14.
Scientists marked and released 100 lizards on 4 islands. Each island is home to predators.
After a month they trapped and counted all surviving marked lizards. They then repeated
the experiment, but after removing all predators from each island. Their results are
summarised below. Which of the following inferences can you make from this plot?
a. Predators have no impact on lizard survival
b. Predators have a bigger impact on female survival than male survival
c. Some islands did not have any predators to begin with
d. Females survival is twice that of males, but only in the presence of predators

Question recap

Scientists released 100 marked lizards on four islands, each with predators present. After one month, they counted surviving marked lizards. Then all predators were removed from the islands and the experiment was repeated. The bar plot (proportion of survivors) compares survival of females and males in predator‑free vs predator conditions.

From the plot, survival is higher on predator‑free islands and lower on islands with predators for both sexes, but the reduction in survival is clearly larger for females than for males.

Correct inference from the plot

The correct answer is: b. Predators have a bigger impact on female survival than male survival.

• In the bar graph, female survival in predator‑free conditions (light bar) is high, while female survival in the presence of predators (dark bar) drops sharply, indicating a strong negative effect of predators on females.

• For males, survival also decreases when predators are present, but the difference between predator‑free and predator bars is smaller than in females, showing that the impact on males, though negative, is less pronounced.

Therefore, predators reduce survival of both sexes, but female survival is more strongly reduced than male survival, which is exactly what option b states.

Why option (a) is incorrect

a. Predators have no impact on lizard survival

• The plot shows clearly that survival proportions (heights of bars) are higher in predator‑free conditions and lower when predators are present for both females and males.

• If predators had no impact, bars for predator and predator‑free treatments would be of similar height for each sex. Because they are not, this statement contradicts the data and is therefore wrong.

Why option (c) is incorrect

c. Some islands did not have any predators to begin with

• The experimental description states that each island is initially home to predators; predators were removed only for the second round of the experiment.

• The plot only compares survival under “predator‑free” vs “predator” treatments; it does not show any information about initial absence of predators on any island. Thus, claiming that some islands originally lacked predators is not supported by the data.

Why option (d) is incorrect

d. Females survival is twice that of males, but only in the presence of predators

• In the presence of predators, the female and male bars are of similar order of magnitude, and female survival is not depicted as twice male survival.

• The figure also does not support a “twice as much” relationship exclusively in predator conditions; rather, it shows that both sexes decline in survival when predators are present, with a larger drop in females, not a simple two‑fold ratio.

SEO‑optimized introduction to the concept

Predators play a crucial role in shaping female and male lizard survival, and understanding these sex‑specific effects is central in ecology and evolution. Exam questions often use bar plots of survival rates to test a student’s ability to draw correct inferences about predator impact on different sexes. This lizard island experiment is a classic example where predators affect survival of both sexes, but the strength of this effect differs between females and males, leading to a nuanced interpretation rather than a simplistic conclusion.