DIVE DRY WITH DR. BILL #866: ZONED OUT
Back in the 60s and 70s when I taught marine biology at the Catalina Island School (Toyon Bay), I created several field laboratory exercises that challenged my students to really think about the ecological concepts affecting marine life along the extensive shoreline of our island. Back then, before the Internet, I knew of no readily available marine ecology texts for high school students. The effort taken to create these new educational materials saved me a fair bit of money.... by keeping me out of the John B. and Chi Chi Club in town.
One of the labs was deemed "idiot's delight" by a group of female students who came from Denver in the very early 1970s to learn marine biology... and serve as guinea pigs to see if the school might survive going coed the following year. This exercise involved taking square templates of varying sizes, placing them over acorn barnacle clusters on the rocks and counting the critters in side the square.
This was done three different times by each group along a line leading into the ocean. One count was done near the upper end of the barnacle distribution, another in the middle and the third at the lower end of their distribution. If all worked out well, the counts at either end of the barnacle range would be lower than the count in the middle. This was to suggest that at the lower and upper ends of their range, the barnacles were experiencing ecological factors that were limiting their distribution whereas in the middle there was a balance between these factors that allowed the barnacles to achieve higher densities.
This exercise was to illustrate some of the basic principles in what was referred to as ecological zonation in the intertidal zone. The key was for the students to understand not just the numbers, but the reasons for them. Of course yours truly gave them a number of clues by asking a series of questions to be answered based upon their three counts.
Barnacles are marine critters that are crustaceans, related to shrimp, crabs and lobster. They live in hard calcium carbonate shells and only come out when submerged so they can feed on plankton. Barnacles do this by essentially standing on their heads and kicking their feathery feet into the surrounding water to capture the tiny critters passing by. When exposed to the air and sun at low tide, they tuck back into their water tight shells.
Barnacles are sessile, living attached to the rock substrate. They cannot move around to adjust to changing conditions. At the upper end of their vertical distribution, barnacles are exposed to terrestrial conditions except at very high tides. Thus they are stressed by two primary factors. First, they have to stay tight inside their shells to keep moisture in and avoid drying out. They use two plates or valves that help seal the opening in the shell. Second, they are submerged for far less time than barnacles at the lower end of the range, and therefore have less time to feed. Both of these factors limit their numbers at the upper end of their world.
So why would there also be lower numbers of barnacles closer to the water? They are exposed less to air and sun and can feed for longer periods of time when submerged by the tide. There must be other limiting factors at play to reduce their numbers. Two immediately come to mind. First, there is more competition for a place to attach in the lower range as more species can survive there. Limpets and mussels represent two competitors. Second, there is a higher likelihood of barnacle predators surviving in this more hospitable level. Barnacle predators include snails like whelks and starfish such as the ochre star.
This lab exercise illustrated the zonation pattern for one species, the acorn barnacle. Of course the intertidal is a complex ecosystem with many different species present. Each one will be affected by similar limiting factors as the barnacle. Thus we often see "bands" of distribution, especially on vertical rock faces emerging from the ocean. Different species will have different tolerances for the varying ecological factors, causing them to sort out into these bands.
© 2020 Dr. Bill Bushing. For the entire archived set of over 850 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: "Idiot's delight" barnacle count and valves used to seal acorn barnacle shell; acorn barnacle feeding while submerged and ecological zones on a vertical rock wall.
Back in the 60s and 70s when I taught marine biology at the Catalina Island School (Toyon Bay), I created several field laboratory exercises that challenged my students to really think about the ecological concepts affecting marine life along the extensive shoreline of our island. Back then, before the Internet, I knew of no readily available marine ecology texts for high school students. The effort taken to create these new educational materials saved me a fair bit of money.... by keeping me out of the John B. and Chi Chi Club in town.
One of the labs was deemed "idiot's delight" by a group of female students who came from Denver in the very early 1970s to learn marine biology... and serve as guinea pigs to see if the school might survive going coed the following year. This exercise involved taking square templates of varying sizes, placing them over acorn barnacle clusters on the rocks and counting the critters in side the square.
This was done three different times by each group along a line leading into the ocean. One count was done near the upper end of the barnacle distribution, another in the middle and the third at the lower end of their distribution. If all worked out well, the counts at either end of the barnacle range would be lower than the count in the middle. This was to suggest that at the lower and upper ends of their range, the barnacles were experiencing ecological factors that were limiting their distribution whereas in the middle there was a balance between these factors that allowed the barnacles to achieve higher densities.
This exercise was to illustrate some of the basic principles in what was referred to as ecological zonation in the intertidal zone. The key was for the students to understand not just the numbers, but the reasons for them. Of course yours truly gave them a number of clues by asking a series of questions to be answered based upon their three counts.
Barnacles are marine critters that are crustaceans, related to shrimp, crabs and lobster. They live in hard calcium carbonate shells and only come out when submerged so they can feed on plankton. Barnacles do this by essentially standing on their heads and kicking their feathery feet into the surrounding water to capture the tiny critters passing by. When exposed to the air and sun at low tide, they tuck back into their water tight shells.
Barnacles are sessile, living attached to the rock substrate. They cannot move around to adjust to changing conditions. At the upper end of their vertical distribution, barnacles are exposed to terrestrial conditions except at very high tides. Thus they are stressed by two primary factors. First, they have to stay tight inside their shells to keep moisture in and avoid drying out. They use two plates or valves that help seal the opening in the shell. Second, they are submerged for far less time than barnacles at the lower end of the range, and therefore have less time to feed. Both of these factors limit their numbers at the upper end of their world.
So why would there also be lower numbers of barnacles closer to the water? They are exposed less to air and sun and can feed for longer periods of time when submerged by the tide. There must be other limiting factors at play to reduce their numbers. Two immediately come to mind. First, there is more competition for a place to attach in the lower range as more species can survive there. Limpets and mussels represent two competitors. Second, there is a higher likelihood of barnacle predators surviving in this more hospitable level. Barnacle predators include snails like whelks and starfish such as the ochre star.
This lab exercise illustrated the zonation pattern for one species, the acorn barnacle. Of course the intertidal is a complex ecosystem with many different species present. Each one will be affected by similar limiting factors as the barnacle. Thus we often see "bands" of distribution, especially on vertical rock faces emerging from the ocean. Different species will have different tolerances for the varying ecological factors, causing them to sort out into these bands.
© 2020 Dr. Bill Bushing. For the entire archived set of over 850 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: "Idiot's delight" barnacle count and valves used to seal acorn barnacle shell; acorn barnacle feeding while submerged and ecological zones on a vertical rock wall.
