Population Density and Cycles

by Jason Amores Sumpter
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factors that affect population growth rates can be said to be either density, independent or density dependent. Now density independent factors will affect population growth rate regardless of the density. This includes stuff like severe weather or natural disasters like this forest fire you see here. Now density dependent factors will change growth rates based on changes in the population density. There are many factors to consider here, including things like competition with other organisms, which could be for resource is or mates or space. Also, diseases spread more easily in denser populations and denser populations can also attract and better sustain predators. Some organisms will also experience intrinsic factors, which are internal changes that will affect its ability to reproduce. Some organisms will actually experience intrinsic factors, uh, due to, uh, high densities in their population. So density dependent intrinsic factors and in some cases thes will actually suppress reproduction so that the organism doesn't add to an already oversaturated population. Now, toxic waste can also accumulate to the point of affecting population growth. For example, yeast which produce alcohol will grow and grow and grow and grow, and if they're in a closed environment, they'll actually grow until they kill themselves with the ethanol they produce. Now here is a graph that just I want you to just think of. This is a generic representation of density dependent factors. So let's just think of the y axis as n and think of. I'm sorry. Think of the X axis is and and the Y axis is are So this is our population growth rate and this is our population size. And all I want you to get out of this is that our growth rate goes down as our size goes up. So that would be illustrating a density dependent factor. So as our population size increases, its growth rate plummets. All right, moving on. Some populations actually experienced regular fluctuations, and these air sometimes referred to as population cycles very famous example of this, although not totally accurate. Um, but for our purposes, it's it's the perfect example. Uh, is the snowshoe hare population and links population in Canada, and these populations actually cycle based on interactions between the two species? The reason I say it's not totally correct is because it's been found out. You know that there's other factors at play here, but we can just make a reductive ist, you know, view of this. It's fine for our purposes. So basically the links this cat you see here, it eats the hairs. As hair populations grow, the lynxes are able thio exploit the availability of their food source and their own population will grow. So as the links food food source becomes more abundant, the lynx population increases. And as the lynx population increases, it means they're going to be eating more and more hairs, which can lead to over predation. And a crash in the hair population with scarce hairs means there's less food available for the lynxes, which will lead to a decrease in their population with a smaller links population. That means there's less predation, which allows the hair population to grow again. And that's why we see this cycle you see here the hair population we use red, so it's easier to see the hair population. This blue line is going to peak and crash just ahead of the lynx population in green, and that's because of the pattern just described so that as the hare population goes up, the lynx population will then go up when links population gets too high and they start overeating. The rabbits, the rabbit population, her hair population is going to go down again. And that's going to cause the lynx population to go down because their food source is gone. So hopefully you can see how these two populations interacting leads to this cycle. With that, let's flip the page.