Study Design Recommendation for a City

Study Design > City

How animals are living with humans in and around our metropolitan areas is an exciting question, and one that could vary from place to place.  We have developed a protocol that samples mammals with citizen science camera trapping along the gradient of human disturbance to evaluate the extent to which different species have adapted to cities. 

Spatial Configuration

Our main question here is to document how close to people different species of animals are living.  To evaluate this, we break up (stratify) the typical city along gradients of development at two scales, and then try to get camera traps spread out evenly among these.   At the larger scale we use housing density to classify zones of the city along a gradient from urban-suburban-exurban-rural-wild.   Within each zone, at a smaller scale, we classify habitats as back yard - open developed - small natural or large natural. 

We use housing density (houses/km2, Silvis housing density. dataset Hammer et al. 2004) to classify development zones. Truly urban habits are very difficult to sample with camera traps because you'll end up with too many pictures of cars and people, plus, it is difficult to ensure the security of a camera trap.  Therefore, we do not sample truly urban areas.  Some cities may not have a truly wild area to include in the study. 

Within each development zone we stratify habitats as back yards, open developed, small natural, and big natural.  Open developed areas typically include golf courses and cemeteries.   Every city has natural areas of native vegetation, forests, grasslands, or deserts.  Small natural is a continuous natural area under 1km2, while large natural is a continuous natural area larger than 1km2.

How many cameras do I need?

Based on our more detailed discussion here, we aim to get cameras run at 50 locations within each development level (i.e. treatment), with each camera run for 3-4 weeks.  This works out to a total of 700 camera sites (Table 1), although some of these might not be available in your city (e.g. large natural areas in suburbia).  We suggest this be accomplished by rotating 50 camera traps across sites (i.e. 14 times each) on a monthly basis, which we suggest could be accomplished over 2 years.  More camera traps could be used to reach this goal more quickly, depending on the success of your volunteer recruitment.  

Example questions and recommended analysis

Results from this type of survey will allow you to describe the medium and large mammals and terrestrial birds that use different parts of a city.  Basic diversity metrics can be used to show the number of species, the species accumulation curve, the Shannon Diversity index, and the relative detection rate for the whole community (all available through eMammal automated analysis here).

Because cameras are set in a stratified random way, without bait, the raw detection rate (sequences/day) can be used to quantify the intensity of use and compare the spatial distribution and habitat preferences of species.  Likewise, the percentage of sites that detected a species can be used to show the spatial spread of a species over an area.  In both cases, simple comparisons across treatments (e.g. yard vs small forest fragments vs. large forests) are possible, as are more sophisticated statistical models evaluating multiple covariates.  Hierarchical occupancy models that account for imperfect detection are now common with camera traps, using each day as separate sampling intervals.

If animals are individually recognizable, individual capture histories can be built to analyzing using capture-recapture analysis to estimate animal density.  There are a variety of experimental approaches to estimate density without recognizing individuals, but (in our opinion) none are yet well-proven or easy to use.

Temporal patterns can also be evaluated by combining the time-stamps from capture events across a study area (available through eMammal automated analysis).

Scroll to the bottom for an example study!

Volunteer Recruitment

Most cities have Museums and Natural Centers that can be good sources for volunteers.  Suburban and exurban volunteers are usually very easy to recruit through a blog or other online advertising, hosted by a local museum or nature center.  Local newspaper and newsletter articles also are of great help when recruiting volunteers in cities.  We have had success recruiting college classes (Natural Resources) and middle school classes to run cameras in cities.  In more rural and wild areas, volunteers can be recruited from hiking groups, Friends groups, Master Naturalist groups and hunters.  Mailing flyers into rural areas advertising the project can also help recruit rural volunteers.

Cost

We recommend Reconyx brand camera traps as they are the most reliable.  They are also the most expensive at about $500 each (including batteries, memory cards and locks).  We suggest that 50 camera traps is a good amount to accomplish this city-wide project in two years, although it could be done with fewer.  Please also take note of the eMammal service charges to cover the cloud-computing expenses of uploading pictures.

Discussion

We would love to hear your thoughts and questions on this advice! Please contact us at emammal@si.edu.