Eco topic 14
what is Succession
the process of change in species composition (which species are present) and community structure (how they're arranged and how many) over time following a disturbance.
what is disturbance
an event that disrupts an ecosystem and causes a change in the landscape which leads to lots of spatial heterogeneity (patchiness)
succession affects the
distrobution and abndance of organisms
in Early 20th-Century Debate Ecologists were split into two main groups
Clements perspective vs Gleason perspective
what is clements perspective
ecosystems behaves like a single unified organisms. wants to move to a stable mature end point (climax equilibrium)
what is Gleason’s Perspective
Argued that species are individualistic. Each species responds to the environment independently. based on individual species tolerances and not a collective goal
clements believed that after a disturbance, species gradually
replace each other over time until the ecosystem becomes stable, mature, and doesn’t change much anymore.
climax equilibrium depends on
temperature, rainfall, soil type, location.
And sometimes it takes so long that it might never actually reach the climax.
What Happens Following Disturbance?
It depends on the ecosystem. Different ecosystems respond to fire in different ways. Ex. prairies, tropical and forest fires
dry grasslands have frequent fires to
encourage new growth for grazers
what happens if you suppress disturbances like fire
because ecosystems rely on these natural disturbance regions to maintain their structure. when you suppress these disturbances, forests start to grow into areas that should naturally remain grasslands. This reduces the species that depend on open habitats and also causes fuel buildup, making fire more dangerous in the future.
species richness is highest when disturbance levels are
intermediate (not too high or low)
high disturbances =
too harsh, things die = low diversity
low disturbances =
competition takes over, dominant species exclude others = low diversity.
does disturbance frequency explain richness?
not always because disturbance is complex
example of high/extreme intensity disturbance
lava fields, severe fires = almost nothing survives = very low diversity
what is primary succession
the change of a biotic community in a newly established area. (start from 0, usually in mineral conditions)
what is secondary succession
the change of a biotic community following a disturbance. (don't start from 0, species can regrow quickly)
explain secondary succession in forests
early successional species arrive quickly and thrive in high-light, low-nutrient environments. As the forest matures, later successional species take over by outcompeting them for light, eventually preventing early species from growing back
explain secondary succession in savannahs
succession is driven by nutrient competition, not light, because fire and grazing keep the habitat open. Early successional species are the best dispersers but weak competitors, while late successional species are poor dispersers but strong nutrient competitors. best nutrient competitors replace early colonizers
the retreat of glaciers create the perfect natural experiment for
primary succession, because life has to start from zero (bare rock, no soil, no plants)
what is the pioneer stage
0-15 years, moss and herb dominant, soil basically non existent, dryas helps fix nitrogen to improve soil.
what is the willow stage
15-35 years, willow dominant, soil gets better, plants get taller= block wind
what is the alder thickets stage
35-80 years, alder and willow dominant, alder fixes lots of nitrogen, massively improves soil, pH drops from 8 to 5
what is the sitka spruce
115-200 years, spruce trees dominant, deep soil so large trees can grow, dense forest starts forming
what is a mixed forest
+200 years, hemlock + spruce + others dominant, mature forest = lots of biodiversity
what is organic carbon
how much dead plant material and organic matter is in the soil
as organic carbon increases =
more biomass