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17 Documented Current Ecological Impacts of Climate Change Given the compounding factors discussed in the preceding section, it is generally difficult to attribute ecological changes directly or solely to the effects of climate change. Evidence of the ecological impacts of climate change becomes more convincing when trends are observed among hundreds of species rather than relying on studies of a few particular species. Two widely documented and well-studied general ecological impacts of climate change that provide a glimpse into the broader issue are climate-induced shifts in species’ ranges and seasonal shifts in biological activities (known as phenology) or events. These types of change have been observed in many species, in many regions, and over long periods of time. Range and seasonal shifts are not the only general impacts of climate change; other impacts that affect many ecosystems are changes in growth rates, the relative abundance of different species, processes like water and nutrient cycling, and the risk of disturbance from fire, insects, and invasive species. Range shifts Climate change is driving the most massive relocation of species to occur without direct human assistance since the beginning of the current interglacial (warm) period (Parmesan 2006). Each species has a range of climates within which it can survive and reproduce. Species can live only in geographic areas where they can tolerate local temperatures, rainfall, and snowfall (see Figure 7). As Earth warms, the tolerable climate ranges for many species are shifting their locations. About 40 percent of wild plants and animals on land that have been followed over decades are relocating in order to remain within suitable climate conditions (Parmesan and Yohe 2003). Maximum range shifts observed during the past 30 years (up to 1000 km poleward and 400 m upward shifts) surpass responses to regional climate variability during the current interglacial (warm) period of the past 10,000 years, and are approaching the magnitudes of range shifts which occurred during the transition from the last glacial maximum to the current interglacial (Coope 1994,1995; Davis and Shaw 2001; Parmesan 2006; Seimon et al. 2007). Populations or entire species that are unable to move become stressed as the climate around them becomes unsuitable, and ultimately are at high risk of extinction if they cannot relocate (Williams et al. 2003; Thomas et al. 2004; Bomhard et al. 2005; Thuiller et al. 2005; Fischlin et al. 2007). For example, several U.S. Fish and Wildlife Service-listed endangered species live on only one or a few mountaintops. When such a restricted species distribution is coupled with poor dispersal abilities, these species are unlikely to be able to colonize new habitats as their current locations become climatically unsuitable.. One obvious consequence of shifting species ranges is that many of the nature preserves, parks, refuges, and marine protected areas may no longer experience the climates required by the very species for which they were founded. In another hundred years the nation’s carefully planned park, preserve, and refuge system may not function as intended (Opdam and Waschler 2004). The movement of species out of the borders of nature preserves is compounded by the fact that some of the preserved areas are also the ones being hardest hit by climate change. For example, the harsh but fragile landscapes of the boreal tundra on the high peaks of the Grand Tetons, the High Sierra, and the Alaska Range, are being strongly affected by human-caused climate change.
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18 Documented Current Ecological Impacts of Climate Change Range shifts acutely affect species in the Arctic and Antarctic. Temperatures are rising more rapidly near the poles—up to 3°C (5.4°F) warming since 1850 (compared with 0.75°C [1.3°F] average global increase) (IPCC 2007b). As sea ice gets thinner and shrinks in area, so too shrink animal populations that use ice as their home, including the polar bear and the ringed seal in the Arctic (Stirling et al. 1999; Derocher et al. 2004; Ferguson et al. 2005). In the Antarctic, declines in Adelié penguin populations reflect warming-induced declines in sea ice and warming-induced increases in precipitation (Croxall et al. 2002; Ducklow et al. 2007). These animals are retreating toward the poles, and are rapidly reaching the end of Earth as they know it. Cold-adapted species living at the tops of mountains are also being stranded with nowhere to move as warmer temperatures—and formerly lower-elevation species—creep up to higher elevations. As these formerly lower-elevation species move into conditions suitable at higher elevations the available land area tends to get smaller as the elevation gets higher (Figure 8). Of course, an upward shift in each forest type means that the next higher type is either eliminated or pushed even higher. The tundra and subalpine plants and animals that grace the tops of the many high peaks and ridges may disappear completely as they are effectively pushed off the tops of the mountains.
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Documented Current Ecological Impacts of Climate Change 19 FIGURE 7 Shifts in plant hardiness zones between 1990 and 2006. Many gardeners rely on plant hardiness zones to determine which plants will grow in their region. Each type of plant will thrive only in certain zones. These zones have changed since the map was established. The hardiness zone is moving north in most areas. This means that a plant that once could be grown only in the south can now be grown successfully in areas that were not suitable 15 years ago. However, it also means that some plants can no longer survive where they were planted. SOURCE: The Arbor Day Foundation. FIGURE 8 This figure shows current and future types of vegetation from north to south and from lower to higher elevation as a result of future warming. Each zone represents a type of ecosystem. In the future these zones move northward but also upward in altitude, replacing existing zones and creating new zones. At an elevation of 1000 m currently one sees subalpine vegetation in the south and fell-field in the north. In a warmer future, at 1000 m one would see
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20 Documented Current Ecological Impacts of Climate Change boreal forest in the south and subarctic forest in the north. This process is called range shift. SOURCE: ACIA 2004. Seasonal Shifts Climate change is also driving changes in phenology. Many biological events are timed based on seasonal cues, with most of the major ones occurring in the spring and autumn. Many studies looking at changes of the timing of spring events have found that over the last 30 to 40 years, various seasonal behaviors of numerous species now occur 15 to 20 days earlier than several decades ago (Parmesan and Yohe 2003; Root et al. 2003; Parmesan 2007). The types of changes include earlier arrival of migrant birds, earlier appearance of butterflies, and earlier flowering and budding of plants. For example, the date when buds open in the spring in aspen trees in Edmonton, Canada, shifted approximately 26 days earlier between 1900 and 2000, in response to a warming of nearly 2°C (Figure 9) (Beaubien and Freeland 2000). Lilacs carefully observed at over 1100 sites in North America expanded leaves and flowered an average of five to six days earlier in 1993 than in 1959. Autumn changes are not as obvious partly because species vary in the way that earlier springs affect their fall behavior. For example, some birds that arrive earlier in the spring also leave earlier in the fall, regardless of the weather. Many trees, on the other hand, respond to a later arrival of fall by delaying the date their leaves turn color. FIGURE 9 This graph shows when the buds on aspen trees opened in Edmonton, Canada during the 20th century. The zero point is the average date (for the entire century) when buds opened. Each circle represents an historical record of when buds opened in that particular year. The dotted line shows the trend; aspen buds are opening on average 25 days earlier than they did a century ago in response to warmer temperatures. The change in blooming date is an example of a seasonal, or phenology, shift. SOURCE: adapted from data in Beaubien and Freedland (2000).
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Documented Current Ecological Impacts of Climate Change 21 If all the different species in an ecosystem shifted their spring behavior in exactly the same way, the impact of warming temperatures might be minimal. But what happens when a species depends upon another for survival (predator on prey, for example) and only one changes the timing of its spring activity? Such a change can disrupt the predator-prey interaction, which in turn can cause a drop in the predator population. For example, in Europe the bird known as the pied flycatcher has not changed the time it arrives on its breeding grounds, but the caterpillars it feeds its young are emerging earlier (Both et al. 2006). Missing the peak of food availability means fewer chicks are surviving and the pied flycatcher population is declining. Another example of mismatched predator-prey emergence is seen in plankton blooms in the North Sea near England. There, many kinds of plankton (small marine organisms) have changed the timing of their major blooms, but not by the same amount. In response to a warming of about 0.9°C (1.6°F), Ceratium fusus, a tiny plant-like organism, shifted its peak bloom about a month earlier in 1981-2002, compared to 1958-1980, but copepods, their shrimp-like predators, shifted by only 10 days. This kind of mismatch appears to be common in the North Sea, with plants generally shifting farther than the animals that feed on them (Edwards and Richardson 2004).