Eight is enough

Plenty of carabids are known to be gregarious. Species of Brachinus, Chlaenius, Bembidion and other, typically riparian species frequently hibernate or aestivate in crowds that are packed gena-by-mandible. Often there can be multiple species all nestled in the same little hidey-hole. But even in these cases, in my experience anyway, it is unusual to find more than three species of the same genus together. In a broader area, which includes a variety of habits, in a single day, e.g. during a bioblitz, I can usually get 30-50 species of carabids that represent an array of tribes and genera. Again, with the possible exception of hygrophilic species like Bembidion or Agonum, it is rare to get more than four species from any one genus. This fits the general notion that secondary sympatry takes millions of years to happen, since there needs to be  time to break competitive barriers and overcome the detrimental effects of incomplete reproductive isolation between closely related taxa. However, I am beginning to see evidence that the expected pattern is often violated for pterostichines, a group much more likely to be found in forests, woodlands and grasslands, rather than areas near surface water. Certainly in the species of the sister clades (and subenera) Pterostichus (Hypherpes) and P.(Leptoferonia) in western North America, sympatry is rampant.

I hit an extreme case of this just last June while on a field trip with my carabidologist colleagues Dave Kavanaugh, David Maddison and Wendy Moore. We made a stop near Carson Spur, El Dorado Co.  and stayed for about an hour. In that time, in an area of about a 50m radius, I collected seven of the eight species in the image below.  The eighth one (P. morionides) is known from the area and probably would have turned up if I had been there longer. So, eight species all active as adults in one forest plot.

8 Pterostichus spp

(Click to Enlarge) Pterostichus species found in sympatry. From left to right: Pterostichus lama, P. morionides, P. tarsalis, P. sp1, P.sp2, P. inanis, P.sp3 (maybe P. tahoensis?), P. hatchi

This array of close relatives appears to have packed the niches by filling the size-bins perfectly. Does this mean they are each targeting a selective and size-specific prey item? Have they divvied up the habitat so precisely that they have little conflicting interaction? What about the larvae? In as much as they are known, larvae are very similar. Given a chance, I expect they would eat each other. In oak woodlands and grasslands I see fewer species living together (sympatric)  and active at the same time (synchronic), but when it does happen, their size is often nearly identical or limited to a few size classes. How does that work?

This case is just one snippet of the many curiosities of Western North America’s extensive Pterostichus fauna, and this event has me taking very careful note of the species that are found together. I hope that I can eventually figure out the details of these patterns.

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About Kip Will

I'm an insect systematist with expertise in carabid beetles, who is always happiest in the bush.
This entry was posted in Carabids, Coleoptera, Entomology, Pterostichines. Bookmark the permalink.

2 Responses to Eight is enough

  1. I can see the adults precisely divying up the resource in a habitat, but the larvae, which must pass through a range of sizes, are hard to explain. Could be some fine microhabitat partioning to go along with the resource partitioning?

  2. Pingback: Shapely beetles on Notes from Nature | pterostichini

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