Ecology: Interactions, Energy Flow, and Human Impact

Ecology: The Study of Interactions

Definition and Scope

Ecology is the scientific study of interactions among organisms and between organisms and their environment. The term was coined by the German biologist Ernst Haeckel, who viewed nature as a house with its own economy. The largest of nature’s “houses” is the biosphere, which refers to the biological component of Earth’s systems—the portion of the planet that can sustain life and all of its interactions.

The prefix "ECO" is derived from the Greek term "oikos," meaning house.

Levels of Organization

Ecology examines life from the level of individual organisms to the entire biosphere:

• Individual: A single organism.

• Population: A group of individuals of the same species living in a defined area.

• Community: Groups of populations of different species living together in a defined area.

• Ecosystem: The community plus the physical (nonliving) environment.

• Biome: Large areas with particular climates and characteristic species.

• Biosphere: The sum of all ecosystems on Earth.

Interdependence in the Biosphere

Organismal Interdependence

Organisms and their environment are interdependent. Every organism relies on others to survive. For example, a snail depends on plants and algae for food, while those plants rely on bacteria to recycle nutrients.

This interdependence forms a complex web, where everything is dependent on something else.

Levels of Ecological Organization

• Species: A group of similar individuals that can breed and produce fertile offspring.

• Population: A group of organisms of one species that interbreed and live together in a defined area (e.g., a herd of sheep, a flock of geese).

• Community: Multiple populations of different species living together.

• Ecosystem: All living and nonliving factors in a particular place (e.g., a pond, a forest, a marsh).

• Biome: Large regions with specific climates and characteristic species (e.g., tundra, rainforest, desert).

Biodiversity

Biodiversity is the variety of organisms, their genetic differences, and the ecosystems in which they occur. It includes:

• Ecological diversity: Different ecosystems in a region.

• Species diversity: Different species within certain ecosystems.

Tropical rainforests have the highest biodiversity of all biomes.

Abiotic and Biotic Factors

Abiotic Factors

Abiotic factors are nonliving components that affect ecosystems, such as soil, pollution, natural disasters, and climate.

Biotic Factors

Biotic factors are living components, such as plants and animals, that affect ecosystems.

Combined Effects

Both biotic and abiotic factors determine the survival and growth of organisms and the structure of ecosystems. For example, the survival of bullfrogs depends on food availability (biotic) and water temperature (abiotic).

Habitats and Succession

Habitats

A habitat is the place where a particular population lives, including both abiotic and biotic factors (e.g., anthills, nests, marshes).

Succession

Succession is the regular progression of species replacement over time in an ecosystem. It can be:

• Primary succession: Occurs where no soil previously existed (e.g., after volcanic eruptions or receding glaciers). Pioneer species like lichens break down rock to form soil.

• Secondary succession: Occurs in areas where soil exists but has been disturbed (e.g., after forest fires).

Succession is typically slow, but natural disasters or human disturbances can accelerate changes.

Energy Flow in Ecosystems

Primary Productivity

All life’s energy originates from the sun. Primary productivity is the rate at which producers (autotrophs) convert sunlight into energy. Factors affecting productivity include water, sunlight, carbon dioxide, and nutrients like nitrogen and potassium.

Producers and Consumers

• Producers (autotrophs): Organisms that produce energy by capturing sunlight or using chemicals (e.g., plants, algae, some bacteria).

• Consumers (heterotrophs): Organisms that consume other organisms for food. Types include:

  • Herbivores: Eat plants.

  • Carnivores: Eat animals.

  • Omnivores: Eat both plants and animals.

  • Detritivores: Consume dead matter or waste (e.g., vultures, earthworms).

  • Decomposers: Cause decay and recycle nutrients (e.g., bacteria, fungi).

Trophic Levels and Food Chains

Trophic levels are assigned based on how organisms obtain energy. The first trophic level is always a producer. Food chains show the path of energy through trophic levels, while food webs illustrate complex interactions among multiple food chains.

Trophic Level	Type of Organism	Example
First	Producer	Sunflower seeds
Second	Primary Consumer	Mouse
Third	Secondary Consumer	Snake
Fourth	Tertiary Consumer	Hawk
Fifth	Quaternary Consumer	Fox

Energy Transfer and the Ten-Percent Law

Energy flows in one direction: from the sun to autotrophs, then to heterotrophs. Only about 10% of energy is transferred from one trophic level to the next; the rest is lost as heat or used for life processes.

For example, if seeds contain 10,000 kcal, the mouse that eats them gets 1,000 kcal, and the snake that eats the mouse gets 100 kcal.

Energy pyramid: A diagram showing the amount of energy at each trophic level, illustrating the ten-percent law.

Biomass: The dry weight of living tissue in a trophic level. Most biomass is found in producers.

Biogeochemical Cycles

Overview

Matter is recycled in ecosystems through biogeochemical cycles, including the water, carbon, nitrogen, and phosphorus cycles.

The Water Cycle

• Evaporation: Liquid water becomes vapor.

• Transpiration: Water evaporates from plants.

• Condensation: Water vapor forms clouds.

• Precipitation: Water returns to Earth as rain, snow, etc.

• Percolation: Water seeps into soil to become groundwater.

• Runoff: Surface water returns to oceans or lakes.

The Carbon Cycle

• Carbon is found in living tissues, rocks, the atmosphere, and oceans.

• Photosynthesizing organisms use CO2 to build organic molecules.

• Carbon returns to the atmosphere via respiration, erosion, and combustion of fossil fuels.

The Nitrogen Cycle

• Nitrogen fixation: Bacteria convert atmospheric nitrogen to ammonia.

• Ammonification: Bacteria produce ammonia from decaying matter.

• Nitrification: Bacteria convert ammonia to nitrates/nitrites.

• Assimilation: Plants absorb nitrogen compounds.

• Denitrification: Bacteria convert nitrates back to nitrogen gas.

The Phosphorus Cycle

• Phosphorus is found in rocks and minerals, not the atmosphere.

• Weathering releases phosphorus into soil and water.

• Decomposers recycle phosphorus from dead organisms.

Nutrient Limitation

The availability of nutrients affects primary productivity. A limiting nutrient is one that is scarce or cycles slowly, restricting ecosystem productivity. Fertilizers supply key nutrients: nitrogen, phosphorus, and potassium.

Population Ecology

Population Size, Density, and Dispersion

• Population size: Number of individuals in a population.

• Population density: Number of individuals per unit area.

• Dispersion: How individuals are arranged in space (random, uniform, clumped).

Population Growth

• Growth rate (r) = birthrate – death rate

• Exponential growth: Population grows rapidly under ideal conditions (J-shaped curve).

• Carrying capacity (K): Maximum population size an environment can sustain.

• Logistic growth: Population growth slows as it approaches carrying capacity (S-shaped curve).

Factors affecting population size include births, deaths, immigration, and emigration.

Limiting Factors

• Density-dependent factors: Depend on population size (e.g., food, water, shelter).

• Density-independent factors: Affect populations regardless of size (e.g., weather, climate).

r- and K-Strategists

• r-strategists: Grow rapidly when conditions are right; population size fluctuates greatly (J-shaped curve).

• K-strategists: Grow slowly, population size remains near carrying capacity (S-shaped curve).

Community Interactions

Competition and Niche

Species compete for resources such as food, water, shelter, and mates. An organism’s niche is its role in the ecosystem, including its habitat, diet, and behavior. The fundamental niche is the full range of conditions an organism can tolerate, while the realized niche is the range it actually occupies due to competition.

Symbiosis

Symbiosis is a close, long-term relationship between species. Types include:

• Predation: One organism kills another for food.

• Parasitism: One benefits, one is harmed (e.g., ticks).

• Mutualism: Both benefit (e.g., bees and flowers).

• Commensalism: One benefits, the other is unaffected (e.g., cactus wren and cactus).

Coevolution

Coevolution is the back-and-forth evolutionary adjustment between species, often seen in symbiotic relationships. Introduction of new species (invasive species) can disrupt this balance.

Human Impact on Ecosystems

Major Human Activities Affecting Ecosystems

• Deforestation

• Pollution and fossil fuel use

• Building dams

• Introducing invasive species

• Overuse of fertilizers

• Overhunting or overfishing

Deforestation leads to resource loss, displacement of animals, and increased competition. It also contributes to climate change by reducing the rate at which CO2 is removed from the atmosphere. Fossil fuel combustion releases greenhouse gases, contributing to global warming. Dams alter water flow, affecting aquatic species and nutrient cycles. Invasive species can outcompete native species, disrupting community balance. Overexploitation, such as overfishing, reduces biodiversity and can threaten species with extinction.

Summary Table: Key Ecological Concepts

Concept	Definition	Example
Biosphere	All regions of Earth that support life	Earth’s surface, atmosphere, and oceans
Biome	Large area with specific climate and species	Tropical rainforest, tundra
Ecosystem	Community plus nonliving environment	Pond, forest
Community	All populations in an area	Forest animals and plants
Population	Group of same species in an area	Herd of deer
Species	Organisms that can breed and produce fertile offspring	Homo sapiens