Pacific Newt Roadkill, Rainfall and Low Temps for 4 seasons (updated Mar 15, 2021)
The following charts show the number of newts killed on Alma Bridge Rd. at Lexington Reservoir and the cumulative number of inches of rainfall for the four seasons we've been collecting data. Notice the following:
(1) The YTD rainfall for this season (2020-2021, purple) is significantly lower than for the previous three migration seasons.
(2) The number of newts killed YTD this season is about 1000 less than last season (2019-2020). Is this due to less rainfall or because HTH hasn't shared all their data with us? Could it be a sign that the Lexington newt population is being extirpated?
(3) The number of juveniles killed this season has significantly increased over previous seasons. Click here for more information about juvenile newt roadkill.
Rainfall Amount
Does the amount of rainfall determine how many newts are found dead on the road the following day?
No - there's no correlation between the amount of rainfall and roadkill numbers, as shown in the following graphs from the 2020-2021 and 2018-2019 migration seasons:
Low Temperatures
How do cold temperatures affect the roadkill numbers? It appears that relatively few newts are killed on the road when overnight temperatures fall below 40 degrees F, as shown in the following graphs. In other words, very few newts come out when it's very cold. The weather service reports air temperatures, not ground temps, and if the ground is at or below 32F, frost can form. According to the National Weather Service, "if the thermometer is even just a few feet above ground, it may not give an accurate reading for frost. Because cool air sinks and the ground can quickly cool, the ground temperature on clear, still winter nights is invariably lower than the temperature only a few feet higher. Thus, frost can form even when a thermometer gives a reading in the upper 30's." Click here for more information.
Kommentit
@merav, @tyap, this is scary. I compared the number of newts killed YTD for 2019-2020 and 2020-2021 and about 1,200 fewer newts have been recorded this year. That's a big difference! It could be due to the lower amount of rainfall, but it could also be an early sign that this population is in a steep decline. @tyap, do you know if Gio is still planning on doing his population study? I would be really interested in his findings.
This is really good @truthseqr - I would be interested to see something that displays the proportional relationship between rainfall and mortality, to compare apples-to-apples, which might bring 2020-2021 more in line.
Second project - to correlate days with the biggest mortality spikes with previous rainy night(s) when nasty accidents/closures meant Highway 17 traffic was diverted entirely to Alma Bridge Road. Presumably newts would be out in force on such a rainy night, and they'd have little chance of survival with such a massive stream of nighttime vehicle traffic.
@anudibranchmom, I've created correlation charts that show dead newts vs. amount of rainfall, and there's very little correlation. In other words, it appears that once the minimum threshold of rainfall is met (4 in?), the actual amount of rainfall doesn't correlate with roadkill numbers. In fact, there's greater correlation between low temperatures and decreased mortality. In other words, the newts don't come out when it gets really cold.
I'm not a statistician, so maybe I'm not setting up these charts correctly. If you want, I could email them to you and you could take a look and perhaps advise me on a better way to display the data.
I've added some charts from the 2018-2019 migration season that show the effect that amount of rainfall and low temperatures have on the number of newts found dead on the road.
If people are interested, I can work on graphs for the current 2020-2021 migration season.
Wow, this is such interesting info, thanks for taking the time @truthseqr ! I'm not a statistician, just curious :-)
@anudibranchmom, curiosity is a wonderful trait. I'm a curious person, too. That's what led me to create these charts in the first place.
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