Into Human Modified Landscapes as Best It

Unbroken tracts of conserved wild area, if they exist, will always be the greatest reservoirs of biodiversity and the most interesting places to visit. But under certain conditions, human-dominated pastoral and agricultural landscapes can also harbor an appreciable amount of biodiversity (Western, 1989; Pimentel et al, 1992; Daily et al, 2001). Simple and inexpensive management techniques, such as maintaining living hedges around agricultural plots (Robinson and Sutherland, 2002) and preserving remnant trees in pasture (Luck and Daily, 2003), can often buttress the biodiversity of these areas.

There are many compelling reasons to conserve countryside biodiversity. One is that most human-dominated landscapes will not revert to wildness anytime soon; enabling wild populations to persist in these areas is the best plausible outcome for biodiversity. Another is that habitat types vary in their tolerance of human activity. Whereas tropical forests are quite sensitive to burning, wood chopping, and hunting, tropical savannas are relatively resilient to anthropogenic disturbance. In many parts of Africa, much or most wildlife occurs outside of nationally protected areas (Western, 1989), and wildlife can coexist alongside limited livestock populations (Georgiadis et al., 2007). That people also share this

338 / Paul R. Ehrlich and Robert M. Pringle space does not necessarily diminish its conservation value. Moreover, maintaining nonconserved areas in biodiversity-friendly ways aids migration and dispersal between protected areas, a process that will become even more important as climate change rearranges species' distributions (Ricketts, 2001; Travis, 2003). Finally, maximizing biodiversity in areas where humans are active in their daily lives increases the frequency of interactions between human and nonhuman organisms, which enhances the potential for ecosystem-service delivery and bioliteracy development (see Onto the Cultural Radar Screen). Economic incentives (or legal strictures) can be developed to encourage (or require) biodiversity-friendly use of privately held lands (Farrier, 1995; Stoneham et al., 2003).

Biodiversity maximization in human-dominated landscapes does not in any respect reduce the need for large conserved wildlands. How to allocate conservation resources among these two different frameworks is a local problem, and answers will vary depending on such factors as the habitat types involved, local land-use history, the state of the region's government and protected-area system, and the availability and price of land for purchase. As in most other respects, Britain is different from Kenya is different from Amazonia. The challenges in planning for conservation in human-dominated landscapes are perhaps most pronounced in fragmented tropical forest-pasture-field mosaics, because tropical-forest biodiversity is so great and the alternate landscape states are so dramatically different from the baseline. One uncertainty is whether the apparently high conservation value of these mosaics [e.g., Daily et al. (2001) and Mayfield and Daily (2005)] will be sustained over centuries, or whether it will ultimately succumb to the ''extinction debt'' (Janzen, 1986; Tilman et al., 1994). A related concern is that the diversity of interspecific interactions in human-dominated landscapes will decline more quickly and less perceptibly than the diversity of populations or species and that this will eventually lead to additional population and species loss. A 300-year-old canopy tree species in a Brazilian pasture may serve as a roost for a diversity of birds, epiphytes, and other organisms. But if its pollinator or seed disperser has been lost or will not venture into the pasture (Cordeiro and Howe, 2003), or if its seeds will not germinate in a pasture, or if its seedling crop will be devastated by pasture-tolerant seedling predators, then it is among the living dead (Janzen, 1986): it will not replace itself, and, when it goes, so go the other species that used it.

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