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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 ffi 'ffi::;,t# jlli:;fi[,:iH::f ::JliH:1ii.ffi1] ECOLOGY AND COMMUNITY POPULATION