Parrotfish are the dominant grazer in most Caribbean reef systems. Scientists we
ID: 100588 • Letter: P
Question
Parrotfish are the dominant grazer in most Caribbean reef systems. Scientists were interested in the potential microhabitat preferences of these territorial fish. Along a series of transects they counted n = 60 individuals and recorded whether each was found on coral, sand, or in seagrass. Sand habitat was twice as common in the environment as either coral or seagrass. A chi-square test comparing the observed number of parrotfish in each habitat versus the expected number of individuals found that 2 = 11.6 and P = 0.003. Answer the following questions:
A. Assuming the fish are distributed randomly across the three habitat types, complete the contingency table below (i.e., what are the expected numbers of parrotfish in each habitat?).
Habitat
Sand
Coral
Seagrass
Observed (n)
17
23
20
Expected (n)
30
15
15
1. Given the above results, is there statistical evidence of non-random habitat association by the fish? Explain.
2. Name two ecological factors that might be important in determining the observed pattern of fish abundances across microhabitats; be specific about how each would work.
Habitat
Sand
Coral
Seagrass
Observed (n)
17
23
20
Expected (n)
30
15
15
Explanation / Answer
Chi square value is summation of (O - E) 2 / E
= (17-30) 2 / 30 + (23-15) 2 / 15 + (20-15) 2 / 15
= 169/30 + 64/15 + 25/15
= 5.63 + 4.26 + 1.66
= 11.58
The chi square value is higher than the expected value. So, the null hypothesis is false and therefore, there is an evidence of significant statistical evidence of non random habitat association by the fish.