Biology · Organisms and Populations · NEET
Parasitism is (+/-). The parasite benefits (+) and the host is harmed (-). (+/0) is commensalism, where one species benefits and the other is neither helped nor harmed. So parasite gets food and a place to live, while the host pays the price. Do not confuse the harmed host of parasitism with the unaffected partner of commensalism.
Both are (+/-): in predation the predator benefits and the prey is harmed; in parasitism the parasite benefits and the host is harmed. The difference is HOW. A predator kills and eats the prey quickly; a parasite lives in or on the host and slowly takes nutrition, usually keeping the host alive for a long time. NCERT groups predation, parasitism and commensalism together because the interacting species live closely together. In a match-the-sign question, if predation is NOT an option, the (+/-) answer is parasitism, and vice versa.
Because they live on or inside a host, parasites evolved: (1) loss of unnecessary sense organs, (2) adhesive organs or suckers to cling to the host, (3) loss of the digestive system (they absorb ready food), and (4) very high reproductive capacity. Their life cycles are often complex, using one or two intermediate hosts or vectors. Example: the human liver fluke needs two intermediate hosts (a snail and a fish); the malarial parasite (Plasmodium) needs a mosquito vector.
Ectoparasites feed on the EXTERNAL surface of the host — examples: lice on humans, ticks on dogs, and marine copepods on fish. Cuscuta, a parasitic plant that lost its chlorophyll and leaves, grows on the outside of host plants and takes their nutrition. Endoparasites live INSIDE the host body at different sites (liver, kidney, lungs, red blood cells). Endoparasites have more complex life cycles, greatly simplified body structure, but very high reproductive potential.
In brood parasitism, the parasitic bird lays its eggs in the nest of a host bird and lets the host incubate them and raise its chicks. The cuckoo (koel) does this in the crow's nest. It is parasitism because the cuckoo benefits (its eggs are raised for free) and the host crow is harmed (it wastes energy raising a foreign chick). During evolution, the cuckoo's eggs came to resemble the host's eggs in size and colour, so the host cannot detect and eject them.
NCERT points out that the female mosquito is not a parasite, even though it needs our blood for reproduction. It does not live in or on the host for a sustained time — it takes a quick blood meal and leaves. A true parasite has a lasting, close association with the host. (Note: the mosquito is instead a vector, e.g. it transmits Plasmodium.)
An ideal parasite would thrive without harming the host. But natural selection favours parasites that reproduce and spread the most, and that usually means taking resources from the host. So most parasites reduce the survival, growth and reproduction of the host, lower its population density, and may even make it physically weak and more vulnerable to predators. Because host and parasite co-evolve, if the host evolves resistance, the parasite evolves ways to counter it — this arms race keeps parasites harmful.
Match List I with List II: List I (Interacting species): A. A Leopard and a Lion in a forest/grassland; B. A Cuckoo laying egg in a Crow's nest; C. Fungi and root of a higher plant in Mycorrhizae; D. A cattle egret and a Cattle in a field List II (Name of Interaction): I. Competition; II. Brood parasitism; III. Mutualism; IV. Commensalism
If '+' sign is assigned to beneficial interaction, '-' sign to detrimental and '0' sign to neutral interaction, then the population interaction represented by '+' '-' refers to
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Parasitism is a (+/-) interaction where the parasite benefits by living in or on a host and getting food and shelter, while the host is harmed.
Loss of unnecessary sense organs, adhesive organs or suckers to cling to the host, loss of the digestive system, and high reproductive capacity.
Ectoparasite: lice on humans, ticks on dogs, or Cuscuta on plants. Endoparasite: liver fluke or Plasmodium living inside the host body.
Brood parasitism is when a bird lays its eggs in another bird's nest and lets the host incubate them. The standard example is the cuckoo (koel) laying eggs in a crow's nest, with eggs evolved to resemble the host's eggs.
Cuscuta is a parasitic plant that lost its chlorophyll and leaves during evolution, so it cannot make its own food. It grows on host plants (an ectoparasite) and takes nutrition from them.
Most parasites are pathogens because they harm the host by living in or on them, but not every harm is a full disease. In ecology, parasitism is defined by the (+/-) sign, not by whether it causes a named disease.
