We have just seen how energy and materials move through the ecosystem. Recall that our primary concern in ecology is to understand the distribution and abundance of organisms. Ultimately, this will relate to organisms' ability to gather and utilize resources. This, in turn, will depend on the availability of the resources in the environment, on the presence of other conditions favorable to organism growth and activity, and on their interactions with other species.
Resources are factors that are consumed by an organism, resulting in their being removed from the environment. These include factors such as nutrients, but also less obvious factors, such as habitats. For example, barnacles require rock surfaces to grow on. As rock surfaces become colonized, growth and recruitment of new individuals is reduced by the crowding. Factors such as temperature are not resources because they are not depleted.
Historically, the effect of resources on growth has been described by Liebig's law of the minimum, which states that a population will increase until a single resource, called the limiting factor, becomes insufficient to support further growth. Thus, for every population in an ecosystem under steady-state conditions, Liebig's law predicts that each would be limited by a single factor. If that factor were known, the population could be increased by supplementing it.
Although it is still a useful concept and often applies to particular situations, Liebig's law is not generally true. For example, it may be possible to increase the growth of grasses in a meadow by supplementing either nitrogen, phosphorus, or water, and supplementing two or three of these together could increase growth even more. One of the factors could become limiting only if the others were present in excess.
Factors can also affect growth in a nonadditive way. For example, growth of Impatiens parviflora was increased 33% by adding nitrogen fertilizer and 19% by adding phosphorus. However, when both were added, the increase was 100%. If there were no interaction, adding both nitrogen and phosphorus would be expected to increase growth 33% + 19% = 52%. This is an example of interaction, which is not accounted for in Lie-big's law. The concept of interaction is important: An interaction is when the sensitivity of a variable to one factor depends on the level of another factor. Here, sensitivity can be given a precise mathematical meaning. It is related to the rate of change of one variable with respect to another. One way to express the sensitivity of variable a to variable b (sab) is da Aa
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