IB Biology HL 4.1.15 Chi-squared analysis in ecology Question Bank

Practise IB Biology HL 4.1.15 by analysing chi-squared calculations and statistical conclusions in ecology.

Exam points

Calculate expected frequencies and degrees of freedom for ecological data.
Use a chi-squared result to evaluate an alternative hypothesis and 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 HL

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 species

b. null hypothesis is that the presence of one is random 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

Marking guidance:

Allow for a word description of the calculation.

Accept vice versa for null hypothesis.

3 max

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C4.1.15—Chi-squared test question 1