APES UNITED 1

Generalists

Specialists

Able to survive on a wide variety of food resources

Specific/limited number of prey

Able to withstand a wide range of environmental conditions

Prone to extinction; sensitive to environmental change

Live in broad niches

Live in narrow niches (e.g., pandas)

Examples: cockroaches, humans, mice

Examples: giant pandas, koalas, mountain gorillasSpecies Interactions

Symbiosis

• Any close, long-term biological interaction between two different organisms (often different species, though the term is sometimes used more broadly).

Key interaction types:

• Amensalism: one species is harmed, the other is unaffected. Example: black walnut trees release a chemical that kills neighboring plants.

• Commensalism: one species benefits, the other is unaffected. Common forms include using another organism for transportation, housing, or using something another organism created.

• Mutualism: both species benefit.

• Parasitism: one benefits while the other is harmed.

• Predation: predators hunt and kill prey.

  • Opportunistic predators eat almost anything

  • Specialist predators prey on certain organisms.

• Competition

  • Can be intraspecific (within the same species) or interspecific (between different species)

  • Driving force of evolution (for food, mates, territory)

  • Predator–prey relationships can also be viewed as competitive in the sense that predators compete for food while prey “compete” for survival.

• Saprotrophism: saprotrophs obtain nutrients from dead or decaying organisms by absorbing soluble organic compounds. (This connects closely to decomposition and nutrient cycling.)

Resource partitioning (how competitors coexist)

When species overlap in resource use, competition can be reduced if they divide the resource in different ways.

• Morphological partitioning: species evolve different structures to use the same resource differently.

• Spatial partitioning: species use the same resource in different areas or microhabitats.

• Temporal partitioning: species use the same resource at different times (day/night, seasons).

Law of Tolerance, limiting factors, and carrying capacity

The Law of Tolerance

• States that the existence, abundance, and distribution of a species depend on its tolerance level to physical and chemical factors.

• If conditions exceed tolerance limits, survival and reproduction decline.

A limiting factor is any environmental factor that restricts population growth, abundance, or distribution.

• Limiting factors can be density-dependent (stronger at higher population densities), such as disease, competition, and predation.

• Limiting factors can be density-independent (affect populations regardless of density), such as drought, storms, fire, and temperature extremes.

Examples of limiting factors:

• In terrestrial ecosystems: soil nutrients, available water, light, temperature.

• In aquatic ecosystems: pH, dissolved oxygen, light (depth/turbidity), and salinity.

The range of tolerance

• Describes how well a species can survive across an abiotic gradient (like temperature or salinity).

• If a habitat becomes drier than a plant’s tolerance, it declines

• If a limiting nutrient increases, productivity can rise.

Carrying capacity is the maximum population size an environment can sustain over time given available resources and limiting factors.

Predator–prey cycles (population dynamics concept)

Predator–prey cycles can occur because predator numbers often track prey availability. If prey rapidly multiply, predator populations can increase due to more available food. Predators may then reduce prey populations, which can later cause predator numbers to decline, allowing prey to rebound.

Feedback loops in ecosystems

Ecosystems include feedbacks that either stabilize or amplify change.

• A negative feedback loop counteracts change and tends to stabilize a system.

  • Example: more plants can remove more CO2 through photosynthesis, which can reduce atmospheric CO2 under some conditions.

• A positive feedback loop amplifies change.

  • Example: warming melts ice, reducing reflectivity (albedo) and increasing heat absorption, causing more warming.

“Positive” does not mean “good”; it means self-reinforcing.

Habitat vs Niche

A habitat

• Where an organism lives (its “address”)

A niche 

• is its role (its “job”):

  • How it gets energy

  • Its place in food webs
    What resources it uses

  • How it interacts with both living and nonliving factors

  • Also reflects the adaptations a species has acquired through evolution

Characteristics of a niche commonly include:

• The habitat used

• Interactions with biotic and abiotic factors (resources and competitors)

• Place/role in the food web

• Types and amounts of resources available

Two species can share a habitat but typically cannot occupy the exact same niche indefinitely; competition tends to push them toward different resource-use patterns. Biodiversity: What It Is, Why It Matters, and What Controls It

Biodiversity is the variety of life in an area. APES emphasizes it because it links to ecosystem resilience, productivity, and ecosystem services.

Levels of biodiversity

• Genetic diversity: variation in genes within a species or population.

• Species diversity: variety of species in an ecosystem.

• Ecosystem diversity: variety of ecosystems across a region.

Genetic diversity increases the chance some individuals survive new stresses. Species diversity can increase resilience because different species often play different roles.

Species richness and evenness

Species diversity commonly includes:

• Species richness: number of species present.

• Species evenness: how evenly individuals are distributed among species.

Biodiversity and ecosystem resilience

Resilience is the ability of an ecosystem to recover after disturbance (fire, storms, human impacts). Biodiversity can increase resilience through functional redundancy and genetic variation, but it does not guarantee stability in every context (keystone species and disturbance type matter).

Habitat fragmentation and edge effects

Fragmentation breaks large habitats into smaller patches, which can reduce biodiversity by lowering population sizes, reducing gene flow, and increasing edge habitat.

The edge effect refers to changes at habitat boundaries (temperature, humidity, predators, invasive species, species interactions) that can make edges ecologically different from interior habitat.

Example: richness vs. evenness

Community A: 10 species, but 95% of individuals are one species.

Community B: 7 species, with individuals more evenly distributed.

Community A has higher richness; Community B has higher evenness. Depending on how “diversity” is defined in the prompt, B may be considered more diverse in terms of evenness, while A is more diverse in richness. Matter Cycling vs. Energy Flow (and why ecosystems need both)

Two big ideas run through ecosystem science

• Energy flows through ecosystems

• Matter cycles within and among ecosystems.

This distinction explains why ecosystems require continuous energy input (mostly sunlight) but can reuse the same atoms (carbon, nitrogen, phosphorus) over and over. If you mix these up, it becomes hard to explain nutrient limitation, eutrophication, and the importance of decomposers.

Energy: one-way movement through trophic levels

Energy enters most ecosystems as sunlight captured by producers. As organisms use energy for metabolism, much of it is released as heat. Because of this, energy does not cycle back into usable chemical energy the way nutrients do.

A useful framing is the Second Law of Thermodynamics:

• As energy is transferred or transformed, more and more becomes unusable (often dispersed as heat).

• Relatedly, entropy is the natural tendency for an isolated system to move from a more ordered state to a more disordered state.

Matter: atoms move among reservoirs

Atoms cycle among living organisms and abiotic reservoirs such as the atmosphere, oceans and lakes, soils and sediments, rocks, and biomass (living and dead). Ecosystems can persist over time because matter is recycled, unless human activity disrupts cycling rates or removes/overloads nutrients faster than they can be balanced.

Decomposers connect energy and matter

Decomposers (many bacteria and fungi) and detritivores (organisms that consume dead organic matter, like earthworms) break down dead biomass and waste.

• They return nutrients to soil and water, making them available to producers again.

• They keep energy transfers moving through detrital pathways, even though energy still dissipates as heat.

Without decomposers, nutrients become locked in dead matter and primary production collapses. Energy in Ecosystems: Producers, Consumers, and the Rules of Transfer

Producers, photosynthesis, and carbon capture

Primary producers (autotrophs) make organic molecules from inorganic carbon. On land, most producers are plants; in aquatic systems, major producers include algae, phytoplankton, and photosynthetic bacteria.

Photosynthesis removes carbon dioxide and uses light energy to produce carbohydrates and other organic compounds, releasing oxygen gas. Producers capture light primarily through chlorophyll in chloroplasts.

A standard way to write photosynthesis is:

• Carbon dioxide + water + light → glucose + oxygen

Producers also carry out cellular respiration, but if they absorb more CO2 than they release, they function as net carbon sinks.

Factors that affect the rate of photosynthesis include CO2 concentration, light amount and wavelength, water availability, and temperature.

Consumers and cellular respiration

Consumers (heterotrophs) depend on photosynthetic organisms, directly or indirectly.

• Primary consumers eat producers (herbivores).

• Secondary consumers eat primary consumers.

• Tertiary consumers eat secondary consumers.

Cellular respiration is often described as the opposite of photosynthesis: glucose is oxidized to produce carbon dioxide, water, and usable chemical energy stored in ATP (adenosine triphosphate).

Trophic levels

A trophic level is the feeding position an organism occupies in a food chain or web, often described as the number of steps it is from the start of the chain.

Food chains vs. food webs

A food chain is a simplified linear pathway of energy transfer. A food web is a network of interconnected food chains that better represents real ecosystems.

In diagrams, arrows typically show the direction of energy transfer (from food to eater), but always check the prompt.

Ecological pyramids (energy, biomass, numbers)

Ecological pyramids place producers at the base and show how energy/biomass/numbers change across trophic levels.

1. Energy pyramids show energy available at each trophic level over time. They are always upright when all sources of energy are included because energy is lost as heat at each transfer. Energy pyramids describe the proportion of energy passed to the next level; energy “temporarily trapped” in biomass is not treated as available unless it is consumed.

2. Biomass pyramids show the mass of living tissue at each trophic level at a moment in time. In aquatic and coral reef ecosystems, biomass pyramids can be inverted.

3. Pyramids of numbers show counts of individuals and can be misleading (one large producer like a tree can support many consumers).

The 10% rule and ecological efficiency

Only about 10% of the energy transferred from one trophic level to the next is converted into new biomass (tissue) available to the next level. The rest is lost as heat and through life processes, including metabolism, temperature control, movement, incomplete digestion, waste production, and decay of waste.

Ecological efficiency is the broader idea that energy transfer between trophic levels is inefficient. Instead of memorizing a single percentage as universally true, focus on the reasoning: organisms must use energy to stay alive, so less remains for growth.

Endotherms (animals that regulate body temperature internally) often lose more energy as heat than ectotherms, which can reduce transfer efficiency in some situations.

Bioaccumulation and biomagnification

Some pollutants are persistent (do not break down easily) and fat-soluble (stored in tissues).

• Bioaccumulation: a chemical builds up in an organism over time because intake exceeds elimination.

• Biomagnification: chemical concentration increases at higher trophic levels because predators eat many contaminated prey.

Primary Productivity: How Fast Ecosystems Capture Energy

Primary productivity is the rate at which producers convert solar energy into chemical energy stored as biomass. It sets the energy budget for entire food webs.

Gross vs. net primary productivity

Producers capture energy through photosynthesis but also use energy for cellular respiration and tissue maintenance.

• Gross primary productivity (GPP): total energy captured by producers through photosynthesis.

• Net primary productivity (NPP): energy remaining after producers meet respiration needs; this is the energy available to consumers and decomposers.

What controls productivity on land?

Terrestrial NPP is strongly influenced by temperature, water availability, soil nutrient availability (especially nitrogen and phosphorus), and sunlight. Warm, wet regions tend to have high NPP (tropical rainforests), while cold or dry regions tend to have low NPP (tundra, deserts).

What controls productivity in water?

Aquatic productivity depends on light availability (depth and turbidity), nutrient availability (often nitrogen or phosphorus), water temperature, and mixing/upwelling that brings nutrients to the surface.

Open ocean regions can be nutrient-poor at the surface, leading to relatively low productivity per unit area, but because the open ocean covers so much of Earth, it contributes a very large share of global total NPP. Trophic Dynamics in Practice: Ecological Efficiency and Ecosystem Stability

Ecological efficiency and transfer constraints

Energy transfers are inefficient because organisms use energy for metabolism and lose energy as heat. If producers capture less energy (drought, shade), higher trophic levels have less energy available. Endotherms may reduce transfer efficiency because maintaining body temperature uses a large share of energy.

Trophic cascades

A trophic cascade is a ripple effect across trophic levels caused by changes at one level. Removing a top predator can increase herbivores, reduce plants, and alter habitat structure, erosion, and nutrient cycling. Cascades are often driven by indirect effects, including changes in prey behavior.

Keystone species

A keystone species has a disproportionately large effect on ecosystem structure relative to its abundance. Losing a keystone species can trigger major ecosystem shifts even if many other species remain.

Indicator species

An indicator species is one whose presence/absence/health reveals environmental conditions. Sensitive species can signal pollution or oxygen stress before broader ecosystem damage is obvious.