Ecology Notes: Limiting Factors

Limiting Factors

  • Organisms are not uniformly distributed; their presence and abundance vary across different locations.

Abiotic and Biotic Forces

  • Abiotic Forces: Environmental conditions that impose limits on organisms.
    • Examples: Temperature, sunlight, soil pH, salinity, atmospheric composition, etc.
  • Biotic Forces: Interactions with other organisms that limit distribution and abundance.
    • Examples: Competition, predation, symbiosis, availability of nest sites, habitat selection, etc.

Role in Different Ecosystems

  • The significance of limiting factors varies among ecosystems.
    • Aquatic Ecosystems: Oxygen levels can be a major limiting factor.
    • Terrestrial Ecosystems: Oxygen is generally abundant and not limiting.
    • Water: Often a limiting factor in terrestrial ecosystems, while abundant in aquatic ones.

Liebig’s Law of the Minimum

  • Proposed by Justus von Liebig (1803-1873), a German chemist considered the "father of the fertilizer industry."
  • The limiting factor determines the potential crop yield; like a barrel with staves of unequal length, the shortest stave limits the water capacity.
  • The law states that plant growth will be limited if one essential nutrient is deficient, even if others are abundant.
Expansion of the Law
  • The concept was expanded beyond crop nutrients to include other environmental factors such as water, light, and temperature.
  • Modern view acknowledges that multiple nutrients or factors can be simultaneously limiting.
Definition of Limiting Condition/Factor
  • Any condition that approaches or exceeds an organism's limits of tolerance.
  • The presence and success of an organism depend on a complex set of conditions.

Shelford’s Law of Tolerance

  • Species can thrive only within a specific range of environmental conditions.
  • Optimum Range: Species are most abundant when environmental variables are within their optimal range.
  • Zone of Physiological Stress: Species become rare when environmental variables are too high or too low, causing stress.
  • Zone of Intolerance: Species are absent in areas where conditions exceed their upper and lower limits of tolerance.

Examples of Limiting Factors

  • Example 1: Thermal Constraints on Sparrow Activity
    • Illustrates the upper and lower limits of tolerance.
  • Example 2: Temperature as Limiting Factor for Marine Fish
    • Shows how population size varies with temperature, with the highest abundance in the optimum range and decreasing abundance in zones of physiological stress and intolerance.
    • Graphically represents the range of tolerance with zones of intolerance, physiological stress, and the optimum range.
  • Example 3: Limiting Factors Controlling Fish Population
    • Demonstrates how factors like decreasing oxygen, low food supply, disease, predators, and limited space can affect population size and stability.
    • Illustrates a population staying stable with regular rises and falls influenced by these limiting factors.

Types of Limiting Factors

  • Abiotic Factors
    • Temperature: Organisms have upper and lower lethal temperature limits and can acclimate to different conditions.
      • Example: Arctic char (Salvelinus alpinus) has an upper lethal temperature of 20∘C20^{\circ}C. Rising temperatures due to climate change can lead to extinction.
      • Example: Willow twigs (Salix spp.) can survive freezing temperatures in winter (below −150∘C-150^{\circ}C) but are killed by −5∘C-5^{\circ}C in summer.
    • Light intensity and requirements: Influence activity cycles, breeding seasons, and depth distribution.
      • Nocturnal desert animals use light as a cue.
      • Seaweeds' depth tolerance is limited by light attenuation.
      • Hemlock seedlings are shade-tolerant but susceptible to drought.
    • Water and moisture: Rainfall distribution and humidity regulate organism activities.
      • Water stress symptoms include decreased growth, small leaves, and off-colored leaves in drought conditions. Waterlogged soils lead to slowed root activity and wilting.
    • Atmospheric gases: Concentrations of carbon dioxide and oxygen can be limiting to higher plants.
    • Macronutrients & Micronutrients: Nitrogen and phosphorus levels are critical limiting factors.
    • Currents and pressures: Affect gas and nutrient concentrations and act as direct limiting factors in aquatic environments.
  • Biotic Factors
    • Competition: Influences how species respond to environmental factors.
    • Predation: Controls population sizes and distributions.
    • Presence or Absence of Food Supply: Directly affects survival and reproduction.

Tolerance Ranges

  • Stenothermic: Organisms with a narrow range of temperature tolerance.
  • Eurythermic: Organisms with a wide range of temperature tolerance.
  • Chionophile: Plants favoring snowy habitats (from the Greek 'chion' meaning snow).
  • Chionophobe: Plants that grow only in windblown, snow-free habitats.

Competitive Exclusion Principle

  • Gause's Experiment (1934) with Protozoans (Paramecium sp.)
    • Demonstrated that “one species, one niche.”
    • When grown separately, both species exhibited similar growth rates.
    • When grown together, P. aurelia outcompeted P. caudatum for bacterial food, leading to the elimination of P. caudatum.
  • Coexistence:
    • In another experiment, P. caudatum coexisted with P. bursaria because P. bursaria fed on yeast at the bottom of the tub, while P. caudatum fed on bacteria in suspension.

Competition for Nest Sites

  • Competition for nest sites can impact breeding success and population dynamics.
  • Example: Starlings competing with great spotted woodpeckers for nest sites in southern England.
    • Nest site interference by starlings was observed to decrease nest survival in woodpeckers. With the decline in starling numbers, woodpecker populations increased due to reduced competition.

Introduced (Exotic) Species

  • Organisms introduced into areas where they were previously absent.
  • Can lead to habitat destruction and extinction of local species.
  • Examples:
    • Kudzu
    • Fire ants
    • Killer bees
    • Zebra mussels
    • Asiatic clams
    • Japanese honeysuckle
    • Water hyacinth
    • Hydrilla
    • Egeria
    • Japanese beetles
    • Honeybee mites
    • Asian Tiger mosquito
  • Examples in Malaysia: To be added.

Examples of Invasive Species

  • Zebra Mussel: Native to southern Russia, has spread invasively.
  • Asian Gypsy Moth (Lymantria dispar): Destructive pest of trees in the Northern Hemisphere, causing defoliation and mortality of host trees.
  • Crazy Ant (Paratrechina longicornis): Native to Africa, an extremely hardy species capable of invading varied habitats. Displaces other ants, transports pathogenic microbes, and is difficult to control with current chemical treatments.