Biodiversity - Week 7

These pages are being generated as part of a senior seminar course directed by Neal Williams at Bryn Mawr College during fall semester, 2007. This week's topic is "Biodiversity and Ecosystem Function"

Reading to be discussed:

  • Schwartz, M.W., Brigham, C.A., Hocksema, J.D., and Kyons, K.G. (2000) Linking biodiversity to ecosystem function: implications for conservation ecology. Oecologia 122: 297-305.

Comments

nwilliams's picture

Effect traits and response traits

Diversity-function relations are often characterized as type A or type B. Although both show significant relations between diversity and function, type B suggests strong redundancy within the system. Schwartz et al (2002) argue that a type A relation provides a strong basis for conservation, whereas a B relation does not. I suggest that a type B relation can easily become b’ if those species that are most important for function are most sensitive to being lost. Ashley points to one factor that could produce the b’ relation (loss of dominant species). Another would be is species that are on a per unit (e.g., individual/ biomass) basis are most important are also most sensitive. These species might not even need to be dominant for a type b’ relation to occur.

diversity_function

I also was very interest by the idea that really if most relations are type b then the most important question we as conservation scientists should be investigating is whether we are on the asymptotic part of the b curve or more to the left.

Neal Williams

maggie_simon's picture

Ecosystem Functioning versus Biodiversity Curves

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.

rkumazaw's picture

Biodiversity and Ecosystem Functioning

Studies on the relationship between biodiversity and ecosystem functioning have become much more prevalent recently due to increasing concerns about the conservation of biodiversity. What is more important than whether or not biodiversity and ecosystem functioning have a positive relationship but whether or not the curve tends to be linear, or whether the curve is type A (linear) or type B (exponential), according to Schwartz et al. The implications of a type B curve, which is the preferred relationship, is that the loss of biodiversity to a certain point will not have a significant impact on ecosystem functioning, hence why it is often used as a conservation argument. However, as we discussed in class, what is even more important is where the present situation lies on the curve, considering that the biodiversity-ecosystem functioning relationship is a type B curve in real life, because the implications depend on that specific point. If the present situation falls in the flat portion of the curve, it implies that we are not facing an immediate collapse of the ecosystem. On the other hand, if our present situation falls in the portion of the curve with a large slope, it implies that we are already in a critical condition and immediate course of action is crucial. In the case that the former is true, a more clear knowledge of a current situation can enable us to know how much time we have until we reach a critical point and how we should design conservation strategies. However, the problem remains in the unfortunate fact that the answer is unattainable and that there is no way to determine this, at least for now. Furthermore, most studies investigating the biodiversity-ecosystem functioning relationship only concerns species richness as the definition of biodiversity and disregard the importance of species evenness. Schwartz et al. criticizes the studies they analyze in that the experiments with equal abundances for each species do not represent real ecosystems. This alters the results to look more like a type A curve then it actually should. Although species richness is the easiest measure of biodiversity, it is most often not enough.

 

Ashley Himelfarb's picture

Effects of Dominance

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.

nwilliams's picture

Dominant or uniquness

Ashley, writes to raise an interesting point and I wondered if you could clarify your understanding of the dominance idea. Is your use (and Bruno et al's) use of it like the point that diversity increases functioning because there is a greater chance of including a species with particularly high function? Seems like it might be a bit different. In any case I agree with your summary of the importance of the order of species loss. You put it nicely that loss of dominant (high funciton species first will cause a dramatic drop in function. --Neal Williams

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