Invasion hypotheses

Question 1

Invasion hypothesis: propagule pressure

Answer 1

• Most invasive plants produce numerous propagules (a vegetative structure that can become detached from a plant and give rise to a new plant, e.g. a bud, sucker, or spore, seeds, vegetative parts such as bulbs, rhizomes) increasing the chance of colonising new sites
• likelihood of establishing new plants is directly proportional to the number of propagules

Question 2

Invasion hypothesis: phenotypic plasticity

Answer 2

• most invasive plants have a high phenotypic plasticity (ability to adapt growth to the environmental conditions)
• a high phenotypic plasticity indicates a broad ecological tolerance, a plant species can therefore establish in a wide range of environmental conditions

Question 3

Invasion hypothesis: disturbance

Answer 3

• highly disturbed ecosystems contain more naturalised and invasive species than less disturbed ecosystems. most invasive plant species grow in disturbed habitats (e.g. urban habitats)
• examples of disturbance promoting the spread of exotic species: overgrazing, habitat fragmentation, logging,general degradation

Question 4

Invasion hypothesis: invasional meltdown

Answer 4

Invasive species attract more invasive species. An invasive species modifies the environment and other invasive species become stablished

Question 5

Invasion hypothesis: limiting similarity

Answer 5

many invasive plant species represent new life forms or new families that are absent in the introduces range where they are invasive

Question 6

Example for invasional meltdown hypothesis

Answer 6

• Zebra mussels rapidly spread in the Great Lakes of North America and established large populations
• the mussels are plankton feeders and filter the water for food uptake
• the the large number of mussels the water became more clear
• sunlight reached deeper waters, promoting the spread of an invasive water plant, Eurasian water-milfoil

Question 7

Example for limiting similarity hypothesis

Answer 7

Opunita stricta cacti in Australia. No native cacti in Australia. The cactus was a weed of rangelands.
Absence of natural enemies. Like soil organisms such as nematodes and Fadenwürmer. Limiting similarity has to do with the absence of natural enemies. If you have an exotic species, that is not around usually, it is very unlikely for a herbivore to attack these invasive species.

Question 8

Which invasion hypotheses are the most confirmed?

Answer 8

Propagule, Plasticity, disturbance