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 30 years, 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, germinates and grows 3 months, flowers and produces fruit and 1000 seeds over 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 a mycorrhiza symbiont, continues to spread to more roots, produces basidia and millions of spores, but the spores die (or are blown away) and the fungus keeps growing (until a severe frost or fire?)
Life history characteristics--what is needed to describe a life history adequately? Growth form unitary: individuals with determinate sizes, like us modular: 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!