(e.g., a protozoan living in the digestive tract of a
flea living on a dog). Social parasites involve
behaviors that benefit the parasite and harm the
host (e.g., cuckoo birds that use other birds to
raise their young). Hosts have evolved defense
mechanisms (e.g., immune systems or plant
toxins) to diminish parasitism whereas parasites
have evolved mechanisms to keep their hosts
alive.Predation Predators hunt and kill prey. Opportunistic
predators (e.g., humans, leopards, dogs, and
alligators) kill and eat almost anything whereas
specialist predators (e.g., some members of the
weasel family) only prey upon certain
organisms.Saprotrophism Saprotrophs obtain their nutrients from dead or
decaying plants or animals through absorption of
soluble organic compounds. Saprotrophs include
many fungi, bacteria, and protozoa. Vultures and
dung beetles are also saprotrophs.Keystone Species
A keystone species is a species whose very presence contributes to a diversity of
life and whose extinction would lead to the extinction of other forms of life.
Through various interactions, a small number of individuals from a keystone
species have a very large and disproportionate impact on how ecosystems
function. An ecosystem may experience a dramatic shift if a keystone species is
removed even though that keystone species was a small part of the ecosystem as
measured by biomass or productivity.
An example of a keystone species is the starfish, also known as a sea star.
Starfish prey on sea urchins, mussels, and other shellfish that have no other
natural predators. If starfish were removed from the ecosystem, mussel
populations would explode and drive out other species, and sea urchin
populations would rise to the point where they would begin to decimate coral
reefs.
Another example of a keystone species is the prairie dog that digs tunnels
that form prairie dog towns, which can be hundreds of acres in size. The “towns”