9.1 - Community and Ecosystem

Habitat, Species, Population, Community, Ecosystem and Niche

Definitions of Key Ecological Terms

Ecology: The study of interactions among organisms and interactions between organisms and their surroundings.
Habitat: The natural surroundings or living place of an organism.
Species: A group of similar organisms that can interbreed and produce offspring.
Population: A group of organisms of the same species living in the same habitat.
Community: All populations of organisms from different species living in the same habitat and interacting with one another.
Ecosystem: A few communities living together in a habitat and interacting with one another and with non-living (abiotic) components such as water, air and soil.
Niche: The role of an organism in an ecosystem, including its behaviour and interactions with biotic and abiotic components in its habitat.

Niche Classifications

Ecological niche: The role of a species in its surroundings.
Species niche: The way a species interacts with biotic and abiotic components in its surroundings.

Biotic and Abiotic Components in an Ecosystem

Biotic components: Organisms in an ecosystem that interact with other organisms; classified as producers, consumers and decomposers.
Abiotic components: Non-living physical and chemical elements that affect organisms in an ecosystem.

Abiotic Components

pH value: Strongly affects organism distribution. Most organisms live in neutral or nearly neutral conditions. Small pH changes affect soil organisms and can reduce soil fertility.
Temperature: Affects physiological activities. Small changes reduce metabolic rate because enzymes are temperature-sensitive. Most organisms live from $2 0^{\circ} C$ to $4 0^{\circ} C$ ; some survive extreme temperatures.
Light intensity: Light intensity and sunlight duration strongly affect distribution, especially of photosynthetic plants. Tropical rainforest canopies create low light beneath them, where only small plants such as ferns grow. Low-light temperate coniferous forests have lower plant density and smaller, shorter trees.
Topography: Physical features that determine humidity, temperature and light intensity.
- Altitude: Relative humidity, atmospheric pressure and oxygen content decrease as altitude increases. Plant type, size and density vary; high-altitude pine trees are smaller than meranti trees in tropical rainforests.
- Gradient: Steep slopes are easily eroded. Soil becomes thin and dry, supporting few trees except short, thorny mountain plants with small, pointed leaves.
- Aspect: Direction of wind and sunlight. Sea-facing slopes receive more rain and have denser vegetation; slopes receiving more sunlight also have denser vegetation.
Microclimate: Climate of a small area that differs from its surroundings, such as beneath a rock or forest canopy. It depends on temperature, humidity, light intensity, heat balance, atmospheric pressure, evaporation and soil water retention.
Air humidity: The quantity of water vapour affects distribution; more organisms inhabit humid areas. Low humidity increases transpiration, water and mineral absorption. Transpiration cools plants and maintains optimum temperature for enzyme action.

Biotic Components

Producers: Autotrophs, mainly green plants, that synthesise organic substances from inorganic substances. Green plants synthesise glucose from water and carbon dioxide using sunlight.
Primary consumers: Herbivores that eat producers.
Secondary consumers: Carnivores that eat primary consumers, or omnivores that eat primary consumers and producers.
Tertiary consumers: Carnivores that eat secondary consumers.
Decomposers: Microorganisms that decompose waste and dead or decaying organisms into simpler substances such as carbon dioxide and ammonia.

Nutrition in Organisms

Nutrition: The way organisms obtain nutrients and energy from food for life processes.

Autotrophic Nutrition

Photoautotrophs: Synthesise organic compounds from carbon dioxide using light energy through photosynthesis; for example, green plants.
Chemoautotrophs: Certain bacteria that synthesise organic compounds without light. They obtain energy by oxidising inorganic substances such as hydrogen sulphide and ammonia through chemosynthesis; for example, Nitrobacter sp.

Heterotrophic Nutrition

Saprotrophs: Saprophytes that obtain nutrients from dead and decaying organic matter. Digestion occurs outside the organism before absorption; for example, fungi.
Holozoic organisms: Ingest solid organic matter, digest it and absorb it into the body; humans and almost all animals are holozoic.
Parasites: Absorb nutrients from a host; for example, ticks and tapeworms absorb nutrients from human hosts.

Biotic Components According to Trophic Levels

Trophic level: The position of an organism linked by energy flow in a food chain.
First: producer; second: primary consumer; third: secondary consumer; fourth: tertiary consumer.
The Sun is the main energy source for all organisms.

Energy Flow in a Food Chain and Food Web

Food chain: A sequence of energy transfer between trophic levels, beginning with a producer and ending with a secondary or tertiary consumer.
- An organism eats one from the previous trophic level.
- Energy from tissues passes after digestion and assimilation to form new tissues.
Food web: Interconnected food chains describing feeding relationships in a community.
- Producers convert sunlight into chemical energy stored as food in roots, fruits, stems or leaves.
- Organisms in all food chains are interdependent for food.

Ecological Pyramids

Represent changes in organism number, biomass and energy at successive trophic levels.

Pyramid of Numbers

Shows organism number at each trophic level.
The producer base normally has the greatest number; number decreases and organism size increases towards the top.

Pyramid of Biomass

Shows total biomass per unit area at each trophic level.
Biomass is measured using dry mass and represents biomass available to the next trophic level.
Total biomass per unit area decreases towards the top.

Pyramid of Energy

Shows total energy available in an ecosystem.
Green plants convert sunlight to chemical energy; feeding transfers energy to the next trophic level.
Energy may be stored in tissues, transferred in urine, eliminated in faeces or released as heat during respiration and other reactions.
Only $10%$ passes to the next level; $90%$ is lost through heat, life processes, excretion and defecation. Lower trophic levels therefore have more available energy.

Types of Interaction among Biotic Components

Main interactions: saprophytism, symbiosis, predation and competition.
Saprophytism: Organisms obtain food from dead organic matter; for example, a mushroom on a dead tree trunk.

Symbiosis

Different species live together and interact.
Mutualism: Benefits both; a myna eats ticks from a buffalo, while the buffalo is freed from ticks.
Commensalism: Benefits one without harming the other; remora fish eat shark food scraps while the shark is unaffected.
Parasitism: Benefits one and harms the other; a tapeworm absorbs nutrients in a human intestine and causes host nutrient deficiency.

Predation and Competition

Predation: A predator eats prey; for example, an owl catches and eats a rat.
Competition: Organisms compete for food, water, light and mates.
- Intraspecific: Between the same species; for example, animals competing for mates.
- Interspecific: Between different species; for example, plants competing for sunlight.

Mangrove Ecosystem

Mangrove trees are tropical plants commonly found in estuaries where rivers meet the sea.

Abiotic Components of Mangrove Swamps

Waves and tides; soft, silty, muddy, poorly aerated soil; seed germination in tidal areas.
High soil salt content; very low dissolved oxygen in water; high sunlight intensity; strong winds.

Biotic Components of Mangrove Swamps

Mangrove trees are dominant producers.
Adapted fauna include egrets, silvered leaf monkeys, proboscis monkeys, snakes, crabs, mudskippers, horseshoe crabs and fireflies.
Bacteria and fungi decompose dead organisms into nutrients for plants.
The food web maintains dynamic balance.

Adaptations of Mangrove Trees

Leaves: Thick cuticles and sunken stomata reduce transpiration. Succulent leaves store water; hydathodes eliminate excess salt. Old leaves store salt and fall when too much accumulates.
Pneumatophores: Short root projections provide aeration in flooded areas; lenticels enable gas exchange with submerged roots. Example: Avicennia sp.
Prop roots: Branch from the lower trunk and grip soil against strong wind and waves. Example: Rhizophora sp.
Buttress roots: Thick plates increase base area and support trees on soft soil bordering land. Example: Bruguiera sp.
Viviparous seedlings: Germinate on the parent; fallen seedlings stick into mud and resist waves.

Colonisation and Succession

Pioneer species: First species in an area where no other living things grow.
Colonisation: Plants conquer an uninhabited area, reproduce and form colonies.
Succession: Dominant plants are gradually replaced by successors.
Coastal zone: Avicennia sp. and Sonneratia sp. trap tidal mud and organic matter. Accumulation raises and compacts soil; Rhizophora sp. succeeds them.
Middle zone: Rhizophora sp. prop roots trap twigs and mud, accelerate sedimentation and make the bank higher and drier. Bruguiera sp. succeeds Rhizophora sp.
Inland zone: Bruguiera sp. buttress roots trap mud and silt. Sedimentation creates new swamp; old shore becomes land for Nypa fruticans and Pandanus sp., which succeed Bruguiera sp.

Importance of Mangrove Ecosystems

Protection: Reduces coastal wave and wind speed; shelters small fish, shrimps and crabs from predators, currents and waves; shelters feeding migratory birds.
Fishery resources: Fish, shrimps, crabs and snails provide income; waters support floating-cage aquaculture and commercial breeding.
Forestry resources: Wood makes boats, fish traps, building frames and handicrafts; furnaces convert it to charcoal.
Food and medicine: Avicennia sp. fruit is eaten as a vegetable, nuts are boiled and flowers produce honey; Sonneratia sp. fruit makes drinks; Nypa sp. fruit is eaten and its liquid makes vinegar and nira; Bruguiera sp. bark treats diarrhoea.