Life Cycles & Reproductive Strategies (College Board AP® Environmental Science) : Study Guide

Alistair Marjot

Written by: Alistair Marjot

Reviewed by: Jacque Cartwright

Updated on

Differences between K-selected & r-selected species

Overview of life cycles

  • Every species has a life cycle that includes growth, reproduction, and death

  • Life cycles vary between species based on:

    • Reproductive behavior

    • Lifespan

    • Investment in offspring

  • K-selected and r-selected species (sometimes referred to as K-strategists and r-strategists) represent two ends of the spectrum in reproductive strategies and adaptations

Awaiting image: r-selected and K-selected species
Image caption: r-selected and K-selected species represent two ends of the spectrum in reproductive strategies of different species

K-selected species

  • Characteristics:

    • Tend to be large in size

    • Produce few offspring per reproduction event

    • Invest significant energy into each offspring

    • Mature after many years with extended youth and parental care

    • Have long life spans and reproduce multiple times in their lifetime

    • Live in stable environments with high competition for resources

  • Reproductive strategies:

    • Focus on quality over quantity of offspring

    • Ensure offspring survival through parental care

  • Examples:

    • Elephants produce one calf at a time and care for it for years

    • Oak trees produce fewer seeds (acorns), but these seeds have a better chance of growing into mature trees due to the stable environment of a mature forest

    • Whales provide extensive parental care to ensure survival of their young

A rhino with a large horn standing in tall green grass, facing the camera, with trees in the blurred background.
Large mammals such as rhinos are K-strategists (Photo by Glen Carrie on Unsplash)

r-selected species

  • Characteristics:

    • Tend to be small in size

    • Produce many offspring per reproduction event

    • Invest minimal energy into each offspring

    • Mature early with little to no parental care

    • Have short life spans and may reproduce only once in their lifetime

    • Live in unstable or unpredictable environments with low competition for resources

  • Reproductive strategies:

    • Focus on quantity over quality of offspring

    • Reproduce quickly to take advantage of temporary resources or new habitats

  • Examples:

    • Flies lay hundreds of eggs without providing care

    • Rabbits reproduce rapidly and in large numbers

    • Dandelions produce numerous seeds that spread widely, thriving in disturbed environments

Close-up of a common housefly with striped body and translucent wings perched on the tip of a twig against a blurred green background.
Flies are r-strategists (Photo by MOHD AZRIEN AWANG BESAR on Unsplash)

Succession and life cycles

  • In early stages of succession, r-selected species dominate:

    • They quickly colonize new or disturbed areas

    • Adapt well to unstable conditions

  • In later stages of succession, K-selected species dominate:

    • Thrive in stable, mature ecosystems

    • Outcompete r-selected species in resource-rich environments

Biotic potential

Biotic potential definition

  • Biotic potential refers to the maximum reproductive rate of a population under ideal environmental conditions

    • Represents the highest possible growth rate for a species

    • Assumes unlimited resources and no environmental resistance (e.g., no predators, competition, disease)

Factors influencing biotic potential

  • Reproductive rate:

    • Species with high reproductive rates (e.g., bacteria, insects) tend to have high biotic potential

  • Number of offspring per reproduction event:

    • Species producing large numbers of offspring at once often have higher biotic potential

    • Example: Frogs laying hundreds of eggs at a time

  • Age at reproductive maturity:

    • Species that reproduce early in life can achieve higher biotic potential

    • Example: Mice can reproduce at a young age, allowing for multiple generations within a year

  • Frequency of reproduction:

    • Species reproducing frequently throughout their lifespan increase biotic potential

    • Example: Rabbits reproducing multiple times per year

  • Lifespan of reproductive individuals:

    • Longer reproductive lifespans allow for more opportunities to produce offspring

Examples of biotic potential

  • High biotic potential:

    • Insects like flies and mosquitoes can rapidly increase their population size in favorable conditions

    • Algae blooms occur when conditions support the rapid reproduction of algae

  • Low biotic potential:

    • Large mammals like elephants produce few offspring, resulting in slower population growth even under ideal conditions

Limits to biotic potential

  • In natural ecosystems, biotic potential is rarely achieved due to environmental resistance:

    • Predation: Natural predators control population growth

    • Competition: Limited resources restrict reproduction and survival

    • Disease: Pathogens reduce population size

    • Carrying capacity: Ecosystems have a maximum number of individuals they can support sustainably

Importance of biotic potential in ecosystems

  • Helps predict population dynamics and population growth rates

  • Provides insights into:

    • Roles of species in ecosystems

    • Responses of species to environmental changes

Variation in reproductive strategies

  • Not all species fit neatly into r-selected or K-selected categories

    • Many species exhibit traits from both strategies depending on environmental conditions

    • Reproductive strategies may also shift over time or in response to habitat changes

Mixed strategies

  • Species with mixed strategies:

    • Some species balance traits of both r- and K-selection to maximize survival

    • Example:

      • Sea turtles lay many eggs (r-selected trait)

      • However, they also invest significant energy in choosing nesting sites to increase offspring survival (K-selected trait)

  • Changing strategies over time:

    • Species may adjust their reproductive strategy based on environmental conditions

    • Example:

      • Frogs may lay hundreds of eggs in temporary ponds produced by heavy rainfall (r-selected trait), even if these are in danger of drying up before the tadpoles metamorphose into frogs

      • In dryer conditions, they may only select safer water sources to increase offspring survival (K-selected trait)

Environmental influence

  • Environmental factors, such as resource availability and competition, drive variation in reproductive strategies

    • In unstable environments, species may adopt r-selected traits to reproduce quickly

    • In stable environments, K-selected traits may dominate to ensure long-term success

Reproductive strategies of invasive species

  • K-selected species are often more negatively affected by invasive species than r-selected species

    • r-selected species are often minimally affected by invasive species

  • Invasive species are themselves often characterized by r-selected strategies

    • They frequently outcompete K-selected species

Impact on K-selected species

Competitive exclusion

  • R-selected invasive species often show rapid growth and high reproductive rates

  • This means they can quickly dominate resources (e.g., space, nutrients, light)

  • This leaves K-selected species with limited access to these resources

    • Example: The invasive zebra mussel (r-selected) outcompetes native freshwater mussels (K-selected) in North America by rapidly colonizing hard surfaces, hindering native mussel attachment and filter feeding

Predator pressure

  • Some invasive species are predators that directly impact K-selected species, particularly those with slow reproductive rates

    • Example: The brown tree snake (r-selected), introduced to Guam, wiped out native bird populations (many K-selected) that had no evolved defenses against this new predator (which predates on birds eggs and nestlings)

Habitat disruption

  • Invasive species can alter habitats in ways that favor r-selected traits, further disadvantaging K-selected species

    • Example: Kudzu (r-selected), a fast-growing vine, smothers native vegetation in the southeastern U.S.

    • This reduces habitat complexity and favors other invasive species that thrive in disturbed environments

Why most invasive species are r-selected

  • Rapid reproduction:

    • High reproductive output allows for quick establishment and population growth in new environments

  • Short generation times:

    • Faster generation times enable rapid adaptation to new conditions and increase the chance of beneficial mutations arising

  • High dispersal ability:

    • Efficient dispersal mechanisms (e.g., wind-dispersed seeds, attachment to animals) facilitate rapid colonization of new areas and over large distances

  • Adaptability:

    • The ability to adjust their physiology and behavior in response to environmental changes increases their adaptability in invaded habitats

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies Content Creator

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Jacque Cartwright

Reviewer: Jacque Cartwright

Expertise: Geography Content Creator

Jacque graduated from the Open University with a BSc in Environmental Science and Geography before doing her PGCE with the University of St David’s, Swansea. Teaching is her passion and has taught across a wide range of specifications – GCSE/IGCSE and IB but particularly loves teaching the A-level Geography. For the past 5 years Jacque has been teaching online for international schools, and she knows what is needed to get the top scores on those pesky geography exams.

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