Transcript Predator-prey interactions: lecture content

Parasitism, Mutualism &
commensalism: lecture content
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Continuum of predation
Parasitism - +
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Parasitoids
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Mutualisms in nature
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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
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Distinguish obligate from facultative mutualism, &
give examples (class discussion)
Mutualisms can be classified ecologically:
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Trophic--specialized partnerships for obtaining
energy and nutrients
 Corals
(algae & zoozanthellae)
 Nitrogen-fixing bacteria (e.g., rhizobium & plant)
 Ectotrophic mycorrhizae & plants
 Lichens (fungus & alga)
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Defensive--partnerships providing protection
against herbivores, predators, or parasites
 Cleaner
fish
 Ant-Acacia
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(ants protect against herbivores)
Dispersive--partnerships in which animals disperse
pollen or seeds of plants, generally for food reward
 Flower-pollinator
 Fruit-seed
disperser
Trophic mutualism formed by coral reef
symbionts: Coelenterates & zoozanthellae
(coralline algae; from Ricklefs, 2001 )
Trophic mutualism comprised of
Rhizobium (bacteria are red, false-color
image in right figure) in soybean root
nodules (left figure; from Ricklefs, 2001)
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)
Defensive mutualism: ants and acacias-e.g., bull’s horn acacia (Acacia cornigera
trees & Pseudomyrmex ants)
•Newly developing bull’s
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)
Older, hollowed-out bull’s horns of
Acacia cornigera, next to main trunk
(Photo by T.W. Sherry)
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)
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)
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)
Ant-acacia system, Costa Rica
Ground cleared by ants around
Acacia tree in Costa Rica
Dan Janzen’s (1966*) experiment, tested
ecological impact of ants on plants
*Co-evolution of mutualism between ants and acacias in
Central America. Evolution 20: 249-275.
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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)
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Results? Next slide...
Table 1. Total wet weigh t of suckers regenerated, and leaf crop as respons e to cutting o f acacia
shrub stems
Unoccupied (treatmnt)
Occupied (control)
N (sample size = nu mber of stems) 66
72
Total r egrowth wet weigh t (grams) 2,900
41,750
Total numb er of leaves
3,460
7,786
Total numb er swollen tho rns
2,596
7,483
Table 2. Incidence of plant-eating insects on shoo ts of Acacia cornigera
Unoccupied (treatmnt)
Occupied (control)
N (no. o f plant shoot s examined)
1,109
1,241
Daytime
% of shoo ts with insects
38.5
2.7
Mean no. in sects per shoo t
0.881
0.039
Nighttime
N (no. o f plant shoot s examined)
793
847
% of shoo ts with insects
58.8
12.9
Mean no. in sects per shoo t
2.707
0.226
Janzen’s conclusions?
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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 don’t fall and water
is limiting to plant growth for up to half a year
Mutualism has evolved here in a stressful
environment for plants
Protective mutualisms
Nutritive mutualisms
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.
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
Melastome fruits (see arrow) eaten by,
and seeds dispersed by, Cocos Finch,
Pinaroloxias inornata (Photo by T.W. Sherry & T.K.
Werner)
Coevolution important in mutualisms
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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
bird’s 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)
Simplistic, but useful model of mutualism
based on expansion of logistic model
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dN1/dt = r1N1[(X1-N1+a12N2)/X1]
dN2/dt = r2N2[(X2-N2+a21N1)/X2]
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 N’s
 Also, K’s replaced by X’s, because mutualists can attain
population size > carrying capacity for each species alone
 All
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How does this model behave? Again, look for isoclines
Species 1 isocline: (X1-N1+a12N2) = 0 implies
N2 = N1/a12 - X1/a12
 Species 2 isocline: (X2-N2+a21N1) = 0 implies
N2 = X2 + a21N1
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Both these isoclines are lines of positive slope
Isoclines --> variety of responses, depending
on parameter values (see Stiling, Fig. 9.10)
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Facultative mutualisms (X1, X2 exist, both >0; i.e.,
each mutualist can live alone, without other mutualist)
 Isoclines
cross ==> stable equilibrium
 Isoclines parallel,not crossing ==> runaway populations
(instability)
 More realistic (curvilinear) crossing isoclines, in which
alphas change with density ==> stable equilibrium
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Obligate mutualisms (X1, X2 do not exist)
 Isoclines
cross ==> unstable “equilibrium”,
unpredictable outcomes
 Isoclines parallel ==> unstability, extinction both spp.
 Curvilinear isoclines ==> region of stability in state
space
Possible explanations for curved isoclines
in Fig. 9.10 c, f?
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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!
What does model tell us?
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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
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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
Aspects of mutualism not included in model?
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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 phosphoruspoor 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)
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Lesson: theory of mutualism needs to incorporate
resource-use dynamics
Another aspect of mutualism not in model
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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)
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Lesson: theory of mutualism needs to incorporate lifehistory characteristics, and negative feedback mitigating
against mutualism at higher population densities
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, sungrown, in Jamaica (coffee
bushes in foreground, and
across hills in distance)
Commensalism
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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
Commensalism between cattle (as food
beaters) and cattle egrets (three white
birds, one sitting on cow) in Jamaica (photo
T.W. Sherry)
Cactus wren
Conclusions:
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Mutualism extremely common, widespread in nature
 Human
agriculture is mutualistic in nature
 Many mutualisms have co-evolved
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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
Acknowledgements:
Some illustrations for this lecture
from R.E. Ricklefs. 2001. The
Economy of Nature, 5th Edition.
W.H. Freeman and Company, New
York.