Off the sand and onto the asphalt: Does the urban heat island impact desert lizards? Synopsis: Lizards provide ecosystem services as pest regulators, and as a cultural icon of Southwestern Native Americans. Unfortunately, climate change has dire implications for these ectotherms. Lizards prefer a narrow range of body temperatures (TB); this limits their activity in space and time. In summer, an increase of 1-2 oC can greatly restrict daily activity, often below a threshold required for survival. As a result, climate change may drive 40% of lizard populations extinct by 20803. This assumes that species will not adapt by shifting their activity patterns. Adaptations may already be observable in Phoenix, AZ, where a heterogeneous urban heat island (UHI) averages +3 oC and can locally exceed +10 oC at night4,5. Ecological consequences of the UHI remain almost completely uninvestigated. I will study how lizards are impacted by the urban thermal landscape, and evaluate a proposal to use ecosystem services for UHI mitigation. This will help us to better predict the consequences of climate change for the future distribution and abundance of ectotherms. Background: In Phoenix, AZ, the urban fringe has been expanding into the Sonoran Desert at a rate of approximately 1 km per year6, creating a large UHI5. Direct societal impacts of the UHI include a doubling of human misery hours per day (temperature-humidity index> 100 C), and increased health risks5,7-9. In Los Angeles, CA, the UHI costs citizens $100 million per year in additional energy use for air conditioning10. Energy use results in further emissions of heat, CO2, volatile organic compounds (VOCS), and nitrous oxides (NOx). The UHI increases the conversion of VOCS and NOx into ground-level ozone, which is a criteria pollutant under the EPAs Clean Air Act and a principal component of smog10. The UHI is strongest in Phoenixs vulnerable low-income, minority neighborhoods. Accordingly, higher incidences of heat stroke and respiratory disease in these areas are environmental justice concerns11. Thermal Ecology: For many physiological processes and behaviors (e.g. sprint speed and predator evasion), lizards perform best only a few degrees below the maximum temperature they can tolerate. As ectothermic TB is closely tied to the environment, an optimal habitat can become a lethal habitat after a small increase in temperature12. Summer surface temperatures often exceed the relatively narrow range of preferred lizard TB (~30-40 oC) at midday, forcing them to retreat underground. Being active above this temperature range can increase a lizards metabolism and daily energy requirements by more than 40%13,14. An increase of only 1-2 oC can reduce potential summer activity time for many species below a temporal threshold required to obtain sufficient resources for survival and reproduction3. An increase of 3 oC can eliminate spatial availability of preferred TB across an entire landscape14. In Phoenix, lizards feed almost exclusively on insects, which, being ectotherms, are similarly restricted by availability of preferred TB. Thus in summer, while urban lizards will have a greater need to forage, they will have less time and space to do so, and their prey may be less abundant as well. However, the simplistic global model used to predict a mass lizard extinction3 assumed that lizards would not be able to trade reduced summer activity for increased winter activity. If lizards can survive the summer, the UHI (and global warming) may actually benefit them in winter by allowing for shorter hibernations, and by increasing the overlap between the activity times of lizards and their insect prey7,14. Study area: The Central Arizona-Phoenix Long-Term Ecological Research project (CAP-LTER) is based at Arizona State University and is one of only two urban LTERs in existence. Despite 20 years of climatological research on the Phoenix UHI, CAP-LTER has not investigated the UHIs ecological impact on animals and only has limited data on the relationship between the UHI and vegetation15,16. One interesting correlation is that a $10,000 increase in median household income yields a 0.3 oC decrease in surface temperature16, presumably due to increased shade and evaporation associated with irrigated vegetation. Thus, Phoenix is inadvertently conducting a natural experiment in anthropogenic warming in disadvantaged neighborhoods, while simultaneously conducting a natural experiment in anthropogenic cooling in affluent neighborhoods. Surprisingly, agricultural areas in Phoenix can actually have lower daytime surface temperatures than the surrounding desert, known as the urban oasis effect. Evaporative cooling is particularly effective in dry desert air, and my study will help predict the effectiveness of this proposed use of ecosystem services for UHI mitigation. The most common land use type in Phoenix is residential, and the majority of residences are situated within private homeowners associations6. Homeowners associations control the vegetation used in communal areas, and restrict the types of vegetation that homeowners may plant on their property. This means that individual households and local communities could play a role in mitigating the UHI through landscaping. Additionally, Phoenixs Tree and Shade Master Plan is deciding how to double urban canopy cover (to 25%) by 2030. Therefore, decision makers ranging from homeowners, to community organizations and city planners, could benefit from my study. Study organisms: The land northwest of Phoenix has similar biophysical characteristics to the land on which the city was built. Approximately 14 native lizard species inhabit this area, but only a subset (11) persists in Phoenixs semi-natural urban desert parks. Four of these species can be found in residential areas, but only one species has a nearly ubiquitous distribution throughout the city and the natural areas surrounding it. This pattern may be explained by two contrasting mechanisms: (1) the effect of personal landscaping on microhabitat temperature and lizard activity, (a bottom-up mechanism), and (2) the effect of broad-scale urban land use/cover variables on lizard habitat suitability (a top-down mechanism).
|Effective start/end date||9/1/12 → 9/30/15|
- US Environmental Protection Agency (EPA): $51,000.00