When a population of a species gets big, sometimes the individuals of the same species will compete for food- causing their fellow species members to die. This is called negative density-dependence. Basically, when the density (e.g. number of tiger beetles per habitat area) gets too high that there isn’t enough food (or space) to go around, it decreases the chance an individual will survive.
I’ve been a little concerned about this idea of negative density-dependence with my Ohlone tiger beetles- not the adults that have lots of room to run and find food, but with the larvae. To lay eggs, the female Ohlone tiger beetles make tiny holes in the soil of bare ground patches and lay just one egg per hole. But, as I’ve talked about in other posts, there aren’t too many bare ground patches left in Ohlone tiger beetle habitat. Thus, a lot of females lay eggs in the same bare ground patch, creating clusters of beetle larvae in their burrows, check it out:
Tiger beetle larvae are predators. They don’t leave their burrow and eat whatever walks by- mostly other insects. You can see here that if they are clustered like this, they might negatively affect each other’s success at catching food! I thought so too. If they do negatively affect each other’s food intake, that means that they negatively affect other larve’s survival to reproductive adults that make more tiger beetles!! This was a litte disheartening to me because we have shown that making more bare ground plots by scraping really increases the number of larval burrows in the new bare ground. But what if those new larvae are starving each other?!?!?!?!
Luckily, a wonderful undergraduate student, Michelle (Shelley) Bennett, wanted to work on the Ohlone tiger beetle for her senior thesis and she was interested in this question about density-dependence. I was already marking the larvae and following their survival over time, so she capitalized on my work (smart!), helped me out in the field (score!), and analyzed whether the number of burrows around a larva impacted its ability to survive and go from 1st stage to 2nd and from 2nd stage to 3rd stage.
My awesome undergrad thesis student Shelley found that there was absolutely NO significant effect of the number of burrows around a larva on whether it survives or not, for either stage transition! She was a little disappointed because of the lack of significance in her study- but I assured her that it was a very good thing for our conservation strategy of habitat creation.
We are not entirely sure of why we didn’t find negative density-dependence. One reason could be that the Ohlone tiger beetle is endangered and so there just aren’t that many larvae to fill the bare ground-the cluster in the picture above is the exception, not the rule, as we found an average of only ~3 burrows in an area. Also, tiger beetle larvae of one stage do not eat the same things as larvae of another stage because their mandibles (fangs) are not the same size and so they can’t catch the same things. Either way, it is good news for Ohlone tiger beetle conservation and habitat restoration. It might even be a good thing if an Ohlone tiger beetle population is high enough to negatively affect itself!