Environmental Problems, Their Causes, and Sustainability 1

How Do Species Replace One Another inEcological Succession? Creating Beneficialand Hostile ConditionsSome species create conditions that favor the speciesthat replace them; some create conditions that hindertheir replacements; and some get along with the nextgroup of species.Ecologists have identified three factors that affect howand at what rate succession occurs. One is facilitation,in which one set of species makes an area suitable forspecies with different niche requirements. For example,as lichens and mosses gradually build up soil on arock in primary succession, herbs and grasses can colonizethe site. Similarly, plants such as legumes add nitrogento the soil, making it more suitable for otherplants found at later stages of succession.A second factor is inhibition, in which early specieshinder the establishment and growth of other species.Inhibition often occurs when plants release toxicchemicals that reduce competition from other plants.Succession then can proceed only when a fire, bulldozer,or other human or natural disturbance removesmost of the inhibiting species.A third factor is tolerance, in which late successionalplants are largely unaffected by plants at earlierstages of succession. Tolerance may explain why latesuccessional plants can thrive in mature communitieswithout eliminating some early successional and midsuccessionalplants.How Do Disturbances Affect Successionand Species Diversity? Setting Back theCommunity ClockChanges in environmental conditions that disrupta community can set back succession.A disturbance is a change in environmental conditionsthat disrupts a community or ecosystem. Examples arefire, drought, flooding, mining, clear-cutting a forest,plowing a grassland, applying pesticides, climatechange, and invasion by nonnative species. Environmentaldisturbances can range from catastrophic tomild and can be caused by natural changes or humanactivities. At any time during primary or secondarysuccession, such disturbances can convert a particularstage of succession to an earlier stage.Many people think of all environmental disturbancesas harmful. Large catastrophic disturbancescan devastate communities and ecosystems. But manyecologists contend that in the long run some types ofdisturbances, even catastrophic ones such as fires andhurricanes, can be beneficial for the species diversityof some communities. Such disturbances create newconditions that can discourage or eliminate somespecies but encourage others by releasing nutrientsand creating unfilled niches.For example, when a large tree falls in a tropicalforest, this local disturbance increases sunlight and nutrientsfor growth of plants in the understory. When alog hits a rock in an intertidal zone, it dislodges or killsmany of the organisms that are growing on the rockand provides space for colonization by new intertidalorganisms.According to the intermediate disturbance hypothesis,communities that experience fairly frequent butmoderate disturbances have the greatest species diversity.Researchers hypothesize that in such communities,moderate disturbances are large enough to createopenings for colonizing species in disturbed areas butmild and infrequent enough to allow the survival ofsome mature species in undisturbed areas. Some fieldexperiments support this hypothesis, but the scientificjury is still out on whether it applies to all types ofcommunities.Does Succession Proceed along an ExpectedPath, and Is Nature in Balance? Things AreAlways ChangingScientists cannot project the course of a given successionor view it as preordained progress toward astable climax community that is in balance with itsenvironment.We may be tempted to conclude that ecological successionis an orderly sequence in which each stage leadsautomatically to the next, more stable stage. Accordingto this classic view, succession proceeds along an expectedpath until a certain stable type of climax communityoccupies an area. Such a community is dominatedby a few long-lived plant species and is in balancewith its environment. This equilibrium model of successionis what ecologists meant years ago when theytalked about the balance of nature.Over the last several decades, many ecologistshave changed their views about balance and equilibriumin nature. When these ecologists look at a communityor ecosystem, such as a young forest, they seecontinuous change and instability instead of equilibriumand stability.Under the old balance-of-nature view, a large terrestrialcommunity undergoing succession eventuallybecame covered with an expected type of climax vegetation.But a close look at almost any community revealsthat it consists of an ever-changing mosaic ofvegetation patches at various stages of succession.These patches result from a variety of mostly unexpectedsmall and medium-sized disturbances.Such research indicates that we cannot project thecourse of a given succession or view it as preordained progresstoward an ideally adapted climax community. Rather, successionreflects the ongoing struggle by differentspecies for enough light, nutrients, food, and space.This allows each to survive and gain reproductivehttp://biology.brookscole.com/miller14159