This always happens — I have grand plans to post while in the field, then things get too crazy and I don’t end up writing anything til I’m back home. But better late than never, I suppose.
Summer 2015 field season was hot, dry, and productive — kinda like the Central Valley! As we all know, California is in a major drought, evidence of which can be found throughout the Kern River Valley — especially looking at Lake Isabella.
It’s definitely affecting Clarkia, too — after promising germination in February (look at all those babies at
right!), the near complete absence of spring precipitation meant most populations were, we’ll say, underwhelming. But I can’t really complain — there were enough plants for an experiment!
Herbivory Experiment
One of Clarkia‘s biotic interactions I’m interested in is herbivory, especially since previous work has indicated that both insect (Eckhart & Moeller, in prep) and mammalian (Geber & Eckhart 2005) herbivory show spatial trends across the subspecies’ range, with potentially large fitness consequences. Over 30% of the C. x. xantiana individuals transplanted outside the eastern range edge in the 1997-98 reciprocal transplant were attacked by small mammals (often lethally). But observational and experimental studies on insect herbivory show that incidence of fruit damage tends to decrease toward the range limit, though this varies year to year; there also seem to be differing levels of herbivory resistance among regional plant genotypes. This summer I used a stem translocation experiment to further explore spatio-temporal patterns in both types of herbivory, with the goal being to revisit a few recurring patterns and also answer a few new questions:
- How does mammal and insect herbivory vary across the growing season?
- How do these types of herbivory vary across and beyond the range of C. x. xantiana?
- What is actually attacking the plants??
- Do we see any differences between central and edge genotypes in regard to these patterns?
To do this I had seven translocation sites: two at the center of the range, three at the range edge, and two beyond the eastern range edge in C. x. parviflora land.
So the question is, why did I need to translocate stems? Why not just use plants in situ? Well, for a few reasons: first (excuse the tautology), there aren’t any C. x. xantiana outside of its range limit. So to get an idea of potential herbivory on plants that might immigrate to the east, I had to bring plants out there. Secondly, to properly analyze any genotypic differences between central and edge plants, I’d need to be able to move them into one another’s region. And even if I wasn’t looking at regional genotypes, and focusing only on natural populations, using translocated stems of controlled provenance gives me a standardized metric by which to gauge herbivore pressure, minimizing confounding genetic and environmental effects inherent in simply doing observations on rooted plants.
One of the nice things about Clarkia is that they hold up pretty well after being cut — even in the 100 degree SoCal summer heat. We just stick them in florists’ water tubes, which are attached to metal rods we secure in the ground, and voila — you can move Clarkia around to your heart’s content.
I put out four rounds of stems during mid-May through the end of June, with two transects of 24 plants at each site during each round. Half of the plants were collected from central sites, and half from peripheral populations. Though the data hasn’t been analyzed yet, there were some definite trends in both insect and mammalian herbivory — results will be in a later post, but here’s a teaser GIF compiled from images taken with a game camera I put out at some sites:
Reciprocal Transplant Experiment
I also set up all my planting grids for the upcoming reciprocal transplant experiment I’ll be putting in come October. For that experiment I’m sowing about 30,000 seeds, including three populations of C. x. xantiana and one of C. x. parviflora, into six sites spread across the range of C. xantiana (three inside subspecies xantiana‘s range, three inside ssp parviflora‘s range). To look at mammalian herbivory I’ll be manipulating herbivore access using cages around plants, and I’m crossing that with a soil microbial treatment applying inoculum sourced from both of the subspecies’ ranges. Why soil microbes? I’ve got some evidence from a greenhouse study (explained in a later post) that geographic variation in soil microbial communities has the potential to affect phenology of C. x. xantiana. See this talk for a better explanation.
I’ll follow all the plants through next summer to record various fitness components (germination, survival, flowering, etc.), and hopefully get an idea of how these biotic factors may be contributing to the eastern range limit of C. x. xantiana. Then we can start asking about traits that may have to respond to enable range expansion, which is where we start getting into all sorts of cool theoretical models and approaching the actual eco / evo mechanisms. More on those later!
John Benning 