|Population||A group of individuals that belong in the same species and live in the same area; for example, the stray cats of New York City|
|Population ecology||The ecological study of how biotic (living) and abiotic (non-living) factors influence the density, dispersion, and size of a population|
|Population size ()||The number of individuals in the population; for example, deer in a forest ()|
|Density||The number of individuals per unit area or volume; for example, deer per acre of land in a forest|
|Density-dependent factor||Referring to any characteristic that changes population size () because it is affected by population density; for example, competition|
|Density-independent factor||Referring to any characteristic that changes population size () because it is not affected by population density; for example, natural disasters like an earthquake|
|Dispersion||The pattern of spacing among individuals within the boundaries of a population; for example, a clumped dispersion|
|Carrying capacity ()||The maximum population size (max ) that can be supported by the available resources in an environment; for example, for the alligators in a swamp|
|Exponential growth||Growth of a population in an ideal, unlimited resources environment; for example, bacteria left on a nutrient-enriched petri dish overnight|
|Logistic growth||Leveling off of exponential growth due to limiting resources; for example, the current human population|
How many bunnies are on an island?
Imagine that you're a population ecologist sent out to a newly discovered island in the Pacific Ocean. Your speciality as a population ecologist is studying various bunny populations on the mainland. However, now your task is to go out and answer a bunch of new questions related to the population ecology of the bunnies on this new island. What are some things that you, as a population ecologist, are interested in figuring out?
First and foremost, you need to figure out just how many bunnies there are in the population. In population ecology, the population size, symbolized as , is the total number of individuals in the population. For example, if there are bunnies on the island, the population size is , or .
However, what if you wanted to know more than just how many bunnies there are on the island? Sometimes a more interesting value to an ecologist is not just the size of the population, but also, how the population size is distributed or dispersed throughout a given area. In this case, some questions of interest would be:
- How exactly is the population spread out within a given area?
- Are there many bunnies tightly packed together in one area of the island? Or are they more evenly spread out all over the island?
In order to answer these new questions, an ecologist would look at the density of the population, or the number of individuals per the unit area or volume.
For example, imagine that the only food source for the bunnies is found on one hilltop at the center of the island. You can now ask (and answer) questions such as:
- How are the bunnies distributed on the island? With only one single food source, they would most likely all be densely packed at the center of the island.
- What if there were many food sources spread throughout the island? How would this change the bunny population's distribution? In this case, we would expect to see a more evenly dispersed bunny population throughout the island.
To make more sense of the different possible ways populations may be distributed within a given area, an ecologist can focus on the dispersion of the population, or the pattern of spacing among individuals within the boundaries of a population. In ecology, individuals in a population may be distributed in a three general ways: uniform, random, or clumped dispersion.
Often times, the dispersion of individuals in a population provides more information about how they interact with each other—and with their environment—than a simple density measurement.
In addition to knowing the density and dispersion of a population, another factor that is important to study in population ecology is how the population size () is changing over time. It's important to remember that ecology is a dynamic study, meaning that it involves looking at the changes that happen to groups of organisms and how those changes affect their constant interactions with the biotic and abiotic factors of their environment.
Ecologists use a variety of mathematical methods to model population dynamics (how populations change in size and composition over time). Let's look at some examples of these dynamics and the how they could apply the island bunnies.
Some models represent population growth without environmental constraints in which the population size () is undergoing exponential growth. In other words, this model shows the growth of a population in an ideal environment with unlimited resources. This can be depicted as a J-shaped curve on a graph that models population growth.
For example, if the bunnies on the island had unlimited resources (e.g. food, water, shelter, mates), their population size () would continue to grow exponentially. In this "perfect" world of unlimited resources, there are no environmental constraints to reduce their population size (), and therefore, the island, given enough time, would be absolutely full of bunnies hopping around everywhere!
In reality, exponential growth is difficult to sustain over long periods of time for any population (including the bunnies) because resources are limited in nature. If this is the case, what would happen to the bunny population after it begins to face such limitations to growth? In order to answer this question, we can use a different growth model used by ecologists known as the logistic growth model.
In the logistic growth model, the population experiences a leveling off of exponential growth due to limiting resources. In turn, the shape of the graph, which was formerly a J-shaped curve, now shifts to an S-shaped curve on a graph that models population growth.
What exactly determines this "leveling off" of the population's growth? In population ecology, carrying capacity, symbolized as , represents the maximum population size that can be supported by the available resources in an environment. Once carrying capacity () has been reached, populations tend to fluctuate around (), periodically going slightly over and under this population size based on the available resources in the environment.
Population growth regulation
As you saw in the logistic growth model, the bunny population's growth, along with all other populations' growth, will always end up having to face some kind of resistance to growth, particularly when the population reaches carrying capacity (). As a population ecologist, it's also worth trying to figure out what exactly are the factors that regulate, or influence, the size and growth of the bunny population?
The bunny population's growth is influenced by two main factors–density-dependent factors, in which the density of the bunny population at a given time affects its growth rate, and density-independent factors, which influence growth rate of the bunny population regardless of its population density.
In the table below, examples of density-dependent factors are shown. With these factors, an increase in population density causes each of these factors to exert an even stronger influence on the population size ().
|Competition||In crowded populations, increasing population density intensifies competition for resources and may cause a decrease in population size ()||Lots of bunnies = lots of competition for limited resources|
|Predation||As a prey population gets more and more dense, predators may have easier access to feed on that species||Lots of bunnies = lots of easy targets for predatory cats on island|
|Disease||In dense populations, diseases can spread more rapidly and may cause a decrease in population size ()||Lots of bunnies = lots of hosts for diseases|
Density-independent factors are quite different. Many abiotic (non-living) factors influence the death rate of a population regardless of its density, including weather, natural disasters, and pollution. An individual bunny may be killed in a catastrophic earthquake on an island regardless of how many bunnies happen to be in that area. Its chances of survival are the same whether the population density is high or low, thus exemplifying a density-independent factor.
As you can see, population ecology is about a lot more than just counting bunnies on an island. Instead, with a deep understanding of all the different factors that affect the bunny population, you, as the population ecologist, can now describe the dynamic changes occurring in the population and, importantly, even predict future changes based on what is already known about the bunnies' past and present population dynamics. Off to the next island!
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- Students might mistakenly think that once a population reaches its carrying capacity (), it no longer changes in population size. This is definitely not true in population ecology! Populations do not permanently remain at carrying capacity (). Remember that ecology is a dynamic study that always involves ever-changing factors influencing populations and their growth. Most stable population sizes fluctuate around , as opposed to remaining exactly at that value.
- Students might mistakenly think population growth models can only be either exponential or logistic...this is not the case! Population growth rates aren't exclusively exponential or logistic in their growth pattern. Population growth models are on a spectrum that can range from exponential to logistic. Growth patterns often fluctuate anywhere between purely exponential and purely logistic growth.
- Students might mistakenly think that population sizes are always increasing. However, population sizes aren't always growing. Sometimes, a population might be experiencing serious enough issues that decrease the population size (). A decrease in population size is caused by a negative growth rate, which can be due to a variety of factors like disease, famine, or natural disasters that lead to widespread deaths over a given time period.