Data and code from: Parasites alter host community structure in a natural experiment

Parasites can profoundly alter host communities. However, the impact of
parasites can vary from one community to another. Understanding why the
impact of parasites varies across communities is challenging because it
requires (i) separating the direct effects of the parasite on the host
species from the indirect effects it exerts through the ecological
interactions among the hosts, and (ii) identifying how the presence of one
host alters rates of infection in another. Freshwater fish communities in
Trinidad have recently been invaded by a generalist parasitic nematode.
This event, combined with our long-term mark-recapture studies of these
communities, which began long before the invasion, presented a replicated
natural experiment. In this experiment, we measured how host demographic
rates responded to two “treatments”: stage of parasite establishment
(before, and 1-2 years, 3-5 years, and 6-8 years after invasion) and type
of community (killifish-guppy communities and killifish-only communities).
This design allowed us to infer the direct and indirect impacts of the
parasite invasion on killifish communities and determine how the presence
of guppies altered infection rates in killifish. The parasitic invasion
drastically altered killifish-guppy communities: the ratio of killifish to
guppies changed from 1:2 before the invasion to 1:16 after the invasion.
Living with guppies amplified the effects of the parasite on killifish,
such that parasite-related mortality rates of large adult killifish were
twice as high in communities with guppies compared to those without. This
effect was driven by a doubling of infection rates in large killifish that
live with guppies. In a parallel study, we identified the same patterns of
size- and community-dependent infection rates of killifish in five
separate river systems, implying that this pattern is general in this
system. Our study provides mechanistic insight into how parasites alter
community structure under natural conditions, via their direct and
indirect impacts on host demographic rates. Our work highlights the value
of long-term field studies for our understanding of the impact of
parasites on community structure and of ecological interactions in
general.