WRL reference
Module
Data set
M02 D04
M02 Ecological Survey Techniques
D04 Camera trapping to assess large mammal populations in
Amazonia
_________________________________________________________________________________
Data collection methods:
The data used from line transects to determine species assemblages and to calculate mammalian
population densities in the Pacaya Samiria Nature Reserve is complemented with information
gathered from camera traps. Such remote surveying techniques are essential for understanding the
abundance of those animals rarely seen by observers walking transects, either because they are
principally nocturnal or because they are well camouflaged and can sense approaching observers
and evade detection. Housed in waterproof casings, digital cameras are equipped with infrared
sensors that take images when triggered by heat or motion (Figure 1). Heat from small mammals
will trigger the camera up to a distance of roughly 10m, although the limited extent of the light
generated by the flash means clear pictures cannot be taken at night beyond a distance of about 6m.
Between the months of June and August in 2009 and 2011, forty camera traps were set in the Várzea
forest of the Pacaya Samiria Nature Reserve along transects that covered an area of approximately
50km2. The cameras were set along transects on trees at a height of c. 1m, and were checked once a
week to download the images and to replace batteries as necessary. Camera traps were set in pairs
on either side of trails so that for mammals with unique markings, such as the coat patterns of
jaguar, individuals could be identified. In addition, the relative changes in the counts of camera trap
images of various species could be used to establish how populations are faring from year to year.
Figure 1: Camera traps in situ on trees in the Pacaya Samiria Reserve
With over 3000 images captured by the cameras across 2009 and 2011, identifying the animals
collected on film by the camera traps is a time-consuming process. Whilst the number of images of
ocelot and red brocket deer caught on camera in these two years are provided, a sub-set of the
Operation Wallacea | www.opwall.com | [email protected]
These data were gathered from the Opwall Peru expedition: http://opwall.com/sixth-form-high-school/locations/peru-school-expeditions/
Copyright: these resources are the sole property of Operation Wallacea although they may be used freely for educational purposes within the classroom
or for internal examinations. Further use will require permission which can be gained by email.
photographs captured by the camera traps are given to illustrate the range of mammals for which
camera traps provide data.
Analysis methods:
The file named [Data set INCOMPLETE] gives the number of red brocket deer and ocelot captured on
film from the camera traps set out in the Pacaya Samiria Nature Reserve in 2009 and 2011. It also
contains the number of days that the camera traps were active in each year. In order for the
number of images of each species to be comparable between years, they must first be standardised
to give the number of images per 1000 camera trap days.
To calculate the number of images of red brocket deer captured per 1000 camera trap days in 2009,
write the following formula in cell D5:
=(B5/C5)*1000
This divides the number of images by the number of camera trap days to give number of images
caught per day, and then multiplies this figure by 1000.
Once the formula is entered, click once in the D5 cell and drag the lower-right corner of the cell
down into cell D6. This will copy the formula down and calculate the number of images of red
brocket deer per 1000 camera trap days for 2011. If you then click on this cell, the formula bar
shows the following:
=(B6/C6)*1000
Repeat this process to calculate the number of images of ocelot per 1000 camera trap days for 2009
and 2011.
These results should now be represented graphically using bar charts.
 Highlight the cells containing the number of red brocket deer images per 1000 camera trap
days, and then hold CTRL and highlight the cells containing the years.
 Click on the Insert tab and select Column Chart. If you are using Microsoft Office 2010 or
2013, you may then have to select Change chart type under the Design tab to get the
correct format.
 Add a title to the graph, and titles to both the horizontal x-axis (year) and the vertical y-axis
(captures/1000 trap days).
 Create a second chart in the same way using the ocelot data.
A statistical test called the chi-square (χ²) test can be used to compare observed and expected
counts, and determine the probability that the differences observed occurred by chance. In this
instance, if the high water levels had no impact the populations of deer and ocelot might be
expected to have remained the same over the course of the study period, and so we would expect
the number of captures on the camera traps to be the same each year.
The formula for the chi-square test:
𝜒2 = ∑
(𝑂𝑏𝑠𝑒𝑟𝑣𝑒𝑑 − 𝐸𝑥𝑝𝑒𝑐𝑡𝑒𝑑)²
𝐸𝑥𝑝𝑒𝑐𝑡𝑒𝑑
Operation Wallacea | www.opwall.com | [email protected]
These data were gathered from the Opwall Peru expedition: http://opwall.com/sixth-form-high-school/locations/peru-school-expeditions/
Copyright: these resources are the sole property of Operation Wallacea although they may be used freely for educational purposes within the classroom
or for internal examinations. Further use will require permission which can be gained by email.
If the red brocket deer population had remained stable, we would expect the same number of
captures per 1000 camera trap days in 2009 and 2011. To calculate this expected value, the
observed numbers of captures from 2009 and 2011 should be added together, and then divided by
2. To calculate this, in cell N2, type
=SUM(M2:M3)/2
The same value should be written in cell N3, as we expect the same number of captures each year.
The value of χ² is calculated in stages. First, the expected value is subtracted from the observed
value. In cell O2, type
=M2-N2
Once the formula is entered, click once in the O2 cell and drag the lower-right corner of the cell
down into cell O3 to copy the formula down and calculate the (observed - expected) for the 2011
values.
Next, the (observed - expected) value is squared to remove any negative numbers. In cell P2, type
=O2^2
This multiplies the value in cell O2 by itself. Again, drag the lower-right corner of the cell down to
copy the formula into cell P3.
In the next stage, the (observed – expected)² value is divided by the expected value. In cell Q2, type
the following and copy the formula down into cell Q3
=P2/M2
The sum of the values in this column is the χ² value. In cell Q3, type
=SUM(Q2:Q3)
What does this χ² value mean? To work out the statistical significance this value of χ² needs to be
compared to a table of critical values (Figure 2).
The df column, the degrees of freedom, is essentially a measure of the number of samples (n),
calculated as n-1. Because there are two samples (2009 and 2011), the df = 2-1 = 1.
The row along the top is the probability, p. For p<0.05, there is less than a 5% probability that the
observed differences in counts from the expected values (in this case in captured images of deer or
ocelot) occurred by chance – this is taken to be a statistically significant result. If p<0.01, there is
less than 1% probability that the observed counts differed from the expected counts differed by
chance. If the χ² value for the red brocket deer data is greater than 3.84 (the value where P = 0.05
for 1 degree of freedom), the number of images captured in 2009 and 2011 are significantly
different.
Operation Wallacea | www.opwall.com | [email protected]
These data were gathered from the Opwall Peru expedition: http://opwall.com/sixth-form-high-school/locations/peru-school-expeditions/
Copyright: these resources are the sole property of Operation Wallacea although they may be used freely for educational purposes within the classroom
or for internal examinations. Further use will require permission which can be gained by email.
Figure 2: Critical values of χ²
Run another χ² test to determine whether the observed counts of ocelot images per 1000 camera
trap days differ from what would be expected if the population had remained constant over the two
years.
Once you have calculated and graphed the number of captured images of red brocket deer and
ocelot per 1000 trap days and determined whether differences in capture rates between 2009 and
2011 are significant, you are ready to consider the research questions.
Operation Wallacea | www.opwall.com | [email protected]
These data were gathered from the Opwall Peru expedition: http://opwall.com/sixth-form-high-school/locations/peru-school-expeditions/
Copyright: these resources are the sole property of Operation Wallacea although they may be used freely for educational purposes within the classroom
or for internal examinations. Further use will require permission which can be gained by email.