Unit 3: Populations - Specialist vs Generalist, Reproductive Strategies, and Demographic Trends

Specialist and Generalist Species

• Specialist Species: These species possess a narrower ecological niche and a smaller range of tolerance. Because of their highly specialized needs and specific requirements, they are more prone to extinction.

  • Specific food requirements: A named example includes the panda, which requires bamboo as a primary food source.
  • Adaptability: They have a lower ability to adapt to new environmental conditions.
  • Resource Use: They utilize a specific, narrow set of resources.
  • Environmental Context: Specialists are easily affected by changing conditions but tend to have a competitive advantage in habitats that remain constant over time.

• Generalist Species: These species have a broader ecological niche and a larger range of tolerance, making them significantly less prone to extinction compared to specialists.

  • Invasiveness: Their broad requirements make them more likely to become invasive species.
  • Food requirements: They have broad dietary needs and can utilize a wide variety of resources.
  • Adaptability: Generalists exhibit high adaptability to many environments.
  • Environmental Context: They have a competitive advantage in habitats that are frequently changing or unstable.

K-selected and r-selected Species

• K-selected Species ("Quality"):

  • Characteristics: Typically large in size with long lifespans and a long life expectancy.
  • Reproduction: They have few offspring per reproduction event and reproduce more than once in their lifetime. They mature slowly after an extended period of youth.
  • Energy Investment: Significant energy and resources are expended for each offspring. High levels of parental care are present to protect and ensure the survival of the young.
  • Ecological Context: These species usually live in stable environments where competition for resources is relatively high.
  • Sensitivity: They are more likely to be disrupted by environmental changes or invasive species. Their low biotic potential results in a slow population growth rate, making it hard for populations to recover after a disturbance.
  • Parental Impact: Because of the high reliance on parental care, the death of a parent often leads to the death of the offspring.
  • Examples: Most mammals and birds (e.g., Chimpanzees, Pumas).

• r-selected Species ("Quantity"):

  • Characteristics: Typically small in size with short lifespans.
  • Reproduction: They have many offspring per reproduction event and may reproduce only once in their lifetime.
  • Energy Investment: Minimal energy or investment is provided for each offspring. Parental care is largely absent. Maturity is reached early and quickly.
  • Ecological Context: They tend to inhabit environments where competition for resources is typically low. They are better suited for rapidly changing conditions.
  • Invasiveness: Most invasive species are r-selected. Their high biotic potential leads to rapid population growth rates.
  • Resilience: They are minimally affected by invasive species because their populations grow quickly. Large populations and fast generation times increase the likelihood of adaptation and decrease the chance of extinction.
  • Examples: Insects, fish, plants (e.g., Oysters, Tuna).

• Biotic Potential: This refers to the maximum reproductive rate of a population under ideal environmental conditions.

• The Reproductive Spectrum: Many species do not fit uniquely into the r-selected or K-selected categories; reproductive strategies exist on a spectrum and can change under different conditions and at different times.

  • Oyster: Approximately $500 \times 10^6$ offspring per year (extreme r-selected).
  • Tuna: Approximately $6,000$ offspring per year.
  • Frog: Approximately $200$ offspring per year.
  • Hare: Approximately $12$ offspring per year.
  • Puma: Approximately $2$ offspring per year.
  • Chimpanzee: One offspring every $5$ years (extreme K-selected).

Comparison Table of K-selected vs. r-selected Traits

Survivorship Curves

• Definition: A survivorship curve is a line displaying the relative survival rates of a cohort (a group of individuals of the same age) in a population from birth to the maximum reached age.

• Interpreting the Graph:

  • Faster drop in line: Quicker die-off of individuals.
  • Slower drop in line: Longer average lifespan.

• Type I Survivorship (Mostly K-selected):

  • High survivorship early in life due to high parental care.
  • High survivorship in mid-life due to large body size and defensive behaviors.
  • Rapid decrease in survivorship in late life as old age sets in.
  • Examples: Humans, most mammals.

• Type II Survivorship (Intermediate):

  • Features a steadily decreasing survivorship throughout the entire life of the cohort.
  • Example: Birds.

• Type III Survivorship (Mostly r-selected):

  • High mortality and low survivorship early in life due to little or no parental care.
  • Few individuals reaching mid-life; however, for those that do, there is a slow, steady decline in survivorship.
  • Even fewer make it to adulthood and old age.
  • Examples: Insects, fish, trees.

Carrying Capacity ($K$)

• Definition: The maximum number of individuals in a population that a specific ecosystem can support based on limiting resources.
• Limiting Resources: Factors that determine carrying capacity include food, water, and habitat (such as nesting sites and physical space).
• Growth Patterns:
  • Biotic Potential: Exponential growth that occurs when there are no limiting resources.
  • Logistic Growth: Initial rapid growth that eventually slows and levels off at the carrying capacity ($K$) due to limiting factors like competition, disease, and predators.
• Overshoot and Die-off:
  • Overshoot: Occurs when a population briefly exceeds its carrying capacity ($K$).
    • Example: Deer breeding in the fall and giving birth to a large number of fawns in the spring, leading to a sudden spike in population.
  • Environmental Impacts of Overshoot: Resource depletion (e.g., deer overgrazing vegetation).
  • Die-off (Dieback): A sharp, often catastrophic decrease in population size that occurs when resource depletion leads to famine, disease, or conflict.
• Historical Case Study: Reindeer of St. Paul Island:
  • $25$ reindeer introduced in $1910$.
  • Gradual growth from $1910$ to $1930$.
  • Exponential growth from $1930$ to $1937$.
  • Carrying capacity was overshot, resulting in a severe population crash because food resources (lichen) were severely depleted.

Population Characteristics and Growth Factors

• Population Size ($N$): Total number of individuals in a given area at a given time. Larger populations are generally safer from decline.
• Density: The number of individuals per unit area (e.g., $12 \, \text{panthers/km}^2$). High density leads to high competition, disease outbreaks, and food depletion.
• Distribution: How individuals are spaced relative to each other.
  • Random: e.g., Trees.
  • Uniform: e.g., Territorial animals.
  • Clumped: e.g., Herds or group animals.
• Sex Ratio: The ratio of males to females. A $50:50$ ratio is usually ideal for breeding. Skewed ratios (e.g., fewer females) can limit growth.
• Density-Dependent Factors: Factors that influence growth based on population density. Examples: Food, water, light, habitat competition, and disease transmission. These mainly limit high-density populations.
• Density-Independent Factors: Factors that influence growth regardless of population density. Examples: Natural disasters such as floods, hurricanes, tornadoes, and fires.

Calculating Population Dynamics

• Population Change Formula:     $\text{Population Change} = (\text{Immigrations} + \text{Births}) - (\text{Emigrations} + \text{Deaths})$
• Growth Rate ($r$): The percentage increase in a population, usually calculated annually.
• Crude Birth Rate (CBR): Number of births per $1,000$ individuals per year.
• Crude Death Rate (CDR): Number of deaths per $1,000$ individuals per year.
• Growth Rate Formula using CBR/CDR:     $r = \frac{\text{CBR} - \text{CDR}}{10}$Note: Dividing by 10 converts a "per 1,000" rate into a percentage "per 100".
• Rule of 70 (Doubling Time):     \text{Doubling time (years)} = \frac{70}{\text{Growth rate (expressed as %)}}     Example: A population growing at $2\%$ per year will double in $35$ years ($70 / 2 = 35$).

Age Structure Diagrams

• Age Cohorts:
  • Prereproductive: Ages $0\text{--}14$.
  • Reproductive: Ages $15\text{--}44$.
  • Post-reproductive: Ages $45+$.
• Growth Prediction: The size difference between the $0\text{--}14$ and $15\text{--}44$ cohorts indicates growth rates.
  • Rapid Growth: Extreme pyramid shape; the $0\text{--}14$ cohort is significantly larger than the reproductive cohort.
  • Slow/Stable Growth: Less extreme pyramid shape or a house-like shape where cohorts are roughly equal in size.
  • Declining Population: The diagram is narrowest at the base (prereproductive cohort is smaller than reproductive cohort).

Total Fertility Rate (TFR) and Infant Mortality

• TFR: The average number of children a woman in a population will bear throughout her lifetime. Higher TFR generally leads to higher birth rates and population growth.
• Replacement Level Fertility: The TFR required to keep a population size stable (offsetting deaths).
  • Approximately $2.1$ in developed countries.
  • Higher in less developed countries due to higher infant mortality rates.
• Infant Mortality Rate (IMR): The number of deaths of children under $1$ year of age per $1,000$ people in a population.
  • Factors in IMR decline: Access to clean water, healthcare (hospitals, vaccines, supplements), and a reliable food supply.
  • Impact on TFR: High IMR often leads to higher TFR as families have "replacement children."

Factors Affecting Human Population Growth

• Development and Affluence: Wealthier nations typically have lower TFR due to:
  • Increased educational and economic opportunities for women.
  • Higher access to family planning and contraceptives.
  • Later age of first pregnancy.
  • Less reliance on children for agricultural labor.
• Government Policy: Can be coercive (forceful) or non-coercive (encouraging).
  • Coercive: China's former One-Child Policy, forced sterilization, financial penalties for extra children.
  • Non-coercive: Tax incentives for fewer children, microcredits for business-owning women without children, free higher education for women, and financial incentives for single-child families.
• Standard of Living Indicators:
  • Gross Domestic Product (GDP): Total value of goods and services produced. Per capita GDP is $\frac{\text{Total GDP}}{\text{Total Population}}$.
  • Life Expectancy: Average age a person lives to. High life expectancy and high GDP indicate high development and low population growth.

Malthusian Theory and Technological Advancement

• Malthusian Theory: Thomas Malthus theorized that human population grows exponentially while food production grows linearly. He predicted humans would reach a carrying capacity limited by food supply, resulting in a "Malthusian catastrophe."
• Technological Innovation: Humans have historically altered Earth's carrying capacity.
  • Named Example: In $1918$, the synthetic fixation of Nitrogen led to the creation of synthetic fertilizers, which dramatically increased the global food supply and cereal yields (e.g., wheat, barley, and oats in the UK from $1270\text{--}2014$).

Demographic Transition Model (DTM)

• Industrialization: The transition from an agrarian (farming) economy to an industrial (manufacturing) one.
• Stage 1: Pre-industrial:
  • High CBR and high CDR/IMR (due to lack of clean water and healthcare).
  • High TFR for replacement and agricultural labor.
  • Little to no population growth.
• Stage 2: Industrializing/Developing:
  • Modernization leads to access to clean water and healthcare; CDR and IMR decline.
  • CBR remains high due to generational lag and a continued need for labor.
  • Rapid population growth occurs.
• Stage 3: Developed/Industrialized:
  • Increased family income and educational opportunities for women.
  • TFR declines; marriage and first children are delayed.
  • Growth slows as CBR drops closer to CDR.
• Stage 4: Post-Industrialized/Highly Developed:
  • Very high affluence and education.
  • TFR drops below replacement level ($2.1$).
  • CBR drops below CDR, leading to negative growth (population decline).

Questions & Discussion

• Identify ONE characteristic of specialist species and explain how that characteristic makes them more likely to become extinct than generalist species.

  • Specialists have a narrow ecological niche. This makes them more likely to become extinct because if their specific resource (such as a specific food source like bamboo) is removed or their constant environment changes, they lack the adaptability to switch to new resources or conditions.

• Identify ONE characteristic of an r-selected species that could increase the likelihood of the r-selected species becoming a more successful invasive species than K-selected species.

  • High biotic potential (reproductive rate) allows r-selected species to grow their populations rapidly, outcompeting native K-selected species for resources before the native population can recover.

• Describe the relationship between Zebra Mussel and Unionid Mussel population density in the Hudson River based on the provided table (1991-2015).

  • In $1991$, Zebra Mussel density was $0$ and Unionid density was $8/m^2$. After Zebra Mussels were introduced (reaching $3,250/m^2$ by $1997$), the Unionid population crashed to $2/m^2$ and remained low as Zebra Mussel density stayed high (between $2,000$ and $2,750/m^2$).

• Calculate the population size of a 14-wolf pack that experiences 5 deaths, 3 births, and 4 new wolves released into the pack.

  • Change: $(3 \, \text{births} + 4 \, \text{immigrants}) - (5 \, \text{deaths} + 0 \, \text{emigrants}) = 7 - 5 = +2$.
  • New population: $14 + 2 = 16 \, \text{wolves}$.

• Identify the country with the slowest population growth rate among Countries X, Y, and Z and explain.

  • Country Z has the slowest growth rate (declining) because its age structure diagram is narrowest at the base, meaning the prereproductive cohort ($0\text{--}14$ individuals) is smaller than the reproductive or post-reproductive cohorts.

• Identify and discuss TWO of the causes for the trend in worldwide TFR (declining from 1950 to 2015).

  • One cause is increased educational opportunities for women, which leads to delayed childbearing. A second cause is increased access to family planning and contraceptives, which reduces unplanned pregnancies.

• Explain the impact that the canine virus had on the moose population based on the provided graph.

  • The canine virus likely reduced the predator (wolf) population, which initially allowed the moose population to increase due to reduced predation pressure. However, this may eventually lead the moose to overshoot their carrying capacity.

• Identify the stage of the Demographic Transition Model in which population grows the fastest and explain why.

  • Stage 2 (Industrializing) has the fastest growth because the death rate drops rapidly due to improved healthcare and sanitation while the birth rate remains high, creating a large "natural increase" gap.