Biodiversity - Week 7

In our discussion of Type A and Type B curves (ecosystem function versus species diversity), we came to the issue of order.  With a Type A curve, the order in which species are lost from the system does not matter, the curve orientation will not change.  However, with Type B, order can be important.  For some cases, where species function is shared (redundantly) among a few species in the ecosystem, the order in which they are lost is unimportant; all that matters is when you come to the last one.  For other cases, where there are a few species that perform fairly different functions from the rest, the Type B curve can be very steep at low diversity, leveling out at greater diversity (that is the different-functioning species are not lost until later); or the Type B curve can be very leveled at low diversity and steep at high diversity (signaling the loss of the different-functioning species earlier on). 

Following this train of thought, we could introduce a third curve type (again order is important) whose shape is parabolic with maximum functioning for the middle range of diversity, and minimum functioning at the extremes (high and low) of diversity.  The problem with this curve is that it depends heavily on what is being measured or defined as ecosystem functioning.  For example, one could find a curve like this if the function was biomass, and if animals were lost first, leaving just plants (maximizing biomass).  As the animals die, ecosystem functioning increases, but as the plants die, functioning decreases.  Of course, if our function was predation for the same situation, then we would not find this curve type.

Returning to Ruri’s point about the relevance of knowing where the ecosystem is on the curve for conservation purposes, order might also an important parameter in conservation decisions.  For the type B curve, if we seem to be in a level situation in which losses of diversity is not heavily changing ecosystem functioning, then one might want to know whether there is a point in the future when that will change, or whether that point has actually already been passed.

Schwartz et al 2000 called to question the hypothesis that there is a strong dependence of ecosystem function on the full complement of diversity within a site. This is based on review of diversity-function papers from 1991-1998. The literature indicates predominance of a type B response curve where an ecosystem can lose much of its diversity without losing function. Regardless of curve type, many experiments have shown a positive relationship between biodiversity and ecosystem function.

There are many possible explanations for this relationship. One that we discussed in class this week is dominance. If a dominant species is responsible for most of the biomass or are better at the ecosystem function being measured they can cause species richness to appear more significant for function than is actually the case. A study of algal species identity and richness by Bruno et al 2006 supports this hypothesis. They found that 75% of total wet algal mass was made up of just 2 or 3 species and that polycultures yielded lower primary productivity than 6 of 8 monocultures. Their analysis indicates that in coral reef ecosystems the influence of magroalgal diversity on primary productivity is negligible while species identity effects were highly significant.

The effects of dominance on the biodiversity ecosytem function relationship are important to consider when making a conservation argument. If the same function can be derived from a system with a few dominants as those with a full compliment of speices it will not prove very convincing. It does, however, suggest that the order in which speices are lost could be important for the biodiversity-function relationship. If we are going to lose a key or dominant speices first because it is particularly sensitive to human disturbance the loss of a few species could have a significant negative effect of function.

Until the many confliciting results and numerous hypothesis about the components of the biodiversity-ecosystem funtion relationship are resolved and elucidated it is difficult to determine its direction, strength, and sensitiviey, and usefullness as an argument for conservation

Bruno JF, Lee SC, Kertesz JS, Carpenter RC, Long ZT, Duffy JE (2006). Paritioning the effects of algal species identity and richness on benthic marine primary production. Oikos. Vol. 115, Issue 1, pg. 170-178.