IB Biology: Ecology, Energy Flow, and Climate Change Study Guide

4.1 Species, Communities, and Ecosystems

Introduction to Sustainability - The survival of living organisms, including humans, depends on sustainable communities.
Defining Species and Populations - Species: A species is a group of organisms that can potentially interbreed to produce fertile, viable offspring. - Interbreeding Constraints: Members of a single species are generally unable to produce fertile, viable offspring with members of a different species. - Hybridization: When two different species produce offspring via cross-breeding, the resulting hybrids are reproductively sterile (e.g., a liger or a mule). - Population: A group of organisms belonging to the same species that live in the same geographic area at the same time. - Reproductive Isolation: Members of the same species living in different regions (different populations) are reproductively isolated and unlikely to interbreed. However, they are still classified as the same species if interbreeding remains functionally possible.
Community, Habitat, and Ecosystem - Community: Formed by populations of different species living and interacting with each other within a given area. - Habitat: The environment in which a species normally lives, or the specific location of a living organism. - Ecosystem: Formed by a community and its interactions with the abiotic (non-living) environment. - Ecology: The study of the relationships between living organisms or between living organisms and their environment.
Modes of Nutrition - Autotrophs (Producers): Organisms that synthesize their own organic molecules from simple inorganic substances (e.g., $CO_2$ ) obtained from the abiotic environment. - Photosynthesis: Most autotrophs derive energy from sunlight (photoautotrophs). - Chemosynthesis: Some autotrophs derive energy from the oxidation of inorganic chemicals (chemoautotrophs). - Heterotrophs (Consumers): Organisms that obtain organic molecules from other organisms (living, recently killed, or non-living remains/detritus). - Mixotrophs: Certain unicellular organisms that use both autotrophic and heterotrophic forms of nutrition depending on resource availability.
Categories of Heterotrophs - Consumers: Feed on living organisms by ingestion. - Herbivores: Feed principally on plant matter (e.g., cows, sheep, rabbits). - Carnivores: Feed principally on animal matter (e.g., crocodiles, wolves, tigers). - Omnivores: Diet composed of both plant and animal matter (e.g., pandas, humans). - Scavengers: A type of consumer that principally feeds on dead and decaying carcasses rather than hunting live prey (e.g., hyenas, vultures, carrion birds like crows). - Detritivores: Heterotrophs that obtain organic nutrients from detritus via internal digestion. - Detritus: Dead, particulate organic matter, such as decaying material and fecal matter. - Humus: Decaying leaf litter intermixed within topsoil. - Examples: Dung beetles, earthworms, woodlice, snails, and crabs. - Saprotrophs (Decomposers): Heterotrophs that obtain organic nutrients from dead organisms via external digestion. - They live on or in non-living organic matter, secrete digestive enzymes into it, and absorb the products. - Unlike other heterotrophs, they do not ingest food. - Examples: Bacteria and fungi.
Nutrient Cycling - Nutrients: Materials required by organisms, including elements like carbon, nitrogen, and phosphorus. - Finity of Nutrients: The supply of inorganic nutrients on Earth is finite; new elements cannot be created. - Recycling Process: 1. Autotrophs obtain inorganic nutrients from air, water, and soil, converting them into organic compounds. 2. Heterotrophs ingest these compounds for growth and respiration, releasing inorganic byproducts. 3. Saprotrophs decompose remains, freeing inorganic materials back into the soil, ensuring a continual supply for autotrophs.
Biogeochemical Cycles - The Water Cycle: Shows continuous movement of water through evaporation (liquid to vapor), condensation (forming clouds), and precipitation (rain or snow). - The Phosphorus Cycle: Phosphorus is a component of DNA, ATP, and phospholipids. It is usually solid and not found as a gas. It exists as phosphates in soil (fixed via weathering of rocks) and enters waterways via erosion and leaching. - The Sulfur Cycle: Sulfur is essential for proteins and enzymes. Atmospheric and soil sulfur can be oxidized to sulfates ( $SO_4^{2-}$ ). Plants/bacteria reduce sulfates into organic molecules. Burning fossil fuels releases sulfur dioxide ( $SO_2$ ), contributing to acid rain. - The Nitrogen Cycle: Atmosphere is mostly nitrogen gas ( $N_2$ ), which is chemically inert. Nitrogen-fixing bacteria process it for plants. Plants absorb nitrogen as nitrate ions ( $NO_3^-$ ) or ammonium ( $NH_4^+$ ). Decomposing organic nitrogen (proteins) is converted back to inorganic forms via ammonification. Denitrifying bacteria convert soil nitrogen back into gaseous $N_2$ .
Ecosystem Sustainability - Ecosystems can be self-sustaining over long periods. Three requirements are: 1. Energy availability: Continuous light from the sun. 2. Nutrient availability: Constant recycling by saprotrophic decomposers. 3. Recycling of wastes: Detoxification of harmful byproducts by bacteria. - Mesocosms: Small, self-contained experimental areas used to model ecosystems.
Species Associations and Sampling - Positive Association: Species found in the same habitat (e.g., predator-prey, symbiosis). - Negative Association: Species that do not occur together due to competition for resources. - Competitive Exclusion: One species uses resources more efficiently, precluding the other. - Resource Partitioning: Species alter environment use to avoid direct competition. - Independent Distribution: No association between species. - Quadrat Sampling: Uses a rectangular frame of known dimensions to establish population densities. Placed randomly or along a belted transect. Not effective for motile organisms.
Symbiotic Relationships - Symbiosis: Close, long-term interaction between species. Can be obligate (essential) or facultative (advantageous). - Mutualism: Both species benefit (e.g., anemone protects clownfish; clownfish provides fecal food). - Commensalism: One species benefits, the other is unaffected (e.g., barnacles transported by whales). - Parasitism: One species benefits at the expense of the other (e.g., ticks/fleas on a canine).
Ecological Niches - Definition: The functional position and role of an organism within its environment. - Components: Habitat, activity patterns (active periods), resources obtained, and species interactions. - Niche Differentiation: How competing species use the environment differently to coexist.

4.2 Energy Flow

Energy Source and Conversion - Most ecosystems rely on sunlight. Photoautotrophs (green plants, some bacteria) convert light energy into chemical energy in carbon compounds via photosynthesis. - In rare cases, chemoautotrophic bacteria use energy from chemical processes. - Chemical energy flows through food chains via feeding.
Trophic Levels - Definition: The position an organism occupies in a feeding sequence. - Levels: Producers (1st), Primary Consumers (2nd), Secondary Consumers (3rd), Tertiary Consumers (4th), etc.
Food Chains and Webs - Food Chain: Linear feeding relationships. Arrows point in the direction of energy flow. - Food Web: Complex diagram showing linked food chains. More representative because organisms can have multiple food sources and predators, allowing them to occupy multiple trophic levels.
Energy Loss and Heat - Energy released from carbon compounds by respiration is used for metabolic processes (growth, homeostasis) and converted to heat. - Forms of Energy Conversion: Kinetic energy (muscle contraction), Electrical energy (nerve impulses), Light energy (bioluminescence). - Exothermic Nature: These reactions release thermal energy (heat). - Heat Constraints: Living organisms cannot convert heat back into usable energy. Heat is lost from the ecosystem. Thus, a continuous influx of solar energy is required. - Efficiency: Energy transformations are ~ $10\%$ efficient; ~ $90\%$ of energy is lost via respiration, heat, excretion (feces), or unconsumed parts.
Biomass and Trophic Level Limits - Biomass: Total mass of a group of organisms, consisting of carbon compounds in cells/tissues. - Biomass diminishes along food chains due to loss of $CO_2$ , $H_2O$ , and waste (e.g., urea). - Trophic Limits: Because energy and biomass decrease at each level, higher levels must eat larger quantities. If hunting energy exceeds food energy, the level becomes unviable.
Pyramids of Energy - Graphical representation of energy at each level in $kJ\,m^{-2}\,year^{-1}$ . - Never inverted. Each level is roughly one-tenth ( $10\%$ ) the size of the previous level.
Ecological Productivity - Primary Production: Generation of chemical energy by producers (Sunlight or chemosynthesis). - Gross Primary Production (GPP): Total chemical energy created in a given time. - Net Primary Production (NPP): Energy not consumed by respiration ( $NPP = GPP - \text{respiration}$ ). - Secondary Production: Generation of biomass by heterotrophs via feeding. - Units: Usually expressed as $kg\,m^{-2}\,day^{-1}$ .
Other Ecological Pyramids - Pyramid of Numbers: Shows relative number of organisms. Can be distorted/inverted (e.g., thousands of caterpillars on one oak tree, or many fleas on one dog). - Pyramid of Biomass: Total mass at each stage. Almost always upright. Exception: Marine ecosystems where zooplankton biomass may exceed phytoplankton biomass because phytoplankton replace biomass very rapidly.
Bioaccumulation and Biomagnification - Bioaccumulation: How pollutants enter a food chain within a single organism. - Biomagnification: Increase in pollutant concentration at higher trophic levels (e.g., DDT causing egg-shell thinning in birds; Mercury in fish).

4.3 Carbon Cycling

Introduction to the Carbon Cycle - Carbon is exchanged between the atmosphere (air), lithosphere (ground), hydrosphere (water), and biosphere (living things). - Forms of Carbon: - Atmospheric gases: $CO_2$ and Methane ( $CH_4$ ). - Oceanic carbonates: Bicarbonates dissolved in water and calcium carbonate in corals/shells. - Organic materials: Carbohydrates, lipids, proteins. - Non-living remains: Detritus and fossil fuels.
Carbon in Aquatic Ecosystems - Carbon exists as dissolved $CO_2$ and hydrogen carbonate ions ( $HCO_3^-$ ). - Calcium carbonate ( $CaCO_3$ ) is used by reef-building corals and Mollusca for hard parts, which can fossilize into limestone. - Ocean Acidification: $CO_2$ combines with water to form carbonic acid ( $H_2CO_3$ ). Increased $CO_2$ leads to more $H^+$ ions, lowering pH (acidification). $H^+$ ions bind with carbonate ions, making them unavailable for corals, leading to thinner shells.
Methane Production and Oxidation - Methanogenesis: Methane ( $CH_4$ ) is produced from organic matter in anaerobic conditions by methanogenic archaeans. - Atmospheric Methane: Methane is oxidized to $CO_2$ and $H_2O$ in the atmosphere.
Fossilization and Combustion - Peat: Forms when organic matter is not fully decomposed due to acidic or anaerobic waterlogged soil conditions. - Fossil Fuels: Partially decomposed organic matter from past eras converted into coal, oil, or gas. - Combustion: Heating hydrocarbons in the presence of oxygen ( $O_2$ ) releases energy, $CO_2$ , and $H_2O$ .

4.4 Climate Change

Greenhouse Gases - Core Gases: Carbon dioxide ( $CO_2$ ) and Water vapor (clouds) have the most significant warming effect. - Secondary Gases: Methane ( $CH_4$ ) and Nitrogen oxides ( $NO_x$ ). - Impact Factors: Ability to absorb long-wave radiation and atmospheric concentration.
The Greenhouse Effect - The Earth absorbs short-wave radiation from the sun and emits longer wavelength (infrared/heat) radiation. - Greenhouse gases trap this long-wave radiation, retaining heat in the atmosphere.
Climate Correlations - There is a correlation between rising atmospheric $CO_2$ since the Industrial Revolution and average global temperatures. - Increases are largely due to increased combustion of fossilized organic matter.
Conservation Strategies - In situ Conservation: Preservation within a natural habitat (Nature reserves, national parks). - Advantages: Maintains normal behavior, preserves food chains, maintains habitat for other species, allows for scientific study. - Ex situ Conservation: Preservation outside natural habitats (Zoos, botanical gardens, seed banks). - Advantages: Greater control of conditions, artificial breeding methods. - Disadvantages: Does not prevent habitat destruction, captive organisms may lose survival skills, restricted gene pool (inbreeding).