IB Biology SL 4.1.15 Chi-squared tests in ecology Question Bank

Practise IB Biology SL 4.1.15 by using chi-squared tests to assess associations in ecological data.

Exam points

Explain what a chi-squared test can test in ecological research.
Use observed and expected frequencies to interpret an ecological association.

[Maximum number: 3]

White sage (Salvia apiana) is a native Californian shrub. Slender oat (Avena barbata) is a grass, originally from the Mediterranean, which was introduced to California. The map shows the distribution of the two species in relation to one another in an area near Santa Ynez, California.

Figure for Question C4.1.15—Chi-squared test question 1 — IB Biology SL

Using the scale bar, determine the area of quadrat A1, giving the units.

Outline how chi-squared can be used to test for an association between the distributions of the two species.

a. in each quadrat determine the presence/absence «of plants» of each type

b. null hypothesis is that the presence of one is random/independent in relation to the presence of the other plant

OR

alternate hypothesis is that the presence of one is associated with the presence or absence of the other

c. $x^{2}=\sum \frac{(O-E)^{2}}{E}$

d. accept alternative hypothesis / reject null hypothesis if the difference between observed and expected is statistically significant / p<0.05 / calculated $X^{2}$ higher than tabulated $X^{2} /$ critical value

OR

it supports the association between the two species if the difference between observed and expected is statistically significant / p<0.05 / calculated $X^{2}$ higher than tabulated $X^{2} /$ critical value

Equation may be written out in words.

3 max

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C4.1.15—Chi-squared test question 1