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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
• 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
The castration barnacle—
Anelasma squalicola – this one’s for
you Steven…
Vampire Catfish
Detects thin trail of urine that
gills secrete (other fish)
Real – life
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 –
• 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.
• Schizophrenia
• More outgoing, warmhearted and
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
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
• Migrates from intestines to lungs
• Ascaris pneumonia – can cause death, or….
• Permanent cough – phlegm – spit – swallow –
Here’s what happens under general
anaesthesia and the worms think
you’re dying…..
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.
Parasites are abundant within the
• 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
• See Kuris et al. (2008) Ecosystem energetic
implications of parasite and free-living biomass in
three estuaries Nature 454, 515-518
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
• 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
– 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
– 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:
– range of density of predators tolerated
– range of concentrations of an allelopathic chemical
– range of intensities of competition from another
species tolerated
– range of temperatures tolerated
– range of pH tolerated
Niche explained (2)
• The requirements of a species can be abiotic and biotic:
– availability of prey
– appropriate ratios and concentrations of nutrients in
– presence of pollinators or other ecological services
– 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
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
Resource partitioning
• Where realised niches are sufficiently different,
potential competitors can co-exist by resource
– 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
Source: Geoff Morgan
• Niche for ectoparasites is on the surface of its
• Many are Arthropoda, such as the lice
mentioned on the previous slide and the ticks
on this kudu
Source: (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
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
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:
Case: Bubonic plague
Pathogen: Yersinia bacterium
Infects lymphatic system
Several major pandemics
Flea is vector
Infected flea cannot feed
effectively, so bites more
• Large reservoir of infection
as many rodent species are
hosts, with differing levels of
• Without treatment 75% of
infected humans die within a
few days
Source: Geoff Morgan
Sex and Parasitism Tutorial
• Read the above article and complete the
Tutorial Worksheet.
Extension Tutorial
• Effect of Parasitic Infection on mate choice
• Read abstract:
Discuss aspects of sexual investment and optimal
reproduction, courtship, female choice and honest
signals of fitness.