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.