a predator or alarm substances produced by conspecifics.
Other behavioral responses include hiding (e.g.,
burrowing by fiddler crabs or burying in the bottom by
flatfish). Many species of fish form schools of large numbers moving together. This safety-in-numbers approach
has been found to be protective.
Many species utilize camouflage as protection. Many
fish use countershading or counterillumination to reduce
their visibility from both above and below. Some fish
resemble a rock or seaweed, and decorator crabs put
pieces of algae on top of their shells.
The mimic octopus can rapidly change its pattern and
color to resemble a seaweed, fish, or rock and thus provide
an excellent defense.
If the animal has been detected by a predator, there are
additional techniques that may be employed, most obviously running or swimming away. Many animals have
highly developed senses of sight, smell, and hearing so
that they can detect danger and escape. However, escape
has rather low odds of being successful, since the predator
is probably larger and faster. This tactic is useful, however,
when the predator is not too close and/or there is a refuge
nearby. Another technique is to form a partnership
(symbiosis) by joining with another species that is well
defended. For example, some crabs carry sea anemones
or sea urchins around, and clown fish (“Nemo”) associate
with sea anemones. Once an animal has failed at avoiding
a predator or warding it off at a distance, self-defense may
be utilized. Crabs and lobsters can use their claws to fight
off predators. Puffer fish swell up and become larger when
threatened. Morphological adaptations to reduce predation include having spines (e.g., sea urchins) that deter
a predator or having a toxin, e.g., jellyfish and sea anemones with stinging cells on their tentacles. Some animals
make use of signals, for example, bright warning coloration, advertising that they are poisonous.
If the animal has already been captured by a predator,
there may still be some ways to survive. Mechanical
defenses, such as armor and spines, can prevent access
to softer edible parts. Many crustaceans have a reflex
called “autotomy.” A crab being picked up by a bird, for
example, can break off its legs at a preformed breakage
plane, thus leaving the predator with only a leg or claw
while the crab scuttles away on its remaining legs. It can
subsequently regenerate the lost appendages. Sea cucumbers under threat can eviscerate, ejecting part or all of their
digestive tract at the predator.
Evolutionary aspects and plasticity
Predation can be an important evolutionary force since
natural selection favors more effective predators and more
evasive prey. “Arms races” have been recorded in some
species. These need not require genetic selection over
many generations, but can occur within an individual animal. These changes can be behavioral or morphological
and are referred to as phenotypic plasticity. For example,
periwinkle snails (Littorina littorea) in the presence of
cues from their predators (green crabs Carcinus maenas)
develop thicker shells, while the green crabs develop
thicker claws with greater crushing power (Trussell and
Smith, 2000; Smith, 2004). Mussels (Mytilus edulis) at
sites with high levels of predation by green crabs developed thicker shells and greater shell mass and were more
tightly attached (with byssus threads) to the substrate than
mussels at sites with low predation. When mussels from
low predation sites were moved to high predation sites,
they produced more byssus threads and attached more
firmly to the substrate. In the laboratory, mussels exposed
to waterborne cues (odors) of the green crab grew thicker
shells that were mechanically stronger and could better
resist crushing (Leonard et al., 1999). However, mussels
are also prey to sea stars, which use a totally different
mode of attack. Instead of crushing the shell, these predators pry the two valves apart, using steady pressure. In
order to defend against this type of predator, the bivalve
needs to strengthen its adductor muscle, rather than
thicken its shell (Freeman, 2007). Thus, induced defenses
in mussels are specific to predators with different predation strategies.
“Bottom-up” and “top-down” effects
Populations are regulated by the amount of available
nutrients. Thus, when nutrients (or prey) are high,
populations of predators can thrive and grow. This is called
“bottom-up regulation.” However, populations can also be
regulated by their predators, which is termed “top-down”
control. Predation is a mechanism of population regulation.
When the number of predators is scarce, the numbers of the
prey species should rise because fewer of them are getting
eaten. When there are more prey individuals around, the
predators will have more to eat and would then be able to
reproduce more and increase their population size. As the
predator species increases, the numbers of prey decline.
This results in food scarcity for predators, which will again
have a decline in their population. Predation can have profound and complex effects on communities. The sea star,
Pisaster, is the top predator in intertidal rocks in the Pacific
Northwest. The rest of the community includes mollusks,
barnacles, and other invertebrates. When sea stars were
removed from an experimental area (Paine, 1974), barnacles and mussels began to occupy most of the available
space on the rocks, outcompeting other species. The sea star
was a keystone predator in this community, keeping the
strongest competitors (barnacles and mussels) in check.
Although it was a predator, the sea star helped to maintain
a greater number of species in the community. Its beneficial
impact on species that would otherwise be outcompeted is
an example of an indirect effect.
A common scenario for observing food web effects is
the invasion of a new species into an ecosystem. When
nonnative species (exotics) invade an area, they often create “domino” effects, causing many other species to
increase or decrease. Grosholz (2005) noted that as the
green crab (Carcinus maenas) invasion in California
498
PREDATOR–PREY RELATIONSHIPS
Other behavioral responses include hiding (e.g.,
burrowing by fiddler crabs or burying in the bottom by
flatfish). Many species of fish form schools of large numbers moving together. This safety-in-numbers approach
has been found to be protective.
Many species utilize camouflage as protection. Many
fish use countershading or counterillumination to reduce
their visibility from both above and below. Some fish
resemble a rock or seaweed, and decorator crabs put
pieces of algae on top of their shells.
The mimic octopus can rapidly change its pattern and
color to resemble a seaweed, fish, or rock and thus provide
an excellent defense.
If the animal has been detected by a predator, there are
additional techniques that may be employed, most obviously running or swimming away. Many animals have
highly developed senses of sight, smell, and hearing so
that they can detect danger and escape. However, escape
has rather low odds of being successful, since the predator
is probably larger and faster. This tactic is useful, however,
when the predator is not too close and/or there is a refuge
nearby. Another technique is to form a partnership
(symbiosis) by joining with another species that is well
defended. For example, some crabs carry sea anemones
or sea urchins around, and clown fish (“Nemo”) associate
with sea anemones. Once an animal has failed at avoiding
a predator or warding it off at a distance, self-defense may
be utilized. Crabs and lobsters can use their claws to fight
off predators. Puffer fish swell up and become larger when
threatened. Morphological adaptations to reduce predation include having spines (e.g., sea urchins) that deter
a predator or having a toxin, e.g., jellyfish and sea anemones with stinging cells on their tentacles. Some animals
make use of signals, for example, bright warning coloration, advertising that they are poisonous.
If the animal has already been captured by a predator,
there may still be some ways to survive. Mechanical
defenses, such as armor and spines, can prevent access
to softer edible parts. Many crustaceans have a reflex
called “autotomy.” A crab being picked up by a bird, for
example, can break off its legs at a preformed breakage
plane, thus leaving the predator with only a leg or claw
while the crab scuttles away on its remaining legs. It can
subsequently regenerate the lost appendages. Sea cucumbers under threat can eviscerate, ejecting part or all of their
digestive tract at the predator.
Evolutionary aspects and plasticity
Predation can be an important evolutionary force since
natural selection favors more effective predators and more
evasive prey. “Arms races” have been recorded in some
species. These need not require genetic selection over
many generations, but can occur within an individual animal. These changes can be behavioral or morphological
and are referred to as phenotypic plasticity. For example,
periwinkle snails (Littorina littorea) in the presence of
cues from their predators (green crabs Carcinus maenas)
develop thicker shells, while the green crabs develop
thicker claws with greater crushing power (Trussell and
Smith, 2000; Smith, 2004). Mussels (Mytilus edulis) at
sites with high levels of predation by green crabs developed thicker shells and greater shell mass and were more
tightly attached (with byssus threads) to the substrate than
mussels at sites with low predation. When mussels from
low predation sites were moved to high predation sites,
they produced more byssus threads and attached more
firmly to the substrate. In the laboratory, mussels exposed
to waterborne cues (odors) of the green crab grew thicker
shells that were mechanically stronger and could better
resist crushing (Leonard et al., 1999). However, mussels
are also prey to sea stars, which use a totally different
mode of attack. Instead of crushing the shell, these predators pry the two valves apart, using steady pressure. In
order to defend against this type of predator, the bivalve
needs to strengthen its adductor muscle, rather than
thicken its shell (Freeman, 2007). Thus, induced defenses
in mussels are specific to predators with different predation strategies.
“Bottom-up” and “top-down” effects
Populations are regulated by the amount of available
nutrients. Thus, when nutrients (or prey) are high,
populations of predators can thrive and grow. This is called
“bottom-up regulation.” However, populations can also be
regulated by their predators, which is termed “top-down”
control. Predation is a mechanism of population regulation.
When the number of predators is scarce, the numbers of the
prey species should rise because fewer of them are getting
eaten. When there are more prey individuals around, the
predators will have more to eat and would then be able to
reproduce more and increase their population size. As the
predator species increases, the numbers of prey decline.
This results in food scarcity for predators, which will again
have a decline in their population. Predation can have profound and complex effects on communities. The sea star,
Pisaster, is the top predator in intertidal rocks in the Pacific
Northwest. The rest of the community includes mollusks,
barnacles, and other invertebrates. When sea stars were
removed from an experimental area (Paine, 1974), barnacles and mussels began to occupy most of the available
space on the rocks, outcompeting other species. The sea star
was a keystone predator in this community, keeping the
strongest competitors (barnacles and mussels) in check.
Although it was a predator, the sea star helped to maintain
a greater number of species in the community. Its beneficial
impact on species that would otherwise be outcompeted is
an example of an indirect effect.
A common scenario for observing food web effects is
the invasion of a new species into an ecosystem. When
nonnative species (exotics) invade an area, they often create “domino” effects, causing many other species to
increase or decrease. Grosholz (2005) noted that as the
green crab (Carcinus maenas) invasion in California
498
PREDATOR–PREY RELATIONSHIPS