Lecture 6: Patterns and rates of spread

Question 1

Explain the gypsy moth example.

Answer 1

• immigrated to America with eggs to breed them with american silk moths to create a better silk worm to get an edge on the silk market
• storm came and the cages broke and they escaped
• NOTHING HAPPENED
• 20 years later, trees in that town were defoliated
• spread to Canada
• time lags are COMMON

Question 2

List the main types of dispersal.

Answer 2

1. Neighbourhood diffusion
2. Jump dispersal
3. Stratified diffusion

Question 3

Explain neighbourhood dispersal.

Answer 3

• dispersal through a homogenous environment
• individuals move equally in all directions
• individuals disperse WHILE the population grows
• *pattern will look like a series of concentric circles from a central point AKA direction is predictable, relative risk of the next neighbouring area can be determined

Question 4

Example of neighbourhood dispersal?

Answer 4

Muskrat in Europe

• range expansion from 1905 to 1927
• five escaped from a fur farm
• started the pop in Europe
• not perfect circles bc environment is obviously not perfectly homogenous
• and movement for them speeds up in wetlands
• if you plot the square root of the area of expansion against time you would get a constant straight line
• *SO circle spread constantly overtime not speeding up
• doesn’t work with things that fly or crawl

Question 5

Explain jump dispersal.

Answer 5

• rapid, long distance dispersal to remote areas
• often through a heterogenous environment (across inhospitable train e.g. dispersal between islands
• *not very distinct

Question 6

Explain stratified diffusion dispersal.

Answer 6

• dispersal by short distance expansion and long distance jumps
• i.e. neighbourhood diffusion combined with jump dispersal
• *most common type of dispersal
• a few long distance jumped coupled with short distance expansion

Question 7

Example of stratified diffusion dispersal?

Answer 7

• spread of the house finch in eastern NA
• released in 1940
• fly a little, establish in a suitable habitat
• then you have rebellious individuals that will be long distance flyers and start a population if enough landed there, and cause a disjunction from the other colonies
• eventually the core colony will engulf the smaller surrounding colonies as it expands
• you will get a sudden change in rate
• it is constant for a point but once it reaches these satellite colonies and absorbs them you see the jump in rate

Question 8

What form of dispersal does cheat grass show?

Answer 8

• stratified dispersal
• went through western NA
• spread by farmers and wind
• many sat colonies around the region
• they fuse
• exponential curve increase
• it gets an asymptote at the top bc they run out of room eventually
• difference here between birds , sat colonies are very common

Question 9

Explain range expansion as a function of the number of colonies

Answer 9

• the number of colonies is more important than their individual size
• it’s better to have a bunch of small colonies for expansion bc they have more edge to expand with vs one big colony
• also good for insurance bc they are spread out

Question 10

Explain the spread of red deer in South Island, New Zealand

Answer 10

• deer were introduced intentionally
• some wondered off and started populations
• filling territory
• similar to the spread of cheat grass
• stratified
• they reach the ocean so they can’t go any further = asymptote
• it’s a kind of middle example btwn the bird ex and cheat grass
• more sats vs birds but less than cheat grass
• • there is more in the way for sexually reproducing sp than plants for ec
• some deer spread further than others so you get a series of broken lines

Question 11

What are the three types of expansion rates?

Answer 11

Type 1: Linear expansion
Type 2: Bi-phasic expansion
Type 3: Exponential expansion rate

Question 12

What are the stages of range expansion?

Answer 12

1. Establishment - may require many introductions
2. Expansion - this stage defines the different types of expansion rates
3. Saturation - occurs when you run out of room, a barrier geographic, physiological

Question 13

Explain the range expansion of tiger pear cactus in South America.

Answer 13

• took 30-40 years before it occupied an area where it was noticeable
• it then levelled off

Question 14

What are the range expansions of European Gypsy Moth in NA and the japanese fungus used to control them.

Answer 14

• 20 year lag for the european gypsy moth
• 80 year lag period for the fungus
• LAGS

Question 15

List the reasons for the lag phase seen in invasions.

Answer 15

1. Limits on the detection of a population’s growth
2. Period of genetic adjustment
3. Density-dependent effects
4. Lagging introductions of mutualists

Question 16

Reasons for lag phase:

1. Limits on detection of a pop growth. Explain

Answer 16

a perceived lag may reflect our inability to detect small populations

Question 17

Reasons for lag phase:

2. Period of genetic adjustment . Explain

Answer 17

• natural selection may destroy all but a few predated genotypes
• time may be needed to evolve new genotypes

**NS for the proper genotype needed for the new region

Question 18

Reasons for lag phase:

3. Density-dependent effects.

Answer 18

• birth rate is correlated with population density
• not many individuals, they don’t reproduce as fact. You don’t notice they are increasing bc they are increasingly slowly. THIS BUILDS UP OVER TIME.

Question 19

Reasons for lag phase:

4. Lagging introductions of mutualists. explain.

Answer 19

• e.g. pollinators, seed disperses, mycorrhizal fungi, N-fixing bacteria.
• ex a plant waiting for pollinator, insect may come later from the same region the plant was from and can spread the plant.
• Ex fig tree and fig wasp. Wasn’t invasive on it’s own but once it’s mutualist came it became it.
• *can be delayed mutualists or environmental condition
• *lag time can also rep a delayed pop

Question 20

Does climatic warming explain why an introduced barnacle finally takes over after a lag of more than 50 years?

Answer 20

• not abundant for years in Europe
• 50 years later it became abundant quickly
• this barnacle came from warmer waters, and temp is changing, so climate change provided an opportunity for it to reproduce more better and thrive. So it spread further
• maybe the change in thermal regime was also detrimental to competitors to the barnacle and they died out
• this lag was a delayed app bc of change in environmental condition

**Key just bc something is not bad initially doesn’t mean it will always be that way. NEED TO UNDERSTAND LAG TIMES AS IMPORTANT

Question 21

What is the fisher-skellam model for neighbourhood diffusion?

Answer 21

• rate of change in local pop size is equal to change due to pop dynamics and change in movement of sp

Rate of change of local population size = Change due to population dynamics (logistic growth) + Change due to random movement of individuals

**D= diffusion coefficient = mean displacement of an individual per unit time.

Question 22

What are the fisher-skellam model assumptions?

Answer 22

1. Every individual moves at random throughout its life. -> not migratory individuals bc they are moving randomly
2. Individuals move through a homogeneous environment -> can move equally in either direction
3. the birth rate and death rate do not vary with the population -> this is not natural

Question 23

What does the fisher-skellam model predict?

Answer 23

• the population will spread like a travelling frontal wave.

Question 24

What is Vf?

Answer 24

• rate of spread, which is proportional to the species population growth rate (r) ad nd it’s diffusion coefficient (D)
  1. r can be derived by measuring the rates of increase of small populations
  2. D is diff to estimate, mark recapture expts measure only short term after dispersal
  3. r and D *What

Question 25

Explain observed rates vs predicted by fisher stellar model with terrestrial sp and marine sp.

Answer 25

• deviations around the line will tell us if it is over or under estimating
• if the model works the dots will be not he line
• model over estimates the movements of marine sp and under estimates terrestrial
• esp gypsy moth
• over estimates marine sp prob bc of d, measured by watching something move. It assumes for ex all larvae move the same BUT in reality currents change this
• does not consider human impact via moving them around either like the gypsy moth

Question 26

Why did the model consistently overestimate rates of spread of marine species?

Answer 26

• possibly due to how the diffusion coefficient (D) was calculated:
• values of D were based on recorded larval swimming speeds in still water
• these are not very meaningful if the larvae are carried by surface water current which vary year to year
• long distance dispersal does not necessarily imply successful range expansion fyi
  L> larvae might be diluted in currents and thus arrive at site in insufficient numbers
  L> this may also explain why spreading rates are not correlated with larval development time in the plankton

Question 27

Explain rate of spread vs larval development time in the plankton

Answer 27

• people assumed if larvae stay in this stage long, clean could move them a lot
• but larval dev is a poor predictor on it’s own bc it is context dependent
• ocean conditions etc
• *mean rate of spread increases with larval dev time ?

Question 28

Explain the mean rates of spread for terrestrial and marine species.

Answer 28

• range shift: edge of a range, spread a little beyond this..natural range expansion. BUT if this expansion is into an area with no evolutionary history..invasion in profs opinion.
• rates of spread seem to be higher in terrestrial than marine. Water current and thermal barriers are props the likely culprit. Not including ships
• water current they cannot disperse in areas equally as well, not supporting neighbourhood dispersion.

Question 29

Explain the probability of dispersal with jump dispersal.

Answer 29

• probability of dispersal to any given site is directly dependent on dispersal opportunity
• also dependent on time and distance.

Question 30

Explain what human vector dispersal looks like.

Answer 30

• a form of long distance jump dispersal
• dispersal depends on vector availability
• probability of dispersal is nearly independent of time and distance
• will look like stratified dispersal BUT the jumps depend on the vector

Question 31

Explain recreational boot traffic between Michigan Lakes as an example of human vector dispersal.

Answer 31

• boats in a weedy lake will prob leave with weeds attached
• can move weeds talk to lake
• can move things attached to them like zebra mussels, they can live a little outside of water for awhile, most common way mussels are transported around
• will depend on what humans choose for boating routes
• need to understand human behaviour, human vector dispersal, kind of start diffusion BUT independent of time and distance, it’s dependent on US