Invasive
species commonly modify native communities of plants and animals, sometimes in drastic
and unexpected ways. Through competition, habitat destruction, and predation,
vertebrate invasive species are a major threat to communities across the globe.
One way invasive species impact communities is through altering the ecosystems’
feeding structure. Increased predation by the invasive on a particular group of
prey times, termed trophic level, can modify the number of species and their
relative population sizes in ways that ripple through the entire community.
Ecologists call such rippling effects a trophic cascade. Consider the
simplified food web:
If a new top predator begins to feed on the 3rd
trophic level predator, the original top predator’s population would decrease. In
response, the mid-level predator would experience decreased levels of predation
and its population would increase. This increased mid-level predator population
would then intensify predation on the prey species, whose population would
experience a decline. Such patterns describe a trophic cascade; these changing
community compositions often accompany the invasion of new species.
Understanding trophic level interactions and trophic cascades is essential for
understanding the intricacies and connections within biological communities
increasingly altered by human actions, including the introduction of new
species.
In “Cats
protecting birds: modeling the mesopredator release effect,” Frank Courchamp,
Michel Langlais, and George Sugihara (1999) investigate the effects of
introducing new species to island communities by modeling a particular type of
trophic cascade, mesopredator release. Invasive species particularly threaten
endemic island natives, species found in only one island locale, because these
isolated species are highly susceptible to extinction due to lack of genetic
diversity or lack of new immigrants to resupply a declining population. Both
invasive rats and feral cats have been unintentially introduced by humans to a
number of islands across the globe. Rats and cats both prey on birds; rats
commonly steal eggs and kill juveniles while cats kill adults. A number of
rare, endemic songbirds and seabirds have gone extinct due to predation by both
or either invasive species. Courchamp et
al. (1999) studied the interactions between the three species through
trophic cascades. In this system cats serve as the top trophic level predator,
rats as the mid-level predator, and birds as the prey species. The major
modification to the previously introduced trophic structure is that the cats
prey on both rats, the mid-level predator, and birds, the prey species.
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Figure 2.
Trophic level interactions between cats, rats and birds, the system studied by
Courchamp et al. (1999). In this
figure, the smaller arrows indicate predation. Cats, the top predator, prey on
both rats, the mid-level predator, and birds, the prey species.
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The mid-level predator is commonly called the mesopredator;
trophic cascades within this system can lead to mesopredator release. If the
top predator’s population declines or is removed, the mesopredator’s population
is “released” from predation and intensifies its predation on the prey species.
Mesopredator release, then, is a trophic cascade that results in decreasing
prey populations due to the decline of the top predator. In this system,
mesopredator release would cause the decline of songbirds if cats were removed
from the system. If cats, though, are not removed from the system and prey more
commonly on rats, the mesopredator’s population should decline. In this way,
higher populations of cats indirectly help to increase bird populations by
suppressing rat populations.
Courchamp
et al. (1999) created a mathematical
model to investigate whether mesopredator release could occur in island
communities containing cats, rats, and birds. Their model indicates that a
mesopredator release effect would be likely if cats were removed; in other
words, they found that rats and birds could not coexist without the presence of
cats because rats would hunt birds to extinction. The authors suggest that the
high probability of trophic cascades demonstrates the necessity of understanding
the indirect effects of species’ interaction. Furthermore, their findings
suggest that the top predator of systems often have extremely important roles
in maintaining the stability of biological communities. Courchamp et al. (1999) explain that these
findings should caution conservation plans that eradicate cats to protect bird
populations because such plans may actually cause further decline of bird
species through increased predation by rats.
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Courchamp
et al. (1999)’s results indicate that
ecological conservation must understand the complex interactions of
communities. Invasive species, though, can greatly modify such interactions,
causing trophic cascades that damage the stability of the community. The ability to
model trophic interactions is an important tool for conservation biology
because such mathematical models generate predictions that help to minimize the
negative impacts of trophic cascades. Courchamp et al. (1999)’s research focused on island populations, but the
trophic interactions among birds, rodents, and cats undoubtedly occur in urban
areas as well. Urban yards are an important habitat for many songbirds; both
cats and rodents are common invasives that threaten bird populations in urban
and suburban areas. Therefore, expanding Courchamp et al. (1999)’s investigation to urban areas would be beneficial to songbird
conservation efforts. Finally, while mathematical models are quite useful,
investigating trophic level interactions such as mesopredator release through
fieldwork is important for testing mathematical predictions against natural
systems. While difficult, such field studies are essential to increase
knowledge of the effects of invading species on communities.
References:
Courchamp, F.,
M. Langlais, and
G. Sugihara. 1999. Cats protecting birds: Modelling the mesopredator
release effect. Journal of Animal Ecology 68:
282-292.
