AGCI Session II: Characterizing and Communicating Scientific Uncertainty
Session Chairs: Dr. Richard H. Moss and Dr. Stephen H. Schneider
July 31 to August 8, 1996
How to Approach Assessing Climate Impacts on Animals
Terry Root
University of Michigan
Ann Arbor, Michigan
The issue of potential effects of climate change on animals (except fish) was entirely left out of the IPCC process and Root argues that this must be remedied. Models that deal with these issues are very immature at this time. Why are we just starting to look at this problem? One reason is that there are strong controversies in ecology regarding what factors shape animal species' ranges. Root believes that both environmental factors and biotic interactions are important, but the conventional wisdom in the community for at least the last 30 years has been that biotic interactions are dominant in shaping species ranges.
Scale is another significant problem. Most ecological studies are done on areas the size of a tennis court. On this small scale, biotic interactions such as competition and predation can be seen, but not environmental factors such as climate. The dogma has been that it is impossible to understand ecology on a large scale because the biotic interactions cannot be observed at this level. It has thus been widely assumed that animal ecology can not be done on the continental scale, though this scale is needed to observe environmental factors like climate.
Species whose
ranges are related primarily to temperature are going to move north.
But those related to both temperature and vegetation cannot move
until the vegetation moves. So communities may be torn apart and some
species may be driven to extinction.
Bucking this trend, Root has begun a study of the effects of climate on passerine birds in North America. Using National Audubon Society Christmas Bird Count data, she has plotted maps for all species that winter in North America. These data begin in 1900 and identify both the species and the number of individuals observed by volunteers. Using ten years of these data and sixty years of climate data, Root plotted maps for all species that winter in North America. For the purposes of this talk, the edge of the distribution range for each species is of central importance. Root wanted to see how strongly environmental factors such as climate were associated with distribution edges, in hopes of challenging the notion that distribution edges are determined strictly by competition and predation. She finds that indeed, there is a strong association between climatic factors and distribution edges, particularly northern edges.
A large percentage of the 250 bird species studied have their northern ranges associated with climatic factors, most often temperature (defined as average minimum January temperature). In addition, some species ranges are largely determined by vegetation, which is itself strongly related to climatic variables. For yet other species, their ranges are associated with both temperature and vegetation. The possibility exists that with global warming, as the temperature rises, species whose ranges are related primarily to temperature are going to move north. But those related to both temperature and vegetation cannot move until the vegetation moves. So communities may be torn apart and some species may be driven to extinction. This is a concern in terms of pest control (certain bird species eat certain insects) as well as seed dispersal (some bird species transport particular seeds).
How to acquire animal data on a continental scale is a significant problem. With no money for such research, volunteers have traditionally been relied upon for data collection. But legislation currently before Congress could disallow the use of data collected by volunteers. This prohibition has been introduced as part of the proposed reauthorization of the Endangered Species Act (ESA) and is based on the notion that volunteers from organizations such as the Audubon Society have a vested interest in saving species, making data they collect invalid. This issue will be decided in the near future and Root makes the point that volunteer data are critically important to research such as she describes here. As further evidence of this, she points to additional volunteer data used in her most recent analysis. In this study, Root plotted 30 years of data for arrival dates for migrants and found a detectable signal suggesting that some migratory birds are returning earlier as temperatures warm.
Root plotted 30
years of data for arrival dates for migrants and found a detectable
signal suggesting that some migratory birds are returning earlier as
temperatures warm.
Given her conclusion that many bird species' northern ranges are associated with climate, the question arises: why are animals' ranges associated with environmental factors? Each animal species has a metabolic response curve. For any given animal, the thermal neutral zone is the temperature range in which the animal doesn't have to raise its metabolism to cool or warm itself. The animal can also live in the wings of this distribution, outside of its optimal range, where it needs to expend energy to warm or cool itself. But when the temperature gets hotter or colder than the zone of tolerance for that animal, it will die or move. This shows that temperature is very important to animal ranges. Species abundances are also highly determined by this curve, as a species will be most abundant in the optimal temperature range, less abundant in the wings, and not exist at all outside of its zone of tolerance.
For some species, the absolute minimum temperature is the key factor, while for others, it's other measures. This is species-dependent for many reasons. One reason is that microclimate effects impact species differently. Another is that one species may simply not be able to physically survive below a certain temperature, while another's survival is based upon whether it has stored up enough fat to last through a cold spell. In some species of reptiles, temperature determines sex ratios. In turtles, for example, the sex of the young is determined by the temperature at which the egg is incubated; higher incubation temperatures yield more female turtles.
Of the 250 bird species Root is looking at, winter physiology studies have only been done on 14. From these studies, she was able to find the basal metabolic rate of the species, the slope of the metabolic response curve, the temperature at the edge of the distribution, and the lower critical temperature (the temperature at which metabolism has to increase in order for the animal to stay warm). From these, she calculated the metabolic rate at the northern boundary for each species. She found that for a little over 50 percent of the species, the northern boundary metabolic rate was equal to 2.5 times the normal basal metabolic rate. This gives us a relationship that can be used in modeling studies; for those species limited by temperature, we can determine the northern range of the species if we know its metabolic rate.
Basal metabolic rate is strongly affected by food availability which in turn is strongly influenced by day length (for foraging). (Passerine birds have to forage all day to accumulate enough fat to survive through the night.) From mid-December to the first of March, Root studied birds at eight sites paired on one hand by latitude (same day length) and on the other by average minimum January temperatures. The question she sought to answer was: are individuals more closely related between sites that have the same temperature or sites that have the same day length? She found that they were more temperature related, suggesting that global warming could have important effects.
Are individuals
more closely related between sites that have the same temperature or
sites that have the same day length? Root found that they were more
temperature related, suggesting that global warming could have
important effects.
Ecological dogma is that bird distributions don't shift, but Root says this is false. Some of her work has thus focused on showing that birds can and do actually move. Temperature time series data for one year show that birds do go north when its warmer and further south when its cooler and that they are indeed shifting in response to climate. This implies that there will be changes in response to global warming. Some birds move when the temperature gets outside their zone of tolerance, while others die. This is species dependent. For example, the eastern phoebe moves, while the field sparrow dies.
Root is beginning to broaden her study to include other organisms, in particular some amphibians and reptiles. It appears that the ranges of the American alligator and the broad headed skink are also shifting. Organisms that are slow dispersers seem to be most at risk. There is a need to add the factor of land use into this study, Root says. For example, if alligators need to move due to climatic change, will changes in land use, such as farm fields, prevent their needed movement? Climate maps and land use maps are needed to further the study in this and other important ways.
A number of factors are impeding progress on the study and modeling of climatic effects on animals. There is a need for historic climate data (past 30-35 years) at a half degree grid, altitudinally adjusted (time series data). There is also a need for general circulation model data downscaled to a half degree grid, at appropriate altitudes. Abundance data on a continental scale exists only for birds at this time; distribution data is available for other animals, but no abundance data exists. Vegetation data and forecasts are needed as well, since vegetation is important to most animals. Complexity among species (many things are species specific) adds to the difficulty in modeling impacts on animals. The fact that there are no obvious feedbacks to climate from animal impacts is another deterrent. And finally, the widely held assumption that species are largely substitutable is an obstacle to furthering research in this area.
Temperature time
series data for one year show that birds do go north when its warmer
and further south when its cooler and that they are indeed shifting
in response to climate. This implies that there will be changes in
response to global warming.