Parasites and host communities
Parasites have the potential to have large effects
on their host communities; they can
- determine the competitive balance
between two species, whether one
species can invade or coexist with
another;
- change the flow of energy through
and relative balance of different
trophic levels;
- act as "ecosystem engineers"
to change the environment in which
other organisms live;
- have cascading effects on entire
communities.
Indirect interactions
Most of what we've talked about so far have
been direct interactions, where
(e.g.) parasites change the fecundity and
mortality of their hosts, leading to
population cycles.
In indirect interactions, the
direct (-/+) interaction between parasites
and one host leads to a change in the interaction
between two hosts, or between one host
and another species in the community.
These interactions can be density-mediated
(the parasite changes the population density
of the target host, benefiting the second species
indirectly) or trait-mediated (the
parasite changes the behavior of its host,
which somehow hurts or helps another species).
For example,
here are the direct effects between deer, moose,
and parasite populations:
In contrast, here are the indirect interactions
(width of the arrow indicates strength of
the interaction):
Here is an example for a parasite that
changes the behavior of its host to encourage
trophic transmission:
Figuring the costs and benefits of parasitism:
distinction between individual-level and population-level
effects
Parasite-mediated coexistence
- Drosophila melanogaster, D. simulans,
L. boulardi (exclusion by melanogaster in absence of
boulardi; coexistence with boulardi; exclusion by simulans
at lower temperature with boulardi)
- Tribolium castaneum, T. confusum,
Adelina tribolii
Parasite-mediated invasion
- human movement: Europeans to the New World,
Europeans to Africa
- introduced parasites: e.g.
Acipenser stellatus (from Caspian to Aral Sea),
carried Nitzchia sturionis (gill monogenean),
reduced populations of A. nudiventris
Parasite-mediated resistance to invasion
Parelaphostrongylus tenuis (meningeal worm):
kills moose (Alces alces) and caribou (Rangifer
tarandus) in clinical infections (by causing brain pathology), but doesn't kill
white-tailed deer.
(moose density is inversely correlated with density of
P. tenuis eggs in deer feces)
P. tenuis has a two-host life cycle, from gastropods which are
eaten accidentally by grazing ungulates and back again (via excreted
eggs which hatch into larvae and bore into the gastropods when they
crawl over the larvae).
In the absence of the worm, moose can outcompete white-tailed deer for forage.
It has been assumed that P. tenuis has caused or assisted the
rise of deer and the decline of moose in the southern boreal forest,
and that the existence of deer and P. tenuis would prevent the
reintroduction of moose. Where moose were seen to persist in the
presence of deer, it was assumed that some kind of habitat separation
was occurring.
Schmidt and Nudds built a model very much like the macroparasite
model we looked at in class; the only difference is that the model has
two possible definitive hosts, moose and deer (all the intermediate
gastropod hosts are lumped into one big box), which compete by
Lotka-Volterra competition in the absence of the parasite.
P. tenuis is assumed to kill moose but have no effect on deer.
The authors then look at the results of the model for different
parameter values to see what the possibilities are.
They are not as clean-cut as the classical dogma would have it; it
looks like (depending on parameters that we don't know), moose could
outcompete deer, be outcompeted by deer, or coexist even in the
presence of deer and P. tenuis.
Major points of the paper are that:
Example: killifish and cestodes (Lafferty and Morris)
trophic cascades (APPARENT mutualism)
Lafferty 1992: not a mutualism
at the individual level?
INDIRECT.
(depends on level of parasitism, costs, benefits)
(predator population size might be max. with no
parasites, but individual decisions maximize
individual fitness)
flow through food webs??
Example: ecosystem engineering
ECOSYSTEM ENGINEERS: cockles
Other examples
protection from other parasites:
cowbirds eat botflies?
concomitant immunity
(heterologous immunity == cross reaction)
Large-scale community structure
- trypanosomiasis keeps out livestock,
horses (and hence humans, or at least Europeans)
- Serengeti: rinderpest, ungulates, vegetation,
trypanosome interaction
- chestnut blight (hypovirulence, fungal superparasites)
- invasions