Environmental Problems, Their Causes, and Sustainability 1

It is tempting to think of mutualism as an exampleof cooperation between species, but actually it involveseach species benefiting by exploiting the other.How Do Species Interact So That One Benefitsbut the Other Is Not Harmed? Do No HarmSome species interact in a way that helps one speciesbut has little if any effect on the other.Commensalism is a species interaction that benefitsone species but has little, if any, effect on the otherspecies. One example is a redwood sorrel, a small herb.It benefits from growing in the shade of tall redwoodtrees, with no known negative effects on the redwoodtrees.Another example is plants called epiphytes (such assome types of orchids and bromeliads) that attachthemselves to the trunks or branches of large trees intropical and subtropical forests. These so-called airplants benefit by having a solid base on which to grow.They also live in an elevated spot that gives them betteraccess to sunlight, water from the humid air andrain, and nutrients falling from the tree’s upper leavesand limbs. This apparently does not harm the tree.8-5 ECOLOGICAL SUCCESSION:COMMUNITIES IN TRANSITIONHow Do Ecosystems Respond to Change?Shifting Community CompositionOver time new environmental conditions can causechanges in community structure that lead to onegroup of species being replaced by other groups.All communities change their structure and compositionover time in response to changing environmentalconditions. The gradual change in species compositionof a given area is called ecological succession. Duringsuccession some species colonize an area and theirpopulations become more numerous, whereas populationsof other species decline and may even disappear.Ecologists recognize two types of ecological succession,depending on the conditions present at the beginningof the process. One is primary succession,which involves the gradual establishment of bioticcommunities on nearly lifeless ground. With the other,more common type, called secondary succession, bioticcommunities are established in an area wheresome type of biotic community is already present.What Is Primary Succession? Establishing Lifeon Lifeless GroundOver long periods, a series of communities with differentspecies can develop in lifeless areas where thereis no soil or bottom sediment.Primary succession begins with an essentially lifelessarea where there is no soil in a terrestrial ecosystem(Figure 8-11) or no bottom sediment in an aquaticecosystem. Examples include bare rock exposed by aretreating glacier or severe soil erosion, newly cooledlava, an abandoned highway or parking lot, or a newlycreated shallow pond.Primary succession usually takes an extremelylong time. One reason is that before a community canbecome established on land, there must be soil. Dependingmostly on the climate, it takes natural processesseveral hundred to several thousand years toproduce fertile soil.Soil formation begins when hardy pioneer speciesattach themselves to inhospitable patches of bare rock.Examples are wind-dispersed lichens and mosses,which can withstand the lack of moisture and soil nutrientsand hot and cold temperature extremes foundin such habitats.These tough species start the soil formationprocess on patches of bare rock by trapping windblownsoil particles and tiny pieces of detritus, producingtiny bits of organic matter, and secreting mildacids that slowly fragment and break down the rock.This chemical breakdown (weathering) is hastened byphysical weathering such as the fragmentation of rockthat occurs when water freezes in cracks and expands.This is a slow process. It may take a lichen 100 years togrow as large as a dinner plate.As patches of soil build up and spread, a newplant community replaces the community of lichensand mosses. Most of these plants are tiny annuals thatlive for only a year. However, they produce flowersand seeds that fall to the ground and can germinate forthe following growing season. These taller plants eliminatethe lichens by depriving them of sunlight.Acommunity of small perennial grasses (plants thatlive for more than 2 years without having to reseed)and herbs or ferns normally replaces the annual plantcommunity. The seeds of these plants germinate afterarriving on the wind and in the rain, in the droppingsof birds, or on the coats of mammals.These early successional plant species growclose to the ground, can establish large populationsquickly under harsh conditions, and have short lives.Some of their roots penetrate the rock and help breakit up into more soil particles. The decay of theirwastes and dead bodies also adds more nutrients tothe soil.After hundreds to a thousand or more years, thesoil may be deep and fertile enough to store enoughmoisture and nutrients to support the growth of lesshardy midsuccessional plant species of herbs, grasses,and low shrubs. Trees that need lots of sunlight and areadapted to the area’s climate and soil usually replacethese species.156 CHAPTER 8 Community Ecology