Outline how four different factors influence the resilience of an ecosystem.

Explain how a community of trees in a woodland may be considered a system.

Disturbance of the composition and processes of the atmospheric system through human activity always disturbs the equilibria of marine systems.

Discuss the validity of this statement with reference to named examples.

Valid factors:
greater diversity of components/species increases resilience;
complexity of interactions/developed food webs increase resilience;
establishment of keystone species increases resilience;
larger storages/stores / more abundant/productive resources (nutrients, water, sunlight, reproductive rates, biomass etc) increase resilience (NB for credit, there must be indication of abundance in these resources, and if multiple examples are given like those in brackets, there is still only [1 max] allowed for this MP);
larger size of the system increases resilience;
strong negative feedback systems increase resilience;
strong positive feedback mechanisms may decrease resilience;
human impact degrading structure/diversity/abundance will decrease resilience;
a steady state equilibrium/balanced inputs and outputs (as in climax communities) increases resilience;
systems being close to a tipping point decrease resilience.

Accept converse statements.
Award [1] for each correct factor identified, up to [4 max].
If valid factors are identified, but their effect on resilience is not, award [1] for each TWO factors up to [2 max] (ie FOUR factors). Eg identifying TWO factors and their effect on resilience, along with TWO factors but no specified effect would score [2+1=3] total OR eg TWO factors and their effect, along with ONE with no effect would score [2+0=2] total OR eg FOUR factors with no effect ([1+1=2] total).

A community of trees in a woodland has the following features of a system:
individuals/species of trees are the components of the system;
these components are interrelated/interdependent/form an integrated whole;
eg may regulate populations through competition / contribute to succession of community;
it has flows/transfers of matter/energy between components/storages;
eg leaf fall may provide nutrients through decomposition to other trees / pollination/genes/food storage in seeds / glucose is transported from leaves around tree;
components carry out processes/transformations;
eg photosynthesis/respiration/growth;
it is an open system exchanging matter and energy with surroundings;
eg absorption of solar energy / provision of nutrients for non-tree species (NB mark for either example of matter or energy, not both);
it has feedback mechanisms to maintain equilibrium/balanced inputs and outputs;
eg more seed production → more competition between seedlings → fewer viable offspring / death of trees → more light entering canopy → more tree growth.

Award [1] for each correct suggestion, up to [7 max].
Award [4 max] for identifying relevant generic features of system (given above) and [4 max] for examples of these within a tree community (beware of responses that are looking at entire woodland ecosystem as a system rather than the tree community alone). Credit alternative examples of equivalent validity/relevance and detail.

The following guide for using the markbands suggests certain features that may be offered in responses. The five headings coincide with the criteria given in each of the markbands (although “ESS terminology” has been conflated with “Understanding concepts”). This guide simply provides some possible inclusions and should not be seen as requisite or comprehensive. It outlines the kind of elements to look for when deciding on the appropriate markband and the specific mark within that band.

Answers may include:

• understanding concepts & terminology of greenhouse gases; global warming; thermal expansion; rising sea-level; melting ice caps; salinity; ocean acidification; ozone depletion; UV radiation; global productivity; tropospheric ozone; acid rain (worth noting here that acid rain is NOT a significant cause/contributor to ocean acidification); etc
• breadth in addressing and linking impacts on atmospheric systems (increased CO₂; global warming; ozone depletion; tropospheric ozone; acid rain; etc) with disturbances in marine systems (rising sea level; ocean currents; fish migration; acidification; destruction of coral reefs; phytoplankton / marine food chains; etc)
• examples of relevant atmospheric pollutants/disturbances; ocean currents; coral communities; migratory fish; marine food chains; etc
• balanced analysis of the extent to which each atmospheric disturbance causes disturbance in marine systems whether significant, minimal or none at all, acknowledging relevant counter-arguments/alternative viewpoints;
• a conclusion that is consistent with, and supported by analysis and examples given e.g. generally, the statement is valid regarding large scale disturbances to the atmosphere such as global warming and ozone depletion but more localized disturbances like tropospheric ozone and acid rain have fairly minimal impact. NB This is only an example of a possible conclusion. Candidates’ conclusions do not have to agree.

Refer to paper 2 markbands, available under the "your tests" tab > supplemental materials

Question 4 was one of the least popular but often produced quite good responses. In part (a), a number of candidates could go no further than biodiversity (in its various forms) as a contributing factor to resilience. Some too were under the false impression that external impacts affect resilience, whereas resilience is not dependent on the absence of threats, but is the inherent ability of the system to resist such threats. Of course, human impacts such as hunting/harvesting etc may affect resilience by depleting those qualities that contribute to resilience (diversity/productivity etc) but this needs to be specified to become a relevant influence on resilience, as such.

Most candidates were able to recognise a tree community as an open system and give examples of exchange of matter/energy and transformations within that system. Some responses became confused by extending the boundary of the system addressed to the entire woodland ecosystem such that some examples given were irrelevant.

Most candidates were able to find at least one or two valid links between atmospheric disturbances and marine systems particularly between global warming, rising sea levels, changing currents/coral bleaching etc. A significant minority mistook “marine” to mean simply “aquatic”, and therefore addressed irrelevant material relating to freshwater systems. There was a quite common confusion that acid rain was a cause of ocean acidification whereas acid rain has negligible impact on marine systems. The inclusion of “always” in the question stem made it possible for candidates to develop counter-examples (like photochemical smog/acid rain) but few took advantage of this opportunity, limiting the depth/balance of their analysis.