Urban Ecology Module- Student Version

Urban Ecology Module- Student Version

Community buzz: conservation of native bees in urban areas

Tara Cornelisse, Mark Weckel, and Andrew Collins

Part 1 - Introduction to Urban Conservation

“The world is increasingly urban, interconnected, and changing. If current trends continue, by 2050 the global urban population is estimated to be 6.3 billion, nearly doubling the 3.5 billion urban dwellers worldwide in 2010. More than 60 percent of the area projected to be urban in 2030 has yet to be built.”

- Cities and Biodiversity Outlook (2012)

What does the growth of cities mean for the conservation of biodiversity internationally? Is there a place for conservation within cities? And if so, what does urban conservation look like? These are very important questions currently being played out on a global scale. Cities are projected to grow. This larger urban footprint will have far-reaching impacts, well beyond city limits. The size of this footprint, the impact of cities on the globe, and the quality of life for urbanites all depend on the emergence of an urban conservation movement. Urban areas are growing rapidly especially in close proximity to biodiversity hotspots and in species-rich coastal areas (Figure 1, CBD 2012).

Figure 1. Projected urban population growth in biodiversity hotspots.

Even protected areas are impacted by urbanization at its borders as cities alter microclimate, increase local temperatures, and alter hydrology (Bolund and Humhammar 1999), all of which change the local ecology. A larger and closer urban populace will place higher demands for resources, which may incentivize both legal and illegal natural resource extraction from hotspot regions, further threatening species with extinction. Furthermore, the price of land adjacent to preserves will increase, making it more difficult to add land to nature preserves and buffer hotspots from the cityscape as it becomes economically more attractive to develop than land.

By destroying natural areas, including wetlands and forested habitat, its clear that the creation and growth of cities imposes major and often irreversible changes to the landscape and its biodiversity. Nevertheless, it may still be helpful to look at cities as ecosystems, albeit ones dominated by humans. The built environment is the defining characteristic of cities, yet more than just remnants of its original biodiversity exist. In fact, many cities are located where they are often because of the original biological diversity and productivity of the land. Cities were, and still are, established in areas with navigable waterways and abundant natural resources.

Cities are a patchwork of human, or anthropogenic habitat (residential, commercial, industrial zones) and “greenspace” (recreational parkland, remnant woodlots, and post-industrial waste places). The biodiversity that emerges “post-urbanization” is a result of the interaction between humans, their industry, trade, culture and travel, and traditional environmental factors we often consider in explaining patterns of biodiversity.

As centers of trade and transport, cities are gateways for establishment of exotic species, a serious conservation and economic challenge (Kiviat and Johnson 2013). Yet on average, 70% and 94% of plant and bird species are native to the region. The result is a ecosystem with a mix of native and non-native species alike. At the same time, there are certain cosmopolitan flora and fauna found in many cities around the world suggesting that urban development may have a homogenizing impact on biodiversity (CBD 2012). Yet urban biodiversity is not uniform throughout a city or over time. For example, older cities have more species than younger ones and wealthier neighborhoods have more floral diversity than poorer ones (CBD 2012).

Much of the urban biodiversity is vital to a resilient, productive city. Many city managers are now realizing that the stability of the “human-side” of cities from neighborhoods to economic development of commercial districts benefits from conservation investment and the construction of green infrastructure (e.g. green roofs, bioswales, parkland). For example, with global climate change resulting in rising sea levels and more frequent, stronger storms, landscape architects are proposing and manufacturing living coastal reefs (Figure 2). Coral reefs protect protect shoreline infrastructure by reducing wave energy and buffering storm surges while simultaneously promoting sea life habitat and increased recreational opportunities (Moberg and Folke 1999).

Figure 2. Protective coastal reef.

Ultimately, with an increasingly urban future, the protection of biodiversity and maximizing human health and well-being - both within and beyond cities - call for cities to place a priority on conservation within cities and developing new cities with conservation objectives in mind.


  1. How does urban conservation differ from traditional conservation?
  1. Why is urban conservation important to a global conservation effort?
  1. Researchers often speak of the ecology of cities, framing the urban landscape as a socio-ecological system.
  2. Identify some of the social and ecological components of a city.
  3. Is there a difference between the statements, “nature of cities” and “nature in cities?” Defend your answer.

Does an urban forest make for a better New York City?

Since 2007, over 200,000 street trees have been planted in New York City through the MillionTreesNYC (MTNYC) campaign of PlaNYC, an initiative to build a greener, more sustainable city by 2030.

In New York City, 25% of the land has been set aside for parks and open space (PlaNYC 2011). Concrete and pavement, buildings and roads, however, dominate the rest. Roads and roofs are dark and dry, absorbing most of the sun’s rays and warming the surrounding air through the process of conduction.

Figure 3. Tree-lined Street

The result is the creation of an urban heat island where local temperatures are 1 to 3°C higher than to the adjacent suburban and rural areas (Akbari 2005). Vegetation, including street trees (Figure 3), can mitigate this warming by changing the microclimate of entire neighborhoods. Through direct shading and the process of evapotranspiration, plants can greatly cool the surrounding air (Figure 4).

Figure 4. Average temperatures in landscapes with varying vegetation cover.

Trees can also influence the microclimate by reducing wind speeds. While single trees have a limited impact, in a residential area of Pennsylvania with 67% tree cover, trees reduced wind speed by 60% during the winter (Heiser 1990). By blocking cold winter winds, an urban forest can help homeowners and landlords reducing heating costs. However, care must be paid to where said trees are planted. Planting a tree that blocks the winter sun, but none of the winter wind can actually increase heating costs in the winter (McPherson 1987, Nowak 2007).

Street trees have a variety of ecological, economic, and social benefits for cities beyond regulating the microclimate. During a rainstorm, a single large tree can temporarily capture up to 100 gallons of water via its leaves and trunk alone (Fazio 2010), known as the “umbrella effect”. In NYC, this is a huge service since the city’s combined sewer overflow (CSO) system does not distinguish between rainwater and sewage. In some neighborhoods of NYC, water treatment facilities become overloaded after as little as 1/10 of inch rain per hour (Riverkeeper). At this point, a mixture of raw sewage and clean rain water bypasses treatment plants and are dumped directly into our local waterways reducing water quality, damaging fisheries, and closing beaches. Over 27 billion gallons of untreated sewage enters NYC waters from CSOs annually (Figure 5, Riverkeeper).

Figure 5. Waste and storm water flow in New York City.

Street trees may also contribute to cleaner, health air by intercepting particulate matter via leaves and bark and by absorbing gaseous compounds through their stomata (Pugh et al. 2012). Urban trees have been shown to greatly reduce several compounds associated with lower air quality including ozone (O3), nitrogen dioxide, and sulfur dioxide (SO2) (Figure 6). Interestly though it is possible for trees to temporarily reduce air quality, by slowing wind speeds and trapping pollutants from idling cars below.

Figure 6. Removal of pollutants increases during the vegetation growing season.

The ability of trees to reduce particulate matter has been hypothesized to reduce the incidence of asthma in children. In fact, a 2008 study conducted in NYC compared rates of asthma across different neighborhoods that varied in street tree density and found lower incidences of childhood asthma in neighborhoods with more street trees (Lovasi et al. 2008). However, a follow-up study in NYC using a finer-scale of sampling and looking at the relationships between asthma and overall tree canopy (from parks, gardens, and street trees) failed to see any correlation (Lovasi et al. 2013). On the contrary, there was some evidence that increased canopy cover was positively associated with allergic sensitization to tree pollen. How and if street trees contribute to a healthy citizenry is up to debate.

In many neighborhoods where there are few parks and private gardens, street trees may be the dominant vegetation. Here, trees greatly increase biodiversity both directly and indirectly by providing habitat to a variety of birds and insects. And where tree corridors connect parks, trees may actually serve wildlife or ecological corridors, providing connectivity between green spaces (Fernadez-Juric 2000). With that said, increased tree diversity need not be native. Many non-native trees are planted in tree pits for their ability to survive the challenging environment of the sidewalk. One of those trees is the Norway maple which several decades ago was widely planted as an urban street tree. Norway maples “escaped” their urban environment and can be found in many urban, suburban, and rural forests (Harrington et al. 2003). Owing to its ability to outcompete native red and sugar maples, Norway maple is generally viewed as invasive and the subject of intense management (Nowak and Rowntree, 1990). Today, in NYC, no invasive or potentially invasive plants can be used as street trees and a minimum of 30% of all trees used should be native (NYC Green Codes Task Force).

Beyond biodiversity, street trees are believed to play an unexpected foresting community empowerment and are even associated with lowered crime rates. Studies have suggested that areas of well-maintained vegetation are indicators of civic engagement and time spent outside by community members (Coley, Kuo, and Sullivan 1997). Some have suggested that well maintained street vegetation might actually reduce crime by serving as a signal to criminals that someone cares and is watching over the neighborhood (Brown and Bentley 1993). Supporting this idea, in Baltimore County, MD, increased urban tree canopies were overall correlated with lower crime rates (Troy et al.2012). However, the the relationship between vegetation and crime and perceived crime risk is not simple. In the same Baltimore study, Troy et al. (2012) also identified neighborhoods where the trend was reversed: more vegetation, more crime. A mixture of industrial and residential housing characterized these neighborhoods where abandoned lots, characterized by weedy, overgrown, and unattended vegetation was common (Troy et al. 2012). As with areas that are completely free of vegetation, areas with unkempt, weedy vegetation can too be viewed as “no-mans land” (Fisher & Nasar 1992; Nasar et al. 1993).The relationship between street trees, crime, civic engagement is all very much connected to the point that attributing cause and effect is very challenging.

While street trees can benefit both individuals and entire communities, it is important to consider that street trees are not planted everywhere and may not always be viewed as beneficial. A single street tree most directly impacts the person, or peoples (in the case of apartments) who live where the tree is planted! Sounds simple, but its’ really not. If a tree is planted on a public sidewalk in front of your home, to whom does the street tree belong to? Who is responsible for the tree? Who benefits? Who might be negatively impacted? Care must be taken to make sure a tree has room to grow, is healthy, and is properly pruned. An old big tree can buckle sidewalks, a concern for the elderly and the young. Roots from a tree with too small of a tree pit can seek out water well beyond its crown and in the process weaken the foundation of buildings and damage pipes. A sick tree or one that is improperly pruned can cause harm to persons or property from falling weakened limbs, and as roots grow in the direction of water (Rae et al. 2011).


  1. What are some benefits and drawbacks of planting urban street trees? Fill in Table 1: From the reading identify 10 ways street trees can impact a city. For each one, identify whether its impact is social, ecological, or economic. Indicate if the impact is a pro (favors street trees), a con (against street trees) or both.
  2. The word stakeholder is used to identify “any individual, group, or institution that has a vested interest in the natural resources of a project area and/or who potentially will be affected by project activities and have something to gain or lose if conditions change or stay the same.” Based on your analysis from Question 1, identify three stakeholders and explain how they might be harmed or benefited by an urban tree project.

Table 1.

Ex. Reduce Air Pollutants / Social / Ecological / Economic / Pro/Con/B

PART 2 - Urban Bee Conservation

Of all the insects, bees are the most widely recognized for their contribution to human well being. Bees are part of the insect order Hymenoptera, which also includes ants and wasps. When you think of “bees” you probably think immediately of “honey bees” (Apis mellifera), but in fact honeybees are only one species of over 20,000 species of bees and, in the United States, have only been residents since the early 1600s, as they are native to Europe. On the other hand, there are more than 4,000 species of native bees in the United States and over 200 in New York City.

All bees are important pollinators but honeybees, as their name suggests, also produce honey and are the most economically important species of bee. Honeybees are social insects that live in hives, making them easier to keep, while the majority of native bees are solitary- meaning they do not live in hives, but as individuals. Honeybees are responsible for pollinating more than 90 crops worldwide and their services are estimated to be worth nearly $15 billion (USDA). Urban beekeeping for honey production has increased in popularity in recent years in cities around the United States, including New York City.

Native bees are bees that are indigenous or naturalized to an area. Unlike honeybees, however, native bees are not amenable to keeping in beehives, nor do they make honey; yet, they are extremely ecologically and economically important. Native bees pollinate and are responsible for the reproduction of 70% of the world’s flowering plants, including 2/3rds of crop species, and worth about $3 billion (The Xerces Society). In fact, native bees pollinate the majority of plants in urban gardens (Matteson 2008) and are 2-3 times more productive at pollinating New York State apple orchards than honey bees (Park et al. 2012). Native bees come in many forms and vary from all black to metallic blue and some have stripes of red, orange, yellow, or white. Some common names of native bees are bumble bees, carpenter bees, mason bees, plaster bees, leafcutter bees, and digger bees. For a guide and photos of many native bees, go here and click on the names of the bee genera on the left.

Both honey bees and native bees are threatened due to human activities. Honey bees are primarily threatened by Colony Collapse Disorder, which, as current knowledge indicates, is caused by a combination of disease, parasites, and pesticides (Lu et al. 2014). Colony Collapse Disorder (CCD) has resulted in both widespread acknowledgements that honeybees are responsible for pollinating the majority of our food and fear that CCD will result in reduced food supply (Wines 2013); but CCD has also resulted in the recognition of native bees as important pollinators (Madrigal 2009, Mims 2009). While not threatened by CCD, native bees are in trouble because of loss of habitat, particularly in urban areas.

Urban native bees need our help. Native bees need floral resources, nesting sites (e.g. wood piles, rock piles, logs), and overwintering sites (e.g. logs, bee “houses”) to survive in urban areas and these resources have been declining with loss of green spaces and homogenization of urban biodiversity, especially of plants (Jha and Kremen 2013). However, there are many ways people can create native bee habitat in their yards or community parks and gardens by planting a diversity of flowering plants as well as providing breeding and overwintering habitats, such as logs or even “bee houses”. There are even manuals that have been developed to assist those in construction of effective native bee habitat (e.g. /uploads/2008/10/nests_for_native_bees1.pdf). With increased habitat for native bees, they have a chance to survive and even thrive in urban areas, further increasing insect and plant diversity and providing pollination services.