So what can we learn from studying the diversity of stream ecosystems? First of all, I don’t think that we gain very much from using conventional “diversity indices”. These have been explored ad nauseum by ecologists, usually for no better reason than that they are easy to calculate. I pointed out in a post in December 2013 (see “A Christmas turkey …”) that calculating diversity of just the diatoms was, in any case, a meaningless exercise as diatoms are part of a community that includes many other algae as well (a paper demonstrating this is referenced below).
As I was thinking about this, I remembered reading an insightful book by an anthropologist, Paul Richards on traditional farming methods in Sierra Leone. He pointed out that subsistence farmers were not impressed by the modern varieties of rice that promised high yields and, instead, preferred traditional “land races”. These had a broader genetic base and, though they may not yield as much in a good year, there was a good chance that there were enough seeds with some drought resistance, flood-resistance, pest resistance and so on to ensure that they would always get a harvest, regardless of any unexpected events that may occur during the farming year. The land race, in other words, was resilient in a way that the highly-bred varieties were not.
Much has been written about how all diatoms have a unique niche and how this makes them extremely sensitive environmental indicators. The problem is that there is not much hard evidence to support such assertions. I do not doubt that most diatoms do have unique requirements; however, there is no particular reason why these niches have to be determined solely by human pressures. Is it not also possible that factors such as fungal resistance might not apply to diatoms just as it does to crop plants? There are a few tantalising hints that this might be the case but what would this mean for ecological status assessment?
The graphs below come from a study I was involved with, in which samples were collected from streams with different levels of human impact. We’ve divided them into two groups: those that are as close to pristine as you can get (“reference”) and those that have a measurable human impact (“non-reference”). The left-hand plots show the number of species belonging to two common diatom genera, Achnanthidium and Gomphonema. The right-hand plots show the percent of the total number of diatoms recorded that belong to these two genera. In both of these instances, the number of species within the two genera is significantly higher (Wicoxon test) in reference sites. I chose these two taxa because I have noticed that they do tend to be more diverse in cleaner sites, which makes it unlikely that we can explain differences in distributions purely in terms of different preferences for chemical variables. My suggestion is that this diversity reflects a more fundamental resilience in the reference assemblages that is lacking in the impacted sites.
Differences in the number of taxa of Achnanthidium and Gomphonema recorded at reference and non-reference sites in an unpublished study of ecological conditions in streams in an EU Member State.
I have used the game Jenga as a visual analogy of what I think is happening in these samples. The left-hand image below shows an intact tower of bricks which is equivalent, in this parable, to an unimpacted community (“high ecological status”). The objective in this game is to remove bricks from the tower without it collapsing. The EU’s definition of “good ecological status” is a slight change in composition and abundance which, in my analogy, equates to having a few bricks removed (as in the middle photograph). The ecosystem continues to function in a near-natural manner because the remaining taxa can fulfil the “services” that the missing taxa once provided. However, if too many bricks are removed (the right hand image), the tower collapses. This is equivalent to lower classes of ecological status (moderate, poor or bad). Other organisms will move in to occupy the space and use the available resources, but this new community will be very different to that expected under natural conditions.
Jenga as a metaphor for ecological status: a. an intact tower of bricks, equivalent to pristine conditons, “high ecological status”; b. a few bricks are missing but the tower is still intact: “good ecological status”; c. several bricks have been removed and the tower has collapsed: “moderate ecological status” or worse.
The previous post described diversity in terms of a huge variety of microhabitats that are difficult for us to comprehend due to the differences in scale. This post has taken a broader view of that hypothesis, suggesting that microhabitats will vary not just in space but also in time and, therefore, that the ecosystem can “bounce back” from short-term shocks rapidly, because other organisms can occupy the spaces left by those that cannot thrive and, as a result, higher trophic levels are still able to feed. In the same way that being able to recover from a cold or other bug is a characteristic of the healthy human, so having “resilience” to a short-term perturbation, whether natural or human-induced, is one property of a healthy ecosystem.
DeNicola, D.M. & Kelly, M.G. (2014). Role of periphyton in ecological assessment of lakes. Freshwater Science 33: 619-638.
Richards, P. (1985). Indigenous Agricultural Revolution. Ecology and Food Crops in West Africa. Hutchinson, London.