Chapter 2: Habitat Fragmentation - USDA

1 Chapter 2: Habitat FragmentationNatural Resources Conservation Service (NRCS)2-1 INTRODUCTIONF ragmentation, the breaking up of large patches ofnative vegetation into smaller and increasingly isolatedpatches, is a process as old as civilization (Figure 2-1). It intensified as hunter/gatherer societies settledin permanent locations and began planting crops andherding livestock. Research suggests that the initialimpacts on biodiversity were minimal, disturbed areaswere small and regenerated when no longer croppedor grazed. But as human populations increased andtechnology became more sophisticated, the effectsof Fragmentation spread across the evidence suggests that many wildlifespecies were displaced and local continues today, driven by an explodinghuman population and growing demand to producemore food and fiber from a finite land resource. Thecontemporary rural landscape is the result of thecumulative impacts of past and present human landuse practices including urbanization, agriculture,ranching, and of a landscape reduces the area oforiginal Habitat and increases the total lineal feet ofedge, favoring species that inhabit edges at theexpense of interior species that require largecontinuous patches.

2 Ecologists, such as Wilcox andMurphy, believe that Habitat Fragmentation is the mostserious threat to biological diversity and is the primarycause of the present extinction crisis. Not only have the fields become vast flat tracts of land exclusively devoted to a single crop, they have become devoidof many traditional features of the rural landscape. In the quest for large uniform farming surfaces, topographicalirregularities such as gullies, washes, sloughs, rises, slopes, and knolls have succumbed to land remodeling. At thesame time, features once essential to rural life such as woodlots, windbreaks, ponds, fences, country schools, ruralchurches, outlying farm buildings are systematically being removed or destroyed. (Carlson 1985)Figure 2-1: In this fragmented landscape, little remains of the prairie and wetlands that once existed Betts NRCSDick RolDick Rol2-2 Habitat FRAGMENTATIONP rior to the age of mechanized agriculture (circa 1890),rural American landscapes were fine often surrounded small fields of diversecrops while wetlands, steep slopes, swales, and rockyareas were left undisturbed (Figure 2-2).

3 Fields of 40,80 and 160 acres were common. With today smechanized agriculture, Fragmentation occurs at amuch coarser scale resulting in more homogenouslandscapes (Figure 2-3). Small fields are combinedto form larger tracts of land to accommodate farmingwith large machinery. Many fields are enlarged at theexpense of windbreaks, fence rows and other valuablewildlife Habitat . Several areas in the Midwest havelost over 60% of their windbreaks due to the declininghealth of windbreak trees, expanding field size, andurban sprawl. The resultant loss of Habitat diversityin agricultural landscapes has adversely impactedwildlife populations. Wildlife biologists studyingbobwhite quail (Colinus virginianus)in Nebraska discovered that acounty with 5 times more acreagein hedgerows than a neighboringcounty also had an estimatedpopulation of quail almost 4 a species to survive in alandscape or watershed, it musthave access to Habitat resourcessufficient to maintain a viablepopulation.

4 A minimum viablepopulation (MVP) is the smallestnumber of individuals required tosustain a population for the long-term. A projected MVP is basedon estimates of a population sizethat can counter the negative effectsof genetic variation loss, populationFigure 2-2 Oblique aerial view of agricultural land with diverse fields,corridors and patchesFigure 2-2: These small Pennsylvania fields have been integrated with patches ofnon-tillable land, providing Habitat for 2-3: Large fields of row crops dominate this North Carolina landscape, leavinglittle Habitat for quail or other , and of a MVP is oftendependent on functioningmetapopulations, wildlife populationsthat are spatially separated but interactthrough the dispersal of in small patches can wink on or off (experience localextinction) due to local variation in sexratios, disturbance such as fire, andother local factors.

5 A metapopulationis more likely to persist if immigrationand colonization are facilitated bycorridors or stepping stone between patches is critical insustaining healthy metapopulations in highlyfragmented landscapes (see the Louisiana Black BearCase Study, pp. 3-9). Habitat Fragmentation diminishes the landscapes capacity to sustain healthy populations ormetapopulations in four primary ways: Loss of original Habitat Reduced Habitat patch size Increased edge Increased isolation of patches Modification of natural disturbance regimesFrank Lucas NRCSN orth Carolina State University2-3 LOSS OF ORIGINAL HABITATP erhaps the most significant adverse impact offragmentation is simply the loss of original findings suggest loss of Habitat has a muchgreater impact on wildlife populations than the changein spatial arrangement of Habitat 90% of the grasslands east of the MississippiRiver are gone, approximately 90% of Iowa s wetlandshave been removed and 80% of Indiana s forests havebeen eliminated (Figure 2-4).

6 Habitat losses of thismagnitude will permanently displace many speciesand dramatically depress the population levels ofothers. It forces remaining species into the fewremnant patches available, increasing competition,crowding, stress, and the potential for diseaseoutbreaks. The number of currently listed federaland state threatened and endangered speciessuggests that many populations are at or near in areas where Fragmentation is not readilyapparent, subtle but equally devastating effects ofhabitat loss can exist. A grassland invaded by exoticgrasses may look natural but be functionallyfragmented. For example grasslands infested bycheatgrass (Bromus tectorum) look similar to nativegrass patches, but provide no Habitat of value forsensitive species such as the pronghorn (Antilocapraamericana) and the greater prairie chicken(Tympanuchus cupido).Figure 2-4: When only disconnected remnants of habitatremain in a watershed, wildlife are often crowded, stressed,and subject to high levels of Habitat PATCH SIZER eduction in Habitat patch size is a principalconsequence of Fragmentation .

7 Biologists MacArthurand Wilson suggested that the rate of speciesextinction in an isolated patch of Habitat is inverselyrelated to its size. As remnants of native habitatsbecome smaller, they are less likely to provide food,cover and the other resources necessary to supportthe native wildlife community. Small patches are alsomore susceptible to catastrophic disturbance eventssuch as fire or severe weather that can decimate also decreases the area of interiorhabitat (Figure 2-5). Interior Habitat is the area farenough from the edge to maintain communities of theoriginal larger Habitat . For example, when large tractsof sage/grassland are cleared and seeded into grassesor alfalfa, sage/grassland patch size and interiorhabitat are reduced. Not surprisingly, populations ofan interior-dwelling cold desert species that requirelarge patches of sage brush like the sage grouse(Centrocercus urophasianus) are in serious 2-5: The fragmented landscape on the left has lessinterior Habitat and over 50% more edge than the block ofhabitat on the : 640 acresEdge: 38,620 lineal feetArea: 640 acresEdge: 21,120 lineal feetINCREASED EDGEA lthough an increase in edge (the boundary betweentwo plant communities) due to Fragmentation maybenefit some species, some researchers believe thatincreasing edge may be detrimental to the protectionof native biodiversity.

8 Edges act as barriers causingsome predators to travel along them. High predatordensities along edges can result in higher mortalityfor edge dwelling prey species or species movingthrough narrow corridors. Nest parasitism by brownheaded cowbirds (Molothrus ater) also appears to behigher in species nesting in edge Habitat . Least bell svireo (Vireo bellii pusillus) is an endangered speciesthat inhabits the edges of riparian corridors in southernCalifornia. Parasitism by cowbirds appears to be assignificant as the loss of riparian Habitat in the declineof the least bell s vireo on Camp Pendleton, RolNRCSC raig Johnson2-4 INCREASED ISOLATIONF ragmentation leads to increased isolation of patches(Figure 2-6). Wildlife populations in isolated patchescan be sustained by immigration of species fromsurrounding patches. However, as fragmentationcontinues, distances between patches get longer anddispersal and immigration rates decrease.

9 Thediversity of species moving between patches alsodecreases; small species with limited mobility areparticularly distance sensitive. As immigration ratesdecrease, factors like inbreeding and catastrophicdisturbances can cause the number of species in apatch to decline to zero over a long enough period studying chaparral bird species extinctionrates in remnant patches in southern California foundthat on average, less than one chaparral bird speciessurvived after 40 years of isolation in canyons lessthan 125 2-6: Patch B is more isolated from the remnants ofpatch A when A is fragmented, limiting movement between Aand B for some species of 2-7: This recently restored riparian corridor is reconnecting the structural elements in an Iowa Betts NRCSMODIFIED DISTURBANCE REGIMESF ragmentation and associated land managementactivities like fire suppression alter the flow of naturaldisturbances.

10 For example, fire, a disturbance factoressential to the maintenance of tall grass prairies,has virtually been eliminated in the Midwest. Remnantprairie plant communities separated by miles of rowcrops and protected from fire are being overtaken byless fire tolerant woody species. Wildlife dependenton prairie ecosystems are being EFFECTSThe cumulative impact of Habitat Fragmentation resultsfrom the combined incremental effects of Habitat loss,reduced patch size, increased edge, and patchisolation. The impacts are cumulative across scalesand over time affect populations of organisms as wellas individuals. These impacts are not related linearlyto the extent of original Habitat . There are thresholdswhere local extinction for a species may be imminenteven though only a small percentage of original habitathas been lost. Unfortunately, understanding of thesethresholds is CONNECTIONSIn many regions of the country agriculture and ur-banization are dominant forces in land conversion;most land is in private ownership, Habitat patchesare small in size and number and they are oftenisolated.