(hii I.h'ini; Resiiidvi's — Global Cliimilc Change 



J,S7 



literature one gets the impression that such 

 changes are the most common and most impor- 

 tant. 



hidirect effects of habitat change, however, 

 are probably just as common and important, and 

 perhaps even more so. although obtaining clear 

 obvious evidence for indirect effects is diftlcult 

 given the fact that other factors are changing at 

 the same time. One such effect is the biotic 

 response to the abiotic changes induced by 

 human disturbance. A good example is changes 

 in birds" ranges in response to increasing tem- 

 perature. 



Range Expansion 



One way to examine the possible importance 

 of global wanning on changing ranges is exam- 

 ining possible physiological mechanisms con- 

 straining birds" ranges to warm areas. Previous 

 work has shown that 50 species of songbirds 

 (e.g., sparrows and warblers) have range bound- 

 aries apparently dictated by average minimum 

 January temperatures (Root 1988b). Ongoing 

 studies of a few of these key species have shown 

 significant changes in the location of northern 

 range boundaries from year to year, and these 

 correspond to annual climate changes. 



Preliminary studies on noilhern cardinals 

 (Carcliiuili.s cunllnalis) suggest that the lack of 

 stored fat, which is needed to fuel increased 

 metabolic rates in colder areas (Root 1991 ), is 

 the primary factor restraining this bird's range. 

 Consequently, as the earth warms, we expect 

 birds with ranges restricted by low temperatures 

 to readily expand their ranges. Such expansions 

 may indeed be already occurring. 



Successfully managed birds show extensive 

 range expansions. Up to 1940, the mute swan 

 (Cygiius olor) was recorded only in 

 Pennsylvania and Michigan (Fig. la), but since 

 then, programs to introduce and establish it — 

 primarily in parks — have allowed it to spread to 

 19 states (Fig. lb). 



The wild turkey {Meleagris gaUopavo) 

 shows even a more dramatic change (Fig. 2). It 

 originally occurred in the Southwest and in all 

 the states east of the 100th meridian, except for 

 North Dakota (Schorger 1966). Hunting pres- 

 sures, habitat loss, and disease spread by 

 domestic poultry all contributed to its dramatic 

 range contraction (Schorger 1966: Hewitt 1967; 

 Lewis 1973). Froin 1901 to 1940 it was record- 

 ed in only 10 states (Fig. 2a). Turkeys were rein- 

 troduced into all but three states within its orig- 

 inal range and introduced into all the states out- 

 side its original range (Fig. 2b). Obviously, 

 management has had a major effect on this 

 gamebird. 



Similarly, people may have contributed to a 

 change in both ranges and abundances of vari- 

 ous seed-eating birds (Fig. 3). On average, a 



Mule swan 



^^ 



■ 0,71-1,00 



m 0.46 - 0,70 



□ 0,26 - 0,45 



□ 0,11-0.25 



□ 0.01-0,10 



□ 



□ No data 



M 



Fig. 1. Range and abundance pattem.s of the mute swan, (a) Data Irom 1901 to 1940. (b) Data 

 froni 1460 to 1989 (except l%9). 



third of the households in North America pro- 

 vide about 60 lb of bird food a year, with the 

 average being even higher in New England 

 (Ehrlich et al. 1988). Consequently, feeders 

 may have contributed to the expansion of win- 

 ter ranges of some birds into the northeastern 

 part of the country (e.g., mourning dove 

 [Zcnaiclci macroiira] Fig. 3; tufted titmouse 

 [Parus hicolor]: noilhern cardinal; and evening 

 grosbeak [Coccothrauste.s vespertiiiiis]). 



Habitat change due to logging may have 

 contributed to the extensive and recent range 

 changes of the barred owl (Stri.x varia; Fig. 4), 

 which tends to prefer mixed-aged forests. 

 Before 1972 no northern populations of this owl 

 were reported west of the 100th meridian (Root 

 1988a). The recent expansion is of concern 

 because this owl's range is now partly sympatric 

 with that of the endangered northern spotted 

 owl (iS. occidentalis caitrina), which prefers 

 ancient forests. The consequences of competi- 

 tion between these two species are not under- 

 stood well yet, but nesting sites, foraging, and 

 diet are similar, particularly in the Northwest 

 (Taylor and Forsman 1976). Anecdotal evi- 

 dence, however, suggests the larger, more 

 aggressive barred owl may be able to displace 

 the smaller spotted owl (Sharp 1989). 



Other raptors (e.g., northern hairier [Circus 

 cyaneus] and feiTuginous hawk [Biiteo regalis]) 

 have also significantly expanded their ranges. In 

 particular, the golden eagle (Aqiilla cluysaetos) 

 has moved east, while the bald eagle 

 {Haliaeetus leucoceplialus: Fig. 5) has spread 

 into the center of the continent. 



Over the years humans have strongly influ- 

 enced the expansion of the bald eagle's range 

 through water-management programs (Root 

 1988a). Large lakes and impoundments built in 

 the 1930's, locks placed on major waterways. 



Barred owl 



Wild turkey 



Fig. 2. Range and abundance pat- 

 terns of the wild turkey, (a) Data 

 from 1901 to 1940, (b) Data from 

 1960 to 1989 (except 1969). 



Mourning dove 



Fig. 3. Range and abundance pat- 

 terns of the mourning dove, (a) 

 Data from 1901 to 1940. (b) Data 

 from 1960 to 1989 (except 1969). 



Fig. 4. Range and abundance patterns of the barred owl. (aj Data from 1901 to 1940. (b) Data 

 from 1960 to 1989 (except 1969). 



