Populations Dynamics

STOP - You must read Chapter 1 before doing this lesson

Introduction

What determines whether a population will increase or decrease abruptly or whether it will maintain a delicate balance with its neighbors? Why some populations grow to enormous numbers while others do not?

One of the most important concepts to be gained from the study of populations dynamics is that different mechanisms regulate population density, depending on the niche an organism occupy in its ecosystem and the stability of that ecosystem.

Exponential Growth and Doubling Time

Exponential Growth - growth at a constant rate of increase per unit of time. It is also called geometric growth. For example 2, 4, 8, 16, 32 ... or 3, 6, 12, 24, 48 ...
Arithmetic Growth - pattern of growth that increases at a constant amount per unit of time. A constant amount is added to the population. For example 1, 2, 3, 4 ... or 1, 3, 5, 7, 9 ...

Population Oscillations

In the real world, there are limits to growth. When a population exceeds the carrying capacity of its environment or some other factor comes into effect, death rates begin to surpass birth rates.

Carrying Capacity of the Environment - How much population the environment can sustain.

Overshoot - When a population exceeds the carrying capacity of its environment.

Dieback - When population decreases as fast, or faster, than it grew.

The growth rates of many species are regulated by both internal and external factors. These species may growth exponentially when resources are unlimited but their growth slows as they approach the carrying capacity of their environment.

Malthusian Strategies

Organism with Malthusian growth patterns often tend to occupy low throphic levels in their ecosystems. They usually do little to care for their offspring or protect then from predation. They depend on sheer number and dispersal mechanisms to ensure that some offspring survive to adulthood. Many insects, rodents, marine invertebrates, parasites, and weeds follow this reproductive strategy. Their numbers generally are limited by predators or other controlling factors in the environment.

Logistic Strategies

While environmental resistance is a factor in controlling population growth in all species, those exhibiting logistic growth tend to grow more slowly and are more likely to be regulated by intrinsic characteristics than those with Malthusian patterns. This organism are usually larger, live longer, mature more slowly, produce fewer offspring each generation, and have fewer natural predators.

Factors that Regulate Population Growth

What factors regulate natality, mortality, and other components of population growth?

Factors can be:

Intrinsic- operating within individual organisms or between organism of the same species.

Extrinsic - imposed from outside the population.

Biotic - caused by a living organism.

Abiotic - caused by a nonliving components of the environment.

Density Dependent - effects are stronger or a higher proportion of the population is affected as population density increases.

Density Independent - the effect is the same or a constant proportion of the population is affected regardless of population density.

Biotic factors tend to be density dependent and abiotic factors tend to be density independent.

Population Ecology web site.