Parasite interactions
Interactions in parasite communities
► Biotic factors
► Parasite interactions within the host
► Interspecific interactions
► Positive - disruption of host defence mechanisms by one
parasite species can facilitate the use of different host
species
► Negative - the presence of one species of parasite leads to
reduction of population size, changes in distribution or
limitation of reproduction of another species
► Intraspecific interactions
Ecological niche of parasites
► Multidimensional space of parasite habitat defined by the
biotic and abiotic variables
► Parasite occupies a specific position in host = habitat
i.e. habitat of endoparasites – intestine
► Niche = determined by the range of all positions of all
individuals of a given species
Niche dimension = mean or median position
(!!! in simple case a niche measured as unidimensional i.e.
lentgh of intestine)
Ecological niche of parasites
Host habitat (gills) →
microhabitats
transversal
longitudinal
vertical
lateral
inner and outer surfaces
Basic versus realized ecological niche
► Hutchinson 1957
► Basic (preinteractive, precompetitive) - virtual range of
positions where the parasite reproduces itself and
survive in the absence of competitor
► Realized (postinteractive, postcompetitive)
- subunit of basic niche reduced due to interspecific
interactions
Fundamental and realized niches, niche
overlap
Ex. Distribution of Hymenolepis diminuta
(Cestoda) and Moniliformis dubius
(Acanthocephala) in the intestine of rats
Experimental infection
- single-species
- double-species
Fundamental niche of parasites
Ex. Basic niches of intestinal helminths (prevalence > 25%)
in two species of grebes: (a) Aechmophorus occidentalis, (b) Podiceps nigricollis
Numerical responses to competition
► reduction of parasite population size in the presence of
other parasite species
asymmetric output - affected only one species
symetric output – reduction of infrapopulation sizes in both
species
2 nematode species
in rats
2 digenean species
in IH (Mollusca)
2 acanthocephalean species
in IH (Amphipoda)
single infection
concurrent
infections
Infraopulationsize
Infraopulationsize
Cystacanthvolume
Functional response to competition
► Shift in realized niches in different species or reduction of
niche overlap due to interactions
► Functional response occurs with or without numerical
effects
Niche space occupied
Relativeinfrapopulationsize
Relationship between the overlap in realized
niche and the overlap in fundamental niche
Ex. 120 pairwise associations among 16 species of intestinal
helminths in Aythya affinis
Overlap of multidimensional niche
► Two-dimensional niche - high overlap only in one dimension
Ex. Overlap between pairs of cestodean species parasitizing in spiral valve of
intestine in two species of elasmobranches (a) 9 species in Urobatis halleri (b) 5
species in Leucoraja naevus
Next type of functional response
► congeneric species with the same size (or morphology) in
basic niches → divergence of size in overlapping niches –
e.g. morphology of beaks of Galapagos finches
Ex. Body length in two co-occuring congeneric digenean species
Pharyngostomoides adenocephala and P. procyonis parasitizing raccoons
Interactive versus non-interactive parasite
communities
► Interactive community – competition
► Non-interactive community – species coexistence
► Continuum between non-interactive and interactive
community depending on niche space
► Saturated community – number of species cannot increase
or number of species increases with decreasing niche size
► Non-saturated community – niche space free for parasite
colonization, absence of competitors
Competitive exclusion principle
► Gause´s law, Gause (1934) – first experimental competiton
- affecting species with similar ecological requirements →
2 species with the same niche cannot coexist permanently
2 species with the same ecology cannot coexist permanently
if 2 species coexist,
there was a differentiation
of realized niches
the coexistence of competing species is allowed to
differentiate niches, otherwise competitive exclusion
Competitive exclusion principle
Ecological niche in parasites: dimensions
Rohde (1979)
► Host specificity
► Microhabitat
► Macrohabitat
► Geographical distribution
► Sex and age of hosts
► Season
► Food
Host specificity
► Restriction of a given parasite species to a given host
species (or range of host species)
► Specialist vs. generalist
Microhabitats
► Preference of a parasite species for a specific microhabitat/-s
(different morphology and physiology of microhabitats)
Macrohabitats
► Habitat of hosts
► Sandy beach, rocky shore, estuary, deep sublittoral…
► Certain macrohabitats of a given host species, or several
hosts from different macrohabitats
► Macrohabitat of parasites - narrower or wider than host
habitat
► Ex. Larvae of nematodes parasitizing marine fish from
Queensland
- Anisacis – open water
- Contracaecum – coastal shallow water
- Pseudoterranova, Thynnascaris – intermediate distribution
Geographical distribution
► The spatial dimension of a niche, sometimes analogous to
a macrohabitat
► Parasites - wider geographical range than host (more host
species in different geographical areas)
- narrow geographical range (one host
species, infection only in part of area of host
distribution)
Ex. Diclidophora dinticulata (Monogenea) – fish Pollachius
virensis in the Barents Sea, Merluccius merluccius and
Gadus minutus in the Atlantic
Ex. Pseudothoracocotyla gigantica (Monogenea) only Heron
Island (Great Barrier Reef) on Scomberomorus commerson
- commonly distributed in the Indian Ocean
Sex of host
► limited evidence
► Different food preferences, different composition of the
epidermis of males and females
► Ex. Discocotyle sagittata (Monogenea) on Salmo trutta
parasitizes 5-7 year old males more often than females
Ex. Calicotyle kroyeri (Monogenea) is not present in gravid
female stingrays Raja radiata
Ex. males of snails Hydrobia ulvae in Britain are more
parasitized by larval stages of digeneans than females of
snails (16:1)
Age of host
► Preference for age category
► More common than gender preferences
► Ex. Bychowsky (1957) – many Gyrodactylus species –
100% mostly on young specimens of host
Ex. Diclybothrium armatum (Monogenea) - absence on
young sturgeons, 70-80% on adults
Food
Niche heterogeneity in parasites
► Interactive niche heterogeneity – shift of niches in different
species and reduction of niche overlap due to competition
► Spatial and temporal niche heterogeneity – i.e. seasonal
occurrence of parasite species
► Niche heterogeneity facilitates parasite species coexistence
in host
► If there is no niche heterogeneity → competitive exclusion
of one species by the other one (e.g. larvae of digeneans
in snails)
Ecological niche of parasite
► Predicted and limited localization on/in host - restricted
(limited) niches
► Separation of niches between different species – niche
segregation
► Limitations of the niche at the level of the microhabitats
Niche restriction in parasites
► Interactive segregation = functional response in the
presence of competitor (i.e. intestinal parasites in
Podiceps) - reduction of overlap in realized niches
► Selective niche segregation = strong competition over
several generations - genetically fixed niche shift or
reduction of overlap of fundamental and realized niches
- evolutionary consequence of competition between
sympatric species
Evolutionary niche restriction in parasites
► Specific niches of parasites in the absence of current
competiton
► product of past competition „ghost of competition from the
past“ – by the mechanisms of selective niche segragation
► independent on competition - many free niche space for
parasite colonization and to facilitate intraspecific contacts
and reproduction
e.g. Monogenea and Crustacea on fish gills – small
infracommunities, many congeneric species on host
Specific niches of congeneric parasites
► Specialization and adaptation
► Morphology of attachment organ (haptor in monogeneans)
Dactylogyrus species in Rutilus rutilus (Cyprinidae)
Segregation of niches by specialization
► Holmes (1990) - intestinal helminths
► Parasites select certain niches for localization, in the
case of introduction to other niches, active movement
to the preferred niche
► The range of niches of adult parasites is smaller than
in larvae - strict requirements for reproduction
► The preferred location does not change in the case of
increasing population density, and expanding the niche
Reinforcement of reproductive barriers in
congeneric parasites
Evolution of preferred niches in
congeneric parasites
Quantification of ecological niche
► Niche width according to Levins (1968)
where pj is the proportion of individuals of a species
found in sector j
1
2
ia jap p
R

 

► Renkonen index of niche overlap (Renkonen, 1938)
where pia is the proportion of individuals of
species i in sector a, and pja is the proportion of
individuals of species j in sector a
2
1
( )j
B
p


Quantification of ecological niche
► Outlying Mean Index
(Dolédec et al., 2000)
► Niche-oriented models
(Tokeshi, 1990)
Coexistence of parasite species
► Niche preference
► Morphological adaptation
► Reproductive isolation
► Agregation - reduction of the overall intensity of
competition through the aggregated use of fragmented
resources (host)
Coexistence and aggregation of parasite
species
► Aggregation model of species coexistence (Shorrocks,
1996)
► applied for parasites (Morand et al. 1999 - ectoparasites
of marine fish)
► Interspecific aggregation is reduced in relation to
intraspecific aggregation
Investigating the coexistence of congeneric
parasites in fish
Agregation model of species coexistence (Shorrocks, 1996)
 
m
m
V
m
m
m
nn
J
p
i
ii
1
1
1
1
1 1
1
11
1
1
1






12C 
1in 2in
1m P
 2mi1
p

2m
 12Cov
1m 2m
Intraspecific aggregation Interspecific aggregation
  
 112
2
21
12
11



C
JJ
ASpecies coexistence
n1i is the number of species 1
in patch i
m1 it the mean number of
species 1 per patch
V1 is the variance in number
of species 1
n1i, n2i,m1i m2i are mean number and variance
in number of species 1 and 2 per patch
P is the number of patch
Cov is covariance between a pair of species
Aggregation model applied for congeneric
monogeneans
9 Dactylogyrus species
24 pairs of Dactylogyrus
positively aggregated
9 pairs of Dactylogyrus
negatively aggregated
3 pairs of
Dactylogyrus
not evaluated
Reduction of interspecific aggregation
in relation to intraspecific aggregation
Species coexistence (A)
Intraspecific aggregation (J) Interspecific aggregation (C)