Why is disease a density-dependent limiting factor on a population but a natural disaster such as a flood is a density-independent limiting factor on a population?

Answer 1

Hi Carly T.,

This is a good question!

The key to understanding why disease is density dependent while most natural disasters are not is to understand the relationship between risk of death and proximity to other individuals.

In the case of disease, the risk of getting the disease and eventually dying is dependent on how close together the individuals are. If they are very close together (dense), then the disease will have an easier time moving from one individual to another and causing more to die. If the individuals are not close together, then the disease does not move as easily, and fewer of the individuals will end up dying. This is in contrast to most natural disasters, where the density of the individuals does not really matter, since the natural disaster effects all individuals equally, no matter how close together they are.

An example might help to clear this up. Since we are in the middle of the COVID-19 coronavirus outbreak, I can use that as an example. If we compare the city of New York, for example, to a rural part of the United States of equal area but identical population size, the key difference as far as disease transmission is that the people are much closer together in New York City than they are in rural parts of the country. As a consequence, COVID-19 is spreading much faster in New York City than it is in rural areas of the United States, and many more people are dying in New York City than in most other places. If we were to compare this to a natural disaster, for example a flood, the difference is that the density of people does not have any effect on their likelihood of dying in the flood; people in rural areas are just as likely to drown as people in a dense city like New York. For example, a person walking in New York City's Central Park next to 20 people is just as likely to drown in a flood as a person walking alone on a rural road. This is the essence of density independence. When there is density independence, no matter how close you are to other individuals, you have the same probability of dying.

For some kinds of disasters, for example a tornado touching down, I can see how it might seem as if there is density dependence. For example, if a tornado struck downtown New York City, a lot more damage and death would result than a tornado hitting a rural part of the state. However, the key is to understand what is happening on a small scale. Within both of these tornado-struck areas, you have to answer the following question: does the physical proximity of the people to one another have any effect on their likelihood of survival? If the answer is no, then this is a density-independent situation. Within the zone hit by the tornado, it does not matter if people are close together or far apart. Everyone in the zone that is hit will have the same likelihood of dying, even though the total number of dead people will be higher in the city than in the rural area.

I hope that this makes things clearer. Please reach out if you are still confused. I can give some more examples that might make this more clear.

Dylan