Ecology - Mod 6 (Ch 7): Life History

PCB 3044 with Klowden mod 6 (ch 7)

population

a group of individuals of a single species that live in a particular area at the same point in time

life history

record of events and landmarks related to an individual’s growth, development, reproduction, and survival

traits show phenotypic variation among individuals

examples of life history

birth month, size at birth, growth rate

age/size at sexual maturity

number of offspring at each reproductive event

interval between reproductive events

age of last offspring (senescence)

age at death

life history strategy

a summary of the typical range or average values for all life history traits for a population or species

unique suite of adaptations which optimize energy use based on the unique abiotic and biotic pressures each population or species faces

represents an attempt to optimally divide limited energy and time between survival, reproduction, and growth

represents a compromise with various costs and benefits

costs and benefits of allocating energy to survival

benefits: permits future reproductive output, increases longevity

costs: decreased reproduction and growth

costs and benefits of allocating energy to reproduction

benefits: increases genetic contribution to future generations

costs: decreased survival (longevity or future reproduction) and growth

costs and benefits of allocating energy to growth

benefits: may allow better future reproduction and survival (longevity)

costs: decreased energy for maintenance (survival) and reproduction

fitness

the measure of an individual’s contribution to future generations

maximized by an optimal life history strategy

greater fitness: reproduce at high rate and/or reproduce for many years (i.e. survive a long time)

different life history strategies among species

may result from different abiotic and biotic pressures

example of difference in life history strategies

wood thrush vs ashy storm petral

wood thrush: reproduce when 1 year old, produce several broods of 3-4 young per year, rarely live beyond 3 or 4 years

ashy storm petral: reproduce at 4-5 years old, produce 1 young per year at most, may live to be 30-40 years old

similarities in life history strategies

may result from similar abiotic or biotic influences in different groups of species

example of similar life history strategies

temperate songbird species have larger clutch sizes than tropical species

Dr. Lack: could reflect parents’ ability to obtain food

Dr. Skutch: could reflect level of nest predation

examples of populations of the same species having different life history strategies

American ginseng

humans

phenotypic plasticity

when a genotype can display a range of phenotypes depending on environmental conditions

occurs due to natural selection for variability in phenotype in an individual

typically only occurs in variable environments

allows organisms to be better tailored to their environment, like natural selection

may cause variation in life history traits among populations/species

required for acclimatization

doesn’t always result in acclimatization, may result in more permanent changes

examples of phenotypic plasticity

fence lizards from Nebraska but not New Jersey

plants that can increase root growth when H2O is scarce

life history traits

phenotypic traits that often show plasticity

e.g. age at first reproduction in sea otters

may vary depending on resource availability, predation rate, or other environmental conditions

e.g. growth rate

growth rate

a life history that may have plasticity due to population differences in abiotic environmental factors

e.g. shape differences resulting from plasticity in relative growth rate between height and width

e.g. between ponderosa pine populations

example of the effect of plasticity in relative growth rate on animal morphology

spadefoot toad populations: 2 different morphs

omnivore morph: usually occurs in permanent ponds, slower growth, better survival after metamorphosis

carnivore morph: usually occurs in more ephemeral ponds, faster growth, worse survival after metamorphosis

polyphenism

distinct, discrete morphologies/morphs arise from a single genotype as a result of differing environmental conditions

e.g. spadefoot toads

2 ecological tradeoffs associated with reproduction

offspring size vs number

current vs future reproduction

offspring size vs number

more seeds/eggs of smaller size:

benefit: higher chance that one will survive

costs: less nutrients stored, dry out faster, shorter dormancy

fewer seeds/eggs of larger size:

benefits: more nutrients per seed, less prone to desiccation

cost: fewer offspring so higher change that none will survive

examples: plant seeds, bow-winged grasshopper, Chinook salmon, western fence lizards, lesser black-backed gulls

current vs future reproduction

associated with reproductive timing

when an individual should begin to reproduce (e.g. relative width of annual tree rings vs mean number of cones per tree, maturity age of different seabirds vs annual survival)

depends on lifespan and if reproductive output varies with age

how often an individual should breed (e.g. red deer, mortality of does that bred two years in a row vs those that didn’t)

semelparity vs iteroparity

semelparity

having one reproductive event before death

no parental care

advantageous for large clutch sizes

seems to reproduce long-term reproductive output, but balances out with Cole’s Law

e.g. salmon, some flowers, agave, giant squid

iteroparity

having multiple reproductive events in an organism’s life

more advantageous for smaller clutch sizes

Cole’s Law

if natural selection adds 1 to a clutch size, reproductive output from semelparity equals iteroparity

if natural selection adds more than 1 to a clutch size, semelparity exceeds iteroparity

(know how to do the math and understand it)

case study: trophy hunting and inadvertent evolution

bighorn sheep have been trophy hunted for horns

largest and strongest males are removed from population by hunting

average size of males and horns decreased over a 30 year period

small populations struggle to recover in abundance, since larger and stronger males are preferred to sire offspring

similar effects seen in other animals hunted, e.g. fish, invertebrates, plants

in cod: cod that mature at younger age and smaller size are more likely to reproduce before getting caught —> genes for smaller size and younger maturity are more likely to be passed down

prediction: over time, more fish will have genes for younger maturity and smaller size