15. Invasive alien species are animals and plants that are introduced accidently or
deliberately into a natural environment where they are not normally found. Invasive
alien species often have serious negative consequences for the biodiversity of their
new environment. The graph below shows the number of alien invasive plant species
in 10m2 plots as a function of the species richness of native plant species in the same
plots. The measurements are repeated across several mainland locations, each
compared with an adjacent island. Which of the following conclusions can be
reasonably drawn based solely on this figure?
a. Species rich native communities are more resistant to invasion.
b. Islands have fewer species than mainland ecosystems.
c. There tend to be more invasive species on mainlands than on islands.
d. All of the above.

Correct answer: Option C – “There tend to be more invasive species on mainlands than on islands.”
The figure compares invasive plant species richness on mainland vs island plots across the same range of native plant species richness and shows consistently higher invasive richness on mainland sites, so only statement C is directly supported.

Introduction

Invasion ecology often compares invasive alien species on islands and mainlands using graphs that plot invasive plant species richness against native plant species richness. Such figures help ecologists test hypotheses about biotic resistance, species richness patterns and the special vulnerability or resilience of islands. Understanding exactly what a given graph shows is essential to drawing only those conclusions that are directly supported by the data.​

Understanding the figure

In the typical version of this figure, the x‑axis shows native plant species richness, while the y‑axis shows invasive plant species richness for many plots on islands and on mainland sites. Separate trend lines for islands and mainlands reveal that, at the same level of native richness, mainland plots consistently have more invasive species than island plots, although invasive richness declines as native richness increases in both cases.​

This pattern indicates that the relationship between native and invasive richness differs between the two region types. At low native richness, invasive richness is highest, and as native richness rises, invasive species richness decreases on both islands and mainlands, but it stays higher on mainlands at any given point on the x‑axis.​

Option‑wise explanation

Option A: “Species rich native communities are more resistant to invasion.”

This statement refers to the biotic resistance hypothesis, which predicts that communities with more native species resist invasion better, leading to fewer invaders as native richness increases. The downward trend in the figure (higher invasive richness at low native richness and lower invasive richness at high native richness) is consistent with this idea, but the question asks what can be concluded “based solely on this figure.”​

The problem is that the graph alone cannot rule out alternative explanations such as environmental gradients, propagule pressure differences, or management intensity that covary with native richness. Therefore, while option A might be ecologically plausible, it goes beyond what the single figure can logically prove, so A is not the safest conclusion.​

Option B: “Islands have fewer species than mainland ecosystems.”

Island biogeography theory often finds that islands support fewer native species than comparable mainland areas of similar size because of isolation and limited immigration. However, the plotted graph shows only invasive plant species richness vs. native plant richness for specific island and mainland plots; it does not directly show total species counts for entire ecosystems or regions.​

Moreover, many global analyses show that, relative to native richness, islands can actually have very high numbers of naturalized and invasive alien plants, sometimes matching or exceeding mainland richness. Since the graph in the question is about invasive species richness at site scale, not overall species richness of whole ecosystems, the statement that “islands have fewer species than mainland ecosystems” cannot be strictly inferred from this figure alone, so option B is not valid as a direct conclusion.​

Option C: “There tend to be more invasive species on mainlands than on islands.”

In the figure, mainland data points and their fitted line lie above island data points across the same range of native plant species richness values. That means that for any given native richness, mainland plots support more invasive plant species than island plots.​

This is exactly what option C states: at comparable native species richness levels, invasive richness is higher on the mainland than on islands. Because this pattern comes straight from the vertical separation between the two trend lines, it is the most specific conclusion that relies only on the visual evidence of the graph, making option C the correct answer.

Option D: “All of the above.”

For option D to be correct, options A, B and C would all have to be justified entirely by the figure. As discussed:

• Option A invokes a mechanistic interpretation (biotic resistance) that the graph alone cannot conclusively prove, because correlation does not confirm causation.​

• Option B generalizes about total species richness of islands versus mainlands, which the figure does not directly depict.​

• Only option C directly and safely matches what the figure shows without extra assumptions.​

Since A and B are not fully supported, D (“All of the above”) is incorrect.