12 October 2016

Koeberg Data Summary - The Numbers

In the next three posts I'll be summarizing my camera trapping results form my time at Koeberg Nature Reserve. (You can click on the images to enlarge the charts.)

The white sand dunes at Koeberg Nature Reserve with Table Mountain in the background

I started camera trapping at the reserve during September 2013 until around November 2014. Earlier this year I returned to try and photograph the Leopard from May 2016 to June 2016.

A good balance of mammal and bird species showed up at the camera traps. The reptile and amphibian species were a pleasant surprise. I saw more snakes at Koeberg than any previous reserve I camera trapped at, but it seems that snakes are not easily camera trapped. The reptile species that showed up on the photographs were of Red-Sided Skink, Cape Skink and Angulate Tortoise.

Looking at the number of observations the mammals have slightly more total observations than the birds. Breaking it down and looking at the number of observations per species, we see that the mammals have a fairly good spread, but for the birds almost half of the camera trap observations were of Cape Francolin, and another quarter was of Cape Robin-Chat.

The species accumulation curve shows the usual trend: An initial ramp as new species are camera trapped for the first time, and then a steady decline as more time passes before encountering the next new species. The smaller "jumps" on the curve are usually due to one or more cameras being moved into a new "micro-habitat" such as a small pond or rodent nest. I'm surprised to see the strong leap in new species during 2016, after my long absence from the reserve. It seems like the reserve still has a lot to offer and new discoveries to be made. A new university study is currently underway and it would be very interesting to compare our results.

Not all camera trap locations are equal and some have a much higher species richness than others. Yet both the number of species per camera trap location and the total number of observations per camera trap location show the expected pattern: A few locations with lots of species / observations and many locations with fairly low amount of species / observations.

The duration of camera trap observations also reflects the expected trend. With a very high number of observations shorter than 10 seconds and then a steady decline. I suspect the small bump around the 1 minute mark is mostly due to the long delay between photographs on the Cuddeback cameras (also the odd time when I set the interval to more than 30 seconds on the Bushnell cameras). At the end of the chart I group all very long (7 minutes or more) observations together, which all add up to more or less the same amount of observations as the 10-20 seconds group.

I always enjoy interpreting the charts below. When a species visits a camera trap, what is the chance of that same species showing up again during the same day-night cycle?

These charts also nicely show the danger of interpreting data in this manner. From the charts the Honey Badger seems to be almost guaranteed to visit a camera trap more than once per day-night cycle, but this was only true for the 3 observations I recorded, and that is far too small a sample size to establish any meaningful trend. Another thing to note is that the Honey Badger observations were in fact "baited" by the presence of a carcass and does not reflect normal foraging behavior. In contrast a species such as the Duiker or Steenbok, with much larger sample sizes, will provide a much more accurate reflection of reality.

The rodent species usually top these charts, but there can be meaningful differences even for larger species like the Steenbok and Duiker.

The Bush Karoo Rat construct and live in large wooden lodges and thus its data pattern is very different to most other species.

One of the Bush Karoo Rat lodges at Koeberg Nature Reserve

As you'll see in the posts to come Steenbok and Duiker, in particular, provided a very good comparison - as both species have decently large sample sizes, but showcase very different behavior.

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