LIFE HISTORY STRATEGIES - University of California, Davis · Life history characteristics--what is needed to describe a life history adequately? Growth form unitary: individuals with

LIFE HISTORY STRATEGIES

LIFE HISTORY STRATEGIES

What characteristics help a population survive and

grow?

What are the dominant species in an ecosystem?

Why doesn't a dominant species (of plant or animal)

crowd out all others?

Why don't all species evolve to look and act alike?

Some answers come from considering "life history

strategies"-- reproductive histories, methods

Examples of life histories:

--you are born, grow 15+ years, reproduce 1 to 10 children over 30years, putting lots of effort into it, live 30 more years, and die

--a tomato plant starts as a seed, is dormant 1 1/2 years, germinatesand grows 3 months, flowers and produces fruit and 1000 seedsover 3 months, dies; most of the seeds are eaten by birds or fungi;most of the seedlings die

--a mushroom germinates from a spore, invades a pine root as amycorrhiza symbiont, continues to spread to more roots, producesbasidia and millions of spores, but the spores die (or are blownaway) and the fungus keeps growing (until a severe frost or fire?)

Life history characteristics--what is needed to describe a life historyadequately?

Growth formunitary: individuals with determinate sizes, like usmodular: individuals of indeterminant size: tomato plant, fern colony

(vegetative reproduction); mycorrhizal fungus

Survivorship (lifespan)

Growth rate: slow vs fast

Age to first reproduction: early vs late

note: human population reproductive rates depend

strongly on the age at which women start conceiving

Mating systems

plants: self-fertilizers vs obligate out-crossers,

apomixis

animals: monogamy, poligamy, polyandry

Reproductive effort: high fertility vs lower

fertility/parental care

high fertility (insects, grasses) usually comes with

high initial mortality

low fertility (mammals, avocados) usually comes with

more energy for survival and growth of young

Number of reproductive episodes: one vs many

plants: annuals, biennials, perennial monocarpics

("semelparous" from Semele, mother of Zeus's son

Dionysus, destroyed by Zeus's lightning) put all

energy into one reproductive burst; perennials

(iteroparous) save energy for repeated reproduction

Each species has characteristic life history, combination

of traits selected for survival in a given environment

Principle of allocation: there are only so many

resources to go around--everything you put into

something has to come from somewhere--there is

no free lunch

Trade-offs:

• number of reproductive episodes vs fertility per

episode

• number of seeds vs storage material per seed

• number of young vs energy spent protecting the

young

Consider birds

Arizona robins vs Argentina robins

Arizona robins have lower

probability of surviving the winter,

higher clutch sizes (5.8 vs 3.7)

Also, Arizona robins react more to

jays (nest robbers), while

Argentina robins react more to

hawks (adult predators)

(Science, April 20, 2001)

Life histories can be associated with population growth patterns,called “K selected” and “r selected”

Life history strategies (syndromes) of populations:

certain combinations of traits tend to occur together:

Trait r-selected K-selected

lifespan short long

growth rate fast slow

first reproduction early late

reproductive effort high low

parental care less more

offspring many, small few, large

reproductive

episodes

semelparous iteroparous

kind of environment variable stable

We may recognize a continuum of strategies, rather than

just two discrete types

And other strategies are possible (fungus, with large

number of stress-resistant spores, low growth rate

when they germinate)

Summary

Why don't all species evolve to look alike?

• there are different "niches" (habit+habitat, life

history strategy)

• different species evolve to fit different niches

Why don't dominant species crowd out all others?

• competition among species in different niches is

limited, escapable

• e.g., in Swedish beech forest, the beech trees

intercept most sunlight, but small herbaceous

plants grow and flower for 2 weeks before

beeches leaf out

Evolution does not select for dominance, but for survival!