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Unformatted text preview: Mutualism; Community structure and dynamics M t alism Comm nit str ct re and d namics Today's questions: T d ' ti I. I II. III. IV. Mutualisms as dynamic interactions M li d i i i Community structure How does disturbance affect communities? How do communities respond to disturbance? I. Mutualisms as dynamic interactions A. Treehopper and ant interaction (in text) 1. Why is this interaction "contingent?" 2. What would happen over time, if spider density were low every year and "honeydew" production expensive? B. Endophytic fungi in plants B E d h ti f ii l t Fungi found growing on leaves, stems Design an experiment to test the hypothesis that a Design an experiment to test the hypothesis that a certain leaf endophytic fungus species is mutualistic with strangler fig tree seedlings in Ecuador. strangler fig tree seedlings in Ecuador. C. Nitrogenfixing bacteria g g Species of Rhizobium and Frankia colonize root cells in legumefamily plants and alder legume family plants and alder g ( ) The bacterial cells convert molecular nitrogen (N2) to amino groups (NH2) that can be used to build proteins, nucleic acids, and other key molecules. Nitrogenfixation is extremely energy intensive (energy is furnished by the host plant). furnished by the host plant). II. Are communities "superorganisms?" p g Community = the collection of species in a particular area. These species interact via: Mutualism Parasitism Competition Consumption Key idea with coevolution and arms races: Key idea with coevolution and arms races: Hypothesis 1: Communities are highly structured (highly coevolved; lots of interactions) Patterns: 1. Communities have a mature ("climax") stage that is predictable. 2. Community composition is predictable because it is dominated by biotic interactions. dominated by biotic interactions. 3. Communities develop over time like an individual organism. If disturbed, the climax community will i If di t b d th li it ill reconstitute itself by reiterating "developmental stages." Hypothesis 2: Communities are loose assemblages of species (little coevolution/dependence) Patterns: 1. Communities are not stable. They are dynamic (always changing). 2. Community composition is not predictable, because the species present in an area are there largely because of the species present in an area are there largely because of chance events (history). 3. If a community is disturbed, a different community is 3 If a community is disturbed a different community is likely to develop. Testing these hypotheses Climate change: Average global temperatures have increased over the past 50 years, and are predicted to p y p increase much more over the next 50 years. Species ranges are changing ... II. How does disturbance affect communities? Disturbance any change that removes biomass Disturbance = any change that removes biomass Types of disturbance: To understand disturbance, need to analyze: 1. ___________ 2. ___________ 3. ___________ e.g. eg Boreal forests burn to the ground every 100300 years Tropical and temperate rainforests: frequent (annual) p p q ( ) gaps created by windblown or diseased large trees Grasslands and savannas: fires every 23 years Consider some species from fireprone ecosystems: Lodgepole pine requires full sun to thrive; cones are sealed shut with resin In most prairie (grassland) plants, the meristematic tissue responsible for new growth is located belowground p g g Some species in California chaparral have seeds that must be exposed to smoke/ash to germinate / Burr oak trees have extraordinarily thick bark compared Burr oak trees have extraordinarily thick bark, compared to other oak species of similar size A case history: Forest fires in the Rocky Mountains Historically, most regions burn every 60100 years Most forests have now gone >120 years without a fire What are the likely consequences of this change in disturbance regime? III. How do communities respond to disturbance? Succession refers to the sequence of species that occupy a site through time, after a disturbance site through time after a disturbance Early successional Late successional _
Small seeds (good colonizers) Large seeds (good competitors) Thrive in harsh abiotic conditions Require high water/nutrients (low water & nutrients; high (organic soils), low temp (l t & t i t hi h ( i il ) l t variation, temp/wind/light) can cope with low light (forests) Rapid growth, shortlived Slow growth, longlived R id th h t li d Sl th l li d Key observations: 1. Early successional species alter the physical environment in ways that favor late successional species. 2. Late successional species are much better competitors than early successional species. than early successional species To explain how and why succession occurs at a particular To explain how and why succession occurs at a particular location, need to focus on: 1. Species traits 1 Species traits 2. Species interactions 3. History of the site ...
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- Fall '07