considered a weed tree until its use as a source of
taxol was discovered. Adraftenvironmental impact
statement, published for public review in January
Status and Trends in Genetic Diversity 1993, discussed alternatives to allow harvest of Pa-
cific yew, balanced with management to protect the
Limited information is available to assess genetic viability of the species for future use. (USDA Forest
diversity systematically over broad geographic re- Service, USDI Bureau of Land Management, and
gions within the United States. Available studies USDHHS Food and Drug Administration 1993).
weregroupedintotwocategories.Thefirstisgenetic Forest trees have been a second focus of genetic
structure,whichincludesstudiesaboutgeneticvaria-
study. Likeagriculturalcrop species, the motivation
tion within and across individuals. The second cat-
for most tree genetic studies has been to breed spe-
egoryispopulationviability,whichincludesstudies cieswithimproved timberproductivity.Unlikecrop
focusing on how population size and distribution
species, thespeciesofinterestareprimarilynativeto
affects genetic variation. the United States. Although many conifer species
have been subjected to genetic analysis, the results
arenoteasilygeneralizedtootherspecies.Foresttree
4