3: Abundance

Question 1

What is abundance a key measure of?

Answer 1

Community structure

Question 2

Describe Preston’s canonical log-normal distribution

Answer 2

→ How the abundance of one species compares to the abundance of others (Relative abundance)
- Argues that the commonest is 2x as abundant as the next commonest etc.
- Plotted on a log2 scale
= Bell shaped curve
- Jury still out as doesn’t seem to be a law that predicts precisely

Question 3

Generally, do communities have more rare species than common ones?

Answer 3

across local to global scales most communities have lots of rare species and few common ones

Question 4

Describe the type of graph that shows abundance relationships

Answer 4

Species rank abundance diagram
- Commonest species on the left, rarest on the right
- Slope tells us how even the distribution of different species is
- Length of X axis gives strong indication to species richness

Question 5

What do diversity indices do?

Answer 5

They combine species richness and relative abundances of species
- Can vary from 0-1 (0 =no diversity)
- Powerful tool for cf. evenness of communities

Question 6

What do diversity incises express?

Answer 6

These diversity indices are essentially expressing the probability that any 2 individuals drawn randomly from a community will belong to the same species

Question 7

What sort of bias might explain the relationship between range size and abundance?

Answer 7

Sampling artefact? Low abundance → easy to miss → range size underestimated → artificial link between abundance and range size?

= But, UK bird data (with no sampling artefacts) shows the pattern is strong

Question 8

Describe Brown’s hypothesis of Niche breadth

Answer 8

Wide niche breadth → can use lots of diff resources at 1 site → high abundance
Wide niche breadth → can tolerate lots of diff environs – Large range
= + correlation between high abundance and niche size

Question 9

Why is Brown’s Niche breadth hypothesis hard to disprove?

Answer 9

• Because niches are hyperdimensional.
• Difficult to separate from resource abundance: if resources that are widespread are also abundant, will also get + range size-abundance relationship

Question 10

What can rareness be defined by?

Answer 10

Small geographic range size
Small pop. size
Habitat specialist