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Some Implications of Paleoecology for Contemporary Ecology
Author(s): Peter K. Schoonmaker and David R. Foster
Source: Botanical Review, Vol. 57, No. 3 (Jul. - Sep., 1991), pp. 204-245
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BOTANICAL
THE
VOL.
57
REVIEW
JULY-SEPrEMBER, 1991
No. 3
Some Implications of Paleoecology for
Contemporary Ecology
AND DAVID R. FOSTER
PETER K. SCHOONMAKER
HarvardUniversity
HarvardForest,Petersham,Massachusetts01366
I. Abstract...--------------------------------------------204
Sommaire.........................205
LI. Introduction
.........................205
III QuaternaryEnvironmentsand Vegetation .............
.
206
..........
A. ClimaticChange
Duringthe
Quaternary..................................................
206
................................
B. Evidence for Biotic Change .................................................
211
C. VegetationChangeDuringthe Late-Glacialand Holocene .......................................
215
IV. PaleoecologicalInsightsfor CommunityEcology ................................................ 220
A. The Nature of the Biotic Community ................................................
220
B. Evidenceof Disturbancein the PaleoecologicalRecord . .................................
223
C. CommunityStabilityand Equilibrium...............
.................................225
D. EvolutionaryConsiderations
227
.............................
E. Refugiain the PaleoecologicalRecord ......
230
........................
V. Prescriptionsand Applications.............................................................................................................
233
VI. Summary .-.............
......................................----------.....235
VII. Acknowledgments...............
236
,,,....
.
VIII. LiteratureCited...........................................................................................
. ................. ..... 236
I. Abstract
The paleoecologicalrecordallows contemporaryecologists to put currentphenomena into the context of a longer time-frame,therebyprovidingthe opportunityto
evaluatethe importanceof slowly operatingprocesses,past cyclic or unusualevents,
disturbanceregimes,and historicallyconstrainedphenomena.We brieflyoutline the
environmentalhistoryof the Quatemary,discuss the spatialand temporalresolution
of the paleoecologicalevidence for biotic change, and summarize data relevant to
such currentissues as the nature of the biotic community, the role of disturbance,
stabilityversus rapid change, evolutionarytheory, explanationsof species diversity,
and refugiatheory. Finally, we offer examples of the utility of paleoecologicaltechniques for ecologists and environmentalscientists.
Copies ofthis issue [57(3)] may be purchasedfrom the Scientific PublicationsDepartment,The New York BotanicalGarden, Bronx, NY 10458-5126 USA. Please inquireas to prices.
TheBotanicalReview57: 204-245, Jul.-Sep., 1991
C 1991 The New York Botanical Garden
204
IMPLICATIONSOF PALEOECOLOGY
205
Sommaire
Le dossier paleoecologiquepeut aider les ecologistes contemporainesa mettre les
phenomenes actuels dans le contexte d'une periode plus longue, ainsi fournissant
l'occasion d'evaluer l'importance des processus qui s'effectuentlentement, d'evenements cycliques ou inhabituelsdu passe de regimes perturbatrices,et du phenomenes historiquementcontraints.Nous traconsbrievementl'histoire du milieu environnement du Quaternaire, discutons la resolution spaciale le temporelle de
l'evidence paleoecologiquedu changementbiotique et resumons les donnees applicable a des issues actuels tel que la nature de la communautebiotique, le role du
derangement,la stabilite contre le changement rapide, la theorie d'evolution, les
explicationsde la diversite des especes, et la theorie d'evolution, les explicationsde
la diversite des especes, et la theorie de refugia. Finalement, nous presentons des
exemplesde l'utilitedes techniquespaleoecologiquepour les ecologisteset les savants
qui etudient l'environnement.
II. Introduction
The earth's biota responds to environmental changes that range in time from
millisecondsto millions of years and affectareas from 10-6 to 10'4 m2(Fig. 1). Most
ecologists study organisms on a time scale of less than 100 years (and are usually
limited to the time span of a field season, grant or dissertation)but many patterns
and processesin naturecan be better understoodover a longerperiod. By providing
information on vegetation and environmental change, especially during the past
15,000 years, paleoecologycan broaden the interpretationof issues that are specifically relevant to modern ecologists, e.g., the prevalence of stability versus rapid
change in ecosystems, the concept of an integratedcommunity, the importanceof
disturbancein regulatingcommunityorganization,alternativeexplanationsof species
diversity, and refugia theory. Paleoecologicalapproachesalso have many uses for
applied ecologists and environmentalscientists concernedwith landscapemanagement, restorationecology, and the design of naturepreserves.
The ecologicalimplicationsof paleoecologicaldata have been noted both by community ecologists and paleoecologists (cf. various authors in Barbour & Billings,
1988; Braun, 1950; Clements, 1924; Davis, 1983; Delcourtet al. 1983; Foster et al.,
1990; Ogden, 1985; Walker, 1982; Watts, 1970; West, 1964). Becausepaleoecology
is essentially community ecology stretchedbackwardthroughthe fourth dimension
of time, it offers the ecologist a vast temporal vista from which to view present
patterns,processesand phenomena.While the numberof paleoecologicalstudiesand
the variety of temporaland spatial scales thereinhas increased,contemporarycommunity ecologistshave become more awareof the importanceof processesoccurring
over extended periods of time. In the past two decades enough paleoecologicaldata
have accumulatedto allow analysis and mapping of multiple data sets, which has
resultedin temporal/spatialmaps relevant to many ecologists.
As ecologistsarebeginningto extend the scalesat which contemporaryphenomena
are addressed(Hubbell & Foster, 1986; Ricklefs, 1987; Urban et al., 1987), and as
paleoecologistsare contributingto ongoingdiscussionsof ecologicaltheory (Bennett,
1988; Davis, 1986; Delcourt & Delcourt, 1987; Prentice, 1988; Huntley & Webb,
1989), the interrelationshipsof the two fields are becoming more apparent.
This paper emphasizes this interdependencewith examples drawn mainly from
REVIEW
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Fig. 1. Spatial and temporal scales at which disturbance,biotic responsesand vegetation
pattems can be detected. Paleoecologicaldata can detect micro- and macro-scaledisturbance
regimes, and are useful at all but the finest scales of biotic responses(productivityand competition) and vegetationpattems (individualorganisms).(From Delcourt et al., 1983.)
eastern North America and western Europe. A brief outline of the environmental
history of the last 1.6 million years (the QuaternaryEra)sets the stage for describing
the response of vegetation to environmentalchange duringthe last 18,000 years of
the recent post-glacialperiod. Within this context we will examine the relevanceof
recent developments in the field of paleoecologyto the fields of community ecology,
evolutionarybiology, and environmentalscience.
III. QuaternaryEnvironmentsand Vegetation
A. CLIMATICCHANGEDURING THE QUATERNARY
The QuaternaryEra encompasses the last 1.6 million years and some 24 glacial
events (van Donk, 1976), and is recognizedas 'apredominantlycold periodfollowing
the relatively warm and equable Tertiary(Nilsson, 1982). Within the Quaternary,
glacialstageshave usuallylasted 50-1 00,000 years,with a 4 1,000 yearcycle dominant
in the early Quaternaryand a 100,000 year cycle dominant after 700,000 years BP
(Shackletonet al., 1988). During glacial stages largeice sheets slowly accumulatein
high latitudesin an oscillatoryfashion. These glacialstagesgenerallyend with a rapid
decay of the ice sheets, followed by relativelywarm interglacialsof 10-20,000 years
similarto the currentHolocene Epoch,which beganabout 10,000 yearsago and may
end within the next few thousand years (Emiliani, 1972; Imbrie & Imbrie, 1986).
Thus glacial stages are periods of relative climatic stability followed by rapid environmental change and relatively brief interglacials. Shorter-termenvironmental
changes occur within this glacial-interglacialtime frame.
The alternatingwarm and cold periods of the Quaternaryare generallybelieved
to result from changesin insolation caused by cyclical alterationin the earth'sorbit
around the sun as set forth by the Milankovich theory (Bergeret al., 1986; Milan-
IMPLICATIONSOF PALEOECOLOGY
U
post,
<
w
207
future5
E
3
zW
24.00
w
ac
250
200
50
150
0
-50
100
-1OO
THOUSANDSOF YEARS AGO
Fig. 2. Changesin the geometry of the earth'sorbit duringthe last 250 ka and projected
for 100 ka into the future.Glacial-interglacialcycles roughlycorrespondto periods when the
extremesof these cycles reinforceone another.(From Imbrie & Imbrie, 1986.)
kovich, 1941). This theorydescribesthreecomponentsof the geometryof the earth's
orbit, each with its own long-term cycle, that combine to give an overall cyclicity.
Eccentricity(the degreeto which the earth'sorbit deviates from being circular)varies
from 0%to 5%or 6%on a 100,000 year cycle. Obliquity(the tilt of the earth'saxis)
changes from 22 to 24.5 degrees on a 41,000 year cycle, and primarilyaffects the
latitudinaldistributionof solarradiation.Precession(thetime of yearwhen the earthsun distanceis a maximum or minimum)altersapproximately10%on a 22,000 year
cycle, and affectsseasonality.Long-termclimatic variationis causedby the interplay
of these three cycles (Figs. 2 and 3). Climatic changesare variablebecausethe three
cycles have differenteffects at various latitudes;they also amplify and/or interfere
with each other at various times and cause time-transgressiveevents like glaciation
and oceanic heating/cooling. In general, warm and cold stages in the Quaternary
roughlycorrespondto periods when these cycles amplify one another,althoughthe
effectsof orbitalforcingare modified by surfaceboundaryconditions, positive feedback, and lag effects.
The occurrenceand durationof glacial cycles duringthe Quatemaryis well documented by the study of glacial geomorphologicalfeatures,fluctuationsin ocean and
lake levels, stable isotope ratios in ocean sediments and ice cores, planktonicassemblages in ocean and lake sediments, plant (especially pollen) and animal fossils in
lake and bog sediments, and changes in sedimentation and chemistry in peat and
lake sediments (Bradley,1985). In particular,radiometricand paleomagneticmethods of dating and stable isotope analysis revolutionizedthe field of paleoecologyin
the 1950's and 1960's (Cox et al., 1963; Godwin, 1980; Shackletonand Opdyke,
1973; Thompson & Oldfield, 1986). These methods helped to extend previous estimates of the number of glacial episodes duringthe Quaternaryfrom 4 to about 24
THE BOTANICALREVIEW
208
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1200
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Fig. 3. Global temperature(a-A)and ice volume (e) on differenttime scales shows that we
are currentlyliving at the end of a briefwarm periodin a predominantlyglacialclimate. Note
that interpretationof apparentpatternsdepends on the scale at which they are viewed. For
example, the dramaticrise in temperaturefrom 1880 to 1960 (a) appearsto be a continuation
of a steady warmingtrend since the end of the little ice age (b), which in turn appearsto be a
briefreversalof a slow cooling trendbeginningabout 6000 yearsago when globaltemperatures
duringthe currentinterglacialreachedtheir maximum. (From Webb, 1986.)
IMPLICATIONSOF PALEOECOLOGY
209
(Fig. 3). They also provided a more detailed chronologyfor these glacial cycles and
for shorter-termevents such as the environmentaland biotic changes since the end
of the most recent glaciation.
A few marine and terrestrialrecords show climatic changes at the frequencyof
Milankovitchcycles for at least the past 3 million years(Hooghiemstra,1984; Shackleton et al., 1988). This suggeststhat orbitalcycles, albeit unaccompaniedby glaciations for the most part, have occurredthroughoutearth history and have gone undetected by temporallycoarse geologic evidence (Bennett, 1990).
The nature of late-glacialand Holocene climate change is being refined through
the use of generalcirculationmodels coupled with a global networkof paleoclimatic
data (Bartleinet al., 1989; COHMAP, 1988; Kutzbach & Guetter, 1986; Webb et
al., 1987). Simulationsrun on these models are in reasonablyclose agreementwith
the paleocimatic data, and the integrationof these data and models are the basis
for the followingbrief summaryof the climatic history of North Americafor the last
18,000 years.
At the time of the last glacial maximum 18,000 years ago, solar insolation was
similar to that of the present (Fig. 4), but global climate was considerablydifferent,
with air and sea-surfacetemperaturesgenerally lower, and global air and ocean
circulationaltered.The large3 km thickLaurentideice sheet coveringmost of Canada
and the northeasternUnited States split the westerlyjet stream,which was stronger
than it is presently,into northernand southernsectors. The more southerlytrackof
the jet stream brought more moisture to the American southwest, while a strong
anticyclone centered on the ice sheet brought drier air to the Pacific Northwest.
Although the central and eastern United States were colder than at present, cold
arctic air masses were trappedbehind the ice sheet in the Arctic Basin. When this
air mass was able to flow down to the mid-continentplains duringwinter, it would
have been warmed adiabaticallyby 200 to 300C.Yet summer temperatureswould
have been cooler nearthe ice sheet. Thus the climateof the easternand centralUnited
States, although cooler than at present, may have been less seasonable and more
equable.
From 18,000 to 11,000 years BP the earth-sun distance decreased in northern
summer and the axial tilt increased,thereby increasingaveragesolar radiationand
seasonalityfor northernlatitudes. Ice sheets stabilized and began to retreatas early
as 16,000 years BP, althoughwidespreadmelting did not begin until 14,000 years
BP. By 12,000 yearsBP increasingsolarinsolation(andperhapsfeedbackmechanisms
such as bedrockrebound,increasedicebergcalving,moisturestarvationand elevated
CO2concentrations)reduced the Laurentideice sheet so that it no longer deflected
a somewhatweakenedwesterlyjet streamnorthward.Maximumratesof glacialdecay
lagged the solar insolation maximum at 11-10,000 years BP by about 1000 years,
as did maximum mid-continentaltemperatures.The transitionfrom late-glacialto
Holocene climates from 14,000 to 10,000 years BP was not continuous spatiallyor
temporally.Climatechangeacceleratedto a maximum towardthe end of this period,
and at least one significantcool interval (possibly resulting from oceanic cooling
caused by rapidly decaying ice-shelves) is recorded for many sites adjacent to the
North Atlantic Ocean before a rapid warming that marked the beginning of the
Holocene about 10,000 years BP (Fig. 3).
From 9000 to 6000 years BP summertemperaturesreachedtheir maximum of 20
to 4?Cabove present(exceptat the boundariesof the rapidlymelting ice sheet), and
thereafterdeclined to present values. During this time climatic regimes in the west
THE BOTANICAL REVIEW
210
18
15
12
9
6
3
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I
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0 ka
1
8
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00
18
15
12
9
6
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O ka
globalclimatesimulain theCOHMAP
conditionsforvariousparameters
Fig.4. Boundary
departure
frompresentvaluesfor:
tion modelforthe past 18 ka, expressedas the percentage
and December-February,
SJJA,SDIF,NorthernHemispheresolarradiationfor June-August
Landice is expressedas the
temperature.
SST,globalmeanannualsea-surface
respectively;
fromthatof 18 ka.CO2iSpartspermillionby volume,andexcessglacialpercentdifference
scale.(FromCOHMAP,1988.)
ageaerosolis drawnon an arbitrary
approached modemnvalues as temperaturesincreased, and precipitation patterns
shifted from predominantlywinter to summer monsoonal in the southwest;in the
PacificNorthwesttemperatureswerewarmerand moisturelevels lower than at present until 6000 years BP (COHMAP, 1988; Ruddiman & Wright, 1987).
Although paleoclimatic data are adequate only for North America and western
Europe,the reasonablyclose agreementof the data with generalcirculationmodels
suggeststhe following:First, complex climatic changeshave occurredover the entire
earth, driven by orbital forcing,yet modified by the interplayand feedbackof such
factors as changes in ice sheet albedo, extent of sea ice, icebergcalving, jet-stream
consequentoceanic
deflectionby ice sheets, sea surface-landtemperaturediffierences,
and atmosphericcirculation,and elementalcycling.Second,climate has variedalong
a continuum of space and time, and climatic changes may have been time-transgressive with lag effects in certain regions such as at the boundaries of ice sheets.
Third, there were past climatic regimesfor which there are no modern analogs.For
example, thereare probablyno modernclimate analogsfor North Americanclimates
from 18,000 to 12,000 BP, or for the increased seasonality in the Northern Hemisphere approximately9000 years BP (Fig. 4).
IMPLICATIONSOF PALEOECOLOGY
211
B. EVIDENCE FOR BIOTIC CHANGE
The evidence for long-termvegetationalchangecomes from fossil plant partssuch
as pollen, seeds, leaves and twigs contained in lake muds and peats, and in drier
environments,pack-ratmiddens (Berglund,1986; Betancourtet al., 1990; Birks &
Birks, 1980). Most late-glacialand Holocene vegetationaldata come from studies of
fossil pollen in which samples are taken from lake and peat sediments. The pollen
within these sediment samplesis concentrated,sub-sampled,counted, and displayed
on a pollen diagram(Fig. 5). Macrofossilssuch as seeds, leaves, and twigs offermore
taxonomicand spatialresolutionthanpollen, but areless abundantin most sediments
so that problemsof samplingand temporalresolutioncan arise. Informationderived
from fossil pollen lacks the spatial, temporal and taxonomic resolution of living
material,but offersa long-termperspectiveunavailableto ecologistswho studyliving
organisms.
The principlesof pollen analysis, brieflyenumeratedbelow, have been rigorously
tested and refinedfor over 70 years.
1. Most pollen and spores are producedin abundance.
2. Most of these fall to the groundfrom a well-mixed pollen rain that represents
the surroundingvegetation.
3. Pollen is preservedunder anaerobicconditions, for example in lake and bog
sediments.
4. Smallamountsof sediment(< 1 cm3)containenoughtaxonomicallyidentifiable
pollen (up to 106 grains)to representthe surroundingvegetationat the time the
sediment was deposited.
5. Pollen spectraobtained from several levels of a sedimentarycolumn represent
the history of the vegetation at that place.
6. Two or more series of pollen spectra obtained from differentsites represent
changesin vegetation at differentplaces.
Pollen analystshave takengreatstridesin understandingdifferentialpollenproduction, dispersal, deposition and preservation (Andersen, 1974a, 1974b; Calcote &
Davis, 1989; Clark, 1988a; Cushing, 1967; Davis, 1973; Davis & Brubaker,1973;
Havinga, 1967; Jackson, 1990; Raynoret al., 1975). Recent advanceshave also been
made in sampling,numericalanalysis and multivariatetechniques(Berglund,1986;
Birks & Birks, 1980; Clark, 1988b; Gordon & Birks, 1985; Green, 1981, 1988;
Jacobson & Grimm, 1986; Maher, 1972, 1981; Prentice & Webb, 1986; Schwartz,
1989; Webb et al., 1981). These studies have helped to define the taxonomic, numerical, spatial, and temporal confidence limits of pollen data, and have provided
methods for reducingthe complexity of the data so that pollen assemblagescan be
interpretedmore clearly with regardto stratigraphicrelationships,environmental
gradients,and rates of change.
Paleoecologistscan addressvarious ecologicalproblemsalong a broad continuum
of spatialand temporalscales throughthe carefulselection of study sites, organisms,
and field, laboratoryand analytical techniques. In general paleoecologistswant to
know about the presence, abundance, location, and temporal occurrenceof taxa,
communitiesand environments.Many studiescombine pollen analysiswith analyses
of plant/animal macrofossils, charcoal, sediment lithology and chemistry, human
artifacts,tree-rings,historical records,and modem vegetation. These studies reveal
LAKE OF THE CLOUDS, MINNESOTA
A. M. Swain 1973
Charcoal Influx
Vorve
Thickness
Conifer/Sprouter
RatRioo_
DEPTH
DATES
(cm)
A.D
.1
./
TOTAL POLLEN INFLUX
Pol Ion
_
_ _ _
>400
000
_ _
_ _
_ _
_ _ _
_ _
lA'
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-490
-4864
-40'___________-
6000
-4739
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40
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00
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0
5
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o
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or
IMPLICATIONSOF PALEOECOLOGY
213
patternsof climate change,naturaland anthropogenicdisturbance,soil development,
erosion, community composition and diversity, migration,invasion and succession
(for examples and techniquessee Berglund,1986; Birks & Birks, 1980; and Huntley
& Webb, 1988).
A few examples illustratethe spatial, temporaland ecologicalresolutionavailable
at various scales (Fig. 1). Tree-ringanalysis, vegetationreconstructionand historical
recordsprovidevery fine resolution,often comparableto data from permanentplots.
Henry and Swan (1974) and Oliver and Stephens (1978) analyzedthe growthrates
of live and dead stems on small plots (0.04 and 0.36 ha respectively)in conjunction
with historicalrecords.They detecteddisturbancessuch as fire,windthrowand treecutting,and subsequentchangesin vegetationcomposition.Manystudies(e.g.,Foster,
1988; Heinselman, 1973; Hemstrom & Franklin, 1982; Romme, 1982) have used
less intensive techniques to document disturbance and vegetation change at the
landscapelevel. Tree-ringsalso provide paleoclimaticinformation at the local, regional and global level, detectingphenomenasuch as the "little ice age" 1450-1850
A.D. (Fritts, 1976).
Annual laminations in lake sediments may provide yearly records of vegetation
and environmental change (Saarnisto, 1986; Clark, 1990). Swain (1978) analyzed
varved sediments from Hell's Kitchen Lake in north-centralWisconsin for pollen,
charcoal and seeds. He found a successional sequence of aspen, birch, white pine,
and hemlock following fires whose frequencyof 100 to 140 years was affectedby
available moisture of the prevailingclimate. He concluded that major vegetational
changeoccurredon two differenttime scales.Short-termchangesrelatedto individual
fireswere superimposedon longer4termchangesresultingfrom increasesor decreases
in the frequencyof these fires.
Basin size (in addition to morphometry,in/outflow, and surroundingvegetation)
determines to a large degree the source area of pollen falling onto the surfaceof a
lake or pond (Fig. 6). Moderatesized lakes (10 to 100 ha) receive pollen mainly from
within a 20 to 30 km radius.Finer spatialresolutionis availablethroughthe analysis
of mor humus and sediments from small forest hollows (Bradshaw, 1988). These
sites, overhungby a tree canopy that contributesabundantpollen and also filtersthe
regionalpollen rain, collect pollen from a smallersource area than ponds and lakes,
allowing stand-level interpretationsof vegetationalchange over thousands of years
in some cases (Andersen, 1970; Jacobson& Bradshaw,1981; Mitchell, 1988). Aaby
(1983) analyzedpollen and fungalhyphae from two soil profilesonly 7 m apartand
found clear differences,substantiatedby historical records, between the two sites.
Andersen (1984) analyzed pollen from a suite of small hollows and soil profilesin
the Eldrupforest in Denmark, producinga detailed history of forest change at the
stand level. Analysis of pollen from small hollows and mor humus is a well known
techniquein northernEurope,but has been adopted by North Americanpaleoecologists only recently(Davidson, 1985;Frelichet al., 1989;Heide, 1984; Schoonmaker
& Foster, 1989; Schwartz,unpubl.).Although mor humus and small forest hollows
have been thoughtto collect most of theirpollenfrom a radiusof 20 to 30 m (Jacobson
Fig. 5. Charcoalinflux, varve thickness,and pollen influx for the last 1000 years of Lake
of the Clouds,Wisconsin. Sedimentin this lake is depositedin discreteannuallaminationsor
varves, which allows for precisedating of events. A fire frequencyof 60 to 70 years (peaksin
the charcoal influx marked by stars or dates) has maintained a relatively stable vegetation
composition. (From Swain, 1973.)
THE BOTANICAL REVIEW
214
(a)
x
I
A
~
4
~~(b)
I
9.4*
A
N
20kA
(c)
a
8~~~~~~~~~
1m20m
Fig. 6. a) Aerialview of hypothetical
studyareawithpond(y) andforesthollows(x). b)
Viewof pondwithpollensourcearea(diameterA-A')andc) hollowwithpollensourcearea
(diameterB-B').(FromBradshaw,
1988.)
& Bradshaw,1981;Prentice,1985), recentevidence suggeststhat the pollen collection
propertiesof these sites vary considerably,and that extra-localand regionalpollen
may contributea substantialamountof pollen input (Calcote& Davis, 1989;Jackson,
1990; Schoonmaker& Foster, 1990). Pollen analysts must be careful to test and
acknowledgethe spatial and temporal limits of data from small hollows and mor
humus.
A combination of fine spatial scale pollen analysis and vegetation reconstruction
should producea long recordof both high spatial and temporalresolution(Schoonmaker & Foster, 1990). Reconstructionof the establishment,growth and death of
live, dead, and downed stems provides a spatially and temporallydetailed 100 to
1000 year history of vegetation dynamics. These data can be used to interpretthe
longer but less spatially precise record available through pollen analysis. Because
seeds, leaves and woody plant partsare less mobile than pollen, macrofossilanalysis
should be included in studies whose aim is high taxonomic and spatial resolution
(e.g., Jackson, 1989, 1991).
Landscape-levelphenomenacan be investigatedby selectingsmall to medium size
basins (that collect pollen from the surrounding10 to 100 km2) at distances such
that pollen sourceareasdo not overlapextensively (Gaudreauet al., 1989). Brubaker
(1975) analyzedpollen from lake sediments on three differentsoil types in Michigan
and found that the same regionalclimate change resultedin differentcommunities
on differentsoils. Gaudreau(1986) compared two sites less than 15 km apart but
with an elevational differenceof 440 m in the northeasternUnited States and found
that physiographicposition affectedthe arrivaldates of migratingplant species.Clark
(1990) combined a stratigraphicanalysis of charcoal on petrographicthin sections
IMPLICATIONSOF PALEOECOLOGY
215
from varved sediments from three small (5 ha) lakes having adjacent catchments
with a study of firescars on red pine trees. This combinationof techniquesproduced
a detailed picture of fire history and environmentalconditions for a 1 km2 area.
Radiocarbon-datedpollen spectrafrom thousandsof basins throughoutthe world
have been analyzedin the last few decades (Fig. 7). This largenumberof late-glacial
and Holocene paleoecologicalstudies has enabled the detailed mapping of the distribution of taxa throughtime on a regionalor subcontinentalscale in Europeand
North America (Figs. 8-11). Huntley and Birks (1983) presentedisopoll maps (contour lines connecting similar pollen percentages)of pollen sampled from 423 sites
in Europe covering a period from 13,000 to 0 years BP with a resolution of 500
years. Bernabo and Webb (1977), Gaudreau(1986, 1988), and Webb et al. (1983)
have drawn similar maps for the easternUnited States (Fig. 8). Davis (1981), Gaudreauand Webb (1985), and Webb et al. (1983) constructedisochronemaps (contour
lines representingmigrationalfronts of taxa at differenttimes in the past) for eastern
North America(Fig. 9). Delcourtand Delcourt(1981, 1985) and Davis and Jacobson
(1985) mapped inferredvegetationfor easternNorth Americaduringthe late-glacial
and Holocene. Most recently, Delcourt and Delcourt (1987) have produceda large
set of maps based on 162 sites in easternNorth Americathat presenttree percentages
at differenttimes during the Holocene as interpretedby modern pollen/vegetation
analogs(Fig. 10). Gaudreau(1984) and Jacobsonet al. (1987) mapped differenttaxa
simultaneouslyusing colored isopolls to reveal population centers and overlap of
differenttaxa at - 500 year intervalsover the past 18,000 years (Fig. 11). This latter
method may offerthe most insight into species interactions.A furtherstep might be
computer enhanced interpolation providing continuous output. These and other
studies carefullyselect the data for properand consistent analyticaltechnique,time
control and resolution. As more sites become available, the spatial and temporal
resolution of the maps will continue to improve.
C. VEGETATION CHANGE DURING THE
LATE-GLACIAL AND HOLOCENE
The maps of changingvegetationas implied by changingpollenpercentagespresent
the following picture:At the time of maximum glaciation 18,000 years ago, when
ice covered much of northwesternEurope, Canada and the northeasternUnited
States, the vegetation in the area immediately south of the ice sheet (the periglacial
area)consisted of species drawn from a rangeof present day environmentsand life
history strategies(pioneer,ruderalherbs, arctic-alpine,steppe).Some of these communities, which included sedges, grassesand taxa such as Alnus, Betula, Dryas, and
Artemisia,might be recognizedas tundraor steppe (Birks, 1986).
Othercommunities such as the open polardesert(dominatedby chenopods,grasses, and Artemisia)that occupied northwesternEuropebetween the ice sheet and the
Alps have no modern analogs (Iversen, 1973). The absence of modern analogs for
fossil assemblagesis not so surprisingwhen we consider that climatic and edaphic
conditions duringthe past were not merely differentfrom those of the present,but
may have combined to create unique habitats resulting in unique vegetation, for
example, spruce-poplarand spruce woodland-tundraassemblages(Huntley, 1990;
King, 1981; McAndrews, 1965; Prentice, 1986; Wright, 1987).
South of the 50- to 100-km-wide periglacialareas, steppe and galleryforests occurredin Europewith true forestsfound south of the Alps, especiallyin southeastern
0~~~~~~~~~~~
F
Cs
aclveaa.
0000n
Mide
Daa
u~~~~~~~
it~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~n
a
D
S
6
(8kaol)
Fig. 7. Location of sites in COHMAP global paleoclimate database,which does not include all rad
number of sites in the NorthernHemisphere, especially in eastern North America and western Europe
(From COHMAP, 1988.)
IMPLICATIONSOF PALEOECOLOGY
217
Europe in locally mesic refugiasurroundedby an otherwise and environment supporting steppe and possibly sclerophyllwoodland (Bennett et al., 1991; Huntley &
Webb, 1988;Van derHammen et al., 1971). In North Americasouth ofthe periglacial
area, an open spruce and larch forest dominated in the central United States well
into the Great Plains, with jack pine becoming more importanttoward the Appalachians and reachingdominance along the Atlantic Coastal Plain and southeastern
United States. Deciduous trees may have occurredas minor elements throughout
much of the conifer forest (Jacobsonet al., 1987). These deciduous species became
more importantin the lower MississippiRiver Valley and had isolated refugiain the
southern Appalachians(Delcourt & Delcourt, 1977; Delcourt et al., 1983; Wright,
1983).
As temperaturesincreased,ice sheets beganto retreatin a spatiallyand temporally
patchy mannerabout 14,000 years BP. Species on the peripheryof the glacialboundary invaded exposed substratefollowed by species from furthersouth. In Europe,
boreal species such as pine and birch invaded followed by elm, beech, hornbeam
and spruce,amongothers.Althoughmost taxa migratedfrom southeastto northwest,
each took a somewhat differentroute, and moved at a differentand variable rate
(Huntley & Birks, 1983). Spruce,a late-comer, migratedalong a double front both
westwardinto centralEuropeand northwestinto Scandinavia.Birch and pine have
continued to migrate southwardalong the Scandinavianpeninsula in the last few
thousand years.
As the ice retreatedin eastern North America, tundra species preceded spruce,
jack pine and balsam fir, which moved northwardat differentrates and in different
directions,creatingopen and then closed conifer forests. Furtherto the south, white
pine and hemlock (Fig. 9) migratedup the Atlantic seaboard,while oak appearsto
have migrated north-northwestwardalong a broad front. Following these species,
elm, maple,beech and chestnutappearto have moved in that orderfromthe southern
Mississippi River Valley or further west, while hickory diverged into two fronts
apparentlysplit by the higher Appalachians.Not all species migratedin a general
south to north direction. Duringthe late-glacial,ash appearsto have been restricted
to small microhabitatssouth of the ice sheet in the midwest. In the late-glacialto
early Holocene, ash expanded its range, becoming a component of a spruce-ashpoplarassemblage,and then partof an ash-ironwood-elm asemblage,beforeits range
began to shrinkagainabout 10,000 years BP. Duringthis time the geographiccenter
of ash's rangeremainedrelativelyfixed,just to the south of the GreatLakes(Jacobson
et al., 1987) (Fig. 11). Birch appearsto have expanded to the southwest from the
northeasternUnited States and then later northward(Gaudreau,1988) (Fig. 8). As
in Europe,each species took a somewhat differentroute (generallynorthward)and
migratedat variable rates (Davis, 198la) (Table I). When we inspect the isochrone
maps (Fig. 9), we can clearly see that differenttaxa migratedat differentrates, from
differentorigins and in differentdirections. Hickory invaded deciduous forests in
Ohio 4000 years before beech, but beech arrived in southern New England 3000
years before hickory. Beech and hemlock arrivedin the AppalachianMountains at
about the same time, but hemlock migrated more quickly thereafterand reached
centralNew Englandand upper Michigan 2000 and 2500 years respectivelybefore
beech. Some species such as chestnut were still migratingnorthwardsas recentlyas
2000 years BP, thousands of years after a well-developed forest had become established and climatic cooling had begun. Most species have retractedtheir ranges
somewhat to the south in response to this recent cooling (Davis, 1983). The inde-
0
:
8~~~
112
~~~
w
IMPLICATIONS
OFPALEOECOLOGY
L
a 0B O
Fm
I
12 1
I
219
I
oto
o~~~~~~~~~~~~~~~~~~~~~~~~~~i
0
Fig.
9.ThemigraionfrontofHemlock
o
400km
40k
6
6
10~~~~~~~~~~~~~~~~~~~~1
10
15
dentoto
Chestnut
0
eoltonr
400 km
Fig. 9. The
isochrone
migrationfrontof
hemlock,
tionsfor
and chestnut
depicted
by
coloicaltheor.
Wht
wesee beech,
odayhickory,
s meely
sna-sho
ofy
aSynm
ic
maps (12 ka to present)with lines connectingarrivaldates of taxa as inferredfrom increases
in pollen or presenceof macrofossilsat 1000 yearintervals.Heavy lines show presentor recent
rangeof taxa. (From Davis, 1982.)
pendent response of organisms to changing environments has far reaching implications for ecological theory. What we see today is merely a snap-shot of a dynamic
and changing environment and biota that have come about through evolutionary
and historical forces operating on several temporal and spatial scales.
Fig. 8. Isopoll map for birch pollen 14 ka to presentwith contourlines connectingvalues
of 5%(stippled),20%(diagonallines), and 40%(black)birchpollen. Birchappearsto have had
a more northerlyrangeduringthe last glacial period than many other arborealtaxa. Ice sheet
appears in the northem part of 14, 12 and 10 ka maps. Dots are fossil-pollen sites. (From
Gaudreau, 1988.)
THE BOTANICALREVIEW
220
*
S
TRU
40
A600pr L-P
A.THWNTREOFTE
ITI
LP
CMUNT
The nature of the biotic community has long been a source of disagreement and
controversy among ecologists (Pickett & Davis, 1990). The "organismal" concept of
Clements (I1916)was criticized by ecologists who recognized the individualistic nature
of species within assemblages (Cain, 1947; Gleason, 1917, 1926; Livingston &Shreve,
192 1; Ramensky, 1924) and the continuous (vs. discrete) distribution of species along
environmental gradients (Curtis, 19 55, 19 59; Curtis & McIntosh, 19 51; Whittaker,
1951, 1956, 1975). It is interesting to note that Clements recognized the importance
of disturbance in structuring some communities (Clements, 1910), and was well aware
of the paleoecological perspective (Clements, 1924).
Recent definitions of "communitiy'' have been purposely vague, ranging from "6a
set of species populations that occur in some place" (Schoener, 1986) to "all orpanisms
in an area" usually restricted for the convenience of the researcher "spatially, trophically, taxonomically or by life form" (Roughgarden & Diamond, 1986). A temporal
restriction is not mentioned, but is often implicit (i.e., the time period during which
the community is studied).
IMPLICATIONSOF PALEOECOLOGY
. ...
*
Fig. 10.
t
IWr --;- l,\
221
wA .
~~~~m -TOP
~~t ::m
Continued.
Severallines of paleoecologicalevidence suggestthat plant communities are loose
and relatively ephemeral assemblages of species populations. The individualistic
response of tree species to climatic change described above, and the apparentease
with which species migrate into and subsequentlyleave assemblagesdemonstrate
the absence of cohesive communities. For example, chestnut invaded the mature
hardwoodforest of the northeasternUnited States 2000 years ago with only modest
readjustmentsin the densities of resident species (Davis, 1983). When chestnutwas
eliminated as a canopy dominant in these forests in the first half of this century,
other species, especially oaks and hickories,merely increasedtheir abundance(Keever, 1953; Mackey & Sivic, 1973). The sudden and temporarydecline of hemlock
in North America, and of elm in Europearound 5000 years BP also resultedin the
increaseddominance of remainingspecies (Allison et al., 1986; Davis, 198lb, 1983;
Iversen, 1973). Both hemlock and elm recoveredwithin a few hundredto a thousand
years and rapidlyregainedtheir formerabundance.Thus forests are relatively open
communitiesthat may sufferapparentlylittle reorganizationor ecosystem disruption
upon the invasion or decline of a dominant species.
The ephemeraland transitorynatureof plant communities is demonstratedby the
independentmigration of species and by historical changes in abundanceand distribution of modern species assemblages. The forests with which we are familiar
222
THE BOTANICAL REVIEW
14
ka
8 ka
ka0 12
6Ska
0
k-
Oska
Fig. 11. Isopoll maps, 14 ka to 0.5 ka, with ice sheet to north stippled.Elm (lightgrey),ash
(heavy outline), and ironwood (symbol < >) showing overlappingranges(e.g., dark grey for
elm-ash). The rangesof all three taxa have decreasedthroughoutthe Holocene, to the point
that their presentrangesmight be consideredrefugia.(From Jacobsonet al., 1987.)
today are composed of species that arrived at differenttimes from various glacial
refugia,respondingindependentlyto changingclimatic and disturbanceregimesand
biotic interactions.As a particularspecies migratedin response to climatic change,
it would have encounteredother species to form dynamic and constantly changing
assemblages.At the height of the last glaciation, a sprucewoodland-tundraassemblagewith no modernanalogexisted in the centraland easternUnited States(Wright,
1984, 1987); several thousand years later, the hardwood(black ash-ironwood-elm)
component of this community was left behind as conifers moved northward(Jacobson et al., 1987). By the time most tree species were migratingnorthward(12,00010,000 yrs BP), ephemeral communities, such as the black ash-oak-spruce-graminoid assemblage in north central Illinois (King, 1981) were not uncommon. The
animal fossil record also has many examples of no-analog assemblages or what
Graham (1986) calls "interminglings"of species that have separate ranges today.
IMPLICATIONSOF PALEOECOLOGY
223
Table I
Averagerates of late-glacialto Holocene rangeextensions in easternNorth America
(modified from Delcourt & Delcourt, 1987)
*
Genus
Rate (m/yr)
Salix
Populus
Betula
Tilia
Tsuga
Larix
Fagus
Abies
Picea
Juglans
Pinus (Northerngroup)
Ulmus
Acer
Quercus
Fraxinus
Carya
Castanea
Pinus (Southemgroup)
Nyssa
287
263
212
209
202
189
169
159
141
140
135
134
126
126
123
119
100*
81
70
From Davis, 1982, roughlycomparableto data from Delcourtand Delcourt, 1987.
This is not to say that all speciesassemblageshave been ephemeralat all times. There
are examples of constancy over long periods of time (Delcourt & Delcourt, 1987;
Green, 1982; Jacobson& Grimm, 1986), but in general,changeappearsto be more
characteristicof the late-Quaternary.
B. EVIDENCE OF DISTURBANCE IN THE
PALEOECOLOGICALRECORD
We define disturbancehere sensu White and Pickett (1985) as "any relatively
discreteevent in time that disruptsecosystem, community or population structure,
and changes resources,substrateavailability or the physical environment"and include both destructiveevents and environmentalfluctuations.Most ecologists consider disturbancein a diurnal, seasonal, decadal, or centennial time frame, yet the
disturbanceregime (sensu Heinselman, 1981) of destructive events and environmental fluctuationswith profoundeffects for the vegetation of an area is best characterizedover longertime periods.The use of the long paleoecologicalrecordenables
ecologists to avoid the space-for-timesubstitutionthat, althoughemployed by most
successionalstudies, may be highly misleadingbecause of the difficultyof assuring
between-siteuniformity(Clark, 1990; Pickett, 1988).
Naturaldisturbancesof varyingsize, frequencyand severityhave beendocumented
throughoutthe paleoecologicalrecord.Fires may be detectablein sediment as charcoal layersand as peaksin charredparticlescountedon slides preparedfrom sediment
(Clark,1988a, 1988b;Pattersonet al., 1987) (Fig. 5). Geomorphologicalfeaturesand
sediment lithology record erosional events (Campbell& Fraily, 1984), and tephra
layers indicate volcanic eruptions (Sarna-Wojcickiet al., 1983). Sudden changes in
pollen assemblagesoften correlatewith catastrophicevents or anthropogenicactivity
224
THE BOTANICALREVIEW
I
,I T p-w1
CI
0
Woodland only
0
W
O
or woodland
0
~
I
?
~~~~Prairie
only
tw
Time
period
--
Fig. 12. SchematicdiWam showinga two-dimensionalcrosssectionofthe subspacesin which
prairieand oak woodland will persist in an n-dimensionalhyperspacein which the axes are
time period and the independentclimatic variablesinhuencingfire frequency.The curves are
the thresholdsfor the changesfrom oak woodland to prairieand the reverse from prairieto
oak woodland. (From Grimm, 1983.)
(Brugam,1978; Green, 1981). The paleoecologicalrecordalso reveals changingpatterns of disturbanceas environmentalconditions change.Grimm (1983) documents
the invasion of oak woodland in southernMinnesotaby more mesic tree species as
the result of decrasing fire frequencycaused by the cooler, moister climate of the
"littleice age."He presentsa model suggestingthat climate,physiography,vegetation
pattern and fire interactto cause thresholdsfor vegetation change to vary in space
and time (Fig. 12). Thus differentvegetationtypes can persistadjacentto one another
in essentiallyidenticalenvironments,their locations bemighistoricallycontingenton
past disturbances.
The paleoecologicalrecord allows us to better understandthe spatial as well as
temporal dynamics of disturbance.The extent and nature of discrete and periodic
events may not be observable over an entire landscapeor region in a lifetime. For
example, in southern New Englandthe overall frequencyof fire is relatively low.
However, studies of charcoaland pollen along a transectof sites suggeststhat a very
stronggradientof decreasingfire frequencyfrom the coastal oak-pine forests to the
IMPLICATIONSOF PALEOECOLOGY
225
northernhardwoodforest parallelsa climate/vegetationgradient(Patterson& Backman, 1988; Winkler, 1986). The history of coastal oak-pine vgetaion repeatedly
returningafterfireillustratesanotherpoint:certaintypes ofvegetation areremarkably
resilient
Recent and paleoecologicalevidence confirm that the regionalvegetation mosaic
in communities with fire-dependentspecies appears to have been maintained by
naturallyrecuning fires(Cwynar,1987; Habeck & Mutch, 1973; Heinselman, 1973;
Minnich, 1988;Romme, 1982;Swain, 1978).In these communities,fireis responsible
for long-termstability;the lack of fire,owing to climatic changemight be considered
more of a disturbanceto the overall system, causingwidespreadspecies replacement
(Grimm, 1983). Conversely,dwing a time of climatic change,a community showing
some inertiamay be opened up to invasion by fire.Green (1982) documentedrapid,
long-term species replacementfollowing fire in the early Holocene in Nova Scotia.
Disturbancein tropical areas has been characterizedas less severe than that in
temperate regions (Connell, 1978; Whitmore, 1975), but there is evidence to the
contrary,such as hurricanesthat have their genesis in the Tropics of Cancer and
Capricorn(Lugo et al., 1983; Wadsworth& Englerth,1954), and the 1983 El Ninio
event that unleased torrential rains along the eastern Pacific rim but resulted in
droughtand fire in the western Pacific (Leighton& Wirawan, 1986). Nevertheless,
stand-leveldisturbancein the tropics has received more attentionin the last decade
(Brokaw, 1985; Denslow, 1980; Liebermanet al., 1985; UhI et al., 1988).
The paleoecological record further extends the evidence for disturbancein the
tropics at several scales. Catastrophicdeluges of rain along the west coast of South
America even strongerthan that in 1983 have occurredperiodicallythroughoutthe
Holocene and are especiallyevident in areasof human settlement(Nialset al., 1979).
Sedimentologicaland geomorphologicalevidence suggestsequallycatastrophicflooding in the Amazonbasinduringthe Holocene(Campbellet al., 1985;Liu&Colinvaux,
1988). River channel meanders revealed by satellite photographyhave inundated
forestswith the resultthat 12 percentof the forestalongriversin the wester Amazon
is successional(Salo et al., 1986). Another 14.6 percent lies on the floodplain and
has been disturbedin the recent past. Thus a quarterof this region of the Amazon
basin has been disturbedby flooding probablywithin the last few hundred years.
Evidence of fire has been found in tropicalrainforestsdated back to 6000 years BP
(Saldarriaga& West, 1986; Sanfordet al., 1985).
Past human activities have had major impacts on the landscapeand often serve
as convenient time horizons. Neolithic man most likely caused the sudden decline
of elm in Europe around 5000 years BP by girdlingtrees and using the foliage for
animal fodder (Birks, 1986; Iversen, 1973). North American Indians are generally
believed to have had a sophisticatedunderstandingof the use of fire as a tool for
hunting and clearing land (Day, 1953; Patterson & Sassaman, 1988; Pyne, 1982).
Human presence drasticallyaltered vegetation in parts of central America during
Mayan times (Binford& Leyden, 1987). A distinct settlement horizon of Ambrosia
in most palynologicalrecords throughoutNorth America is a consequence of the
clearanceof forests for agricultureby colonists from Europe.
C. COMMUNITY STABILITY AND EQUILIBRIUM
With the recognitionthat disturbanceis an integralpart of many ecosystems and
that climate has been changingcontinuously in the past, ecologists have begun to
226
THE BOTANICALREVIEW
debate the relative importance of equilibrium vs. nonequilibriumexplanations of
communitystructure(Connell,1978;Hubbell& Foster, 1986;Pickett& Davis, 1990).
Organismsencounterand respond to environmentalchanges throughoutthe entire
spectrumof spatialand temporalscales. Indeed, we must firstdefine the taxonomic,
spatialand temporalscales at which we wish to investigatequestions of stabilityand
equilibrium.We might ask if two populations are in equilibriumwith each other.
Or whetheran entire community is at an equilibriumstate. Or whethera species is
in equilibriumwith its environment.The answermay be positive at one intersection
of the various scales under considerationand negative at another.
A furthercomplication with pollen data is determininghow to quantify stability
and change. Paleoecologists traditionallyhave recognized different pollen assemblagesor zones, and numerousmultivariatestatisticalapproacheshave been adapted
to allow for more objective zonation (Gordon& Birks, 1985). Jacobsonand Grimm
(1986) and othershave applieddetrendedcorrespondenceanalysis(DCA) to modem
and fossil pollen data matrices. With appropriateapplication,the results show the
rate and direction of change (Figs. 13 and 14). Using this method, Jacobson et al.
(1987) have shownthat the highestratesof widespreadvegetationalchangein eastern
North America occurredat two discrete times: 11-9000 years ago, reflectingrapid
migration of taxa in response to climate change, and since European settlement,
reflectinga response to pervasive anthropogenicdisturbance.
An ongoingdebateamongbiogeographersand paleoecologistsconcernshow closely
vegetationtracksenvironmentalchange.The consensus seems to be that the answer
depends on the time scale under consideration,the rapidity and amplitude of the
climate changeand the particularvegetationunderinvestigation(Prentice,1986). At
a frequencyof 1000 years, vegetation usually appearsto be in dynamic equilibrium
with climate (Birks, 1989; Webb, 1986). On shortertime scales, between 1 and 500
years, vegetation may often lag behind climatic change (Davis, 1986).
In an environmentthat varies between long periods of glacial stasis and relatively
abrupt interglacials,organisms must cope with conditions that may favor stable
equilibriumcommunitiesduringglacialtimes and moredynamiccommunitiesduring
interglacials.The paleoecologicalrecordprovides evidence that communities in the
past and present have been both stable and unstable, and in disequilibriumand
equilibriumstates(e.g.,Delcourt& Delcourt, 1987;Green, 1981;Jacobson& Grimm,
1986; McAndrews,1965; Payette & Filion, 1985; Ritchie, 1986). However, ecologically dynamic and nonequilibriumcommunities probablyhave been more characteristic throughoutthe Holocene as species migratedindependentlyin response to
climatic change.
The independent migration of tree species and their penetration of established
deciduous forests in the northeasternU.S., the hemlock and elm declines, and the
transient nature of species assemblagesin general demonstratethe disequilibrium
nature of communities during the Holocene. Time lags shown by organismsin response to environmentalchange are evidence for both stability (in the sense that
organismsremainwherethey are-inertia, sensu Gorham, 1957) and disequilibrium
(in the sense that other organismsjoin or departfrom the assemblage).Cole (1985)
has shown that some plant species growingnear the GrandCanyon at the beginning
of the Holocene were slow to move northwardwith warmerclimatesand werejoined
by fastermoving arrivalsfrom the south resultingin a transientspecies-richassemblage. Davis and Botkin (1985) have modeled tree species' responses to climate
changein computersimulationsand predicted200 year responselags. Davis (1986)
IMPLICATIONSOF PALEOECOLOGY
227
-0.2
0.0 _
Oucus-Gromho
oN
/D
0.2 0.4
O0.6
Gr ~~AdU1~O4K
?0
0
0u
5k
*
K
~~~~BeAWi-Cyperoceae
BezPirws
0.8
1.0~~~~~~~~~~~~~~~~~~~~
1.0 _
12
1.4
AXISI
Fig. 13. Directionand rateof vegetationchangein centralMinnesotaduringthe past 10,000
years. Ordinationof 100 yearsBP analogpollen assemblages(open and half-filledcircles),and
a five-samplerunningaverageof fossil pollen for the last 10,000 yearsfrom Billy'sLake,central
Minnesota(dots connectedby line). Points on the runningaveragecurverepresentthe position
every 100 years (ratherthan the position of each sample).(From Jacobson& Grimm, 1986.)
has also summarizeddata showing that various animals includingbarnacles,birds,
insects and small mammalshave laggedanywherefrom a few yearsto decadesbehind
environmentalchanges. Migrationmaps do show, however, that presenttemperate
communities probablyhave been roughlyin equilibriumwith the environment on
a regionalscale since about 2000 years BP.
If instability and disequilibrium are characteristicof the Holocene, models of
populationand communitydynamicsbased on equilibriumconditions,while heuristicallyuseful,may be even less realisticthanempiricalecologistsoftenclaim (Franklin
et al., 1987). Most models of population growth,competition and predationignore
migrationfor simplicity'ssake,but the paleoecologicalrecordsuggeststhat migration
and dispersalare fundamentalto understandingthe above phenomena.Thus modelers should be exlicit about the time and space scales under consideration,and recognize that models of short time-scale phenomena may not extrapolateto longterm ecological or evolutionarytime scales.
D. EVOLUTIONARY CONSIDERATIONS
Paleoecologicaldata from the Quaternaryshould be of considerableinterest to
those concerned with rates of evolution and speciation, and related processes includingthe evolution of diversity,coevolution,selection,competition,and extinction.
Bennett (1990) has even suggestedthat Gould's (1985) three-tiertemporalhierarchy
of evolutionary processes should be expanded to include glacial-interglacialcycles,
thus yielding a four-tierhierarchyconsistingof: ecologicaltime (thousandsof years),
228
o
0
THEBOTANICAL
REVIEW
0.20
c1 0.15S
0
o 0.10
z
w
o- 0.05
z.
0j
1
2
3
4
5
6
7
8
9
10O
AGE(IOOO14Cyr BP)
Fig. 14. Rate of vegetationchangein centralMinnesotaduringthe past 10,000 years.Graph
of distance(numberof standarddeviations)moved every 100 yearsin the firstthreedimensions
of the ordinationdepicted in Figure 13 vs. age. Greaterdistance indicates greaterchange in
pollen spectrain 100 years. The graphshows high rates of vegetationchangefrom 10 to 8 ka,
as foresttaxa were replacedby prairietaxa (see Fig. 13), relativestasis from 8 to 4 ka, and then
higherrates of changeagain as differentforest taxa invaded from 4 ka to the present.Highest
rates of changeare associatedwith settlement.(From Jacobson& Grimm, 1986.)
orbitallyforcedclimate changes(20,000-100,000 years),geologicaltime (millions of
years), and mass extinctions (approximately26 million years).
Viewed over the entire Quaternary,taxa have constantlymigratedin responseto
glacial-interglacialcycles, often resultingin alternationbetween widespreadpopulations in favorable times, and small, fragmentedpopulations during unfavorable
periods.These oscillationsin populationsize, locationand geneticcompositionshould
favor rapid evolution throughgeographicisolation and founder effects (Huntley &
Webb, 1989).
The rapidmigrationof tree species duringthe Holocene providesexamplesof how
allopatric speciation by geographicalisolation might occur. Migration maps show
taxa moving in two differentdirectionsat differentrates (e.g., Figs. 8 and 9). Rapid
speciationduringthe last glacial-interglacialcycle is suggestedin the paleoecological
record,e.g., the recentdivergenceof jack pine and lodgepolepine (Critchfield,1985;
Cwynar& McDonald, 1987). On a longer time scale, sprucehas become rarersince
the early interglacials,failing to reach Englandduringthe Holocene, while hazel has
become more common. West (1980) speculatesthat these trends are likely a result
of evolutionarychangesin thesetwo speciesbasedon the observationthatabundances
of other species have remainedroughlysimilarduringQuaternaryinterglacials.Mengel (1964) shows how modern species of wood warblerscould have arisen from
repeated migration of an ancestral species northwestwardfrom the southeastern
United States during interglacialsand subsequentisolation of western populations
upon glacialadvance.These examplessuggestthat environmentalchangesthat induce
varyingratesand directionsof migrationand other individualisticresponsesplay an
importantrole in determiningrates and directionsof speciation.Webb (1988), however, points out that the time scale of glacial-interglacialcycles may be too short for
manypopulationsto develop majormorphologicaldifferencesand becomegenetically
isolated.
Speciesinteractionsshould be profoundlyaffectedby glacial-interglacialcycles. In
contrastto a more stable environmentwhere species may have time to adapt to one
IMPLICATIONSOF PALEOECOLOGY
229
another, species reacting individualisticallyto oscillating environments may form
looser associations.A particularspecies migratingnorthwardin the Holocene would
have competed with several species along its route, and even may have occupied
various ephemeral communities in different places at different times or simultaneously. Thus while speciation may be facilitatedby repeatedisolation and contact
of populations and species, the individualistic nature of species responses implies
diffuseinteractionswith less time for coevolutionaryrelationshipsto develop. Furthermore, some microevolutionarychangesthat arise in a stable environmentmay be
lost upon widespreadenvironmentalchangeduringmore dynamicphases of glacialinterglacialcycles (Bennett, 1990). Diffuseinteractionsand loss of microevolutionary
changes may be common to both temperateand tropical regions, since widespread
environmentalchange has been documented for both areas.
This last point is particularlyrelevant to current thinking on the evolution of
diversity in the tropics. If species responses are as individualistic in the tropics as
they are in temperateregions, and species interactionsare at times diffuse,then the
hypothesizedevolution of diversitythroughcompetition and niche differentiationin
a stable environment becomes questionable as does the importance of long-term
coevolutionaryrelationships.Instead, speciation by repeatedgeographicalisolation
wouldbe the drivingevolutionaryforce.Van Devander(1986) speculatesthatbecause
climatic fluctuationsare less severe towardthe equator,species with refugiaat lower
latitudes are less likely to go extinct. Hence they should survive to speciate allopatrically,with intense competition after sympatryleading to niche separationand
high diversity in the tropics. Alternatively,the majorityof evolutionarydifferentiation could have takenplace in stableenvironmentsbeforethe onset of the Quaternary
glacial-interglacialoscillations.Morepaleoecologicaldataareneededfromthe tropics
to test these hypotheses, but we must recognize the limits of the paleoecological
recordin this regard.Any searchfor the ghost of competitionpast is likely to produce
only circumstantialevidence. Fine resolutionpollen analysismay suggestcompetitive
interactions(Bennett, 1986), and paleoecologistshave appliedcompetitionequations
to fossil pollen data (Delcourt& Delcourt, 1987). But with the difficultyof defining
competition and proving it has happened in extant populations, it is unlikely that
most sites provide the taxonomic and spatial resolution to infer mechanisms of
competitive interactionsusing pollen data as a proxy for biomass or reproductive
success. We can, however, document that taxa occurredin the same locality at the
same time, that their abundanceschanged,and that their rangesdiverged,separated,
and coalesced. This provides us with circumstantialevidence that competition or
speciation by geographicalisolation has had the chance to occur, and with a timeframe within which to place these processes.
Given the fluctuatingenvironments of the Quaternary,we can probablyassume
that selection pressureand direction on species have been in a state of flux. During
a long stable glacialperiod competition for resourcesmay be of primaryimportance
in areas where the environment is not extreme. But during a rapid environmental
change,ability to adaptquicklyor disperseto more suitablehabitatsmay be favored.
This should be true in the tropicsas well as in temperateregions.The variablenature
of selection pressure suggests that those species (especially plants) that are most
flexible in their response to environmentalchange may have been favored during
the Quaternary.Some organismsthat exist today may have been able to rapidlyalter
theirlife historystrategiesto adaptto changingenvironmentsand selectionpressures.
This would be possible in populationswherealleles for alternatelife historystrategies
230
THE BOTANICALREVIEW
exist in a small percentageof the population.However, some strategiessuch as rapid
dispersalmight be at least adequateat all times, and other more stress tolerantand
competitive strategies(sensu Grime, 1977) might be viable for organismsinhabiting
regions where the environment has fluctuatedless rapidly or severely than others.
But the recordof environmentaloscillation duringthe Quaternaryindicates that, in
general,those species or populations unable to respond rapidly to a changingenvironment should be the ones most likely destined for extinction.
The paleoecologicalrecord also demonstratesthe stochasticand historicallyconstrainednatureof evolution and community structure.The structureof some communities can be traced in part back to climatic fluctuationsthat began and ended
hundredsof years ago (Clark, 1990; Grimm, 1983) (Fig. 12). Periodic global events
with regionallycatastrophiceffects, such as the 1983 El Niino event (as yet poorly
understoodon a paleoecologicalscale) may have importanteffectson the evolution
of species (Gibbs & Grant, 1987). Species also may migrateinto areasthat constrain
their evolutionary future. An example of this is the cloud forest of Amazonian
affinitiesthat is isolated in pocketsalong the westernfoothills of the Andes (Koepcke
& Koepcke, 1958).
Tree species diversity in Europe is much lower than in eastern North America,
quite possibly because the east-west trendingAlps trappedspecies retreatingsouthwardfrom advancingglaciersand causedtheir extinction,while small relict southern
populations also became extinct (Bennett, 1991; Davis, 1983; Reid, 1935). Slowmigratingand late-arrivingspecies were probablyunable to spread throughoutthe
British Isles at the beginning of the most recent interglacialbecause the English
Channeland IrishSea were floodedabout 8000 yearsBP by the risingAtlanticOcean
(Birks, 1989; Godwin, 1975).
Becauseof the complexnatureof past climatechangeand biotic response,historical
constraintshave probablycompounded in many cases so that it is highly unlikely
that species responses to past glacial-interglacialcycles have been identical to responsesduringthe most recentWisconsinglacial-Holoceneinterglacial.Speciesprobably encounteredvarious competitors, predators,mutualists and pathogensduring
differentglacial stages that have influenced their course of evolution. West (1970)
presentssome evidence for differentcommunity composition duringeach of the last
7 interglacials(TableII). Thus, on an evolutionarytime scale, we might think of past
climaticallydynamicinterglacialsas times when the biotic cardsarereshuffled,setting
the stage for glacial periods when species in more stable assemblageshave a better
chanceof playingan extendedgame. If Milankovitchcycles do indeed extendbeyond
the Quaternary,this scenariomight be characteristicof biotic interactionsthroughout
earth history.
E. REFUGIA IN THE PALEOECOLOGICALRECORD
The individual nature of species as revealed by the paleoecologicalrecord has
importantimplications for refugetheory, which has gained more attention with the
acceptanceof wide-spreadclimatic changein the tropicsduringthe Pleistocene(Binford & Leyden, 1987; Colinvaux, 1987; Van der Hammen, 1988; Walker& Chen,
1987). The refuge hypothesis states that climatic oscillations have caused species
rangesto contractand expand, and has focused on tropicalrainforestspecies whose
ranges are thought to have contractedand fragmentedduring the cool, dry glacial
periodsand expandedand agglomeratedduringwarm,moist interglacials(Whitmore
231
IMPLICATIONS OF PALEOECOLOGY
Table II
Occurrencesof generain fossil deposits from seven interglacialstages,includingthe
Holocene, in England(modified from West, 1970)
stage
Interglacial
Genus
Quercus
Ulmus
Tilia
Carpinus
Picea
Abies
Tsuga
Ludhamian
Antian
Pastonian
Cromerian
+
+
+
+
+
+
-
-
-
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
+
+
+
IpHoxnian swichian Holocene
+
+
+
+
+
+
+
+
+
+
+
+
+
& Prance, 1987). Thus forest communities would have been constantly changing,
and the opportunityfor speciation by geographicisolation would have arisen many
times duringthe Pleistocene.Various areashave been identifiedas probablerefugia
in the Amazon, based on present diversity, endemism, contact zones and climatic
conditions (Prance, 1982). This approachis probablytoo simplistic in light of what
we know of long-term forest dynamics in temperate zones and the complexity of
climatic change. Global climate simulations of full glacial climate suggest, for example, that much of the tropics was cooler, but that rainfallwas similar or slightly
less than at present,with decreasedseasonalityand less intense summerrains in the
southerntropics (COHMAP, 1988). These climate simulationsdo not disprove the
refugehypothesis, but neither do they presentcompelling evidence for locating specific refugialareas in the tropics.
The paleoecologicalevidencefor rainforestcontractionand expansionin the tropics
is scanty. Walker and Chen (1987) compiled a list of 118 sites that have been
investigatedby pollen analysis in the tropics (Table III). Only 24 of these sites are
from lowlandeverwetrainforests,and of these sites, 13 contain less desirablerecords
fromswampsand riverinelakes.None of these containevidence of full-glacialrefugia.
In the Amazon, for example, there is no complete palynologicalrecord back to
full-glacialtimes for lowland rainforest.However, Liu and Colinvaux (1985) producedradiocarbon-datedevidencefor a 700-meterloweringofthe lowlandrainforestlower montane rainforestboundarybetween 33,000 and 26,000 years BP. Absy and
Van der Hammen's (1976) pollen diagramfor Rond6nia shows that land currently
supportingseasonal rainforestwas occupied by savanna sometime in the early Holocene. Hence, the palynologicalevidence for lowlandrainforestrefugiain Amazonia
is circumstantial,suggestingonly that refugia probably occurred. Paleoecological
evidence does suggest,however,intermediatedisturbanceprocessessuch as flooding,
erosion,and fireas possiblemechanismsforgeneratingdiversity(Campbell& Frailey,
1984; Colinvaux et al., 1985; Salo et al., 1986; Sanfordet al., 1985; Saldarriaga&
West, 1986).
Elsewhere,some lowland rainforestin Africa occupies what was savanna or dry
woodland only 10,000 years BP (Livingstone, 1975), and an apparentPleistocene
refugehas been documentedfor montane rainforestin east Africa (Hamilton, 1982).
In northeasternAustralia, savanna woodland changed to seasonal rainforestfrom
10,000 to 5000 years BP (Chen, 1986; Kershaw, 1983). Clearevidence exists for the
232
THE BOTANICALREVIEW
Table III
Numberof radiocarbondatedpollen recordsfor non-aridenvironmentsin the tropics
(modified from Walker& Chen, 1987)
Lowlandeverwet rainforest
Lowlandeverwet swamps and riverinelakes
Lowlandseasonal forest
Otherlow to mid altitude forestsand woodlands
Savanna,shruband grassland,some secondaryforest
Lowermontane rainforest
Upper montane and subalpineforest
Subalpineand alpine nonforest
Total
11
13
7
7
6
28
17
29
118
loweringof forest zones in the upperAmazon, centralAmerica,tropicalAfrica and
southeastAsia duringglacialtimes (Bushand Colinvaux, 1990; Flenley, 1979, 1984;
Hamilton, 1982; Liu and Colinvaux, 1985).
The tropics,especially,requireextensivepalynologicalandgeologicalinvestigation,
such as the ongoingwork of Colinvaux(1991) and co-workersin the Amazon basin,
before any hypothesis of refugia composition, size, location, and history can be
confidentlyaccepted (Campbellet al., 1985; Colinvaux, 1987; Connor, 1986). Examples of testing the locations of proposed refugiaare provided by Campbellet al.
(1985) who found one proposed refugium overlain by Holocene alluvial deposits,
and by Bush and Colinvaux (1990) who producedpalynologicalevidence suggesting
climatic cooling inconsistent with proposed refugialareas in Panama.
In summary,no tropicalrainforestrefugiahave been documented,and some postulatedrefugialareashave been disproven.Manypollen studieshave found,however,
that non-rainforestvegetationwas more widespreadduringthe late-glacial,implying
that rainforestrangeswere restricted,perhapsto the point of functioningas refugia.
Althoughmost interestin the refugehypothesishas been in connectionwith tropical
rainforestspecies, refugia (or at least sites where sparse populations occurred)are
best documented by paleoecologicalevidence in temperateregions. The proposed
Mixed Mesophytic Forest refugium,believed by Braun (1950) and others to have
included most tree species presentlygrowingin eastern North America, has never
been documented in the paleoecologicalrecord. Instead it appears that taxa had
various refugiaindependentof each other (Davis, 1983; Delcourt& Delcourt, 1987).
Refugia may have occurredin some surprisingplaces. For example, white pine and
hemlock appearto have had a refugiumlocated partiallyon the continentalshelf of
the Carolinaswhich was above sea level duringglacial times (Davis, 1983). Many
hardwoodsmay have occurredalong the bluffs of the southernMississippi Valley,
and in isolated pockets of suitable habitat in the southernAppalachians(Delcourt
& Delcourt, 1977; Delcourt et al., 1983).
The higherdiversityof tree species in easternNorth Americacomparedto Europe
generally has been attributed to tree species being able to migrate independently
down the Appalachianmountain chain and find appropriaterefugiathroughoutthe
south. Europeantrees on the other hand, would have had to migratesouthwardover
or aroundthe Alps duringclimatic deterioration(Campbell,1982; Davis, 1983). The
difficultyof reachingproposedrefugianear the Mediterraneanand Asia Minor may
have resultedin a significantloss of species. Bennettet al. (1991), however, maintain
that species repeatedlybecame extinct in their northern ranges upon initiation of
IMPLICATIONS OF PALEOECOLOGY
233
glacial stages, while relict populationssurvived in the south. Thus the crucialfactor
in the elimination of a species would be the extinction of its relict populationsin the
Balkansand southernEurope.There is evidence that the biogeographicbarnierpresented by high mountain ranges can be breached (Davis et al., 1988), but rapid
climate change may precludethis for some species.
Refuge theory should take into account the dynamic nature of species behavior
and the complexity and interdependenceof past climates, edaphic factors and disturbanceregimes.It also shouldrecognizethatrefugiafor variousspeciesareprobably
constantlyformingand expandingand contracting:species that are limited to refugia
today (e.g., plant and animal species isolated in mountainrangesand surroundedby
desert in the Great Basin, Fraserfir in the centralAppalachianmountains)may be
widespreadduringglacialstagesand vice versa.Finally,given the independentnature
of species' migrationand life history traits (fecundity,wind vs. animal pollination,
seed dispersalmechanisms, environmentaltolerances,etc.) the concept of a 'refuge'
may not be appropriatefor many species. The response of species to unfavorable
conditions probablyspans a continuum from migrationtowards refugiahaving no
overlapwith the formerrange,to rangecontractions,to survivalof sparsepopulations
in suitable microhabitatsthroughoutthe former range. Many other scenarios are
possible and indeed can be observed in contemporarybiota.
V. Prescriptionsand Applications
The greatestadvantageof a paleoecologicalperspectiveis that it extendsknowledge
of spatial patterns into the very useful fourth dimension of time. Patterns can be
observedover long periodsof time and processescan be inferredfrom these patterns.
These patterns and inferred processes can be compared with currentlyoccurring
phenomena to interpretand perhapspredict environmentaland biotic change.
Most ecologistsare awareof the geologyand recenthistoryof their study sites, but
few take full advantage of the available paleoecologicaldata to better understand
disturbanceregimes and other historical constraints that may operate at several
spatialand temporalscales (Hamburg& Sanford,1986). For example,Hamburgand
Cogbill (1988) present long-termdata that suggestclimatic warming(in addition to
recent pollution) as the causal factor for red sprucedecline in New Hampshire.
Paleoecological data also can provide tests for hypotheses that infer temporal
processesfrom spatialpatterns.Jacksonet al. (1988) cored a dune pond on the shore
of Lake Michigan and showed that Cowles' (1899, 1901) classical hydroserewas a
product of human disturbancecommencing 150 years ago. Campbell et al. (1985)
demonstratedthat the diverse biota of a proposedrefugiumin the westernAmazon
must have invaded the site after several meters of Holocene-agealluvium were laid
down.
The comparisonof paleoecologicaland currentdatanot only allows the community
ecologist and evolutionarybiologist to place currentphenomenainto the context of
a longertime-frame,it also can be helpfulin the areaof appliedecology.For example,
globalclimate circulationmodels that relateorbitalgeometryof the earthand climate
can be tested with paleoecologicaldata to refine the models and help predict future
climate and biota (Davis, 1990; Davis & Zabinski, 1991). We also know from geomorphologicaldata, and from increasesin paludificationand retrogressionof certain
forested areas that soils have become increasingly leached and nutrient poor in
northerntemperateareasduringthe latterhalf of the Holocene (Engstrom& Hansen,
234
THE BOTANICAL REVIEW
1985; Godwin, 1975; Iversen, 1964). Long-termecosystem acidificationdriven by
climate and vegetation change during interglacialperiods has been shown to occur
especiallyundercertainconditions (Ford, 1990). Vegetationallymediatedsoil change
is an ongoing process and should have important implications for foresters and
agronomists(Birks, 1986). On a shortertime scale, tree-ringchronologiesand historical recordsmay be helpful in characterizingcurrentclimate in relationto recent
climatic changes,and in determiningwhat changesconstitutea long-termtrend(such
as globalwarming)vs. a recentcycle (such as the 20-yearprecipitationpatternacross
much of SaharanAfrica) vs. random short-termvariation (a two-year drought in
parts of North America).
Informationon species responsesto past environmentalchangecan help us predict
futurebehavior of species. One area of much concernis the possible warmingof the
earth'satmosphereowing to increasedemissions from the burningof fossil fuels. In
theory, it should be possible to suggestfuture scenariosof community change and
species migration by integratingpast vegetation changes with a global circulation
model that includes anthropogenicallygeneratedglobalwarming(Davis & Zabinski,
1991). In practicethis may not be so easy. First, the predictedclimatic changeswill
take place over a time span of decades to a few hundredyears instead of hundreds
to thousandsof years.Second,the predictedglobalwarmingwill be causedby changes
in atmosphericchemistry affectingradiationtransmittance,reflectanceand absorption ratherthan by orbitallyinduced changesas in the past. Third, the effects of an
alteredglobal climate will be confoundedby the physiologicaleffectsof greenhouse
gases, such as CO2,on plants. Fourth, the earth's predictedfuture orbital geometry
indicates global cooling, a trend reflectedin global climate and vegetation over the
last 2000 years up to the 19th century.Furthermore,generalcirculationmodels and
paleoecologicalevidence suggest that disturbanceregimes will likely change with
changingclimate and vegetation (Grimm, 1983; Rind et al., 1989). Predictions of
futuregreenhouseeffectson the earth'sbiota must take into accountall these factors,
and recognize that future climate may not simply be warmer or drier but entirely
differentacross a continuum of spatial scales.
In light of the difficultiesof predictingclimate change, what generalizationscan
we make about biotic responses?First, the type and rate of response will vary taxonomically,even phenotypically.Second, responsewill vary locally with the severity
of climatic change. Third, where climatic change is severe response rates will be at
or near their maximum. Fourth,even at a maximum responserate, the earth'sbiota
will likely lag behind climate change. Fifth, in generalmost species will move polewardor upwardin altitude.Sixth, populationsat polewardextremitiesof landmasses
and near mountaintopswill be the most likely to become extinct. Seventh, species
that are more phenotypicallyand genotypicallyflexible in their responses will be
favored over less flexible ones. Eighth, as species migrateat differentrates, assemblages of species will change.
The prediction of species responses to impending environmentalchanges is particularlyimportantwith regardto wildernesspreservesand refugesthat protect endangeredspecies (Graham, 1988; Hunter et al., 1988). Rapid migration of species
in response to environmental change and the variable nature of past Quaternary
refugiasuggestthat planning refugesis a more complicatedtask than most conservationists realize. Communities as we know and classify them will add and lose
species, perhapsproducingassemblagesthat have never occurredin the earth'shistory. Simply markingoff an area may not be enough. Instead, nature preservation
IMPLICATIONSOF PALEOECOLOGY
235
will have to take on a fourth, temporal dimension. Hunter et al. (1988) emphasize
continental scale migration corridorsand large reserves encompassinga variety of
physicalenvironments.This contrastswith the presentstrategyof preservingspecific
communities or ecosystems.
One practicalapplicationof paleoecologicaltechniques that should be employed
more widely is the documentationof an area'shistory before a decision is made to
preserveit in its assumed naturalstate. Backmanand Patterson(1984) found that a
scrub oak-pitch pine forest in southeasternMassachusetts,which was managedto
maintainthat uniquehabitat,had been a white pine-hardwoodsforestin pre-colonial
times, and was likelyalteredby land-clearingassociatedwith settlementand extensive
fire. Such informationmay not deterthe effortto preservean area,but might greatly
alter the managementprocedures.
Fire histories can be reconstructedto help the forest managerunderstandnatural
firecycles and plan accordingly(Wright,1974). A largebody of such literatureexists
for manyregionsin NorthAmerica(e.g.,Harmon, 1983;Heinselman, 1973;Morrison
& Swanson, 1990; Romme, 1982), and has been employed in the implementation
of National Parkmanagementstrategies(e.g., Hemstrom & Franklin,1982; Kilgore,
1973; Kilgore& Taylor, 1979; Romme, 1982).
Anotherapplied use of paleoecologicaldata is as baseline informationwith which
to compare environmentalchangescaused by humans. One may want to know, for
example, the various characteristicsof the previous natural community before attemptingto restorea naturalecosystem(Engstrom& Wright,1984). Past abundances
of organismssensitive to acidity, toxic compounds and eutrophicationcan give us
an idea of naturalconditions in presentlypolluted lakes (Battarbee,1984; Battarbee
et al., 1989; Davis, 1987). Brugam(1978) documentedpast limnologicalconditions
in LinsleyPond usingpollen, diatoms, otheralgae,chironomids,cladocerans,rotifers,
and chemical analysis of the sediments. By studyingdiatom assemblagesburied in
lake muds, Stocknerand Benson (1967) detected changesin eutrophicationin Lake
Washingtonassociated with the development of and sewage input from the city of
Seattle. After sewage was diverted from the lake, Shapiro et al. (1971) recorded
improvementsin waterquality by comparingthe chemistryof pre- and post-sewage
sediment cores. Davis (1987) describesan extensive effort to document changes in
lake acidificationby analyzingpast diatom populations from a network of lakes in
the northeasternUnited States.
Paleolimnologicalstudiesin CentralAmericaand the Caribbeansuggestthat widespreadenvironmentaldestructionin some over-exploitedareas occurredin Mayan
times (Binford & Leyden, 1987). Contemporarypatterns similar to those in early
Mayan times should help to alert us to areas prone to incipient environmental
destruction.In summarypaleoecologicaldata can help answer the question, "what
is a 'natural'ecosystem?"in addition to such questions as, "how did the ecosystem
arise, what is its history, and which processesare implied by past patterns?"
VI. Summary
Most ecologistsstudyorganismson a scaleof less than 100 years,but many patterns
in naturecan be better understoodon a scale of hundredsto thousands of years or
more. The paleoecologicalrecordcan help contemporaryecologistsput currentphenomena into the context of a longer time-frame,allowing us to see processestaking
longer than a lifetime, and to gaugethe importanceof past cyclic or unusualevents,
disturbanceregimes,andhistoricallyconstrainedphenomena.The Holocenehas been
236
THE BOTANICAL REVIEW
a time of rapid environmental change following a more stable glacial. The biotic
responseto this changehas been complex and probablydifferentin detail in each of
the precedingglacial-interglacialcycles. Species react in an individualistic manner
to long-term environmental change; when considered on a paleoecological scale,
communities are transient assemblagesof species. Species populations and ranges
have respondedto and sometimes laggedbehind environmentalchangesin the Quaternary;these populations and rangescan vary between states of stasis and equilibrium vs. instability and disequilibrium.Paleoecologicalstudies indicate that disturbances of various sizes and intensities play an important role in structuring
communities. The environmenthas been highly cyclical duringthe Quaternary,and
perhaps throughoutearth history. Such cyclicity should: 1) select for flexibility in
species responses to environmental changes and biotic interactions;2) allow for
allopatricspeciationby geographicisolation;3) cause some loss of microevolutionary
change as biotic assemblages are reorganized.Paleoecology can be helpful in explaining present processes in light of past patterns,making comparisons,and predicting futurepatterns.
VII. Acknowledgments
We thankD. Gaudreau,H. D. Hammond, A. Knoll, D. Perlman,T. Sipe, V. Viana
and G. Whitney for critically reading the manuscript,B. Flye for assistance with
typing, and M. Fluet for assistancewith the figures.Schoonmakerwas supportedby
fellowships from the National Science Foundation and the R. T. Fisher Fund of
Harvard University. This work is a contribution from the Long Term Ecological
ResearchProgramat the HarvardForest (National Science FoundationGrant BSR
8811764).
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