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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) 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. 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 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 regrowth 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 shoots 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 shoots 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? 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+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 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 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 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) 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 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 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.