Ecology: Lecture 16 Parasitism and Mutualism

Lecture overview  Basics of parasitism  Characteristics of parasites  Hosts as habitat  Life cycles  Dynamics of transmission  Host responses

Basics of parasitism  Defined: Condition whereby two organisms live closely together and one derives its nourishment (or other resource) at the expense of the other.  Impacts on individuals  How do parasites affect their hosts?  What determines the degree of impact?

Basics of parasitism  Impacts on populations  What did early ecologists think about the role of parasites?  What did Aldo Leopold hypothesize?  Current: Many studies show effects of parasites on  Host’s birth, death and growth rates  Host’s mating success  Host’s susceptibility to predation

Characteristics of parasites  Key groups of parasites  Micro vs. macroparasites  Location of parasites  Ectoparasites: outside, may burrow (i.e.ticks, fleas, gill parasites)  Endoparasites: live inside body (i.e. tapeworms) British Broadcasting Company (BBC)

Parasitoids  Intermediate between predators and parasites  Eventually kill their host, but it takes many to kill one (and only one…) Parasitoid wasps on caterpillar WSU Cooperative Extension

Hosts as habitat Location as related to needs/strategies  Trematode parasites in snail gonads  Lipid-rich environment! Photos: USGS “Soundwave” program

Hosts as habitat Location as related to needs/strategies  Tapeworms in digestive tract  Absorbs ready-to- use nutrients!  No digestive tract of its own

Hosts as habitat Location as related to needs/strategies  Schistosoma “blood fluke” in intestinal blood vessels  Direct access to nutrients  Location by intestine: eggs released into intestine/exit with feces University of York Department of Biology

Host entry  Via food and drink  Trichinosis: Eating raw or undercooked pork  Cholera: contaminated water supply and food Campbell, Reese “Biology” M. Courtney-Clarke, Photo Researchers, Inc.

Host entry  Via burrowing  Schistosoma mansoni burrows through feet or ankles of wading person

Host entry  Via insect bite  Plasmodium, the malaria parasite, enters from the salivary glands of the mosquito into the bloodstream

Host entry: Candiru  Via body passageways…

Host exit  Via the feces  Schistosoma mansoni

Host exit: bot fly

Host entry  Via insect bite  Malaria: A few Plasmodium in, many Plasmodium out…

Life cycle: single host “Direct transmission”  Single host; may exit host  Mechanisms of transfer  Direct host-to-host contact (lice)  Bites (rabies)  Vectors: transfers parasite from host to host  Example: Bot fly uses mosquito vector for transfer to mammalian host… Mosquito not infected.  NOTE: For malaria, the mosquito is infected; it is a secondary host as well as a vector.

Life cycle: multiple hosts “Indirect transmission”  Human  Definitive host: location of sexual reproduction  Snail  Intermediate host: asexual reproduction only  See outline and own notes for more detail

Dynamics of transmission  Direct transmission tends to favor high population densities.  Exception: introduced parasites may initially spread rapidly due to lack of developed defenses.  In these cases, high rate of spread may be independent of density

Dynamics of transmission  Multiple host parasites: Success linked to  Effectiveness of transfer  Availability of both species to complete life cycle

Dynamics of transmission  Advantages of multiple hosts  One host scarce; parasite can persist in other host  Both asexual and sexual reproduction occur  Why an advantage to have both types?  Disadvantages of multiple hosts  Disruption of transmission prevents completion of life cycle  Schistosomiasis can be prevented by wearing waders  Loss of one host  eventual crash of the parasite population.  Eradication of mosquitoes reduces malaria.

Host response to parasitism  Biochemical  Inflammation  Immune response  In some cases, hosts can become resistant to the parasite (Schistosomiasis)  Abnormal growths  Cysts may form around the invading parasite (Plant galls [Fig ])

Galls on plants

Host response to parasitism  Sterility  Example: parasite within snail gonad tissue  Also see text for nematode parasite in fungus-eating flies.  Behavioral changes  Example 1: Ant parasitized by the liver fluke Dicrocoelium dendriticum  Example 2: Killfish infected with a particular trematode (fluke relative)

Mutualism (briefly)  Defined: A relationship between two species in which both benefit  Types of mutualistic relationships  Obligate symbiotic mutualism: a permanent and obligatory relationship where it is sometimes difficult to tell where one organism ends and the other begins  Example: coral animals and their protist (zooxanthellae) symbionts

Obligate symbiotic mutualism: coral reefs Reef and coral photos courtesy of NOAA

Coral: a closer look

Mutualism  A brief survey of mutualistic relationships (cont.)  Obligate non-symbiotic mutualism: two organisms live physically separate lives, but cannot survive without each other  Example: Pollination (some cases)  Non-obligatory (facultative) mutualism  Example: Seed dispersal by animals (sometimes)  Defensive mutualism  Example: Alkaloid-producing fungus that lives within grass