Other Species: Predation, Competition, Parasitism
PREDATION
Predation can also limit the distribution of a species. The example below depicts research
performed on the mussel, Mytilus edelis. Mytilus are distributed
world-wide and are found on rocky coasts. They are good subjects for ecological studies
because as soon as they attach to a rock as a larval form, they remain attached to the
same rock for life. Thus, by mapping the distribution of mussels on rocks, you can follow
individuals over time. If an individual is missing you are confident that it didn't just
get up and move someplace else- it is dead. While mapping the distribution of mussels,
researchers noted that small mussels (<25 mm; light blue on the map) were found only on
the wave-torn rocky coasts, not in the calm inlets. Similarly, larger mussels (red on the
map) were confined to the exposed coast. If they moved a rock with mussels to the inlet,
and returned after several weeks, the old transplanted individuals survived (green), but
all the little ones were dead (white). If they moved a rock from the coast to the inlet
and then covered it in a wire cage to exclude potential predators, then both the young and
the old mussels survived, proving that the inlet abiotic factors were within the tolerance
limits for the young mussels. Finally, they transferred young mussels to the inlet and sat
down to watch what happened to them. It turned out that crabs were eating the mussels. The
crabs were not powerful enough to pull the older mussels from the rocks, so they only ate
the little ones. Additionally, the crabs were unable to hold on to the rocks along the
shore when waves hit, which explains why mussels weren't removed from the shoreline. Here
is an example of predation limiting the distribution of a species.
The snowshoe hare and Canadian lynx populations cycle together. The lynx feeds almost exclusively on the hares. Click here for data from 1845 to 1937. Note HERE, however! Lynx/Hare pix are HERE.
COMPETITION
The Tricolor Blackbird and Redwing Blackbird are closely related species with
similar ecological requirements. In the absence of Tricolors, the Redwing will chop up the
habitat into a series of territories as shown below in figure A. If Tricolors move in,
however, they are more demanding as to their territorial needs and will displace the
Redwings as shown in "B". Tricolors out-compete the Redwings and limit their
distribution. Other examples of competition displacing or limiting the distribution of a
species are shown below. These include an introduced plant (Kudzu) and the
competition between sage and other species in a mixed grassland.
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Redwing Blackbird |
Kudzu out-competes native plants in the south (YouTube)
The image shown above depicts competition between sage (on the left) and a mixed grassland. A 2-meter wide swath of bare ground surrounds the sage (A) while an even wider area where the growth of wild oat and bromegrass is inhibited is seen at "B". Full-growth mixed grassland is shown at "C". It is clear that the successful species is "using up" some resource in the environment that is in short supply. The sage is removing nutrients from the soil that are needed by the other grasses (the sage and zone A). In the B zone, the nutrients are depressed, but grass species other than wild oat and bromegrass are able to grow at low concentrations. The second image shows an aerial view of this competition.
PARASITISM
Tsetse flies (Glossinia) occupy 4 million square miles of Africa between 15° N
and 28° S. The tsetse fly is a vector for African Sleeping Sickness in humans and nagana
in domestic animals. Because of nagana, 1/4 of Africa can have no domestic animals other
than poultry.
American Chestnut (Castanea dentata)
