Lecture 9: Disturbance, Vacant Niches and Enemy Release

Question 1

How does a disturbance contribute to a communities vulnerability to invasion?

Answer 1

• frees up resources by disrupting resident species

Question 2

What are alternative explanations as to why disturbances contribute to invasions?

Answer 2

• many disturbed habitats are important to humans, so exotic species are more likely to be noticed there
• opp for human vector dispersal are high in these habitats

Question 3

Explain the relationship between Anthoxanthum colonists vs biomass of resident veg ?

Answer 3

• if there is a higher amount of community biomass, there is less leaf area seen in Anthoxanthum bc comp is likely happening

Question 4

What are the main habitats for exotic plants and animals, where disturbances facilitate establishment ?

Answer 4

• grassland and islands = plants

- islands and aquatic = animals

Question 5

Explain the general relationship between invasion success and disturbance with plants vs animals

Answer 5

• establishment facilitated by disturbance is much higher in plants vs animals
• **due to the fact plants are sessile for the most part and undergo more comp for space.
• animals can move away (minus fouling animals)

Question 6

Is disturbance essential for range expansion?

Answer 6

• most animal invaders that are established in the absence of disturbance do not require subsequent disturbance for range expansion
• 60% of plant invaders require disturbance for initial establishment and subsequent range expansion
• *bottom line = comp for space is hella important in plants

Question 7

What is the vacant niche hypothesis?

Answer 7

• invasions are more likely to occur in habitats with vacant niches (i.e. where various resources are unused)

Question 8

What is a niche?

Answer 8

• the total set of physical and biological requirements of a species and its ecological role in the environment (Elton def)
• the combination of conditions and resources that allow a species to maintain a viable population (Hutch def)

Question 9

What is the rationale behind the vacant niche hypothesis?

Answer 9

• competitive exclusion principle
  L> no competing species can occupy the same niche in a stable environment
  *Problem: since no two species are completely identical, we can always find a niche dimension along which they could be separated

Question 10

Explain the relationship between risk of extinction and morphological difference (bill size) of introduced birds vs native in Hawaii.

Answer 10

• risk of extinction goes down, the more morphologically different the exotic birds are from the native birds

**of limiting similarity, which predicts that there is a maximum level of how phenotypically similar two coexisting species can be, to allow for coexistence in the same habitat together without exclusion occurring

Question 11

What is the relationship between number of bird invaders vs extinctions of native birds on islands?

Answer 11

• extinctions increase with more invaders
• *not a casual relationship
• *not telling you if they occurred after invasion

Question 12

Explain the relationship of extrpated/imperiled fish versus number of invading sp in NA rivers

Answer 12

• not one to one relaitonship
• there are more coming than leaving
• tells you there were vacant niches?

Question 13

Explain Darwin’s naturalization hypothesis

Answer 13

• introduced species belonging to novel genera are more likely to establish
• rationale: they avoid competitive exclusion
• evidence: studies of plants — inconsistent

**when two things are closely related, they are more likely similar bc of phylogenic relationship is nicely related to ecological role. SO RESISTANCE WILL OCCUR.

Question 14

Explain the Enemy Release Hypothesis.

Answer 14

• species are suppressed in their native range by natural enemies = assumption
• escape from these enemies enables introduced populations to grow explosively (in communities where no similar enemies exist = hypothesis
• successful biological control using co evolved predators/parasites and lower parasite loads in successful invaders = evidence

Question 15

Explain the relationship between natural enemies and movement across biogeographic regions.

Answer 15

• many filters between time the propagules are picked up and travel the vector path dumping them in a new area
• parasites, enemies etc may get lost on this travel.
• movement of a species also is only taking a parasite load that is a sample of whats avail in the whole native range
• more over if the parasite needs an intermediate host, if that something isn’t present, the life cycle won’t work
• SO you expect fewer to be in the non native range
• vectors act asa filter
• *makes sense exotic sp would do well if they aren’t bringing these enemies with them and there isn’t anything similar in the new region

Question 16

Relationship btwn insect damage to native and nonnative congeners of Eugenia?

Answer 16

• native species suffered more damage bc the insects were ADAPTED to foraging on them
• it would take them time to adapt to the exotic species
• maybe the insects aren’t generalists ?

**however in cultivated areas the damage is almost equal, suggesting loss of enemies is not the cause of their invasiveness

Question 17

What are some other considerations of the enemy release hypothesis?

Answer 17

• natural enemies may have different effects in the invaded range vs native range
• enemies native to the introduced range may offset the effects of lost natural enemies (generalist predator )

Question 18

Explain the evolution of increased competitive ability (EICA)

Answer 18

• when released from specialist herbivores, introduced plants can allocate more resource to growth vs defence i.e. can grow larger, produce more biomass and seeds (ex think a rabbit eating roses and the dev of thorns)
• Explains: why introduced plants often grow more vigorously in invaded habitats vs native habitats AND the lag time between introduction to invasiveness (adapting)

Question 19

What are other considerations for the evolution of increased competitive ability?

Answer 19

• not usually observed probs bc of lag time
• differs from the ERH with respect to the mechanism of increased competitive ability (ecological vs evolutionary)
• conversely the presence of generalist herbivores in an invaded area may reduce the success of a plant invader

Question 20

Is the evidence for evolution of increased competitive ability concrete?

Answer 20

• naw its hella mixed
• some support it like L. salicaria
• Ex 2: seedlings from china and invasive USA populations were grown together and while invasive plants ahd more biomass, they suffered more from herbivores BUT still outperformed natives
  Ex3: mile a minute weed had greater reproductive output but also lower biomass and greater physical defences vs native plants = partial support. Introduce plants suffered greater damage
  Ex4:
  no difference in competitive ability btwn NA and Euro plant species (L. draba)