Eco - Parasitism

symbionts

organisms that live in or on other organisms

• more than half of Earth’s species are symbionts

pathogens

parasites that cause diseases - often a medical definition vs an ecological definition

symbiosis

an intimate association between organisms of different species - living together

parasitism

• interactions similar to predation (+,-)

• one organism draws nutrients from the other diseases (viruses, bacterial pathogens), ticks, tapeworms

mutualism

• interactions with mutual benefit (+,+)

• note that mutualism does not require close physical association (plants/pollinators)

commensalism

• benefit to one members of association, no effect on the other (+,0)

• many epiphytes on trees are commensals

reciprocal selection

evolutionary process where two or more species exert mutual selective pressures on each other, acting as the primary driving force behind coevolution

strict coevolution

• between 2 closely interacting species

• example - a parasite and its host (1:1 interaction)

diffuse coevolution

• interactions spread among many species

• examples - plants and herbivores; plants and pollinators; N:N interactions

non-symbiotic interactions

• organisms do not live together in a close physical association, but still interact

• plants and pollinators

facultative (opportunistic) interactions

each members of the interactions can survive independently

obligate interaction

1 or both members of the interaction require the interaction to survive

hemiparasites

parasitic plants that conduct photosynthesis

holoparasites

parasitic plants that lack photosynthesis

macroparasites

• large; grow but do not multiple within host (or external)

• animals: tapeworms, roundworms, lice, ticks

• plants: fungi (mildews, rusts), gall-forming insects

• longer generation times

• part of life cycle outside of hosts

• produce infective stages that are released to infect new hosts

• external, in gut or other cavity, not inside cells

• easier to count number of macroparasites (parasite load)

microparasites

• small, extremely numerous, multiple directly within host

• viruses, bacteria, protozoans

• short generation time

• difficult to study number of microparasites, instead study number of infected hosts

• examples - viruses like measles, covid-19, or HIV

• viral load = # of virus particles

• transmitted by direct contact between hosts or through a vector (intermediate host)

• example of vector/intermediate host - mosquitos transmit Plasmodium which cause malaria

ectoparasites

live on the outer body surface of the host

endoparasites

live inside their hosts, within cells or tissues or in the alimentary canal

example of plant ectoparasites

mistletoes are hemiparasitic - they get water and nutrients from the host but can also photosynthesize

where is the best place for endoparasites to live

the alimentary canal is an excellent habitat for many parasites; most do not eat host tissue, but rob the host of nutrients

advantages and disadvantages of ectoparasitism

advantages:

• ease of dispersal

• safe from host’s immune system

disadvantages:

• vulnerability to natural enemies

• exposure to external environment

• feeding more difficult

advantages and disadvantages of endoparasitism

advantages:

• ease of feeding

• protected from external environment

• safer from natural enemies

• vulnerability to host’s immune system

disadvantages:

• dispersal more difficult

memory cells

present in vertebrate immune systems; recognize microparasites from previous exposures

plant defense systems

• resistance genes

• nonspecific immune responses such as antimicrobial and antifungal compounds

• chemical signals that ‘warn” nearby cells of imminent attack

• chemicals that stimulate deposition of lignin, making a barrier to an invader’s spread

parasite counterdefenses

• ectoparasites must penetrate external defenses and toxic compounds produced by plants, challenges similar to those faced by herbivores and predators

• endoparasites must cope with defenses found inside the host

parasites are under strong selection pressure to develop counterdefenses:

• parasitoid wasps avoid encapsulation by injecting virus-like particles that infect lamellocytes and cause them to self-destruct

• other parasitoids lay eggs covered with filaments that become embedded in the host’s fat cells where they are not detected by circulating lamellocytes

lamellocytes

blood cells that can form multicellular capsules around large objects such as nematodes

coevoluton

occurs when populations of two interacting species evolve together, each in response to selection pressure imposed by the other

hosts are reactive environments

• grooming by birds and mammals is a common behavior to remove ectoparasites

• animals respond to bacterial infections with inflammation, which can yield hardened cysts that isolate the parasite

  • plants can also respond to infections by forming cysts, removing contact between healthy and infected tissues

  • animals also have an immune response, producing antibodies against previously encountered pathogens

  • in plants, this process is completely different, and not as well understood: systemic acquired resistance

mathematical models of host-pathogen population dynamics

• host population is divided into susceptible individuals (S), infected individuals (I), and recovered and immune individuals (R)

• a simple model shows that a disease will spread only if the density of susceptible hosts exceeds a critical threshold density

host parasite population dynamic equation

dI/dt = beta SI - mI

• S = density of susceptible individuals

• I = density of infected individuals

• SI = probability of infected individuals encountering susceptible individuals

• beta = transmission coefficient (how effectively the disease spreads)

• beta SI = probability (or rate) of disease transmission

• a disease will spread when dl/dt=0

• a disease will establish and spread when the number of susceptible individuals exceeds threshold density: S(subscript 1) = m/beta