Parasitism, Mutualism

1
Parasitism, Mutualism commensalism lecture
content

• Continuum of predation
• Parasitism -
• Parasitoids

2
Mutualisms in nature

• Mutualism is an interaction between two species
  in which both participants benefit
• Mutualism thus a , interaction, to contrast
  with competition (-,-), predation, parasitism
  (both ,-)
• Mutualism is one kind of symbiosis
• Latter defined as close (ecologically
  interdependent) relationship of two or more
  species
• Other kinds symbiosis involve parasites,
  predators
• Distinguish obligate from facultative mutualism,
  give examples (class discussion)

3
Mutualisms can be classified ecologically

• Trophic--specialized partnerships for obtaining
  energy and nutrients
• Corals (algae zoozanthellae)
• Nitrogen-fixing bacteria (e.g., rhizobium
  plant)
• Ectotrophic mycorrhizae plants
• Lichens (fungus alga)
• Defensive--partnerships providing protection
  against herbivores, predators, or parasites
• Cleaner fish
• Ant-Acacia (ants protect against herbivores)
• Dispersive--partnerships in which animals
  disperse pollen or seeds of plants, generally for
  food reward
• Flower-pollinator
• Fruit-seed disperser

4
Trophic mutualism formed by coral reef symbionts
Coelenterates zoozanthellae (coralline algae
from Ricklefs, 2001 )
5
Trophic mutualism comprised of Rhizobium
(bacteria are red, false-color image in right
figure) in soybean root nodules (left figure
from Ricklefs, 2001)
6
Defensve mutualism between cleaner organism in
this case a prawn (Lysmata amboiensis, a shrimp
relative) and moray eel prawn gets food, eel
gets parasites removed (from Ricklefs, 2001)
7
Defensive mutualism ants and acacias--e.g.,
bulls horn acacia (Acacia cornigera trees
Pseudomyrmex ants)

• Newly developing bulls horns (evolutionarily
  enlarged thorns)
• Filled with a pith that ants easily remove,
  creating hollow interiors
• Ants chew small hole into each thorn for use as
  home
• Plants also provide ants with extra-floral
  nectar, secreted from glands at base of leaves
  (arrows)

8
Older, hollowed-out bulls horns of Acacia
cornigera, next to main trunk (Photo by T.W.
Sherry)
9
Plants also supply ants with protein and fat-rich
food in the form of Beltian bodies, shown here
being harvested by ants (arrows) from the tips of
newly expanding leaflets of Acacia cornigera
(Photo by T.W. Sherry)
10
Small grove of Acacia cornigera trees in Costa
Rica, showing ground cleared around base of trees
by a single colony of Pseudomyrmex ants (Photo
by T.W. Sherry)

• Pseudomyrmex ants provide two services to Acacia
  trees
• 24-hour patrolling of leaves for protection
  against herbivorous animals (insects and mammals)
  by stinging biting
• Clearing of plants from ground and from Acacia
  trees themselves as protection from competitors
  (for water, nutrients)

11
Ant-acacia system, Costa Rica
12
Ground cleared by ants around Acacia tree in
Costa Rica
13
Dan Janzens (1966) experiment, tested
ecological impact of ants on plantsCo-evolution
of mutualism between ants and acacias in Central
America. Evolution 20 249-275.

• Methods
• Fumigated randomly selected sample of Acacia
  cornigera trees to remove Pseudomyrmex ants
• Kept ants from re-colonizing experimental trees
  using tanglefoot (sticky goo) at base of trees
• Monitored plant growth of cut, re-growing suckers
  (stems), and ant activity at experimental
  (defaunated) versus control trees (containing
  normal densities of ants)
• Results? Next slide...

14
(No Transcript)
15
Janzens conclusions?

• Ants definitely play active role in protecting
  plants from herbivory by insects (and other
  animals)
• Both ants and acacias involved in co-evolved,
  obligate relationship (each depends on other
  species, in specialized, one-to-one relationship)
• Value of ants to plants is particularly great in
  tropical dry forests, where rains dont fall and
  water is limiting to plant growth for up to half
  a year
• Mutualism has evolved here in a stressful
  environment for plants

16
Protective mutualisms Nutritive mutualisms
17
Other facultative mutualisms with extrafloral
nectary plants Ipomoea (Morning glory), various
legumes (Mung Beans etc), Cotton and other
mallows, lots of tropical trees like Balsa.
18
Dispersive mutualism Flowers of Penstemon sp. in
the Sonoran Desert pollinated by the rufous
hummingbird(Photo from www.desertmuseum.org
)Below is another Penstemon sp. being
pollinated by a bee (from helios.bto.ed.ac.uk/
bto/desertecology/bees.htm)
Pollination is an extraordinarily important
mutualism
19
Melastome fruits (see arrow) eaten by, and seeds
dispersed by, Cocos Finch, Pinaroloxias inornata
(Photo by T.W. Sherry T.K. Werner)
20
Coevolution important in mutualisms

• Define Coevolution as reciprocal evolutionary
  adaptations involving both partners of
  ecologically interacting species (often difficult
  to document in nature)
• Coevolution well documented in a few cases
• In Ant-Acacia system, both participants have
  traits that are unique to the interaction, and
  that facilitate the mutualism
• Unique Acacia traits include Beltian bodies,
  hollow thorns
• Ant traits include high running speed, stinging
  ferocity, 24-hour activity patrolling plant,
  attacks on plants
• Dodo birds extinction on Island of Mauritius
  jeopardized survival of its coevolved tree,
  Calvaria major, indicating obligate relationship
  of tree to bird (bird evolved to abrade seed in
  gut, helping germination)

21
Simplistic, but useful model of mutualism based
on expansion of logistic model

• dN1/dt r1N1(X1-N1a12N2)/X1
• dN2/dt r2N2(X2-N2a21N1)/X2
• All variables same as in logistic model, except
  a21 is mutualistic per capita effect of species 1
  on species 2, and a12 is effect of species 2 on
  species 1 these alphas increase Ns
• Also, Ks replaced by Xs, because mutualists can
  attain population size gt carrying capacity for
  each species alone
• How does this model behave? Again, look for
  isoclines
• Species 1 isocline (X1-N1a12N2) 0 implies
  N2 N1/a12 - X1/a12
• Species 2 isocline (X2-N2a21N1) 0 implies
  N2 X2 a21N1
• Both these isoclines are lines of positive slope

22
Isoclines --gt variety of responses, depending on
parameter values (see Stiling, Fig. 9.10)

• Facultative mutualisms (X1, X2 exist, both gt0
  i.e., each mutualist can live alone, without
  other mutualist)
• Isoclines cross gt stable equilibrium
• Isoclines parallel,not crossing gt runaway
  populations (instability)
• More realistic (curvilinear) crossing isoclines,
  in which alphas change with density gt stable
  equilibrium
• Obligate mutualisms (X1, X2 do not exist)
• Isoclines cross gt unstable equilibrium,
  unpredictable outcomes
• Isoclines parallel gt unstability, extinction
  both spp.
• Curvilinear isoclines gt region of stability in
  state space

23
Possible explanations for curved isoclines in
Fig. 9.10 c, f?

• Optimal allocation of energy by species
  interacting mutualistically Excessive resources
  allocated to symbiont will be penalized by
  natural selection
• E.g., plants must produce nectar just sweet
  enough to attract pollinator, but no sweeter
• Similarly, plants must produce fruits just
  attractive enough to be eaten by seed-dispersal
  agent
• This would explain diminishing benefits (and
  reduced population growth) of each species as the
  other increases
• Alternatively, cost of mutualism is substantial
• If cost of mutualism increased with density of
  mutualist, then benefit would be reduced, leading
  to curvilinear isoclines
• E.g. 50 of fig seeds destroyed by larvae of fig
  wasp pollinator (Bronstein)
• Conclusion Mutualism is more complicated than
  just linear positive feedback of each species on
  the other!

24
What does model tell us?

• A variety of outcomes of mutualism are possible,
  all consistent with positive slopes of isoclines
• Outcome depends on parameter values, which
  determine slopes and y-intercepts of isoclines
• Mutualistic organisms may either coexist stably
  at fixed densities, populations spiral upwards,
  or populations collapse to extinction
• Obligate mutualisms should be less stable than
  facultative
• Indeed, some obligate mutualisms fall apart in
  changing environments (e.g., coral bleaching,
  Ingas at higher altitudes, Cecropia on islands)
• Facultative mutualism can be stabilized by
  changing alphas, such that benefit to each
  partner decreases with density

25
Aspects of mutualism not included in model?

• Benefit of mutualism increases with decreased
  resource availability
• Examples
• Nitrogen-fixing Alders in nutrient-stressed bogs
• Many legumes in tropics dominate in nitrogen-poor
  soils
• Plants with mycorrhizal fungi prevalent in
  phosphorus- poor soils
• Corals prevalent in nutrient-poor
  (carbon-limited) tropical water
• Termites cattle use microbial mutualists to
  digest cellulose (plant cell walls wood,
  difficult-to-digest)
• Lesson theory of mutualism needs to incorporate
  resource-use dynamics

26
Another aspect of mutualism not in model

• Mutualism often found in stressed habitats (In
  favorable environments, by contrast, species can
  make it on their own, without expending energy on
  behalf of mutualist)
• Examples
• Ant-acacia mutualism in tropical deciduous
  forests (seasonally water-stressed soils)
• Other nectary and domatia mediated mutualisms
  common on white sand (low nutrient) tropical
  soils.
• Lichens (association of fungus with alga) live in
  physically, and nutrient-stressed environments
  (e.g., arctic tundra, dry soils, water-stressed
  tree canopies, rocks)
• Lesson theory of mutualism needs to incorporate
  life-history characteristics, and negative
  feedback mitigating against mutualism at higher
  population densities

27
Applied ecology humans have developed extensive
mutualisms with plants animals that provide us
with food and other resources. In turn, we
provide nutrients, water, and protection from
herbivores. (Photo by T.W. Sherry)
Blue Mountain Coffee, sun-grown, in Jamaica
(coffee bushes in foreground, and across hills in
distance)
28
Commensalism

• Defined as an ecological relationship in which
  one species benefits from other species, which is
  itself not affected one way or the other by the
  relationship
• This is thus a , 0 relationship
• Examples include spanish moss (epiphyte) on trees
  in Louisiana, cattle egrets, and cactus wren
  nesting in ant acacia trees
• Next few slides illustrate some examples

29
Commensalism between cattle (as food beaters) and
cattle egrets (three white birds, one sitting on
cow) in Jamaica (photo T.W. Sherry)
30
Cactus wren
31
Conclusions

• Mutualism extremely common, widespread in nature
• Human agriculture is mutualistic in nature
• Many mutualisms have co-evolved
• Mutualism ranges from facultative to obligate
• Model of mutualism, based on Logistic model,
  helps explain some aspects of mutualism, but does
  not really explain when they are stable obligate
  mutualism should be less stable than facultative,
  according to theory
• Natural history of mutualism indicates a variety
  of factors that will make models more realistic
  consumer-resource dynamics, tradeoffs, habitat
  stress
• Commensalism also widespread, not well understood

32
Acknowledgements Some illustrations for this
lecture from R.E. Ricklefs. 2001. The Economy
of Nature, 5th Edition. W.H. Freeman and
Company, New York.