Parasite-mediated interspecific interactions in a community of crustaceans

Abstract

Differential impacts of parasitism among sympatric species influence important interactions among hosts, both directly and indirectly, which can translate to impacts on communities and ecosystems. This thesis investigated the role and strength of parasite-mediation on interspecific interactions in a community of freshwater crustaceans. Specifically, I examined the impacts of the trematodes Coitocaecum parvum and Maritrema poulini, acanthocephalan Acanthocephalus galaxii, and nematode Hedruris spinigera, on four crustacean hosts: the amphipods, Paracalliope fluviatilis and Paracorophium excavatum, and isopods, Austridotea annectens and A. lacustris.

Direct effects of parasites on host survival, behaviour, and reproductive parameters were assessed in three host species. Hosts varied in their susceptibility and tolerance to parasites. Maritrema poulini and C. parvum had no effect on P. excavatum survival despite a high abundance, whereas they negatively affected P. fluviatilis survival. Behaviour of A. annectens was affected by M. poulini, with more heavily infected individuals being more active.

Then, the role of M. poulini in mediating microhabitat use by congeneric isopod species was tested in laboratory experiments. Austridotea annectens showed a clear preference for one microhabitat type regardless of competition or infection. Conversely, A. lacustris showed little habitat selection without competition. However, A. lacustris clearly favoured sandy habitats when competing with uninfected A. annectens and rocky habitats when competing with infected A. annectens. We also tested the role of M. poulini infection on predation risk of A. annectens hosts by a dragonfly nymph, a ‘dead-end’ host for the parasite. Isopods with higher parasite abundance were less likely to be caught by dragonfly predators.

To quantitatively test the role of parasites in regulating host populations and structuring communities, I used a mesocosm experiment with the four-host species and with or without exposure to M. poulini. This parasite was found to influence population dynamics of hosts in species-specific ways. In particular, the survival and recruitment of the amphipod P. fluviatilis were greatly reduced. As a consequence, the relative abundance of each species was also influenced by parasite exposure, thus demonstrating the role of parasitism in host community structuring.

Climate change is likely to substantially impact ecosystem structure and dynamics, yet our understanding of the complex interactions between multiple parasites and other abiotic factors remains limited. I set up a mesocosm experiment to examine the combined role of parasites and temperature on community structure. Parasite exposure and temperature had additive effects on community composition. Higher temperatures strongly impacted both species of amphipods. However, isopod hosts were less affected, suggesting that predicted temperature rise, and higher parasite exposure may increase the relative abundance of isopods in the community.

Overall, this thesis showed that variation in how parasites affect their different host species results in community-wide effects. One trematode species was shown to affect the distribution of another species, and mediate competition between these species, as well as alter the hosts’ susceptibility to predation. Variable exposure to the trematode M. poulini influenced population dynamics of these hosts. The combination of temperature and parasitism was found to have an independent and sometimes additive effect on the abundance of crustacean species. My results provide new evidence that, by influencing key interactions among hosts, both directly and indirectly, parasites can affect relative abundance of host and non-host species, ultimately altering community and ecosystem structuring.