Powerpoint: Feb 16 Population Growth and Decline

Learning Objectives

• Explain an exponential growth curve and a logistic growth curve
• Describe how Density dependent and density independent factors impact population growth
• Interpret a population vs. time graph
• Identify and define carrying capacity and steady state
• Evaluate what happens when species are removed from their natural habitats

Imagine you had an infinite amount of food and comfort. Everything that you ever want, all the food you could eat, all the space you need, all the luxury you can think of. It’s no wonder that you might think, “What a great world, time to reproduce!”
And so you do. But you don’t just have one child, why stop there when there’s so much space and room for all? Why not have two? Three? Five?
Later on, your children have many children and their children have many children and it keeps going on and on.

Notice that for every generation, you’re getting more and more individuals being born than in the last generation. If we were to graph this growth in individuals, it would look something like this:

Exponential Growth Curve

If nothing stops the population from growing, the population would just keep expanding and expanding faster and faster. If we were to graph this, we would produce an exponential growth curve.

Its calculated that if eastern cottontail rabbits were allowed to reproduce to their fullest capacity (20 kits a year/pair), in seven years, we’d have 184, 597, 433, 860 rabbits!

Thankfully, our world is not yet flooded with rabbits and will likely not be. Realistically, a population cannot keep growing unchecked. Something will come along and beat the population down, as it grows too large. We call these factors density- dependent limiting factors.

Density-Dependent Limiting Factors

Density dependent limiting factors are factors that control population size more strongly on large populations than on smaller ones.

1. Competition: when populations become crowded both plants and animals compete, or struggle, with one another for food, water, space, sunlight, and other essentials of life. The more individuals the less space and resources per individual.
2. Predation: As predators become more numerous, they eat more prey than are born and the population of prey decreases. As the population of prey decreases, there is less food for the predators, and their numbers decrease. This predator-prey relationship keeps both species in check. We call this relationship cyclic growth.
3. Parasitism: Parasites are much like predators, but instead of killing them; they live off of them and weaken them. When the population is very large and crowded, parasites are able to travel from one individual to the other faster and the population would decrease. Parasites are detrimental to their prey, but often not deadly. Why? If the prey were to die the parasite would die too.
4. Crowding and Stress: Crowding creates stress and could lead to lowered health that would be detrimental to the population. Some fishes, birds and mammals are also extremely territorial. When population numbers increase, the amount of fighting for space will likewise increase and so will stress.

Thanks to the above four density-dependent limiting factors, organisms do not normally exhibit exponential growth. The growth curve looks more like this:

                                                                                                                Logistc Growth

Parts A and B still look like the exponential growth curve above. This is where there is still lots of space and resource for everyone and crowding has not become a problem. Birth rate >> death rate.

Part C the growth curve begins to level as less births and death of individuals due to predation, parasitism and competition increases. There is still growth though, so birth rate > death rate.

Part D at this population size, the population birth rate = death rate. For every one individual born, one dies. Which means, the population is not growing. Therefore, this part of the graph is called the steady state.

Since in the environment it is in, the population does not generally increase past this number, the number of individuals at the steady state is called the carrying capacity. It is, theoretically, the MAXIMUM number of individuals that can be held.

Density Independent Factors

Not all organisms have their numbers limited by density dependent factors though. Some are limited by factors that have nothing to do with their numbers.

1. Boom and bust populations: locusts and algae for example, grow in great numbers when conditions are right, but die in huge numbers suddenly (population crash).
2. Natural disasters: natural disasters such as floods, rainstorms etc. The population can essentially be wiped out. It doesn’t matter how large the population is at that point. 

Apply your knowledge to a new situation

In any one environment, organisms that have evolved in relation to each other have evolved to deal with each other’s strengths and weaknesses. For example, the lynx and hare each evolves over time to compete with each other, the hare evolving traits to run from the lynx and the lynx evolving traits that allow them to hunt hare down. Similarly in an environment where organisms have evolved together (co-evolved) for a long time, they help to keep each other in check.

However, when organisms are torn away from their environments, the checks and balances are also taken away and in some cases, the population has exploded past control.

We see this in invasive species, such as the scotch broom, European starling and House sparrow. These species, which were introduced from the British Isles, have since become pests that compete with and threaten native species.