Energy Flow in Urban Ecosystems
As human civilization progressed throughout time, humans congregated in towns and villages. These gave rise to cities, and eventually megacities. This paper describes a megacitiy as an urban area that has more than 10 million inhabitants. Heavy urbanization can lead to a host of dramatic environmental changes. Environments can be physically altered by human development, land can become polluted, and air and water can be subject to increases in pollutants and toxic substances. This particular paper looked at how urban areas (specifically megacities) affect the energy flow in their respective ecosystem. They looked at the energy and material inputs and outputs in urban settings. Inputs were classified as stored inputs (construction materials for urban infrastructure and solid waste in landfills), transformed inputs (food, fuel, and water), or passive inputs (air, water, and solar radiation). Outputs were classified as atmospheric or aquatic and marine. Exported goods were not examined, as they are thought to compromise a very small portion of energy flow. The author noted that major cities are often located near a large water source. This makes sense, as a large population would have a tremendous demand for water for both consumption and general use. Aside from being located near a body of water, megacities are quite diverse in their physical surroundings. Urban ecosystems generally gain materials and energy as inputs through human action. Energy is stored in materials used to build structures (wood, stone, metals) and are used until they become no longer viable and are considered waste, which goes to landfills. Urban ecosystems also gain materials and energy passively through precipitation, gas flow, and solar radiation. Stored inputs in urban areas have a significant effect on the ecosystem. Trees must be cut down to provide wood for buildings. Metals are often mined and then processed to provide materials for structures. Both of these processes require energy and this often comes from fossil fuels. Burning fossils fuels provides energy for a multitude of human activities. However, this also leads to an output of carbon into the atmosphere in the form of carbon dioxide. This has detrimental effects on ecosystems, as carbon dioxide can trap radiated heat in the atmosphere, contributing to warmer temperatures and air pollution.
Decker, E. H., Elliott, S., Smith, F. A., Blake, D. R., & Rowland, F. S. (2000). ENERGY ANDMATERIALFLOWTHROUGH THEURBANECOSYSTEM. Annual Review of Energy and the Environment, 25(1), 685–740. doi:10.1146/annurev.energy.25.1.685
Element Cycling in Urban Ecosystems
Throughout the semester I have been investigating how urbanization alters ecosystems. In this blog post, I read an article that examined elemental fluxes in Quercus ilex, a species of evergreen tree. The research was done in an urban area in Naples. Leaf content was analyzed from trees from urban sites and compared with leaves from rural areas. The main elements examined were C, N, and S. However, several other micro-nutrients were also analyzed including Na, Fe, Cu, and Pb. It was thought that some of the nutrients would be found in increased amounts in urban areas due to various forms of pollutants.
The paper sought to investigate how humans have impacted urban ecosystems through pollution. Some of the main sources of urban pollution included industrial plants, power stations, domestic heating systems and motor vehicles. Most of the contaminants were believed to be airborne, and the authors wanted to examine how prolonged exposure to airborne pollutants affected this species of evergreen tree. They chose to use this species of tree because they hypothesized that organisms with large surface areas would accumulate particulates from the air at a relatively high rate. Naples was chosen as the area for urban research because it is heavily populated, has a very high vehicular traffic rate, and an abundance of large scale industrial plants. The authors hypothesized that nitrogen oxides, carbon monoxide, and other trace elements would be found at an increased rate in the urban areas. Leaves from both urban and rural settings were examined at various sites and collected in leaf litter bags for later analysis. Carbon levels in rural leaves were found to be higher than carbon levels in urban leaves. Nitrogen levels were substantially higher in urban leaves compared to rural leaves. Levels of metals such as Fe and Pb were much higher in leaves from urban ecosystems versus leaves from rural areas. This article emphasizes how important it is to study the impact that humans have on ecosystems, especially from pollution.
Alfani, A., Baldantoni, D., Maisto, G., Bartoli, G., & Santo, A. V. (2000). Temporal and spatial variation in C, N, S and trace element contents in the leaves of Quercus ilex within the urban area of Naples. Environmental Pollution, 109(1), 119-129. doi:10.1016/s0269-7491(99)00234-1
Urban Ecosystems and the Phosphorus Cycle
Throughout this semester I have been examining how humans affect their ecosystem, especially in urban settings. Urbanization can dramatically change many aspects of an ecosystem. The phosphorus cycle can be altered in urban settings, and this can lead to differences in phosphorus fluctuations in the ecosystem. This paper focused on the Greater Phoenix Metropolitan Area. Some human impacts considered include agriculture, urban infrastructure, soil composition, and water usage. It is important to understand how humans influence element cycling. Phosphorus is a key resource for living organisms, thus it is important to understand how to use this resource to ensure food security and maintain water quality. It is important to note that phosphorus is available in large amounts in the Earth’s crust, and therefore can be altered by human activities. Agriculture is one major area that impacts phosphorus levels. Most fertilizers contain phosphorus as well as nitrogen and other vital nutrients for primary producers.An increased human population leads to increased food demand. More fertilizer is required to increase crop yield, and in order to facilitate this need, phosphorus is often mined from the Earth. Increased fertilizer use can lead to an increase in nutrient runoff into local waterways. This is important to note, because watersheds often run into larger aquatic environments. This can lead to increased phosphorus levels in aquatic ecosystems. The increased nutrient load may not seem like a big deal, but an overabundance of nutrients in aquatic ecosystems can lead to eutrophication. When eutrophication occurs, plant production flourishes, leading to an aquatic environment that is void of oxygen. This can affect fish and other organisms, and often leads to them dying off from lack of oxygen. Agriculture is not the only culprit however. Urban ecosystems require more food per capita, and therefore more phosphorus. This additional introduced phosphorus is often disposed of in the form of food, or human waste. The waste becomes concentrated in wastewater, leading to phosphorus pollution in aquatic environments downstream of urban areas.
Metson, G. S., Hale, R. L., Iwaniec, D. M., Cook, E. M., Corman, J. R., Galletti, C. S., & Childers, D. L. (2012). Phosphorus in Phoenix: A budget and spatial representation of phosphorus in an urban ecosystem. Ecological Applications,22(2), 705-721. doi:10.1890/11-0865.1
The Nitrogen Cycle and Humans
This semester I have examined how various aspects of ecosystem ecology are affected or different in urban ecosystems. For this blog post I chose to examine how humans affect nitrogen and the nitrogen cycle. It is no secret that nitrogen is a critical element for primary producers as well as organisms that occupy higher trophic levels. Nitrogen availability is often a factor that limits the growth of primary producers. Humans impact nitrogen levels through a variety of methods including but not limited to, agriculture, burning of fossil fuels, urbanization processes, and waste runoff. These processes interfere with the normal nitrogen cycle in the environment. By either inputting or removing nitrogen, the cycle is disrupted and this alters the ecosystem directly at many different trophic levels. The paper came to several conclusions:
- Human alterations have nearly doubled the nitrogen input into terrestrial nitrogen cycles
- Human burning of fossil fuels has increased levels of gaseous nitrogen, altering global nitrogen fluxes
- Human agriculture has contributed to the leeching of nutrients from soil via over-farming. Nitrogen supplementation (fertilizer) has made this possible
- Humans have greatly increased the flow of nitrogen from soil to lakes, rivers, estuaries, and oceans. Tile drainage in fields is an excellent example of this.
- Human alteration of the nitrogen cycle has had effects on the carbon cycle, and these combined effects have limited biodiversity in both terrestrial and especially marine ecosystems.
Overall, human activity has altered the global nitrogen cycle. This impact is shown in smaller ecosystems, along with larger scale communities.
Vitousek, P. M., Aber, J. D., Howarth, R. W., Likens, G. E., Matson, P. A., Schindler, D. W., . . . Tilman, D. G. (1997). Technical Report: Human Alteration of the Global Nitrogen Cycle: Sources and Consequences. Ecological Applications, 7(3), 737. doi:10.2307/2269431
The Urban Stream Syndrome
My area of focus for my blog has been on urban ecosystems and the various factors related to them. This paper examines how streams impact the dynamics of urban ecosystems. They describe streams impact on ecosystems in three ways; first as habitats for a potentially diverse and productive biota, second as carriers of water and processors of the materials the water is carrying, and finally as important cultural and social foci for the human inhabitants in the respective ecosystem. They go on to discuss “Urban Stream Sydrome”, which they describe through a list of symptoms that rivers in urban ecosystems often display. One symptom of urban stream syndrome is an increased concentration of nutrients and contaminants. Another symptom is reduced biotic richness, along with an increased dominance by tolerant species. Tolerant species can be described as species that have demonstrated tolerance to the increased load of both nutrients and bio-contaminants. An example of this can be seem when examining macroinvertebrates in urban river ecosystems. Macroinvertebrates normally make up and large, diverse, and critically important trophic level in river ecosystems. They help to decompose organic matter as well as provide food for fish. This study found that in urban river ecosystems most macroinvertebrates were sensitive to the levels of contaminants in the river, and were not present. Only a few species of oligochaetes were present in significant numbers, which suggests that their dominance in urban river ecosystems can be attributed at least in part to their tolerance of the increased particles in the river. Studies in various urban regions including Atlanta and Maryland have shown that fish in urban ecosystems respond in the same manner as macroinvertebrates. Fish species that were sensitive to increased levels of contaminants were far less present and often displayed signs of poisoning. Only a few species displayed tolerance, and they were found to be present in large numbers.
Walsh, C. J., Roy, A. H., Feminella, J. W., Cottingham, P. D., Groffman, P. M., & Morgan, R. P. (2005). The urban stream syndrome: Current knowledge and the search for a cure. Journal of the North American Benthological Society, 24(3), 706-723. doi:10.1899/04-028.1
Urbanization Alters Trophic Dynamics
This paper focuses on how urbanization affects the food web and its dynamics. A vast majority of the planet is urbanized, or under the process of becoming urbanized, so it is critical that we study how this drastic change impacts the food webs of the respective environment. Research in urban ecosystems have presented ecologists with a paradox involving higher trophic level predators. Appropriately titled, the “Predator Paradox” is related to the rise of vertebrate predators in urban ecosystems compared to areas that have not been affected by humans. However, this rise in vertebrate predators has not led to an increase in predation. In fact, predation rates in urban ecosystems are much lower than in non-urban areas. The researchers in this paper hoped to understand how urbanization has shifting the control of the urban food web from top-down control to bottom-up control through various mechanisms. One mechanism for this is the removal of apex predators such as bears and mountain lions. Studies have shown that as the size of predators increases, humans acceptance of the predator decreases. Apex predators also require a large range to hunt for prey because of their large prey requirement. These two factors help to explain the general lack of apex predators in urban ecosystems. This leads to an opportunity for mid trophic level predators to have access to more resources. However, mid trophic level predators such as coyotes and raccoons are not as adept at preying on various food sources as apex predators. It is believed that this may contribute to the decrease in predation rates that have been observed in urban ecosystems. Another explanation for the decreased rates of predation in urban ecosystems is the increased resource availability. Lower trophic level vertebrates often have access to considerable amounts of food, and therefore spend less time foraging and making themselves available to be preyed upon.
Urbanization and the Predation Paradox: The Role of Trophic Dynamics in Structuring Vertebrate Communities. (2012). BioScience, 62(9), 809-818. doi:10.1525/bio.2012.62.9.6
What Affects Food Webs in an Urban Ecosystem?
The impact of humans on the environment is frequently the subject of media scrutiny, but almost exclusively in regard to climate change. While this is important, humans also impact their environment by altering ecosystems via the process of urbanization. The paper goes into detail about how a food web has many levels. In order to understand how we impact urban ecosystems, we must understand the various levels of the food web. It is important to determine how many levels a food web can accommodate, along with the level of primary production consumed by herbivores. It is also important to consider how the loss of a top level predator can affect an ecosystem. Urbanization can affect an ecosystem and its food web in many ways. Habitat fragmentation is one such way. Adding urban settings can alter an ecosystem by making it less contiguous. Induction of exotic species can affect the food web of an urban ecosystem as well. If the exotic species in invasive, it can have a devastating effect on the food web. Primary production in urban ecosystems is impacted by urbanization at a large level. One such example is in the American Southwest. Humans have taken desert plants that typically go through senescence during extremely dry periods, and watering them year round in order to keep them green. This takes an incredible amount of water, and impacts the urban food web. However, the constant water does increase primary production in such an ecosystem. Humans also alter the food web by eliminating some species. By removing certain species, levels of the food web are altered both above and below.
Faeth, S. H., Warren, P. S., Shochat, E., & Marussich, W. A. (2005). Trophic Dynamics in Urban Communities. BioScience, 55(5), 399. doi:10.1641/0006-3568(2005)055[0399:tdiuc]2.0.co;2