Ecosystems and Ecological Principles and Ecology

Basic Concepts and Levels of Organization in Ecology

• Ecology is defined as the scientific study of the interrelationships between organisms and their environment, and the interrelationships among different organisms.
• Levels of organization in ecological studies progress hierarchically from simple units to the global scale:
  • Individual: A single organism of a specific species.
  • Population: A group of individuals of the same species living in the same habitat at the same time.
  • Community: All the populations of different species living and interacting within the same habitat.
  • Ecosystem: The basic unit of ecological study, consisting of communities of organisms interacting with each other and with their physical (abiotic) environment. It is characterized by the flow of energy and the cycling of materials.
  • Biome: A collection of similar ecosystems covering a large geographic area on Earth.
  • Biosphere: The entire surface and atmosphere of the Earth that contains living organisms.
• A stable, self-supporting, and dynamic ecosystem is maintained through the interdependence of organisms, the continuous flow of energy, and the efficient cycling of materials.

Major Ecosystem Types in Hong Kong

• Freshwater Stream: Natural flowing water bodies supporting aquatic life adapted to currents.
• Rocky Shore: Coastal areas dominated by rock substrates, subject to tidal actions.
• Mangrove: Coastal wetlands characterized by salt-tolerant trees in intertidal zones.
• Grassland: Terrestrial areas dominated by grasses and non-woody plants.
• Woodland: Areas dominated by trees, such as the evergreen broad-leaved forests common in Hong Kong.

Components of an Ecosystem

• Abiotic Factors: The non-living components that influence the survival and distribution of organisms.
• Biotic Community: The living component consisting of various species interacting within the habitat.

Detailed Analysis of Abiotic Factors

• Temperature:
  • Affects the distribution and activity of organisms because cellular reactions are catalyzed by enzymes which function within a narrow temperature range.
  • Endotherms (Homeotherms): Mammals and birds that maintain a constant body temperature through physiological means, allowing them to remain active in a wider range of environmental temperatures.
  • Ectotherms (Poikilotherms): Reptiles, amphibians, and fish that cannot maintain a constant body temperature and are restricted to habitats with narrower temperature ranges.
  • Plant Distribution: Temperature influences plant growth; for example, apple trees cannot grow in tropical regions.
  • Median Lethal Temperature ($LT_{50}$): The temperature at which $50\%$ of individuals die when exposed for $24$ hours.
• Light:
  • Light intensity and duration (day-length) affect the rate of photosynthesis and the timing of flowering in plants.
  • Animals active at night (nocturnal) develop adaptations such as enhanced vision (e.g., owls) or echolocation (e.g., bats) to navigate and hunt.
• Rainfall and Humidity:
  • Both factors determine water availability in a habitat.
  • Humidity (the amount of water vapor in the air) affects the transpiration rate in plants and the evaporation rate of sweat in animals.
• Wind Speed:
  • Important for the pollination of flowers and dispersal of seeds.
  • High wind speeds increase the transpiration rate of plants and can physically blow organisms away from their habitats.
• Water Current:
  • Strong currents and wave action can wash organisms away. Adaptations include:
    • Zebra loach (Beaufortia kweichowensis): Features a flattened body and fins modified into suckers for attachment.
    • Chitons: Utilize powerful muscular feet to attach to rocks.
    • Rock Oysters: Sessile organisms that cement themselves to rocky surfaces.
• Oxygen Concentration:
  • Critical for aquatic and soil-dwelling organisms.
  • Factors affecting oxygen solubility in water:
    • Temperature: As temperature increases, oxygen concentration decreases (solubility decreases).
    • Water Current: Faster flow and splashes increase the contact between air and water, increasing oxygen levels.
    • Organic Matter: Decomposition of organic matter by microorganisms consumes oxygen, lowering the concentration.
  • Adaptations to low oxygen:
    • Water Scorpions: Possess long breathing tubes to reach the air surface.
    • Mangroves: Develop pneumatophores (breathing roots) that grow out of waterlogged soil.
• Salinity:
  • The concentration of dissolved salts in water, affecting the osmotic balance and availability of minerals.
  • Estuarine organisms must tolerate wide fluctuations in salinity. Some mangroves have salt glands in their leaves to excrete excess salt.
• Soil Factors:
  • Provides anchorage, water, and minerals for plants.
  • Particle Size: Silt and clay have small, tightly packed particles with tiny air spaces that are easily flooded (waterlogged), leading to low oxygen levels. Sand has larger particles with better aeration.
  • Organic Matter and Humus: Microorganisms decompose dead plants, animals, and droppings to form humus, a nutrient-rich dark brown substance.

Biotic Community Dynamics

• Habitat: The specific physical place where an organism lives.
• Ecological Niche: The functional role of an organism in its community, defined by the physical space it occupies, the food it eats, and its timing of activity (e.g., a Great White Shark acting as a tertiary consumer/predator).
• Species Diversity: The variety of organisms in a community, determined by:
  • Species Richness: The total number of different species.
  • Species Evenness: The relative abundance and distribution of individuals among the species.
• Dominant Species: Species that exert strong control over the composition and diversity of a community, often the most abundant or influential type (e.g., evergreen broad-leaved trees in Hong Kong woodlands).

Relationships Between Organisms

• Predation: An interaction where a predator hunts and kills a prey species for food ($+/-$ relationship).
  • Population cycles: An increase in prey leads to an increase in predators; subsequently, predators over-consume prey, causing a decline in the prey population, which eventually leads to a predator decline due to limited food.
• Competition: Fighting for shared resources like food, shelter, mates, or sunlight ($-/-$ relationship).
  • Intraspecific Competition: Competition between members of the same species (typically more intense due to identical requirements).
  • Interspecific Competition: Competition between different species.
  • Competitive Exclusion: Two species with identical niches cannot coexist; one will eventually outcompete the other.
• Symbiosis: Close and long-term biological interactions including:
  • Commensalism ($+/0$): One organism benefits while the other is unaffected. Example: Barnacles on crab shells.
  • Mutualism ($+/+$): Both organisms benefit. Examples:
    • Sea anemones and hermit crabs (protection and transport/food leftovers).
    • Nitrogen-fixing bacteria (Rhizobium) and leguminous plants (nitrates provided to plants/carbohydrates and shelter for bacteria).
    • Lichens: Mutualism between algae (photosynthesis) and fungi (protection and water absorption).
  • Parasitism ($+/-$): A parasite gains nutrients at the expense of a host. Example: Tapeworms in mammal intestines; Dodder coiling around Mikania.

Ecological Succession

• Definition: The gradual change in community composition over time in response to interactions between organisms and the physical environment.
• Primary Succession:
  • Occurs in barren areas where no soil or organisms previously existed (e.g., bare rock, volcanic islands).
  • Process: Pioneer species (lichens/mosses) stabilize the environment -> biological weathering forms soil -> grasses and ferns replace pioneers -> shrubs grow -> trees establish -> Climax Community reached.
  • Pioneer species: The first organisms to colonize a barren area.
  • Climax Community: A stable, steady-state ecosystem with maximum supported biomass, usually dominated by specific tree types in forests.
• Secondary Succession:
  • Occurs in areas where a previous community existed but was disturbed (e.g., by fire or deforestation).
  • Reaches climax faster than primary succession because soil, seeds, and underground vegetative organs are already present.

Energy Flow in Ecosystems

• Source: The sun is the ultimate source of energy.
• Producers: Capture light energy through photosynthesis and convert it into chemical energy in the form of organic matter.
• Consumers:
  • Primary Consumers: Herbivores that feed on producers.
  • Secondary/Tertiary Consumers: Carnivores that feed on other animals.
• Decomposers: Bacteria and fungi that break down dead bodies and waste into inorganic minerals. They are essential for material cycling but are not typically listed as a trophic level within a food chain.
• Food Chain: A linear sequence of feeding relationships.
• Food Web: Complex, interconnected food chains within an ecosystem.
• Trophic Level: The position an organism occupies in a food chain. Energy flows from lower to higher levels.
• Energy Loss: Energy is lost at each level ($90\%$ loss is common) through:
  • Respiration (heat loss).
  • Uneaten material.
  • Undigested and egested material (faeces).
  • Excretion.
• Ecological Pyramids:
  • Pyramid of Numbers: Represents the number of individuals at each level. Can be inverted if the producer is very large (e.g., many insects on one tree).
  • Pyramid of Biomass: Represents the total dry mass at each level. Usually more accurate than numbers, but can be temporary inverted in aquatic systems if producers reproduce very quickly.
  • Pyramid of Energy: Always upright; represents the rate of energy flow through each level.

Cycling of Materials

• The Carbon Cycle:
  • Removal: Photosynthesis by producers.
  • Release: Respiration of all organisms, decomposition of organic matter, and combustion of fossil fuels.
  • Transfer: Carbon moves through the food chain via feeding.
• The Nitrogen Cycle:
  • Nitrogen Fixation: Conversion of atmospheric $N_2$ into ammonium or nitrates by nitrogen-fixing bacteria or lightning.
  • Ammonification: Decomposers (putrefying bacteria) convert organic nitrogen from waste and dead bodies into ammonium compounds.
  • Nitrification: Nitrifying bacteria convert ammonium into nitrites ($NO_2^-$) and then into nitrates ($NO_3^-$).
  • Denitrification: Denitrifiying bacteria convert nitrates back into atmospheric $N_2$ (typically in anaerobic/waterlogged conditions).
  • Uptake: Plants absorb nitrates to build proteins.

Conservation and Sampling Methods

• Conservation Principles: Reducing waste, reusing resources, recycling non-renewable resources, and replacing them with renewable ones.
• Ecological Study Strategies:
  • Random Sampling: Used in uniform habitats to avoid bias.
  • Systematic Sampling: Used transitionally where there is an environmental gradient.
• Sampling Tools:
  • Quadrat: A square frame used to estimate population abundance or percentage cover in fairly uniform habitats.
  • Line Transect: Recording the presence or absence of species along a line.
  • Belt Transect: Recording abundance and distribution within a specific width along a line to correlate with environmental factors (e.g., distance from the shore).

Questions & Discussion

• Question: Why do some plants grow poorly in water-logged soil?
• Answer: Water-logged soil has very low oxygen content because air spaces between soil particles are filled with water. This inhibits the aerobic respiration of roots, leading to poor growth.
• Question: Why are quadrats unsuitable for studying the abundance of crabs on a rocky shore?
• Answer: Crabs are mobile animals. Quadrats are intended for sessile or slow-moving organisms. Mobile animals may move in or out of the quadrat during sampling or hide in crevices, leading to inaccurate counts.
• Question: Describe the journey of a carbon atom from a dinosaur to a human's breath.
• Answer: The dinosaur's body could be decomposed into $CO_2$ or turned into fossil fuel. Carbon is released to the atmosphere via decomposition or combustion. It is then captured by plants through photosynthesis to form organic matter. Humans obtain the carbon by eating the plants or animals in the food chain. Inside the human body, the organic matter is digested, absorbed, and transported to cells where it undergoes respiration to form $CO_2$, which is then breathed out.