Kelp Forest Dysfunction Flashcards
Bottom-Up Forcing
Lower trophic levels affect the community by means of resource restriction
Abiotic factors have a huge impact
Related to production and food supply
Top-Down Forcing
The top consumer controls the primary consumer population
Predators induce population change, they do this by predation
Robert T. Paine
Introduced the concept of top-down forcing in marine ecosystems looking at Pisaster
This idea formed the basis of keystone species
Top-down vs Bottom-up forcing in kelp forests
Natural and human-caused
Natural Bottom Up: Thermal stress linked to natural temperature and nutrient fluctuations
Human-Caused Bottom Up: Climate change
Natural Top Down: Disease-induced mass mortalities of urchins
Human-caused Top Down: Exploitations leading to trophic cascades and other complex interactions
Phase Shift
When the community composition completely changes due to some change in the environment
This is normally temporary
Alternative Stable State
If there are positive feedback mechanisms phase shifts can last indefinitely leading to these alternative stable states
Urchins
Detrital herbivores can be scavengers Highly mobile Tasty egg masses (gonads) 20-30 lifespan Distributed in every ocean and every climate Intertidal to 7000 m depth
Abalone
Better defence but more sessile
Marine snails
Lose food competition to urchins
Sea Otters
Currently extirpated from the Salish Sea
Locally on the Western Side of the Island
Live in tight family groups, that don’t disperse very much
Sea Otter Hunting
1741: Russia discovery of sea otters
Used Aleut slave hunters in 1700s
1790: Hunting mad its way down to northern California
Nearly 9 million otters are killed annually at its peak
early 1900s most were extinct
Estes and Palmisano
Compare Islands with and without otters
Studied the effects that otters had on structuring nearshore ecosystems
Top-down impact on kelp forests
The first real suggestion of this
No Otters: Kelp rare, urchins large and evenly distributed from 0-25m, few fish and eagles
Otters: Kelp abundant, urchins small and restricted to depths >25m, abundant fish and eagles
Trophic Cascade
Interaction across two or more trophic levels
Indirect species interactions that originate with predators and spread downward through food webs
Sea-otter driven trophic cascade
By grazing down the urchins sea-otters have a positive impact on kelp numbers and indirectly fish and bald eagle numbers
This change can be very fast occurring across a decade
Functional Diversity in kelp forests
Otters were functionally extinct on the North American coast in the 1800s, kelp forest persisted for another 150 years.
Other predators of the urchins like fish can help fill this role
Modern overfishing, potentially non-industrial destroyed fish populations
This removed the functional redundancy
Functional Redundancy in Modern Northwest Atlantic kelp forests
Modern human fishing of sea urchins has reduced their numbers and helped the kelp recover
This caused a population surge of the Jonah crab
