Unit 2- Chapter 3 - Life History Strategies Flashcards
Life History Strategy : the suite of traits and reproductive strategies that organisms adopt over its lifetime and shaped by natural selection in order to maximize its lifetime reproductive success (pattern of growth, reproduction, survival, development, resource allocation). Organisms can exhibit different lifetime history strategies based on their environmental conditions and their evolutionary pressures. Organisms have a limited amount of resources that can be allocated to specific aspects of their LHS.
MAIN TRAITS:
- Maturation: the time and readiness and age that an organism will be to start reproducing.
- Fecundity: how many offspring will an organism produce (organisms potential reproductive capacity).
- Parity: how many times to attempt reproduction (an organism may only have 1 chance to reproduce, one time)
- Parental Investment: time that parents put into offspring
- Longevity: how long an organism lives
Not one of these is better or more successful than the others – just one works better for an organism.
Ex: Could have 100 offspring (fecundity) but less time (parental investment) to allocate to each offspring.
Life Histories:
What is meant by evolutionary trade-off?
Principle of Allocation (aka. Principle of Trade-Offs or Principle of Energy Allocation)?
Evolutionary trade-off : Improvement in one trait or feature comes at the expense of another. Inescapable compromise between conflicting demands due to limited resources or physiological constraints. These trade-offs shape the life history of an organism. An example is a trade-off between egg size & clutch size - bigger but fewer eggs (size increases survivability) vs. smaller, but more eggs (more offspring but less survivability)
There is the Principle of Allocation: an organisms limited resources (energy, time, nutrients) must be allocated among different competing life functions or traits. Resources are finite - allocating resources to one function…means it’s diverted from another (between reproduction, growth, maintenance).
Ex : Having 100 eggs (fecundity) but having less time (parental investment) per egg.
Ex: allocating more resources to reproduction…diverts it from growth or survival. (the mayfly!)
Regarding LHS:
What is Intrinsic and Extrinsic factors?
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INTRINSIC:
* Genetic Factors: genetic variations within a population. Because it determines growth rate, reproductive timing, lifespan –which ultimately shape an organisms life history.
* Physiological Constraints : metabolic rate, body size, reproductive physiology can influence factors such as growth rate, energy allocation, and reproductive output
* Age & Size : younger organisms may prioritize growth & development. Older organisms may invest more in reproduction. Size can influence factors such as reproductie output, survival, & resource requirements. -
EXTRINSIC:
* Environmental Conditions : temperature, precipitation, resource availability (food, water, shelter). Less food? more growth/maintenance, rather than reproduction.
* Predation Risk: presence and intensity of predators. Can influence when an organism reproduces, shorter lifespan (Mayfly!), more defense mechanisms.
* Competition : for limited resources can lead to delayed reproduction, increased resources to growth, or more competitive traits.
* Disturbances/Habitat Variability : natural disasters or changes in habitat can have organisms adjusting their reproductive timing, investment in offspring, or migration patterns in response.
Example of a trade-off with egg size?
What is Clutch Size?
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Bigger eggs? more nutrient resources allocated to the bigger eggs.
Giving up? the # of eggs. Can make 1 giant egg or make a bunch of smaller ones.
Optimal egg size? can measure how many eggs a female makes and if it correlates with the size of the eggs. Is there a relationship? Then can take the eggs and watch if the bigger or smaller eggs have better survivability. Do an OBSERVATIONAL study. An addition or subtraction experimental study can be done such as taking out or placing in more eggs into a birds nest. Which survive the best ..and which survive the least.
NATURAL SELECTION SHAPES THIS STRATEGY done by the organism.
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- Clutch Size: # of eggs a bird produces in a single nesting attempt. Numbers based on factors such as ecological niche, environmental conditions, and life history strategies.
Looking more into Life History Strategies and Conditions they are more effective/less effective:
THE NEXT 3 EXAMPLES ARE EXAMPLES OF TRADE-OFFS :
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(Trait #4) -PARENTAL INVESTMENT -
#1 :Number of offspring vs. Size of offspring
- Higher # of offspring = more opportunities for offspring to be more successful. Higher probability for More offspring out into environment to use resources. Why successful? a better strategy when there’s a large amount of resources available for a short amount of time
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Size of Offspring = more survivability, can compete better for resources because they are born in a larger size (compared to smaller animals, let’s say)
Why successful? a better strategy when there is more competition over possibly a limited amount of resources.
Example : (an example of phenotypic plasticity, btw) — Acorn mass is largest with the least water. Acorn mass is smallest when there is lots of preciptation. Why? The larger seed has more energy inside to survive the inclement conditions because it’s not getting them from the environment. More resources are allocated to the offspring. This is an example showing an extrinsic factor (Environment)
PARENTAL INVESTMENT:
#2 : Parental Care
Defined as behaviours and actions undertaken by parents to provide protection, food, shelter, and other forms of assistance to their offspring (incubation, feeding, grooming, teaching, defending) Why? Increases reproductive success.
Ex: When one reptile lays many, many eggs and then leaves. Compared to another reptile that lays a few eggs and then stays to care for them.
a) ALTRICIAL YOUNG: offspring born or hatched in a underdeveloped and helpless state. Typically immobile, closed eyes, lack well-developed sensory organs, highly dependent on parental care such as heat/food/protection. Significant parental investment before they are independent. Therefore, parents spend less time on incubation, more time on post-care.
Ex: humans, rodents, primates, songbirds.
b) PRECOCIAL YOUNG: offspring born or hatched in more advanced and self-sufficient state. Usually mobile, open eyes, possess sensory organs, capable of walking or running shortly after birth or hatching, feed themselves somewhat. Still benefit from parental care but level of dependency is significantly lower compared to altricial young. Therefore, parents spend more time on incubation, and less time on post-care. Ex : ducks, geese, chickens, horses, deer, elephants.
WHY? A successful strategy because of extrinsic factor (Predation)!
PARENTAL INVESTMENT:
#3 : Current vs. Future Reproduction
Q: why might parents produce fewer offspring than they could theoretically support in a single reproductive bout?
Reproduction can directly decrease an individuals ability to produce future offspring because available energy resources are reduced.
Producing fewer offspring currently than their maximum potential may increase reproductive success for the future in several ways:
- Parental Investment: : allocates resources to smaller # of offspring. Meaning, parents are providing high quality care & investement currently (providing more food, attention, protection) to increase survivability in the future for the few offspring
- Offspring Quality: fewer offspring currently means enhanced physical development in the future (increasing survivability, competiveness and reproductive success) due to having less mouths to feed.
- Resource Availability: could be an extrinsic factor (environmental) where there is food lacking so the parents are ensuring enough food for fewer offspring. Increases survivability.
- Parental Survival: there is wear and tear on the parents. More babies means more work from the parents that season, and if there are too many offspring, this may have the parent not having babies the following year.
Examples:
- In a 2007 study, Trinidadian guppies that were in high-predation areas allocated more resources to earlier & more frequent reproduction BUT they had a shorter lifespan compared to guppies in a low-predation area.
- In a 2001 study by Brown & Brown, female cliff swallows that were older, allocated more resources to self-survival & maintenance BUT they had reduced reproductive output compared to younger females (an example of senescence – intrinsic factor – demonstrating trade-off between current reproductive effort and future survival)
THE NEXT 3 EXAMPLES ARE EXAMPLES OF TRADE-OFFS OF L.H.S. :
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(Trait #2) -FECUNDITY -
#1 : Fecundity vs Body size
Fecundity can change over an organisms lifetime, as the pic below shows. The older the Hen, the less eggs it lays. Fecundity also increases with the size of an animal.
Ex: a 2011 study published in the journal Ecology showed that larger Atlantic cod fish produced significantly more eggs than smaller sized cod (positive correlation)
Ex: A 2006 study published in the journal Nature showed that larger female house sparrows laid more eggs and had higher reproductive success (positive correlation)
Determinate vs Indeterminate Growth:
- DETERMINATE GROWTH : reach a specific size or body mass – after maturity is reached, no more growth occurs. Ex : humans stop growing after reaching adulthood and body size remains relatively constant.
- INDETERMINATE GROWTH : continue to grow throughout life, even after sexual maturity is reached. No predefined max size or body mass. Ex : certain fish, such as salmon, some reptiles, and some invertebrates (lobsters, crabs, shrimps, snails, clams, sea stars, sea urchins, sea cucumbers, earthworms– can add segments to their body–, some jellyfish)
NOTE : while these examples represent animals for indeterminate growth, the extent and pattern of growth can still vary among species within each group. Additionally, factors such as resource availability, environmental conditions, and physiological constraints can influence the growth rates and overall size attained by these invertebrates
(Trait #2) -FECUNDITY -
#2 : Fecundity vs Age of Maturity
Age of Maturity : the age at 1st reproduction and enables them to contribute offspring to the next generation. It’s the transition from juvenile to adult.
Influenced by environmental factors, genetic factors, ecological contraints.
Why would this matter with Natural Selection? Natural Selection favours and acts on traits that influence an organism’s FITNESS (ability to survive, reproduce, and pass on genes!) & REPRODUCTIVE SUCCESS. So if the animal matures earlier, it will have a higher lifetime reproductive success (more offspring).
Why would age of maturity be delayed?
* larger size & reproductive investment (resources go 1st towards growing size and later, resources go towards reproduction (larger organism, higher fecundity, larger offspring, more resources towards parental care) (instrinsic factors)
Why would age of maturity be earlier?
* higher predation pressures & unpredictable/unstable environments & high competition for resources (extrinsic factors)
What is SENESCENCE ?
- the process of aging in organisms
Involves:
* gradual decline in physiological function
* increased susceptibility to age-related diseases
* increase in mortality BUT decrease in fecundity
* Investment in reproduction should always exceed investment in self-maintenance
What happens to organisms that invest more in self-maintenance than breeding?
* enhance own survival and longevity
* may save energy for reproduction in future seasons
What happens to organisms that invest more in breeding than self-maintenance?
* shorter life-span so more resources to reproduction
* younger organisms generally have higher reproductive value and they may take advantage of their prime reproductive years – increases fitness.
* As reproductive value decreases with age, they may prioritize maintenance
Semelparity vs Iteroparity ?
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SEMELPARITY : single reproductive event followed by death. Conditions that favour semelparity include:
a) unpredictable/harsh environments: lower chances of survival - leaves offpsring before environment becomes unsuitable or offsprings’s chance of survival declines
b) low adult survival rates: they may have inherently limited lifespan
c) Limited resources : food, nesting sites, shelter
Ex: Semelparous plants such as agave or bamboo, produce large numbers of offspring in a single event, and die shortly afterwards. Pacific salmon too – produce once and die.
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- ITEROPARITY : reproduce multiple times in lifetime. This trade-off allows organisms to adapt to different environmental conditions and maximise reproductive success (based on resource availability)
Ex: Iteroparous plants such as perennial plants. Humans, nearly all mammals, nearly all birds.
What about cessation of reproductive capabilities?
MENOPAUSE
Only 3 species go through menopause:
1. Humans
2. Killing whales
3. Short-finned Pilot whales
TWO BENEFITS:
- lowered risk of certain reproductive-related diseases, such as uterine cancer and ovarian cancer. Hormonal decreases (estrogen) also reduces the potential for abnormal cell growth. AND why important? this leads to the organism not being able to help with taking care of existing offspring.
- Allocate resources from reproduction (menstruation, pregnancy, lactation) to other aspects. AND why important? helps existing offspring survive, find food, navigate social dynamics within the pod, passing info such as migration routes, foraging techniques, & social behaviour. The experienced older females help their survival, ultimately increasing fitness of the pod.
Looking at one of the extrinsic factors of Life History Strategy:
How does environmental factors affect L.H.S?
- Resource Availability (food, water, nesting sites). More resources allow higher fecundity (more offspring) & higher parental investment & higher size at the age of maturity. Less resources encourage survival instead (less offspring, maybe delayed reproduction)
- Predation Pressures : Higher forces early reproduction. Lower favours delayed reproduction, more parental care, possibly larger offspring.
- Stable Environment : when stable, organisms will adopt strategies to optimize long-term survival and reproductive success. In unpredictable/unstable times, organisms will prioritize faster reproduction.
- Weather/Climate : temperature & rainfall patterns affect development rates, metabolic processes, reproductive cycles. Ex: cold environments =slower growth rates + delayed reproduction to ensure offspring have resources to survive harsher conditions.
- Ecological Interactions : with other species within an ecosystem, can influence reproductive timing, mate selection, parental care.
It’s important to note that these factors do not act in isolation, and their effects can interact and vary across different species and populations. Organisms often exhibit phenotypic plasticity in their life history strategies, allowing them to adjust their reproductive efforts and investment based on the prevailing environmental conditions and selective pressures they experience.
r-strategists vs K-strategists
r-strategists would be most successful under variable evironmental conditions where they can take advantage of any favourable times and create offspring quickly.
k-strategists need stable environments.
The terms “r-strategists” and “K-strategists” come from the logistic growth equation, where “r” represents the intrinsic rate of increase and “K” represents the carrying capacity of the environment. The distinction between these reproductive strategies is based on the trade-off between maximizing reproductive output (r-strategists) and optimizing offspring survival and population stability (K-strategists). It’s important to note that the classification as r-strategist or K-strategist is not mutually exclusive, and many organisms fall on a continuum between these extremes, exhibiting varying degrees of both strategies depending on environmental conditions and selective pressures.
The plant strategy spectrum, also known as the CSR (Competitor-Stress tolerator-Ruderal) theory, is a framework that categorizes plant species based on their life history strategies and adaptations to different environmental conditions. This spectrum describes three broad ecological strategies that plants adopt in response to varying levels of stress and disturbance in their habitats
It’s important to note that the plant strategy spectrum is not a strict categorization, but rather a continuum. Many plant species exhibit a combination of strategies or can shift their strategy in response to changing environmental conditions (phenotypic plasticity)
So their names are literally these 3 names below and their chosen ‘life strategy’ is what is described in the picture:
1. STRESS TOLERATORS
2. COMPETITORS
3. RUDERALS
