Human Impact on the environment question

I'm stuck on the following question below:

Some farmers in the developing countries of Southeast Asia use an agricultural system known as “shifting cultivation”. This involves felling and burning a plot of forest to provide ash in which to grow food crops. After one to three years, as weeds flourish and fertility declines, the plot is abandoned for a fallow period of about 20 years.
However recently, where the human population density is high, fallow periods have been reduced and food yields have dropped significantly. In some places the cultivation pattern has been replaced with permanent agriculture, such as rubber plantations.

‘Where the human population density is high, fallow periods have been reduced and food yields have dropped significantly.’
Explain why the food yields have decreased. (3 marks)

I wrote that there would be less time for regeneration so less material to burn and less fertile ash is available to grow the food crops. The mark-scheme, however, has something completely different:

Increased growth of crops (to meet increased demand for food) removes more minerals from soil(1)
Crops are removed so do not decompose to replace minerals in the soil(1)
So soil minerals are depleted/ soil becomes less fertile(1)

I don't really understand how increased crop growth is directly related to what is being asked in the question, which is fallow periods.

Please can someone explain the mark-scheme to me.

Thanks

The fallow period lets the region of land which is to be cultivated or has been cultivated to replenish, as you have stated. This replenishment brings back nutrients and minerals to the soil, allowing fertile topsoils to regenerate. During fallow, the soil will gain minerals such as nitrates or phosphates, which are used for growth. The longer the period, the stronger the replenishment. This results in more nutrients for crop growth, and more nutrients results in more crop growth, in turn increasing yield. This may or may not be true, but the longer the fallow, the more nitrogen can be fixed, which is why I made that point. As fallow has been reduced and more crops are being removed, they do not directly return to the site, which means that not only is there a lower time for replenishment, the intensive agriculture results in the minerals being removed from what you may consider the local nutrient cycle for the field: nutrients would cycle back and forth, i.e. from the ground, plant, animals/insects, decomposers, and so forth: they end up being taken away from that area, and aren't replenished: fallow usually allows this to happen naturally, and thus with it reduced, **** them sideways it's over.

That is an amazing explanation!

Could you also help me on this question:

A pharmaceutical company uses genetically modified bacteria to produce human insulin. It is important that they can monitor the numbers of bacteria in their culture vessels to ensure optimum production of the human protein.

The graph shows the growth of a colony of bacteria in the laboratory.


At each time interval, using aseptic technique, a sample was taken from the culture and serially diluted before counting the bacteria under a microscope.

In order to establish a new culture of these bacteria, a laboratory technician transferred a sample of the bacteria to a fresh culture medium. The technician decided to take the sample from phase B. Justify the decision to take the sample from this phase and not the other phases.


I wrote that in phase B the reproductive rate is greater than the death rate, so rapid growth of the colony. In phase C reproductive rate = death rate so growth rate slows and in phase D most cells are dead/ dying. Phase A can't be used as the growth rate is too slow as enzymes are still being synthesised and most cells are at interphase.

This is what the markscheme said:
In phase B (nearly) all cells are viable/ living(1)
Phase A : slow cell division would result in the slow production of a viable/ living culture(1)
Phase C : slow cell division/ many non-viable cells + Phase D: most cells are dying/ non-viable(1)

Surely reproductive ability is more important in establishing a new culture, rather than whether cells are living or not.

Thanks again

*Grabs cognac, slips on glasses*
Okay let's see here...

We need living cells to actually produce the secondary metabolite that yields the insulin. It's given off as a 'waste product' as far as I am aware. I agree with you completely on the idea that a new culture should be fast growing and the MS should have included this, as the converse seems to be emphasised for parts A,C and D.

The MS seems to be a little bit BS imo. Here's what I'd have put for the MS:

Cells in A are viable, however are slowly reproducing and hence would not produce a viable culture in a suitable period for culture vessels/industrial production.

Cells in B are adapted and quickly reproducing and hence will quickly reproduce in the new culture vessel, alongside being viable, and hence will produce the most secondary metabolite containing insulin, the desired product.

Cells in C aren't reproducing at a high rate in comparison to those in B and a growing proportion will not be viable due to the death rate being far more similar to the birth rate.

Cells in D are mostly unviable as death rate far exceeds the birth rate, leading to an unsuitable set of cells for culture.

Those are the answers that were given for a similar question on one of my papers for EDUQAS. In all seriousness, the fact that they didn't state that phase B had quickly reproducing cells but stated the converse for A,C, and even D, is totally preposterous.