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Pisaster seastar as keystone species

Predation

Top down control of rocky intertidal community structure

 

 

Site 1

This area went thru dramatic changes over the season

 

This page follows how the barnacle Semibalanus cariosa goes from juveniles to 100% coverage and then gets eaten

 

Spat: early May

Rapid growth June

Predation by seastars removals by July

 

Links: TPL_04_May_breccia_pass.htm

Semibalanus Spat

Luck: it was the quantity of young barnacles that became evident in early May that led me to select several sites to follow over time. I was not sure what barnacle the ‘spat’ was when I started.

Below is higher magnification showing typical adult Semibalanus cariosus and the ‘spat’. The limpet (at L) has kept its feeding area clear of spat presumably by ‘bull dozing them away

Semibalanus ‘spat’ grew extremely fast

“Spat” is several centimeters high after only 5 weeks as below

 

Seastars begin ‘eating’ the Semibalanus

I noticed in early July that some of the thick spat layer was beginning to disappear but could not
explain why – after all the seastars were well below site; so I began ‘monitoring’ this rock using photography.
When these pictures taken the seastars had (apparently) already ‘cleaned off’ the south end

Monitoring ‘Site 1’

Monitoring the Pisaster feeding on barnacles

By using the cluster of Alaria (“A”) as a reference point took pictures every few days

But little change seen at short invervals (compare the ‘spat’ above “1” on 7/1 and its gone by 16th)

Video shows area around the ‘1’ better 3.2 Megs MOV01614.MPG or Site1_pisaster_July.wmv

Still change not dramatic

Dramatic Change in Months time

Gone

Only the silhouettes remain a month later

 

Pisaster on Semibalanus

 

Lessons: Semibalanus grows very quickly till July—(strategy: grows too large for easy snail predation) but spreading clusters attracted Pisaster ‘gangs’

Keystone species

Pisaster is text book example of ‘top down’ control and its feeding structure the rocky shore line at wave exposed intertidal sites

 

 

 

 

 

 

 

 

 

Created by LG on 9/10/2004

Last updated on Saturday, September 11, 2004