restricted access IV.4 Biodiversity Patterns in Managed and Natural Landscapes
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IV.4 Biodiversity Patterns in Managed and Natural Landscapes Paul R. Moorcroft OUTLINE 1. Habitat loss 2. Habitat fragmentation 3. Not all species are equal 4. Invasive species and climate change Human activities are profoundly altering the biodiversity of the earth. The principal drivers of change thus far have been the transformation of lands for human use, accompanying fragmentation of remaining natural habitats, hunting, and modification of native disturbance regimes. These forces have resulted in the extinction of numerous species and radically altered the abundances of countless others. Empirical and theoretical studies imply that many more of the world’s species will experience the same fate as humanity’s collective impacts on the planet further expand and intensify. Long-lived plants and animals and animal species in which individuals range widely in space appear to be particularly vulnerable because of the strong dependence of their populations on the dwindling number of regions that are free of significant human influence. In numerical terms, the impacts of humans on terrestrial biodiversity are generally larger in the tropics because of the restricted spatial distributions of many tropical species and the high species diversity of many tropical ecosystems compared to their high-latitude counterparts. Two additional , and increasingly important, modifiers of terrestrial ecosystem biodiversity are the introduction of exotic species into ecosystems and human-induced changes in climate . These more recently recognized agents of change may act independently of, or synergistically with, land cover change, habitat fragmentation, hunting, and altered disturbance regimes to yet further modify the composition of the world’s ecosystems over the coming century and beyond. GLOSSARY early successional species. Species that appear in an ecosystem following a disturbance event, such as a fire, landslide, or logging. Early successional species typically possess r-selected traits, such as high dispersal ability, short generation time, and rapid growth, but at the expense of having a short lifespan and poor competitive ability. As a result, their population sizes usually increase immediately after disturbances , and then decline later as conditions become more crowded and they are competitively replaced by late successional species. endemics. Species that have a relatively narrow geographic range, such as species found only on a particular island, or in a particular habitat or region. fire return interval. The number of years between two successive fire events at a particular location. invasive species. Introduced or nonindigenous species that are rapidly expanding outside of their native range. late successional species. Species found in an ecosystem that has not experienced a disturbance for a long period of time. Late successional species typically have K-selected traits, such as long generation time, slow rates of growth, but long lifespan and strong competitive ability. As a result, late successional species come to dominate an ecosystem when no further disturbances occur. species–area curve. A graph showing the number of species found in an area as a function of the area’s size. Human population growth and economic development have led to a radical transformation of the earth’s land surface. As plate 8 illustrates, humanity’s footprint on the planet is now pervasive, with 83% of the earth’s surface being significantly affected by one or more of human land transformation, population density, power infrastructure, and transportation networks. In this chapter, I review how these and other human activities are affecting the biodiversity of terrestrial ecosystems. The focus of this chapter is on ecosystem biodiversity; however, as discussed in more detail in chapters IV.2, IV.6, and IV.8, an ecosystem’s species composition has important consequences for its biophysical and biogeochemical functioning. 1. HABITAT LOSS One of the most significant ways in which humans affect terrestrial biodiversity is the loss of species that accompanies the destruction of natural habitats. A particularly compelling study of this phenomenon is the Biological Dynamics of Forest Fragments Project (BDFFP) initiated by James Lovejoy and others in the late 1970s (Bierregaard et al., 2001), which is examining the consequences of habitat destruction in the tropical forests of the Amazon (figure 1A). Figure 1B– D shows the number of understory bird species in three forest fragments of different sizes experimentally created as part of the BDFFP. The initial species diversity of the understory bird community in the forest fragments was high, with the 3-, 11-, and 100-ha fragments, respectively, containing 90, 92, and 111 bird species (Ferray et al., 2003). However, following their creation , species diversity in the fragments declined rapidly: in a 13...