Parasitism

What is Parasitism?

Parasitism is a close relationship between species where one organism lives on or inside another, causing harm. This relationship can be seen in various forms, from single-celled protozoans to complex insects and fungi. Parasites are not just a nuisance; they play significant roles in the ecological balance of our world.

Types of Parasitism

Parasites use six major strategies to exploit their hosts: parasitic castration, directly transmitted parasitism, trophically-transmitted parasitism, vector-transmitted parasitism, parasitoidism, and micropredation. Each strategy represents an adaptive peak in the evolutionary arms race between parasite and host.

Endoparasites vs Ectoparasites

Parasites can be classified into endoparasites (living inside the host’s body) or ectoparasites (living outside on the host’s surface). These parasites reduce host fitness through pathology and modification of behavior, while increasing their own fitness by feeding on hosts and using intermediate hosts to transmit themselves.

Parasite Strategies

The six major parasitic strategies are:

• Parasitic castration: Feeding on the host’s reproductive organs or secreting chemicals that destroy them.
• Directly transmitted parasitism: Parasites are passed from one host to another without a vector, such as lice and mites.
• Trophically-transmitted parasitism: Parasites are ingested by the host, like trematodes and cestodes.
• Vector-transmitted parasitism: Parasites rely on an intermediate host to carry them from one definitive host to another, such as mosquitoes carrying malaria.
• Parasitoidism: Insects that kill their hosts and can be divided into idiobionts (sting their prey) and koinobionts (lay eggs inside young hosts).
• Micropredation: Parasites attack more than one host, reducing each host’s fitness by at least a small amount.

Parasite Life Cycles

Parasites can be obligate or facultative. Obligate parasites depend on hosts to complete their life cycles, while facultative parasites do not. Parasite life cycles can be direct or indirect, with endoparasites living inside hosts and ectoparasites living outside.

Host Cues

Parasites use various methods to infect hosts, including physical contact, the fecal-oral route, free-living infectious stages, and vectors. Host cues used by parasites include vibration, exhaled carbon dioxide, skin odours, visual and heat signatures, and moisture.

Variations on Parasitic Strategies

There are variations on parasitic strategies such as hyperparasitism (feeding on another parasite), social parasitism (taking advantage of interspecific interactions between eusocial animals), brood parasitism (laying eggs in nests of other species and increasing parental investment and energy expenditure), and kleptoparasitism (stealing food gathered by the host).

Examples of Parasites

Parasites include hyperparasites, social parasites like large blue butterflies and bumblebees that invade other colonies without a queen, brood parasites such as cowbirds and cuckoos that lay eggs in nests of other species, and kleptoparasites like birds that steal food gathered by the host. Sexual parasitism occurs where males are reduced to tiny parasites that depend on females for survival.

Parasitic Plants and Fungi

In the plant kingdom, parasitic plants derive nutrients from other living plants (hemiparasites) or rely entirely on another plant for nutrition (holoparasites). Parasitic fungi are classified into biotrophs, hemibiotrophs, and necrotrophs. Pathogenic fungi can cause serious diseases in animals as well as humans.

Parasitism in Evolutionary Ecology

Parasitism is a major aspect of evolutionary ecology; almost all free-living animals are host to at least one species of parasite. Vertebrates, the best-studied group, are hosts to between 75,000 and 300,000 species of helminths and an uncounted number of parasitic microorganisms.

Parasites in History

The study of parasites is important for understanding their role in ecosystems and for developing effective control strategies. Parasites occupy multiple positions in food webs, causing energy loops and making it difficult to analyze the system. The eradication of a single parasite species can have unforeseen consequences.

Parasite Ecology

In parasite ecology, traditional statistical methods are often invalid due to the aggregated distribution of parasites across hosts. Instead, advanced biostatistical methods are used to analyze data. Parasites have been documented throughout history and modern parasitology developed in the 19th century with accurate observations and experiments by researchers such as James Annersley, James Paget, and Patrick Manson.

Conclusion

The study of parasites is crucial for understanding their role in ecosystems. Parasites are not just harmful but also play significant roles in ecological balance. By studying these fascinating organisms, we can better understand the complex relationships that shape our world.