Identify four impacts on an ecosystem that may result from the introduction of an invasive species of herbivore.

Explain how both positive and negative feedback mechanisms may play a role in producing a typical S population growth curve for a species.

Technocentrists may support the belief that technological development has always been able to overcome limits to human population growth.

To what extent do the patterns of growth and development in human populations, as demonstrated in the Demographic Transition Model, support this claim?

reduction in plant species (through feeding);
…(causing) reduction in overall productivity;
reduction/loss of native herbivore species (through competition);
…(causing) reduction/loss of carnivore species/higher trophic levels;
…(causing) reduction in species diversity;
shift of equilibrium/toward tipping point;
introduction of new diseases/pathogens/parasites (carried by invasive species);
in some circumstances may benefit ecosystem by eg increasing food for higher trophic levels/increasing diversity;

[4 max]

positive feedback will occur at start of S curve where numbers are small;

Award [3 max] for following marking points as statements OR shown in a diagram. See example below:
as populations increase/reproduce they increase the number of reproducing individuals;
…which will further increase the growth rate;
ie positive feedback …a change promoting further change in same direction;

negative feedback occurs as graph approaches maximum/carrying capacity/plateau;

Award [3 max] for following marking points as statements OR shown in a diagram. See example below:
Limiting / density dependent factors / predation/food/water availability/disease;
…will become increasingly limiting, reducing growth rate;
…stabilizing population at around carrying capacity
ie negative feedback …a change leading to reduction of further change;

Award [3 max] if responses do not clearly identify +ve and -ve feedback.

[7 max]

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 demographic transition model, birth/death/growth rates, FR, DT, NIR, pre-industrial/expanding/stationary/declining stages of transition, age pyramids, phases of exponential growth curves, limits to growth, carrying capacity, agricultural/industrial revolutions, pro/anti-natal policies, urbanisation, economic development, lifestyle choices, Malthus/Boserup theories, technocentric/cornucopian views, medical advancements etc;
• breadth in addressing and linking earlier stages of demographic transition/population growth with technological developments in eg agriculture, industry, medicine, social welfare, family planning, globalisation, addressing a range of limits to growth eg food, shelter, disease, wealth, energy, in MEDCs, LEDCs, and linking later stages with limits of lifestyle choices, economic independence, population policies, historical events, international collaboration, etc;
• examples of populations in expanding stages overcoming limits to growth through technology eg fertilisers, pesticides, agricultural mechanisation, fossil fuels, antibiotics/antiseptics/sanitation, immunisation, contraception, urban technology, alternative energies etc and populations in stationary/declining stages being limited by family costs, anti-natal policies, gender equality, urban/industrial lifestyles etc;
• balanced analysis of the extent to which technological developments have overcome or failed to overcome limits to growth in a diversity in human populations in various stages of demographic transition, etc;
• a conclusion that is consistent with, and supported by, analysis and examples given eg “it is true that technological developments have played a large role in overcoming limits to population growth in the earlier stages of demographic transition; however, there is a finite limit to the Earth’s carrying capacity, and many populations are now stabilising due to self-imposed economic and political constraints”;

[9]

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

Great majority scored some credit, usually for addressing depletion of plants and outcompeting other herbivores … but few went on to consider further impacts of these phenomena and so were limited to two marks.

Many candidates scored some credit here, though usually rather precariously through identifying some limiting factor or describing predator–prey relationships, but rarely with a sound understanding of the role of positive and negative feedback in population growth curves.

Many candidates could identify ways in which medical/agricultural technology are significant in overcoming limits to growth during early development but few acknowledged opposing influences later on, or related their response very directly to all stages of the DTM.