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.

https://c2.staticflickr.com/4/3212/4562537127_3247924143.jpg

                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.

Since when are leaves magnetized?

A close up of U. carpinifolia leaves

https://www.portlandoregon.gov/parks/article/479621?

       Since when are leave                        magnetized?
                         
                         By: Andrea Semlow


       Air pollution is a prominent concern in urban areas around the world. Dry regions in particular have higher particulate matter air pollution in high traffic areas. These high traffic areas are usually areas near streets with high vehicle density that kick-up dust containing heavy metals from brake systems and industrial facilities. These tiny airborne particles are highly hazardous because they can lead to lethal respiratory and cardiovascular diseases. Due to these pressing concerns, researchers have been searching for inexpensive, rapid air pollution monitoring techniques.  Over the past twenty years, studies on tree leaves' ability to proxy as a bio-monitoring systems have resulted in a variety of new techniques. Tree leaves in urban and roadside areas have been proven to be good accumulators of atmosphere dusts. Recent studies have even shown that along with dust particles, leaves also accumulate magnetic particles. So leaves are magnetized, so what?

A zoomed in photo of a leaf surface 

with arrows indicating magnetic particles.

http://www.academia.edu/7334220

/BIOMAGENTIC_MONITORING_OF_AIR_POLLUTION_USING_DUST_

PARTICLES_OF_URBAN_TREE_LEAVES_AT_UPPER_EGYPT

            Mohammad Mehdi Sadeghian conducted a study in 2011 that utilized this magnetized quality of leaves to find out the quantity and quality of vegetation in different areas of Isfahan, Iran, specifically looking at the magnetic properties of Elm (Ulmus carpinifolia) leaves. His goal was to determine if magnetic material on leaves could be a possible proxy to monitor regional distribution of air particulate matter (PM) pollution. A close up of the magnetic particles on a leaf surface is depicted on the right. The magnetic particles associated with atmospheric particles arise from domestic heating, vehicle exhaust and brake systems, and industrial facilities (Sadeghian et al 2011).This method is non-destructive, inexpensive, easily detected, and rapid. Sadeghian collected from four spaces, a park, square, street, and control station, in order to determine urban landscape areas with the highest density of air pollutants. This method is unique because he has accounted for the leaf surface composition change due to rain as well as water evaporation (Sadeghian et al 2011). Leaves from each site were collected and a magnetometer, an instrument used for measuring magnetic forces, measured magnetized susceptibility with a pulsed field (Sadeghian et al 2011). The study found the highest concentration of magnetic properties were trees found in squares and streets. Streets had the highest concentration of particles most likely because high traffic roads re-suspend road dust resulting in higher magnetization of the U. carpinifolia leaves (Sadeghian et al 2011).

            

        Future use of this method will hopefully allow for cities to develop less expensive monitoring systems; however, it appears this particularly approach would be quite labor intensive. My potential future project would be in a similar vein utilizing tree leaves as a bio-monitoring system, only a little closer to home in Phoenix, Arizona. If you have never heard of a COTS monitoring system, it stands for commercial off the shelf monitoring system. Basically a miniaturized version of the really expensive air pollution monitor currently in place in urban areas around the world. These devices will allow for more localized air pollutant detection, increasing our knowledge of on the ground conditions in cities. This will be extremely beneficial to updating the urban landscapes of not only Phoenix, but numerous dry urban areas.

View article at: http://www.academicjournals.org/article/article1380792211_Sadeghian.pdf

Reference: Sedeghian, M.M. (2011) “Biomonitoring of particulate matter by magnetic properties of Ulmus carpinifolia leaves” African Journal of Biotechnology 11 (73): 13827-13830.

            

The Disturbing Invasion of Sweet Clover

            Rocky Mountain National Park was established in 1915. Situated in Colorado, its main goal has been to preserve areas for the enjoyment of people. However, adding people to any environment tends to create disturbances. These disturbances, caused in part by the addition of roads, trails, and human activity, are called anthropogenic disturbances. Recent research has illuminated connections between the colonization of exotic species and these human disturbances, yet the impacts of these invasions on the native ecosystem are hard to quantify. Thus, Wolf et al (2003) investigate the impacts of sweet clover (Melilotus officinalis and Melilotus alba) invasion on the montane grasslands, specifically how invasive patches differ in species richness and community structure from control patches.

Images of Melilotus officinalis (left) and Melilotus alba (right) which are invasive species under examination in Wolf et al’s (2003) research. Image courtesy of http://ohioplants.org/wp-content/uploads/2012/04/Melilotus-officinalis-and-alba.jpg

            For their experiment, Wolf et al (2003) sampled areas that had been invaded by sweet clover, measuring each invaded patch’s total size. These patches were paired with an adjacent patch of comparable size that retained the native community (no invasives). Within these patches, the researches established plots to estimate the species cover and composition, noting also the percent cover of bare ground. Additional plots were placed along the edges of the invaded patches to look at the edge effects.

           

The researchers found several interesting relationships. First, total species richness varied as a function of time. Yet native species richness varied over the growing season while the exotic species richness remained the same. Second, invaded patches had a higher percentage of cover attributed to forbs where as the native patches had a higher percentage of grass cover. Third, more perennials were present in the control plots. The invaded plots had a higher richness of annual and biennial species (Figure 1). With respect to the edge experiments, Wolf et al found a gradient, with many of the aspects characterizing invaded plots concentrated at the center and diffusing outwards. Sweet clover spread 0.8m from the original patch boundaries in 1998 and 1.8m in 1999.

Figure 1: Summary of Wolf et al (2003) results from experiments comparing control patches to sweet clover invaded patches in Rocky Mountain National Park.

            The findings of Wolf et al signify the potential for sweet clover invasion to change the community composition of native systems. These differences in species richness and composition may be due to life history characteristics of sweet clover or inherent competitive advantages in comparison to native species. These different interactions, introduced by sweet clover colonization, may induce community composition shifts. The fact that patches invaded by sweet clover were dominated by exotic species is frightening, as it may indicate that invasion fragments existing native communities. Therefore, the invasion of an exotic species may constitute a different type of anthropogenic disturbance. As land stewards plan conservation projects, understanding invasions as a type of anthropogenic disturbance will be important in ensuring any project succeeds in its goals.

Read the full article:

Wolf, J.J., Beatty, S.W., and Carey, G. (2003). Invasion by Sweet Clover ( Melilotus ) in Montane Grasslands, Rocky Mountain National Park. Ann. Assoc. Am. Geogr. 93, 531–543.