Chapter: Population Ecology

The ecological definition of a population is a group of individuals of the same species in the same area the same thing. Population ecology studies how and why a population changes over time. And very commonly, population ecologists use mathematics as a predictive tool.

Demography

Demography is the study of factors that determine the population size and structure through time. It looks at how populations grow and decline. Populations increase through births and immigration (with an I), where as population decline by deaths and emigration with an E. Put on your math brains, because this is where we begin. The equation on this slide represents all of these principles. A new population is a function of the old population plus the amount of births and deaths and how much immigration has happened. The new population is defined as P2, whereas the old population is P1.

Survivorship curve

Population ecologists often look at a survivorship curve of a population. A survivorship is a graph of the number of survivors in a population through their life span.  In other words, it is the proportion of offspring that survive at a specific age. And in general there are three types of survivorship curves. The rarest of the survivorship curves is Type I. This is where survivorship is high through the life span of the organism, and suddenly drops toward the limits of the organism’s life span. This is the typical survivorship curve for humans. Once born, we have a relatively high rate of survival until old age. In Type 2 survivorship,  there is steady survivorship throughout the lifespan of a species. This means that there is a equal chance of dying through the lifespan of that species lifespan.

Many organisms are type three. In this, there are high deaths in early age. However, once these organisms are established, individuals of this species tend to live for a long time. The classic example of these are trees. An oak tree will produce thousands of acorns, but chances are only a few of these will live beyond being seedlings. And even fewer will grow to maturity. Population ecologists are also concerned with fecundity. Fecundity is the number of offspring produced by female in a population at a particular age. Species can either produce many species at a time. Rabbits are a great example of this. In this case, they are limited to the amount of energy they can provide an individual offspring. Other species have far fewer numbers of offspring at a time, but are able to invest far more energy into their growth and development. Elephants are a great example of this form of fecundity. Population ecologists can use the survivorship curve and fecundity in calculating the growth rate of a population.

Fitness tradeoffs

Fitness is defined as the ability of an organism to live and reproduce. Organisms all exist somewhere on a continuum of fitness. Since resources are limited, organisms can not maximize both reproduction and survivorship. They can either devote most of the energy towards survival. However, their ability to reproduce is severely compensated. Other organisms may have high fecundity, but they risk death themselves if they over extend their abilities to reproduce. However, most organisms exist somewhere in the middle, with moderate fecundity and moderate survivorship. In this way they can persist in the hard times, yet have relatively high reproduction in good times. If you look at the graph, you can see the effect of resource allocation on fecundity. On the independent variable is the average clutch size of a species of birds. This is the number of eggs a birds lays. The dependent variable is the probability of survival to the next year. As you can see, birds with very low clutch sizes have a very high chance of living through the next year. In contrast, birds that have very high clutch sizes have a very low chance of surviving through to the next year.

Population growth

Population growth is a primary component of population ecology. The growth rate of a population is symbolized by the letter r. And r is defined as the change in the numbers of individuals of a population (delta N) through time (delta t). r is calculated as the number of births minus the number of deaths in a population through time. And r varies through time. If it is positive, the population is growing. If it is negative the population is shrinking. And if r  is zero the population is neither growing nor shrinking.

Exponential growth

The human population is currently experiencing population growth. Exponential growth occurs when the growth rate (r) doesn’t change over time. Think of it this way. 2 become 4. 4 become 16. 16 become 256. So in just a few generations, populations explode. In ecology, this is known as a density independent growth. What this means is that the growth rate, r, does not depend on the density of the population. Density independent populations do occur in nature under certain conditions. These kinds of growth rates are found when individuals found a new habitat, such as when the ancestor of Darwin’s finches first reached the Galapagos islands. Another example of density dependence is when population are completely devastated by a major disturbance such as a hurricane and then recover from a few individuals. However, density independence growth rates can’t continue forever, because once the population reaches a certain number the growth rate decrease due to competition for resources. At this point, the population becomes density dependent. 

Logistic growth

The maximum number of individuals that a species in a given area can hold is known as that population’s carrying capacity, and it is symbolized with the letter K. And quite simply, if the population is less than K then the population increases, and when the population is greater than K the population decreases. On the right of this slide is  simplified example of a logistic growth equation of a density dependent population. The section of the logistic growth curve where early growth is very rapid is known as its r phase. And it is where r is constant and the growth of the population is constant. This is also known as the density independent stage of population growth. Once the growth begins to slow, the population begins to become density dependent and enters the K phase. As the growth rate, r, begins to reach the carrying capacity, K, the population size begins to level off. In mathematical terms, r begins to reach zero as it approaches K. 

Density dependence

Perhaps the most important factors that determine the value of K is density dependence. And some of the factors that affect density are predation and over-exploitation of resources. In the example of predation the higher the population of the prey becomes the easier it is for predators to capture them. In this way, predation affects density dependence.

Another major determinant of K is resource availability. As a population is colonized resources are abundant and growth is exponential. In other words it is density dependent. However, once food, space or light become limited the exponential growth slows yet continues growing towards K. If you have ever garden, you know well about density dependence. You know that after your seeds have sprouted, you must thin your garden plants. If you don’t all of the seedlings will complete for limited light and space. You may end up with more plants, but they won’t produce well at all. That is because of density dependence.

 K varies

So what number is the carrying capacity. Well, it varies. It varies among different species. It can also vary within population due to a variety of factors such as resource availability and predation. And not all habitats are the same. Some habitats can have higher population density, such as those that have more available resources or lower predation rates. And also, carrying capacity is not constant for a given area. There are better years than others, and this has a direct effect on the carrying capacity.

Metapopulation

Population ecologists also think beyond population. Metapopulations are populations of populations. An ecologists can look at the numbers of individuals of population of several populations and compare them. This is known as metapopulation ecology. And in metapopulation ecology, some patches can go extinct, redeveloped and the reestablish over time.

Age structure

Age structure is the proportion of individuals in a population at each age group. And looking at an age structure of a population can tell us a lot about the nature of that population. By looking at an age structure, a population ecologist can predict how a population’s growth will be affected over time.   And the age structure of a population varies over time. For example, an age structure was determined at after a tree fell over creating a tree gap in the above graph. In this graph, you can see that there were many more seedlings than other ages. However, as the tree grows, the population as a whole declines leaving mostly adults. In this way, we can predict future age structures based on past age structures.

Humans age structure

Here is a comparison of age structures of two populations of humans. On the left is a typically age structure of a developed country. There is a fairly even distribution of age groups in the population until later in life. In comparison, the graph of the right is a developing country. It has a very unequal distribution of age groups. This shows you that the likelihood of surviving childhood is far lower than that of a developed country.

 Human population growth rate

Humans are currently in an r phase of population growth. In other words, our population growth is expanding exponentially. The current estimate put the global population at approximately 7 billion. Whereas, at the beginning of the 20th century, the population was less than 2 billion. This has had very significant effects on the global ecosystem, including the loss of over 80% of habitat on earth through urbanization and the conversion of wildlands to agricultural lands. And this may come as a surprise to you. We currently live on Earth in the time of the highest extinction rate ever. Extinction rates are nearly double of what they were when the dinosaurs went extinct. So what is going to happen. The answer is all determined on what the carrying capacity of spaceship Earth. However, at a certain point, we will not be able to sustain more humans and this curve will taper off towards human’s K. What is the carrying capacity of the human species? …. Only time will tell.