Editing Parasitism (section)

== Biology and conservation ==

=== Ecology and parasitology ===

[[File:Condor in flight.JPG|thumb|upright|The rescuing from extinction of the [[California condor]] was a successful and expensive project, but its [[ectoparasite]], the louse ''[[Colpocephalum californici]]'', was made extinct.]]

Parasitism and parasite evolution were until the twenty-first century studied by [[parasitologist]]s, in a science dominated by medicine, rather than by [[ecologist]]s or [[evolutionary biologists]]. Even though parasite-host interactions were plainly ecological and important in evolution, the history of parasitology caused what the evolutionary ecologist Robert Poulin called a "takeover of parasitism by parasitologists", leading ecologists to ignore the area. This was in his opinion "unfortunate", as parasites are "omnipresent agents of natural selection" and significant forces in evolution and ecology.{{sfn|Poulin|2007|pages=x, 1–2}} In his view, the long-standing split between the sciences limited the exchange of ideas, with separate conferences and separate journals. The technical languages of ecology and parasitology sometimes involved different meanings for the same words. There were philosophical differences, too: Poulin notes that, influenced by medicine, "many parasitologists accepted that evolution led to a decrease in parasite virulence, whereas modern evolutionary theory would have predicted a greater range of outcomes".{{sfn|Poulin|2007|pages=x, 1–2}}

Their complex relationships make parasites difficult to place in food webs: a trematode with multiple hosts for its various life cycle stages would occupy many positions in a [[food web]] simultaneously, and would set up loops of energy flow, confusing the analysis. Further, since nearly every animal has (multiple) parasites, parasites would occupy the top levels of every food web.<ref name="Sukhdeo2012">{{cite journal |last=Sukhdeo |first=Michael V.K. |title=Where are the parasites in food webs? |journal=Parasites & Vectors |volume=5 |issue=1 |year=2012 |doi=10.1186/1756-3305-5-239 |pmid=23092160 |pmc=3523981 |page=239 |doi-access=free }}</ref>

Parasites can play a role in the proliferation of non-native species. For example, invasive [[Carcinus maenas|green crabs]] are minimally affected by native [[Trematoda|trematodes]] on the Eastern Atlantic coast. This helps them outcompete native crabs such as the [[Cancer irroratus|Atlantic Rock]] and [[Jonah crab]]s.<ref>{{cite journal |last1=Blakeslee |first1=April M. H. |last2=Keogh |first2=Carolyn L. |last3=Fowler |first3=Amy E. |last4=Griffen |first4=Blaine D. |last5=Todd |first5=Peter Alan |title=Assessing the Effects of Trematode Infection on Invasive Green Crabs in Eastern North America |journal=PLOS ONE |date=1 June 2015 |volume=10 |issue=6 |pages=e0128674 |doi=10.1371/journal.pone.0128674|pmid=26030816 |pmc=4451766 |bibcode=2015PLoSO..1028674B |doi-access=free }} {{open access}}</ref>

Ecological parasitology can be important to attempts at control, like during the [[Eradication of dracunculiasis|campaign for eradicating the Guinea worm]]. Even though the parasite was eradicated in all but four countries, the worm began using frogs as an intermediary host before infecting dogs, making control more difficult than it would have been if the relationships had been better understood.<ref>{{cite journal |last=Eberhard |first=M. L. |title=Possible Role of Fish and Frogs as Paratenic Hosts of Dracunculus medinensis, Chad |volume=22 |issue=8 |pages=1428–1430 |date=August 2016 |journal=Emerging Infectious Diseases |doi=10.3201/eid2208.160043|pmid=27434418 |pmc=4982183 }}</ref>

=== Rationale for conservation ===

{{further|Conservation biology of parasites}}

{{external media |width = 210px |float = right |headerimage= |video1 = [https://knowablemagazine.org/article/living-world/2018/why-you-should-care-about-parasites "Why you should care about parasites"], 14 December 2018, ''[[Knowable Magazine]]'' }}

Although parasites are widely considered to be harmful, the eradication of all parasites would not be beneficial. Parasites account for at least half of life's diversity; they perform important ecological roles; and without parasites, organisms might tend to asexual reproduction, diminishing the diversity of traits brought about by sexual reproduction.<ref name="Holt2010">{{cite journal |last=Holt |first=R. D. |url=https://people.clas.ufl.edu/rdholt/files/247c.pdf |archive-url=https://web.archive.org/web/20150908060622/http://people.clas.ufl.edu/rdholt/files/247c.pdf |archive-date=8 September 2015 |url-status=live |title=IJEE Soapbox: World free of parasites and vectors: Would it be heaven, or would it be hell? |journal=Israel Journal of Ecology and Evolution |volume=56 |issue=3 |pages=239–250 |year=2010 |doi=10.1560/IJEE.56.3-4.239 }}</ref> Parasites provide an opportunity for the transfer of genetic material between species, facilitating evolutionary change.<ref name="Combes">{{cite book |last=Combes |first=Claude |title=The Art of being a Parasite |publisher=University of Chicago Press |year=2005 |isbn=978-0-226-11438-5}}</ref> Many parasites require multiple hosts of different species to complete their life cycles and rely on predator-prey or other stable ecological interactions to get from one host to another. The presence of parasites thus indicates that an ecosystem is healthy.<ref name="HudsonDobson2006">{{cite journal |last1=Hudson |first1=Peter J. |last2=Dobson |first2=Andrew P. |last3=Lafferty |first3=Kevin D. |url=http://www.personal.psu.edu/users/p/j/pjh18/downloads/184_Hudson_2006_Healthy_ecosystem_is_infected_TREE.pdf |title=Is a healthy ecosystem one that is rich in parasites? |journal=Trends in Ecology & Evolution |volume=21 |issue=7 |year=2006 |pages=381–385 |doi=10.1016/j.tree.2006.04.007|pmid=16713014 |archive-url=https://web.archive.org/web/20170810105426/http://www.personal.psu.edu/users/p/j/pjh18/downloads/184_Hudson_2006_Healthy_ecosystem_is_infected_TREE.pdf |archive-date=10 August 2017 }}</ref>

An ectoparasite, the California condor louse, ''[[Colpocephalum californici]]'', became a well-known conservation issue. A large and costly captive breeding program was run in the United States to rescue the [[California condor]]. It was host to a louse, which lived only on it. Any lice found were "deliberately killed" during the program, to keep the condors in the best possible health. The result was that one species, the condor, was saved and returned to the wild, while another species, the parasite, became extinct.<ref name="Stringer2014">{{cite journal |last1=Stringer |first1=Andrew Paul |last2=Linklater |first2=Wayne |title=Everything in Moderation: Principles of Parasite Control for Wildlife Conservation |journal=BioScience |date=2014 |volume=64 |issue=10 |pages=932–937 |doi=10.1093/biosci/biu135 |doi-access=free }}</ref>

Although parasites are often omitted in depictions of [[food web]]s, they usually occupy the top position. Parasites can function like [[keystone species]], reducing the dominance of superior competitors and allowing [[competition (biology)|competing species]] to co-exist.<ref name="Sukhdeo2012"/><ref name="LaffertyAllesina2008">{{cite journal |last1=Lafferty |first1=Kevin D. |last2=Allesina |first2=Stefano |last3=Arim |first3=Matias |last4=Briggs |first4=Cherie J. |display-authors=etal |title=Parasites in food webs: the ultimate missing links |journal=Ecology Letters |volume=11 |issue=6|year=2008 |pages=533–546 |doi=10.1111/j.1461-0248.2008.01174.x|pmid=18462196 |pmc=2408649 |bibcode=2008EcolL..11..533L }}</ref><ref name="Chase2013">{{cite journal |last1=Chase |first1=Jonathan |title=Parasites in Food Webs: Untangling the Entangled Bank |journal=PLOS Biology |volume=11 |issue=6 |year=2013 |pages=e1001580 |doi=10.1371/journal.pbio.1001580|pmid=23776405 |pmc=3678997 |doi-access=free }}</ref>

[[File:Aggregated distribution of parasites on hosts.svg|thumb|upright=1.35|Parasites are [[Aggregated distribution|distributed unevenly]] among their hosts, most hosts having no parasites, and a few hosts harbouring most of the parasite population. This distribution makes sampling difficult and requires careful use of statistics.]]

===Quantitative ecology===

{{further|Aggregated distribution}}

A single parasite species usually has an aggregated distribution across host animals, which means that most hosts carry few parasites, while a few hosts carry the vast majority of parasite individuals. This poses considerable problems for students of parasite ecology, as it renders [[parametric statistics]] as commonly used by biologists invalid. [[Data transformation (statistics)|Log-transformation]] of data before the application of parametric test, or the use of [[non-parametric statistics]] is recommended by several authors, but this can give rise to further problems, so quantitative parasitology is based on more advanced biostatistical methods.<ref name="pmid10780537">{{cite journal |last1=Rózsa |first1=L. |last2=Reiczigel |first2=J. |last3=Majoros |first3=G. |url=http://www.zoologia.hu/list/quant_large.pdf |title=Quantifying parasites in samples of hosts |journal=J. Parasitol. |volume=86 |issue=2 |pages=228–32 |year=2000 |pmid=10780537 |doi =10.1645/0022-3395(2000)086[0228:QPISOH]2.0.CO;2 |s2cid=16228008 |archive-url=https://web.archive.org/web/20180619111223/http://www.zoologia.hu/list/quant_large.pdf |archive-date=19 June 2018 }}</ref>