Ecology: Tolerance Limits of Organisms

Tolerance and Limiting Factors

• The limits of tolerance for reproductive individuals, seeds, eggs, embryos, and larvae are usually narrower than for non-reproducing adult plants or animals.
• Adult blue crab and many other marine animals can tolerate brackish water, but their larvae cannot live in such waters, making it impossible for the species to reproduce in the river environment.
• Terms used to express the relative degree of tolerance:
  • Stenothermal: narrow temperature tolerance
  • Eurythermal: wide temperature tolerance
  • Stenohaline: narrow salinity tolerance
  • Euryhaline: wide salinity tolerance
  • Stenohydric: narrow water tolerance
  • Euryhydric: wide water tolerance
  • Stenophagic: narrow food tolerance
  • Euryphagic: wide food tolerance

Physical Factors as Limiting Factors

• Temperature is one of the essential environmental factors, and life can only exist within a tiny range of about 300 degrees centigrade (from -200°C to 100°C).
• Most species and most activity are restricted to an even narrower band of temperature.
• Temperature is often responsible for zonation and stratification in both land and water environments.
• Temperature, light, and water largely control the seasonal and daily activities of plants and animals.

Temperature Regulation in Animals

• Animals can be classified into three groups relative to temperature regulation:
  • Endotherms (generate heat metabolically): maintain a fairly constant body temperature independent of external temperatures (e.g., birds, mammals).
  • Ectotherms (acquire heat from the environment): have a variable body temperature (e.g., fish, amphibians, reptiles, insects, other invertebrates).
  • Heterotherms (regulate body temperature by endothermy at some times and ectothermy at other times): have a mix of both endothermic and ectothermic traits.

Dispersal Mechanisms

• Dispersal mechanisms allow organisms to "test" new environments for their suitability.
• Movement is usually guided by inherited behaviors.
• Plant dispersal mechanisms:
  • Seed dispersal: movement or transport of seeds away from the parent plant.
  • Four main modes of seed dispersal: gravity, wind, water, and by animals.
• Animal dispersal mechanisms:
  • Non-motile animals: produce dispersal units, such as specialized "buds" or motile sexual reproduction products.
  • Motile animals: majority of animals are motile, and their movements are guided by inherited behaviors.

Population Growth and Reproductive Strategies

• Logistic growth curve: population growth slows down as the carrying capacity is reached, eventually leveling off.
• Carrying capacity (K): maximum number of individuals that the environment can support.
• Logistic growth equation: ΔN/Δt = r max N (K-N/K)
• Reproductive strategies:
  • r-selection: species adapted to thrive when the population is growing rapidly.
  • K-selection: species adapted to thrive when the population is near its carrying capacity.
  • Characteristics of r-selected species:
    • Short life
    • Faster growth
    • Early maturity
    • Many, small offspring
    • Low parental care and protection
    • Adapted to unstable environments
    • Low trophic level
  • Characteristics of K-selected species:
    • Long life
    • Slower growth
    • Later maturity
    • Few, large offspring
    • High parental care and protection
    • Adapted to stable environments
    • High trophic level

Food Chains and Food Webs

• Food chains: linear sequences of organisms that eat other organisms, with each level representing a trophic level.
• Food webs: networks of interconnected food chains in an ecosystem.
• Types of food chains:
  • Grazing food chains: start from green plants and move to herbivores and then to carnivores.
  • Detritus food chains: start from dead organic matter and move to detrivores and then to carnivores.
• Significance of studying food chains and food webs:
  • Understand feeding relationships and interactions among organisms.
  • Explain the flow of energy and circulation of materials in ecosystems.
  • Help understand the concept of biomagnification in ecosystems.