Measuring population

Population A group of individual organisms of one species living in the same place at the same time and interbreeding Populations can be defined by natural boundaries (such as an island) or artificially determined boundaries (such as a portion of a forest) How do scientists measure populations in the wild?

Population Density The number of organisms in a particular unit area Mathematically: Number of individuals Unit area 500 earthworms = 25 earthworms 20 square meters square meter

Why is population density so useful? It allows us to compare populations in different places Example: There are 250 deer in Forest A which is 25 square miles in area. There are 40 deer in Forest B which is 10 square miles in area. What is the population density of each?

Forest A 250 deer = 10 deer 25 sq m sq m Forest B 40 deer = 4 deer 10 sq m sq m So Forest A is more than twice as densely populated with deer as Forest B.

Why would anyone investigate something like the density of deer populations? When deer are too dense they eat young plants and understory and then those plants are not able to recover. They also become prone to disease. So population density is also a way to monitor the health of populations and their impact on resources

How do scientists actually measure populations? Do they literally go out and count each deer? No – they use sampling techniques to estimate.

1. Quadrat sampling A certain area is marked off. All organisms of the species being investigated present within the quadrat are counted. This is repeated several times and averaged. The value is multiplied by the number of quadrats in the whole space. What kind of organisms would this work best for?

Limitations of quadrat sampling Can only be used for organisms that are stationary or move very little/very slowly Requires that organisms be evenly spread out (“uniformly dispersed”)

2. Mark-recapture method Catch a certain number of organisms of a given species and tag them or mark them with paint and release them. Return another time and catch another set of organisms: count how many of this group are marked. Use proportions to estimate the total population What kind of organisms would this be useful for?

Limitations of mark-recapture method Animals might move into or out of area between mark and recapture Animals might lose their marks Marking might affect the animals: make them harder to catch next time, make it easier for predators to spot them

3. Indirect counting Count something that represents individual organisms—such as nests, burrows, tracks What kind of organisms would this be useful for?

Limitations of indirect counting What you are counting (such as a nest or tracks) may be old and the organism that made it no longer there If you count burrows or nests, you still don’t know how many occupy each