Parasitism - Wikipedia
That is the picture of the human parasite, who drains, not his victim's . that there is something fundamentally amiss in the relationship between host and. A parasitic relationship is one in which one organism, the parasite, lives off of another organism, the host, harming it and possibly causing death. The parasite. Definitive & Intermediate Hosts in Parasitic Life Cycles . Sometimes the host is harmed by the parasite, and sometimes the relationship is neutral. Helminth parasites might be what most people picture parasites would look.
The generalist parasite may infect all, or only a subset, of the hosts in the community. To account for the generalist parasite, the equations for any host species k that can be infected by the generalist become: We assume, for simplicity, that the contact rates of the generalist parasite with hosts are the same as those of the specialist parasite.
We also allow hosts that are singly infected with the specialist parasite to become co-infected on contact with the generalist parasite, with being the probability of co-infection, given contact. Hosts that are doubly infected with the specialist parasite cannot be infected by the generalist parasite. Because we assume that the total abundance of parasites is set by host traits, we introduce the parameter xk to account for competition between the strains for host resources.
As above, if a parasite in the environment does not avoid contact with co-infected hosts, it is removed from the environment at the rate. We also need to consider the dynamics of host species k individuals that are singly infected with the generalist parasite Ik,g or are co-infected with the specialist and generalist parasite Ck,s,g: This is because, in an invasion analysis, we are interested in whether the generalist parasite can invade the community when it is very rare soand Pg are all assumed to be very close to 0.
In such an analysis, we can ignore the dynamics of any variable that depends on products ofor Pg [ 39 ]. Since double infections require contact between hosts that are singly infected with the generalist parasite and generalist parasites in the environment, we can ignore this variable.
Finally, we consider the dynamics of the generalist parasite in the environment: In the absence of such costs, the generalist would always be able to invade. Note that such costs could be accounted for by assuming that the contact rates for generalist parasites were lower than those of specialist parasites. We are interested in knowing when this equilibrium is unstable, that is, when the generalist parasite can increase from rarity and invade the system.
Mathematically, this is governed by the eigenvalues of the Jacobian matrix for the full system equations 2. This expression is complex, but has an intuitive biological interpretation. The first term is the probability that a generalist parasite infects a susceptible host of species k.
The second term is the probability that a singly infected host remains singly infected for its lifetime, multiplied by the expected number of parasites that a singly infected host will shed.
The third term is the probability that the host becomes co-infected, multiplied by the expected number of parasites shed by a co-infected host.
The fourth term is the probability that a parasite co-infects a host infected by the specialist, multiplied by the expected number of parasites shed by a co-infected host. The above derivation is general for a community with any number of hosts, but we focus our analysis on the simpler case where there are only two hosts.
We are interested in understanding how host traits, parasite traits and the environment influence the magnitude of Rm. Males attach themselves to females with their jawsand in some cases the tissues and circulatory systems between the sexes are joined. Thereafter, the male serves as a sperm -producing organ on the female, since transformation makes him completely dependent upon her.
The Tomato Hornworm
Other forms of sexual parasitism also exist, including those in which the genetic material from one parent is discarded by the other parent despite the effort made by the other parent to produce and deliver it. For example, young resulting from the pairing of sailfin mollies Poecilia latipinna and Atlantic mollies P. They need sperm from males of either of the two species to start the process; however, since all offspring are clones of their mother, no male DNA is passed on.
Parasitism differs from parasitoidisma relationship in which the parasite always kills the host. Parasitoidism occurs in some Hymenoptera ants, waspsand beesDiptera fliesand a few Lepidoptera butterflies and moths: Left A female ichneumonid, or ichneumon, wasp family Ichneumonidae lays her eggs in the host larvae by means of her ovipositor.
Right A wasp from genus Trichogramma family Trichogrammatidae develops within a host egg and emerges as an adult. The Brown-Headed Cowbirds above are nest parasites. They originally followed the bison on the Great Plains, feeding on insects kicked up by the large herds. Since the bison themselves migrated, following the melting snows and eating the fresh spring grass, the cowbirds had to move as well. This presented a problem, as it's hard to incubate eggs on the move.
Lay the eggs in other birds' nests, and trick the other birds into raising your young. The cowbirds hatch out first, push the other eggs out of the nest, and the nest-builders often much smaller than the rapidly growing cowbird end up feeding it instead of their own young.
Even though the other birds may pattern their eggs the cowbirds are up to the challenge. Cowbirds hesitate entering forests, but roads, farms, powerlines and other human intrusions give them a pathway deep into the woods where they are free to parasitize the nests of birds which until the arrival of humans didn't have to worry about the cowbirds.
Some of these bird species are on the verge of extinction as a result. Bromeliads left, above left avoid the hassle of crating a trunk to lift their leaves above the forest floor and closer to the sun.
They simply grow on the branches of trees. Since the bromeliads don't take any nutrients from the trees this is usually classified as a commensalism, but if there are a lot of bromeliads left the tree will need to add extra wood to support the weight a bromeliad can trap up to 10 gallons 80 pounds of water in its leaves.
So, if there are a lot of bromeliads the relationship overall turns into a negative for the tree.
The bromeliads also host a number of organisms in the water they trap; the wastes from the animals living there undoubtedly fertilizes the bromeliad in a mutualistic relationship. The tree at lower left is absolutely covered with epiphytes. Leeches below left are usually thought of as ectoparasites although some are predators.
They attach to a vertebrate host and take a blood meal before dropping off. Most aren't adapted to a single vertebrate host, but they are highly adapted to sucking blood; their saliva includes anesthetics to help keep the host from noticing the bite, as well as anticoagulants to keep the blood flowing. Below is a larval mussel freshwater clam. If there is any case of "good" parasitism, this may be it.
The little mussels go into the mouth and pass over the gills. Here, they clamp down by closing the shell and digging in with the little teeth pictured at the edge of the shell. The fish provides a meal and transport upstream moving is not something mussels do well over long distances, particularly upstream. Lichens above and left are mutualistic associations between a fungus and an algae or cyanobacteria. They are the terrestrial equivalents in some ways of corals.
The fungus provides a tough, waterproof body able to withstand extreme environments on rocks and tree trunks. It is good at obtaining water and secretes acids to dissolve minerals from the rocks. It also produces carbon dioxide. All of these materials are then funneled to the endosymbiotic algae or cyanobacteria, which use the materials in photosynthesis and produce sugars which are then shared with the fungus.
Some studies have shown that the fungus benefits from this relationship more so than the algae; at least under good conditions algae grown without the fungus grow faster then they do with the fungus.
This wasp has stung and paralyzed a stink bug and is dragging it to its underground lair. Here it will deposit an egg and the larvae that hatches from the egg will eventually consume the bug.
Keeping the bug alive but paralyzed ensures it doesn't rot. The two lice to the right parasitize humans. The body louse above can attach to hairs of the body or head and then suck blood from the host. While it is relatively easy to remove the adults particularly if your hair is thinthe eggs are another story.
The eggs are called nits and are glued to the hairs, the careful search for these tiny eggs has given us the term "nitpicking". The larger claws of the crab louse allow it to grasp the thicker pubic hairs. Overall, lice aren't the biggest health concenr humans face; on their own they do relatively little damage.
The diseases they can transmit, however, can cause devastating epidemics and many deaths. Fleas below are adapted to live in mammals with thicker hair. The comb-like structures help them hang on. The mosquito above is a very temporary exoparasite; it probably shouldn't be counted as a symbiont so much as a predator.
Not all situations are readily apparent. The mites on the bumblebee at left are in fact sucking fluids from it; mites have been implicated in the decline of our commercial honeybees.
HOST PARASITE RELATIONSHIPS
This is a clear case of ectoparasitism. On the other hand, the mites in the image above left are merely hitching a ride on the Carrion Beetle. This beetle locates dead animals and flies to the carcasses to lay its eggs, which hatch and feed on maggots on the carcass.
The mites are interesting. Often, they feed on fly eggs and small maggots; this reduces competition for the carrion by the flies, and thus actually helps the beetles out a bit. The mites do NOT suck fluids from the beetle; they merely hitch a ride and thus make a trip they would not be able to make on their own. This hitchhiking is called phoresy, and as long as the phoretic animals are much smaller than their hosts - and there aren't too many of them - this would qualify as a commensal relationship.
If the mites help to reduce the maggot population and thus reduce competition for the beetle, they may actually be benefiting the beetle and thus move this relationship into mutualism.
Right - A leafcutter ant tending fungus in its underground nest. The fungus is almost completely dependent on the ants. The ants bring in nutrients bits of plant leavesprune the fungus back, transfer it to new bits of leaves and even to new ant nestsremove competing fungi, bring in only leaf bits from trees without chemicals which would hurt the fungus, etc.
Perhaps most amazing is the fact that the ants enlist a second symbiont - bacteria of the genus Streptomyces that the ants grow in specially modified areas of their own exoskeletons.