The latest Parasitology Paper of the Month is “Consistent patterns of trophic niche specialization in host populations infected with a non-native copepod parasite” by J. Pegg, D. Andreou, C. F. Williams and J. R. Britton.

Differences in the ecology of individuals within animal populations are now receiving a great deal of research attention. It is increasingly recognised that the variability between individuals is as important as looking at population averages. In the context of the ‘trophic niche’ (the food resources consumed by a population), it is now recognised that the population niche often comprises of several smaller niches of specialised individuals. Despite a great deal of knowledge of how parasites affect food-web metrics [1,2], there has been relatively little consideration of how parasites affect population trophic niches and their specialised subsets [3]. This is surprising, given the strong influence that parasite infections can have on the host phenotypes, including how and for how long they search for food.

We started to address this in our field-based study. We selected the copepod Ergasilus briani as our model parasite. This parasite, alien to Britain, can result in chronic infections that can induce relatively subtle phenotypic alterations to host behaviour. We used five infected British fish populations across two species as the model hosts. In the host populations, E. briani feeds on gill tissues. We used stable isotope analyses to quantify the trophic niche of the fishes (the ‘isotopic niche’).

Through analysis of individuals with and without E. briani infections, we demonstrated that hosts of both fish species consistently had smaller niches than uninfected conspecifics. Importantly, this smaller niche of the infected fish sat within the larger niche of their uninfected conspecifics, indicating that the shift in host resource use was to a sub-set of prey utilised by the population overall. Thus, infection did not result in the exploitation of alternative resources. Although not explored in this paper, we initially hypothesise this is due to the energy costs of infection, with hosts specialising their foraging on less motile prey, whereas uninfected conspecifics remain more generalist.

An area of work we have yet to explore is the basis of this dietary shift, i.e. the physiological processes by which infection affects the energetics and foraging of hosts. It is this aspect of our work that we suggest has the potential to really illuminate the ecology of this host-parasite system. We showed a strong and consistent pattern of niche specialisation in our field data, but we now need to understand the processes driving this. For example, an alternative hypothesis is that the patterns of niche specialisation were driven by behavioural variability in the individuals that preceded infection, i.e. their diet was already specialised and this elevated their infection probability.

The next steps in this infection ecology research are the testing of the two alternative hypotheses that stem from our study. We suggest the way to do this is focusing on analysing patterns in field data with understanding processes from manipulative laboratory experiments, and by using a wider range of parasite-host systems.

Read the full article “Consistent patterns of trophic niche specialization in host populations infected with a non-native copepod parasite” in full for free until 31st May 2017.



  1. Britton JR (2013) Introduced parasites in food-webs: new species, shifting structures? Trends in Ecology & Evolution 28, 93-99
  2. Médoc, V., Firmat, C., Sheath, D. J., Pegg, J., Andreou, D., & Britton, J. R. (2017). Parasites and Biological Invasions: Predicting Ecological Alterations at Levels From Individual Hosts to Whole Networks. Advances in Ecological Research, 57, 1-54.
  3. Britton JR, Andreou D (2016) Parasitism as a driver of trophic niche specialisation. Trends in Parasitology 32, 437-444

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