Struggle for Existence pt. 6 Flashcards
How are age differences treated in age structured populations?
Population is divided into a convenient number of different age classes, each of which has different, AGE SPECIFIC, prospects of death and reproduction.
Age structured models only account for….
females, as they are the only propulsive force of a population
First age class is denoted by subscript…
0
- N_0 = # newborns
- N_1 = # of one year olds
age structure of a population
set of N_x values for a population
Ix
survivorship at age x (probability that an individual is still alive at age x), thus values range from 0 to 1.
What is the Ix value for the last age class in a life table?
0
senescence
generalized breakdown of organisms
Hypothetically, what does it mean if the probability of dying is constant across all ages? What does the graph look like?
External sources cause death rather than intrinsic biological characteristics. The graph would mimic exponential decay.
Type I Survivorship
convex curve; low early mortality
Type II Survivorship
Exponential decay but on a logarithmic scale, a linear downwards slope.
Type III Survivorship
Concave curve; a lot of early death.
bx
average number of daughters produced by a female in her xth year of life
Why is b0 typically 0?
Newborns need to pass through a period of resource acquisition and growth after birth before they are capable of reproducing themselves.
First age of reproduction
waiting period before reproduction can begin
Why are age-structured models are not density dependent?
Because fecundity and survivorship are both treated as constants.
Why do we need to consider survivorship when calculating how many daughters a female will have?
Fecundity and survivorship interact. A female will have more daughters the longer she lives but the older she is the less likely it is that she will be alive. You must down-weight the extra births with the lesser probability of being alive at x age.
net reproductive rate/replacement rate
sum of ( survivorship probability x the average number of births) for all age classes. Ro = ∑Ixbx (still a step function)
How are Ro and lamda different?
Their time scale. Lamda is the amount of growth that occurs over one unit of time (like a year) while Ro is the amount of growth that occurs over one generation
How is generation time calculated?
T = ∑x Ixbx/ Ro
- sum of age multiplied by Ro all over Ro
Stable age distribution
proportion of the population in each class stabilizes. Value of lamda also stabilizes.
Examples of life history characteristics
lifespan, first age of reproduction, number and timing of offspring
Constraints and tradeoffs
size-number tradeoff
- many small offspring/seeds or few large offspring/seed
early vs late reproduction
- organisms that reproduce earlier do not have as long a period as resource accumulation
cost of reproduction
- offspring require energy and nutrients
- parental care
reproductive value
expected number of daughters to be produced by a female of age x, now and for the rest of her lifetime (all previous births do not matter). Denoted by Vx
Why is the graph of reproductive value often bell curve shaped?
Newborn must go through resource acquisition stage before she can begin reproducing and she encounters some risk of dying during that period.
What do you expect in a species that forms lasting pair bonds?
strong selection for males to prefer female mates with high reproductive values. Produces tendency for traits associated with the onset of reproductive age to become associated with sexual attractiveness.
semelparity
Big bang reproduction. Accumulate resources for their whole life, reproduce and then die.
iteroparity
reproduce numerous times in life
What are iteoparous plants called?
usually called perennials
What are semelparous plants called?
Monocarpic (making fruit once)
- annuals if they only live one season
- biennials if they grow vegetatively in first year then flower and die in second
Monocarpic perennials
plants that live for more than two years before flowering and dying . Resource accumulation stage is long
What kind of plants are common in desert conditions?
Annuals are common in deserts because they are small and fast growing. They can grow and flower in the brief, wet season and produce seeds that spend dry season dormant.
- seeds germinate only after heavy rains that leach out germination inhibitors from seed coat
- Don’t really need water conservation strategy because all active growth is during period of non desert conditions.
Name classes of plants where annuals are prominent. Why are they prominent?
Plants that take advantage of brief windows of time (milder conditions)
- desert plants
- weeds
annual strategy selected for by humans to maximize seed output (ancestors were likely perennials)
- crop plants: rice, maize, wheat
What are weeds adapted for?
Growing in transient habitats that have been disturbed (either by animals burrowing or human activity like agriculture). Soil disturbance allows a brief period for small annual plants to occupy an uncrowded bare site before taller perennials outcompete them.
- seeds germinate when struck by sunlight
What’s another name for weeds?
Fugitive species
Characteristic of desert annuals and weeds?
Have long lasting seed banks in the ground. Large proportion of the population is underground at any one time.
Century plants
ex. agaves
grow for decades without flowering
Why do plants bloom in synch?
Might use same cues to induce flowering. Would suck if you bloomed before everyone else and there were no pollinators.
Examples of synchronized blooming/growth
- Frasera speciosa (monument plant)
- some bamboos
Why synchronicity?
- predator satiation
- ability to attract pollination as a whole group is greater
mast crops
synchronization of seed production in regular iteroparous perennials (nut-bearing trees like oaks) (predator satiation)
Semelparous animals
- salmon
- antechinus
R-strategist/ R selected species
- high fecundity
- poor competitors
- low survival
- short generation times
- small but numerous seeds
- good dispersal ability
ex. fugitive annual weeds
These adaptations are good in low-density situations
K-strategist
- low fecundity
- good competitors
- high survival rate
- longer generations (slower growing)
- few, large seeds
- poor dispersal ability
These adaptations are good in high-density, competitive situations (carrying capacity)
J.P. Grime
advocated for three categories “CSR trichotomy”
- competitive species (slow k strategists)
- stress tolerators (grow in hostile environments that remain uncrowded)
- Ruderal species (fugitive weedy species)
Deterministic models
parameters that determine births and deaths are constants (determined by starting conditions)
What are stochastic models and why have they replaced deterministic models?
Accounts for random variation in parameters in Nx, bx, and Ix that could result in chance extinction (other than just relying on average values).
