In general, populations have the ability to increase exponentially, but why do you think that most populations are fairly constant in size?

You might want to question that question!

http://www.globalchange.umich.edu/globalchange2/current/lectures/human_pop/human_pop.html

and others: http://www.google.com/search?q=population+growth&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7SUNA

The stability of population sizes is influenced by various factors, including limiting factors and ecological mechanisms that regulate population growth. These mechanisms can counteract the inherent potential for exponential growth.

To understand why most populations are fairly constant in size, we need to consider two important concepts: limiting factors and population regulation.

1. Limiting Factors: These are environmental factors that prevent populations from growing indefinitely. Limiting factors can be biotic (e.g., availability of resources, competition, predation) or abiotic (e.g., climate, temperature, availability of space). For example, if a population's food supply is limited, the population growth may be hindered despite the potential for exponential increase.

2. Population Regulation: Populations are subject to various ecological mechanisms that work to stabilize their sizes. Some of these mechanisms include:

a. Density-Dependent Factors: As a population size increases, the impact of certain factors becomes more pronounced. For instance, competition for resources intensifies, disease transmission may increase, or predation pressure becomes more significant. These density-dependent factors can limit population growth and maintain a relatively constant population size.

b. Density-Independent Factors: These factors can influence population size regardless of the population density. Examples include natural disasters, extreme weather events, and habitat destruction. These factors can cause sudden decreases in population size, preventing a sustained exponential increase.

c. Life History Traits: Many species have evolved life history traits that help stabilize populations. For instance, some organisms have high reproductive rates but short lifespans, ensuring that population growth is sustainable but not excessive.

d. Evolutionary Processes: Over time, populations may evolve adaptations that help them cope with density-dependent and density-independent factors. This process can lead to a balance between population growth and the limiting factors, maintaining a relatively constant population size.

In summary, although populations have the potential for exponential growth, various factors including limiting factors, density-dependent and density-independent mechanisms, life history traits, and evolutionary processes work together to regulate population sizes, resulting in a fairly constant population size in many cases.