Includes the Allee effect, intrinsic and extrinsic factors affecting population growth, life-history traits, r- and K- selected species, and reproduction-survival trade-offs.
Ecology
Lecture 2 Population Ecology and Life History Evolution 17/02/21
The Allee Effect
- Where the population growth rate drops at low population densities.
- Most often seen in small populations.
- Can drive species to extinction.
Population growth in real populations
- If we look at nature, we find populations tend to fluctuate.
Intrinsic factors
- Factors within the model itself such as time lags and values of r or λ – our population growth
rates.
Extrinsic factors
- Factors outside of the model, such as changes in abiotic conditions, or biotic factors such as
interactions with other species, including predators and competitors.
Monotonic damping
Damped oscillations
Stable limit cycles
Chaos
Delayed density dependence
- Delayed density dependence can cause populations to fluctuate in size.
- For example, lab populations of Daphnia show marked population cycles when in laboratory
cultures.
- As the population grew, reproduction decreased.
- But survival was less sensitive to density, and so adults continued to survive even though
resources were scarce
- As resources became more abundant, reproduction could once more take place….but the
population was made up of old, senescent individuals who laid very few eggs.
- Thus, the next up cycle of population growth was delayed, because the ↑ in fecundity at
lower density was lower than predicted (because old individuals produce few young).
Life history traits
- In many respects we have two ends of a continuum.
- The mouse can be described as an r-selected species (the r relates to the intrinsic rate of
increase – as per our last lecture).
- r-selected refers to selection for high population growth rates.
- Generally found towards the bottom of the S-shaped growth curve.
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