Outline how species diversity and population size influence the resilience of an ecosystem.

Describe the similarities and differences in using a biotic index and a diversity index to assess ecosystems.

With reference to named examples, discuss the significance of diversity in the sustainability of food production systems.

greater species diversity/greater population size usually lead to greater resilience;
with more species, it is more likely others can take over the role/niche of any lost/declining species;
more food chains/energy/biogeochemical pathways in an ecosystem provides redundancy therefore greater stability;
a variety of species is more likely to include those resistant to environmental change;
larger populations provide greater storages that can last over periods of lower productivity; larger populations generally carry greater genetic diversity;
larger populations of invasive species may lead to reduced diversity/resilience;
lower populations are more prone to extinction after a disturbance (e.g. habitat fragmentation) / or due to stochastic fluctuations;
an ecosystem may be more resilient if there are many small populations of different species than one large population of a single dominating species;
large populations of foundation/keystone species may be crucial for resilience of certain ecosystems (e.g. corals, kelp, beavers, elephants, pines, hemlock);

Note: Award credit to any valid arguments providing they directly relate species diversity or population size to resilience.
Award [3] max for responses addressing only diversity or only population size.

Similarities:
both may involve species identification;
both may involve quantitative sampling/estimating the abundance of living organisms;
both require multiple samples for effective comparisons/reliability;
both indices involve evaluating the range/variety of different species;
both involve calculating a single figure (index) from a collection of data;

Differences:
only biotic indices (not diversity indices) give different values to sensitive and tolerant species;
biotic indices are specifically used for evaluating impact of pollution whereas diversity indices just measure variety and evenness of species/general condition/maturity of an ecosystem;
diversity indices are appropriate for most ecosystems whereas biotic indices are usually for aquatic systems;
biotic indices usually focus on animal species/macroinvertebrates alone whereas diversity indices can be used for plant or animal communities;
diversity indices usually address all species present in system whereas biotic index focuses on a particular selection;

Note: Award [4] max if only similarities or only differences are given.
Only credit differences where both sides of the contrast are given or clearly implicit. No credit for naming indices eg Simpson’s, Trent etc.

Refer to paper 2 markbands, available under “your tests”  tab > supplemental materials.

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 and terminology of genetic/species/habitat diversity; cultural / political diversity; sustainability; ecological footprint; yield per unit area; aquatic and terrestrial food production; commercial vs. subsistence; multinational vs. local production; monoculture v polyculture; organic farming; GMOs; selective breeding; impact of escapees on wild populations; integrated agriculture; habitat loss/degradation; air/water/soil pollution; threats to biodiversity/pollinators; pesticide/fertilizer/antibiotic use; biological pest control; food choice; buffer zones; mineral cycles; crop rotation, etc.;
• breadth in addressing and linking technological and management strategies of terrestrial and aquatic food production systems with genetic/species/habitat diversity and significance of this in terms of sustainability and ecological footprint in the context of a range of geographical locations, social settings, levels of economic development, traditional values, international relations, legislations, personal attitudes and EVSs, etc.;
• examples of named food production systems and strategies involved in monoculture, polyculture and integrated agriculture, wild fisheries, aquaculture etc.;
• balanced analysis evaluating the extent to which diversity is significant in determining the sustainability of food production systems along with relevant limitations and counterarguments, etc.;
• a conclusion that is consistent with, and supported by, analysis and examples given e.g. “because diversity is such a significant factor in maintaining stability of systems, it is inevitably of great significance in maintaining both a sustainable production system and the wider environment in which the production takes place”;

Most candidates had some idea regarding the influence of species diversity and population density on resilience but few were able to gain the full credit available.

Although the majority of candidates had some idea of a 'diversity index', few had sufficient understanding of a 'biotic index' to effectively compare and contrast them. 

This was generally well-answered with many relevant examples of terrestrial and aquatic food production systems and particularly the distinction between examples of polyculture v monoculture.