BIOL1907 Chap.7 Trophic Ecology and Ecosystem Change
Trophic Ecology and Ecosystem Change
This chapter follows energy and matter through food webs, then examines how disturbance changes communities and ecosystems. It distinguishes autotrophs from heterotrophs, food chains from networks and energy flow from nutrient cycling.
Trophic transfer efficiency provides a quantitative method, while succession and resilience questions require a named baseline, response variable and time window before claims about recovery can be evaluated. Autotrophs introduce organic matter into food webs by using light or chemical energy to fix inorganic carbon.
Heterotrophs obtain matter and energy from producers or other consumers, while decomposers process material from many trophic positions. A chain displays one route, but a web better represents multiple resources, consumers and indirect effects. Define the arrow convention because arrows for resource transfer and arrows for causal influence may point differently. Energy and nutrients require separate ledgers.
Organisms transfer some stored chemical energy to consumers, use some for metabolism and lose some as heat. Less usable energy generally remains at higher trophic positions. Elements can return through waste and decomposition and be taken up again, so nutrients cycle while energy flows. Standing biomass is a stock, whereas productivity is a rate of new material formation.
Trophic transfer efficiency divides production at a receiving level by production at its resource level over matched area and time. The percentage is conditional on organisms, digestibility, metabolism and interval. It should not be applied as a universal constant to every link.
A change in a top consumer can propagate indirectly through prey or resources, but alternative environmental drivers still need to be controlled before a field pattern is declared a trophic cascade. Communities include co-occurring species considered as an interacting unit; an assemblage can describe species grouped together without assuming all interactions. Disturbance changes organisms or resources and can begin succession.
Colonisation, local extinction and priority effects shape the later community, while repeated disturbance or altered conditions can redirect rather than simply reset the trajectory. Resilience is meaningful only after the response variable is named. Plant cover can recover while species composition, productivity or interactions remain different.
A claim of restoration should therefore specify the baseline, metric and time window. Stability also does not mean immobility because turnover can continue beneath a similar aggregate value. Conservation reasoning joins pressure, mechanism, target and monitoring. Protecting area can help but may not address altered fire, invasive species, disrupted connectivity or pollution.
Propose an action at the causal link and a measurement that would reveal whether the intended response occurred. Then check for trade-offs elsewhere in the web. Revision should use perturbation. Remove one species or change one resource, trace the direct link first, add only supported indirect effects and state the likely short-term versus longer-term response.
This keeps the explanation connected to energy, matter and interaction rather than a list of ecosystem vocabulary.
What this chapter covers
- 01
Autotrophs, heterotrophs and decomposers
- 02
Food chains, food webs and trophic levels
- 03
Species interactions and trophic cascades
- 04
Disturbance, succession and resilience
- 05
Productivity, nutrient cycling and conservation
Worked example · free
Calculate trophic transfer efficiency
- +1Use consumer production divided by resource production times 100.
- +1Substitute 1,800 divided by 18,000.
- +1The efficiency is 10%.
- +1State that the matched area and time units cancel in the ratio.
Key terms
- Autotroph
- An organism that builds organic matter using inorganic carbon.
- Trophic level
- A feeding position in an energy-transfer network.
- Food web
- A network of feeding relationships among organisms.
- Succession
- Directional change in community composition through time.
- Resilience
- Recovery or persistence of a named system property after disturbance.
Trophic Ecology and Ecosystem Change FAQ
Why does energy not cycle like nutrients?
Organisms transfer and transform energy, with some dissipated as heat at each step. Elements can be returned through decomposition and reused, so nutrient cycling and energy flow require different language.
What is a trophic cascade?
It is an indirect effect that propagates across feeding levels, such as predator decline releasing herbivores and increasing pressure on plants. Controls are still needed before a field pattern is assigned to that pathway.
How do I decide whether an ecosystem recovered?
Name the reference state, response variable and time interval. Biomass can return while composition or interactions remain altered, so a single unqualified label may hide different recovery outcomes.
Exam move
Redraw a food web with arrows explicitly defined as resource-to-consumer transfer. Perturb one species and trace only the direct link before adding supported indirect effects. Complete one efficiency calculation, then compare two resilience measures such as cover and species composition. Keep separate ledgers for energy loss and nutrient recycling, and label productivity as a rate rather than a standing stock.
For disturbance questions, write the baseline, response variable and time window before judging recovery. Test a conservation proposal by naming the pressure it changes, the predicted response, the monitoring indicator and a plausible trade-off elsewhere in the web. Use matched area and time units for transfer efficiency.
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