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Communities 3 - Mutualisms
Mutualisms Outline I. Terms and concepts Symbiosis vs. Mutualism Obligate vs. facultative: Degree of dependence: Specialists vs. generalists: Degree of specialization: What types of "currency" Coevolution Effects on realized niche II. Examples A. Mycorrhizae - fungi and plants Ectomycorrhizae Endomycorrhizae (Arbuscular) B. Nitrogen fixation C. Corals- zooxanthellae, anthozoans This is a difficult topic in which to draw generalizations. Species specific interactions, so the details of how the interactions work differ from example to example. Some very interesting biology. Suggest Giselle MuellerParker's class on Symbiosis. I. Terms and concepts A. Symbiosis vs. mutualism "Symbiosis" means, literally, "together living". Implies intimate physical association, usually with one spp. serving as host and other living inside. Could be mutualistic, parasitic, or commensal. "mutualism" means both spp. benefit (+/+ interaction), mutual exploitation; "enlightened self-interest". Could be symbiotic, as just described, or free-living - e.g., pollinators, seed dispersers. - Important point: symbioses can be either mutualistic or otherwise; mutualisms can be either symbiotic or free-living. B. Degree of dependence: Symbiotic mutualisms are usually obligate for at least one of the species, i.e., that species could not grow and reproduce without its partner species. Examples: N-fixing bacteria, mycorrhizae, corals, gut symbionts (protozoans) of cattle, termites, etc. for digesting cellulose. Free-living may be obligate, or facultative - organism does better with partner, but can still survive without. Example: Defense mutualism: Ants/plants: Pseudomyrmex spp./Acacia collinsii Photos from Costa Rica- bullhorn acacia, w/ How does this affect performance? (Molles, fig. 15.8, 15.9) C. Degree of specialization: free-living can be very specialized - only one species performs function for only one other species; or generalized - multiple species can perform function. Example: Pollination mutualisms: insects/plants, mammals/plants 1. the benefits: plants get pollination, pollinators get nectar or use extra pollen for food. 2. Degree of specialization a. Some obligate - yucca moths, orchids - good pictures in the book i. Orchids - flower shapes mimic female bees, males attempt to mate Males pick up scent useful for attracting females to mate. Orchids deliver pollen sack to specific part of bees body - where only an anther of the same species of flower will pick it up.

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b. Heliconia - specialization for hummingbirds of different bill shapes. Slides from Costa Rica c. most plants visited by a variety of insects, diffuse coevolution - most nectarivores 3. Interesting question: to what extent is plant reproduction "pollinator limited"? Will a plant population increase in abundance if more pollinators are around? The main point: the degree of specialization varies This is important for conservation. D. What types of currency? Mutualisms are +/+ interactions, not because the species involved “want” to help the other, but because there is a trade involved. It’s worth them giving up something that they have in exchange for something that the partner has. This goes back to what we talked about with plant physiological ecology – investing resources that an individual has to get resources that are limiting. Nutrition - includes both energy (fixed carbon) and nutrients (N, P) (see N-fixer and mycorrhizal mutualisms, below). Protection - protection from predation, competition, physical environment (e.g., ants on acacaias) Fertilization/gamete dispersal – e.g., pollination Seed dispersal - fruits, etc.

E. Co-evolution: reciprocal selection pressure on two interacting species. Traits of one species evolve in response to traits of the other. In the context of mutualisms, this might occur as the closeness of an association increases over evolutionary time because beneficial mutuations in one species lead to greater advantage in the long run for both spp. - Important point: Probably started as exploitation of one by another, e.g., for dispersal, animals came to eat plant fruits for food, and in process seeds got dispersed. Co-evolution also happens in other community interactions as well, such as predation, parasitism, herbivory. For example, the term was first coined by Paul Ehrlich and Peter Raven (when both at Stanford) to describe the "arms race" between insect herbivores and host plant chemical defenses. Plants evolve chemical defenses against an herbivore. This acts as a selective pressure on the insect - those individuals having resistance to that chemical defense are selected for, others perish. Eventually the whole population becomes resistant. There is then selection pressure on the plant species to evolve a different chemical defense. And so on and on. Not so different from what humans and pests (diseases, insects, etc.) are doing now. F. Effects on realized niche – mutualisms can increase the range of a species (realized niche) compared to expectations of from that species’ range when free-living.

II. Examples of important mutualisms A. Plants/Mycorrhizae - fungus growing in close association with plant root. "mycor" - fungus "rhizae" - root Yes, there are two r's when you spell it. a. How important are they? How common is this? i. VERY!! Found in almost all families of vascular plants, including some very old evolutionarily (e.g., Psilotum). Only a few families don't: Sedges, Mustards. (REC, p.237) ii. revegetation – many conifers can’t grow without their fungal mutualists. iii. Food webs - for those with animal inclinations. Many of the fungal symbionts produce sporocarps (e.g., mushrooms, truffles), which are fed upon by voles (California red-backed vole), squirrels (northern flying squirrel), (over 90% of diet!) which in turn disperse the spores. Those animals are also primary food for Northern spotted owl. (smith, fig.27.6, p.584) b. What is the basic tradeoff?

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i. Fungus gets carbon; ii. Plant gets: - nutrients, especially P. P is not very mobile in soil, mycorrhizae essentially extend the root system of the host plant - photo of root extension by mycorrhizae (Paul&Clark, fig.11.3, p.252 or Read (1991) Experientia, 47: 376-391) or "Mycorrhizas in Ecosystems", QK604.2.M92 M965 1992 3 points: 1. size of a.g. portion of plant relative to fungal mycelium REC, 3rd ed., fig.12-35, p.237 2. Mycorrhizae vastly increasing the surface area for uptake of nutrients and water: mycelia length:root length is about 10,000 to 1!! 3. Fungi also have extracellular enzymes that help break down organic matter, releasing the nutrients, which the fungus then takes up and transfers to the plant. - parasite protection, Especially for ectomycorrhizae (see below) Photo from web - Lactarius rubrilacteus on Pseudotsuga menziesii d. Types of mycorrhizae -2 primary types, i. Endomycorrhizae - Arbuscular - most common: 80 % of all vascular plant species have endomyccorhizae!!! - Most common type in the tropics - about 30 spp. of zygomycetes: not very specific. - structure: vesicles and arbuscules penetrate root cells of the host (Smith, fig. 27.4, p.583) ii. Ectomycorrhizae - primarily temperate zone spp.: conifers, willows, oaks - Basidiomycetes, some Ascomycetes - at least 5000 spp. of fungi, often very species specific. Many different fungi on same spp. of tree: can differ with site conditions, successional stage (Perry ref.) - structures: mantle and Hartig net - don't penetrate cells (Smith,fig. 27.3, p.583) 2. Nitrogen-fixation: bacteria and plants a. what is this? Tradeoff plants get N, bacteria get fixed carbon (energy) the problem: lots of N in atmosphere (N2), but not available biologically (N=N triple bond is very stable, difficult to break. the solutions: nitrogenase enzyme in some types of bacteria is able to break the N=N triple bond the problem with the solution: very energy intensive; it takes lots of fixed carbon to generate enough ATP’s for the nitrogenase enzyme to work. the solution to that problem: mutualism (there are some free-living bacteria too). Plant provides bacteria with fixed carbon from photosynthesis and gets NH4+ in exchange once bacteria convert N2 amino acids. b. importance succession – often some of the first plants to colonize after disturbances. High light availability is important for the plants to keep photosynthetic rates high, allowing them to “feed” the N-fixing bacteria in their roots. Agriculture – beans, clover, and many other species are N-fixers and help to increase the fertility of soils once they die (or are plowed under) and decompose. c. types Rhizobial - usually with bacteria in the genus Rhizobium or Bradyrhizobium

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- plants in the Fabaceae (Leguminosae) - bean family, legumes. Found in about 700 genera. Actinorhizal - actinomycetes from the genus Frankia (bacteria, not to be confused with Ascomycete fungi) - diverse families of plants (25 genera in 8 families), but all are perennial dicots, most are woody trees or shrubs: e.g., Alnus (alder), Myrica (bayberry), Casuarina (beef wood). 3. Coral: zooxanthellae/anthozoan a. Importance: i. responsible for large physical structures (coral reefs) that host huge diversity of marine life. Colonial schleractinian corals: many polyps cemented together by calcium carbonate. (brain coral from web) ii. Protection of coasts from storms The photosynthetic algae provide the energy (fixed carbon) that allows the coral to grow: must keep pace with storm destruction, as well as longer term changes such as subsidence of islands and/or sea level rise - or else coral will be below photic zone. This is an issue with future sea-level rise – will rates of coral growth be able to keep up with potential sea level rise. Sensitive to physical disturbance - but will usually grow back Less resilient in face of sedimentation (blocks light), eutrophication (allows fast growing, free living algae to overgrow, shade out corals) b. Tradeoff Zooxanthellae - photosynthetic dinoflagellates (genus Symbiodinium), provide fixed carbon energy Colonial Anthozoans - gather nutrients; degree of dependence on zooxanthellae varies. (Molles, OH#57) c. Types/Structure (smith, fig. 27.5, p.584)