Why not a line plot? A line plot seems pretty fitting if you'd like to a general trend in mean and SD if individual species is not the focal point.

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Anyway, here is an alternative, which strictly speaking is still a line plot but time is not one of the axes. It is also good for discussing individual species. x-axis is your variable 1 (I guessed that's mean), y-axis is the variable 2 (SD).

![enter image description here][1]

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And yes, heat map maybe too much, but I think it's worth a try to show 3D kernel density if you can provide the actual data as well. Codes are taken from [this thread](http://stats.stackexchange.com/questions/31726/scatterplot-with-contour-heat-overlay).

![enter image description here][2]

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Another possibility is to make three plots:

  1. t1 SD vs. mean, use black symbol
  2. t1 SD vs. mean, but this time the symbol turned to light grey; overlay t2 SD vs. mean, use black symbol. Connect each species pair with an arrow.
  3. t2 SD vs. mean, but this time the symbol turned to light grey; overlay t3 SD vs. mean, use symbol. Connect each species pair with an arrow.


  [1]: https://i.sstatic.net/kfsnH.png
  [2]: https://i.sstatic.net/if8kj.png