Well, it’s been a few weeks since I put the revised version of my diversity-disturbance experiment out in the field. I completed sampling on Monday, and, *whew*, my power analysis paid off! Although not entirely how I expected…
First off, kicking the maximum urchin density up to 50 per cage (as opposed to 16) was indeed helpful. Even after 1 week, there was a strong urchin effect. And after three weeks, well…
What is perhaps most interesting is what one sees in looking at the relationship between the number or urchins added to a cage and the change in the amount of cover. I sampled by counting the amount of cover of benthic sessile species under points in a fixed grids. Looking at the change in total number of points we see the following relationship:
A far cry from the results in trial 1. What’s fascinating, however, is to look at the points in the 0-16 urchin range in trial 2. Take a gander at the above figure, and, as you can see, there appears to be no real pattern – another scattershot. It really did require high densities of the spiny little buggers to generate a strong grazing effect. The guidance from my power analysis was right on!
What is perhaps the most exciting (and puzzling) is my preliminary results regarding diversity and disturbance. The following only really shows up when looking at algal cover and algal species richness (i.e., number of species in a plot). It shows that, yes, the effect of urchin grazing appears to change with initial algal species richness – but not in the way one would expect. Basically, low diversity plots with low number of urchins GREW. High diversity plots with few urchins in them LOST COVER. However, at high densities of urchins, everything got chomped, and pretty evenly.
What does this mean? I have no clue as of right now! As soon as the data was collected, I began feverishly entering and analyzing. Why such a rush? I had to decide whether to move the cages and run yet a third trial, or, just cut off the fencing on the cages, and use the weighted frames to mark the plots in order to follow how the community in each plot recovers. After seeing the preceeding graph, and weighing many costs and benefits, I’m sticking with my single trial (the data from trial 1 is being treated as pilot data and won’t be used in the final analysis). So, yesterday was a crazy day of cutting hundred of cable ties and herding thousands (about 1500) of urchins underwater.
I’ll be following up with recovery, but for now, the really heavy lifting is finis. And it looks quite intriguing, although I emphasize that this is all VERY preliminary.