BIOS 6150: Ecology
Dr. Stephen Malcolm, Department of Biological Sciences
•  Week 8: Parasitism and
Herbivory.
•  Lecture summary:
•  Parasitism:
•  Micro- & macroparasites.
•  Dynamics of host-parasite
interactions.
•  Epidemiology.
•  Herbivory:
•  Examples.
•  Effects of herbivory.
•  Plant defense.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 1
2. What is a parasite?:
•  “An organism that obtains its nutrients from one or
a very few host individuals, normally causing
harm but not causing death immediately.”
•  Intimate association between parasite and host.
•  Dependence upon host for “habitat” stability.
•  Species without parasites are rare
•  probably don't exist!
•  Many parasites and pathogens (microparasites)
are host specific.
•  More than 50% of all species are parasites.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 2
3. Microparasites and macroparasites:
•  Microparasites multiply directly within their host
(intracellular):
•  e.g. bacteria, viruses, protozoa, and fungi
•  Direct and indirect transmission.
•  Macroparasites grow in their host, but multiply
with infective stages released to infect new hosts
(intercellular):
•  e.g. helminth worms (tapeworms, trematodes),
acanthocephalans, nematodes, lice, ticks, fleas & fungi,
plant holoparasites and hemiparasites.
•  Direct and indirect transmission.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 3
4. Population dynamics of disease epidemiology
•  Methods vary according to parasite
type because:
•  Macroparasites can be counted and so
are the units of study.
•  Microparasites are difficult to count
and so infected hosts are the units of
study.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 4
5. Transmission of parasites from one host
to another:
•  Directly transmitted microparasites:
•  Short-lived infective agents:
•  Transmission rate directly proportional to frequency of
encounters between infected and susceptible (uninfected)
hosts.
•  Longer-lived infective agents:
•  Transmission rate proportional to frequency of contact
between hosts and infective stages.
•  Indirectly transmitted microparasites:
•  Vector transmission rate is proportional to:
•  Host biting rate.
•  Proportion of a host population that is susceptible.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 5
6. Density dependent transmission:
hosts as “islands”:
•  Density-dependent transmission (Fig. 12.6).
•  Density dependent reproduction of parasites
(Fig. 12.8).
•  Use of mixtures of resistant and susceptible
individuals in populations (Fig. 12.16):
•  Relevant to immunization programs that dilute the
density, or contact rate, of susceptible individuals in a
high density population:
•  Measles requires 92%-94% vaccination for sufficient
dilution to prevent an epidemic.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 6
7. Parasite distributions:
•  Can be highly aggregated & correspond well with a
negative binomial distribution (Figs 12.10).
•  Prevalence:
•  Proportion of a host population infected:
•  Most widely used epidemiological statistic for microparasites.
•  Intensity:
•  Number of parasites per host:
•  Measurement of severity of infection.
•  These measures compared in Fig. 12.11 for
different distributions:
•  Regular, Poisson & binomial/aggregated.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 7
8. Negative impact of parasitism:
•  Parasites have a negative impact on the
survivorship, growth and fecundity of hosts
(Figs 12.17 and 12.18).
•  Effects of parasitism also commonly interact
with other processes, such as:
•  Competition and predation:
•  e.g. oat competition, or predation of parasitized
red grouse by foxes and harriers.
•  Or disease and malnourishment in lessdeveloped countries.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 8
9. Population dynamics of parasitism:
•  Mathematical models have helped to
understand the population dynamics of
parasitism more effectively than for any
other ecological process.
•  Cycles of measles outbreaks are just like
Lotka-Volterra cycles for predators and
prey because acquired immunity in the
host population has the same effect on
parasite populations as the removal of
prey has on predator populations.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 9
10. Host reproductive rate:
•  The best descriptor of microparasite-host
dynamics is basic reproductive rate of hosts:
•  Ro (= ∑lxmx)
•  For directly transmitted microparasites Ro is:
•  Rp = the average number of newly infected hosts that
arise from each currently infected host.
•  Transmission threshold for spread of the disease:
•  Rp = 1
•  Rp > 1
•  Rp < 1
remains constant
infection spreads
infection dies out
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 10
11. Effects on reproductive rate:
•  Rp increases with:
•  Density of susceptible hosts (S) in the population
•  Immunity will reduce S
•  Transmission rate β
•  β also increases with both the frequency of host
contact and disease infectiousness.
•  Average time (L) an infected host remains
infectious.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 11
12. Host reproductive rate:
•  Therefore:
•  Rp = SβL
•  When Rp = 1 (endemic) the transmission
threshold density of hosts ST is:
•  ST = 1/βL
•  If Rp is high (epidemic) then ST will be low.
•  Cycle in Fig. 12.17 is from “high incidence/few
susceptibles” to “low incidence/many susceptibles.”
•  Critical level of vaccination required to halt disease
transmission (Fig. 12.18).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 12
13. Vector-transmitted microparasites:
•  Rp = average number of new disease cases that
arise from each infected host and vector:
•  Rp = β2 · Nv/Nh · fvfhLvLh
where:
•  Nv & Nh = densities of vector & host.
•  fv & fh = fractions of infected vectors & hosts that survive to
become infectious.
•  Lv & Lh = time that vectors & hosts remain infectious.
•  β = transmission rate
•  Squared because infection is transmitted to and from host.
•  Transmission threshold at Rp = 1 is a ratio,
•  Nv/Nh = 1/β2fvfhLvLh
•  Thus vectors are the target of control & not the parasite.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 13
14. Directly transmitted macroparasites:
•  Can count individual macroparasites, so Rp is a
measure of parasite increase:
•  Rp = (λ Lafa) x (β NLifi)
•
•
•
•
•
where,
adult
infective
a = adult parasite;
i = infective stage.
λ = rate of egg production per adult (often very high as in many
helminths);
N = host density.
L = expected life-span of parasite (a in host and i outside host).
f = proportion of parasites (a in host reach sexual maturity, i that
are infective).
β = transmission rate.
•  High densities result in intense density-dependent
control and stability
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 14
15. Indirectly transmitted macroparasites
(via a vector):
•  Rp = (λ1La1fa1)(β1N1Li1fi1) x (λ2La2fa2)(β2N2Li2fi2)
adult
infective
1 = host
adult
infective
2 = vector
•  λ = rate of parasite offspring production.
•  β1 = transmission to man; β2 = transmission to vector.
•  L = expected life-span of parasite in/out of host & in/out
of vector.
•  f = proportions surviving to infectivity.
•  e.g. low survivorship of infected snails (fa2) limits the
spread of schistosomiasis.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 15
16. Impact of
parasites on
population
dynamics:
Figure 12.27 (3rd
ed): Dynamics of
(a) flour beetleprotozoan, (b)
meal moth-virus (c)
larch budmothvirus dynamics
show good fit to
Anderson & May's
(1980) model, (d)
grouse with or
without nematodes
(see Fig 12.23 &
12.24, 4th ed.)
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 16
17. Anderson’s perspective:
•  Anderson, Roy M. (1991) Populations and infectious
diseases: Ecology or epidemiology? Journal of Animal
Ecology 60: 1-50.
•  “More recently, there has been an encouraging trend for
convergence in the concepts employed by ecologists in
thinking about the transmission and persistence of infectious
agents in natural or managed plant and animal communities,
and those employed by epidemiologists concerned with the
study of infection and disease in human communities.”.......
•  “The similarity in the population-based theories that underpin
the disciplines of ecology and epidemiology, is the central
theme of this paper.”
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 17
18. Long term epidemiology studies:
•  Long term studies are valuable such as the measles data for
England & Wales shown in Anderson's Figure 1. These
data have been described accurately by a simple LotkaVolterra model.
Figure 1.
The incidence of
measles (reported
cases per year) in
England and Wales
1940-88. Mass
immunization was
introduced in 1967.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 18
19. Interdisciplinary approaches to epidemiology:
•  One problem we need to overcome is to narrow the
gap between population genetics and population
ecology because genetic changes associated with
changes in population abundance have important
implications for the dynamics of disease parasites.
•  For example, parasite-host coevolution is dynamic such
as that between humans and the flu virus:
•  Techniques like the polymerase chain reaction (PCR) could
detect amplified changes and help understand the dynamics of
evolving host-parasite interactions.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 19
20. Anderson's summary of models for a directly
transmitted microparasite:
•  Ro = β X/(α + b + σ )
•  This is the same as the Populus relationship,
where,
•  X = density of hosts.
•  β = transmission probability (infected to
susceptible).
•  α = disease-induced mortality rate.
•  b = is per capita mortality of uninfected hosts.
•  σ = rate of recovery from infection.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 20
21. Population dynamics of HIV and AIDS:
•  Anderson’s (1991) AIDS model predictions (Fig. 5).
•  Mittler, Antia & Levin, 1995. Population dynamics of
HIV pathogenesis. TREE 10(6): 224-227.
•  “During the past few months major advances have been
made in elucidating the mechanisms by which HIV
causes AIDS. These experiments illustrate how central
the population dynamics, the within-host ecology and
evolution of this retrovirus and immune system cells are
to the etiology of AIDS and the treatment of HIV infections
by chemotherapy.” (Fig. 1.).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 21
22. Herbivory:
•  Herbivory considered subset of predation based on:
•  (1) Taxonomic classification:
•  Carnivores consume animals, herbivores consume plants and
omnivores consume both.
•  (2) Functional classification:
•  True predators:
•  kill and consume prey immediately; kill many prey.
•  Grazers:
•  attack many "prey"; rarely lethal; only partially consume.
•  Parasitoids:
•  attack single "prey", always lethal, complete consumption.
•  Parasites (micro and macro):
•  attack few or single "prey"; rarely lethal; only partially consume.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 22
23. Basic kinds of herbivores:
•  Grazers
•  Browsers
•  Leaf miners
•  Borers
sheep, bison, rabbits & grasshoppers.
deer, goats and hares.
many insects.
(leaves, stems, trunks, buds, seeds
and fruits) many insects.
•  Root feeders
nematodes, insects, mammals.
•  Sap suckers
many insects, birds and mammals.
•  Gallers
many insects, mites, nematodes and
bacteria.
•  In addition, frugivores, seed predators, pollinators
and nectarivores all feed on plant parts.
•  see Fig. 12.7 from Begon, Harper & Townsend (1996).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 23
24. Effects of herbivores on plants that influence
distribution and abundance:
•  1. Compensation:
•  Despite some compensation herbivores almost always harm
plants (Figures 8.2 & 8.3).
•  2. Herbivory can enhance negative competitive
effects:
•  Fig. 8.4, 2nd ed. & Fig. 8.7.
•  3. Defense:
•  Repeated defoliation by herbivores can kill plants or make
them more susceptible to death:
•  But they can defend:
•  e.g. West's leaf miners and inducible defenses (Fig. 8.4).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 24
25. Effects of herbivores on plants that influence
distribution and abundance:
•  4. Survivorship:
•  Mature plants are usually not killed (although repeated
herbivory can increase mortality).
•  Recruitment can be slowed by herbivores killing seeds/
seedlings - Charles Darwin found 83% mortality.
•  5. Growth:
•  Herbivory can slow or stop plant growth.
•  Timing is important.
•  Grasses tend to be resistant to the effects of grazing
because the low meristem is unaffected
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 25
26. Effects of herbivores on plants that influence
distribution and abundance
•  6. Fecundity can be reduced :
•  Growth related:
•  Smaller plants produce fewer or less viable seeds.
•  Plants may flower later:
•  Can turn annuals into perennials by repeated grazing or
mowing e.g. Poa annua
•  Herbivores can eat reproductive parts (flowers)
directly:
•  Excluding mutualistic pollen or nectar feeding or
exploitative seed predation.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 26
27. Effects of herbivory on plant populations:
•  Impact greatest on stressed individuals
•  Compensation by unaffected individuals because of reduced
intraspecific competition:
•  Thus herbivory and competition can balance each other out and
result in similar densities before and after the event - because net
recruitment/productivity increase.
•  Negative effects of herbivory are modified to some extent in
modular plants:
•  Thus compensation is important.
•  Threshold for compensation is important to consider for
repeated harvests or exploitation by natural herbivores:
•  e.g. locust plagues and herbivore mobility.
•  Herbivore compensation.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 27
28. Functional responses of herbivores:
•  Imply satiation at high levels of food availability.
•  May explain unpredictable masting by trees subject to high
levels of herbivory to swamp herbivores and ensure high
seed/seedling survivorship.
•  But should plants also enhance dispersion to reduce the impact of
intraspecific competition in these years? (Figs 8.11 & 8.12).
•  Herbivore life histories cannot respond?
•  But masting is expensive for the plant!
•  Perhaps not as costly as the impact of severe herbivory!
•  Temporal scaling of life histories:
•  Herbivores with short generation times can track resource quantity
fluctuations more effectively than herbivores with long generation
times.
•  Opposite is probably true for differences in spatial scaling!
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 28
29. Plant defense and herbivore foraging:
•  Herbivory is the
ecological process
that describes the
nature and dynamics
of the interaction
between plant defense
and herbivore foraging
•  Fig. 20.1
Malcolm (1992).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 29
30. Plant apparency and optimal defense:
•  Feeny (1976) and Rhoades & Cates (1976) argued that plant
defenses vary according to plant life history (see summary).
•  “Unapparent” or smaller, weedy plants have:
•  Toxins or qualitative defenses:
•  Like alkaloids, saponins, cardenolides & cyanogenic glycosides
•  Effective against abundant, generalist herbivores, and may
account for the effectiveness of some specialist herbivores.
•  “Apparent” or large, woody plants have:
•  Digestibility reducers, or quantitative defenses:
•  Like tannins.
•  Effective against both specialists and generalists by making
nutrients less available to herbivores.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 30
31. Conclusions:
•  “effects of consumption on a consumer are
not simply and unqualifiedly beneficial.”
•  “effects on a consumed population are not
simply and unqualifiedly harmful.”
•  “consumption is never beneficial to the
individual consumed.”
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 31
Figure 12.6 (3rd ed): Rate of advance of
Pythium through cress seedlings.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 32
Figure 12.8
(3rd ed):
•  Density
dependence in
(a) roundworm
egg production,
(b) tapeworm
egg production,
(c) mean
tapeworm
infection weight,
and (d) mean
tapeworm
individual weight
in mice
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 33
Figure 12.16 (3rd ed.): Influence of potato
variety on rate of spread of potato blight
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 34
Figure 12.10 (3rd ed): Fit of negative binomial to
aggregated distributions of parasite numbers per
host for (a) fox nematode, (b) human lice.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 35
Figure 12.11 (3rd ed): Relationship between prevalence and mean
infection intensity for different distributions.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 36
Figure 12.17 (3rd ed): (a) Parasite effects on host mortality
for (i) nematodes in mosquitoes, (ii) flukes in sheep;
(b) human survivorship in rich and poor countries.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 37
Figure 12.18 (3rd ed): Effects of parasitic mites on
life history fitness measures in a water bug.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 38
Figure 12.17: Cases of (a) measles and
(b) pertussis in England and Wales.
Mass vaccination
started in 1956
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 39
Figure 12.18: Dependence of critical vaccination density
on basic reproductive rate for common human diseases.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 40
Figure 5
(Anderson, 1991):
•  Demographic impact of
AIDS on human
population growth and
age structure as
predicted by a simple
model of the transmission
dynamics of HIV-1 and
human demography.
•  (a) Time-dependent
changes in total age
structure.
•  (b) Changes in age
distribution of people
with AIDS.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 41
Figure 1 (Mittler et al. 1995):
(a) Density of CD4+ lymphocytes (dashed line) and HIV infection (solid
line) in plasma with time since initial infection.
(b) Changes in virus and CD4+ density following administration of antiviral
drugs (protease inhibitors) at the circle in (a).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 42
Figure 12.7
(3rd ed.):
•  Niche diversity in a
plant for insect
herbivores
(predators and
parasites) and
fungal pathogens.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 43
Figure 8.2 (3rd ed): Regrowth after
defoliation in two grass varieties.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 44
Figure 8.3 (3rd ed): Compensatory fruit production in
wild parsnip after defoliation by the parsnip webworm
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 45
Figure 8.4 (Begon et al. 2nd ed.): Effect of parasitic nematodes
on the outcome of competition between two grasses.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 46
Figure 8.7 (3rd ed): Effect of a combination of interspecific
competition and beetle herbivory on Rumex fitness.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 47
Figure 8.4 (3rd ed): Larval leaf miner survivorship
decreases with increasing oak leaf damage.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 48
Figure 8.11 (3rd ed): Masting in Scots
pine and Norway spruce.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 49
Figure 8.12 (3rd ed): Inverse density dependence weevils attacking witch hazel fruits.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 8: Parasitism and herbivory
Slide - 50