The Homogenization of Urban Macro-systems

*This post is by Rabia Ahmed, a student in Marc's 'Causes and Consequences of Diversity' class.

If you have ever walked along a residential street in the city or suburbs you will notice many similar features in each backyard. Often times personal gardens are representative of peoples’ identities and reflect their membership in the neighbourhood. With the expansion of the urban population, an increasing area is covered by personal yards. While each homeowner views their yard to be small and therefore quite insignificant to the overall ecosystem- aggregated across the country, this area quickly adds up. 

 Despite the expansion of urban ecosystems little research has been devoted to understanding the patterns of ecosystem biodiversity, function and assembly. The findings of a recent paper by Pearse et al. (2018) investigated the extent to which “residential macro-systems” are the same across different US cities. The main focus of the paper was to compare the diversity, composition and structure of cultivated yards to the natural ecosystem in different climates across the US.

The results of the study showed that indeed the phylogenetic and species composition in yards had greater homogenization across regions compared to the corresponding natural ecosystems. There was also evidence of homogenization in vegetation as the tree density in yards remained similar across regions, despite the fact that, due to environmental filters, the tree densities in the different urban climates varied significantly. For example, the natural ecosystems in Salt Lake City and Los Angeles almost had no trees, but the tree density in the yards was well above zero.

Figure 1. The above diagram shows the convex hulls (dashed line) for three species pools: cultivated (orange), spontaneous (blue) and natural (green). The regions are abbreviated, Boston, Baltimore, Los Angeles, Miami, Minneapolis–St. Paul, Phoenix, and Salt Lake City as BOS, BA, LA, MI, MSP, PHX, and SL, respectively. The data shows that cultivated and spontaneous pools are more similar across regions than natural area pools, and in all cases, pools in the same geographical area are more similar than pools across a geographical region.

(Retrieved from Pearse et al. 2018)

Surprisingly, however, it was found that urban vegetation whether directly planted or spontaneously growing in the yards, had greater species richness than the comparative natural areas. The greatest phylogenetic diversity (MPD) was found within the fully cultivated yards, suggesting that these species would be better suited to future climate stressors due to their evolutionary distinctiveness. This variation in species lineages provides evidence that people prefer to have a variety of plants and flowers in their backyards which are not often found in the species pool.

Overall the data suggests that similarities in land cover and residential structural characteristics lead to a decrease in microclimate divergence at a continental scale.

These results underscore the common human preference for maintaining yards that are aesthetically pleasing and low maintenance. This homogenization has broad implications as it takes effort to keep these ecosystems the same, across forests, deserts and planes. For example, it has been observed that there is little difference between the amount of irrigation and fertilizers used by homeowners in the driest (Phoenix) or the wettest (Miami) cities.

While many argue that urban and suburban habitats do not compare to natural landscapes, recent research shows that they are more biologically diverse than previously assumed. The increased biodiversity is mostly because of the fact that people plant non-native species along with the native species, and artificial maintenance is used to overcome the environmental filter. Therefore, artificially enriched environments such as yards have both positive and negative consequences on the surrounding environment. For instance, researchers at Boston University found that trees in urban yards grow twice as fast as those in nearby forests, and store carbon at a faster rate. On the other hand, it was found that the rich mulched soils in suburban yards emitted twice as much CO2 as the soil in rural forests.

In conclusion, although yards have been given diminished importance in the study of human-dominated environments, they can provide great insight into how we can make our communities more sustainable. Residents, municipalities and neighbourhood associations can help reshape their residential macro-system into a thriving eco-system one backyard at a time. The key is to keep a balance between human preferences and other organisms’ needs, thus designing landscapes that are not only aesthetically pleasing but also support pollinators and birds.

References

Groffman, Peter M., et al. “Satisfaction, water and fertilizer use in the American residential macrosystem.” Environment Research Letters, vol.11, 29 Feb. 2016, doi:10.1088/1748-9326/11/3/034004

Humphries, Courtney. “The Residential Macrosystem.” Anthropocene, 21 June 2017, www.anthropocenemagazine.org/2017/06/residential-macrosystem-backyard-science/.

Pearse, William D., et al. “Homogenization of Plant Diversity, Composition, and Structure in North American Urban Yards.” Ecosphere, vol. 9, no. 2, 15 Feb. 2018, doi:10.1002/ecs2.2105.

Losing the rainforest of the sea: Coral reef decline and loss of future ecosystem benefits and services

*This is a guest post by Karuna Sehgal - student in my 'Causes & Consequences of Biodiversity' course. 

The past three decades of human activity has altered the earth in more ways than one. The Earth is losing species, ecosystems and biodiversity because of warming climates, among other factors. Coral reefs, in particular, are greatly impacted by the rise of global surface temperatures.

Coral Reefs throughout tropical and sub-tropical oceans are under tremendous heat stress resulting in coral bleaching and mortality. Corals are animals that live in a symbiotic relationship with microscopic dinoflagellate algae that inhabit the coral tissues (Baker et al., 2008). Increased water temperatures result in corals expelling dinoflagellates living in their tissues, causing the coral to turn white, ending its symbiotic relationship (Heron et al., 2017). This does not necessarily mean death for the coral; however bleaching still adversely impacts corals by inhibiting growth and reproduction (Heron et al., 2017). This symbiotic relationship provides the coral with about 90% of the energy it needs to thrive, it also enables corals to construct limestone skeletons that form the three-dimensional structure of reefs, which provides habitat for over a million species (Heron et al., 2017. They are referred to as the Rainforests of the Sea because they are the most bio-diverse ecosystem in the ocean, comparable to rainforests on land. Species richness and the diversity found in these systems are phenomenal and breathtaking, and yet they are dying at an alarming rate.

Fig. 1: Examples of a healthy and a bleached coral reef (images modified from Wikipedia pages on coral reefs and reef degradation, respectively)

Coral Reefs provide a lot of ecological and economically important services; they gross an estimated value of over $1 trillion (USD) globally, because of their social, economic and cultural services (Heron et al., 2017). With that being said, reefs only account for less than 0.1% of the ocean floor, but host more than one-quarter of all marine fish species (Heron et al., 2017). Climate change alters the pristine attractiveness of coral reefs to tourists, which directly affects low-income coastal countries and small developing islands within coral reef regions (Hoegh-Guldberg et al., 2007). Developing countries are not equipped to respond to climate change, and many rely on tourism for the majority of their economies (Hoegh-Guldberg et al., 2007). But tourist visits are one form of valuation, coral reefs are also critical for supporting fisheries and protecting shorelines from erosion,  For the loss of reef ecosystem services it is going to cost the US about $500 billion per year by 2100 (Hoegh-Guldberg et al., 2015).

This loss of economic value through bleaching is ultimately caused by our activities. Anthropogenic activity has resulted in rising temperatures and increases in the atmospheric concentration of carbon dioxide; this has been the largest increase in global temperature since the pre-industrial times (Stocker et al., 2013). Widespread mass coral bleaching was first documented in 1983 at the time of an extremely strong El Nino (Cofroth et al., 1989). It is important to note that coral reefs have been around a long time and residing in oceans since at least the Triassic period over 200 million years ago, and are well adapted to specific environmental conditions and human activity has damaged them in a matter of 30 years. Therefore water temperatures of even 1-2^oC above the normal temperature would result in severe coral bleaching (Heron et al., 2017). It was estimated that coral reefs would take approximately 15- 25 years to recover from mass mortality, but if the frequency of mass mortality events increases to a point where the return time of mortality event is less than the time it takes to recover, the abundance of corals on reefs will decline (Heron et al., 2017).

Ocean acidification is another factor affecting coral reefs because it hinders the coral's ability to build their limestone skeletons and increases bio-erosion of reefs (Heron et al., 2017). With approximately 25% of the emitted CO₂ from anthropogenic sources entering the ocean and producing carbonic acid, which then dissociates to form bicarbonate ions and protons, reducing the availability of carbonate to biological systems (Hoegh-Guldberg et al., 2007). These high CO₂ levels and ocean acidification are expected to cause coral reefs to erode. A number of studies have determined that the doubling of pre-industrial atmospheric CO₂ to 560 ppm decreases coral calcification and growth by up to 40% through the inhibition of aragonite formation as carbonate-ion concentrations decrease (Hoegh-Guldberg et al., 2007). Studies have concluded that the corals will not thrive again until the atmospheric CO₂ has been reduced to 320-350 ppm (Heron et al., 2017).

Building the resilience of these reefs by reducing human impacts is now the main focus of organisations like the World Heritage Committee of UNESCO and the Reef Resilience Network. A World Heritage Committee analysis showed that nearly all of the 29 World Heritage coral reef sites were exposed to levels of heat stress that cause coral bleaching, more than twice per decade during the 1985-2013 period (Heron et al., 2017). Roughly 21 of the World Heritage reef properties have been exposed to repeated heat stress during the past three years (Heron et al., 2017), threatening the long-term persists of these unique and valuable places.

Fig. 2: Satellite image of coral bleaching alerts from  2014–2017 (image from NOAA Coral Reef Watch)

Bleaching and heat stress spread across tropical oceans and intensified during El Niño, and continued from La Niña and beyond (Heron et al., 2017). This period has included the three warmest years on record: 2014, 2015, and 2016 (Heron et al., 2017). Figure 2 shows that more than 70% of the global coral reef locations have experienced bleaching and most of these have experienced it twice or more, since June 2014 (Heron et al., 2017).

What is the future of these reefs? Will the next generation be able to see and explore them as we have or will they have to watch documentaries of what used to be? Coral Reefs are the most biologically diverse and economically important ecosystem on the planet, providing ecosystem services, essential to human societies and they are at danger (Hoegh-Guldberg et al., 2007).

References

Baker AC, Glynn PW, Riegl B (2008) Climate change and coral reef bleaching: An ecological assessment of long-term impacts, recovery trends and future outlook. Estuarine, Coastal and Shelf Science 80:435-471.

Cofroth MA, Lasker HR, Oliver JK (1989) Coral mortality outside of the eastern Pacific during 1982-83: Relationship to El Niño. In: Global Ecological Consequences of the 1982-83 El Niño-Southern Oscillation. Glynn, PW. (ed.). Elsevier.

Heron et al. 2017. Impacts of Climate Change on World Heritage Coral Reefs : A First Global Scientific Assessment. Paris, UNESCO World Heritage Centre.

Hoegh-Guldberg O, et al. (2015) Reviving the Ocean Economy: the case for action - 2015. WWF International, Gland, Switzerland.Geneva, 60p.

O. Hoegh-Guldberg, P. J. Mumby, A. J. Hooten, R. S. Steneck. (2007). Coral Reefs Under Rapid Climate Change and Ocean Acidificaition. Science, 318, 1-7. Doi: 10.1126/science.1152509

Stocker TF, et al. (2013) Climate Change 2013: The Physical Science Basis. Working Group 1 (WG1) Contribution to the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (AR5), Cambridge University Press. 
 

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