Monday, April 10, 2006

The HSS Hypothesis ~ the world is green

Today we read one of the seminal papers in all of ecology by Hairston, Smith and Slobodkin published in the American Naturalist in 1960. This paper is cited in every modern ecology text book. It is often called 'the HSS hypothesis' or 'the green world hypothesis."

The premise of the paper states that plants:

are limit by their ability to pick up nutrients {mainly nitrogen}:

Nitrogen is a key component in the most abundant protein on earth, RuBisCO. Ribulose-1,5-bisphosphate carboxylase/oxygenase is the protein singularly responsible for the anabolic reaction photosynthesis, which reduces carbon dioxide for cell stuff and energy as cartooned in the picture of the chloroplast below:

Without nitrogen, and thus photosynthesis, we would all cease to exist. The HSS hypothesis states that plants could grow ad infinitum as long as they {the plants} have N.

However, as we all know, there are a lot of animals out there who love to eat plants, such as myself and the horses pictured below:

There are many, many, many animals out there who enjoy a vegetarian diet. One might predict that all the animals might eat all the plants over time. Thus, one might expect the world to become denuded of plants. So why is the world green?

The HHS hypothesis predicts the world is green is due to the presence of carnivores, like this tiger:

Often tigers like to eat juicy herbivores such as deer and cows, as do I. The HHS hypothesis suggests that the world continues to be green because the carnivores keep the herbivore populations in check, thus allowing the world to remain green.

There are several problems with this. Experimental data {which are very rare} seldom back up the predictions of the HSS hypothesis for a variety of reasons {primarily scalar and logistical). Secondly, can we really measure any ecosystem in the world these days without accounting for man's fingerprint? From large-scale effects on biogeochemical cycling, medium scale habitat loss and fragmentation for a vast numbers of organisms, and small scale human disturbances {i.e. paving}, I ask, can we ever again even think about measuring a pristine ecosystem without considering the human covariate?

So I ask, why does the world appear to be green? Or does it really? What you all think?

This is great thought exercise considering the upcoming EarthDay holiday.