Which is better: a bunch of small wetlands spread around a landscape, or one large wetland with an equivalent total area? At Dan Green Slough in Clay County, I pondered the question of how size might relate to wetland function. This 350-acre (142-hectare) wetland must be one of the largest natural wetlands in Iowa!
It seems logical that a larger wetland would have greater ecological (or other) value than would a smaller one. But…if you could choose between building this wetland, or 35 wetlands each 10% as large as this one…which would provide greater functionality (ecologically, or otherwise)?
“But Paul,” you say. “What an esoteric question!” But consider this: we must make decisions about acquiring natural areas and we have only so much resources. We also are routinely building wetland ecosystems all over Iowa and elsewhere. Shouldn’t their design and construction incorporate our best understanding of ecological function, so we derive the most benefit from the investment?
The ecological literature has considered the question extensively, trying to optimize our time, money, and effort in preserving natural areas or when attempting ecological restoration. Sufficient work has been done, that researchers have an acronym for this phenomenon: SLOSS (Single Large, Or Several Small). The arguments have raged for decades, and that’s merely considering numbers of species, not other ecological functions. Even such a narrowly-defined focus on biodiversity is by no means definitively understood. We don’t have a simple rule to apply…no simple rule may even exist.
For wetlands, it gets much more complicated: we need to promote biodiversity, sure…and yet we need so much more. We want wetlands to “clean the water” through holding sediments and phosphorus, transforming nitrogen, and reducing biochemical oxygen demand. We want wetlands to store excess water, lessening downstream flooding during wet times. We want to support wildlife, especially waterfowl. And on and on. Five years ago, a leading wetlands expert, Paul Adamus, published a summary of this. Spoiler: it’s hard to even define the parameters of the question. Sigh. What exactly do we mean by “function” or “ecosystem health,” anyway?
So, what to do in the face of this uncertainty? First, “do no harm.” Existing natural wetlands, of any size, should be preserved. Natural wetlands are connected to waters moving through a landscape, are used by animals in ways we may not be able to observe, have microbes or seeds in the soil of which we are unaware…they are valuable, and functional, whether we can observe all that function or not.
What about creating wetlands? I suppose the adage “follow the money” will apply: if we build a “retention wetland” to hold back water in a flood-prone area, or if the bills are being paid by duck hunters…the goals are obvious. In the absence of a specific goal (financial incentive) directing the construction, I humbly suggest we concentrate on building high-quality wetlands. I have seen many wetlands around Iowa, and elsewhere, built by humans. (Check out blog entries on Indian Slough, Neff Wetlands, or Heron Marsh for examples in Iowa). Some I’ve observed are pretty good, others are shaky at best, and some are clearly failures. Wetland professionals in other locations report mixed results, too. I’m sorry to say, we might have to think about what we can actually accomplish, as well as (or before??) ecological function. Realistically, we need to ask what is feasible. In the future, we really can, and should, insist on preserving and maintaining with an eye on what is optimal. In the meantime, I’m sorry to ask if we are more likely to properly engineer and construct and monitor fewer, larger wetlands than many, smaller wetlands? It’s unlikely the situation will ever be a case of “all things being equal…” so maybe the best bet is to really focus, and do the job right. My observations to date suggest that smaller projects tend to be lower-quality, and are quickly forgotten. If there’s a minimum size before we take our task seriously…then let’s only build wetlands that size, or larger. It pains me to suggest such an approach, but it’s not an original argument, and it is often made in the context of mitigation banking…a topic for another day.
Back to Dan Green Slough: yes, this is a big wetland! I saw evidence it supports lots of waterfowl (I saw many ducks and geese), non-game birds (red-winged and yellow-headed blackbirds), and muskrats. The wetland had large open water areas, but also smaller coves and backwaters. I presume water depths varied around the basin. So, this large wetland has a variety of habitat within it. A large system with heterogeneity is A Good Thing, simply put.
This particular wetland, and this journey of 99 wetlands, asks you to appreciate these valuable ecosystems. “Large or small, 99wetlands treasures them all.”