Disease and parasitism cause more widespread changes in ecosystems than predation. For example, in Scottish river estuary systems Hydrobia mud snails lose 1-2% of their population to predation from shorebirds, whereas parasitism kills 8-10% of the same population. In addition to this, disease and parasitism can have more indirect impacts upon population size by affecting reproduction rates.
Despite disease being a prominent cause of death, individuals may be able to develop an immunity to a certain disease. For example, in 1998 an outbreak of the Phocine distemper virus resulted in the death of 95% of European harbour seals Phoca vitulina. While many of the seals were killed by the virus, seals which did recover gained immunity to the virus.
Not only are species able to generate immunity to a certain strain of a disease, but diseases are also able to overcome such immunity through the spawning of a new strain of the disease. This can often cause both the disease and the species it affects to co-evolve in an “evolutionary arms race”, with an example of this being seen in the relationship between the Myxomatosis virus and the UK rabbit population.
Myxomatosis was introduced into the UK in 1953 in order to control the rapidly increasing rabbit population which had become detrimental to crop production. This lead to a mortality rate of 90% in the rabbit population between 1950 and 1960. Although immunity to certain strains of the disease has been developed in the rabbit population, this has also been matched with mutations in the virus, therefore, causing the two species to coevolve. Recent studies have found that the virus still acts to limit the rabbit population, 65 years since its introduction. This, therefore, highlights a long time scale over which the impacts of population controls such as disease introduction can operate.
Figure 1 – A European Harbour Seal
“Harbour Seal”, 2016, Graphic, Anders Galatius. Source: http://www.waddensea-secretariat.org/news-and-service/news/16-19-12seal-counts-2016-harbour-seal-population-may-have-reached-carrying
Ecosystem – A community of interdependent organisms and the physical/chemical environment in which they inhabit
Parasitism – A relationship where one organism benefits and the other is negatively impacted (see Non-mutualistic Interactions article for more information)
Population – A group of individuals of the same species occupying a particular geographic area
Predation – The act of an animal preying on another