Lecture 16 Community Structure- Interactions within assemblages of co-occurring species (Nov. 9)
1. Community structure
- A community is a complex of species living in a particular habitat/location, linked to another through feeding relationships and other interactions that govern the flow of energy and cycling of nutrients.
- Key topics of community ecology:
a. What determines the number of coexisting species
b. The degree to which all members of a community influence one another
c. The controls on overall community structure and function
d. The replace-ability of species
2. “Community” vs “Assemblage”
- Clements advocated tightly integrated species associations (holistic concept).
- Gleason advocated individualistic response of species resulting in loose associations.
- Never fully resolved. Species distributions frequently imply independence. Experimental studies often document strong functional linkages.
- Ecotones are regions of rapid replacement of species along a gradient
- In open communities, species are distributed independently with respect to one another
- Strong geological and soil influences can produce ecotones in plant communities. Concentrations of elements in the soil determine the plants that make up each community.
- But gradient analyses of multiple plant species often indicate that each species’ distribution is independent of other species’ boundaries. Each species has its greatest abundance at a different point on the moisture gradient, for example.
5. Trophic levels
- It describes the position that an organism occupies in a food chain- what an organism eats, and what eats the organism. (from Wikipedia)
- Primary producer (basal resources), primary consumer, secondary consumer, tertiary consumer, top predator.
- Omnivores feed at more than one level. Omnivory is much more common than previously thought.
- Detritivores feed on the feces and carcasses of all
6. Food web analysis
- Most food webs are horribly complex
- Connectance is the ratio of number of links to number of possible links (which increases with number of species)
- By their very existence, food webs are presumed to have/exhibit stability.
- They exhibit stability because they include species level information and include more diversity. So, if a species is unable to get energy from one source due to sudden deaths in that species, it has many alternatives to receive energy from.
- However it can be argued that foodwebs also reduce stability because of the large number of interactions between species. In omnivory, one species feeds from many trophic levels. These kinds of interactions are believed to bring down stability.
7. Type of food webs
- Connectedness webs emphasize feeding relationships, based on observations of who ingests what.
- Energy flow webs weight interaction strengths by flux of energy, require measures of food and energy consumed, and generally show dominance of a small number of species.
- Functional webs emphasize influence of individual species on the growth rate of other species. Revealed by removal experiments and may not correspond to energy flux.
8. Trophical cascades
- Trophic cascades occur when predators in a food web suppress the abundance of their prey, thereby releasing the next lower trophic level from predation (or herbivory if the intermediate trophic level is an herbivore). For example, if the abundance of large piscivorous fish is increased in a lake, the abundance of their prey, zooplanktivorous fish, should decrease, large zooplankton abundance should increase, and phytoplankton biomass should decrease. (from Wikipedia)
- “The world is green” because predators control herbivore abundances, thus plants are not limited by herbivory.
- Bottom up: nutrients (or plants) control abundances of upper trophic levels. With bottom-up control, increased production results in greater productivity at all higher trophic levels.
- Top down: predators control trophic level below, benefiting second trophic level down. With top-down control, consumers depress the trophic level on which they feed, indirectly increasing the next lower trophic level. The relative biomass of trophic levels alternate under top-down controls.
- Consumers can depress the size of the trophic level immediately below, and increase populations two levels below.
- Small, numerically abundant species occur at low trophic level (“height”)
- Note the strong inverse relationship between numerical abundance and average body mass of a species.
9. (Cross- System) Subsidies
- Food supply (energy, organic matter) inputs to ecosystems are important to many ecosystems
- Streams receive numerous inputs: leaf fall, POM from upstream and floodplains, insects, fish carcasses and reproductive products
- Streams also subsidize lateral and downstream ecosystems: POM, emerging insects, salmon carcasses
- Fish migrations (migratory salmon) bring nutrients from marine to fresh water system, as well as terrestrial system.
10. Food webs and species interactions – take away messages
- A food web reminds us of the many ways that species interact with one another.
- In addition to direct, two-way interactions and trophic cascades, there are ample opportunities for more subtle, indirect effects, as when species A affects species C via an intermediary, species B.
- Such effects may interact with trophic cascades to create complex, system-wide effects.
- An important force in most biological communities: Fire, floods, droughts, hurricanes
- Unpredictable in frequency and magnitude
- May affect species differentially, due to innate differences in resistance or recovery rate
- May mediate strong biological interactions: Where disturbance is frequent, one expects high species turnover, species succession following disturbance, and a greater preponderance of ‘fast’ species (small size, rapid maturation, high fecundity)
12. Disturbance can act as a “switch”
- During flood-free periods, large grazing caddis (Dicosmoecus) becomes abundant, and is relatively immune to predation by juvenile steelhead because of heavy case
- During floods, caddis is displaced, mobile small mayflies become dominant grazers, and good fish food
- Below dams, floods are eliminated and the caddis takes over (too bad for the steelhead)
13. Non-Native predators cause Domino Effects
- Opposum shrimp, introduced to Flathead Lake, Montana, have strong indirect effects. Kokanee salmon declined, eagles no longer frequent the area, and grizzlies may be affected.
- Prior to shrimp introduction, lake trout and kokanee salmon (also introduced) fed on small zooplankton
- spawning runs of kokanee into rivers provided food for eagles, bears
- opossum shrimp, introduced as “fish-food”, upset the system
- shrimp preyed upon and out-competed native zooplankton
- shrimp migrate to deep waters by day, so inaccessible to kokanee
- kokanee collapsed, eagles no longer stop over, and bears lack important fall food supply
14. Ecological succession
- Any new habitat attracts species adapted to be good pioneers
- These in turn modify the environment, by creating shade, adding detritus to soil, stabilizing the soil and increasing its moisture content. This may make the environment more suitable for other species, which then replace the pioneering species
- The transition form abandoned field to mature forest passed through multiple stages (seres) and may take hundreds of years
15. Succession: definitions
- Primary succession is the establishment and development of communities in newly formed or disturbed habitats previously devoid of life. (A stream or new soil as a glacier retreats. A sand dune devoid of vegetation. Krakatau)
- Secondary generation is the regeneration of a community following a disturbance, but where some species persist (A stream after a severe flood, a forest after a fire that leaves the seed bank largely intact, or a hurricane that opens canopy gaps)
- The climax community is one that has no further replacement (the eventual final stage)
16. Succession: mechanisms
- Species re-establish from survivors and seed banks, or re-colonize via dispersal
- Existing species may facilitate new colonists by adding nutrients, detritus to soil, modifying soil moisture and shade
- Existing species may inhibit future colonists by predation, chemical secretions, etc.
- Mutualisms, such as those between plant roots and mycorrhyzal fungi