Friday, January 22, 2010

The Ups and Downs of Hydrologic Connectivity

So yesterday I linked to an article about the benefits of dams. Then I read through my emails, and find someone has sent me this article, entitled:

"Ecological benefits of reduced hydrologic connectivity in intensely developed landscapes."


Ok, so the fundamental point of this article is actually pretty obvious. Take a look at the following figure from that paper:
If you think about the caption here, I think you'll find that it doesn't make any sense unless you replace 'hydrologic connectivity ' in the first sentence with 'aquatic ecosystems' or something similar. The entire paper treats hydrologic connectivity as an essentially 'physical' process that affects the entire ecosystem, so I think that is what is being implied here.

What are we seeing in this figure? What we are seeing is a simplistic representation of multi-dimensional environmental space. Instead of an n-dimensional space (where each dimension is a different environmental variable), this figure has been condensed down to just three dimensions: Biological variables, environmental variables, and physical variables. I think this is a good way to think about the effects of different management practices.

Now the first example the authors describe involves changing just the physical characteristics of a particular location. So, for instance, you restore natural stream flows to an area otherwise regulated by a dam. Assuming you are successful at doing this, you've moved the ecosystem properties towards the 'desired' portion for physical characteristics, but you haven't actually gotten into that area because the biological and chemical characteristics are still way outside that 'idea' space.

This is the only idea the authors treat explicitly with this figure, but almost everything else they talk about can be related back to this figure. That example shown in the figure itself (moving from X to X') implies that it is possible to change the physical environment without changing the biological and chemical environment. That's probably not the case very often. As the article I linked to yesterday shows (and numerous examples are also in this paper), removing a physical barrier can open up a previously isolated reach to invasion by non-indigenous species. When you move closer to the 'ideal' space by moving along the physical axis on this figure, you're actually moving further away on the biological side.

That's just one example of how things can go wrong. There are lots more in this paper. The authors break them into 7 categories, but I really think you could group them into 3:
  1. Attracting native species to bad habitats
  2. Allowing non-indigenous species to invade native refuges
  3. Radically altering watershed biogeochemical dynamics (This one is tenuous to me, they talk about removing farm ponds and consequently losing all the sediment and nutrient removal those ponds do. I'm skeptical that this is really occurring, but I can get into that later. They equate these farm ponds to fulfilling the same functions beaver dams used to do...again...I'm skeptical.)
Unlike the paper I referred to the other day, this paper seems to fit a need in the literature. There's been a general sense in the stream ecology community that if you fix the physical habitat then the ecological community will follow along. This has led to work by people like David Rosgen to restore streams so that they function morphologically like natural streams. This view has also been supported by ecological theory (e.g., Poff and others suggesting discharge is a master variable controlling stream ecosystems). With the examples in this paper, Jackson and Pringle suggest it isn't going to be that simple:

Restoration of hydrologic connectivity in a disturbed landscape moves an aquatic system towards a new ecological state, with which we often have little experience and which may have undesirable ecological attributes. (pg 44)

They also quote Saunders and Tyus (1998):

The potential for success of flow management strategies will depend on the extent to which target species or communities are limited by other factors, such as contaminants or the presence of nonnative species, that may not be responsive to changes in the flow regime (pg. 427)

This kind of thinking can lead very quickly to paralysis. The reality is that almost all our ecosystems are under assault from some anthropogenic stressor. Figuring out the right way to alleviate these stressors almost always involves 1) great cost, 2) great societal commitment, and/or 3) great desire. There's never much difficulty finding people who care greatly about fixing these problems, but the money and commitment tend to be more scarce. So the people who care tend to focus on the easiest problems to fix. Removing a dam is easy. All you need is a back-hoe and an operator. Removing an invasive species is hard. In the U.S. we spend billions of dollars just to keep invasives at bay, I'm not aware of any that have actually been eliminated.

So as a result, people tend to plunge ahead fixing what they can (without thinking about the other impacts) or they tend to get bogged down by the myriad of other possible negative outcomes and don't do anything.

I'm not really sure there's a great solution here.

Jackson, R.C. and C.M. Pringle. 2010. Ecological benefits of reduced hydrologic connectivity in intensively developed landscapes. BioScience 60:37-46. doi:10.1525/bio.2010.60.1.8

PS. Let's think about those axes a little differently. How easy do you think it is to move an ecosystem along those axes? My feeling is that moving something along the biological aspect is impossible for some biological variables: Once a disease or invasive species becomes established, it's just there forever. On the chemical side, we probably can change the chemistry of an aquatic ecosystem pretty readily, but it isn't going to be cheap. The physical axis, on the other hand? Well, we move dirt all the's practically the default state of being for humans. That's why we see the emphasis in restoration on physical restoration.

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