Definitions • What is the definition – quick review… • An area where plants, animals and microbes interact with each other
Examples • Within a grassland biome, you have grassland ecosystems, river ecosystems, etc. • Within a grassland ecosystem, you can have a short grass community, a tall grass community, a riparian (area next to stream) forest community
Community Structure • If we look at a grassland community, we can look at what organisms fill the producer role • Big Bluestem, Little Bluestem, Switchgrass, Indiangrass, Dropseed, Lovegrass • What organisms fill the herbivore or primary consumer role • Bison, grasshoppers, mice, voles, prairie dogs • What organisms are carnivores • Coyotes, snakes, hawks, skunks, racoons, etc.
Communities and Food Webs • Food webs show the interconnection between the organisms in a community • See that in “real world” there is no such thing as a food chain
More Community Structure • Food webs deal only with one type of interaction between organisms – predation • Other interactions are also important • Competition • Symbiosis • Commensalism • Parasitic
Community change over time -- succession • If you have a community that is disturbed by some agent (e.g. fire, plowing, landslide, flooding), the community structure in that community is altered. Gradually the community will rebuild itself, tending towards a more stable structure that can be supported by the environment in that particular climate – The Climatic Climax
Forest Succession Pioneer Comm. Climax Comm.
Secondary Succession • Succession that occurs following a disturbance is called Secondary Succession
Community change over time -- succession • Organisms also invade spaces that did not previously support life, e.g. bare rocks, sand dunes, volcanic flows. • This is known as Primary Succession
Pioneer organisms are lichens and mosses Takes very long to reach climatic climax community stage because soil must be created through physical and biotic interactions Pioneer organisms are typically small, weedy plants Time it takes to reach climatic climax stage variable, dependant on climate (short in tropics, long in arctic) Comparison of Primary and Secondary Succession Primary Secondary
Details on Succession • Intervening communities found in succession after pioneer and before climax community are known as “seres”. • Types of communities that area grows through during succession are important, will work on during group exercise • Succession is driven by competition between organisms in each sere.
Community Structure Details • Each sere has a different structure than the one preceeding it. • Pioneer sere tends to be made of small plants with little control over their own “microenvironment”. • How do ecologists quantify community structure?
Dominance • Dominance – how prevalent a species is, how many individuals there are. • The most dominant species are usually the ones whose name is given to the community, e.g. redwood forest • The redwood forest has lots of species in it besides redwoods, but redwoods are the dominant tree.
Diversity • Diversity = how many different kinds of species are present. • Usually find highest diversity in tropical, moist areas and lowest diversity in arctic or dry areas.
Diversity & Stability • There is a huge controversy in ecology right now about whether or not diversity increases ecosystem stability.
Diversity & Stability • Stability in an ecosystem is defined as • Resistance: The ability to resist change • Resilience: The ability to recover from change
Diversity & Stability • If you look at two extremes, a corn field and a tallgrass prairie, which is most diverse? • Which would be the most stable if a virus attacked the dominant plant species?
Diversity & Stability • We see that stability does not increase with the addition of more and more species, but rather there is a point above which – more species make a system less stable. Stability Species number
Species distribution Random Uniform Clumped
Group Exercise Assignment 1 • Succession can take over 100 years to re-establish a climax community. You are a young assistant professor wanting to get tenure at a major University. You have 6 years to publish several interesting and important papers to accomplish this. How could you possibly figure out what the successional seres are for a grassland in Central Oklahoma in time to get tenure?
Group Exercise Assignment 2 • You are interested in determining what types of interactions are going on among several coral reef fish species. It’s too expensive for you to go to the Bahamas all the time, so how would you determine what interactions they have in aquariums here in Central Oklahoma? (remember you need to keep enough alive to do multiple experiments).
The steps in a good experiment • Observe the system • Make a hypothesis on how it works • Design a method that will determine whether or not that hypothesis is false • Implement your methods & collect data • Analyze data • Conclusions • Share your information with others