Option C: Ecology and conservation (Core topics) :C.3 Impacts of humans on ecosystems SL Paper 3

Date	November 2017	Marks available	2	Reference code	17N.3.SL.TZ0.13
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	Compare and contrast	Question number	13	Adapted from	N/A

Question

The sea snail Nucella ostrina and the sea star Pisaster ochraceus are predators of the mussel Mytilus trossulus. The mussels live on rocks at the edge of the sea and feed on phytoplankton and zooplankton. The zooplankton feed on the phytoplankton.

Groups of 50 mussels were transplanted to an experimental area and protected from predation until the start of the experiment. Researchers then investigated the effect of the predators on the population of the mussels over a period of 60 days.

[Source: Republished with permission of John Wiley and Sons, from Navarrete, S. A. and Menge, B. A. (1996),
Keystone Predation and Interaction Strength: Interactive Effects of Predators on Their Main Prey. Ecological Monographs, 66: 409–429. doi:10.2307/2963488; permission conveyed through Copyright Clearance Center, Inc.]

Compare and contrast the effects of the predators on the population of the mussels.

[2]

The sea star also eats the sea snails. Construct a food web to show the feeding relationships between these five organisms in the ecosystem.

[2]

Markscheme

a. both cause the frequency of the mussel to decrease

b. sea star affects the mussel population more than the sea snail

c. when both are together the effect of the sea snail is low

Accept binomial names

Allow numerical answers if expressed as comparisons and the candidates are not simply stating numbers

[Max 2 Marks]

Award [2] for a correct food web

Award [1] for phytoplankton, zooplankton and mussel with correct arrows

Award [1] for mussel, sea snail and sea star with correct arrows

Award [0] if arrows are in wrong direction

Accept binomial or scientific names

Date	November 2017	Marks available	2	Reference code	17N.3.SL.TZ0.12
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	Explain	Question number	12	Adapted from	N/A

Question

Calculate the energy loss due to respiration in primary consumers in kJ m–2 y–1.

[1]

Outline why a year is more suitable than a month for the measurement of energy flow.

[1]

Explain how pesticides may undergo biomagnification in the lake.

[2]

Markscheme

190

accounts for different productivity at different times of year/seasonal variations

more data collected

to increase reliability

trends over time more easily detected

a. the increase in concentration of the pesticide at higher trophic levels

b. taken in by organisms low in the food chain

c. cannot be excreted so remains in tissues

accumulates as more organisms from lower levels are eaten

d. pesticides do not degrade/degrade very slowly

[Max 2 Marks]

Date	May 2017	Marks available	3	Reference code	17M.3.SL.TZ1.15
Level	Standard level	Paper	Paper 3	Time zone	Time zone 1
Command term	Evaluate	Question number	15	Adapted from	N/A

Question

After their introduction cane toads (Rhinella marina) have become a serious pest in many parts of Australia. In an attempt to control them scientists set traps to which they added toxins produced by native species of toad to capture cane toad tadpoles. The toad toxin attracts the cane toad tadpoles without killing them.

Outline one consequence of introducing an alien species into an ecosystem.

[2]

State the origin of cane toads.

[1]

Evaluate the use of traps containing toxin as a means of cane toad control.

[3]

Markscheme

a. uncontrolled increase of numbers «in alien species»
OR
become invasive
OR
have no «natural» predators

b. outcompetes native species / reduces biodiversity
OR
carries disease
OR
preys on local species decreasing population size
OR
disrupts food chains/webs

Central/South/Latin America

a. the baited traps catch a lot more tadpoles than the unbaited traps / traps with bait are more effective than those without

b. baited traps are almost 20 times more effective

c. traps without toxin reach capacity sooner than those with toxin

d. there may be environmental/health/safety problems with the toad toxin used

e. there is no information on how toads are controlled since the toxin does not kill the tadpoles

Date	November 2016	Marks available	4	Reference code	16N.3.SL.TZ0.14
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	Discuss	Question number	14	Adapted from	N/A

Question

In 1997 in South Africa, a decision was made to decrease the use of mosquito-killing pesticides due to their negative effect on the environment. Mosquitoes are known to be responsible for the spread of malaria. In 2001 the decision was reversed and the use of pesticides was increased. The graph shows the estimated numbers of people with malaria in each year.

Outline the trend in the number of people with malaria during the period when the use of pesticides was decreased in South Africa.

[1]

One pesticide used in killing mosquitoes was DDT. Considering its harmful effects, discuss whether the decision to reintroduce it was justified.

[4]

Markscheme

the number of people with malaria increased

a. choice has to be made between damage to environment or increase in malaria

b. DT may lead to biomagnification/bioaccumulation in food chains
OR
taken up by species in lower trophic levels becoming more concentrated at higher trophic levels

c. causes harm to consumers at end of food chain
OR
example «eg: thin egg shells of falcons»

d. DDT is shown to be effective in reducing malaria

e. possible partial solution to be selective in areas sprayed with DDT

f. may kill insects that are not pests

Date	May 2016	Marks available	4	Reference code	16M.3.SL.TZ0.16
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	Explain	Question number	16	Adapted from	N/A

Question

Explain how alien species can affect community structure in an ecosystem.

Markscheme

Species introduced into habitat/ecosystem

Disrupt food chains

Reduce the number of organism that occupy similar niches

They can overconsume prey species

Reduce availability of prey species for other consumers

They can overconsume a native predator

Leading to loss of control on numbers of prey species

Their impact will reduce the biodiversity

Can lead to extinction of some species

May have no natural predators/control

Date	May 2016	Marks available	2	Reference code	16M.3.SL.TZ0.15
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	Deduce	Question number	15	Adapted from	N/A

Question

The images show three predator–prey relationships.

Biomagnification of two groups of organic pollutants was investigated in three predator–prey relationships. BDEs and PCBs are broadly used in industry. The biomagnification factor is a ratio of the amount of pollutant in predator tissue compared to the amount of pollutant in prey tissue.

Outline how biomagnification occurs.

[2]

(i) Identify the predator with the least biomagnification of pollutants.

(ii) Suggest a reason for the species identified in (b)(i) having the lowest biomagnification factor.

[2]

Deduce two conclusions about PCBs that are supported by the data.

[2]

Markscheme

Toxin at lowest concentrations in organisms at lowest trophic level

Toxin concentration builds/is magnified in organisms at each successively higher trophic level

Toxins often fat-soluble
OR
can accumulate in body tissues

Toxin/chemical is not metabolized/excreted

(i) Fox

(ii) Unlike the other two predators, it is a mammal
OR
has other sources of food
OR
different biochemistry/metabolism

PCBs biomagnify in all three predator–prey relationships

PCBs biomagnify most in rodent–buzzard/least in rodent–fox relationship

Greatest range of PCB biomagnification occurs in rodent–buzzard

Biomagnification in birds is higher than in mammals

Date	May 2015	Marks available	2	Reference code	15M.3.SL.TZ2.21
Level	Standard level	Paper	Paper 3	Time zone	Time zone 2
Command term	Outline	Question number	21	Adapted from	N/A

Question

Outline one example of biological control of a named invasive species.

[2]

a .

Explain the cause and consequences of biomagnification

[4]

Markscheme

a. named example of invasive species;
b. named example of biological control;

possible examples:

a .

a. substances/pesticides/heavy metals accumulate up the food chain / at each trophic level;

b. substances cannot be metabolized/excreted;

c. these substances become concentrated in (fatty) tissues/internal organs;

d. magnified in organisms in upper part of chain as they eat more/larger organisms;

e. increase in concentration may become toxic in higher trophic levels;

f. example of a consequence of biomagnification e.g (DDT causes) thinning of egg shells (in birds);

Date	May 2015	Marks available	3	Reference code	15M.3.SL.TZ1.21
Level	Standard level	Paper	Paper 3	Time zone	Time zone 1
Command term	Explain	Question number	21	Adapted from	N/A

Question

Explain the causes and consequences of biomagnification of a named chemical.

[3]

Explain the concept of niche.

[2]

Markscheme

a. named example of chemical (for example: mercury, DDT, PCBs, TBT, PAHs,heavy metals, selenium);
b. long lived / do not biodegrade / stored in body tissues / fat soluble;
c. present in small concentration in the environment;
d. chemical becomes more concentrated in the bodies of organisms at each successive trophic level;
e. reach toxic levels in top consumer/organisms near the end of the food chain / example of top consumer affected;
Award [2 max] if no named example of chemical.

a. includes all aspects of way of life of a species/role of the species in the ecosystem;
b. includes relationships within the community;
c. feeding relationships;
d. interaction with environment/spatial habitat;

Date	November 2015	Marks available	1	Reference code	15N.3.SL.TZ0.21
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	State	Question number	21	Adapted from	N/A

Question

State the process where pesticides such as DDT become more concentrated at each trophic level.

[1]

Explain what is meant by the niche concept.

[3]

Markscheme

biomagnification/bioaccumulation

a. every organism in an ecosystem has their own role;

b. (includes) spatial habitat/space inhabited by organism;

c. (includes) feeding activities of organism;

d. (includes) interactions with other species;

e. valid description of an organism’s niche including habitat, feeding activities and interaction with other species;

Date	May 2013	Marks available	4	Reference code	13M.3.SL.TZ1.21
Level	Standard level	Paper	Paper 3	Time zone	Time zone 1
Command term	Explain	Question number	21	Adapted from	N/A

Question

Discuss the difficulties of classifying organisms into trophic levels.

[2]

Explain the cause and consequences of biomagnification, using a named example.

[4]

Markscheme

a. consumer may have more than one food source;
b. organisms eaten may be at different trophic levels;
c. may change their trophic level over time;
d. different stages in life cycle might exist in different trophic level (eg frog);
e. example of organism (presenting such a difficulty);

a. a process in which chemical substances become more concentrated at each trophic level;
b. valid named example (for example mercury, organophosphorous pesticides, DDT, TBT);
c. these substances cannot be broken down / are broken down slowly by metabolism;
d. are often stored in (adipose) tissues;
e. each organism consumes large quantities of the trophic level below it;
f. so substance accumulates/increases to reach toxic levels;
If no example is given award [3 max].

Date	May 2011	Marks available	3	Reference code	11M.3.SL.TZ1.20
Level	Standard level	Paper	Paper 3	Time zone	Time zone 1
Command term	Explain	Question number	20	Adapted from	N/A

Question

Define the terms fundamental niche and realized niche.

Fundamental niche: …………………………………………….. ……………………………………….

Realized niche: …………………………………………….. ……………………………………………..

[2]

Explain why the carnivores in an ecosystem tend to be fewer in number and have a smaller biomass than the herbivores in the same ecosystem.

[2]

Explain why carnivores tend to be more affected by biomagnification than organisms lower down the food chain.

[3]

Markscheme

Fundamental niche:
the potential niche / the niche the organism could occupy under ideal conditions / the full mode of existence given the adaptations of the species / OWTTE;

Realized niche:
the actual niche / the niche restricted by competition and environmental variables / the niche resulting from the limits placed on the species / OWTTE;
Responses must distinguish between the two types to gain credit.

energy transfer along the food chain is less than 100 % efficient;
10 % energy transfer between trophic levels;
nutrient transfer is less than 100 % efficient;
each carnivore needs to consume many prey organisms;
tendency for size of organisms to increase as trophic level increases;

mercury / DDT / other named example;
biomagnification is the accumulation of chemicals through the food chain;
chemicals that undergo biomagnification are stored/not broken down (in the bodies of the organisms that consume them);
chemicals are passed (unaltered) from one trophic level to the next;
chemicals become more concentrated in the bodies of each (subsequent) trophic level;
organisms higher up the food chain consume larger amounts of the chemical;

Date	May 2011	Marks available	2	Reference code	11M.3.SL.TZ1.21
Level	Standard level	Paper	Paper 3	Time zone	Time zone 1
Command term	State	Question number	21	Adapted from	N/A

Question

The introduction of alien species and the release of environmental pollutants are examples of human activities that have an impact on the environment.

Alien species may arrive in an ecosystem due to accidental or deliberate release. State the impact of one named example of deliberate release.

Markscheme

example e.g. Cane Toads/Bufo marinus;
effect e.g. predation of native invertebrates;

Other possible examples:

Accept other suitable examples. Accept common name or systematic name.

Google to check others.

Date	May 2012	Marks available	1	Reference code	12M.3.SL.TZ2.21
Level	Standard level	Paper	Paper 3	Time zone	Time zone 2
Command term	State	Question number	21	Adapted from	N/A

Question

Explain how living organisms can affect the abiotic environment during primary succession.

[3]

State one example of biological control of an invasive species.

Invasive species: ………………………………………………

Biological control: ………………………………………………

[1]

Define biomagnification.

[1]

Markscheme

remains/debris/litter from growth/death of plants can increase soil depth;
remains/debris/litter from growth/death of plants can increase soil mineral content;
remains/debris/litter from growth/death of plants can alter soil pH;
remains/debris/litter from growth/death of plants can improve soil water retention and reduce drainage;
growth of (larger) plants can reduce erosion though binding action of roots;

example of invasive species and an example of its biological control
e.g.:
prickly pear cactus is controlled by moth (Cactoblastis cactorum)
Accept other suitable example.

a process when chemical substances become more concentrated at each trophic level

Date	May 2010	Marks available	1	Reference code	10M.3.SL.TZ1.28
Level	Standard level	Paper	Paper 3	Time zone	Time zone 1
Command term	State	Question number	28	Adapted from	N/A

Question

State one example of the accidental release of an alien species that has had a significant impact on an ecosystem.

[1]

a .

Discuss the impact of alien species on ecosystems.

[3]

Markscheme

Answer needs to be specific giving name of organism, where it was released and where it came from. If unsure, check answer with Google, but examples must be accidental releases.

e.g. zebra mussel (Dreissena polymorpha) originally from Russia/Caspian carried in ships’ ballast water and introduced into Great Lakes / rats accidentally introduced to mainland of New Zealand from visiting ships / Africanized honey bees introduced to Brazil (“killer bees”)

a .

inter-specific competition/alien species have characteristics that may enable them to out-compete native species;
lack of predators may allow alien species to reproduce more rapidly;
alien species may utilize areas or resources that native species cannot;
predation by invasive species can cause loss of biodiversity;
can lead to species extinction, especially of endangered species;
alien species may introduce new diseases;
use of alien species for biological control can be ineffective or negative;

Date	November 2012	Marks available	2	Reference code	12N.3.SL.TZ0.20
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	Explain	Question number	20	Adapted from	N/A

Question

Explain the principal of competitive exclusion.

[2]

Using a named example, explain a consequence of biomagnification.

[2]

b (ii).

Markscheme

no two species can occupy the same niche/role in an ecosystem;
too much competition occurs until one is eliminated / one out-competes the other;
competition is for habitat/nutrition/relationships/space/limited resources;

name of chemical and top consumer affected; (both needed)
consequence;
eg:
DDT, affecting bird of prey;
fragile egg shells which did not hatch affecting population size;

b (ii).

Date	November 2010	Marks available	3	Reference code	10N.3.SL.TZ0.22
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	Discuss	Question number	22	Adapted from	N/A

Question

Discuss the impact of alien species on the environment.

Markscheme

a. (interspecific) competition with native species;

b. does not have natural predators so may survive more;

c. can be a predator difficult to control;

d. reproduce faster/more;

e. may cause the extinction of native species;

f. most are benign/some may be beneficial (e.g. honeybee introduced to the Americas from Europe in the 1600s);

Date	November 2009	Marks available	3	Reference code	09N.3.SL.TZ0.21
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	Discuss	Question number	21	Adapted from	N/A

Question

Discuss the impacts of a named alien species introduced as a biological control measure.

Markscheme

named alien species e.g. western mosquitofish / small Indian mongoose / rosy wolfsnail;

introduced for removal of pests;

competes with native species;

excessive predation on native species that do not have defences;

impacts industry such as tourism/harvesting of natural resources;

Accept other specific examples of effects.