AP Environmental Science:: Unit 1 - Ecosystems

An ecosystem, a foundational concept in environmental science, is defined as:

- **A)** Only the living (biotic) organisms present in a given area, with no reference whatsoever to any nonliving (abiotic) environmental component
- **B)** Only the nonliving (abiotic) physical/chemical components of a given environment, with no reference whatsoever to any living organism present in that same environment
- **C)** A community of interacting living organisms (biotic components) together with the nonliving physical and chemical components (abiotic components, such as sunlight, temperature, water, and soil/mineral nutrients) of their shared physical environment, all interacting together as one integrated functional system - energy and matter flow through an ecosystem via the interactions between these biotic and abiotic components
- **D)** A concept that has no meaningful, describable relationship whatsoever to any living organism or nonliving environmental component of any kind

Producers (autotrophs), the foundational trophic level in most ecosystems, obtain their own energy primarily by:

- **A)** Directly converting an external energy source (most commonly sunlight, via photosynthesis, though some specialized producers instead use chemical energy via chemosynthesis) into chemical energy stored within organic molecules - producers form the base of virtually every ecosystem's food chain/web, since they are the only organisms capable of directly capturing external energy and converting it into a biologically usable chemical form that other organisms (consumers) can subsequently obtain by consuming them
- **B)** Consuming other already-living organisms as their primary energy source, a description that instead corresponds to consumers (heterotrophs, discussed further below), rather than accurately describing the genuinely distinct producer/autotroph category itself
- **C)** A trophic category that has no meaningful, describable relationship whatsoever to energy capture, photosynthesis, or chemosynthesis of any kind
- **D)** Breaking down already-dead organic matter specifically, a description that instead corresponds to decomposers (discussed further below), rather than accurately describing the genuinely distinct producer/autotroph category itself

The 10% rule (ecological efficiency), an important general principle describing energy flow through an ecosystem's trophic levels, states that:

- **A)** Essentially all (100%) of the energy available at one trophic level is successfully transferred to and available at the next higher trophic level, with no meaningful energy loss whatsoever occurring between successive trophic levels
- **B)** On average, only approximately 10% of the energy available at one trophic level is successfully transferred to and becomes available at the next higher trophic level - the remaining approximately 90% of energy at each level is generally lost, primarily as metabolic heat released during cellular respiration, along with additional energy lost to incomplete digestion/undigested waste and energy used for movement/other organism-level metabolic activities not directly available to be passed on to a predator/consumer - this substantial and progressive energy loss at each successive trophic level directly explains why ecosystems generally support progressively FEWER total organisms (and typically fewer total distinct trophic levels) at progressively higher trophic levels
- **C)** A principle that has no meaningful, describable relationship whatsoever to energy transfer between different trophic levels of any kind
- **D)** Energy transfer efficiency between trophic levels that instead INCREASES progressively at each successive higher trophic level, the general opposite of the 10% rule's own actual, well-established core defining relationship/pattern

A food web, distinguished from a simpler food chain, more accurately represents real-world ecosystem trophic relationships because a food web:

- **A)** Represents only a single, simple, linear sequence of feeding relationships (e.g., grass eaten by a rabbit, which is in turn eaten by a fox), a description that instead corresponds to a simpler food chain, rather than accurately describing the genuinely distinct, more complex food-web concept itself
- **B)** Has no meaningful, describable relationship whatsoever to any feeding relationship or trophic interaction of any kind
- **C)** Only applies to aquatic ecosystems specifically, with no meaningful, describable applicability whatsoever to terrestrial ecosystems
- **D)** Depicts the numerous, interconnected feeding relationships among the many different organisms within an ecosystem, showing that most real-world organisms actually consume (and are consumed by) MULTIPLE different other species, rather than participating in just one single, simple linear food chain - this more complex, interconnected web structure has an important practical ecological implication: it generally provides greater overall ecosystem resilience/stability, since if one particular specific food source/species becomes scarce or is otherwise lost, many consumers can potentially shift toward consuming various different alternative food sources instead, rather than that entire simple chain of dependent species collapsing entirely, as might instead occur within a much simpler, more linear, single-path food chain

Primary consumers (herbivores), the trophic level directly above producers in a typical food chain, obtain their energy by:

- **A)** Directly consuming producers (plants, algae, or other autotrophs) as their primary food source - primary consumers occupy the second trophic level, converting the chemical energy stored within producer biomass into forms usable for their own growth, maintenance, and reproduction, while also becoming, in turn, a potential food source for the next higher trophic level (secondary consumers)
- **B)** Directly capturing sunlight or other external energy via photosynthesis or chemosynthesis, a description that instead corresponds to producers, rather than accurately describing the genuinely distinct primary-consumer trophic category itself
- **C)** Consuming only already-dead organic matter specifically, a description that instead corresponds more closely to decomposers/detritivores (discussed further below), rather than accurately describing primary consumers
- **D)** A trophic category that has no meaningful, describable relationship whatsoever to producers, plants, or energy consumption of any kind

Decomposers (and the closely related detritivores), an essential ecological functional group distinct from producers and consumers, serve the critical ecosystem function of:

- **A)** Directly capturing sunlight via photosynthesis, identical to producers, rather than accurately describing the genuinely distinct decomposer functional group itself
- **B)** Only consuming living producers directly, identical to primary consumers, rather than accurately describing the genuinely distinct decomposer functional group itself
- **C)** A functional group that has no meaningful, describable relationship whatsoever to dead organic matter, nutrient cycling, or ecosystem recycling of any kind
- **D)** Breaking down dead organic matter (the remains of dead organisms, and organic waste products) into simpler inorganic molecules/nutrients (such as ammonia, nitrate, phosphate, and other simple inorganic compounds) - this critical decomposition process releases nutrients that were locked up within dead organic matter back into the surrounding physical environment (soil, water) in a chemical form that producers can then directly take up and reuse, making decomposers absolutely essential for the ongoing cycling of nutrients through an ecosystem, without which nutrients would remain permanently locked up in accumulating dead organic matter rather than continuing to cycle through the ecosystem indefinitely

The carbon cycle, one of several important biogeochemical cycles studied in environmental science, describes how carbon moves between:

- **A)** Only the atmosphere, with no meaningful, describable movement whatsoever between the atmosphere and any other environmental reservoir (such as living organisms, the ocean, or soil/rock)
- **B)** A cycle that has no meaningful, describable relationship whatsoever to living organisms, the atmosphere, the ocean, or any other environmental reservoir
- **C)** Multiple interconnected environmental reservoirs - the atmosphere (primarily as carbon dioxide gas), living organisms (biomass, via photosynthesis removing atmospheric CO2 and cellular respiration releasing CO2 back to the atmosphere), the ocean (which absorbs and releases substantial amounts of CO2, and also stores dissolved inorganic carbon), and long-term geologic reservoirs (such as fossil fuel deposits and carbonate rock formations, which can store carbon for extremely long timescales) - human activities, particularly the combustion of fossil fuels, have significantly increased the rate at which carbon moves from these long-term geologic reservoirs into the atmosphere, a major contributing factor to the enhanced greenhouse effect and global climate change (discussed further in later units)
- **D)** Only living organisms, with no meaningful, describable movement whatsoever between living organisms and the atmosphere, ocean, or geologic reservoirs

The nitrogen cycle, another important biogeochemical cycle, is notable because most organisms cannot directly use atmospheric nitrogen gas (N2, which makes up about 78% of Earth's atmosphere) despite its overwhelming abundance, since:

- **A)** Atmospheric nitrogen gas is actually quite rare/scarce in Earth's atmosphere, contradicting the well-established fact that N2 is actually the single most abundant gas in Earth's atmosphere by a considerable margin
- **B)** The strong triple covalent bond holding the two nitrogen atoms together in N2 gas is extremely stable and requires a substantial energy input to break - most organisms lack the specific biochemical machinery needed to break this strong bond, so atmospheric N2 must first be converted ('fixed') into a more biologically usable chemical form (such as ammonia or nitrate) via a process called nitrogen fixation, carried out primarily by certain specialized nitrogen-fixing bacteria (some of which live freely in soil, while others live in a mutualistic symbiotic relationship with the root nodules of leguminous plants), before that nitrogen becomes available for most other organisms (including plants generally, and, indirectly, animals that consume those plants) to actually incorporate into their own biological molecules
- **C)** A cycle that has no meaningful, describable relationship whatsoever to any chemical bond, biological usability, or nitrogen-fixing bacteria of any kind
- **D)** Most organisms actually can directly and easily use atmospheric nitrogen gas without any need whatsoever for prior nitrogen fixation, directly contradicting the nitrogen cycle's own actual, well-established core defining premise regarding the specific biochemical necessity of nitrogen fixation