BP#3 : Rush Hour

Rush Hour: Fighting the Clock to Protect Against Invasive Species in the City

As was hopefully elucidated in the previous blog posts, plants face myriad problems due to urbanization. Urban areas tend to have rather poor conditions for plants, such as poor soil quality; in addition to these conditions, plants must also compete with other plants that want to gain a footing in the city (given the limited area of exposed soil). A major problem for not only plants in a city, but also other city organisms, is the threat of invasion. Due to humans, urban areas are inundated with invasive species (Kowarik 2011) (See Figure 1; Hulme 2009). 

Figure 1: a figure of annual increases of new species of mammals, invertebrates, and plants in Europe. 

This is likely due to the exotic pet trade, shipping of items, inadvertent planting of non-natives in gardens, et cetera. Imagine a basket of fruit that is about to be shipped overseas; it is not a big stretch in thought to also imagine some other organism feeding on said fruit, and then being shipped internationally. Once unloaded, the insect or whatnot can just ditch the fruit and begin its life in a new ‘world’. If this new traveler can find a mate and produce offspring, they can greatly rise in numbers in this new place, given they can acclimate to the change in climate. The problem with this is that they often outcompete native species, due to a lack of natural predators in the new land, a lack of limitations to feeding or breeding, and so on. Superficially, this shows an increase in species richness in cities (See Figure 2; McKinney 2006); a high species richness is seemingly a good thing, but it is often at the expense of native species. 

Figure 2: this figure shows the differences between species richness in US National Parks and in eight major US cities.

So, while species richness may be higher at the level of any particular city, global species richness is declining as natives are becoming extinct (Sax & Gaines 2003). Another symptom of urbanization is called biotic homogenization. Globally, due to species being shipped abroad and propagating in places other than where they have evolved, flora and fauna are starting to look eerily similar, and this homogenization is in large part due to urbanization (McKinney 2006). The days of exploring another continent and taking in the sights of exotic and unknown plants, fungi, and animals may soon be coming to an end, unfortunately. As globalization and urbanization increases, places once so different from one another will likely continue to look more similar. This is terrifying, as an aspiring scientist. Part of the thrills of being a scientist is discovering something new, or seeing something completely foreign to what one is used to. If this trend continues, the world will seem a lot duller, in my opinion. It is an unfortunate byproduct of living in a global society; so, explore as much as you can, and as quickly as you can!

Invasive species cause a lot of damage, and they can often adversely affect native species—not entirely through competition for resources alone, but also through unsustainable predation. For instance, the lionfish introduced to Floridian waters has no natural predators, allowing populations to soar. They are also ferocious predators; due to this, they are having a very large, negative impact on native fish populations off of Florida’s coast. But I am not here to discuss animals. My main focus is on plants. Plants don’t necessarily predate on other organisms (except, perhaps, the carnivorous plants), but they do compete with other plants for light, space, nutrients, water, etc. Unfortunately, research has shown that invasive plants often outcompete native plants when their niches overlap. Invasive species frequently displace native ones, and when they are relatively closely related, hybridization can often lead to displacement by another means (Huxel 1998).This may seem counterintuitive; one may think, “Hey, wouldn’t the native species be better adapted to a place that it has been evolving in for (potentially) millennia?” (this, of course, is contingent on environmental differences). Well, in many cases, this holds true. But in other cases, this doesn’t appear to give the native much of an edge. The reasons are likely numerous, but the more obvious ones are the fact that invasive species evolved elsewhere, and they evolved methods of dealing with predation from their natural predators. Alongside them, their predators have been evolving to counteract those measures. In another area, the natural predator of an invasive may be completely absent. Their methods of dealing with predation will still be available, however, and this may give them the competitive edge against the native species. This predicament is seemingly amplified in urban areas. Airing on the side of caution, so as not to overstep our conclusions, it is important to bring up an apparent bias: it may only seem that invasive species do better because so-called ‘super invasive’ species gain the bulk of the attention (Daehler 2003) (one such example is Kudzu, a U.S. invasive from Japan, which is scary abundant in the southeastern U.S., including Tennessee).

One final bit before we wrap this up, it is also important to note another type of plant-plant interaction that is not competition: facilitation. Plants are also known to facilitate other plants, and in one incredibly interesting circumstance, a native plant was not competing with an invasive weed, rather it was transferring nutrients over to it (Carey et al. 2004)! If that isn’t contrary to common sense, I don’t know what is. Why would this plant help out an invasive weed that has the potential to outcompete it? Furthermore, how is this even possible? Well, the easy answer to the second question is this: most plants form symbiotic relationships with certain organisms, and one such relationship is with fungi (mycorrhizal relationships). Mycorrhizae have the ability to transfer nutrients from plant to plant, via their expansive systems of hyphae. A possible answer to the first question is that they do this to decrease competition. Normally, plants closely related to one another use a lot of the same resources, and those more distantly related use different resources. Plants may have the capacity to preferentially transfer more nutrients to distantly related species via their shared mycorrhizal partner. This effectively allows the more distantly related plant to have an edge over the one that is more closely related to the plant dishing out these nutrients. This ensures that the distantly related plant is more prevalent, and the more closely related one (the one that would otherwise compete with the plant of focus) less abundant. This is a known, albeit fairly underrepresented, area of study in the scientific literature.

Finally we must conclude on a somewhat optimistic note. Invasive species are a challenge that we must face, especially in city centers, where they are most abundant. Many modes of dealing with this issue, such as using Kudzu as a means of producing biofuel (Sage et al. 2009), have started to rise from the shadows. Other more practical methods exist, too. For instance, if you garden, try to plant species endemic to the area in which you live, rather than planting exotics from elsewhere. Also, if you own a pet that is considered exotic, do not release it into the wild; in the wild, it has a chance at breeding, so populations could emerge and cause issues. These methods are surely needed: invasive species not only harm native flora and fauna, but they also cause billions of dollars in damages. If we are to protect the species that we love, and if we are to ensure that they are still here for our great grandchildren to also love, more research needs to be done. This sort of research is paramount to coming up with solutions to the problems that we as a global community face.

Works Cited

Carey, Eileen V., Marilyn J. Marler, and Ragan M. Callaway. "Mycorrhizae transfer carbon from a native grass to an invasive weed: evidence from stable isotopes and physiology." Plant Ecology 172.1 (2004): 133-141.

Daehler, Curtis C. "Performance comparisons of co-occurring native and alien invasive plants: implications for conservation and restoration." Annual Review of Ecology, Evolution, and Systematics 34.1 (2003): 183-211.

Hulme, Philip E. "Trade, transport and trouble: managing invasive species pathways in an era of globalization." Journal of applied ecology 46.1 (2009): 10-18.

Huxel, Gary R. "Rapid displacement of native species by invasive species: effects of hybridization." Biological conservation 89.2 (1999): 143-152.

Kowarik, Ingo. "Novel urban ecosystems, biodiversity, and conservation." Environmental Pollution 159.8 (2011): 1974-1983.

McKinney, Michael L. "Urbanization as a major cause of biotic homogenization." Biological conservation 127.3 (2006): 247-260.

Sage, Rowan F., et al. "Kudzu [Pueraria montana (Lour.) Merr. Variety lobata]: A new source of carbohydrate for bioethanol production." Biomass and bioenergy 33.1 (2009): 57-61.

Sax, Dov F., and Steven D. Gaines. "Species diversity: from global decreases to local increases." Trends in Ecology & Evolution 18.11 (2003): 561-566.