Extinction, viral theory of background extinction
While the presence of a planktonic larval stage in the life cycle reduces the probability of extinction, the planktonic life style of adults, which is generally connected with the formation of extensive mutually interrelated populations, is, on the other hand, connected with a greater tendency towards background extinction. It has been found for Foraminifera that planktonic species exist an average of 7 million years, while benthic species exist for an average of 20 million years (Emiliani 1993).
This could be explained by the viral theory of background extinction. Detailed analysis of paleontological data indicates that, while larger groups of species become extinct at once at some moments, frequently accompanied by a temporary decrease in the size of the populations of other species, at other times only extinction of individual species occurs and the other species occurring at the same time in the same environment remain practically untouched by extinction. In the former case, the cause of simultaneous extinction of several species could lie in sudden changes in the environment. It is difficult to find a possible explanation for the second type of extinction, i.e. for selective extinction that affects only one species. There are very few factors in the environment that are so specific that they are capable of destroying only one species and not affecting other species.
One of the considered possibilities is a pandemic caused by a parasitic organism, most probably a virus. A number of parasites, including viruses, have absolute host specificity, so that they attack and destroy only the members of a single species (Emiliani 1993). Viruses occur in very high concentrations in a marine environment, in numbers from millions to billions of viral particles per mm3 of water. It is assumed that they have a fundamental effect in regulating the population size of individual species and thus on ecological interspecies interactions. Some authors are of the opinion that, from time to time, a lethal viral variant can emerge against which the host species cannot defend itself and that can therefore spread uncontrollably throughout the entire population of this species and thus cause its extinction. From the viewpoint of theoretical parasitology, this phenomenon is possible but can occur only under very specific conditions (see XIX.1.4). In most cases, the parasite dies out before it can manage to exterminate its host. The effectiveness of the transmission of the infection from host to host generally decreases with decreasing density of the host population, and as soon as the actual reproduction rate of the parasite decreases below a value of 1, i.e. as soon as one infected host infects an average of less than one uninfected host, the epidemic comes to an end. Even if the parasite initially exhibits great virulence and kills a great many infected hosts, the virulence in the population will most probably gradually decrease over time to a value that corresponds to its maximum basic reproduction rate (see XIX.4). However, a different situation can occur if the parasite forms resistant stages that are capable of surviving in the environment even for a long time after they are released from an infected host. In this case, a delay occurs in the parasite-host system, so that the number of infected individuals increases even after a decrease in the size of the host population. This can lead to progressively increasing oscillations that can result in a decrease in the numbers of the host population to zero.
Parasites with several or even many alternative host species can also exterminate a host. These parasites can survive for a long time in the population of a certain species without in any way harming this species. Simultaneously, they can completely eliminate the population of some other species. A well-known contemporary example is the helminth Parelaphostrongylus tenuis, which does not basically harm its natural host, the white-tailed deer (Odocoileus virginianus) but causes very serious diseases in the members of other deer and elk species. Because of this parasite, no other species of deer or elk are found in areas where white-tailed deer occur.
Species forming dense, spatially unstructured populations would apparently be exposed to a greater risk of extermination by a parasite than species forming equally large, but less dense, spatially structured populations. Planktonic species are typical examples of the former kind, while the latter kind tends to be found amongst benthic species.
The contribution of a viral epidemic could also explain the partial taxonomic specificity of some waves of extinction. A great many viruses have a broader host spectrum and can attack a greater range of phylogenetically related species. In contrast to the situation related to most abiotic factors, the risk of infection as well as the sensitivity of the individual species to the negative action of a parasite could actually be correlated with their phylogenetic relatedness rather than with the similarity of their ecological niches.
The important role of parasitic organisms in the extinction of species was also confirmed for some extinctions observed in recent times. It is assumed that this was important for some species of birds in Hawaii, the Tasmanian tiger, and a number of species of snails of the Partula genus. At the present time, a great many species of amphibians seem to be dying out as a consequence of a pandemic of parasitic chytridiomycota (Daszak & Cunningham 1999).