Community Dynamics

Community structure

Community structure

• Describes how an ecological community is organized, including the number of species it contains as well as how evenly individuals are distributed among species

• Includes a description of species’ feeding relationshipsSpecies richness

disturbance

any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical evironment

succession

the repeatable change in community composition through time following a disturbance

Resistance

the ability to withstand a disturbance

Allogenic engineers

alter environments through the structures they build

Trophic cascades

occur when predators indirectly limit the size of a population that they are not directly feeding upon

Species richness

Number of different species

Community dynamics

• Describes how an ecological community’s structure changes over time

• Changes driven by disturbances that originate outside the community or by interactions among member of the community

• Early succession

  • Annuals, forbs, grass

• Mid succession

  • Pine trees

• Late succession

  • Steady pine tree density

Serotinous cones

Only open when heat is applied

Communities are affected by disturbance and succession

Successional sequence is driven by

Thinning and trade-offs

Life history - early successional

• Having seeds/roots that can survive fires or are serotinous

  • These species are easily able to recolonize post-fire

• Quickly dispersing seeds

  • Grow quickly in high-light environments

Life history - late successional

• Poor dispersers

• Grow slowly

• Grow in low-light environments

life history traits and succession

Three mechanisms responsible for succession

• Assume that succession is initiated by a disturbance and proceeds without further changes in the biotic environment

• Assume that species are stationary

  • Only focus on plant communities

• Assume that initial colonists are good dispersers with life-history trait that help them reach new habitats quickly

  • Change the conditions where they live

    • Creating deeper litter, shading the ground, not well suited to grow under these changed conditions 

Autogenic succession

Internally produced or generated independently of external influence

Primary succession

Disturbance removes the soil and all existing organisms

Facilitation

• Primary succession

• Barren ground is uninhabitable by all but the most stress tolerant of colonists

• Over time, early stress tolerant colonists make the announcement more suitable for successive species by increasing nutrient availability, developing soils, reducing pH, or providing chase from the sun and shelter from the wind

• Sequence continues until the more competitively dominant species no longer facilitate the invasion and growth of any other species

Inhibition

• All species arrive on an unoccupied site and survive. Thus the initial community composition is a function of who gets there first

• Once a colonist becomes established, it inhibits growth of subsequent arrivals by monopolizing space and/or other resources

• Only when space and/or resources are released through the death or decay of dominant residents van new colonists invade and grow

• Because short lived early species die more frequently, succession slowly progresses from short lives to long lived species

Tolerance

• All species arriving on an unoccupied site can survive. Thus the initial community composition is simply a function of who gets there first

• Species that appear later simply arrived later or arrives early but grew more slowly

• Late arriving species tolerate the presence of early species and grow despite the presence of early successional species because they are better competitors for light and nutrients. Over time late successional species exclude other species

• Early successional species have no effect on late successional species

Communities are not closed systems

• External factors affect communities as much as internal dynamics

  • Allogeneic process ensure communities are almost never in equilibrium

Disturbance regime

• Describes the characteristic pattern of disturbances experiences by a given ecosystem

  • Defined in terms of the timing, magnitude, frequency, and predictability of disturbance 

Fire helps credit habitat diversity

• Density is the greeted in patches that were burned

  • Only severe surface fire generate sufficient heat to open serotinous cones

Intermediate disturbance hypothesis

Predicts that species diversity will be greatest when disturbances are of intermediate frequency or of intermediate magnitude

Food chain

• Description of how food/energy moves through populations in a community or ecosystem

• Each trophic level contains only one population

Food web

• Contains multiple populations at each trophic level and provides more detail about paths that every can take

Trophic level

• Describes an organism’s position in the food chain

• PP > primary consumers > secondary consumers > tertiary consumers

Ecosystem engineers

• Indirectly affect other members of the community

• Physically alter their environment and as a result alter resource availability

• Can impact the success of other species

Top-down control

• Higher trophic level determines the structure or function of a lower trophic level

Behavioral cascade

• Specific type of top-down effect

• The population size of an herbivore is not directly affected by predation. Instead the predator alters the behavior of the herbivore

Ecology of fear

• A conceptual framework describes how predation risk can indirectly affect population, communities, and ecosystems

Autogenic engineers

• Change the environment through their own physical structures

  • Trees

    • Produce shade, dams, alter hydrology and nutrient cycling, food, nests, shelter

• Must alter the availability of environmental resources

  • Coral

Top-down control

• Primary producers

  • Base of the food chain

  • Only PP? Limited by environmental conditions and resource availability

• Consumers

  • Limit the pop size of PP

  • Competition between PP becomes unimportant

• Trophic levels at the top are limited by food availability

• Lower trophic levels alternate between being consumer-controlled and resource limited

• Odd number of links

  • PP limited by competition

• Even number

  • PP limited by consumers

Green world hypothesis

• Since the world is green, then herbivores are not limiting PP

Bottom-Up control

• Lower trophic level influence the structure of function of higher levels via resource restriction

• Organism on each trophic level are resource limited

• If more energy is moving through the community, then there should be more trophic levels

• Even when PP is high enough to support grazers, bottom-up control persists because many plants are low quality food

  • Communities support low numbers of herbivores because of the low quality food

What determines food chain length

• Lake are good for studies

  • Food-chain length is variable

  • Most important PP is algae, correlated with phosphorus concentration

  • Relatively discrete boundaries , easy to estimate their size

Productivity hypothesis

• More productive ecosystems should have longer food chains. Lakes with higher phosphorus concentrations would have longer food chains but lake size would not matter

Ecosystem-size hypothesis

• Food-chain length should increase with ecosystem size. As ecosystem size increases, species diversity, habitat diversity, and habitat availability also increase. Larger lakes would have longer food chains but that phosphorus concentration would not matter

Productive-space hypothesis

Both productivity and ecosystem size are important. Food chain length would increase with both phosphorus concentration and lake size

• Resources that scale with ecosystem can affect community structure

• Bottom up forces can control different aspects of community structure

• In unproductive communities food chains are short

Questions asked about a disturbance

• How big a change did the disturbance cause?

• How quickly did the community recover from this change?

• How closely did the post-recover community resemble the pre-disturbance community?

Stability

• Its response to a disturbance, as characterized by resistance, return time, and resilience

  • Return time - how quickly a community returns to its initial condition after a disturbance

  • Amount of time it takes for the community to stop changing after the disturbance

Its persistence as an identifiable system through time, especially after a disturbance

Overall degree to which community stays the same over time

• Its constancy, independent of disturbances

Constancy - describes how much a community varies over time, irrespective of disturbance. Communities that are less variable are more constant and thus more stable

Resilience

• Should measure population sizes and species composition

• how closely the post-recovery community resembles the pre-disturbance community

Alternative state

• A community that can exist in a particular environment and is different from the current community

• The alternative state is stable if it does not readily transition back to the original state

  • If it is relatively persistent following further small disturbances

Diversity promotes stability

Ecological communities with higher connectance should be more stable

decreasing biodiversity appears to lower community stability, sometimes

Some species were more important than others, removal caused bigger changes

Dominant species

• Due to its large biomass or abundance, has a large impact on community structure and function

Keystone species

Large impact on community structure or function dewspure having a low biomass/abundance

Community importance

• Compares a measurable ecosystem trait before and after the species is removed from the community

  • Indicated whether a species’ impact is greater or less than would be expected based solely on its proportional abundance

Total impact is the magnitude of the change