Editing Parasitism (section)

=== 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>