BP #4: Urban Soil

Urban Soil: Like a New York City Apartment, It Is Cruddy and Hard to come by

Finally we reach the end. This will be the final bit of fun we have for these topics, and it will end with dirt—just like everything eventually does, anyway. Well, not dirt exclusively, but any type of soil: sand, silt, clay, et cetera. Plants are entirely reliant on soils (well, most are, anyway; and that depends on your definition of ‘plant’), because they are the substrate in which plants are grown. Not only do plants rely on soils for life, but they also rely on the composition of those soils, such as what type of soil it is, what nutrients the soil contains, what other organisms live in those soils, and much more. Going even farther, plants have a large impact on the soils in which they are grown, too; they can even change the chemistry of soils (Khalid et al. 2007; Angers & Caron 1998). Soils in urban areas are rather poor: nutrient poor and poor in that they are normally heavily contaminated. Another thing regarding soils and plants in the city is that most of the available space in cities are dedicated to humans, whether that space is used as roads, sidewalks, buildings, etc. So plants are working hard to fit into these spaces that they can occupy; but once they’ve done that, they must contend with soils that are terrible for growing anything other than trash piles. It is true: city life is tough. But plants seem to be ever so resilient in dealing with these issues. I mean, they have been around for millions of years, and they have suffered through terrible mass extinctions; not much should destroy them completely—well, we hope that we won’t destroy them, at least.

‘So, what about urban soil is so bad?’, one might ask. It is a reasonable question, and there are tons of answers. First off, city soils tend to be incredibly dirty—I say this tongue-in-cheek, because that is obvious; I mean, it isn’t called dirt for no reason—or polluted, rather. Most urban soils are covered with pollution, whether it be plastic, paper, or what have you. This much is obvious, but one thing that is not as obvious is that they contain heavy metal pollutants such as lead, copper, cadmium, cobalt, zinc, etc. (Wilcke et al. 1998; Jim et al. 1998). Many of these metals, such as mercury and lead, are incredibly poisonous to many animals, and they are also not good for plants (Patra & Sharma 2000). These pollutants likely originate from industrial waste, leaded gas, oil from cars, and so on, but trash and heavy metals are not the only pollutants; there is also agricultural pollution from pesticides, sewage from agriculture and even humans, and so much more. If you think about it, urban soils are really quite disgusting. But that isn’t even all of the story. Adding to pollution, urban soils are rough and coarse: they are littered with stones, brick fragments, and building debris (Jim 1998). Do you think that’s the whole of it? Surely there can’t be more things wrong with urban soils, can there? Well, unfortunately there is. In addition to all of the pollution and the coarseness, they also have terrible infiltration rates due to compaction of the soil (Gregory et al. 2006). What does this mean? It means that water cannot easily penetrate the soil due to the fact that it has lost its permeability from being compact. This leads to less water seeping into the soil, and thus less water for plants to take in. I hope this has driven one idea into your head: city soils suck (See Figure 1 for some of the typical characteristics of urban soil and Figure 2 for a picture of some urban soil with different pollutants highlighted). I mean, they really suck. They are just (seemingly) unusable for anything other than as a receptacle for even more garbage—well, that and more space for even more buildings that we apparently need.

Figure 1: This shows a few of the typical characteristics of urban soils.

Figure 2: this shows some urban soil with different pollutants in it.

So, one might wonder how these soils affect plant growth. Well, it really depends on the plant; they are incredibly variable in their tolerances to all types of soils and levels of pollution (I mean, some plants live in sandy deserts; if that fact isn’t telling, I don’t know what is): some do really well in urban soils, and some fair incredibly poorly (Sainz et al. 1998), and this all depends on the type of plant and the soil components—and the symbionts of the plants, of course. Generally, though, it seems that plants do not enjoy urban soils, but some have no choice—think city parks where populations of plants and animals become separated from their original population due to increased urbanization. Despite all of this, some plants can and do thrive in these terrible soils; many fast-growing plants alter polluted soils to the point that they help in soil remediation by sequestering metals (McIntyre 2003), absorbing them, or translocating them (Krumins et al. 2015). This has been a proposed, and executed, method of remediating soils in a cost-effective manner, and it could be used heavily in the future to rid urban soils of many pollutants. Plants are truly astounding, aren’t they? To be able to adapt to the most hostile conditions is truly awe-inspiring. It makes you ask, aloud, “How did they do it?!?”, as if they are magicians putting on a street show. 

So we have learned throughout these past four blogs that plants have to contend with some pretty tough conditions that we impose on them due to our continuing urbanization. Some of these conditions are a byproduct of the materials we utilize in urban areas, such as asphalt and concrete contributing to the urban heat island effect or the materials that leak into the soil as product of industry. And some conditions are because of how cities affect other organisms that pants utilize for pollination, reproduction, and the uptake of nutrients. And some conditions even come from other plants, such as the introduction of invasive species through importation, leading to global biotic homogenization. We, as contributors to these problems, must take responsibility for our actions; we must work with one another to come up with solutions to these problems and to combat the ill effects of urbanization. Some problems may never be solved, but there are many others that can be, and through research and implementation of the knowledge gained from said research, we can indeed help the group of organisms to which we owe our lives.

Works Cited

Angers, Denis A., and Jean Caron. "Plant-induced changes in soil structure: processes and feedbacks." Plant-induced soil changes: processes and feedbacks. Springer Netherlands, 1998. 55-72.

Gregory, Justin H., et al. "Effect of urban soil compaction on infiltration rate." Journal of soil and water conservation 61.3 (2006): 117-124.

Jim, C. Y. "Urban soil characteristics and limitations for landscape planting in Hong Kong." Landscape and Urban Planning 40.4 (1998): 235-249.

Khalid, M., N. Soleman, and D. L. Jones. "Grassland plants affect dissolved organic carbon and nitrogen dynamics in soil." Soil Biology and Biochemistry 39.1 (2007): 378-381.

Krumins, Jennifer Adams, Nina M. Goodey, and Frank Gallagher. "Plant–soil interactions in metal contaminated soils." Soil Biology and Biochemistry 80 (2015): 224-231.

McIntyre, Terry. "Phytoremediation of heavy metals from soils." Phytoremediation. Springer Berlin Heidelberg, 2003. 97-123.

Patra, Manomita, and Archana Sharma. "Mercury toxicity in plants." The Botanical Review 66.3 (2000): 379-422.

Sainz, M. J., M. T. Taboada-Castro, and A. Vilarino. "Growth, mineral nutrition and mycorrhizal colonization of red clover and cucumber plants grown in a soil amended with composted urban wastes." Plant and soil 205.1 (1998): 85-92.

Wilcke, Wolfgang, et al. "Urban soil contamination in Bangkok: heavy metal and aluminium partitioning in topsoils." Geoderma 86.3 (1998): 211-228.

Figure Credits

Figure 1 Credit: https://www.slideshare.net/watershedprotection/planting-trees-in-urban-areas-presentation

Figure 2 Credit: https://www.soils.org/discover-soils/story/studying-urban-soil-processes-natural-laboratory-setting

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.

BP#2 : Vital Interactions, of the Urban Sort

Vital Interactions, of the Urban Sort

Urbanization is of increasing interest to ecologists globally. With most people living in urban areas—at least in the industrialized world—there is a lot of area covered by concrete, asphalt, garbage, and buildings. Even in urban places plants have managed to squeeze into the areas where there is still exposed soil. Life in the city is difficult and stressful, for humans and plants alike, but many plants are able to thrive alongside of us; and like the resilient New Yorker struggling to find an apartment in the Big Apple’s unconscionably-priced real estate market, plants are paying the price of city living, too.

There are many factors affecting a plant’s success in an urban area, and they are many of the same factors that affect plant success in rural areas as well: reproduction, herbivory, and nutrient uptake aided by interactions with fungi (mycorrhizal interactions) and bacteria. These factors in urban areas, when compared to rural areas, tend to deviate from the norm to varying degrees, depending on the degree to which an area is urbanized, the amount of habitat fragmentation, and the abundance of both herbivores and pollinators.

Most plant species found on land are angiosperms, which are plants that produce flowers and seeds, and includes the grasses, most trees, shrubs, and other herbaceous plants. Most angiosperms reproduce via pollination by other organisms, and many even rely on other organisms for seed dispersal, too. So, as you may have gathered, most plants rely heavily on other organisms in order to reproduce and create viable progeny. Examples for this include bees dispersing pollen after gathering nectar, thereby inadvertently lending to a plant’s reproduction; and later on an embryo (the seed) develops inside of a fruit, and that fruit is eventually eaten by a passing deer, the seeds inside being dispersed in a spot that is already pre-fertilized—isn’t nature interesting?! Anyway, this process is vital to the continuity of most plants, but how different is this beautiful process in a city, where pollinators may be picky, seed dispersers may be few, and soil space may be limited? One major pollinator for these plants is the humble bumble bee (genus: Bombus—isn’t that an adorable genus name?). It is known that bumble bee diversity and abundance decreases as urbanization increases (Ahrne et al. 2009), and that decrease likely has an effect on an urban plant’s reproductive success. These decreases are likely due to habitat fragmentation (Cane et al. 2006).  This is unfortunate, but there is some hope for plants: visitation frequency of plants by pollinators increases with larger green spaces (see Figure 1) (Hennig & Ghazoul 2011)—so, that could be in the form of large urban gardens, large parks, etc. Pollination inhibition isn’t the only thing holding plants back, though; seed dispersal is an important process necessary for reproductive success, but it is a process that is compromised. Once a seed is formed by fertilization, the seed needs to make it toward a suitable area in order to germinate. In cities, a large portion of the ground is covered by concrete and asphalt, so there aren’t too many areas where plant seeds can germinate properly—in fact, seeds dispersed in urban areas have a 55 per cent lower chance of settling into a suitable area (Cheptou et al. 2008). Fortunately, humans can contribute to these reproductive efforts. It has been suggested that humans can intentionally disperse seeds in urban areas in order to help plants that are having trouble doing it by other means. This process, though expensive, has been undertaken in a few areas, and most notably in Stockholm, Sweden (Hougner et al. 2006), and the economic benefit could far outweigh the initial costs.

Figure 1: the important thing to note here is that pollinating visitors tend to increase in frequency as green area increases in size, suggesting larger parks/gardens could increase pollination effectiveness in urban areas, thus increasing plant fitness (Hennig & Ghazoul 2011).

Herbivory is also important to a plant’s success. Both invertebrates (such as beetles, grasshoppers, and insect larvae) and vertebrates (such as deer, humans, and birds) use plants as a means of attaining energy. When an organism eats bits of a plant, whether it be leaves or seeds, it may have a negative impact on a plant’s fitness. Consider this: when an organism feeds on a plant’s leaves, it has an impact on a plant’s photosynthetic capacity. In short, a plant needs sunlight to produce its food; in most cases, the leaf is the organ in which this production occurs, and the more leaf area available to photosynthesize, the more food a plant will produce. Now, if a plant’s leaves are being eaten, it severely reduces that plant’s ability to photosynthesize, in turn limiting its food production and ultimately its fitness. A similar thing happens when a plant’s seeds are being eaten, though this affects its ability to reproduce. So, how does this relate to urbanization? Well, research has been conducted in an attempt to answer these questions, specifically, ‘How does herbivory differ between rural and urban areas?’ It turns out that herbivory tends to increase in urban areas, at least as far as the research has shown. Christie and Hochuli (2005) show that trees in urban sites have higher degrees of leaf damage due to herbivory than trees of the same species in forests. One explanation outlined in their research could be the fact that insectivorous birds are negatively impacted by the habitat fragmentation that often occurs due to urbanization (in spite of this, bird densities in urban areas are rather high; although, most of these birds feed on seeds, rather than insects). This would naturally lead to fewer birds and more insects that would have otherwise been eliminated due to predation. Fewer birds lead to more insects which leads to increases in herbivory. Lucky for the insects, huh?

The last factor that we will discuss is the relationship plants have with other organisms that help them to obtain important nutrients, such as nitrogen and phosphorus. As many of us know, most plants form symbiotic relationships with different organisms, such as fungi and other microorganisms, in order for them to obtain nutrients that would otherwise be difficult if not impossible to incorporate into their bodies. In most cases, these other organisms provide the plant with some limiting resource in exchange for carbohydrates. Plants in urban areas need these interactions just like plants elsewhere, but soil is particularly poor in and around larger cities; they are contaminated with heavy metals (Wilcke et al. 1998) and are relatively poor in nutrients. How will these organisms do in heavily polluted, low nutrient soils, and how will this affect the plants with whom they form relationships? It turn out that, generally, mycorrhizal colonization of plants is lower in urban areas compared to rural ones (see Figure 2) (Bainard et al. 2011); the reason(s) why this is so are not yet clear, but soil quality likely plays a role. With lower levels of mycorrhizal colonization, plants are likely to suffer at some level when it comes to nutrient uptake, and this likely limits their growth and/or fitness. More research needs to be done in order to understand how exactly this will impact plants, and it has been proposed by Cousins et al. (2003) that plant primary productivity could be affected.

Figure 2: shows the per cent colonization of mycorrhizae in both urban and rural areas (Bainard et al. 2011).

Urbanization will continue to grow in the following decades, bringing with it many issues for the organisms we share this planet with. It is our duty, as the main cause of these issues, to help mitigate the ill effects we inflict on the natural world, as well as to conserve the species that fill our existence with inexplicable wonder. It is up to us to insure that these species are not swept under the rug—or paved over, for that matter—but rather emboldened; we need the research, we need the innovation that will solve practical issues that would benefit not only our lives, but also the lives of plants and animals. Plants especially need our support, for they provide the bulk of our caloric needs, and they are those that are often most affected by urbanization. Whether we create larger green spaces in which they can reproduce easier, limit herbivory by reintroducing insectivorous birds to urban areas, or cleaning up soil pollution to foster better symbiotic interactions among plants and fungi, the answers can only be found—problems only solved—once people begin to realize their impact on the environment and respect other organisms.

Works Cited

Bainard, Luke D., John N. Klironomos, and Andrew M. Gordon. "The mycorrhizal status and colonization of 26 tree species growing in urban and rural environments." Mycorrhiza 21.2 (2011): 91-96.

Cane, James H., et al. "Complex responses within a desert bee guild (Hymenoptera: Apiformes) to urban habitat fragmentation." Ecological Applications 16.2 (2006): 632-644.

Cheptou, P-O., et al. "Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta." Proceedings of the National Academy of Sciences 105.10 (2008): 3796-3799.

Christie, Fiona, and Dieter Hochuli. "Elevated levels of herbivory in urban landscapes: are declines in tree health more than an edge effect?" Ecology and Society 10.1 (2005).

Cousins, Jamaica R., et al. "Preliminary assessment of arbuscular mycorrhizal fungal diversity and community structure in an urban ecosystem." Mycorrhiza 13.6 (2003): 319-326.

Hennig, Ernest Ireneusz, and Jaboury Ghazoul. "Plant–pollinator interactions within the urban environment." Perspectives in Plant Ecology, Evolution and Systematics 13.2 (2011): 137-150.

Hougner, Cajsa, Johan Colding, and Tore Söderqvist. "Economic valuation of a seed dispersal service in the Stockholm National Urban Park, Sweden." Ecological Economics 59.3 (2006): 364-374.

Wilcke, Wolfgang, et al. "Urban soil contamination in Bangkok: heavy metal and aluminium partitioning in topsoils." Geoderma 86.3 (1998): 211-228.

Big City Botany, Blog #1

Big City Botany

Plant resilience and adjusting to urban life

           

As of 2008, over half of the world’s population lives in cities. Never before in the history of our species has this occurred, because the bulk of the world’s population has lived in rural areas, farming, raising cattle, and providing for their immediate needs. Now, though, this trend of urbanization is only going to increase in the coming decades.

As more people move into cities, and as cities spread farther out and build higher up, they encroach on the territories of plants, fungi, and other (non-human) animals. Along with this encroachment comes a new, ‘hot’ area of study: urban ecology. When one envisions ecological study, one often pictures scientists—pens and notebooks in hand—trudging through a densely-packed, dimly-lit jungle looking for samples to collect in order to gather data relevant to their respective academic interests. One does not imagine a scientist scouring through concrete jungles, such as New York City or Shanghai. However, this is (more or less) what urban ecologists do. Because cities continue to spill into the areas in their periphery, many organisms are eradicated as construction destroys their homes and food supplies; however, many either become trapped in little pockets of nature (such as in parks) or adjust to living in and around areas heavily modified by humans—and some species even thrive in these environments (think of the New York City subway rat or pigeon).

Often somewhat overlooked are plants in these areas, since the biological sciences are largely animal-biased; admittedly, I am in the same boat, as my interest is in urban Herpetology, the study of reptiles and amphibians in urban environments. Plants can be some of the most resilient organisms on the planet, however, and their importance cannot be understated: they, along with other photosynthetic organisms, are responsible for the continuity of animal life on earth, and we (along with all other animals) would surely die without them. A testament to their resilience in this day and age is their ability to adapt to heavily modified environments as humans continue to wreak havoc on the natural world. Plants must conquer a variety of issues that are associated with living in urban areas, from dealing with life in the shadows of skyscrapers to finding enough nutrients in soils that are often fairly infertile and heavily polluted—among other issues. And yet, cities are still fairly inundated with plants, whether they be in a large park, or lining the sides of streets. So, what strategies have plants come up with in order to deal with these sorts of situations? What have they done to adapt to urbanization? These are big questions, and there are many answers; let’s talk about a few.

For one, the species of flora in a city are not exclusively the ones native to that area. Humans directly or indirectly introducing non-native species to areas has caused some serious problems, because non-native species can oftentimes out-compete native species for resources. This appears to be a trend in urban areas, too. Many studies have found that, while biodiversity has decreased in urban areas relative to rural areas, species richness tends to increase in cities (McKinney 2008; Pautasso 2007). Urban and suburban areas have also been central to the spread of non-native species (Duguay et al. 2007). See the figure below this section for a bit of data on the trend showing non-natives becoming more predominant in cities (Godefroid 2001). Without going into much detail—as the focus of this blog entry will be on plants and how they have adapted to urban environments—it is important to note that, while species richness appears to increase, we must not take that as a sign that native plants are doing particularly well in the urban environment; rather, the increase in non-natives is offsetting the losses of the natives (Dolan et al. 2011).

Figure 1: shows the change in numbers of native and alien species in Brussels, Belgium from two different time periods (Godefroid 2001).

An interesting phenomenon that occurs in cities is something called the ‘urban heat island effect’: when a city is much warmer than its surrounding areas due to human influence. It is caused by the replacement of soil and plants with concrete, asphalt, and other materials used for roads, buildings, sidewalks, and so forth. These human-built structures absorb light energy, rather than reflecting it, which causes large cites to heat up to a higher temperature than the surrounding rural areas. Plants must deal with these higher temperatures if they are to adjust to living in urban areas. One aspect of a plant’s life can be greatly affected by this increase in heat, and that is its phenology, or the timing of an organism’s life cycle events—such as the timing of when a tree flowers or when a tadpole goes through metamorphosis. In the case of some plants, the heat island effect seems to have worked in their favor in the form of extended growing seasons due in large part to increased air temperature (Menzel & Fabian 1999). This increase in temperature allows some plants to start growing earlier and to continue growing when they would normally halt growth due to decreasing temperatures as the seasons progressed (Lu et al. 2006; Neil et al. 2010). This is one unexpected example of how urbanization, at least by increasing ambient temperature, has actually helped some plant species. Along with this comes some not-so-obvious consequences, such as the relatively innocuous consequence of extended allergy seasons (Neil & Wu 2006), but there are likely others that have not been brought into the light just yet. One possible solution to the issue of the urban heat island effect is implementation of so-called ‘green roofs’. These are roofs that have been covered completely or partially by plants and soil in order to create green gaps in an otherwise grey, concrete-dominated jungle of rooftops. Another way is to simply paint surfaces in white or other colors that are lighter; this will aid in reflecting more sunlight.

Another problem that plants must face is soil pollution. Soils in heavily-urbanized areas are poor and heavily polluted by heavy metals (Möller et al. 2005; Wilcke et al. 1998); they are also more hydrophobic—they tend to collect water on their surface, rather than absorbing it—and acidic than rural soils (Pouyat et al. 1995). Plants normally need nutrient-rich soils to grow properly, and they are often severely limited by these nutrients. Many plants also rely on nitrogen-fixing bacteria in order to attain nitrogen, which is not only incredibly limited to plants, but also essential in the production of proteins and chlorophyll. Soils in urban areas are very poor in these limiting nutrients, so it is difficult to imagine plants that are doing exceedingly well in these areas, despite the environmental pollutants that they are exposed to. However, many plants can and do exist in these soils. Surprisingly, we see the emergence of a recurring theme: non-native species, more so than their native counterparts, tend to be more tolerant of these more harsh conditions (Godefroid 2001). It is not yet clear how exactly plants have adapted to living in such unfriendly conditions, but this is definitely an interesting area of research. The need for conservation in the coming decades will make this sort of research incredibly valuable in helping to preserve the native species that inhabit cities—those of which may be unable to adapt fast enough to changes brought about by anthropogenic climate change and continuing urbanization.

In order to become a more informed citizen in the coming years, as science denial permeates the political and social realm, it is a good idea to understand the impact we have on the species with whom we share this planet. They, like us, require homes and resources, and our increasing need for land, as we continue to increase our numbers and as we continue to become more urbanized, will ultimately compromise their livelihood. The most pragmatic solutions will need to be implemented in order to alleviate the harm we cause this planet. Since the bulk of our discussion started with our leafy cousins, it is prudent to end the discussion with them, too. That being said, plants face many challenges as we continue to push them to their limits in the urban jungle; challenges like invasive species, the urban heat island effect, and poor soil conditions will likely continue to grow in severity if nothing is done. And these are, by no means, the only challenges plants face—more are included in the figure below, which shows changes in conditions on a rural-urban gradient (Johnson et al. 2015). But innovative people are continuing to fight these issues by implementing their ideas and becoming more aware of the problems faced by organisms other than ourselves.

Figure 2: shows biotic and abiotic changes on a rural to urban gradient (Johnson et al. 2015).

Works Cited

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