Pond Boss Forums INTRO Questions & Observations Carrying capacity

Carrying capacity is one of the most misunderstood concepts in biology. Usually people use the term to measure the biomass of a species, group of species or all living things that a specific area can support. An example would be, “our lake has a carrying capacity of…” The thinking is if we want to change the carrying capacity, we have to change the environment. This is a useful way of thinking in most cases.

However useful it is to think of carrying capacity in this manner, I think it is important to understand that the exact opposite is true. It is the ability of the different species to use the resources of the environment that determines carrying capacity.

This can be demonstrated in many ways.

Imagine we have ten ponds, identical in every way. Now, let’s imagine putting a different species of fish in each pond. LMB in one, BG in another, gizzard shad in another, carp in another, trout in still another etc. If the environment determines carrying capacity (as measured by biomass), then there should be exactly the same biomass in all ten ponds. After all, the environment is exactly the same in all ten ponds. Obviously, the biomass of the ten species will be very different depending on each species ability to use that environment.

Now, let’s look at this the opposite way. Rather than fish, let’s use cattle because they have been domesticated longer. Again, the measure of carrying capacity is the amount of biomass per area. With cattle, it would commonly be head per acre. Lets use longhorn cattle as the wild type. They produce a certain amount of biomass per acre of pasture. Then we look at a beef type of cattle and see that they produce much more biomass per acre on the same pasture. The pasture hasn’t changed meaning the environment hasn’t changed. It is the ability of the beef cattle to use the pasture has changed. Now, let’s put dairy cattle on the pasture. They produce much more milk than the longhorns on the same amount of pasture. Again, the pasture hasn’t changed; it is the ability of the dairy cattle to convert this pasture to milk that has changed. The point here is that the environment hasn’t changed, it is the ability of the animals to use the environment that has changed.

Does this mean the environment isn’t important? Absolutely not! If we add resources the species can use, there will be an increase in biomass. But if we add a resource the species can NOT use, we can add all we want and there will be no increase in biomass.

For much of our discussions, this distinction doesn’t matter. However, I think it is always important to understand clearly what we are doing.

I think of carrying capacity mainly as the amount of food, oxygen, and waste absorption/conversion available. When 1 or more of the above become deficient, things can be done to increase it. Then, of course, you limit out on something else. That's why taking heroic measures to increase carrying capacity has a high risk.