Comprehensive Study Guide for IB Biology Ecology

Fundamental Concepts in Ecology: Species, Communities, and Ecosystems

Species Definition: A species is defined as a group of organisms that have the capacity to interbreed and produce fertile offspring.
Genetic and Functional Distinction: Every species is genetically distinct from others. Furthermore, each species occupies a specific ecological niche, which encompasses its functional role in the environment, its methods for obtaining energy and nutrients, and its interactions with other biological entities.
Communities: A community consists of all the different species residing together in a specific geographic area at the same time.
Community Interactions: Species within a community engage in various interactions, including competition, predation, and mutualism.
Dynamic Nature of Communities: Communities are not static; they change over time due to factors such as environmental shifts, species migration, and the process of natural selection.
Ecosystems: An ecosystem represents the integration of biotic (living) components, such as species and communities, and abiotic (non-living) components, including climate, soil, water, and nutrients.
Scalability and Characteristics of Ecosystems: Ecosystems can range in size from a small pond to a vast forest. They are fundamentally characterized by the one-way flow of energy and the cyclic movement of nutrients (specifically carbon and nitrogen) between organisms and the environment.

Detailed Modes of Species Interactions

Competition: This occurs when two or more species compete for the same limited resources, such as food, light, or physical space.
Competitive Exclusion: A potential outcome of competition where one species outcompetes another, potentially causing the local extinction of the less successful species.
Predation: An interaction where a predator hunts and consumes its prey. This relationship is a key regulator of prey population sizes and can stimulate evolutionary adaptations, such as the development of camouflage or increased speed.
Mutualism: A cooperative interaction resulting in benefits for both species involved.
Example of Mutualism: Bees pollinate flowers while feeding on their nectar, which facilitates plant reproduction while providing food for the bee.
Commensalism: An interaction where one species benefits while the other is neither helped nor harmed.
Example of Commensalism: Barnacles attaching themselves to whales. The barnacles gain transport to nutrient-rich waters (‘a free ride’), while the whale remains unaffected.
Parasitism: An interaction where the parasite benefits at the metabolic expense of the host. Though it causes harm, it typically does not lead to immediate death.
Example of Parasitism: Ticks feeding on the blood of mammals.

Factors Influencing Community Structure and Succession

Environmental Factors: Factors such as climate, the availability of resources, and physical topographical features dictate which species are capable of surviving within a specific ecosystem.
Biotic Factors: The presence of other organisms and the nature of their interactions (e.g., predator-prey dynamics or resource competition) serve to shape the community structure.
Disturbances: Major events such as fires, floods, or human-induced activities can drastically alter a community by removing dominant species, thereby allowing new species to colonize the space.
Ecological Succession: This describes the process where ecosystems undergo gradual changes in species composition over time, eventually leading to a more stable community state.

Dynamics of Energy Flow in Ecosystems

Primary Energy Source: The sun is the primary source of energy for the vast majority of ecosystems.
Photosynthetic Capture: Producers—including plants, algae, and specific bacteria—capture solar energy through photosynthesis to convert it into chemical energy stored in glucose ( $C_6H_{12}O_6$ ).
Trophic Levels: These levels represent the sequential stages of energy flow: * Producers (Trophic Level 1): Photosynthetic organisms that establish the chemical energy base. * Primary Consumers (Trophic Level 2): Herbivores that feed directly on producers. * Secondary Consumers (Trophic Level 3): Carnivores that consume herbivores. * Tertiary Consumers (Trophic Level 4): Top predators that consume other carnivores.
Energy Transfer Efficiency: The transfer of energy between levels is highly inefficient. Only approximately 10% of the energy from one trophic level is passed to the next.
Energy Loss: About 90% of energy is lost at each level, primarily as heat resulting from metabolic processes like respiration.
Limitation on Food Chains: This 90% loss limits the possible number of trophic levels in an ecosystem because energy availability diminishes rapidly at higher levels.

Modeling Energy: Food Chains, Webs, and Pyramids

Food Chain: A linear representation showing the sequence of energy flow.
Example of Food Chain: $\text{grass} \rightarrow \text{rabbit} \rightarrow \text{fox}$ .
Food Webs: Complex networks of multiple interconnected food chains. They illustrate species interdependence and the multidirectional flow of energy within a community.
Decomposers: Organisms like bacteria and fungi that break down dead matter and waste. They return nutrients to the environment and release energy, making nutrients available for producers again.
Energy Pyramids: These are visual models representing the energy available at each level.
Structure of the Pyramid: The base is broad, representing the high energy at the producer level, and it narrows toward the top as energy levels decrease.

The Biogeochemical Carbon Cycle

Cycle Overview: Describes how carbon atoms move through the atmosphere, hydrosphere, lithosphere, and biosphere.
Atmospheric Carbon: Carbon dioxide ( $CO_2$ ) is the primary atmospheric form. It enters the biosphere via photosynthesis.
Photosynthesis and Carbon Fixation: Producers use sunlight to convert $CO_2$ and water ( $H_2O$ ) into glucose and oxygen ( $O_2$ ). This fixes atmospheric carbon into organic molecules.
Respiration: Both producers and consumers break down glucose for energy, releasing $CO_2$ back into the atmosphere as a byproduct. This occurs in plants (continuously, including at night), animals, fungi, and microorganisms.
Feeding and Transfer: Carbon is transferred through the food web as animals consume plants or other animals, incorporating the carbon into their own tissues.
Decomposition and Carbon Release: When organisms die, decomposers release carbon back into the atmosphere as $CO_2$ through respiration. Some carbon may be stored in soil as organic matter.
Long-Term Carbon Sequestration: * Fossil Fuels: Ancient dead organic matter may be converted into coal, oil, or natural gas over millions of years under high heat and pressure. * Oceans: The oceans absorb large quantities of $CO_2$ . Marine organisms utilize carbon for shells and skeletons, which can eventually form sedimentary rock such as limestone ( $CaCO_3$ ). * Forests: Trees and biomass act as significant carbon sinks, storing carbon for long durations.
Human Impact: Activities like burning fossil fuels, deforestation, and industrialization have increased atmospheric $CO_2$ , exacerbating the greenhouse effect and leading to global climate change.

Population Ecology and Growth Dynamics

Population Size: The total count of individuals in a defined area.
Population Density: The number of individuals per unit of area or volume.
Fluctuation Factors: Size and density vary based on resource availability, predation, disease, and environmental conditions.
Exponential Growth: Rapid growth where the population size doubles at a consistent rate, occurring when resources are abundant and competition is minimal. It is represented by a J-shaped curve.
Logistic Growth: Growth that slows as resources become limited and stabilizes at the carrying capacity. It is represented by an S-shaped curve.
Carrying Capacity ( $K$ ): The maximum number of individuals of a species that an environment can support indefinitely without environmental degradation. Growth levels off at this point due to restricted food, space, and resources.

Regulation of Population Size

Density-Dependent Factors: Factors whose impact intensifies as population density increases. These include resource competition, predation, disease, and the accumulation of waste.
Density-Independent Factors: Factors that affect population size regardless of its density. These include natural disasters (floods, hurricanes, fires), climate changes, and human activities like pollution or deforestation.
Demographic Rates: Population growth is determined by the balance between birth rates and death rates. * Growth occurs if \text{Birth Rate} > \text{Death Rate}. * Decline occurs if \text{Death Rate} > \text{Birth Rate}.
Migration: Population size is also influenced by immigration (movement into a population) and emigration (movement out of a population).
Age Structure: A population with many young individuals tends to grow faster than a population composed of older individuals.
Feedback Mechanisms: * Negative Feedback: Regulates size as a population nears $K$ by increasing mortality or reducing birth rates (e.g., via resource limits). * Positive Feedback: Where increased density accelerates growth (e.g., easier mate finding).
Predation Control: Predators can prevent prey populations from exceeding carrying capacity.
Conservation Application: Population ecology is vital for wildlife conservation, pest control, and resource management, helping predict changes and develop protection strategies for endangered or invasive species.

The Biogeochemical Nitrogen Cycle

Importance of Nitrogen: Essential for the synthesis of proteins, DNA, and other biological molecules.
Atmospheric Composition: $N_2$ gas makes up approximately 78% of the atmosphere, though it is unusable by most organisms in this form.
Nitrogen Fixation: The conversion of $N_2$ into ammonia ( $NH_3$ ) or related compounds. * Biological Fixation: Done by bacteria like Rhizobium in the root nodules of legumes. * Industrial Fixation: The Haber-Bosch process synthesizes ammonia from $N_2$ and hydrogen ( $H_2$ ) for fertilizers. * Lightning: High energy breaks nitrogen bonds, allowing it to form nitrogen oxides that enter the soil via rain.
Nitrification: The conversion of ammonia into nitrites ( $NO_2^-$ ) and then into nitrates ( $NO_3^-$ ) by nitrifying bacteria. Nitrates are the primary form absorbed by plants.
Assimilation: Plants absorb nitrates or ammonium ( $NH_4^+$ ) to build amino acids and proteins. Animals acquire this nitrogen by eating plants or other animals.
Ammonification: Decomposers convert organic nitrogen from dead matter back into ammonia ( $NH_3$ ).
Denitrification: Bacteria convert nitrates back into nitrogen gas ( $N_2$ ), releasing it into the atmosphere and closing the cycle.

The Biogeochemical Phosphorus Cycle and Human Impact

Biological Role of Phosphorus: crucial for DNA, RNA, ATP, and cell membranes.
State of Matter: Unlike nitrogen, phosphorus lacks a gaseous phase; it is found in rocks, soil, and water.
Weathering of Rocks: Phosphate ions ( $PO_4^{3-}$ ) are released from rocks through weathering. This is a slow process, often making phosphorus a limiting nutrient.
Absorption and Food Chain: Plants absorb $PO_4^{3-}$ to build nucleic acids and ATP, which then move through the food chain via consumption.
Decomposition: Fungi and bacteria release phosphorus back into the soil as phosphate ions when organisms die.
Sedimentation: In water, phosphorus settles into sediments. Over long geological periods, these form new rocks that eventually surface to begin the cycle again.
Human Anthropogenic Impact: * Nitrogen Cycle: Excessive use of synthetic nitrogen fertilizers leads to runoff and eutrophication (nutrient over-enrichment). * Environmental Consequences: Eutrophication causes algal blooms and the creation of "dead zones," which are areas of extremely low oxygen levels. * Phosphorus Cycle: Overuse of phosphorus-containing fertilizers and detergents similarly contributes to eutrophication and harms aquatic ecosystems.