Community ecology

lecture 38

Ecological community

• group of species that coexist and interact in a defined area

  • community studies on basis of energy and biomass

• Vary in size, scope

  • number of species determined by available energy

primary producers

• convert sunlight (or inorganic molecules) into chemical energy—> energy-rich organic molecules

• Autotrophs: make own food

• consumed by heterotrphs

Gross primary. production (GPP)

• the amount of stored chemical energy from photosynthesis

• not all GPP is available to consumers

  • some used by producers to maintain bodies and reproduce

  • the net primary production is what is available/used by consumers

Higher tropic levels

• have fewer individuals

conservation of energy through trophic levels

• amount of energy is constant

• transformation of energy to new form loses some

• 10% at one level transfered to next

why is there loss in energy transfer between trophic levels?

• due to heat

• availability - not all is ingestible or harvested

• digestibility- not all parts of all bodies can be digested

“ecological efficiencyPri”

Productivity and species richness

• amount of energy (sun) available limits productivity

• higher productivity supports high diversity of species

• however, at high productivity, diversity declines

  • competitive exclusion: carrying capacity

trophic cascades

interaction of one consumer with other species —> indirect effects on many other species

keystone species

• outside role on stabilizing community

• effect is to increase resources (space and food) that are otherwise limiting

Diversity index

• quantify diversity with Shannon index (H)

  • high H means high diversity

• based on predictability of random sampling picking a particular species

• calculate from sum of proportion of total individuals /(all species) that are of each species

What are the three different patterns of diversity

• alpha: a single community

• beta: multiple communities

• gamma: a regions communities

Alpha: A single community

• diversity of a community

• can be more or less diverse

beta: multiple communities

• diversity between communities

• can have the same or different diversity

diversity in rivers

• high species richness but most had some set of species

• low beta diversity (every single river basically the same)

diversity in ponds

• were more variable- some had many species, some had few, but less overlap

• high beta diversity

diversity in ditches

• had fewest species: due to sometimes lack of water or under water (have to be able to deal with both)

• lowest alpha diversity

• supported species not found elsewhere and contributes to gamma diversity

What determines how diverse a community is?

• geographical : distance from equator

  • energy, water that supports primary productivity

• Size of landmass

  • on continents: can migrate, invade new locales

  • on island: movement less likely

Species pool

• source of new recruits

• the collection of all species in a larger region that are capable of dispersing to and establishing in a specific, focal site, acting as the "source" from which local communities are assembled

Island biogeography

• number of species on an island: balance between species add (immigration) and lost (extinction) expected to stabilize

Immigration rate is dependent on what?

• dependent on species pool (from neighboring landmasses) —> further away, smaller species pool

Island biogeography equilibrium:

• when number of species arriving equals number of species going extinct

cyclical disturbance

• some species rely on disturbances to become established

• a recurring event, often natural, that repeatedly alters an ecosystem, resetting it to earlier stages

• Some disturbances like fire enhance seed germination and growth

succession

• when disturbance occurs, especially on large scales, communities reassemble themselves in a predictable sequences shaped by the local climate and soil

• process begins with fast-growing annual plants, which add biomass and nutrients to the soil.

• over time food webs grow more complex as additional species arrive

• succession ends in a relatively stable climax community

primary succession

starts on newly exposed surfaces with little or no soil, such as land uncovered by retreating galciers

secondary succession

begins in areas where a disturbance has occurred but soil remains, such as abandoned agricultural fields.

(more localized)—> retired agricultural fields

shade-tolerant

seedlings can grow up in shade of adult tree

efforts to increase diversity

• plants strips of pairie plants in corn fields

• perennial plants —> food for animals