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.
Why might masting be favored in plants?
Masting is the phenomenon where populations of certain plant species produce large seed crops simultaneously at irregular, often multi-year intervals. It involves periods of low or no seed production followed by a sudden, massive reproductive event across a wide area. Numerous ecological, evolutionary, and environmental factors can favor masting.
Key Reasons Why Masting May Be Favored:
Predator Satiation:
By producing seeds in great abundance all at once, plants can overwhelm seed predators (such as rodents, birds, or insects). In mast years, the sudden and vast availability of seeds allows some to escape being eaten simply because predators cannot consume them all. During non-mast years, low seed production prevents predator populations from building up to large numbers, ensuring that when the mast event occurs, it exceeds their capacity to consume all seeds.
Pollination Efficiency:
Many masting species rely on wind pollination. When trees flower simultaneously and heavily, they create a dense cloud of pollen in the air, increasing the chance that female flowers receive sufficient pollen to set seed. In contrast, sparse and asynchronous flowering leads to lower pollination success.
Resource Accumulation and Synchronization:
Plants may need several years to build up the necessary resources (e.g., carbohydrates, nutrients) to produce a large seed crop. Environmental cues such as temperature fluctuations, rainfall patterns, or El Niño events may synchronize resource accumulation across a population, triggering a collective mast event.
Genetic Advantages of Synchrony:
By synchronizing reproduction, plants reduce inbreeding and increase genetic diversity. The increased cross-pollination among many individuals blooming at once enhances the genetic health of the population, potentially leading to more robust offspring.
Avoidance of Environmental Hazards:
Some environmental disturbances (harsh winters, droughts, fire regimes) may limit when plants can successfully reproduce. By waiting for particularly favorable conditions, multiple individuals can capitalize on these rare “windows” of environmental suitability, resulting in synchronized, high-yield seed production events.
What is life history?
A: Strategies organisms use to allocate resources to growth, reproduction, and survival.
What is an age-structured population?
A: A population divided into age classes to study dynamics where individuals contribute differently based on age.
What does a life table summarize?
age-specific survivorship (lx) and fecundity (mx)
what does lx represent in a life table
The proportion of individuals surviving to age x
what does mx represent in a life table
the number of offspring produced by an individual of age x
what does R0 measure
the average number of offspring a female produces over her lifetime
what is generation time (T)
the average age of reproduction
what is a survivorship curve
a graphical representation of lx over x, showing how survival changes with age
What characterizes a Type I survivorship curve?
A: Low mortality early in life, increasing with age (e.g., humans)
What characterizes a Type II survivorship curve?
A: Constant mortality rate across all ages (e.g., birds).
What characterizes a Type III survivorship curve?
A: High early mortality with few survivors (e.g., fish, plants).
what is semelparity
A reproductive strategy where an organism reproduces once and then dies (e.g., salmon).
iteroparity
A reproductive strategy where an organism reproduces multiple times over its life (e.g., humans).
masting
Synchronous reproduction in plants, often to overwhelm predators (e.g., oak trees).
reproductive value (vx)
expected number of future offspring for an individual of age x
what does vx represent
the expected number of future offspring for an individual of age x
what is an r-strategy
A strategy characterized by high reproductive output and low parental care (e.g., mice).
what is a k-strategy
A strategy characterized by low reproductive output and high parental investment (e.g., elephants).
formula for R0
r0= signma (lxmx)
what does R0>1 indicate
pop is growing
What key factors must organisms allocate energy to?
A: Growth, reproduction, and survival.
Give an example of a species with Type I survivorship.
A: Humans or large mammals.
Give an example of a species with Type III survivorship.
A: Fish or plants.
Give an example of an iteroparous species.
A: Humans or elephants.
how are life tables used in conservation bio
to predict pop trends and assess risks for endangered species
How do survivorship curves shift with environmental changes?
A: Increased mortality rates can shift populations toward Type II or III curves.
why is vx important in pop management
A: It helps target age classes critical for population recovery.
