integrated lec 18 Flashcards
life history and key components
Strategies organisms use to allocate resources to growth, reproduction, and survival.
Key components:
Lifespan
Timing of reproduction
Number of offspring
Parental investment
age-structured populations
Recognizes that individuals of different ages contribute differently to population dynamics.
Requires breaking populations into age classes
life tables
Summarize age-specific survivorship (lx) and fecundity (mx)
Key metrics derived from life tables:
-Net reproductive rate (R0)
-Generation time (T)
-Reproductive value (vx)
survivorship curves
types:
-Type l: High survivorship early, mortality increases with age (e.g., humans).
-Type II: Constant mortality rate (e.g., birds).
-Type III: High early mortality, few survivors (e.g., fish, plants).
fecundity schedules
mx: number of female offspring produced by a female of age x
-reflects reproductive potential across lifespan
semelparity vs. iteroparity
Semelparity: Single reproductive episode (e.g., salmon, agave).
Iteroparity: Multiple reproductive episodes over lifespan (e.g., humans, elephants).
masting and reproductive synchrony
masting: coordinated reproduction in plants (e.g. oak trees)
-synchrony may be favoured to satisfy predators or to increase reproductive success
net reproductive rate (R0)
formula: R0=sigma(lxmx)
meaning: average number of daughters produced by a female in her lifetime
generation time (T)
formula: T= (sigmaxl(x)m(x))/R0
(x’s in brackets are subscripts)
meaning: average age of reproduction
reproductive value (vx)
expected number of future offspring for an individual of age x
life history strategies- key trade-offs:
-Current vs. future reproduction:
High investment now may reduce survival or future reproduction.
-Number vs. quality of offspring:
Many low-investment offspring vs. few high-investment offspring.
r-strategy vs. K-strategy
r-strategy: Short life, fast reproduction, many offspring (e.g., mice).
K-strategy: Long life, slow reproduction, few offspring (e.g., elephants).
age-structured pop and why it matters
Traditional models (e.g., exponential/logistic growth) assume all individuals are equal.
Why Age Structure Matters:
-Fecundity and survivorship vary by age.
-Species differ in life history traits (e.g., elephants vs. salmon).
life tables
columns:
-age class(x), survivorship(lx), fecundity(mx)
Use Cases:
Conservation: Predict population growth or decline.
Demography: Analyze human population trends.
survivorship curves
-type 1: Low early mortality; seen in organisms with high parental investment.
-type II: Constant mortality rate regardless of age.
-type III: High early mortality; typical for species producing many offspring with little care.
semelparity
All resources allocated to a single reproductive event.
Favored when larger size increases reproductive success (e.g., salmon, agave).
iteroparity
Multiple reproductive events over time.
Favored when survival chances increase with age or size.
why is semelparity referred to as big bang reproduction
Semelparity is often called “big bang reproduction” because it involves an organism investing all of its available resources and energy into a single, monumental reproductive event, after which it typically dies
In semelparous species, the individual holds off on reproduction until it reaches a life stage of maximal energy reserves and physiological capability. At this point, it channels virtually all of its stored resources into producing as many offspring as possible. The scale of the effort—from energy expenditure to the sheer number of offspring—is often spectacular.
Physiological Sacrifice:
Because the adult invests all its resources into a one-time reproductive burst, it cannot survive afterward. This “all-in” strategy maximizes the short-term reproductive output but leaves no energy for post-reproductive maintenance or survival.
Evolutionary Explanation:
In environments where mortality is high and the chance of surviving to another breeding season is low, it can be advantageous to devote everything to one grand reproductive event. By making a singular, explosive investment, the organism ensures it leaves the maximum genetic legacy behind in its single opportunity.
examples of semelparous species
Pacific salmon: After migrating upstream and expending tremendous effort to spawn, they die shortly after laying or fertilizing eggs.
Agave (“Century Plant”): After spending years accumulating energy to produce a giant flowering stalk, the plant blooms once and then perishes.
net reproductive rate (R0)
indicates whether a pop is growing (R0>1) or declining (R0<1)
generation time (T)
determines the average time for one gen to replace itself
reproductive value (vx)
reflects contribution of individuals at age x to future pop growth
how do survivorship curves differ across species
Survivorship curves are graphical representations that show the proportion of individuals in a cohort (a group of individuals of the same age) that remain alive at each successive age. These curves vary greatly across species due to differences in life history strategies, reproductive patterns, parental care, and environmental pressures. Biologists often describe three generalized types of survivorship curves—Type I, Type II, and Type III—to highlight typical patterns observed in nature.
Type I Survivorship Curves:
Characteristics: High survival rates during early and middle life, followed by a sharp decline in survival in older age classes.
Implications: Species with Type I curves typically produce relatively few offspring but invest substantial time and resources into each one. This heavy parental care and resource allocation allow offspring to survive to adulthood at high rates.
Examples: Many large mammals (including humans, elephants, and some primates), certain large birds, and organisms in stable, resource-rich environments.
Type II Survivorship Curves:
Characteristics: A relatively constant rate of mortality or survival probability throughout an organism’s life. There is no particular age at which the individual is more likely to die.
Implications: Species with Type II curves may have some parental care or stable habitats, but their risk of death is spread more evenly across all ages.
Examples: Many species of birds (e.g., robins), some small mammals, and certain reptiles.
Type III Survivorship Curves:
Characteristics: Extremely high mortality rates among juveniles, with only a small fraction of the cohort surviving to adulthood. However, those that do reach adulthood often enjoy relatively high survival rates for the rest of their lives.
Implications: Species that follow Type III curves often produce large numbers of offspring but invest little to no parental care in each one. This “shotgun” strategy relies on sheer numbers to ensure that some individuals survive the perilous juvenile stage.
Examples: Many fish (such as sunfish), marine invertebrates (like oysters), plants that produce massive numbers of seeds (e.g., dandelions), and amphibians.
Intermediate or Mixed Types:
Not all species neatly fit into these three categories. Some populations may display a mix of patterns or have curves that vary based on environmental conditions, sex, resource availability, or predation pressures.
In essence, survivorship curves differ across species primarily because of varying strategies in reproduction, parental investment, and ecological interactions. Species with high parental care and stable environments tend to exhibit Type I curves, those with steady mortality often show Type II patterns, and those relying on mass offspring production with little parental care generally fall into Type III.
explain how R0, T and vx are derived and their ecological significance
R₀: Measures the average lifetime reproductive output; indicates whether a population is growing or shrinking.
T: Provides the typical time frame between generations; informs how quickly populations can shift in response to environmental changes.
vₓ: Highlights the age classes most important for future population growth; valuable for understanding life-history evolution and guiding conservation efforts.
