Image by Alloporus
Back when I was a bushy-tailed research student, life was a breeze that flew by without a thought.
It was a time of naivety disguised as the fearlessness of youth.
There were times I had to make some decisions but, fortunately, most were trivial and few animals were harmed in the making of them. In my case, this was a quirk of the University ethics committee. They decided that the invertebrates that were the subject of my experiments were not animals.
My research that I imagined was significant, attempted to build evidence for the importance of competition for food in the population biology of woodlice. Yes, staggeringly important information destined to change the world order and make a fortune for its finder.
Well no, neither was ever the intent, for all that I wanted at the time was to stay at University for as long as possible. It was such a cool place.
I justified this want by claiming to myself that my motivation was part progress up the academic ladder and part avoidance of the real world. And to this day, a stroll through any university campus easily confirms the prevalence of the latter. There is a heap of real world denial in the ivory towers.
But I digress.
The point here is that woodlice are animals and they are important decomposers.
As they consume dead leaves and other detritus, they recycle organic matter and make nutrients available to plants. They are members of an army of organisms we cannot live without.
In my research, woodlice were model organisms used to test the ecological theory of density-dependent competition. It is as nerdy as it sounds.
The idea is that competition for food is one of the mechanisms for natural selection that ecologists have tried to prove ever since Darwin first put a name to it. The recycling credentials of woodlice and their soil animal cousins I studied later. At the time of my research degree, I needed a way to test if woodlice compete for food to add some more evidence in support of evolutionary and ecological theory.
To do this, I had to make a decision on how to manipulate the availability of food on the assumption that it was a limiting resource. If the assumption was correct, theory suggested there would be competition for high-quality food and the woodlice would respond through changes in their patterns of growth and reproduction.
One manipulation option was to exclude (that is to keep out) rabbit grazing from an area of our study site. Rabbits! Where did they come from? Even the ethics dons would say these were animals. In the chalky grasslands of eastern England where woodlice are abundant, rabbits are crucial to the supply of high-quality food to detritivores, the woodlice.
Rabbit grazing alters the structure of the grassland. The attention of many thousands of cute bunnies grazing on the grasses keeps the coarse grasses from taking over. Grazing opens enough light and space for forbs and herbs to flourish. Exclude rabbits, and tough grasses soon dominate in a thicker, dense layer. Dead herbs are the preferred high-quality food of woodlice so when the rabbits are first removed there is a spike in the availability of high quality detritus. A bonanza for the woodlice. Later when the thicker grasses took over the herb and forb food source was reduced, so, in theory, the woodlice would become food limited and compete with each other.
So a rabbit-proof fence was constructed around part of the habitat and, sure enough, the grasses grew at the expense of the herbs and forbs sending through the pulse of high quality woodlouse food from the dead herbs and forbs. The area of rabbit exclusion became the Weeting Heath exclosure experiment. The driver we wanted to control was excluded.
Ecological research often works this way. In order to understand one species, you have to change things up with another, apparently unrelated species.
But this was only part of the evidence needed to test the density-dependence hypothesis of food limitation. I was keen to find out what would happen if we increased the numbers of woodlice in habitat with rabbits. The assumption here was that crowding them out would force them to compete for food.
In the second experimental option, rabbits would crop the grass and maintain the supply of herbs, but there would be an artificially high number of woodlice. Would that make these small critters compete for high-quality food?
This experiment is different.
It required an increase in woodlouse numbers. Such manipulation is not easy to do over large areas. So I decided to create enclosures to keep high numbers of woodlice together with woodlouse proof fences. The fences had to be low enough to let the rabbits in to graze down the grass, high enough to stop the woodlice escaping, and surround an area big enough for the woodlice to behave normally, more or less.
The fenced-in areas became the Weeting Heath enclosure experiment. Here is what it looked like. The rabbit-proof fence of the ‘exclosure’ is in the background.
Keeping things out (the exclosure) and keeping things in (the enclosure) was an obvious solution to an experimental manipulation conundrum — two different ways to manipulate the supply of high-quality food for a wild population of woodlice.
And just to be sure in some of the enclosures I added extra high-quality woodlouse food in the form of ground up leaf litter from alder trees. They love that stuff and grow exceptionally well on it.
Here you can see the darker colour of the grass in one of the enclosures where the extra food was added.
Here are two of the conclusions we published in the Journal of Animal Ecology
(5) When an experimental exclosure was erected which prevented rabbit grazing, the availability of high-quality foods increased. Isopods within the exclosure grew larger, became more fecund, and consequently increased in density.
(6) In isopod enclosures to which high-quality food was added, growth rates of isopods also increased. In other enclosures to which sub-adult A. vulgare were experimentally added, growth rates of new recruits decreased.
Hassall, M., & Dangerfield, J. M. (1990). Density-dependent processes in the population dynamics of Armadillidium vulgare (Isopoda: Oniscidae). The Journal of Animal Ecology, 941-958.
In less jargonese, the woodlice were bigger, reproduced more and their numbers increased in the exclosure without rabbits.
Adding food in the enclosure also got the woodlice to grow faster but they grew more slowly when they were crowded.
Amazing, just the confirmation bias we were looking for and here is how we summed it up in the journal article
We conclude that intra-specific competition is important in regulating the density of this population and that populations of this macro-decomposer are more likely to be regulated from ‘below’ by competing for limited food than from ‘above’ by natural enemies. The relaxation of competition at low densities with the consequent positive effects on natality rates provides an effective ‘floor’ which-reduces the probability of population extinctions.
This is all a little grandiose. It initially seemed remarkably that these animals are sensitive to food supply but as every organism is the idea seems trite. Proof of sorts was worthy of a formal statement.
What I learned from exclosures and enclosures
Ecology is a messy subject with many challenges to the principles behind the scientific method. Experiments are never easy and here will always be criticism of most attempts.
My woodlouse attempts at experimentation were pseudo replicated, failed to measure controlling variables (food availability in the exclosure) and needed a much long run of observations. Just three obvious criticisms.
But I learned a great deal about these innate complexities and the difficulties of real world experiments. That was, after all, one of the reasons to take on a research degree.
I also learned that the theory holds. Organisms can be food limited with consequences for their survival, growth, and reproduction. Homo sapiens take note.
Mostly though I found that scratching intellectual itches is great fun and immensely satisfying, so much so that I have kept doing it to this day and am unlikely to stop until my faculties do.
What a blessing it is to have an enquiring mind.
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