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

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

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

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