Counting What Is Actually There: Sampling, Richness and Diversity Indices
π― Today's mission briefing
We are learning to describe biodiversity in terms of richness and evenness, to design a sampling method that will not fool us, and to calculate and interpret a diversity index from data we collected ourselves.
You'll know you've got it when:
- I can distinguish species richness, species evenness, genetic diversity and ecosystem diversity, and explain why a count of species alone is an incomplete description
- I can design a quadrat or belt transect survey with justified quadrat size, quadrat number and placement method, and state why each decision was made
- I can calculate Simpson's diversity index and a capture-mark-recapture population estimate, and interpret both against the assumptions they depend on
- I can identify at least three specific sources of error in my own sampling and explain the direction each one would push my result
Two green patches, and the survey that disagreed with everybody
Connects to what your guest already knows and makes them curious. Activating prior knowledge is one of the strongest predictors of new learning.
Stand at the back fence of an ordinary Queensland block. On your left, the mown lawn: green, healthy, unremarkable. On your right, a two-metre strip along the creek that nobody has touched in three years: also green, and considerably scruffier. Ask ten people which patch has more life in it and you will get ten answers based on vibes. Now do it properly. Drop a one-metre quadrat on each, get down on your knees, and count every plant individual you can identify. The lawn gives you four species and fifty plants β but forty of those fifty are the same couch grass. The creek strip gives you six species and fifty plants, spread far more evenly. Four species against six. That sounds like a small difference. By the time you have run the numbers this lesson, you will find those two patches sit at 0.35 and 0.82 on the same index β a gap so large that the species count alone was nearly useless as a description. Biodiversity has never been a headcount.