Lecture 6: Patterns and rates of spread Flashcards
Explain the gypsy moth example.
- 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
List the main types of dispersal.
- Neighbourhood diffusion
- Jump dispersal
- Stratified diffusion
Explain neighbourhood dispersal.
- 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
Example of neighbourhood dispersal?
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
Explain jump dispersal.
- rapid, long distance dispersal to remote areas
- often through a heterogenous environment (across inhospitable train e.g. dispersal between islands
- *not very distinct
Explain stratified diffusion dispersal.
- 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
Example of stratified diffusion dispersal?
- 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
What form of dispersal does cheat grass show?
- 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
Explain range expansion as a function of the number of colonies
- 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
Explain the spread of red deer in South Island, New Zealand
- 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
What are the three types of expansion rates?
Type 1: Linear expansion
Type 2: Bi-phasic expansion
Type 3: Exponential expansion rate
What are the stages of range expansion?
- Establishment - may require many introductions
- Expansion - this stage defines the different types of expansion rates
- Saturation - occurs when you run out of room, a barrier geographic, physiological
Explain the range expansion of tiger pear cactus in South America.
- took 30-40 years before it occupied an area where it was noticeable
- it then levelled off
What are the range expansions of European Gypsy Moth in NA and the japanese fungus used to control them.
- 20 year lag for the european gypsy moth
- 80 year lag period for the fungus
- LAGS
List the reasons for the lag phase seen in invasions.
- Limits on the detection of a population’s growth
- Period of genetic adjustment
- Density-dependent effects
- Lagging introductions of mutualists
Reasons for lag phase:
1. Limits on detection of a pop growth. Explain
a perceived lag may reflect our inability to detect small populations
Reasons for lag phase:
2. Period of genetic adjustment . Explain
- 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
Reasons for lag phase:
3. Density-dependent effects.
- 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.
Reasons for lag phase:
4. Lagging introductions of mutualists. explain.
- 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
Does climatic warming explain why an introduced barnacle finally takes over after a lag of more than 50 years?
- 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
What is the fisher-skellam model for neighbourhood diffusion?
- 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.
What are the fisher-skellam model assumptions?
- Every individual moves at random throughout its life. -> not migratory individuals bc they are moving randomly
- Individuals move through a homogeneous environment -> can move equally in either direction
- the birth rate and death rate do not vary with the population -> this is not natural
What does the fisher-skellam model predict?
- the population will spread like a travelling frontal wave.
What is Vf?
- 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
Explain observed rates vs predicted by fisher stellar model with terrestrial sp and marine sp.
- 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
Why did the model consistently overestimate rates of spread of marine species?
- 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
Explain rate of spread vs larval development time in the plankton
- 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 ?
Explain the mean rates of spread for terrestrial and marine species.
- 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.
Explain the probability of dispersal with jump dispersal.
- probability of dispersal to any given site is directly dependent on dispersal opportunity
- also dependent on time and distance.
Explain what human vector dispersal looks like.
- 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
Explain recreational boot traffic between Michigan Lakes as an example of human vector dispersal.
- 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
