Ecologists use experiments, observations, and models to answer ecological questions

In an ecological experiment, an investigator alters one or more features of the environment and observes the effect of that change, a procedure that allows scientists to test whether one factor has a cause-and-effect relationship with another.

lakes or forests, are manipulated (FIGURE 1.12).

FIGURE 1.12 EcologicalExperiments The spatial scale of experiments in ecology ranges from (A) laboratory experiments to (B) small-scale field experiments conducted in natural or artificial environments to (C) large-scale experiments that alter major components of an ecosystem, as seen in this clear-cut watershed. View larger image

In some cases, however, it can be difficult or impossible to perform an appropriate experiment. For example, when ecologists are seeking to understand events that cover large geographic regions or occur over long periods, experiments can provide useful information, but they cannot provide convincing answers to the underlying questions of interest. As an example, let's consider global warming.

Climate Change Connection

Approaches used to Study Global Warming

As we will see in Figure 25.11, Earth's climate is warming as a result of the emission of greenhouse gases, but the future magnitude and effects of global warming remain uncertain. We are not sure, for example, how the geographic ranges of different species will change as a result of the projected temperature increases.

Instead, we must approach such problems using a mixture of observational studies, experiments, and modeling approaches. Field observations reveal that many species have shifted their ranges poleward or up the sides of mountains in a manner that is consistent with the amount of global warming that has already occurred (Lenoir and Svenning 2015). Field observations can also be used to summarize the environmental conditions under which species are currently found, and experiments can be used to examine the performance of species under different environmental conditions. To put all this information together, scientists can use results from observational studies and experiments to develop quantitative models that predict how the geographic ranges of species will change depending on how much the planet actually warms in the future.

The observation that global warming has already altered the geographic ranges of species brings us to a topic addressed in many chapters of this book: climate change. This term refers to a directional change in climate (such as warming and/or precipitation) that occurs over three decades or longer. As you'll read in later chapters, climate affects nearly all aspects of ecology, such as the growth and survival of individuals, interactions between members of different species, the relative abundances of species in ecological communities, and the functioning of ecosystems. These observations suggest that changes to climate have far-reaching effects, as shown by the changes that have already occurred in the physiology, survival, reproduction, or geographic ranges of hundreds of species (Parmesan 2006).