Small, not fine, but powerful
Imagine building a beautiful model of a relationship - and then forgetting a key stakeholder. What is left of the quality? Exactly.
The scheme is in every biology textbook: On the broad base of producers - algae and green plants - rests the narrower block of the first consumer level of herbivores, who feast on it and in turn become victims of the even narrower second consumer level, the carnivores or predators. Sometimes the diagram is adjusted in such a way that the decomposers still appear, those small and microorganisms that break up organic residues and make them available again as nutrients for the producers. And very occasionally, links between all those involved are shown, because there are bacteria on a leaf that are also consumed, or some second-order consumers also grab something salad-like - not everything fits into one drawer. In any case, one wants to show the coexistence and coexistence of the living world and use it to analyze nutrient and energy flows.
Wait – isn't someone missing? The colleague from the office next door would just shrug his shoulders in resignation and say: Sure. After all, there is no world without parasites. However, his special field certainly makes headlines when it comes to medical topics, in ecology the miniature pests have so far aroused little interest: too small, too inconspicuous, too difficult to grasp - and therefore probably negligible in the big questions of the natural balance.
Far from it, says Kevin Lafferty of the US Geological Survey and his colleagues. Food webs, along with their analyzes of stability, energy flow, biomass and more, are completely wrong if the influence of parasites is not taken into account.
All in all, parasites have a crucial influence on the structure of a food web
(Lafferty et al.) Because the researchers come to completely different conclusions. In four known food webs of different ecosystems, they integrated all possible relationships that were additionally created by parasites as players. And there were quite a few: the flukes of a Californian snail species alone enriched the system by 977 links to other participants. The snail, however, is only an intermediate station on the way of the parasites into birds - and it is therefore clear that the small subtenants have an effect up to the highest levels of the entire system. In the same ecosystem, parasites ended up being involved in 78 percent of the interconnections between living beings. Overall, there were far more relationships between predators and parasites than between predators and their prey. So leaving the pests out actually meant excluding a crucial part of the reality in the model.
Parasites not only intensified the connections, they also changed some patterns. Previously, the danger from natural enemies decreased with increasing levels in the food pyramid. However, if parasites come into play, representatives of the middle levels suddenly find themselves in great danger: in addition to the life-threatening predators, pathogens and weakening lodgers are now also making life difficult for them. And the top predators actually lose their prominent position in the pyramid - since there is probably no one who is completely free of parasites, no one is left who does not serve as food for anyone else in living form. That knocks even the last tiger off the pedestal.
And what about the flow of energy? Here, too, parasites should not be neglected: no matter how small they may be, with the right frequency, their total biomass is quite close to that of top predators. The extent to which parasites influence the survival and reproduction of their hosts should also not be underestimated – after all, they suddenly have to reallocate their own resources into defences, are less likely to be successful in mating competitions or simply, weakened as they are, fall more quickly to a hungry enemy to the victim. If the subtenants don't resort to very nasty means and steer their intermediate hosts right under the nose of the robber by influencing his behavior - just so that they can continue their life cycle in the new host. For example, the fluke Euhaplorchis californiensis confuses infested killifish to such an extent that the prey rate for birds increases 30-fold-certainly no small effect in a food web model.
Perhaps parasites are ideally suited to assessing the stability of such a system – the researchers continue to speculate that someone who is so closely linked to others and plays such a key role should be particularly sensitive to unpleasant changes. For a long time now, population dynamics have not only been measured in terms of the large, colourful, easily countable participants. The step towards not even the smallest player is missing so far.
"All in all, parasites have a crucial influence on the structure of a food web," the researchers explain."Not to say: They even dominate them disproportionately." It is therefore probably high time to dust off many a model. And to improve.
