Mutualism and Commensalism

What two services to Pseudomyrmex ants provide to Acacia trees

1. 24-hour patrolling of leaves for protection against herbivores (insects and mammals) by stinging & biting 2. Clearing of plants from ground and from Acacia trees themselves as protection from competitors (for water, nutrients)

What were Janzen's conclusions? (4)

1. Ants definitely play active role in protecting plants from herbivory by insects (and other animals) 2. Both ants and acacias co-evolved, creating obligate relationship (each depends on other species, in specialized, one-to-one ecological interaction) 3. 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 4. Mutualism has evolved here in a stressful environment for plants in terms of water stress of tropical dry forest biome

What aspects of mutualism have not been modeled? (2; first with 5 examples and second with 2)

1. Benefit of mutualism increases with decreased resource availability in environment a. Nitrogen-fixing alders in nutrient-poor bogs b. Legumes in tropics often dominate in nitrogen-poor soils c. Plants with mycorrhizal fungi prevalent in phosphorus-poor soils d. Corals prevalent in nutrient-poor (carbon-limited) tropical water e. Termites, cattle use microbial mutualists to digest cellulose (plant cell walls & wood, difficult-to-digest) 2. Mutualism often found in stressed habitats a. Ant-acacia mutualism in tropical deciduous forests (seasonally drought-stressed soils) b. Lichens (association of fungus with alga) often live in physically, and nutrient-stressed, environments (e.g., arctic tundra, dry soils, water-stressed tree canopies, rocks)

Comment on the stability of mutualism models (2 examples of less stable type of mutualism) and how mutualism in general can become more stable (2)

1. Indeed, some obligate mutualisms observed to fall apart when environment changes: e.g., coral bleaching -> coral death 2. Fragile obligate mutualism of Dodo bird and trees it disperses? 3. Facultative mutualism can be stabilized by alphas (interaction coefficients) that vary with density, such that benefit to each partner decreases with density of partner 4. Growing evidence that mutualistic partners adjust their ecological services depending on whether or not partner cooperates or exploits (e.g., figs and fig wasps), and manipulate their partner (like Acacia)

What are the three lessons relating to the aspect of mutualism that has not been modeled yet?

1. Theory of mutualism needs to incorporate resource-use dynamics and manipulations of one partner by another 2. Mutualisms can be complex 3. theory of mutualism just beginning to incorporate life-history characteristics, and negative feedback mitigating against mutualism at higher population densities, i.e., flexible and manipulative responses of mutualists to each other

Three types of mutualism in terms of the ecological roles of the participants

1. Trophic - partnerships for obtaining energy, nutrients 2. Defensive - partnerships providing protection against herbivores, predators, or parasites 3. Dispersive - partnerships in which animals disperse pollen or seeds of plants, generally for food reward

Why do Acacia need to protect their investment (in Pseudomyrmex ants) & keep the ants faithful? (4)

1. ants clearly vital to Acacia fitness 2. expensive to produce Beltian bodies and nectary secretions year-round 3. In seasonally parched Central American dry forests, lots of other animals could exploit Acacia rewards 4. Arguably: Acacia needs ants more than vice versa

What are two bird example of commensalism?

1. cactus wren nesting in ant acacia trees 2. cattle egrets follow around cattle when they stir up bugs

Commensalism

A +,0 ecological relationship in which one species benefits from another species, which is itself not affected one way or the other

What kind of mutualism do we see in coral reefs?

A trophic mutualism

Describe our major example of a trophic mutualism and how our understanding of it has evolved (4 complications)

A trophic mutualism composed of leaf cutter ants and fungi. We once believed this was simple mutualism: ants benefit fungi, and fungi digest leaves providing fruiting bodies the ants can eat. However, it's much more complicated: 1. Parasitic fungi (Escovopsis) can infest the ant garden and destroy the fungi (+/-) 2. However, the ants house a mutualistic streptomyces bacteria on their bodies that produces antibiotic that fight Escovopsis (+/-/+) 3. Plants fight ants with chemical defenses in leaves (+/-) Plants have endophytic fungi, that may play a role in protecting some plants from ants (research of Sunshine Van Bael) 4. Ants circumvent plant defense by using fungus to digest leaves (+/-/+)

What trophic mutualism do we see in soybeans?

A trophic mutualism comprised of nitrogen-fixing Rhizobium bacteria in soybean root nodules

Examples of defensive mutualisms

Ant-plant mutualisms (myrmecophytes = ant plants defended by ants); but this mutualism is also trophic, illustrating arbitrariness of categories Cleaner-fish (& shrimp) relationships

Examples of trophic mutualisms

Corals & coral reefs Mycorrhizae & plants Lichens (fungus & algae) Ruminant ungulates (like cows; use gut microbes to digest cellulose involving foregut fermentation) Leaf-cutter ant

Describe the basic aspects of the major example of a defensive mutualism that we discussed that is characteristic of many seasonally dry forests

Ants and acacias: Bull's horn acacia trees (e.g., Acacia cornigera) & Pseudomyrmex ants in Central America. Newly developing bull's horns (evolutionarily enlarged thorns) filled with a pith that ants easily remove, creating hollow interiors Plants also provide ants with "extra-floral nectar" secreted from glands at base of leaves Ants chew small hole into each thorn for use as home

What are some other organisms that could potentially exploit Acacia trees?

Ants and other insects that could use nectar and Beltian bodies, not protect the plants, and keep Pseudomyrmex from the plants Fungus would happily infect Beltian bodies

What is an example of a facultative mutualism

Ants protecting aphids for sugar excretions

Describe the controversy that we discussed with cleaner organisms

Are oxpeckers mutualistic or parasitic with African ungulates? Mutualistic: eat ticks and other ectoparasites Parasitic: can draw blood and eat earwax

Why might the ant-Acacia mutualism actually not be a mutualism?

Because the plant is manipulating the ants by causing them to lose their ability to digest sucrose

Describe the experiment related to commensalism (confusing and not covered in class)

Bertness & Hacker 1994: Salt marsh commensalism and trophic effects Marsh elder (Iva frutescens) is commensal with Juncus With Juncus removed, photosynthetic rate went down - Iva eventually died Saltmarsh rush (Juncus gerardii) when removed led to soil salinity doubled & soil oxygen decreased

Who participate in mutualisms in coral reefs?

Cnidarians (animals) and zooxanthellae (protists)

From an evolutionary perspective, what is important in mutualisms?

Coevolution

What study looked into the Ant-Acacia mutualism?

Dan Janzen's 1966 experiment, which tested the ecological impact of ants on plants

What potential coevolved mutualism is still being argued about by researchers?

Did the dodo bird's extinction on the Island of Mauritius jeopardize the survival of its coevolved Tambalcoque tree? Was there an obligate relationship of tree to bird (bird evolved to abrade seed in gut, helping germination)?

Conclusions

Extremely common, widespread, diverse in nature: Many mutualisms co-evolved (sometimes obligate) and even human agriculture is mutualistic in nature Model of mutualism helps explain some aspects of mutualism, but doesn't 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, flexible response to partner, habitat stress Commensalism also widespread, not as well studied or understood as mutualism

What types of mutualisms are involved in the ant acacia mutualism? What does each partner get?

For ants, it is a trophic mutualism because they get nectar. For the acacias, it is a defensive mutualism since plants protect the foliage and the acacias don't have to invest in producing secondary compounds

Describe the methods of Janzen (1966)

Fumigated 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)

Why don't we see as many mutualisms in favorable environments?

In favorable environments, species can make it on their own, without expending energy on behalf of mutualist individuals

How do Acacia protect their investment (in Pseudomyrmex ants)? (2)

In short: chemistry 1. Acacia Beltian bodies (BB) contain Kunitz-type protease (protein digesting) inhibitors (PIs) These inhibitors prevent BB digestion by almost all arthropods, which use Trypsin and Elastase proteases Pseudomyrmex use Chymotrypsin to digest Beltian bodies--that's not inhibited by Kunitz-type PIs Thus Pseudomyrmex alone can access Acacia's "exclusive rewards" 2. Acacia nectaries also secrete Chitinase, which breaks down fungal hyphae, preventing fungal exploitation

What chemical compound do Beltian bodies contain?

Kunitz-type protease (protein digesting) inhibitors (PIs)

Describe the results of Janzen (1966)

Much fewer plant-eating insects were found on the control Acacia plants and control plants had much higher regowth, leaves, and swollen thorns

Is mutualism or commensalism better studied?

Mutualism has been studied more

Extra-floral nectar

Nectar not associated with flowers. It is often made by leaves

What kind of mutualism is potentially a kind of symbiosis?

Obligate mutualism

What type of mutualism is most stable? Why?

Obligate mutualism should be less stable than facultative, because conditions more restrictive for stable coexistence

Examples of dispersive mutualisms

Plant-pollination (e.g., figs & fig wasps, hummingbird-flower, etc.) Plant-seed dispersal

What could obligate mutualisms potentially be?

Potentially one kind of symbiosis = a tight relationship of two or more species that are ecologically interdependent and live together in close proximity (symbiosis can be non-mutualistic, e.g., parasite-host symbiosis)

How do Acacia keep the ants faithful (first identify problem)?

Problem: Most nectar-feeding insects, including Pseudomyrmex, attracted to Sucrose (12-carbon), digested using invertase = enzyme that yields glucose & fructose (6 carbon sugars) Most plants produce sucrose nectar, which Pseudomyrmex ants encounter (on plants other than Acacia) and could potentially use; & which other insects require However: Acacia nectaries manipulate Pseudomyrmex ants Nectary secretions include Invertase and Chitinase = invertase inhibitor By consuming this chitinase, Pseudomyrmex inadvertently lose ability to digest sucrose, and must rely on glucose & fructose Acacia sugars pre-digested for the Pseudomyrmex ants via the invertase secretions, and no sucrose available in nectaries for other insects

Describe the particularly bizarre mutualism we discussed

Salamander-algae mutualism Salamander embryos (and cells in adults!) provide CO2 and nutrients, and the algae provide O2! (algae is found inside the salamander egg mass)

Example of commensalism in Audubon Park

Spanish moss growing on live oak

What are Beltian bodies?

Swollen glands that are a form of protein and fat-rich ant food supplied by acacia

What is an example of a well-documented coevolution?

The ant-acacia mutualism

Why are mutualisms and symbioses important to humans?

These relationships are critical to humans, considering how many beneficial and also disease-related symbioses we have with diverse microbes

What do slopes look like in models of mutualism?

They are positive (as opposed to the negative ones that we see in the Lotka-Volterra interspecific competition model)

What type of mutualism are myrmecophytes invovled in?

Trophic and defensive

What type of mutualism exists between ants and aphids

Trophic/defensive mutualism

What kind of mutualism are cleaner organisms involved in?

Trophic/defensive mutualisms

Two kinds of mutualism based on how interdependent the species are

Two extremes in terms of species interdependence: Facultative mutualism - partners can survive independently but still benefit from mutualism Obligate mutualism - partners cannot survive without each other

Describe the unique traits of both members of the ant-acacia mutualism (Acacia= 3; ants= 4)

Unique ant Acacia traits (compared to other species of Acacia without ants) include Beltian bodies (with particular protease inhibitors), hollow thorns, and nectaries with Sucrose, invertase, and chitinase secretions Pseudomyrmex Ant traits (compared to other species of genus not associated with Acacia) include high running speed, stinging ferocity, 24-hour activity patrolling plant, attacks on plants

Describe how mutualisms have been modeled

Variety of outcomes possible, all consistent with positive slopes of isoclines (vs. negative slopes of isoclines in Lotka-Volterra interspecific competition model) The outcome depends on parameter values - determine slopes and y-intercepts of isoclines Mutualistic organisms may coexist stably at fixed densities, populations may spiral upwards (unstable), or populations may collapse to extinction (unstable)

Symbiosis

a tight relationship of two or more species that are ecologically interdependent and live together in close proximity (symbiosis can be non-mutualistic, e.g., parasite-host symbiosis)

Mutualism includes ____ ecological roles

diverse

What about humans in mutualisms (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

Mutualism

is +,+ interaction, i.e. both species benefit

Give some examples of cleaner organisms

prawn (or remoras) and moray eel, buffalo and oxpecker (bird), manta ray gets parasites removed and eels or remoras get parasites

Coevolution

reciprocal evolutionary adaptations involving both partners of ecologically interacting species (can be difficult to document in nature)

What secretions are found in Acacia nectaries?

sucrose, invertase, chitinase