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.