Life History

Introduction Different species reproduce at vastly different rates over lifetimes that may differ dramatically. Life history consists of the adaptations of an organism that influence the number of offspring it will produce, size and age at reproductive maturity, number of reproductive events, etc.

Introduction Why does diversity in life histories exist? Why haven't rapidly reproducing asexual bacteria taken over the world? The study of life history investigates the underlying strategies that have generated the enormous diversity found among organisms.

Life History Variation Arises from Constraints The limited resources available must be divided among all of an organism's biological needs for survival and reproduction (e.g., maintenance, defense, growth). The need to allocate limited resources generates trade-offs. For example, energy spent on growth cannot be spent on producing eggs.

Trade-Offs Maximizing one life history trait may come at a cost to another. Grow large OR reproduce early Attract mates OR hide from predators

How Many Eggs? In birds, clutch size (the number of eggs laid in one reproductive bout) increases with increasing latitude and day-length. More food available. Unpredictable from year to year.

Successful Life Histories A successful life history strategy is indicated by a stable or growing population; or in other words, a population is successful if r ≥ 0. where r is traditionally used to represent the rate of growth of a population. Values for r can be categorized like this: Positive (r > 0): the population is growing. Negative (r < 0): the population is shrinking. Near 0 (r = 0): the population is stable.

Age Structure A graph of age structure can give us information about whether a population is growing, remaining stable or declining.

Life Tables Detailed, age-specific population data can be summarized for analysis using a life table, which categorizes the probabilities of reproduction, death, and survivorship for different ages or age groups. Survivorship is roughly the opposite of mortality—it is the proportion of a cohort of individuals that survives to a given age. 

Survivorship Curves Information in the life tables can be used to plot survivorship curves.

Ecological Management Using Life Table Data Ecologists build and use life tables to address important management questions. What is the best life stage to protect loggerhead turtles?

Trade-Offs Animals only have a certain amount of resources to divide between growth, reproduction, etc. Results in trade-offs A strategy that works well in one environment may work poorly in another. Within limits, individuals can modify their strategies to respond to ever-changing external factors. Natural Selection

Trade-Offs Many organisms (like the SimSturgeon) face a trade-off between fecundity and age of maturation.

Trade-Offs A trade-off that is seen with real sturgeon is between fecundity and body size. Larger females can produce more eggs. Waiting to reproduce can allow the female to have more offspring, But, she has a greater chance of dying before reproducing.

Adult Survival and Reproductive Allocation Reproductive effort: the allocation of energy, time, and other resources to the production and care of offspring. Any energy or biomass used for one function, such as growth, reduces the amount of energy available for another function, such as reproduction. Trade-offs between reproduction, growth, and maintenance.

Trade-Offs Trade-off between reproduction and growth/survival. Douglas Firs grow less in the years that they reproduce more (many cones). Red deer have a higher winter mortality when they are raising calves.

Classifying Life Histories In areas that are frequently disturbed through fire, floods, storms, etc. organisms that could reach maturity and reproduce quickly, producing many offspring would be favored. (r-selected) Weedy species In stable environments, it would be better to grow larger, reproduce later, having fewer, larger offspring. (K-selected) Beter competitors

Life History Classification r vs K selection r selection (per capita rate of increase) Characteristic high population growth rate. Strongest in species colonizing new or disturbed habitats. Type III survivorship curve K selection (carrying capacity) Characteristic efficient resource use. Most prominent in species whose populations are near the carrying capacity much of the time. Type I or II survivorship curve

Life History Classification r and K are ends of a continuum, while most organisms are in-between. r selection: favored in unpredictable environments. K selection: favored in predictable environments.

r and K: Fundamental Contrasts r-selected species: High intrinsic rate of natural increase (r) Low competitive ability Rapid development Early reproduction Small body size at first reproduction Semelparous – single reproductive event Many small offspring produced

r and K: Fundamental Contrasts K-selected species: Low intrinsic rate of natural increase (r) Strong competitive ability (K) Slow development Late reproduction Large body size Iteroparous – repeated reproduction Few large offspring produced Parental care

Plant Life Histories Two most important variables exerting selective pressures in plants: Intensity of disturbance: Any process limiting plants by destroying biomass. Intensity of stress: External constraints limiting rate of dry matter production.

Plant Life Histories Four Environmental Extremes: Low Disturbance : Low Stress Low Disturbance : High Stress High Disturbance : Low Stress High Disturbance : High Stress No viable strategy here.

Plant Life Histories Ruderals (highly disturbed habitats) Grow rapidly and produce seeds quickly. Disturbance frees them from competition. Stress-Tolerant (high stress - no disturbance) Grow slowly - conserve resources. Competitive (low disturbance low stress) Grow well, but eventually compete with others for resources.

Opportunistic, Equilibrium, and Periodic Life Histories Winemiller and Rose proposed new classification scheme based on age of reproductive maturity (α), juvenile survivorship (lx) and fecundity (mx). Opportunistic: low lx - low mx - early α Equilibrium: high lx - low mx - late α Periodic: low lx - high mx - late α

Complex Trade-Offs What happens when the benefits of raising a brood are reduced when some offspring are lost? Continue care of remaining offspring? Or, start over?

Complex Trade-Offs To deal with trade-offs, some species morph into completely different forms, either as alternatives or sequentially through one lifetime.  Sneaky male bluegill sunfish look like females.