Parasitism

Parasitism is similar to predation, although the host (prey) is rarely killed

Parasitism There are many forms of parasitism with Batesian mimicry and herbivory being two special case, while parasitoids actually act more like predators (killing their hosts)

Parasitism Parasitism has been defined using several criteria: – 1) physiologically dependent upon host – 2) usually higher reproductive than host – 3) potentially can kill highly infected host – 4) infection process tends to produce an overdispersed distribution of parasites within the host population – 5) substainally smaller than host

Parasitism As a result, parasites typically evolve faster than their hosts Ecoparasites (e.g. ticks and mites) exploit the outer surface(s) of their hosts while endoparasites live inside their host organisms Many ecological characteristics associated with the gradient of micro- to macro-parasite

Parasitism

Another phenomena is social parasitism (e.g. thievery, slavery, and brood parasitism)

Parasitism Because of the antagonistic relationship between the two species, natural selection should favor hosts that are resistant It may be in some cases the parasitic interaction may evolve into a commensalistic or even a mutualistic relationship (of course, organisms in these relationships may evolve into a parasitic one). How might that occur?

Parasitism Most parasites are very specialized Endoparasites ultimately become dependent upon their hosts (e.g. tapeworms have lost their digestive system; viruses cannot ‘live’ outside their hosts) Many parasites have evolved species-specific host requirements

Parasitism One result of this intimate relationship is the phenomenon of molecular mimicry Antigenic determinants resemble host antigens and as a result, do not elicit the formation of host antibodies against them Another trick parasites utilize is to simply shed its skin when a serious attack from antigens Finally, some tapeworms secret a sticky substance (glycocalyx) that simply gum-up hosts antigens as they attempt to attack the tapeworm

Parasitism Some parasites have the ability to alter host behavior and in some cases, increase the likelihood being infected (or being passed along to the next host) One parasite whose intermediate host is a fish causes it to swim upside down near the surface of the water. Guess what species is the next host in this complex life cycle?

Parasitism Another example: the lancet fluke primary host is the sheep and an intermediate host are ants (the brain) The fluke actually gets the ant to crawl to the top of grass, locks its jaws and get stuck as the temperature drops Ultimately, they may be ‘eaten’

Parasitism A common view among parasitologists is the very virulent parasites represent a recently evolved host-parasite interaction. Why? There is some evidence for reduced virulence in older lineages E.g. the non-lethal carcinogenic myxoma virus which attacks South American rabbits

Parasitism In the 1950’s, this virus was introduced as a biological control agent in Australia and Europe It is passed between rabbits by fleas or mosquitoes Because these hosts were ‘naïve’, there was an initially kill rate 99%...too effective Many hosts killed before they transmitted it

Parasitism Of course, natural selection quickly favored those more resistant to the virus as well as strains that would not kill hosts so quickly (so they could be passed along, thus surviving themselves)

Parasitism Influenza is similar. If you feel too sick, you stay at home, thus not giving it the opportunity to spread via your coughing and sneezing

Parasitism However, a vector-borne disease, such as malaria, the virulence incapacitates the host and thus makes it easier for the vector (mosquito) to find and bite you, thus passing it along

Parasitism Parasites can have dramatic effects on the ecology of their hosts Consider a malarial parasite and the Sceloporus occidentalis lizard The parasite impacted hemoglobin levels, fat reserves, O 2 consumption, speed, and ultimately their reproductive success and fitness

Parasitism In a similar study, it was found another species of malarial parasite allowed the coexistence of two species of Caribbean Anolis lizards. How? Thus, parasites can impact community structure

Parasitism Darwinian medicine (or the lack thereof) Unfortunately, medicine has long attempted to simply suppress the symptoms rather than attack the cause E.g. aspirin reduces fever, but studies have shown when desert iguana are infected, they attempt to elevate their body temperature!! (and if unable or prevented, actually do worse)

Parasitism In humans, when fever is suppressed in humans with malaria, it takes longer to get rid of the malarial parasites One of the mechanisms involved is the lowering of iron levels, which can starve the microbes of necessary minerals Of course, many stupid doctors actually prescribe iron additives to ‘help’ their patients

Parasitism Many microparasites with extremely short generation times actually evolve within a single host (e.g. HIV virus) Overuse of antibiotics selects for resistant strains (e.g. there are strains of ‘staff’ that are resistant to 3 of the 4 major antibiotics due to doctors giving antibiotics out to anyone who asks for them, including patients with viral infections!)

Parasitism Perhaps there are ways in which humans can manipulate natural selection and take advantage of this process Can you think of any?

Parasitism: coevolution Coevolution refers to the joint evolution of two or more taxa that have a close ecological relationship that share reciprocal selective pressures (e.g. changes in A result in changes in B). See Fig

Parasitism

Parasitism: coevolution Thus coevolution could include forms of population interactions Frequently the term is reserved for the interaction between plants and animals, especially between their herbivores or pollinators

Parasitism: coevolution For herbivore defense, remember the structural and chemical defenses plants typically employ Organisms respond with developing the ability to detoxify the chemicals

Parasitism: coevolution Rare and ephemeral plants are hard to find and hence are protected by escape in time and space In fact, these plants should evolve very differently than plants who may be easy to find and exploit (qualitative vs. quantitative) There are suggested correlates between plant apparency and ecological characteristics (Table 15.3)

Parasitism: coevolution Some evidence (e.g. slugs) suggest good support that early successional species are more palatable (although a similar grasshopper study yielded the opposite conclusion)

Parasitism: coevolution Ginger is polymorphic with the presence of predators (slugs) altering the life-history strategy of this plant

Parasitism: coevolution Some plants and animals have developed cooperative relationships in an attempt to avoid other herbivores Consider Acacia and ants

Parasitism: coevolution To avoid seed depredation, plants may use a very hard shell or even lace the seed with a toxin (seed very nutritious) Predation and seed density may be related (generally highest right under the tree) Thus, recruitment may be maximized at intermediate distance from parent tree

Janzen’s Model

Parasitism: coevolution Trees can reduce the effectiveness of squirrel predation in many ways 1) producing difficult to reach, open cones 2) put fewer seeds in each cone 3) increase thickness of seed coat 4) invest less energy in each seed 5) shed seeds early (before juvenile squirrels start foraging) 6) with periodic ‘cone failures’, squirrel pop’s decline, reducing predation intensity next year

Parasitism