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Parasitism (i): The parasite niche CfE Advanced Higher Unit 2 Organisms and Evolution SQA mandatory key information • Parasites tend to have a narrow niche as they have high host specificity and may lead to parasites which are degenerate. They are lacking in structures and organs found in other organisms. • Ectoparasite and endoparasite niches. Life cycles, definitive hosts, intermediate hosts and vectors. • Fundamental and realised niches, interspecific competition and competitive exclusion. Key concepts 1 • At least half of all species are parasitic, and all free-living species are thought to host parasites. A parasite is a symbiont that gains benefit in terms of nutrients at the expense of its host. Unlike in a predator–prey relationship, the reproductive potential of the parasite is greater than that of the host. • An ecological niche is a multidimensional summary of tolerances and requirements of a species. As the host provides so many of the parasite’s needs, many parasites are degenerate. The niche for an ectoparasite is on the surface of its host, whereas an endoparasite lives within the host. The organism on or in which the parasite reaches sexual maturity is the definitive host. Intermediate hosts may also be required for the parasite to complete its life cycle. A vector plays an active role in the transmission of the parasite and may also be a host. Key Concepts 2 • In ecology, a species has a fundamental niche that it occupies in the absence of any interspecific competing influences. A realised niche is occupied in response to interspecific competition. As a result of interspecific competition, competitive exclusion can occur where the niches of two species are so similar that one declines to local extinction. Where the realised niches are sufficiently different, potential competitors can co-exist by resource partitioning. Why study parasite ecology? • There are many good reasons to study parasite ecology: – There are so many species of parasite – They are abundant in ecosystems – They are the cause of major human diseases – They can give us an insight into evolution. Some good parasites….. The tongue-eating louse—Cymothoa exigua https://www.youtube.com/watch?feature=pla yer_embedded&v=327-bwMQI-Y The castration barnacle— Anelasma squalicola – this one’s for you Steven… Vampire Catfish Detects thin trail of urine that gills secrete (other fish) Or…… https://www.youtube.com/watch?v=V6EdS8yey Fg Real – life example….https://www.youtube.com/watch?v=1b2hbY 4PbSQ Toxoplasmosa gondii will take over your brain….. This mouse isn’t scared….because it’s infected with the most common parasite in the developed world! On humans? Depends on your sex – men? • Schizophrenia • Men who have the parasite become lethargic and unmotivated. They typically gain weight and perform worse on cognitive tests. Essentially, the parasite makes men clumsy, fat, stupid, and lazy. They’re also found to be more jealous, suspicious, and dogmatic, with less regard for the rules of civilized behaviour. Women? • Schizophrenia • More outgoing, warmhearted and conscientious Ascarid Worms • Sent to earth by an evil force…… Some are helpful to a species - eg. Bayliscaris to racoons The racoon’s very own biological Bayliscaris is harmless to weapon…. racoons but causes crippling seizures and death to birds, rodents and humans. Easier for the racoon to catch. Some are not so helpful….. • Ascaris Ascari – pigs and humans • Food and water contaminated with faecal matter • Migrates from intestines to lungs • Ascaris pneumonia – can cause death, or…. • Permanent cough – phlegm – spit – swallow – stomach….. Here’s what happens under general anaesthesia and the worms think you’re dying….. TASK – RESEARCH 3 MORE PARASITES THAT YOU FIND PARTICULARLY INTERESTING – POWERPOINT PRESENTATION ON THEM. For each - Number of species of parasites • The number of species of parasites is unknown, but estimates suggest that: – All free-living species are thought to be hosts for several species of parasites – At least half of all species are parasitic – See, for example, Dobson et al. (2008) Homage to Linnaeus: How many parasites? How many hosts? Proc. Natl Acad. Sci. 105, 11482–11489. http://www.pnas.org/content/105/suppl.1/11482.full Parasites are abundant within the ecosystem • Traditionally, parasites have wrongly been considered to play a minor role in ecosystems • A recent study of temperate estuary ecosystem revealed parasite biomass to be equivalent to that of the predators (birds) within the same ecosystem • See Kuris et al. (2008) Ecosystem energetic implications of parasite and free-living biomass in three estuaries Nature 454, 515-518 http://hdl.handle.net/10088/11982 Parasites are a major cause of human disease • Parasites are the cause of many of the globally important diseases: – 30 million people have HIV – 300–500 million acute cases of malaria each year – 2 billion people are infected with helminths (flatworms/roundworms). • Parasites also affect our crops and stocks of wild and domestic animals The evolutionary importance of parasites • The study of parasites can teach us much about evolution: – Parasites and hosts often show very strong co-evolution – The study of genomes of parasitic and non-parasitic species gives us evidence of the mechanisms of gene loss and diversification – It is thought that parasites may have played a major role in the evolutionary appearance and maintenance of sexual reproduction in organisms. Parasite definition • A parasite is a symbiont that gains benefit in terms of nutrients at the expense of its host • The host is harmed as it loses energy or resources – A symbiont is an organism that lives in close association with another – Non-symbiotic organisms that gain benefits in terms of nutrients at the expense of others are… predators! Reproductive potential of parasites • To help distinguish the parasitic mode of life it is helpful to consider that parasites have a higher reproductive potential than their host • Like parasites, predators can also have topdown ecosystem effects but they tend to have a lower reproductive rate than their prey The giraffe is clearly the host. The oxpecker feeds on ticks, which are clearly parasites. The oxpecker is not causing harm to the host. But, if ticks are scarce, the oxpecker will feed directly on mammalian blood by opening wounds in the giraffe. The oxpecker is now causing harm! Source: Geoff Morgan Parasite definitions can be messy (1) These sawfly larvae are closely associated with this species of poplar tree. The larvae cause harm to the tree and also have a greater reproductive potential than the tree. Traditionally, these would be called grazers, but are they any different ecologically than ticks or lice on mammals? Source: Geoff Morgan Parasite definitions can be messy (2) Degenerate nature of parasites • As a host provides so many of the parasite’s needs, many parasites are degenerate, lacking in structures and organs found in other organisms: – Tapeworms or Cestoda (Order Platyhelminthes) are gut parasites famed for their lack of digestive organs. They also lack various metabolic pathways found in most free-living animals. – Of course, the lack of certain characteristics is more than compensated for by adaptations for parasitism. Parasite taxonomy • The degenerate nature of parasites obscures their evolutionary relationships and initially made them difficult to classify using phenotypic characteristics • Molecular methods have made these relationships much clearer, particularly for the microparasites, and now most important parasites have had their genomes sequenced • Typical parasitic animals include: – Arthropoda (eg insects: fleas, lice; arachnids: ticks; amphipods: sea lice) – Nematoda (eg round worms such as Ascaris) – Platyhelminthes (eg cestodes: tape worms; trematodes: flukes). Ecological niche • In ecology, the term ‘niche’ is used to describe the role an organism plays within an ecosystem – Initially it was considered sufficient to define this role simply as ‘Parasite’, for example. • However, as there are many types of organisms that could fit into a broad category like this, we now tend to define niches much more specifically: – Niche is the multidimensional summary of tolerances and requirements of a species Niche explained (1) • The tolerances of a species can be biotic or abiotic: Biotic – range of density of predators tolerated – range of concentrations of an allelopathic chemical tolerated – range of intensities of competition from another species tolerated Abiotic – range of temperatures tolerated – range of pH tolerated Niche explained (2) • The requirements of a species can be abiotic and biotic: Biotic – availability of prey – appropriate ratios and concentrations of nutrients in food – presence of pollinators or other ecological services Abiotic – presence of suitable habitat features, such as flat rocks for basking Niche explained (3) • Each of these tolerances and requirements is considered a dimension, but they all operate at the same time and determine whether a species can survive in a particular location – this is the multidimensional summary • It is only easy to imagine or plot one, two or even three of these factors in a single graph… unless you can imagine multidimensional graph paper Niche and competition • A fundamental niche of a species is the niche that it occupies in the absence of any interspecific competing influences • A realised niche is the narrower niche occupied in response to interspecific competition • If the niches of two different species are very similar then competition between them will cause the population decline and local extinction of one. This is termed competitive exclusion Invasive birds and avian malaria In many parts of the world the European House Sparrow is an invasive introduced species Native species have declined in the US, for example, as a result of the spread of the European House Sparrow It had been hypothesised that the introduction of the sparrow had also introduced a European species of avian malaria that the native birds species were particularly susceptible to The study below showed that, in fact, the sparrow had lost its Source: Geoff Morgan avian malaria and this increased fitness had given it a competitive advantage Parasite niche is narrow • Parasites tend to have very narrow niches as they are specialist organisms • They may have a very specific host specificity • A good example of this are the lice of birds and mammals. Many species are restricted to living on part of a single host species. See also http://www.nhm.ac.uk/aboutus/news/2011/april/lice-thrived-before-dinosaurextinction96154.html Resource partitioning • Where realised niches are sufficiently different, potential competitors can co-exist by resource partitioning – For example, bird lice are either ‘body lice’ or ‘wing lice’. The resource that is the bird’s skin and feathers has been partitioned into two different niches with lice species adapted to each – The various common internal parasites of a host tend to parasitise different internal organs Ectoparasites Source: Geoff Morgan • Niche for ectoparasites is on the surface of its host • Many are Arthropoda, such as the lice mentioned on the previous slide and the ticks on this kudu Endoparasites Source: phil.cdc.gov (Wikimedia) • The niche for endoparasites lies within the body of the host The beef tapeworm Taenia saginata; Scale bar = 30 cm For example, the beef tapeworm (Taenia saginata) is a parasitic platyhelminth that can grow to 10 m in length within the human gut. The tegument (outer layer) of the parasite is highly adapted to absorb nutrients from the host gut and its metabolism is adapted to the anaerobic conditions found there Host definitions • Parasite life cycles may involve more than one host • The definitive (or primary) host: the organism on or in which the parasite reaches sexual maturity • An intermediate (or secondary) host: these may be required for a parasite to complete its life cycle • A vector: plays an active role in the transmission of the parasite and in different cases may, or may not, be a host • Other host definitions include dead-end hosts and reservoir hosts Source: CDC Case: Sleeping sickness Sexual reproduction of the Trypanosoma parasite occurs in the tsetse fly, which makes the latter the definitive host and humans the intermediate host.See: http://www.sciencedaily.com/releases/2011/02/110217125121.htm Case: Bubonic plague Pathogen: Yersinia bacterium Infects lymphatic system Several major pandemics Flea is vector Infected flea cannot feed effectively, so bites more hosts • Large reservoir of infection as many rodent species are hosts, with differing levels of susceptibilty • Without treatment 75% of infected humans die within a few days Source: Geoff Morgan • • • • • Sex and Parasitism Tutorial • http://journals.plos.org/plosone/article?id=10 .1371/journal.pone.0039506 • Read the above article and complete the Tutorial Worksheet. Extension Tutorial • Effect of Parasitic Infection on mate choice • Read abstract: • http://beheco.oxfordjournals.org/content/3/4/34 6.abstract Discuss aspects of sexual investment and optimal reproduction, courtship, female choice and honest signals of fitness.