Populations in Ecosystems
Unit Three
Generalist and Specialist Species
Generalist Species: These species have a broad ecological niche, meaning they can thrive in various environments and utilize different resources.
Raccoons are generalists because they eat diverse foods and adapt to both urban and wild areas.
Specialist Species: These species have a narrow niche and rely on specific resources or conditions for survival. They are more vulnerable to environmental changes.
Koalas are specialists that only eat eucalyptus leaves, so habitat loss directly impacts their survival.
K-Selected and r-Selected Species
K-Selected Species: Species with slow reproductive rates, fewer offspring, and high parental investment. They often reach a stable population size near the carrying capacity.
Characteristics:
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Long maturation period: Offspring take longer to reach adulthood.
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Higher survivorship: More likely to survive to older ages (Type I survivorship curve).
r-Selected Species: Species that reproduce quickly with little parental care. They produce many offspring, though survival rates are typically low.
Characteristics:
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High birth rates: Often exponential growth in ideal conditions.
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Short lifespan: Lower survivorship with most individuals dying young (Type III survivorship curve).
Survivorship Curves
Survivorship curves illustrate the proportion of individuals surviving at each age for a given species. They help ecologists understand population dynamics and life history strategies.
Cohort: A group of individuals of the same age within a population.
Mortality rate: The proportion of individuals dying within a specified time period.
Type I Survivorship Curve: Shows high survival rates for young and middle-aged individuals, with most mortality occurring at older ages.
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Examples: Humans, elephants, and other mammals that invest heavily in parental care.
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Characteristics: Reflects a K-selected life history strategy.
Type II Survivorship Curve: Depicts a constant mortality rate throughout an organism’s life.
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Examples: Birds, some reptiles, and rodents.
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Characteristics: Mortality is not heavily age-dependent.
Type III Survivorship Curve: Represents species with high mortality rates at young ages, but those that survive early stages often live long lives.
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Examples: Fish, amphibians, and plants producing many seeds.
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Characteristics: Reflects an r-selected strategy.
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Carrying Capacity
Carrying capacity (K) refers to the maximum population size that an environment can sustain over time without degrading resources. It’s influenced by factors such as resource availability, competition, predation, and disease.
Example: A lake can only support a certain number of fish due to limited food and oxygen. If the fish population exceeds this capacity, the fish may face starvation and disease.
Consequences of Exceeding Carrying Capacity:
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Resource depletion: Overpopulation leads to food shortages, lower water quality, and habitat destruction.
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Population crash: Once resources become unsustainable, the population may decrease sharply.
Key Vocabulary:
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Limiting factors: Environmental factors that limit population growth, such as food, water, and shelter.
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Overshoot: When a population temporarily exceeds its carrying capacity, often followed by a population crash.
Population Growth and Resource Availability
Population growth patterns vary based on environmental conditions and resource availability.
Exponential Growth:
Population increases by a constant percentage over time, leading to rapid, unchecked growth.
Exponential growth rate: Growth without limitations, resulting in a J-shaped curve.
Logistic Growth:
Growth rate slows as the population approaches carrying capacity, forming an S-shaped curve.
Logistic growth rate: Growth that slows as resources become limited, resulting in an S-shaped curve.
Since the Industrial Revolution, human populations have grown exponentially, largely due to advances in medicine, agriculture, and sanitation. For example, the global population took until the early 1800s to reach 1 billion but then grew to over 7 billion in just the next two centuries.
Deer populations in a forest may initially grow rapidly due to an abundance of resources such as food, water, and habitat space. As the population increases, however, the competition for these resources intensifies.
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Population Growth and Resource Availability
Age structure diagrams (population pyramids) visualize the age distribution within a population and can indicate whether a population is growing, stable, or declining.
Types of Age Structure Diagrams:
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Expansive: Broad base, indicating high birth rates and rapid growth (e.g., many developing countries).
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Constrictive: Narrow base, suggesting low birth rates and potential population decline (e.g., Japan).
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Stationary: Similar proportions across age groups, indicating a stable population (e.g., the U.S.).
Key Vocabulary:
Cohort analysis: Analyzing specific age groups within a population.
Demographic transition: The shift from high birth and death rates to lower birth and death rates in a population.
Population Growth and Resource Availability
Total fertility rate (TFR) is the average number of children a woman is expected to have over her lifetime.
Replacement Level Fertility: The TFR needed to maintain a stable population size, typically around 2.1 children per woman in developed countries.
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Example: A TFR of 2.1 is needed to replace parents and account for child mortality, ensuring population stability.
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Factors Influencing TFR:
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Social norms: Cultural and societal expectations about family size.
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Economic factors: Wealthier countries tend to have lower TFRs due to career opportunities and access to family planning.
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Government policies: Policies like China’s one-child policy or family planning incentives can influence TFR.
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Key Vocabulary:
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Crude birth rate: The number of births per 1,000 individuals in a population per year.
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Infant mortality rate: The number of infant deaths per 1,000 live births per year.
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