Lecture 10: Population Dynamics Flashcards
What does population dynamics refer to?
the spatial and temporal changes in the characteristics of populations
What 2 main factors cause fluctuations in population size and density?
- extrinsic/environmental changes
2. internal stability
What is internal stability?
A measure of how quickly populations can respond to environmental change
What types of species would you expect to have higher internal stability? why?
LARGE, LONGER lived organisms respond more slowly to environmental change
- lower surface area to volume ratio and are more likely to be endothermic (better able to maintain homeostasis and survive enviro fluctuations)
- longer generation times have less fluctuation in birth rate - less able to recover after large decline in population
ex. wolverines in Canada
What types of species would you expect to have lower internal stability? why? Give an example of a species
SMALL organisms with SHORT generation times respond rapidly to environmental change
- higher surface area-volume ratio so they are more vulnerable to changes in temperature
- short generation times mean they reproduce quickly and can quickly adapt to change
ex. phytoplankton in Lake Eerie
T or F: most populations in nature smoothly reach their carrying capacity. Why/why not?
FALSE.
Most overshoot K and experience die-off because their internal stability is high and therefore their response to their own density is delayed
Describe population cycle
A regular pattern of oscillation in population size
How is harvesting data used to monitor population dynamics? How can this be misleading?
The number of individuals harvested in a given time period can give a good estimate of the population size
This can be misleading because the popularity of hunting a particular species may change over time, and if there are less hunters, less individuals are being hunted and counted so the population size will appear small but this may not reflect the reality
ex. Gyrfalcons: when harvesting intensity was high, there was a clear population cycle but when it was low it looks like the population cycle doesn’t exist, even though it still would
T or F: population cycles can be synchronized across species
TRUE
ex. the 3 species of Grouse in Finland with 6-7 year pop cycles
If environmental variation is typically not regular or predictable, why do populations respond cyclically?
some experience predictable delays in response to their own density
there may be some intrinsic quality of the species that prevents birth rates from falling and death rates from rising as the pop growth approaches K
What are the 3 responses to approaching and reaching carrying capacity?
- Logistic growth (theoretical)
- Natural response
- Delayed response
Describe the expected response of a population as it approaches K if it were following the logistic growth model?
Pop growth would start small, increase rapidly, and then plateau and level off before reaching K
Describe the natural response for populations as size approaches K?
Slow start Rapid growth Slight overshoot of K Slight die off increase and smaller overshoot of K Slight die off
Fluctuation around K that gradually levels out just below K
Describe the delayed response for populations as size approaches K?
slow start rapid growth massive overshoot of K massive die off rapid increase and overshoot of K
repeatedly massively overshoots K and experiences large die offs
What 3 kinds of populations may experience delayed responses to environmental change?
- those with discrete breeding intervals
- ex. large mammals (moose, deer) - those that can store energy resources
- ex. daphnia water fleas - those that experience time delays between developmental stages with different resource requirements
- ex. blowflies
Explain an example of how populations with discrete breeding intervals can experience delayed responses to environmental change
Moose and deer mate in the Fall and give birth in the Spring
- if K drops between mating and birthing dates, the population cannot respond in time (ie., the babies have to be born and there will not be enough resources to feed them = massive die off)
the population will overshoot K and crash
= cyclic response
Explain an example of how populations that can store energy resources can experience delayed responses to environmental change
Daphnia water fleas can store energy in lipid droplets so that they can continue to survive and reproduce after the population has overshot K
By the time the food store has been depleted, the population has overshot K and the population will crash
= cyclic response
How do the daphnia water flea population cycles compare to the bosmina water fleas?
Bosmina water fleas store energy too, but much less than the Daphnia
The energy store in Bosminas is depleted before the population reaches K or just after K and will experience reduced survival and reproduction but will be able to recover more quickly than Daphnia which experience a dramatic overshoot and die off
the Bosmina cycle is most representative of the natural response graph because there is less oscillation around K
Describe the blowfly experiment
EXPERIMENT 1:
- adult population kept with unlimited source of food
- larval population kept with limited source of food
EXPERIMENT 2:
- adult population kept with limited food
Explain the results of the blowfly experiment with unlimited adult food
Having unlimited food, the adults were able to survive and reproduce many eggs
But the larvae that will hatch will have limited food resources = many larvae will die
When the larvae die, the adult population will crash because less are surviving to maturity
If less larvae grow to adults, there will be less reproductive females = less eggs produced
Now that there are less larvae, there will be more food for them = adult population will increase
Now many adults making many eggs
Many larvae with little food = population crash
repeat repeat repeat
Explain the results of the blowfly experiment with limited adult food
The adults produced fewer offspring = fewer larvae grew into adults = fewer adults
there was some oscillation but overall the density was high and generally around or below K
Explain how the results of the blowfly experiment show the relationship between time delays and population cycles
The response to limited resources by the larval life stage can be delayed in the adult life stage that experiences unlimited resources
AND
If different life stages experience similar resource availability, the population DOES NOT CYCLE
Describe habitat heterogeneity and give an example
Suitable habitat generally occurs in patches surrounded by a matrix of unsuitable habitat
ex. suitable habitat for aquatic organisms would be the patches of water (ponds) between a matrix of unsuitable terrestrial habitats (land)
In what two ways can habitat heterogeneity occur?
- naturally (ex. ponds and land)
2. a result of habitat fragmentation (ex. patches of forest between agriculture)
Describe habitat fragmentation
Human imposed division of natural habitats
Define metapopulation
A group of interbreeding members of a species divided into subgroups in different habitat patches that are connected by infrequent dispersal
What are the 4 criterion for a metapopulation to exist?
- suitable habitat occurs in discrete patches that can be occupied by breeding populations
- even the largest subpopulations have a substantial risk of extinction
- habitat patches must not be too isolated to prevent recolonization after local extinction
- the dynamics of local, subpopulations are not synchronized
What is a metapopulation model?
Models that describe the spatial structure of metapopulations
What are the 3 ways ecologists can model metapopulations?
- Simple model
- Source-sink model
- landscape model
Explain the simple metapopulation model by describing the assumptions it makes
Assumes:
- habitats within the patches are of equal quality (and are equally likely to be occupied)
- migration between the patches is ONLY determined by the arrangement and distance between the patches
- the features of the surrounding landscape are not considered
Explain the Source-Sink metapopulation model by describing the assumptions it makes
dense subpopulations with individuals of high fitness from patches of high habitat quality (source) will migrate to less dense patches of lower habitat quality (sink) as the density of the source population continues to increase
- without source populations, the sink populations would go extinct
Assumes:
- habitat quality varies among patches
- individuals have greater fitness in patches of high quality habitat
- subpopulations in high quality habitat patches tend to increase in density
- subpopulations in low-quality patches tend to decline in density
Explain the Landscape metapopulation model by describing the assumptions it makes
The most realistic model that takes into account the source-sink model AND the influence of the variation in habitat surrounding the patches and how that can affect migration between patches
ex. mice from the source patch would be more likely to move through a forested area vs an open area or water patch to get to the sink patch
At what two scales must metapopulation dynamics be considered?
- local (within a patch)
2. Regional (amongst the patches)
At the local scale, what do metapopulation dynamics depend on?
population growth and regulation depends on BIRTHS and DEATHS
At the regional scale, what do metapopulation dynamics depend on?
Interaction among subpopulations depends on COLONIZATION and EXTINCTION
What keeps the probability of regional extinction low?
Dispersal between occupied and unoccupied patches
What are the 3 assumptions of the metapopulation model? Are these usually met by real populations?
- patches are all equal in quality, size, and distance apart
- Each occupied patch contributes equally to colonization of empty patches
- There is a fixed probability of extinction and colonization for all patches
NO not usually met by real populations
