How many beetles does it take?

When people find out that I study the Ohlone tiger beetle, many of them ask “how many are there?”. This is an obvious and good question to ask, but it is very difficult to answer and actually may not even be that important. Of course, you need a healthy (i.e. large) population size to make sure you have enough individuals to breed and make sure you have a genetically diverse population, but what does that really mean? 50 individuals? 100? 1,000? 10,000? One way conservation biologists figure out if a population is healthy and viable, or will persist in the future, is by using population viability analysis.

Population viability analysis not only tells you if a population is actually increasing or decreasing but it also tells you which stage or age in an organisms life cycle is the one that limits population growth AND you can model extinction probability into the future. On top of all that, you can even add in to population models how different management actions- like creating bare ground scrapes or making bikes slow down– will affect the population growth rate. Pretty cool. Of course, it is a model and so isn’t always entirely accurate, but it is a great tool to look at relative population growth rates.

Me checking to see if a larva transitioned to the next stage

Me checking to see if a larva transitioned to the next stage- I marked the corners of this gridded quadrat with metal markers, which I later found with a metal detector!

Creating a population viability model is not super simple because it requires a lot of data…a lot. That is because you need information on every life stage or age and how each of those stages grow and survive to be the next stage or age and once the organisms reach reproductive age, how many babies they have and if those babies survive. Whew. Luckily, tiger beetles have a life history that is just begging to be modeled- their growth and survival are so easy to track because once an egg is laid, the larva stays in the same place until it pupates and becomes

1st, 2nd, and 3rd stage (instar) larval burrows!

1st, 2nd, and 3rd stage (instar) larval burrows!

an adult beetle AND you can tell if it has grown by the size of the burrow opening. So I just marked the areas with eggs and came back every so often to check to see if it was alive and if it transitioned to the next larval stage- of which tiger beetles have 3. Then, I counted the adults and put it all into a matrix model!- yes, it is fancy.

Surprisingly and happily, I found that most (but not all) of the beetle populations are viable and will not go extinct- as long as they continue to be managed- in the near future. Also, it turns out that the survival of the eggs is a limiting factor to Ohlone tiger beetle population growth. I also found that while making bikes slow down does help, creating bare ground habitat for the beetles really increases their population growth.

So yes, there are probably around 2,000 beetles at any given time in the whole of Santa Cruz County (~100-700/population), but the more important question is: will there be more in the future? Let’s hope so.

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About tcornelisse

taracornelisse.wordpress.com
This entry was posted in Conservation solutions, Insects!, The Ohlone tiger beetle and tagged , , , , , , , , , , . Bookmark the permalink.

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