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Biology 2483A
Mark Moscicki

Ecology-Lecture 14 Oct 29 2013 The First Farmers-Case Study  The fungus-growing ants in the tribe Attini started cultivating fungi for food at least 50 million years before the first human farmers.  The ant farmers nourish, protect, and eat the fungal species they grow, forming a relationship that benefits both the farmer and the crop.  The ants cannot survive without their fungi; many of the fungi cannot survive without the ants.  A virgin queen leaves her mother's nest to begin a new colony, carrying fungi from her birth colony. The fungi are cultivated in underground gardens. A colony may contain hundreds of gardens, each the size of a football; they can feed 2–8 million ants.  Other species such as leaf cutter ants do not cultivate fungi found in the environment. Instead, the fungi in their gardens comes only from propagules passed from a parent ant to each of its descendant colonies. Leaf-cutter ants cut bits of leaves from plants and feed them to the fungi. The ants chew the leaves to a pulp, fertilize them with their own droppings, and “weed” (pull out bacteria and things that invade and compete with crop) the fungal gardens to help control bacterial and fungal invaders. In response, the fungi produce specialized structures called gongylidia, on which the ants feed.  Both ants and fungi benefit from the relationship (unholy alliance) Each partner helps the other overcome the formidable defenses that protect plants from being eaten.  The ants scrape a waxy covering from the leaves that the fungi have difficulty penetrating. (processing leaf material into something fungi can access/digest)  The fungus digests and detoxifies the chemicals that plants use to deter insect herbivores. (allowing ants to feed)  Nonresident fungi, pathogens, and parasites can sometimes invade the colonies. What prevents invaders from destroying the gardens? Ants increase weeding rates & also enlist help from other species.  In the 1990s, a parasitic fungus (Escovopsis) was discovered that attacks the fungal gardens of leaf-cutter ants. The parasite can be transmitted from one garden to another, and rapidly destroys the gardens, leading to death of the ant colony  Ants respond to Escovopsis by increasing garden weeding rate.  They also enlist the help of other species. The ants carry a bacterium that makes chemicals that inhibit Escovopsis. The bacteria also secrete compounds that promote the growth of the cultivated fungi. The bacteria also benefit. They get a place to live (in specialized structures called crypts on the ant’s exoskeleton), and a source of food (glandular secretions) from the ants. Thus, the bacterium is a third mutualist. (third species benefitting/contributing to relationship) Positive Interactions  Positive interactions occur when neither species is harmed and the benefits of the interaction are greater than the costs for at least one species.  Facilitation is a synonym for positive interactions. (defines mutualism and commensalism as a whole)  Mutualism: Mutually beneficial interaction between individuals of two species (+/+). (not always symbiosis)  Commensalism: Individuals of one species benefit, while individuals of the other species do not benefit and are not harmed (+/0).  During a flood, a mouse hops onto a frog (frog has no benefit) More realistic commensalism is birds hanging around herbivores. Herbivores kick insects up which birds eat and benefit from. Herbivore is just a bystander  Symbiosis: A relationship in which the two species live in close physiological contact with each other, such as corals and algae or humans and bacteria (bacteria in our gut which is largely helpful)  Symbioses can include parasitism (+/–), commensalism (+/0), and mutualism (+/+).  The benefits of positive interactions can take many forms. In mutualism and commensalism, the growth, survival or reproduction of one or both species is increased by their interaction with the other species.  species may provide its partner with food/shelter or a substrate to grow on  transport seeds/pollen  reduce heat/water stress  reduce negative effects of competitors/herbivores/predators/parasites  Sometimes there is a cost to one or both partners, but the net effect is positive. (cost benefit analysis) For each species, the benefits are greater than the costs. Mutualism and Commensalism are Ubiquitous  Mutualistic associations are everywhere.  Most plants form mycorrhizae: Symbiotic associations between the roots and various fungi. These are usually mutualistic. (fungal filaments help roots extend in soil helping take up water and nutrients from soil; fungi may protect from pathogens; fungus in turn gets carbs)  The fungi increase the surface area for the plant to take up water and soil nutrients (over 3 m of fungal hyphae may extend from 1 cm of plant root).  There are two major types of mycorrhizae.  Ectomycorrhizae: The fungus grows between root cells and forms a mantle/sheet around the root. The hyphae extend short distances into the soil.  Arbuscular mycorrhizae: The fungus grows into the soil, extending away from the root; and also penetrates into some of the plant root cells. (tree like structure) It grows between some root cells while penetrating cell walls of others. Most angiosperms and gymnosperms form these associations.  Mutualistic associations are found in many other organisms and habitats. Corals form a mutualism with symbiotic algae.  The coral provides the alga with a home, nutrients (nitrogen and phosphorus), and access to sunlight. The alga provides the coral with carbohydrates produced by photosynthesis.  All of the numerous invertebrate and vertebrate species that live in and around coral reefs depend directly or indirectly on this mutualism.  Herbivores such as cattle and sheep depend on bacteria and protists that live in their guts to help metabolize cellulose. (eat woody materials full of cellulose. They can't actually digest cellulose, but mutualists in gut help with metabolism). Wood-eating insects also have gut protists that can digest cellulose. (wood eating cockroach has protist in gut helping it digest cellulose so it can eat wood)  Commensalism is also everywhere.  Millions of species form +/0 relationships with organisms that provide habitat. A species that depends on the habitat often has little effect on the species that provides the habitat  Lichens that grow on trees or bacteria on your skin. (no consequence for tree). In forests, many species depend on the trees for habitat, and do no harm to the trees. Evolution of Mutualism and Commensalism  Different types of ecological interactions can evolve into commensalism or mutualism.  For example, a lichen that grows on a trees leaves may initially harm the tree by reducing its access to light. However, over time this relationship may evolve towards a commensalism if the tree gains the ability to tolerate the lichens presence.  Mutualism can arise from a host–parasite interaction.  In a strain of Amoeba proteus that was infected by a bacterium, the bacterium initially caused the host to be smaller, grow slowly, and often killed it.  However, parasites and hosts often coevolve. Five years later, the bacterium had evolved to be harmless to the amoeba; the amoeba had evolved to be dependent on the bacterium for metabolic functions.  Neither species could survive without the other! (anecdotal example of how this relationship forms mutualism) Mutualism and Commensalism-Broad Set of Interactions  Some positive interactions are highly species- specific, and required/obligate (not optional for either species).  The leaf cutter ants and fungus cannot survive without each other, and both have evolved unique features that benefit the other species.  Tropical figs are pollinated by fig wasps. Neither species can reproduce without the other. The wasps and the figs have coevolved with regards to reproductive strategies. Fig flowers are contained within the receptacle. In monoecious figs (separate male and female flowers) the male and female flowers are located in different parts of the receptacle, and male flowers mature after the female flowers. Some of the female flowers have short styles and some have long styles. The wasps have complex reproductive behaviors in the fig receptacle that ensures pollination. A female fig wasp enters the receptacle carrying pollen from male flowers in another receptacle. Once inside, the wasp inserts her ovipositor through the styles to lay her eggs in the ovary, but the wasp can't effectively lay eggs in long style. Therefore, larvae are typically only formed within short styled flowers. The wasp deposits pollen on the stigmas of the short and long stem flowers. After seeds are produced, females get pollen from male flowers before leaving the receptacle and start the process again.  Many mutualisms and commensalisms are facultative (not obligate) and show few signs of coevolution. In a commensalism, the relationship is always facultative for the species that does not benefit. In addition, coevolution does not occur as natural selection does not act on a species that does not benefit from the relationship.  In deserts, the shade of adult plants creates cooler, moister conditions (compared to soil of an adjacent open area). Seeds of many plants can only germinate in this shade. The adult is called a nurse plant.(provides microclimate for other plants to establish). One species of nurse plant may protect the seedlings of many other species.  Desert ironwood serves as a nurse plant for 165 different species. Most of the species it nurses can also germinate and grow under other plant species. This is typical of facultative interactions. A species that requires nursing may be found under a variety of nurse plant species. The nurse plant and the beneficiary species may evolve little in response to one another.  Facultative interactions are also seen in forests. Large herbivores such as deer or moose may accidentally consume seeds of herbaceous plants as they eat the leaves. Many seeds pass through unharmed, and are deposited with feces. Thus, it becomes a dispersal mechanism. Such interactions are sporadic and facultative; there is little evidence to suggest that the species have coevolved.  The below diagram shows how far deer can travel compared to ants (red line shows max distance) with regards to Trillium seed dispersal. The graph shows where seeds are dropped by deer. The trillium benefits greatly by deer due to dispersal (carries seeds far in gut) The deer benefits because it is eating the leaves
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