Community Properties

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

Definition of Interactions Interaction Type Organism 1 Organism 2 Competition negative Symbiosis positive Commensalism neutral Parasitic Predation

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

Comparison of Primary and Secondary Succession Primary Secondary 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)

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

Odum’s laws about succession Ecosystem/Community property Pioneer Climax Control over abiotic environ. Low High Species diversity Productivity Production Leakiness of Nutrient Cycles

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