Cats Helping Birds: Introduced Species Impact Trophic Cascades

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 3^rd 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.

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.

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.