Download biological diversity and its loss

BIOLOGICAL DIVERSITY AND ITS LOSS
flan,ai'i has rlrr lri~lrcsrbiolugiccil dii~erti!,,pcrr o d arcw
it1 rlre Utrired Srorcs.
Lloyd L. Loope and Samuel M. Gon 111
WHAT IS BIOLOGICAL DIVERSITY?
Bioloeical diversity is a measure of the abundance of snecies of plants
and a n i m a . ~in a given area. '1'11~arc:{ can [he a sni:111 portion of ;I fared o r
field. or ;I csn cnconip;,ic entire ihnd,. conlincnr\, ur the ir~irldhiosphere.
Clearlv. there are manv plants and animals on the danet. and there are
HOW IS BIOLOGICAL DNERSITY DESCRIBED?
The evenness of the mix of species in an area is another way to measure
biological diversity. If one area has three species, but one of those species
makes up 99% of all individuals, that area IS quite different from another
area. also with three s~ecies.but where each makes up 33% of the individuals.
extinction.
WHAT FACTORS AFFECT BIOLOGICAL DIVERSITY?
Limiting Factors
Availabilitv of water. nutrients and mace can limit the maximum diversity
Biolo@calDiversity\Lmpe and Gon
ecosystem cannot support species. Limiting factors determine an area's
carrying capacity.
Physical Complexity
Several studres have shown that habitats with greater complexity support
greater numbers of species. When species are adapted to occupy specific
parts of a physically complex area, more species can occur. The concept of
"ecological niche" is relevant to biological diversity in that the more available
niches that are present, the higher is the ootentialdiversity of an area. Although the ecological niche includes physical and biological o portunities for
species, the hysical parameters are the most easily measured? For example,
a low, dry is and offers fewer opportunities than a high island that rises
into the cloud zone, even if the overall size of the two islands is the same.
The high island would have dry, moist, and wet areas capable of supporting
species adapted to those different conditions. The low island could only
& p p m pe'cie, adapted to d y condirions. Hhh:t3t comple\ity ma! ;A6 be increased through per~odicdisturbance. In this way, the physical and hiological
opportunities at a riven site change with time suhtequent to disturhance, producing ecological s?iuccession of c&nmunities.
f
Biological Interactions
Bioloeical comolexitv suo~ortsa diversitv of soecies because most soecies
depend (lithe pre&ce ;)f &er specier to g;e& or l e w r extent. I n
simplc examplc, predators cmnot per\ict in ;in area :~nlcsstheir pre\ is
oreient. In a more complex exam~le,a slow-mowing plant species iften canalso exist to
hot persist in an area where plantka~ersexis~~mless~redators
keep the plant-c~tcrsin zheck. Interactions between spcciec that benefit both
inter~ctinaspecie\ are called mutunlivn. There ;,re m:mv cases oi mutua!im
in trooicairain forests and other areas with hieh hinloeicil diversitv. As
more;pccies enter into direct and indirect intkaction; the biological diversity of :In ecosvstem grows upon itself. creating an extremely complex cystern
that rcqmnds 1 0 i h a n p in ways !ha1 are,not r u v to prccl~ct.Our understanbin$ of the role o b~ul+~cal
1nler.lctrons i n determ~ninghtol,og~cnl
d~vers~ty
1s quite mdimentar)..
\\'HAT ARE SOME RIA.IOR IJ:4TTERR'S
OF \\'ORLD BIOLOCIC.4L I)I\'ERSI11'?
Tropical versus Tempcrate Zone Diversity
'The land ares hetwec~ithe Tropic of Cancer (23.9' Uj and the I'ropic r
Capricorn (23.5" S) compriw much leis than Mi the e;,rrh's land circa but
may hold 80'7~of terrestrial species. E.O. Wilson (1985) cited a study in
Bornro in which 700 trec species were found in 25 acres (10 ha) of rain
I
Anorlwr of
foretr Only 70rl rrce specie, occur in i!l of N I ~Anerica'
Wilt~m',example$ \\as thc ~Jrntific.,t~~m
of 43 i n 1 spec)?>in 26 genera in a
s i d e tree in a rain forest in I'eru. This is aooroximatelv the numhcr of ant
sp&ies in the entire British Isles!
.
~ i
Island versus Continental 1)iversity
I h n d s gr.ner;tlly hold i e w r speck\ t h a n c o m p a r a b l e a r e a s oi continents.
b u t rule.; g o v e r n i n g t h ~ hpecies
d
diversity a r c not sinlplu, a n d t h e r e a r e
c v e r u l c u c r ~ p t i o n i . Ihc I l s u a i i : t n Islxnds a r e r i c h e r in species than m i g h ~h e
e x p e c l e d , hecawe i s u l a l i m a n d considcrilhle t i m e h a v e a l l o w e d the e v o l u t i o n
o f a b i n l a th;ll i endemic ('l'ahle 1). I n t r o d u c e d \ p e c k s s r e n o t i n c l u d e d
in e,tirnatc\ o f nat.lral hiologiclil di\.errity.
Table 1. Distribution of species in major taxonomic groups, with a Hawaiian perspective.'
Taxonomic
category
Species deserihed
worldwide"
Narivc to
Hawai'i
Introduced
and reproducing
in Hawai'i
Mammals
Birds
Reptiles
Amphibians
Fishes
Arthropods
Mollusks
Flowering plants
Conifers
Ferns and allies
Masses, livcrworls
Algac
Fungi
Bacteria
...................... ..........................................................................................
Total
ca. 1,700,WO
ca.9,000
ca.3,WO
~
*
~
.....
Onlv land inommarine) soecies are included for the Hawaiian Islands
~
~
numbers of insect soecies in the world now reach 15-30 million, due orimarilv to rcccnt
Erwin of the Smithsonian Institution. over 1.200 new soecies of bectlcs alone were found
acres) of Ironical seasonal forest in Panama. and as many as 30,000,000 insect soecies on
earth. Other groups thought to have many undescribcd species are fungi and decp-sca
fishes. Even in such a sedentary and canspicuaus group as flowering plants, many species
await description. especially in thc Tropics. Gentry (1986) estimated that lfl.000 species
of flowering plants await discovery in the New World Tropics alone.
BiolodcoI Divenip\ Loope and Con
WHAT IS ENDEMISM?
Endemic specie, are those restr~ctedto a y~ecificarea. A species may be
endemic ro the Western Hemisphere, lo North America, to thc southwestern
United States, to a single state,br to a single mountain range or drainage
basin. Gentry 41986) defined a I o ~ a l i z dendemic as one with a range of less
than 20,000 mi (about 50,000 km ). All land speciy endemic tq the
Hawaiian Islands are confined to less than 6,500 mi (16,800 km ), the
total land area of the Archipelago. Som of these species are confined to very
small areas, sometimes to less than 1mi (2.6 km2).
4
WHAT DOES ENDEhlIShI HAVE TO DO
WITH LOSS OF HIOLOGICAL DTVERSITY?
Species with small ranges (localized endemics) are generally more vulnerable to extinction than species with large ranges. Locally endemic species in
tropical forests and on oceanic islands are particularly vulnerahle to extinction. Gentry (1986) suggested that much local endemism occurs in the Tropics. A perspective on the uneven distribution of endemism is given by the
following analysis of endemism in flowering plants of the United States,
adapted from Gentry (1986):
13 states have no endemic species.
16 states have 1-5endemic soecies.
3 states have GI0 cndcmic specits.
4 rtalcs hwe 11-25 cndcmic spccics.
7 states have 26-100 endemic swcies.
3 rratcs (Utah, Arizona, 0rego.n) have 101-303endemicspecies.
2 states (Florida, Texas) hahaw.331-400 endemic spccies.
1stale (Hawai'i) has about 1,WO endemic s p i e s .
1 state (California) has over 1,MOcndcmic species.
Notice that Hawai'i, with 1,000 or more endemic plant species, is much
smaller in area than California. Hawai'i has the highest biological diversity
per unit area in the United States. The states with the most endemic plant
s eciec also generally have the greatest number of Threatened and
fndangered plant species.
WHAT IS THE ROLE OF EXTINCTION
IN THE HISTORY OF LIFE 01 EARTH?
The earth is thought to be nearlv 5 billion vears old. The first life
appeared about 4 billion years ago, h the formof single-celled ~ ~ r g ~ n i s m s .
Multicellular life prohably np eared .>bout 3 billion y a r s ago hut left little
fossil evidence until about 53tmillion years ago. Geologim hnve established
a worldwide stratigrnphy of fosd-bearing tedimentun. rucks bilsed on the
principle that certain widely distributed forms becanie extinct over time and
new forms evolved. Gcolocists hnve lone rccocnizetl that some transitions
were relatively rapid and ikolved an en61moui turnover of species. For example, at the end of the Permian geological eriod, about 225 million years
ago, 90% of all forms of life disappeared, foljbwed by relatively rapid evolution of new forms. The end of the Cretaceous period 65 million years ago
was another 'crisis" in geologic time, with the diqappearance of dinosaurs
followed by the proliferation of mnmm11l5.
WHAT HAS CAUSED MASS EXTINCTIONS I N THE PAST?
Asteroid I m p a c t Theory
In 1980, a theory dealing with crises in the history of life arose which has
revolutionized historical geology. Alvarez et a[. (1980) found a thin stratigraphic layer that was rich in the relatively rare element iridium. This layer
occurred at the Cretaceous-Tertiary strailgraphic bounda in a number of
widely separated sites, and led the Alvarezes and their cof(eagues to postulate
an extraterrestrial source for this iridium--specifically,meteorites that
stmck the earth in a cataclysm that produced mass extinctions. Others have
postulated a 26-million-year periodicity of extinctions, based on the recurrent
approach of andeathstar," perhaps a twin of the sun, which has repeatedly
triggered such cataclysms. These theories, particularly the latter, remain
highly controversial. Major points of discussion involve whether or not
extinction events are indeed periodic and whether there is evidence that loss
of diversity in major taxa, including dinosaurs, occurred rapidly (versus
gradually).
Pleistocene Overkill
Man's role in the extinction of continental mammals and island birds is
gradually being realized. North America 15,000 years ago, at the time when
humans mav have first crossed the Berine land bridee. had a laree mammal
fauna at le&t as rich as the modern African faunal-~bundantfksils d w w
mentirig this fauna are preserved in such sites as the La Rrca tar pit\ i n In\
Anuele.;. I'aul S. .Martin of the Universitv of Arizona oost~rlatcdi n the 1960s
t h d i the advancrng front as man moved ohward in thd New World was the vre
or n u , , niammal extinctions. This theow has not been proved or d~sproved
but has manv suooorters. Similar extinciions occurred during the late Ice Ages
on a11 continenti ixcept Africa. It has been suggested that /\Trim has retainsd a relativelv rich mammal fauna because mammals evolved a resistance
to human deoredations over a ~ e r i o dof a million vears or more while man
was also evofving.
Prehistoric ~slandextinctions of moas (giant flightless birds) in New
Zealand. over half the bird fauna of the Hawaiian Islands. and birds on islands
worldwide are becoming increasinglv well documented. Ihe majority of recorded extinctions in historic times (for example. m3nv llawaiian honeycreeoers. the dodo (Rha~huscuc~rlldrus)of hlauritius. and so forth) have
occLrred on islands: '
-
WHAT F O R C E S I N TllE \\'OKI.D TODAY
THREATEN RIOLOGICAL DI\'EKSI'I1'?
Human Overpopulation
Intense concern among conservationists over the human population explosion arose in the 1950s and perhaps peaked in the late 1960s with publication
of Paul Ehrlich's ThePopulation Bomb in 1968. Most of us tend to articulate our concern less often now, not because the problem has disappeared,
hut because we feel hel~lessin dealine with such a mmolex and intractable
probl& I h e earth's human populathn has indeed dobhled in the past 25
)cars and now exceeds 5 hillion. Although the immedi~telydire cm~equences
oredicted in some envlronnlental~stcircles (such as "Faniine 1975 I have not
kterislized on schedule. this i.; largely because agricultural "green revolutions" have heroically managed to keep a fairly steady diet for much of the
bureeonine human vo~ulatibn.exceedine oroductiviiv estimates made in the
IY&
lickevcr, I&
monocultures d & m i \ e to n h v e ecosystems and
w~lnerableto insects and diseases have heen required. Spreahng deserts
and chronic famines in certain Darts of the elohd are further remiinden that
agricultural efforts hwe limits.'~urthermo;e. "green rev~~lutions"
are largely
dependent upon high-cnerby inpuls such ac ecticidec, fertilizer,, and fuel for
mechan~zecleaoioment: these mav not be a fordahle in the future.. ,oarticularlv
in Third World cbuntries.
Vitousek et aL (1986) have recently calculated that humans are now
diverting 40% of the potential terrestrial net ~ r i m a r v~roductionof the earth
to theirises. The growing human population' is a rdoi cause of the destruction
of wildlife, of the loss of biological diversity ingeneral, and of the destruction of tro ical forests at a rate, according to the United Nations, of about
24,000 mi2' (62,000 km ) a year--an area the size of West Virginia). There
are other causes, described below, but even were those eliminated today, global biological diversity would be threatened ultimately by the rapid increase
in numbers, needs, and aspirations of the human population.
?
1
F r a g m e n t a t i o n of H a b i t a t
Island-like conservation areas (for example, natural areas surrounded by
areas used for other purposes by humans) tend to lose species through local
eainction of small populations that fluctuate in size (for example, as a
result of local disease epidemics). Species are temporarily lost through the
same processes in reserves that are not island-like; however, recolonization
frequently occurs from adjacent habitat. Another reason island-like
conservation areas lose specles is that manv species reauire more than one
hhbitat. sometimes many'miles apart. for chmpletm of their life cycles. A
good example of this is seen in migratoly Imds that alternate hetwcn hahitats
in the United Statec and tropical America in an annual cvcle. l a s s of even
one habitat can be critical iri perpetuating the species.
Invasion of Alien P l a n t s and Animals
Human-assisted biological invasions are having a homogenizing and disrupting effect on ecosystems worldwide. Classic examples include the feral
goat (Copra h i m ) and the Argentine ant (Iridompner hurnilis). Both these
aliens are established on islands and portions of continental areas over much
of the world. Wherever they establish, they reduce the diversity of the native
biota over time through persistent browsing (goats) and aggressive predation
and competition (ants).
Nuclear War a n d Nuclear Winter
"Nuclear winter" is a term used by Paul Ehrlich, Carl Sagan, and others.
It is thought that nuclear war could destroy much of the life on earth by
creating a dense cloud of ash, dust, and smoke that would prevent sunlight
from reaching the earth's surface, resulting in a sharp drop in temperature of
the planet for an extended period. This phenomenon is undoubtedly a threat
to biological diversity, but its immediate implications for mankind are of
higher concern for most of us. The Alvarez n a/. (1980)'imoact thcorv" of
extinction in the distant $enlogical past suggests a'catadysm'in many respects
very similar to nuclear u.~ntcr--perhapsproviding a theorctiral consolation to
the extent that life on earth h x ~ l l ~ i v such
e d cataclysms hefore.
Air Pollution and Climatic Chan e
!
The effects of air pollution (break own of the ozone layer, global warming, acid rain, and so forth) pose serious ecological problems with biospherewide consequences. Although these effects may seem to pose asmaller threat
to biological diversity than nuclear winter, it may be that human modification
of the atmosphere is among the greatest threats to the biosphere. When air
pollution problems get so bad that humans are being severely affected, the industrial nations may take steps to do something about the cause of the prohlem. The severity of the situation in Europe, especially in Czechoslovakia,
Poland, and East Germany, suggests that by the time this happens, much of
the biological diversity will already have been severely reduced.
HOW RAPIDLY ARE SPECIES BEING LOST?
The lack of an accurate estimate of how many s ecies exist is one reason
e most conservative
we don't reallv know how fast thev are beine lost.
estimates of tiopical rain forest d&tructionh gest that all will be gone by
sometime around 2100 A.D. if current rates ofdestruction continue; other
estimates suggest they will all be gone much sooner. A report in the journal
Bioscience about a "National Forum on Biodiversity" held in September 1986
cited a suggestion by Norman Myers that a third to a half of all species would
be gone by the end of this century (Tangley 1986). A more optimistic assessment by Ariel Lugo of the U.S. Forest Service was that 9% may be a more
reasonable guess.
?%
WHY IS LOSS OF BIOLOGICAL DIVERSITY A PROBLEM?
Cunningham (1987) suggested fnur reasons wh the public should lrc conccrned about loss of species: 1) ethical reasons (ot {er organirms hmc a right
tv cuiwncc); 2) aesthetic reaton, (beauty, symholicvalue of spccie,, :tnd so
forthj: 3) economic reason? (actwl or potential economic uses and henefih to
es
"ecosystem services"-human\); and 4 ) ecological reasons ( ~ [ ~ c c iperform
producing oxygen, maiitaining the chemical composition df the atmosphere,
pollinating plants, and so forth).
Cunnmgham suggested that although most National Park Service natural
resource interpreters and educators may subscribe to reasons 1.2, and 4, the
economic reasons may be the most effective for getting the attention of the
average park visitor. We would agree that using classical economic arguments
In interpreting biological diversity is probably appropriate but would tend to
recommend more stress on "ecosystem services," which really transcend
economics. What is the monetary value of an intact ozone layer? Or of the
bufferine effect of rain forests on the earth's climate? Ecoloeists and
economks can only answer in broad terms. Ehrlich (1086) Gade a persu:s,iw
al
could quickl!
argument that vastly increased efforts :n e c ~ l n ~ i cresearch
ennancc mankind's abilitv to deal with the real oroblems of the world. I le
cited the absurdity of s o k e of our society's prio;ities--for example, devoting
over 100 times as nlwh funding for biomedical re,carch as for ecological
research. ' f i e so-called 'war on cancer' is a notorious example. 'lhe existence
of society would not he threatened if no progress toward a smccr cure were
made over the next 50 cars. But, says Ehrl~ch. there is a deadline for
solving humanity's eco oe~cal
among
-. problems,
.
- which is the irreversible losr
of bioiogical diversity.
The extinction of one particular species probably isn't going to have much
effect on the future of mankind. But with continual and accelerating species
loss, at what point will communities and ecosystems start to break down?
How rapidly will climate and agriculture be affected? How long can modern
civilization sunrive with a shrinking agricultural base? Is Ehrlich's view,
largely shared by us, unrealistic and unnecessarily alarmist? We doubt it, but
we suggest that scientists, educators, and concerned citizens become familiar
with the evidence and decide what reasons for protecting biological diversity
have merit. Each of us also needs t o decide what we are ethically comfortable
with.
r'
AS 1NDIVII)UALS CONCERNED ABOUT
THE "RIOLOGICAL 1)IVERSITY CRISIS "
CAN WE AVOID F ~ L I N G OF
S FUTILI~Y?
At a meeting in India, Michacl Soul2 gave a vivid analogy of the problem of lash of biological diversly that may be constructive i u r us. Consider
that all the plant and animal species in the world are an a runaway train. No
one is in the conductor's seat, and a lre\tIe is nut over a botttmlezs chasm
up ahead. No one of us is capable of stopping the train. But each of us
can throw rocks and sticks onto the tracks t u slou the train down so that
not all the cars will go into the chasm. Individual efforts may not seem to
have much effect in slowing the train, but our collective efforts can make
quite a difference.
Important References
Alvarcz L.W.,W. Ahareq F. Asaro, and H.V. Michel. 1980. Exrraterrcstrial cause for
cretaceous-terlialyextinction. Science 208:1095-n08.
CunninrdIam. R. 1987. Bioloeical diveniw N 1989 initiative forinternretation fexcemis
~~
~~from March '87 memorandum). ~ a r Science
k
8(1):23.
Ehrlich, P.R. 1968. The Population Bomb. Ballanfine Books, New York.
Ehrlich, P.R. 1986, nze Machinery of Na1114. Simon and Schuster, New York.
Ehrlich, P.R., and A.H. Ehrlich. 1981. h-nction: The Causes and Consequences of the
Disappearance of Species. Random House, Ncw York.
Erwin,T.L. 1983. Tropical forest canopies: the last biotic frontier. Bullefin,
EnfomologicaiSocieryOfAmlerico,Spring 1983, pp. 14-19,
Gentry, A.H. 1986. Endemism in tropical versus tempcrate plant communities. Pp. 153-181IN
M.E. Soulk (ed.), Co,rsensofionB i o l o ~ ,771eScience of Scarcfp and Diversip.
Sinauer Associates, Inc., Sunderland,Massachuselts.
lanzen, D.H. 1986. The eternal external threat. Pp. ZCi303 IN M.E.Soulk (cd.),
ColtservalionB i o l o ~ :77ze Science of Scarcily andDiversiry. Sinauer Associates Inc.,
Sunderland, Massachusetts.
MIT Press, Carnbridgc,
Kaufman,L., and K. Mallory. 1986. nze Losf Enir~clio!~.
Massachusetts.
~~
Oldfield, M.L. 1984. ?he Volue of Conserving Genetic Resources. U.S. Dept. Interior,
National Park Senicc.
Tangley, L. 1986. Biodiversity goes public. Bioscience 36(11):708-711,715.
Vitousek, P.M., P.R. Ehrlkh, A.H. Ehrlich, and P A . Matson. 1986. Human appropriation of
the products of photosynthesis. Bioscience 36(6):368-373.
Wileove, D.S., C.H. McClellan, and A.P. Dobson. 1986. Habitat fragmentation in the
temperate zone. Pp. 237-256 IN M.E. Soul&(ed.), Conservation Biology; The Science of
Scar@ and Diversig. Sinauer Associates, Inc., Sunderland, Massachusetts.
Wilson, E.O. 1985. The biological diversity crisis. Bioscience 35(11):700-706.