Biodiversity Guide - The Intertwine

egional conservation strategybiodiversity guideChapter 7 Threats and Challengesnative fish and wildlife. What is the scale of suchhabitat loss? Worldwide, approximately half theEarth’s land area already has been transformedfor human use: 11 percent each for farming andforestry, 26 percent for livestock pasture, and 2to 3 percent for development (housing, industry,infrastructure, services, and transportation).In the greater Portland-Vancouver region, 22percent of the land is identified as agriculture, 13percent as developed, and an unknown but largenumber of acres is managed for forestry(see Table 1-1). Loss and degradation of habitathas resulted in the regional decline and extirpationof many plant, fish, and wildlife species,including the spotted frog, Lewis’ woodpecker,western rattlesnake, black bear, and many plantsand Neotropical migratory birds The most commontypes of habitat loss are habitat conversion,habitat fragmentation, and habitat degradation:as removing standing dead trees), and disruptionof ecosystem processes such as natural hydrologicalfluctuations and fire are some of the ways thathabitats can become so degraded they no longersupport native wildlife.Of all land uses, development is considered themost lasting form of habitat loss, because thepresence of pavement and buildings practicallyprecludes a return to natural conditions.Not all human modifications of the landscapeare harmful to fish and wildlife. In some cases,human manipulation of land can improve habitatquality for some species. For example, agriculturalareas and flooded fields have created habitatfor some bird species, and structures such as tallbuildings, cell phone towers, power line supports,and bridges have proven valuable for nestingraptors. Warm water created by dams and otherimpoundments are excellent habitat for warmwaterfish such as perch and bass (although mostwarm-water species are non-native ).ecological connectivity is a fundamental aspectof maintaining or restoring regional biodiversity.Thus, identifying and removing barriers thatreduce ecological connectivity must be a highconservation priority. In some cases, federaltransportation funding sources will increaseproject dollars for projects that retain or improvewildlife connectivity.find refuge in neighboring habitat patches whenthe one they are occupying becomes unsuitablebecause of reductions in food and nutrientresources, shelter, breeding conditions, or otherfactors. Isolation also reduces breeding interactionamong subpopulations, thus fosteringinbreeding, which over time reduces the geneticdiversity, vigor, and adaptability of the regionalmetapopulation.Natural Versus Artificial BarriersAnything that prevents or reduces the free Types and Impacts of Artificial Barriersmovement of native organisms among appropriateUrbanization and land use changes in the greaterhabitat patches is a barrier. Barriers reduce Portland-Vancouver region have created a highlylandscape permeability, which refers to ecological fragmented landscape with many human-madeconnectivity and an organism’s ability to move barriers and declining ecological permeability.freely within the landscape to meet its basic life The two most common types of human-madeneeds.barriers stem from structural development, suchNatural barriers, such as mountain ranges, as housing, and transportation infrastructure,large bodies of water, and areas of unsuitable habitatsuch as roadways, railroads, and trails. Otherbetween suitable habitat patches, are a normal types of artificial barriers that effectively discon-n Habitat conversion. Habitat conversion refersaspect of landscape-level biology and contribute nect habitat patches include large agriculturalto the outright loss of habitat and includes constructionto the evolution of species and the creation of a fields without brushy margins and hedgerows.of roadways, conversion to farms, andindustrial, commercial, and residential development.The activities typically involve filling in Landscape PermeabilityBarriers and Decliningdiversity of habitats. Natural barriers generally For species that depend on old growth, largeexist at the landscape scale and can take hundreds blocks of commercial forest can act as barriers toor even hundreds of thousands of years to divide movement because they represent large areas ofwetlands, dredging rivers, mowing fields, and Curt Zonick, Metroa large population into two smaller but still quite unsuitable habitat.cutting down trees.Most biological communities remain stable onlylarge subpopulations. In contrast, human-maden Habitat fragmentation. Habitat fragmentation when they exist as a network of many smaller,barriers such as roads, subdivisions, cities, andrefers to conversion that results in larger, connectedfunctionally connected subpopulations. Together,farms can carve a population into hundreds ofhabitat patches being split into smaller, the linked subpopulations form a regional meta-very small subpopulations in just a few years ormore isolated ones. Development and roadways—especiallypopulation that is able to withstand occasionaldecades.those without adequate wildlife local extirpations by reoccupying empty habitatThe isolation of small, remnant subpopulapopulationcrossings—have been the major cause of habitat patches that have remained in spite of the extirpations.tions can reduce the landscape’s ability to supportfragmentation in the greater Portland-VancouverIn addition, a biological community gener-native fish and wildlife as much as habitat lossregion. For aquatic species, habitat also has been ally is healthier if it has abundant suitable habitatfrom development can. However, if habitat fragmentsremain functionally connected, nativefragmented by dams, improper culverts, andin large patches. As habitat patches becomewater diversions. The loss and fragmentation ofsmaller in size and the number of suitable patchesplant and animal species that might otherwise behabitat make it difficult for migratory species todeclines, chances increase that the subpopulationsoccupying those patches will disappear. Thising, despite declines that result from habitat lossextirpated will instead have a chance of persist-find places to rest and feed along their migrationroutes and reduces the viability of local residentpopulations.is basic island biogeography theory. However, justand associated factors, such as edge effects.as important as the number or size of patches isConnectivity creates options, while barriersn Habitat degradation. Habitat degradation how well they are connected. A patch of suitableremove them. When organisms are able to moverefers to actions that, although they do not eliminatehabitat isolated from other patches may notfreely among remaining fragments, they have ahabitat, reduce the value of a given habitat serve a true functional role for native species orgreater chance of responding to stress and locallypatch for supporting biodiversity. Pollution, regional biodiversity.harsh conditions. Restricting or eliminatinginvasive species, structural simplification (such Given the dynamics of metapopulationsmovement among habitat patches reduces the124and habitat patches, preserving and improvingability of subpopulations of plants and animals to125