Parasitism & Disease

5.1 Direct and indirect effects

This figure shows two scenarios (1 and 2) for the impact of parasites on community processes.  Red arrows indicate key interactions; thick arrows show stronger interactions than thin arrows; direction of arrows show positive (+) or negative (-) direct effects.

For each scenario, explain how the relationship between Hosts A and B are affected by the parasite.

Source: Hatcher et al 2012.  Frontiers in Ecology & the Environment; 10: 186-194

Figure 1 redrawn with permission of John Wiley & Sons.

© The Ecological Society of America.

Model answer:

Parasites can have an important role in structuring communities but their impact has often been overlooked.  The figure shows 2 scenarios in which 2 host species (A and B) share a common parasite.  In scenario 1, they use different resources but in scenario 2, they share a resource.  In scenario 1, there is a strong positive direct effect between the numbers of Host A and the parasite.  This means that if numbers of Host A increase, so do the parasites.  There is a strong direct negative effect between numbers of parasites and Host B, so if parasites increase, Host B will decrease.  Thus Hosts A and B are linked indirectly through the parasite, even though they are unlikely to compete as they use different resources.  This is known as apparent competition.  In scenario 2, increased numbers of the parasite lead to reduced numbers of Host A, due the direct, strong, negative effect between them.  Decreases in Host A lead to some increases in the resource, although this isn’t a strong effect.  Increases in the resource result in an increase in Host B as there is a strong, direct, positive effect between the resource and Host B.  Thus the parasite regulates one host strongly which results in increases in the alternative host.  If Host A is a stronger competitor, this interaction would enable Host B to persist in this environment despite being competitively inferior to Host A.  This is known as parasite-mediated coexistence.

5.2 Trematode parasites in lakes

In lakes in New Zealand the trematode worm Microphallus sp. infects the snail Potamopyrgus antipodarum.  The parasite castrates male snails and sterilises females so it has a major impact on populations of snails.  The worm has a very short generation time in comparison with the snail.  Lively (1989) tested the ability of worms from different lakes to infect snails from other lakes.  The figure shows his results.  Explain what is happening and why.

Source: Lively. 1989. Evolution, 43: 1663-1671.

Figure reproduced with permission of John Wiley & Sons.

© Society for the Study of Evolution


5.3 Impact of parasite on coastal foodweb

The figure shows results from an experiment to investigate the impact of infection by the trematode worm Cryptocotyle lingua on the herbivore Littorina littorea (periwinkle).  The researchers set up containers of seawater to compare amounts of Ulva lactua (sea lettuce) eaten by  snails with a high infection (predicted by dark foot colour) and no infection (light foot colour) over 13 days.  An additional container held Ulva with no Littorina present (control).

What do the results in the bar chart tell us about the effect of trematode infection?

Source: Wood et al. 2007. PNAS.  104: 9335-9339

Copyright (2007) National Academy of Sciences.


Use the foodweb diagram to consider the implications of infection with Cryptocotyle lingua on the wider community of species found in these intertidal communities.

Source: Berness. 1984. Ecology 65, 370-381.


5.4 Great spotted cuckoos in Spain

The great spotted cuckoo (Clamator glandarius) is found across Africa and the Mediterranean Basin.  It is a brood parasite of crows.  The young cuckoos don’t remove host eggs from their nests but host fledglings often die from competition with their more aggressive nest-mates.  Young cuckoos are known to produce a smelly cloacal secretion when threatened by predators. Levels of predation can vary widely from year to year.

Why don’t host species evolve to recognise an reject cuckoo eggs and young?

Use the data in the table to explain what’s going on.

Parasitised nest Non-parasitised nest
Likelihood of surviving to hatching 75% 75%
Proportion of nests raising at least one crow fledgling 76% 53%
Mean number of fledgling crows/nest (in successful nests) 2.073 2.564

Source: Canestrari et al 2014.  Science, 343: 1350-1352

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