Life Histories- allocation and tradeoffs

Learning goals:
* be able to describe the traits that influence the intrinsic growth rate of populations= life history traits
* understand the variation and tradeoffs in life history traits and how they relate to ecological condiitons

Life history is the description of the order of events in an organisms life, and how much and when it devotes resources to growth, defense, and reproduction
Major characteristics of an organism’s life history include:
* Age and size at sexual maturity
* Amount and timing of reproduction
* Survival and mortality rates
* Investment into defense

Life history stages that influence an organisms ability to survive and reproduce in an environment- each stage has a genetic basis to its expression, and is subject to natural selection
Environmental variation determines whether an approach (e.g. early versus late reproductive maturity) will yield greater reproductive output, and therefore population growth or stability/ persistence
What environmental characteristics are important?  The availability of resources (energy, nutrients, space), competition for those resources, climate (e.g. how stressful, tropical vs. temperate), and disturbances (regular or stochastic, frequency, severity)

Timing and amount of reproductive output
Resources are limited; allocating to one function compromises allocation of resources to another
Maximum reproductive output and survival of offspring may involve delaying reproduction, and investing into growth earlier in life; larger individuals produce more offspring
Goal is maximum lifetime reproduction, not just reproduction at a single point in the lifetime
Survivorship curves provide a useful framework for thinking about general trends in life history
Two general strategies for number of reproductive events:
Semelparous species reproduce only once
Iteroparous species can reproduce multiple times

Amount of investment into a reproductive episode depends on size of offspring and amount of parental care invested; Trade-off between size and number of offspring
Producing fewer, large offspring may enhance their survival under certain environmental conditions e.g. if competition for resources is high
Producing more, smaller individuals may be better under other environmental conditions- e.g. if predation or herbivory is high

Parental care by animals varies according to environment:
where competition for resources is high, parental investment is favored
where stressful conditions occur or disturbances are frequent, low parental investment is favored

r versus K selection
Life history strategies represent a continuum of traits between maximizing reproductive output in uncrowded, often regularly disturbed environments and maximizing the survival of a few offspring in crowded habitats with few disturbances
At one extreme, traits should lead to high population growth rates; therefore called r-selection
At the other extreme, traits should lead to maintenance of population near the carrying capacity; therefore called K-selection
r-selected traits include short life spans, rapid development, early maturation, low parental investment, high rates of reproduction
K-selected traits include long-life spans, slow development, delayed maturation, high investment into each offspring, and low rates of reproduction