Lecture 10: Populations: Growth potential, limits, & dynamics Flashcards
Density independent factors
- Factors affecting population growth that are unrelated to population size
- ex: natural disasters
Negative density dependent factors
Factors negatively affecting population growth that have increasing strength with increasing population size
why would a population become self-limiting
a variety of consequences associated with too many individuals
Logistic growth curve
- s-shaped curve of density-dependent population growth
- Formalizes the relationship between increasing population size and decreasing growth rate, as the carrying capacity is approached
Inflection point
- Separates early accelerating phase from the later decelerating phase
- located in the middle of the graoh
carrying capacity
- The maximum number of organisms the environment can support
- the point where growth falls to zero in the logistic growth curve.
where on the density-dependent curve is r > 0
- before / to the left of the inflection point
- early growth is exponential
where on the curve is r approaching 0
- after / to the right of the inflection point
- as it reaches carrying capacity
where on the curve is r = 0
at the carrying capacity
density dependence in plants
- slow growth
- smaller size at higher density -> reduced survival and fecundity
Self thinning
- Negative effects of high density cause some individuals to die, allowing remaining plants to increase in average size
- Consistent negative relationship established between plant density and average plant size
Positive density dependence
Increasing population size has a positive
influence of growth rate (up to a point!)
Causes of positive density dependence
- Larger populations are more genetically diverse
- Allee effect
define the Allee effect
Beneficial social interactions at larger group sizes increases individual fitness
lemming population cycle
- Rapid population growth leads to depletion of resources.
- Triggers group migration behavior to locate new resources.
- Obstacles are overcome by a collective behavior/mass action.
- Population crashes (but not all die) and a new cycle begins
why does the lemming population cycle happen
Migratory behavior boosts reproductive output (increases fitness) in period before they fall off the cliff bc they mate along the way
why may a population fluctuate
- Near carrying capacity, birth and death rates follow environmental fluctuations.
- Biology of the organism may favor instability in population size
if fluctuations are the norm in population growth, then what varies between populations
the magnitude and frequency of the fluctuations
biology favoring intrinsic population stability
- Large body size.
- Homeostasis (ability to resist external environmental fluctuations).
- Long life.
- Significant overlap of generations.
biology favoring intrinsic fluctuations/instability
- Minute size and no homeostasis.
- Very short lived.
- High turnover - high mortality rate, so organisms at any time are mostly those from last reproductive event
Periodic population cycling
Repeated, regular fluctuations in population size
Overshooting
More individuals are produced than can be sustained at adulthood
Die-off
- caused by strong effects of overpopulation
- defined as a population decline that drives the population well below carrying capacity
- population crash following density dependent effects
what generates the overshoot and the subsequent die-off
momentum from birth and death rates
what drives increased oscillations around the carrying capacity
Increasing time delay in population response to carrying capacity
how are oscillations amplified and when do they stop
- amplified by an increased time delay
- stops when the population growth responds to the approach of the carrying capacity
how can the Allee effect be in reverse?
Populations that experience the allee effect also suffer sudden collapse once population size declines below the critical minimum
