Questions: Invasive Species Establishment and Mechanisms of Impact

Question 1

A generalist herbivore from Continent A, where its population was regulated by a specialist parasite, is accidentally introduced to Continent B, where that parasite is absent. What does the enemy release hypothesis predict will happen?

• A. The herbivore will fail to establish because it lacks ecological familiarity with the new environment
• B. The herbivore's population will grow unconstrained, as the key ecological brake on its population is gone
• C. Native herbivores will quickly outcompete and extirpate the invader through superior local adaptation
• D. The herbivore will rapidly evolve new competitive traits within one generation

Answer: (B)

The enemy release hypothesis predicts explosive population growth when a species escapes the parasites, predators, and pathogens that regulated it at home. The herbivore is not inherently superior — it simply lost its ecological constraints. The brown tree snake on Guam exemplifies this: with no natural predators, a species that was regulated at home drove multiple native bird species to extinction. Ecological success here is relational, not intrinsic.

Question 2

Which factor best explains why high-diversity, undisturbed native communities are more resistant to invasion than disturbed, species-poor communities?

• A. Native species produce chemical defenses that are toxic to invaders
• B. High diversity and intact trophic structure leave fewer open resource niches for an invader to exploit
• C. Undisturbed communities are geographically isolated from invasion pathways
• D. High-diversity communities have more predators that specifically target invaders

Answer: (B)

Community assembly theory predicts that well-occupied resource space resists new arrivals — there are fewer empty niches for an invader to fill. Disturbed habitats with simplified food webs, open resource space, and reduced competition are invasion-prone. This is why roadsides, agricultural margins, and post-fire areas are invasion hotspots: disturbance opens resource space that invaders can exploit.

Question 3

A species that becomes highly invasive in a new region should have been a dominant competitor in its native range.

• T. True
• F. False

Answer: False

Invasive success often has nothing to do with inherent competitive superiority. The enemy release hypothesis shows that a species can be thoroughly ordinary in its native range — kept in check by parasites, predators, or pathogens — and become explosively successful elsewhere simply because those constraints are absent. Invasiveness is relational: it emerges from the interaction between what the invader lacks (natural enemies) and what the new environment lacks (the agents that would impose those constraints).

Question 4

Invasiveness depends on both the traits of the invading species and the characteristics of the receiving community, not on invader traits alone.

• T. True
• F. False

Answer: True

This is the interaction principle of invasion ecology. A generalist invader with rapid reproduction may fail to establish in a high-diversity, undisturbed community with few empty niches, but explode in a disturbed, simplified ecosystem. Neither invader traits alone nor community vulnerability alone predicts invasion outcome — it is the fit between a well-suited invader and a susceptible community that transforms an introduction into an invasion.

Question 5

Explain the enemy release hypothesis and why it suggests that invasiveness is not simply an intrinsic property of the invading species.

Model answer: The enemy release hypothesis holds that many invaders succeed because they leave behind the parasites, predators, and pathogens that regulated their population in their native range. In the new environment, without these ecological constraints, population growth accelerates dramatically — not because the species is inherently superior but because a key demographic brake is missing. This makes invasiveness relational rather than intrinsic: the same species that is ecologically ordinary at home can be devastatingly successful elsewhere when its natural enemies are absent.

This framework has significant management implications: it predicts that biological control (introducing natural enemies from the invader's home range) can reduce invasive populations, as has succeeded with some weed biocontrol programs. It also explains why eradication is so difficult once a species is established — without the enemy community that kept it in check, even small residual populations can rebound rapidly.