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Refer to one or more of the following readings from Gathering Moss to
answer the following questions. Gathering Moss readings are: In the
Forest of the Waterbear; Choices; Portrait of Splachnum.
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Provide 3 examples of non-competitive interspecific interactions that
are described in the moss readings. Identify the type of interaction
(e.g., commensalism? predation? parasitism?) and which organisms play
which roles in the interaction.
Lots of possible responses
here!
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How would you describe the niche of a moss? Does it vary for different
species of mosses?
This is most clearly covered
in the Portrait of Splachnum chapter; what follows is my response but
yours may vary! Mosses can live in very cold places and some very hot
places but only if there’s enough water (at least periodically). Many
species can survive drought periods in a dormant state and then perk up
again when water becomes available. They can’t compete with faster
growing angiosperms or gymnosperms but can grow in places with fewer
nutrients than most of those competitors. Some mosses are generalists,
growing in a wide variety of habitats and conditions while others are
specialists, such as the Splachnum ampullulaceum species described in
this chapter. Splachnum requires the dung of particular deer species in
order to germinate spores, having a preference for “animal nitrogen”.
This species then also requires the help of a fly to transport its
spores to new dung piles. So yes, the specifics of the niche do differ
for different moss species!
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In these readings from Gathering Moss, do you notice any connections to
other topics we’ve covered this semester?
Lots of possible responses
here!
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As described in the content video on Competition, in Gauss’ experiments
with competition in Paramecium, the same species always drove the other
to extinction. Why is this consistent result expected from competition
theory?
Each species has its own
specific resource needs and efficiency in acquiring and using those
resources. Along with that, that means each different species will have
its own carrying capacity (K) for a given environment, containing
particular amounts of the resources it needs. As a result, when two
species need the same resources but one of them is able to grow more on
the same amount of resource (higher resource-use efficiency), then we
expect that the more efficient user of resources (more individuals per
unit resource per unit time) will “win” in competition. As long as
conditions remain the same, we always expect this interaction to end the
same way. HOWEVER, if the amount of different resources is changed, that
could lead to a different outcome, if the most limited resource now
favors the other species. The basic underlying issue is that resources
are limited – the same atom of carbon or nitrogen cannot be used for two
different purposes at the same time.
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Consider the different effects of intraspecific versus interspecific
competition. Intraspecific competition occurs between members of the
same species and tends to result in increased niche breadth of the
species. In contrast, interspecific competition (between individuals of
different species) is more likely to lead to each competitor species
having narrower niches. Explain why intraspecific competition and
interspecific competition lead to opposite effects on a species’ niche
breadth.
Consider competition within
one species (intraspecific). In that case, everyone has extremely
similar resource needs so competition is likely to be strong between
individuals for any of the most important or most limited resources. As
a result, any individual who can use a resource (such as a different
food source) that cannot be used by everyone else in that species will
have an advantage. This means competition with others of your own
species favors broadening your niche, being able to use parts of the
environment with less competition.
Now, when we consider
competition between two species A and B (interspecific), each species
has its own resource needs and efficiency of using those resources. When
there are alternative ways to acquire resources within the niche, and
some parts of the niche overlap with that of species B, whichever
species has higher resource-use efficiency in the niche overlap space
will do better in that area. So, if species A only does better than
species B at temperatures above 75 deg F, then species A is likely to
dominate when the temperature is higher. This will favor species A
becoming even better above that temperature, narrowing its niche.
Similarly, species B is then at a disadvantage above 75 deg F and it
will be favorable for species B to use niche space below that
temperature, narrowing its niche.
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We think of invasive species as a human-facilitated phenomenon (and in
many cases that’s totally accurate), but is it really true that the
geographic range of a given species has been constant over time? That
is, surely natural species have expanded their ranges in the past, such
that range expansion is not inherently unnatural. After all, the species
that are considered native in this part of Ohio could not have lived
here 13,000 years ago when this place was covered with a mile of
ice—they must have moved here from elsewhere, without human
intervention. Given this, what do you think happens in a community
when a new species is introduced? Discuss what types of species
interactions might occur and how that might influence the ability of the
introduced species to persist in the new area. (You may wish to
focus on a particular example to help you illustrate these points.)
A newly arrived species is
likely to face competition but perhaps less likely to experience
predation/herbivory, parasitism, or mutualism.
When a new species (invasive
or not) arrives in a community, it is likely to experience competition
with some of the existing species. Interestingly, when a new species
first arrives any competition will be interspecies (with other species)
and not intraspecific (with itself) since there will not be many
individuals of the new species around. The existing species will not
have a history of interaction with the new species and so it’s unlikely
that the new species experiences mutualism or parasitism when it first
arrives (these interactions usually exist between species who have
coexisted for a long time). Similarly, the new species may not
experience much predation or herbivory if the consumers in the community
do not recognize it as a food resource. This might mean that the new
species, as long as it can compete for resources, can grow quickly in
its new community. However, if the new species required a specific
pollinator or a particular microbial partner that it did not bring with
it, that could mean the new species would NOT persist well in its new
community.
We can also consider not
just what happens to the new species (which might evolve rapidly in
response to the new conditions), but also what happens to the existing
community. Given that the balance of competition for resources may be
disrupted by the arrival of a new species, some native species may
experience decreases in survival and reproduction if they can no longer
acquire the same amount of important resources. Depending on the other
interactions that the native species engages in, this could have
cascading results for other taxa in the community, for example if a
hummingbird-pollinated plant began to decline, that might mean less food
for the hummingbird population, which might also then decline. Other
species might benefit from the newly introduced species if it does not
have much niche overlap but perhaps the introduced species competes with
its predator, thus when the predator numbers are reduced, the prey
species may increase in numbers – which of course could have cascading
effects on the food resource of the prey species!
The underlying point is that
the consequences of species introductions are typically not very
predictable and can have dramatic impacts on the native
community.