A very dilute compost heap …

It is hard to believe that this idyllic view is within a kilometre of the centre of Newcastle.   We are standing in Jesmond Dene, a steep-sided valley that is now, thanks to the largesse of Lord Armstrong in the 19th century, now a public park.  From the point of view of someone teaching freshwater ecology to undergraduates it is a godsend, as it means that we have a fine location for fieldwork within walking distance of the university.  And, because I teach Geography undergraduates, all I need to do is tell them the location and assume that their spatial awareness will lead them to the right place at roughly the right time.  It usually works.

Don’t be misled by the Arcadian scene in this photograph: for most of its length the Ouseburn is an unprepossessing stream with a multitude of problems, which is one of the reasons why I bring the students here in the first place.  We get them to sample the water, which they analyse in the laboratory over the next couple of weeks, and also collect a sample of invertebrates from the stream bed.  But, most importantly, I just want to get them to start thinking about the factors that driver a river ecosystem.

In the lectures beforehand, I make the point that we need to think beyond the stream channel itself if we are to understand its ecology and the visit to the Ouseburn helps to reinforce this.   That sun-dappled scene above is conveying an important truth: that a lot of the sunlight is intercepted by the leaves of the surrounding trees before it can reach the stream itself.   If we thought about ecology solely in terms of the stream channel we might conclude that this means less energy to fuel the stream ecosystem.  However, look at the photograph below of some of my students peering into the tray containing the invertebrates they have just collected.  Around them in the stream are leaves shed by the surrounding trees.   And, in that tray, we find a range of invertebrates but, most commonly, freshwater shrimp (Gammarus pulex), freshwater hoglouse (Asellus aquaticus) both of which are as happy feeding on the rotting remains of leaves from the surrounding trees as they are on food produced within the stream.

Left: Invertebrate sampling at Jesmond Dene, October 2017. Note the dead leaves in the stream, creating a natural food supply for the bugs.  Right: a Petri dish containing the contents from one pond net.  Note the large numbers of freshwater shrimp, Gammarus pulex (see inset).  

This is the time of year when gardeners are raking up piles of leaves and dumping them on their compost heaps.   When I peer into our compost heap I see a writhing mass of invertebrate animals also feeding on dead and decaying vegetation.  There are many segmented worms in both our compost heap and the samples we get from the Ouseburn, although most of the other animals are quite different (slugs, mostly, in our compost heap).   They may not be as exciting as the hyenas and vultures that perform similar functions on African savannahs but they play an essential role in driving nature’s cycles, turning death back into life (or, at least, into the raw materials on which new life will grow).  All flesh is grass…

Two compost bins: useful metaphors for how energy flows through stream ecosystems.

That’s the first lesson that I want to get across to the students: a river, in its natural state, is really a very, very dilute compost heap, full of organisms custom-built to recycle dead and decaying organic matter.   What I don’t tell them is that bringing them to the Ouseburn is a cop-out for me, as a lecturer whose real skills lay with algae rather than invertebrates.  If I took them to a stream further up the Tyne Valley where the hand of man was less obvious, we would have found many more types of invertebrates, and would have been able to demonstrate a much wider range of ways of feeding than we saw in the Ouseburn.  In particular, I would have expected to see stonefly nymphs and caddisfly larvae, some of which have tough jaws capable of ripping apart leaves, as well as mayfly nymphs, some of which will graze directly on algae.

The idea of a “grazer”, however, needs a little qualification.   Freshwater ecologists like to classify bugs into neat categories based on their food preferences as this helps them understand how energy flows through ecosystems.  The bugs-eye view of algae, however is that they are just one of many types of digestible energy found on and around the stream beds they inhabit.  Some ecologists prefer to lump “grazers” into a larger category of “collector-gatherers” that are relatively unfussy about what type of organic matter they eat and will cheerfully hoover up detritus that other organisms have left behind.

That “detritus” is, by the way, a euphemism for, amongst other things, the downloaded remains of the stonefly nymph’s vegetarian dinner.  Freshwater ecologists refer to this as “fine particulate organic matter” but the rest of us have a wealth of scatological language on which to draw.   That’s another lesson that I want to convey to my students: streams contain a lot of small organisms investing a lot of their valuable time searching for and eating other animal’s poo.   And that means that trout and other predators in these aquatic food-webs are eating a mixture of herbivores (the “shredders” plus the bugs that feed directly on algae) plus a lot of invertebrates that are a lot less fussy about food hygiene.   Next time you sit down to eat grilled trout, remember that you are basically eating reprocessed poo.

References

Bloor, M.C., (2011).  Dietary Preference of Gammarus pulex and Asellus aquaticus during a laboratory breeding programme for ecotoxicological studies. International Journal of Zoology 2011: article ID 294394, http://dx.doi.org/10.1155/2011/294394.

Cummins, K.W. (1983).  Trophic relations of aquatic insects.  Annual Review of Entomology 18: 183-206.

Kelly, D.W., Dick, J.T.A. & Montgomery, W.I.  (2002). The functional role of Gammarus (Crustacea, Amphipoda): shredders, predators, or both?   Hydrobiologia 485: 199-203.

Macus, J.H., Sutcliffe, D.W. & Willoughby, L.G. (1978).  Feeding and growth of Asellus aquaticus (Isopoda) on food items from the littoral of Windermere, including green leaves of Elodea Canadensis.  Freshwater Biology 8: 505-519.

Willoughby, L.G. (1983).  Feeding behaviour of Gammarus pulex (L.) (Amphipoda) on Nitella.  Crustaceana 44: 245-250.

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