parasitism, mutualism & commensalism: lecture content n continuum of predation n parasitism - +...
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Parasitism, Mutualism & commensalism: lecture content
Continuum of predation Parasitism - +
Parasitoids
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)
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
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)
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)
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. 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...
Table 1. Total wet weight of suckers regenerated, and leaf crop as response to cutting of acacia
shrub stems
Unoccupied (treatmnt) Occupied (control)
N (sample size = number of stems) 66 72
Total regrowth wet weight (grams) 2,900 41,750Total number of leaves 3,460 7,786
Total number swollen thorns 2,596 7,483
Table 2. Incidence of plant-eating insects on shoots of Acacia cornigera
Unoccupied (treatmnt) Occupied (control)
N (no. of plant shoots examined) 1,109 1,241
Daytime
% of shoots with insects 38.5 2.7
Mean no. insects per shoot 0.881 0.039
Nighttime
N (no. of plant shoots examined) 793 847
% of shoots with insects 58.8 12.9Mean no. insects per shoot 2.707 0.226
Janzen’s 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 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
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
dN1/dt = r1N1[(X1-N1+12N2)/X1]
dN2/dt = r2N2[(X2-N2+21N1)/X2]
All variables same as in logistic model, except 21 is mutualistic per capita effect of species 1 on species 2, and 12 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
How does this model behave? Again, look for isoclines Species 1 isocline: (X1-N1+12N2) = 0 implies N2
= N1/12 - X1/12
Species 2 isocline: (X2-N2+21N1) = 0 implies N2 = X2 + 21N1
Both these isoclines are lines of positive slope
Isoclines --> variety of responses, depending on parameter values (see Stiling, Fig. 9.10) 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 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?
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? 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
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
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
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)
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
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: 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
Acknowledgements: Some illustrations for this lecture from R.E. Ricklefs. 2001. The Economy of Nature, 5th Edition. W.H. Freeman and Company, New York.