BI107 quiz 5

41.1 Communities contain species that colonize and persist

Community = a set of species that occur together at the same time and place 

  • Complex community within an ant nest (smallest farmer)

    • Cut but not eat leaves - cultivate fungus to feed their offspring 


Community structure

  • Characterized by species composition and diversity

    • Which species, how many species, abundance of each species

  • Organized by trophic levels: based on source of energy 


Food chain (more simple than food web)

Links the trophic levels from producers to top carnivores and decomposers 

  1. Primary producer

  2. primary/ secondary/ tertiary/ quaternary consumers 

41.2 Communities change over space and time

Turnover = patterns of spatial and temporal change of species composition within a community 

  1. Temporal: changes in community composition over time 

    1. Mostly natural

    2. Krakatan eruption in a small island → plants disperse from neighboring islands → new plants adapt (complete turnover of plant communities) → new environment

  2. Spacial: environmental gradient (elevation, soil moisture, nutrients) 

    1. In marine environment: surface has different environment from deep ocean 

    2. Could be vertical (in ocean) or horizontal (in continent)


How communities change over time?

  1. Ongoing colonization events and local extinction 

    1. Colonization: new occupation of a habitat or territory by a biological community

    2. Dispersal: constant influx of new individuals 

  2. After natural disturbance

    1. Succession: a predictable sequence of change in community structure 

    2. Ex: wildfire → all gone → succession (group of species tend to grow first: do well in harsh environment or easily dispersed seed) → make new environment → other species can inhabit there! 

    3. Many communities will return to pre disturbed state, but not always!

    4. Ex: dung beetle (feed at different time for rollers, dwellers, and tunnelers) - succession but not after natural disturbance

  3. Global change: natural and anthropogenic 

    1. Packrat nest (find fossil) - can be dated (different environments in different dates)

41.3 Community structure affects community function

Community function - measured by amount of energy or matter that moves into and out of the community per unit of time

How to enter?

  • GPP: total amount of carbon fixed by primary producers (inaccessible to accessible)

  • NPP: energy (carbon-carbon bonds) contained in tissues of primary producers and available to other trophic levels 


Energy transfer between trophic levels

  • Ecological efficiency: overall transfer of energy from one trophic level to the next

    • Biomass decreases with each trophic level in most terrestrial communities

  • ~10% of energy in one trophic is transferred to next level → limits the number of trophic levels in a community 

Two reasons for low ecological efficiency

  1. Organisms use most of the energy they take in to fuel their own metabolism, this energy is lost as heat and is not available to next trophic level

  2. Not all biomass at one trophic level is ingested by the next one, and some ingested matter is indigestible and is excreted as waste 

    1. Excreted or dead material used primarily by decomposers 


Community structure

All species occupy a specific ecological niche

  • Physical and biological environment a species occupies

  • the ecological role - how their actions affect other trophic levels/ communities

  • allow for greater species diversity and represent the functional role of a species 

  • Birds found in the different area of a tree (so there are different niches in a tree)


Species diversity

  1. Species richness: total number of different species in the community

  2. Relative abundance (evenness): proportion each species represents of the total individuals in the community 

Shannon diversity index: higher the H value, higher the diversity 




Species diversity affects community function

  • NPP is generally greater and more stable as species diversity (richness) increases 

  • Competition of space or food (why is it stable)



What if trophic structure changes

A trophic cascade!

  • A series of direct and indirect effects when the actions of a top consumer in a food web impacts other trophic levels 

  • Ex: wolves population reintroduction to stream hydrology (in yellowstone) in 1995

    • Wolves entirely extinct → elk population (zero in 195-1996, generally increasing)

    • In 1995 reintroduced wolves

    • Wolves eat elk → willow and aspen (what elk eats) increases → beaver population increases!

    • Beaver dams have effects on stream hydrology!!!

41.4 Diversity patterns provide clues to what determines diversity 

Patterns in diversity

  1. Global: latitudinal gradients

    1. Stable climate in the tropics → tropical species experienced few extinctions and more speciation events

    2. Solar energy and high productivity → greater energy flow through communities could support longer food webs 

    3. Habitat complexity → structurally complex habitats could lead to greater niche specialization 

  2. Regional: geographic (affect diversity) 

    1. Species-area relationship

      1. Diversity increases with increasing area of patch

      2. Distance to other patches also important 

    2. Island biogeography: The number of species on an island represents an equilibrium between the rate at which species migrate to the island and the rate at which resident species go locally extinct

      1. Equilibrium: the point at which colonization and extinction rates are equal

      2. Species itchiness on island depends on 

        1. Colonization: influenced by distance from the mainland (farther → fewer species will make the trip)

        2. Extinction: influenced by island size (smaller → fewer resources and greater competition → increase extinction)

      3. Ex: species on islands in tropical eastern pacific

      4. Applicable to ANY ecological island (can be urban island - any isolated area) 




A: just one species - example of population

B: groups of species at the same place at the same time 

C: mammals is a clade



39.1 Populations are patchy in space and dynamic over time

Population = groups of individuals of the same species that interact with one another within a given area at a particular time 

  1. Population size: number of individuals

  2. Population density: number of individuals per unit area or volume 

Consider the “Scale” - what species we are studying (so it gets a different scale!)


Population ecology

  • Study of population dynamics - how populations interact with their environment

  • Patterns and processes of change in population 

39.2 Births increase and deaths decrease population size

Populations change over time

  • Births and immigration

  • Death and emigration 

→ rate of population growth or decrease

“Birth-death” model 

Computational model: a quantitative hypothesis about the components of a system and their interactions that can be used to predict the properties of the system (predict the changes)


Growth rate (represent one time interval!)

  1. Change in population size can be measured directly only for very small populations

  2. Need to keep track of a sample of individuals over time

  3. Can estimate

    1. The number of offspring that average individual produces

      1. Per capita birth rate (b) = number of births per individual per unit of time

    2. The average individual’s chance of dying

      1. Per capita death rate (d) = number of deaths per individual per unit of time 

Change in population size over time = per capita births and deaths multiplied by population size at time t

⇒ Per capita growth rate 

  • Intrinsic rate of increase (r)

    • Maximum rate of increase of a population per individual

    • r = b - d