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Questions and answers

on sand dunes, saltmarsh, succession topics and sediment shores

Johnny Brown of Swainby asks: Can primary succession only take place if there is no life in the sere beforehand? If so is it only plants and microbes that are capable of primary succession? Presumably humans have never indulged in primary succession if it's always defined so strictly? Even in Antarctica one could argue that the penguins got there first.

I think the problem here Johnny is that you are thinking in terms of individual species. Succession refers to development of a community. Quite when that development starts is up to the investigator to define. So if you take our example of salt marsh succession on this site; we defined the beginning of the flowering plant stage as the true start of succession. It was primary succession because it was a community developing from scratch, rather than a cyclic or seasonal change or development resulting from a change to an already established community. It is an error to suggest that individual species are "capable" of succession. As to whether humans count in successional development; this is a philosophical problem along the lines of "do volcanic eruptions count as pollution or does it have to be human produced?" It's probably true to say that people who study succession tend more to the botanical side of things and a lot of what you read about in succession refers to plants. There are however studies of animal community development with time. Studies of deer and bird species in developing woodland in Virginia USA for instance were carried out between 1990 and 1998.

Meredith asks “i hope that i have found the right place to ask questions if not, sorry.
why is it that marram grass generally grows longer at the top of sand dunes than at the bottom? i thought it would be the other way around.”


There could be several reasons. Marram thrives in unstable, wind-blown areas. Sand striking it simulates growth. As a coloniser it does well without competition, and when competition occurs it is soon replaced. 1. At the top of dunes there will be more wind blown sand stimulating growth, too sheltered at the bottom. 2. More water at the bottom along with shelter will encourage other plants which will out-compete the marram. 3. Transpiration rates will be highest at the top, that is better growth.

Amy Burlingham asks about her data: From my soil samples of a sand dune i found that there was more moisture and nutrients at the top of the dune rather than the bottom, but from what ive read there should be more at the base. How come this was the case and could this be the reason why Marram grass was tallest at the top for this particular dune?Does Marram grass have a temperature it grows best at? Would the Marram grass at the top or bottom of the dune get covered in more sand due to the wind and other factors?

You’re right, the results are not the way you would imagine. Marram is tallest at the top because of the wind-blown sand – the factor that controls it most. Marram is tolerant of high temperatures, not low ones. It does not occur for example in Iceland but does in north Africa. I do not know what ideal temperatures there are.

The more wind-blown sand the more vigorous the growth and it will be the windward side of the dune that has most rather than say top or bottom. Bottom facing the sea might have most. Trouble is I am generalizing and it depends how you did your survey and the topography around. Largely I would expect the top.

Andrew Holloway asks What organisms prey upon the thin tellin?  Are there any references i can investigate in order to find a detailed description of tellin behaviour?
Small bivalves like tellins are attacked by necklace shells (Polinices catena). These use chemicals and mechanical means to drill a hole near the point where the two shells attach. You can find the empty shells lying like butterflies on the surface of the sand and on one side a hole bored through to cut the muscle that holds the shells together. Flatfish eat the siphons as they emerge from the sand and these have to be regrown but does not kill the tellin.

here are several books that cover burrowing etc. One is Living Marine Molluscs by (Yonge and Thompson) pub. Collins

 

 


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