The parasite connection

For most of us, endoparasites and pathogens have a negative connotation, and we think that the world would be better without them. In a less anthropocentric view, however, they appear to be essential for the long-term survival of all ecosystems. Like the arms race between predator and prey species at the trophic level, a constant race takes place between hosts and their parasites. But as we know, there is no ultimate winner, because most defence mechanisms (and drugs) work only at the level of the individual host and are eventually superseded by the genetic flexibility of rapidly reproducing parasites.

The positive effect of parasites derives from their vital interest in maintaining the status quo. To this end, all of them dampen fluctuations in population size caused by predator-prey cycles. In addition, heteroxenic parasites (protozoans and metazoans that reproduce asexually in the intermediate host and sexually in a quite different final one) balance the system between the two kinds of hosts by doing less harm to the weaker than the stronger partner. For instance, in the case of the lion-gnu system (Seilacher et al., 2007; Figure 10.8), the herbivores suffer little from their sarcocystid infection, except that the most-loaded individuals become behaviourally conditioned for falling prey. In contrast, only one out of five lion cubs reaches the second year in its natural environment, largely due to the protozoan parasites.

Similarly, trematodes (liver worms) spare the intermediate host, a freshwater snail, by stopping their own reproduction when the host switches to hunger metabolism (Seilacher et al. (2007), and literature cited therein).

Another important feature is the host specificity of parasites. In the lion-gnu example, another sarcocystid species cohabiting in the same herbivore cannot develop in the lion but needs to reach a hyena sharing the same prey. This specificity, however, is at a high taxonomic level. The lions' parasite may also develop in any other member of the cat family and the one of the hyena in any canid, because all constituent species share a similar histological and physiological outfit. In this way, higher taxonomic categories may become units of selection (for instance in mass extinctions). On the other hand, such flexibility makes parasites tolerant against minor changes of host species due to immigration, speciation, or extinction. Parasitic partnerships also may survive dramatic habitat changes of the host, as long as adequate transfer mechanisms can be established in the new environment. Thus, sarcocystids similar to the ones in the lion-gnu system went to sea with the whales in the Early Tertiary. Now baleens (derived from herbivorous ungulates) serve as their intermediate hosts, and the orcas (probably derived from terrestrial carnivores) as final hosts, while herrings may be the transport hosts back to baleens. The parasites' fidelity may last until a whole clade of hosts becomes eradicated in a mass extinction. On the other hand, their time-constant and well-isolated environment allows endoparasites themselves to radiate in coevolution with their hosts, as shown by the large number of sibling species.

In conclusion, probably no animal (or plant) is without endoparasites and, as the same organism hosts different kinds of them, parasites form networks glueing together established ecosystems.

Although we are far from understanding all these complex relationships (parasites that have no direct or indirect effect on humans remain underexplored), they possibly account for the stasis of ecosystems observed in the fossil record. Cases concerned range from the long-term identity of benthic communities ('coordinated stasis' (Brett and Baird, 1995), to the evolution of 'exclusive clubs' in ancient lakes and semi-restricted basins (snails in the Miocene Steinheim Lake and ceratites in

occyst 2 sporo-cysts species separate occyst 2 sporo-cysts species separate sexual process (gamogony); resulting oocysts multiply and are released into the intestine

SARCO CYSTID CYCLE

ungulate herbivores are little harmed; diaplacentar and/or lactogen transfer to calves species coexist a merozoites enter tissue cells, induce cyst and divide into cystozoites ready to be eaten

Fig. 10.8. Like other heteroxenic parasites, sarcocystids balance host populations by damaging the stronger carnivores more than the herbivores. (From Seilacher et al. (2007). Drawings by P. Wenk.)

canid carnivores suffer decimation macrofaeces sexual process (gamogony); resulting oocysts multiply and are released into the intestine canid carnivores suffer decimation macrofaeces

merozoites released into blood sporozoites enter cells of blood vessels and multiply into merozoites

ungulate herbivores are little harmed; diaplacentar and/or lactogen transfer to calves a merozoites enter tissue cells, induce cyst and divide into cystozoites ready to be eaten merozoites released into blood species coexist sporozoites enter cells of blood vessels and multiply into merozoites

Fig. 10.8. Like other heteroxenic parasites, sarcocystids balance host populations by damaging the stronger carnivores more than the herbivores. (From Seilacher et al. (2007). Drawings by P. Wenk.)

the Triassic Muschelkalk Basin (Seilacher et al., 2007)), and to the incumbency of ecological power structures at the largest scale.

Possibly the persistence of the Ediacaran biota has to do with such connections. Although the events in the Precambrian-Cambrian transition were different from the ones that led to the extinction of the dinosaurs, they resulted in the collapse of established ecosystems and allowed multicellular animals to become the rulers.

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    What is parasite connection?
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