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These ponds also flood rnany hectares of forest, causing the trees ro die and creating habitat for animals that
rely on dead trees. Several species of woodpeckers and
some species of ducks make their nests in Lavities that
are carved into the dead trees. Alligators play a similar
role in their communities by diggt"g deep-,,gatorholes"
in sumrner.These holes serve as critical ,orrri., of water
for manyother animalsduring dry months.
/ What are the various ways in which species
interact with one another?
/ What are the four rypes of predators?
/ What roles might a keystone species play in an
ecosystem?
Thecomposition
of a community
changesovertime
Even without human activity,natural communiciesdo
not staythe sameforever.Changein the speciescomposition of communitiesover time is a perpJtualprocesiin
nature.One rype of changethat occursin virtually every
communiry is
-ecological succession, the predictable
replacementof one group of speciesby -oih.. groop
of speciesover cime, Depending on the corrun-unity
type, ecological successioncan occur over time spans
from decadesto centuries.In terrestrial communitieq
ecologicalsuccessioncanbeprimaryor secondarydepend_
ing on the starringpoint of the communiry.
PrimarySuccession
Prirnary succession occurs on sufaces that are ini_
tially devoid of soil, such as an abandonedparking loq
newly exposedrock left behind afteta glacialretreat,or
newly cooled lava.
FIGURE
6.23showsthe processof primary successionIt begins when bare rock is colonized by organisms
such as algae,lichens,and mosses-organismsth"t .*
survivewith litde or no soil.As theseearly-successional
speciesgroq rhey excrete acids that
them to
"llo*
take up nutrients direcdy from the rock.
The resulting
cfremicalalteration of the rock alsomakesit more susceptibleto erosion.'W'henthe algae,lichens,and mosses
die, they become the organic matter that mixes with
mineralseroded from the rock to createnew soil.
Over time, soil developson the bare surface,and it
becomesa hospitableenvironmentfor plantswith deep
@Primarysuccession.Primarysuccessionoccursinareasdevoidofsoil'
plantsandalgaecancolonize
the
barerockandbeginto formsoil,making
Early-arriving
of distinct
forotherspecies
to colonize
later.0vertime,a series
sitemorehospitable
barerock
representing
anareain NewEngland,
develops.
Inthisitlustration,
communities
shrubs,
andtrees.
is initiatly
andlaterbygrasses,
colonized
bylichens
andmosses
ANDcoMMUNtry
EcoLoGY
168 r cHAprER
6 popuLATI0N
and
Mid-successionalplantssuchasgrasses
root systems.
speareas'These
such
to
dispersed
wildflowers areexily
ciesarerypicallywell adaptedto exploiting open' sunny
areasand are able to survive in the young, nutrientpoor soil.Thelivesand deathsof thesemid-successional
speciesgradually improve the quality of-the soil by
increasingits abfity to retain nutrients and water.As a
result, new speciescolonize the area and outcompete
sPecies.
the mid-successional
The tlpe of comrnunity' that eventually develops
is determined by the temperature and rainfall of the
producesforest
succession
region.In the United States,
co-rnmunitiesin the East,grasslandcommunities in the
Midwest, and shrublandcommunitiesin the Southwest'
assuccesIn someareas,the number of speciesincreases
communities
late-successional
In
others,
sion proceeds.
communities'
haveiewer speciesthan early-successional
occurred
succession
primary
of
example
A dramaiic
Krakatau
volcano
island
the
when
1883,
in
Indonesia
in
erupted and coveredthe island with lava and hot ash'
The ashkilled all living organismsand buried the soil on
had become
rhe island.By late 1884,algaeand grasses
just
followed
were
ash.They
and
rock
establishedon the
forest
island
The
shrubs.
and
a few yearslater by trees
continued to undergo successionas additional species
of trees and shrubs establishedthemselvesover time'
Becausethe number of specieson Krakatau has not
reachedthe number living on undisturbedneighboring
islandsof similar size,it is likely that primary succession
continuesthere todaY.
SecondarySuccession
Secondary succession occurs in areasthat havebeen
disturbedbut havenot lost their soil. Secondarysuccession follows an event,such asa forest fire or hurricane,
that removes vegetation but leaves the soil mostly
intact. Secondarysuccessionalso occurs on abandoned
agricultural fieids,such as the New England farms we
discussedin this chapter'sopening story'
6.24 showsthe processof secondarysuccesFIGURE
begins with rapid colonizacionby plants
usually
sion.It
rhat can easilydisperseto the disturbed area'The fitst
to arrive are typically plants such as grassesand wildflowersthat havelight,wind-borne seeds'Asin primary
thesespeciesare eventuallyreplacedby spesuccession,
bettei comPetitorsfor sunlight,water' and
are
that
cies
soil nutrients. In regioni that receivesufficient rainfall'
trees replacegrassesand flowers'The first tree species
to colonize aie those that can disperseeasilyand grow
rapidlv.For example,the seedsof aspentreesare carried
o" ttt. wind, and cherry tree seedsare carried by birds
that consume the fruit and excrete the seedson the
ground.Treessuch asaspenand cherry are often called
wheresoiIis present'
occurs
succession
Secondary
succession.
-llit#LL Secondary
on."Pitl,"l-^--^ptants
areas
set
theseEarly-arriving
nuu.beenremoved.
ffi
withgrasses
begins
forest
Engtand
New
a
in
succession
Secondary
;;;;;;;;rc.ssion.
whicharelaterreplacedby trees'
ad witdflowers,
SV E RT I M E S 1 6 9
ONFA C O M M U N I TCYH A N G EO
T H EC O M P O S I T I O
pioneer species becauseof their abiiity to colonize
and grow well in tull. sunshine'tu
;;;*;apidly
th. pion.e, o.t itt.t""t" in number and grow larger'
one
ho*lrrer, they start to shadeone another' reducing
more
are
that
species
Other
another'sgroovthand fitness.
and
,frra" tot!o"t, including many species of beech
piothe
of
shade
the
in
*"pt , survive and grow well
These shade-tolerantspeciesgrovr up
,r.i *. species.
through the pione., canopy' wenlu{Y outcomPeting
,h, pio.r.., tr.es attd dominating the forest.communiry'
on
Th. p-."tt of secondary succesion frappens
such
**V tp'"rid scales,from small-scaledisturbances
tittgl. treefall to huge areascleared
"
"t
disasterssuch as hurricanes
naturd
bv
The process is similar in
*ildfit"t.
oi
example,if a large tree
For
cases.
both
an opening for light to
creates
and
falls
floor, the seedlingsof
forest
the
reach
in full sunlight
rapidly
grow
ffees that
and secseedlinS,
ourcomPeteother
again'
begins
ondary succession
Historically, ecologists have described successionas ending with a
climax stage.In forests, for example,
ecologistsconsidered the oldest fores$to be climaxforu*.It is now recognized that,becausenatural disturbances
such as fire, wind, and outbreaks of
insect herbivoresare a regular part of
most cotnmuniries,a late-successional
stageis rypically not 6nal becauseat
anv moment it can be reset to an earlier stage.
barnaclesand
dominated by severalspeciesof atached
mussels.
tn
FIGURE6.25 shows the pattern oi successron
basin'
lake
a
out
ft.rh*"te. lakes.A glacier rn:ly carve
scouring it of sedimentsand vegetetion'-Over time'
.l;. ;d aquatic plants colonize the lake'The growth
the erosionof surroundingrock
*i dg".
"ttd
"fpft""
organic
aod soil slowly fill the basin with sediment and
shallow'This
increasingly
Processmay
matter, making it
the lake
Eventualiy'
years'
of
trk hundr.dior thousands
terrestrid
a
become
and
-"y U. filled in completely
habitat.
AquaticSuccession
Disturbancesalso create opportunities
in aquaticenvironments'
for succession
For example,the rocky intertidal zone
alongthe Pacificcoastof NorthAmerica ii exposed to the air during low
tide and jies under water during high
tide. From time to dme, major storms
turn over rocks or otherwise clear
them of living things.Thesebare rocks
can then be colonized through the
Diatoms
processof primary succession'
red and
of
sPecies
and short-lived
first to
the
usuallY
are
algae
ereen
rocks
the
afterward,
long
I.iot
Irrive.
several
and
barnacles
by
are colonized
speciesof long-lived red algae'This
stagedevelopsrather
rnid-successional
15 months' Finally'
about
in
quickly,
disrurbed again for
not
is
ii the'rock
areamaYbecome
the
3 yearsor more,
I7O .
6
CHAPTER
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ECOLOGY
AND COMMUNITY
POPULATION