The keystone

HOW TO BUILD AN ARCH:

Step 1: Construct a frame in the form of an arch

Step 2: Endorse the foundation

Step 3: Stack stone blocks along the top of the framework

Step 4: Set a keystone at the top of the arch (see image 1)

(Ryan 2009)

Image 1: Position of the keystone in an arch (unknown artist 2012)

You may ask why I am yammering on about how to construct an arch. Well, apart from being pretty nifty, the structure of an arch is actually, in my view, very similar to the structure of an ecosystem. Let me explain.

The framework is the location – the landscape and climate. The foundations are the primary producers who provide the fundamental support, both in structure and nutrition. The layers of stone blocks depict trophic levels and the keystones are those species whose presence maintains the stability of the system (indeed the term ‘keystone’ species was borrowed from the stone used in the construction of arches).

When a keystone species is removed from the ecosystem, the system collapses. This collapse is what we call a trophic pyramid. How is it that keystone species have evolved, when their demise means the disruption of an entire ecosystem or a cascade through the trophic levels?

I would first like to point out that evolution is not conscious. There is no mind behind the scenes wondering what feature would be practical, what would look stunning, or what skill would be handy. Change overtime is simply driven by a response to external pressures.

Coevolution, in particular, is the evolutionary change (physical or behavioural) that one species undergoes in response to another species (Richard & Lockshin 2007).

I would like to use the trophic cascade seen in the Yellowstone National Park when wolves were removed from the ecosystem as an example. In this scenario, coevolution can be seen between the wolves and the deer. The wolves hunt the deer. In response, deer have the stamina to run (physical change) and tend to stay away from certain areas to avoid the presence of the wolves (behavioural change). These adaptations in return influence the physiology and behaviour of trophic levels below. So wolves, in this ecosystem, and other species in other ecosystems, have become keystones because of their influence on the physiology and behaviour of the trophic level below and beyond (Fortin et al. 2005).

Image source

Unknown artist 2012, ‘Position of the keystone in an arch’, The Keystone Ukulele, 15 March, 2014, <http://iriguchiukuleles.com/ukuleles-2/the-keystone-ukulele-our-signature-ukulele/>

Other sources

Fortin, D, Beyer, H.L, Boyce, M.S, Smith, D.W, Duchesne, T, Mao, J.S 2005, ‘Wolves influence elk movements: behavior shapes a trophic cascade in Yellowstone national park’, Ecological Society of America, vol. 86, no. 5, viewed 15 March 2014, <http://www.esajournals.org>

Richard, A & Lockshin, Ph.D 2007, Coevolution: An Examination of How Scientists Ask and Answer Questions Using the Story of Evolution as a Paradigm , Springer, Netherlands.

Ryan, V 2009, Technology Student, 15 March 2014, <http://www.technologystudent.com/struct1/roman1.htm>