Interdependence Among Organisms Within Their Environment
Senior 5 · Biology
Interdependence Among Organisms Within Their Environment
Lesson 3 of 4
25 min
Look at a hedge in late summer and you may find it covered in thin, yellow-orange threads, like spilled spaghetti. The threads are a flowering plant called dodder (Cuscuta). It has no roots, no proper leaves and very little chlorophyll. It survives by pushing feeding pegs into the stems of other plants and drawing off their sap. Dodder is a parasite, and so are the tick on a cow's ear and the tapeworm in a human gut. This lesson asks how parasites are built for their way of life, and what they cost their hosts.
Definition
Parasitism: a close association in which one organism, the parasite, lives on or in another, the host, and obtains food from it, harming it. Effect: + for the parasite, − for the host.
Ectoparasite: lives on the outside of its host (ticks, lice, fleas, dodder). Endoparasite: lives inside its host (tapeworms, Plasmodium, hookworms).
Obligate parasite: can live only as a parasite. Facultative parasite: can also live independently, for example as a saprotroph.
Primary (definitive) host: the host in which the parasite reproduces sexually. Intermediate (secondary) host: a host in which a larval stage develops. Vector: an organism that carries a parasite from one host to another, such as the Anopheles mosquito for Plasmodium.
A parasite has three problems: to stay attached or hidden in its host, to resist the host's defences, and to reach a new host before this one dies. Its adaptations solve each one.
| Problem | Adaptation | Example |
|---|---|---|
| Staying in place | Hooks, suckers, claws or mouthparts that grip | Scolex of a tapeworm; mouthparts of a tick |
| Feeding | Absorbs food already digested by the host; organs it does not need are reduced | Tapeworm has no gut and absorbs nutrients through its whole surface |
| Resisting the host | Thick protective covering; enzyme inhibitors; hiding inside cells; changing surface proteins | Tapeworm tegument resists digestion; Plasmodium hides inside red blood cells |
| Living with little oxygen | Anaerobic respiration | Gut parasites in the oxygen-poor small intestine |
| Reaching a new host | Huge numbers of eggs; resistant eggs or cysts; intermediate hosts and vectors | A tapeworm releases thousands of eggs per proglottid |
| Reproducing alone | Hermaphrodite: male and female organs in one body | Each mature tapeworm proglottid can fertilise itself |
The adult pork tapeworm lives in the human small intestine. Its body is a chain of segments called proglottids, budded off from a narrow neck behind the head, or scolex.
The life cycle needs two hosts. The human is the primary host, because the adult reproduces sexually there; the pig is the intermediate host.
The danger shown by the dashed arrow is serious. If a person swallows tapeworm eggs, from hands or food contaminated with human faeces, the larvae develop in that person as if they were a pig. Cysts in the brain, called neurocysticercosis, are a common cause of epilepsy in places where pork tapeworm is found, according to the World Health Organization.
Breaking the cycle: cook pork until no pink remains; inspect meat at slaughter; use latrines so that pigs cannot reach human faeces; keep pigs penned rather than free-roaming; wash hands with soap after using the toilet; and treat infected people and pigs.
Worked example 1: why so many eggs?
Illustrative figures: an infected person passes 4 gravid proglottids a day, each containing 40 000 eggs. Suppose only one egg in two million is eaten by a pig and develops into a cysticercus.
(a) How many eggs are released in a year? (b) How many cysticerci result? (c) What does this show?
Plants can be parasites too. They attach with a haustorium, a peg-like organ that grows into the host and joins its vascular tissue.
Definition
Haustorium (plural haustoria): the specialised organ by which a parasitic plant or fungus penetrates its host's tissues and absorbs water and nutrients.
Holoparasite: a parasitic plant with little or no chlorophyll, which gets all its food from its host. Hemiparasite: a parasitic plant that photosynthesises but takes water and minerals (and some food) from its host.
Worked example 2: measuring a parasite's cost
In an illustrative sorghum trial, plots free of Striga gave 2.4 tonnes of grain per hectare, and heavily infested plots gave 0.9 tonnes per hectare. Calculate the percentage loss and suggest why the loss is so large.
Parasites take food, damage tissues (hookworms cause bleeding and anaemia), release toxic wastes, and open the way to other infections. Yet most parasites do not kill their host quickly: a parasite that kills its host loses its home and food before it has passed on its offspring. Hosts fight back with immune responses, grooming and, in cattle, by twitching the skin and swishing the tail. Many parasites have evolved ways round these defences, and the hosts in turn evolve new ones: an evolutionary arms race.
World Organisation for Animal Health (WOAH): "A One Health Approach to Tackling the Pork Tapeworm". Watch for how the parasite passes between pigs and people, and why controlling it needs farmers, vets and health workers together.
Warning
Common mistakes
Tip
Adaptation answers: link each feature to the problem it solves, e.g. "hooks and suckers so that the tapeworm is not swept out by peristalsis". A list of features without the reason scores little.
Practice: parasitism
Key points
Sources and credits
Figures 1 and 2, the tables, the worked examples and the practice questions are written by Ganzaa. The data in the worked examples and practice questions are illustrative.
