(a) (i) Niche

• An ecological niche is a multi-dimensional summary of tolerances and requirements of a species

• A species has a fundamental niche that it occupies in the absence of any interspecific competition

• 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

(ii) The parasite niche

• Parasitism is a symbiotic interaction between a parasite and its host (+/-)

• A parasite gains benefit in terms of nutrients at the expense of its host

• Research the ecology, evolution, reproduction, and physiology of a selected human parasite.

• Unlike in a predator–prey relationship, the reproductive potential of the parasite is greater than that of the host

• Most parasites have a narrow (specialised) niche as they are very host-specific

• As the host provides so many of the parasite’s needs, many parasites are degenerate, lacking structures and organs found in other organisms

• An ectoparasite lives on the surface of its host, whereas an endoparasite lives within the tissues of its host

(b) Parasitic life cycles

• Some parasites require only one host to complete their life cycle

• Many parasites require more than one host to complete their life cycle

  • The definitive host is the organism on or in which the parasite reaches sexual maturity. 

  • 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

• The human disease malaria is caused by Plasmodium

  • An infected mosquito, acting as a vector, bites a human. 

  • Plasmodium enters the human bloodstream. 

  • Asexual reproduction occurs in the liver and then in the red blood cells. 

  • When the red blood cells burst gametocytes are released into the bloodstream. 

  • Another mosquito bites an infected human and the gametocytes enter the mosquito, maturing into male and female gametes, allowing sexual reproduction to now occur. 

  • The mosquito can then infect another human host.

• Schistosomes cause the human disease schistosomiasis

  • Schistosomes reproduce sexually in the human intestine. 

  • The fertilised eggs pass out via faeces into water where they develop into larvae. 

  • The larvae then infect water snails, where asexual reproduction occurs. 

  • This produces another type of motile larvae, which escape the snail and penetrate the skin of a human, entering the bloodstream.

  • Viruses are parasites that can only replicate inside a host cell

    • Specific examples of viral life cycles are not required.

  • Viruses contain genetic material in the form of DNA or RNA, packaged in a protective protein coat

  • Some viruses are surrounded by a phospholipid membrane derived from host cell materials

  • The outer surface of a virus contains antigens that a host cell may or may not be able to detect as foreign

  • Viral life cycle stages: infection of host cell with genetic material, host cell enzymes replicate viral genome, transcription of viral genes and translation of viral proteins, assembly and release of new viral particles

  • RNA retroviruses use the enzyme reverse transcriptase to form DNA, which is then inserted into the genome of the host cell

  • Viral genes can then be expressed to form new viral particles

(c) Transmission and virulence

• Transmission is the spread of a parasite to a host

• Investigate the spread of a plant pathogen in a variety of planting densities and humidities.

• Virulence is the harm caused to a host species by a parasite

• Ectoparasites are generally transmitted through direct contact or by consumption of intermediate hosts

• Endoparasites of the body tissues are often transmitted by vectors

• Factors that increase transmission rates:

  • the overcrowding of hosts when they are at high density

  • mechanisms, such as vectors and waterborne dispersal stages, that allow the parasite to spread even if infected hosts are incapacitated

• Host behaviour is often exploited and modified by parasites to maximise transmission

  • Alteration of host foraging, movement, sexual behaviour, habitat choice or anti-predator behaviour.

• The host behaviour becomes part of the extended phenotype of the parasite

• Parasites often suppress the host immune system and modify host size and reproductive rate in ways that benefit the parasite growth, reproduction or transmission

(d) Defence against parasitic attack

• Immune response in mammals has both non-specific and specific aspects

• Non-specific defences

• Physical barriers, chemical secretions, inflammatory response, phagocytes, and natural killer cells destroying cells infected with viruses are examples of non-specific defences

  • Epithelial tissue blocks the entry of parasites; hydrolytic enzymes in mucus, saliva and tears destroy bacterial cell walls; low pH environments of the secretions of stomach, vagina and sweat glands denatures cellular proteins of pathogens.

  • Injured cells release signalling molecules. 

  • This results in enhanced blood flow to the site, bringing antimicrobial proteins and phagocytes.

  • Killing of parasites using powerful enzymes contained in lysosomes, by engulfing them and storing them inside a vacuole in the process of phagocytosis.

  • Natural killer cells can identify and attach to cells infected with viruses, releasing chemicals that lead to cell death by inducing apoptosis.

• Specific cellular defences

• A range of white blood cells constantly circulate, monitoring the tissues

• If tissues become damaged or invaded, cells release cytokines that increase blood flow resulting in non-specific and specific white blood cells accumulating at the site of infection or tissue damage

• Mammals contain many different lymphocytes, each possessing a receptor on its surface, which can potentially recognise a parasite antigen

• Binding of an antigen to a lymphocyte’s receptor selects that lymphocyte to then divide and produce a clonal population of this lymphocyte

• Some selected lymphocytes will produce antibodies, others can induce apoptosis in parasite-infected cells

• Antibodies possess regions where the amino acid sequence varies greatly between different antibodies

• This variable region gives the antibody its specificity for binding antigen

• When the antigen binds to this binding site the antigen-antibody complex formed can result in inactivation of the parasite, rendering it susceptible to a phagocyte, or can stimulate a response that results in cell lysis

• Memory lymphocyte cells are also formed

  • Initial antigen exposure produces memory lymphocyte cells specific for that antigen that can produce a secondary response when the same antigen enters the body in the future. 

  • When this occurs antibody production is enhanced in terms of speed of production, concentration in blood and duration.

(e) Immune evasion

• Parasites have evolved ways of evading the immune system

• Endoparasites mimic host antigens to evade detection and modify host immune response to reduce their chances of destruction

• Antigenic variation in some parasites allows them to change between different antigens during the course of infection of a host

• Compare antigenic variation in trypanosomes with antigenic variation in the influenza virus.

• It may also allow re-infection of the same host with the new variant

• Some viruses escape immune surveillance by integrating their genome into host genomes, existing in an inactive state known as latency

• The virus becomes active again when favourable conditions arise

(f) Challenges in treatment and control

• Research how attempts to disrupt the lifecycle of Plasmodium in the control of malaria have resulted in the loss of apex predators due to bio-magnification of the organochloride insecticide, DDT.

• Epidemiology is the study of the outbreak and spread of infectious disease

• The herd immunity threshold is the density of resistant hosts in the population required to prevent an epidemic

• Vaccines contain antigens that will elicit an immune response

• The similarities between host and parasite metabolism makes it difficult to find drug compounds that only target the parasite

• Antigenic variation has to be reflected in the design of vaccines

• Some parasites are difficult to culture in the laboratory making it difficult to design vaccines

• Challenges arise where parasites spread most rapidly as a result of overcrowding or tropical climates

  • Overcrowding can occur in refugee camps that result from war or natural disaster or rapidly growing cities in LEDCs.

• These conditions make co-ordinated treatment and control programs difficult to achieve

• Civil engineering projects to improve sanitation combined with co-ordinated vector control may often be the only practical control strategies

• Improvements in parasite control reduce child mortality and result in population-wide improvements in child development and intelligence, as individuals have more resources for growth and development