Contributions to abundance at the local scale, in my experience, are two part:
Part 1 is connectivity of habitats... If they have good connectivity of habitat in the stream, they'll be able to move around a lot better, and may be why you see the huge populations in the Till Plain streams of Ohio, for example, since they're one long riffle with some pools interspersed among them. However, on a old river like some of the streams like the Kentucky or Licking Rivers, riffles in big stream orders can be amazingly far apart, and this may begin to explain the extirpation or absence of
maculatum in those systems. In the Ohio River drainages, I would hypothesize
camurum is the most tolerant, followed by
tippecanoe and then
maculatum, and I have data to support this at the scale of the State of Ohio, but I need to flush some more things out before I can publish it.
Part 2 is more theoretical... If we think about this in terms of their ecological niche, these species may be considered specialists. What makes them a specialist? That comes down to the difference between the functional niche (all places they could occur) vs the realized niche (where they occur). For a species like a johnny darter, the functional niche is enormous (can occur on riffles in big rivers, lakes, tiny headwaters), but they may be limited at the local scale to a very thin piece of the pie and only have a small realized niche. However, with our friend
maculatum, the functional niche and realized niche are a much much much more close an approximation of each other - either the habitat is there, or they're not there (and sometimes this goes back to factor 1).
I can show this with rocks (Wentworth scale), depth and flow, which you saw the collection of this data on the Licking, Lance. This is from 2200 observations of darters including the species "No Fish", as absences are just as informative as presences.
nonigrum.jpg 764.78KB
8 downloadsDistances along those axes represent the strength of the interactions on those measurements. Notice how the size of substrates corresponds with depth and flow.
And I can also generate some pretty interesting histograms that suggest some of this also dynamic, esp if the abiotic factor they're responding to is flow and it ceases, and they're intolerant of sand and silt and it's increasing (which is roughly correlated with flow).
sand.jpg 39.36KB
3 downloadsThe two circles are all statistically significant from each other at the 0.05 alpha, and then there's
nigrum lol. These tolerances of sand may explain some northward migration and perhaps some of the long segments of the Tippecanoe River where
Nothonotus are absent.
Flabellare, of course, was able to resolve that issue by living in riffle crests in millimeters of water where sand is always washed away.
It's pretty slick stuff. I need to get the CCA axes to be fixed so I can look at different rivers to look for niche shifts for
caeruleum,
blennioides and
zonale. My hypothesis is that
caeruleum will shift around to the cobble axes in the absence of the Nothos,
zonale will shift to gravel and
blennioides will stay about put... And they'll have much weaker interactions on those axes. I'll be submitting this for publication after Christmas, so hopefully we'll have the whole story together shortly.
We're also starting to work on this in the Great White North where it's basically
caeruleum,
blennioides,
flabellare, and
nigrum, all four have been known to live in wave zones in bigger lakes, for example, and have been extirpated in some places by round gobies (extreme edges of functional niches, or were the gobies really just big meanies?
). Again, I expect the same pattern, weak associations etc.
So back to your question and more straight forward speculation... My guess is that under the heavy flows, they were able to live within their tolerances for local velocity and rock type at your site, and didn't use the Wabash or downstream riffles until much later when the rains backed off. Under normal flow regimes, the habitat would have become much more marginal for
camurum during the summer months, and they would get out of there and move downstream. That is not to say that some prime real estate in the
camurum world isn't available... There might be that anomaly that the king of the riffle can stay on, and you would check that off in your head as a "hit", or "presence". But I guarantee if you were writing down lengths and sex at each sample, you'd see it change under normal conditions, and wouldn't see much of a change under this past summer's extreme conditions.
Now on to Bruce's point... Yes, food stuff is probably a correlate of rocks, and vice versa. Next undergrad I get is going to do a review of all that old skool lit that documented life history, and we'll pull it into the present and see if we can't tie it to rocks. I'm using rocks because they're much more simple to quantify, and I'm explaining away a lot of the variation using these somewhat expulsive methods, so why get precise?
Todd
NANFA Youtube Channel 
