394 



Global Climate Chani^c — Our Living Resources 



Fig. 3. The proportion of species 

 on the dispersal-ability scale that 

 are out of their climate envelopes 

 (vulnerable) or in their climate 

 envelopes (not vulnerable) with a 

 +3°C (+5.4°F) temperature 

 change. Full data were available 

 for 8.668 species. 



3 4 5 



Dispersal ability 



7 High 



survival. Rate plants would thus potentially 

 have tiouble niigiating to comparable new sites, 

 regardless of their ability to disperse. For exam- 

 ple. Boott's rattlesnake-root (Prenanlhes hoot- 

 tii) and mountain avens {Geum peckii). endem- 

 ic to alpine habitats in the northeastern United 

 States, would be pai'ticularly sensitive to global 

 warming. 



Migration Rate 



During the warming at the end of the last 

 glacial period, plant migration rates, as calculat- 

 ed from the fossil pollen record, ranged from 

 about 5 to 130 km (3-90 mi) per century 

 (Shugart et al. 1986). Human-caused climate 

 change may occur at rates more than five times 

 faster than any changes since the last glacial 

 maximum, including the period of most rapid 

 deglaciation (Oveipeck et al. 1991). Various 

 studies have suggested that such rapid climate 

 changes would require shifts of plant ranges of 

 up to 500 km (300 mi) within the next century, 

 exceeding the known rates of migration for 

 many plant species (Davis 1984; Davis and 

 Zabinski 1992). 



Since species respond individually to cli- 

 mate change, migration rates will vary within 

 and among natural coinmunities. It is unlikely 

 that entire biological communities would move 

 together in response to climate changes 

 (Graham and Grimm 1990). Some plants may 

 respond rapidly to changes: others may survive 

 for several generations in place or persist as 

 long-lived clones despite significant climate 

 change. The fossil record provides evidence of 

 decade- or even century-long time lags in 

 species migration (Davis 1989). The process of 

 changing community composition in response 

 to climate change has been documented in the 

 fossil record through the disassociation and 

 reassembly of plant and aniinal taxa (Graham 

 and Grimm 1990). This variation in species 

 assemblages displays the transitory nature of 

 former as well as existing and future communi- 

 ty types. 



Temperature extremes and changes in the 

 frequency and .severity of local disturbances 

 may have a greater influence on the survival of 



plant species at particular locations than small 

 shifts in the average climate. More frequent 

 droughts, fires, and pest and pathogen outbreaks 

 are predicted to act in conjunction with climate 

 change to significantly transfomi the landscape 

 (Peters 1992). This prediction is supported by 

 paleoecological evidence that altered distur- 

 bance regiines can intensify the effects of cli- 

 inate change on plants and increase the amount 

 of overall vegetational change (Davis 1989). 



Threats by Weedy Exotics 



With global climate change, some exotic 

 weeds may be favored over native species. 

 Many weeds are able to expand relatively 

 quickly, posing serious threats to existing 

 species and overall biodiversity (Schwartz 

 1992). Many weedy species are widespread, 

 prolific, fast-growing annuals capable of estab- 

 lishing in disturbed habitats and are often 

 favored by disturbances. Climate-induced 

 changes could expose native plants to non- 

 native competitors for the first tiine (Peters 

 1992). stressing the balance established 

 between native plants and their habitat. Exotic 

 weeds may become a greater problem in the 

 management of many preserves and natural 

 areas. 



Landscape Fragmentation 



The potentially rapid rates of warming, com- 

 bined with habitat loss and fragmentation from 

 human development, suggest that many species 

 will not adjust as successfully to climate change 

 as in the past. Most native plant species exist in 

 a highly fragmented landscape that further sep- 

 arates appropriate habitat patches, increasing 

 the dependence of many species on relatively 

 rare events of long-distance dispersal. 

 Furthermore, species often must disperse across 

 hostile habitats, including I'oads. cities and sub- 

 urbs, and farmland (Peters 1992). Finally, plants 

 would need to establish themselves in land- 

 scapes where many of the open or disturbed 

 areas have been colonized by aggressive weedy 

 exotics. 



Climate Change and 

 Conservation Planning 



Rapid climate change could place novel 

 demands and constraints on plant species con- 

 servation. Vulnerability to climate change could 

 affect selection and design of new preserves and 

 management procedures in existing preserves, 

 especially in southern or low-elevation portions 

 of species" ranges. Management of species 

 threatened by climate change could involve 



