Introduction
One of the most important concepts in ecology, and one that is relevant to the natural historian, is the idea of 'environmental gradients'. Webster's dictionary defines 'gradient' as "change in the value of a quantity (as temperature, pressure, or concentration) with change in a given variable and especially per unit distance in a specified direction". Environmental gradients, then, are changes in measurable characteristics of the environment, often with respect to direction.
In ecology, there are a number of gradients that have special importance. Temperature gradients are important to the abundance and distribution of species because temperature affects metabolic activity. This occurs because the activity of molecular processes in an organism is sensitive to changes in temperature and often there is a narrow range of 'optimal temperature'. Low temperature may mean an enzyme operates slowly and the organism may result deficient in the product of a reaction that is promoted (catalyzed) by the enzyme. The deficiency may make it impossible for the organism to perform certain functions. This is what happens when a frog moves slowly or a wasp cannot fly on a cold day. On the other hand, an animal in a hot environment may have to expend energy or evaporate scarce water in order to not overheat (e.g. panting), or may have to remain inactive in a cooler area (desert lizards). Gradients in moisture, sunlight, pH, or soil fertility can affect the distribution of plant species because they affect plants' abilities to carry out processes necessary for life. Different species often have distinct optimal ranges of environmental variables and so species often change in relative abundance along gradients. Additional processes can be involved in determining what range of conditions a species inhabits, such as predation or competition.
Multiple gradients
One interesting aspect of environmental gradients is that they often vary together in a predictable way. For example, by climbing an elevation gradient one comes to areas of cooler temperatures, more rainfall, less oxygen and higher winds. These factors are often only partially separable and it may be possible or impossible to distinguish their effects on the activities of organisms. Additionally, the effects of these correlated factors need not be in the same direction. For example, lower light levels inside a forest may reduce the photosynthetic potential for an understory herb compared to when it grows in a forest opening but the higher humidity and soil moisture of the closed forest environment may allow the herb to photosynthesize over a greater portion of the day.
Environmental gradients may be especially strong on the border between two types of habitat. For instance, the edge between subalpine forest and alpine tundra is an area where the exposure of an organism to wind (and concomitant desiccation) changes greatly over a very short distance. In another couple of examples, imagine the multitude of environmental factors that change when one goes from a clearcut into a stand of ancient forest or when dropping from high ground into a shaded, humid ravine. These examples suggest two things. First, gradients vary in their severity. We can observe strong changes over the space of a few meters, or we detect changes over several kilometers (as when ascending a mountain). Once again, scale of variation is important. You might ask yourself how this relates to variation in scale of habitat and microhabitats within habitats. Secondly, the biota of an area can play a large role in determining environmental gradients; clearcutting quickly establishes the importance of large trees and a closed canopy to light, humidity, and soil moisture regimes.
Disturbance gradients
One of the most important types of gradients in conservation and management of forests is comprised by 'disturbance gradients'. These gradients often manifest in time as well as space. For example, some forests burn with a higher frequency than others; there may be a gradient in fire frequency (and intensity!). Humans may establish disturbance gradients by varying the intensity or frequency of tree harvest or the harvest of other forest products. You might consider a gradient in the degree of urbanization. If one considers urbanization or tree harvest in a number of distinct sites, one notices that disturbance gradients may not have a strong directional component. Disturbance gradients, not surprisingly, have substantive effects on many environmental factors.
Goal and objectives
The goal of this exercise is that you become more aware of environmental gradients and the changes in animal and plant populations that accompany them. There are several objectives that will lead you to a deeper understanding of environmental gradients and their significance to influencing what grows or lives in a particular place. First, you should define and explore an environmental gradient that interests you. What are the environmental factors that comprise the gradient? What are some of the reasons that these factors vary in a gradual way? Is the gradient caused by protection from prevailing winds afforded by a large boulder? Does it come from a decrease in the height and density of forest as you move into pasture? Is it caused by decreasing ice throw as one moves away from the edge of a large lake? Over what distance does the gradient become evident? Is it several centimeters as one emerges from beneath a fallen log, or several meters as you come out from underneath an patch of brush? Perhaps you define the gradient that occurs as you descend by trail off Hurricane Ridge or perhaps you choose the gradient that exists when different rocky cliffs vary in their exposure (aspect) to wind or ocean waves! Describe the gradient in words: what environmental characteristic(s) varies and how come? You may choose to measure the gradient with a thermometer, light meter, anemometer (wind gauge), altimeter, a tape measure, compass, historical fire records, or something else. These tools can help assure that your subjective impressions will be agreed upon by other observers.
Gradients and places
Next, you should identify a number of sites along the environmental or disturbance gradient. These should be particular places. What size should these places be? Are we considering 3 cm x 3 cm plots to look at isopod density or one square meter plots to look at avalanche lilies on a mountain slope? Perhaps you want to consider 8m x 8m areas to look at how salamander density changes with leaf litter depth from conifer forest to adjacent hardwood forest. How far apart should these places be in order to capture the variation that occurs along your gradient? Centimeters, meters, or kilometers? How are these sites different with respect to the gradient at hand? Describe these differences in detail. You may choose to measure differences among the places (sites). You should make your observations along a large enough portion of the gradient so that your organism or community of interest (plant or animal) is confronted with relevant environmental variation. To do this, you'll have to pick an animal or plant (species or community!) that finds the gradient you've defined as being relevant.
Response to gradients
What plants or animals respond to the gradient (as distinguished from organisms that might play a major role in producing the gradient)? Identify these as best you can, hopefully specifying the genus and species. Are you looking at the change in understory plant species? The density of isopods? The number of varied thrushes you find? Song sparrows? Maiden hair ferns? What is the pattern of change among the affected species along the gradient? Are there species substitutions in which one ecologically related species replaces another along the gradient (substitution from within a 'guild') or does one species just appear to become [smaller, larger, more common, sparser, different color, more insect damage, age distribution, that is, different] without another species appearing?
Notes
As alluded to above, your definition of the gradient is something that may have to be repeated if there aren't any species that find your gradient of choice to be relevant to their experience. Your increasing ability to identify relevant gradients indicates that you are learning about the natural world.
As with the previous exercises, use this assignment as a motivator to get outside and look at things. Document your travels, experiences, observations, data, etc. in your journal. Since this is for natural history credit, you may choose to focus your learning in that direction. But above all else, document what you do and what you learn from doing it!!