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Community Ecology
Interactions among species
living in the same place
Some individuals eat other individuals, but cooperation is also known. Populations interact with one another as well. Today we consider two interactions between populations: competition (-,-) and mutualism (+,+).
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Many species don’t interact at all. The symbol ‘0’ is used for no effect.
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Reading Assignment
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Today’s reading is Chapter 53, pages 1196 -1202, 1207-1209 plus pages 571-573 on How small cells affect global change.
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Lab 13 on Interspecific Interactions covers much of this material.
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Species Interactions
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There are normally many species living together in the same area. What are the possible interactions among species in a community?
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A simple categorization of effects is to consider species in pairs and to categorize effects on population size as increasing N (+) or decreasing N (-).
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In this lecture we consider -,- and +,+ population size effects, by comparing N when species are grown together versus alone.
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Describing Interactions
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Biologists sometimes measure the interaction between species as the effect on the population sizes of the species, but …
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Other times the interactions are descriptions of events in which individuals of two species interact.
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The relation of individuals and populations is not always simple.
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The results looking at individuals is not easily converted into population consequences. A clear example is the relationship of humans to cattle.
Relations of humans and cattle
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People eat cattle, so one usually thinks of humans as predators and cattle as prey, but...
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The size of the human population has been increased by cattle and the abundance of cattle has definitely been increased by people, so
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Human, cattle relationship is mutualism.
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Biological Competition
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The operational definition of biological competition is to compare population size when grown together (mixed) versus alone (on the same resource). This used in your lab.
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Two species are said to be competitors if both species have smaller populations (-, -) when grown together compared to when grown alone.
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The time element is not well defined in this model, that is how long do they have to be grown together?
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Alone versus Together
Lower together = -,-
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Both the ‘blue’ and ‘red’ types have lower abundance when grow together.
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Interactions among Algal species
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Some algae can be cultured in the lab and you used such species in your lab.
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When a single resource is limiting growth of both species, the species are expected to compete, i.e., be less abundant together than alone.
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What are the resources algae need?
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What are the resources that algae need for growth.
Resources that might limit algae
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If one species could grow on the wall of the tube, it could limit access to light.
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Algae could compete for nutrients in water.
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What else do you think algae might compete for?
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Maybe all resources are present in excess.
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Population Size Outcomes
when two species are grown together
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What do you think is the relationship of raccoons and opossums to humans?
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Other ways to look at competition
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Niche overlap
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Niche is range of resources a species can use.
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Overlap of resource use and access to same resources leads to expectation that species will interfere with each others growth.
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The greater the overlap the more the interference.
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Niche Overlap
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Mechanisms of Competition
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Consumptive
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Preemptive
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Overgrowth
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Chemical
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Territorial
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Encounter
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Competitive Exclusion Principle
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Formulated by G. F. Gause based on experiments with Paramecium and yeast, it states “It is not possible for species with the same niche to coexist”.
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It has primarily motivated studies looking for differences between two species that at least superficially coexist with the same way of life.
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Clearly individuals with the same niche persist, that is the nature of a population, so this idea may not survive well in the future.
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Mutualism
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Interactions between two species in which both species have larger populations when together than when alone (+, +).
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Some pairs of species are so dependent on one another that one or both species can not survive alone much less be grown alone.
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The +, + interaction does not imply that the benefits to both species are equal.
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Mutualism Examples
Distantly Related
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The species involved in a mutualism may be far apart taxonomically
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Ants & Fungi
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Bees & Plants
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Fungi & protists Lichens, so distinct that they are given ‘species’ names
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There are ants that have a fungal farm fed with leaf parts the ants cut.
Plants are often dependent of insects for pollination services.
Lichens are mutualisms between fungi and algae.
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Rhizobium-Legume Mutualism
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Nitrogen used to be a limiting nutrient for plants in many terrestrial environments.
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Lightening is a major source of nitrogen compounds.
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Nitrogen fixation by bacteria is another major source of nitrogen compounds.
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The symbiosis between legumes and Rhizobium is a source of nitrogen for plants.
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Rhizobium is a bacteria. It lives in nodules formed in the roots of the legume.
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Mutualism Examples
Closely Related
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Mutualisms can be between closely related species
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Fish-fish
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Insect-insect (Ants-treehoppers)
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Ants protect the treehoppers from predators and benefit by eating excretions of the treehoppers
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(Similar to human relations with domestic animals)
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Big fish encourage the presence of smaller fish that eat parasites of the bigger fish.
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Mutualisms involving Ants
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Ants protect the Acacia (a tree) by biting herbivores and by clearing area around tree. Tree provides a home and food for the ant. Both species benefit.
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Ants-treehoppers and trees. Treehoppers are herbivores that suck the sugar out of the sap, but end up excreting a lot of sugar. Ants live off the excreted sugar.
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What macromolecules have a lot of nitrogen?
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The ants protect the treehoppers from predators so that the ants can use the leafhoppers themselves.
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Ants & Treehoppers
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The ants population is increased by the treehoppers. Do the ants increase the treehoppers population?
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The ants protect the treehoppers from spider predators, but the protection is only measurable when the spiders are common.
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Relationships among species may be conditional on the environment.
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Problem/Solution
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If species A grown alone reaches 1000 cells per ml,
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If species B grown alone reaches 2000 cells per ml,
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What would you conclude if grown together they had densities of 1000 and 2000 cells respectively?
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Conclusion: The growth of species A does not reduce the resources available to species B and vice-versa.
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Extracellular secretions are one way a species can benefit another species.
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Vocabulary
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Competition
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Niche
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Nice Overlap
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Population size
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Types of Interactions
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Competitive Exclusion
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Nitrogen fixation
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Lichen
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Mutualism
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Kinds of resources
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