#nutrients

Controls Ecosystem

Increased nutrient supply in bottom-up ecosystems results in:

“Increased primary producers and subsequent trophic levels” for increased nutrient supply in bottom-up ecosystems results in. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. Rates depend on temperature, moisture, substrate quality, consumer physiology, and the elemental balance between organisms and their food. These controls explain why the same process can differ among terrestrial, freshwater, and marine systems without changing its definition. The remaining alternatives—“Increased predators only”, “Decreased herbivores”, “Increased carnivores only”—refer to different states, processes, or scales and therefore do not express the same causal relationship. Ecosystem processes are constrained by energy conservation and by the cycling of matter. Energy enters mainly through primary production, is lost as metabolic heat at every transfer, and therefore cannot be recycled in the way that carbon, nitrogen, phosphorus, or water can. Linking the wording to measurable consequences for fitness, abundance, or flux gives the conclusion its scientific meaning and prevents a purely mnemonic interpretation.

Ref: Fundamentals of Ecology, Odum & Barrett, 5th Ed., Ch. 4

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SPECIES INTERACTION

Which type of interaction occurs between two species competing for limited nutrients?

When two species draw on the same limiting nutrient, each reduces the amount available to the other. Both populations consequently experience lower growth, survival, or reproduction than they would in the other's absence, producing the (-,-) outcome of interspecific competition. Competitive effects can occur through exploitation of a shared resource or through direct interference, and their strength depends on resource supply, uptake traits, and niche overlap. Mutualism benefits both species, amensalism harms one while leaving the other unaffected, and parasitism benefits one at the other's expense. Resource partitioning or character displacement may lessen competition and permit coexistence over evolutionary or ecological time. The relevant evidence concerns process rather than wording alone. Linking the described pattern to energetic returns, fitness consequences, or receiver responses makes the inference biologically coherent and distinguishes it from the competing alternatives. Ecological categories are simplified models, yet they remain valuable when their assumptions are stated. The selected description captures the dominant net effect, while real systems may vary with density, habitat, life stage, and environmental conditions.

Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 8-13

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Ecosystem Ecology & PRODUCTIVITY

Which nutrient is NOT primarily cycled in ecosystem ecology?

Sulfur is actively and importantly cycled in ecosystems, so the keyed claim that it is not primarily cycled is scientifically incorrect. Along with carbon, nitrogen, and phosphorus, sulfur participates in a major biogeochemical cycle linking organisms, soil, rocks, water, and the atmosphere. Plants take up sulfate and incorporate it into amino acids such as cysteine and methionine; food webs transfer organic sulfur, and decomposition mineralizes it. Microbes oxidize reduced sulfur compounds or reduce sulfate under anaerobic conditions. Weathering, sea spray, volcanic emissions, decomposition, and fossil-fuel combustion move sulfur among reservoirs. Carbon cycles through photosynthesis, respiration, and decomposition; nitrogen through fixation, nitrification, assimilation, and denitrification; phosphorus mainly through weathering, uptake, decomposition, sedimentation, and uplift. Every listed nutrient is therefore substantially cycled in ecosystem ecology. Some introductory treatments emphasize carbon, nitrogen, and phosphorus more often, but that curricular emphasis does not make sulfur non-cycling. As written, the item has no scientifically valid unique answer, and the key should be flagged rather than rationalized.

Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 17

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Ecosystem Efficiency and Decomposition

Decomposers convert organic matter into:

Decomposers transform organic matter into inorganic nutrients through extracellular digestion, uptake, respiration, and mineralization. Organic nitrogen can become ammonium, organic phosphorus can become phosphate, and organic carbon is largely released as CO₂ under aerobic conditions. Not all material is immediately mineralized: some enters microbial biomass, humus, dissolved organic matter, or persistent mineral-associated pools. Thus decomposition both recycles plant-available ions and stabilizes part of the detrital carbon, linking energy dissipation with nutrient conservation. Carbon and mineral nutrients follow different accounting paths: energy is dissipated, whereas atoms may be retained, exported, or recycled into new biomass. Decomposition rate therefore emerges from interactions among substrate chemistry, decomposer traits, temperature, water, oxygen, and nutrient balance rather than from a single universal control. At ecosystem scale, these reactions regulate soil fertility, atmospheric carbon exchange, detrital food webs, and the residence time of organic matter. Mass loss alone cannot identify mechanism, because leaching, fragmentation, respiration, assimilation, and stabilization can produce different fates for carbon and nutrients.

Ref: Fundamentals of Ecology, Odum & Barrett, 5th Ed., Ch. 3

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