Parasitism describes a biological interaction where one organism, the parasite, lives on or in another, the host. The parasite benefits by obtaining resources, while the host is harmed. This ecological relationship is found across diverse life forms.
Characteristics of Parasitism
A parasitic relationship is a close, long-term association where the parasite gains sustenance and resources like nutrients, shelter, or reproduction from the host. The host experiences negative effects like weakened health, disease, or reduced reproduction. Parasites generally do not kill their hosts immediately, as the host remains essential for the parasite’s survival. This distinguishes parasitism from predation, where the predator kills its prey, and from mutualism, where both organisms benefit, as only one organism gains an advantage.
Examples in Animal Hosts
Ectoparasites live on the external surface of their hosts. Fleas and ticks are common ectoparasites on mammals and birds, feeding on blood or skin. They can cause discomfort, allergic reactions, and transmit diseases. Lice also spend their entire life cycle on the host, feeding on blood.
Endoparasites reside inside the host’s body. Tapeworms, for example, are flat, segmented worms living in animal intestines, including humans. They absorb nutrients from digested food, leading to nutritional deficiencies. Roundworms are another endoparasite type inhabiting various organs and tissues.
Brood parasitism occurs in some bird species, where the parasite manipulates another species to raise its young. Cuckoos are well-known brood parasites; the female lays eggs in a host bird’s nest, which incubates and raises the cuckoo chick. Brown-headed cowbirds also lay eggs in many host species’ nests. Their nestlings often compete with host nestlings, reducing host reproductive success.
Parasitism Beyond the Animal Kingdom
Plant Parasites
Plant parasites derive water and nutrients from host plants. Mistletoe is an obligate hemiparasite that photosynthesizes but also siphons water and minerals from host trees via haustoria. Mistletoe can impede host tree growth and increase drought susceptibility. Dodder is another plant parasite lacking chlorophyll, relying on its host for food. It uses haustoria to penetrate host tissue and extract carbohydrates. Dodder can reduce crop yields and kill its host.
Fungal Parasites
Fungal parasites also cause diseases in plants. Rust fungi, for example, are obligate plant pathogens that infect living plants, causing pustules and reducing host vigor and productivity. Smut fungi are another group that parasitizes plants, particularly grasses. They colonize plant tissues, causing stunted growth or distorted organs, impacting crop quality.
Microbial Parasites
Microbial organisms like bacteria and viruses also exhibit parasitic behaviors. Viruses are obligate intracellular parasites, replicating only inside host cells using host cellular machinery. This can lead to cellular damage or disease. Similarly, some bacteria act as parasites by invading host cells. They can produce toxins or manipulate host cell behavior, contributing to bacterial infections.