“Size Matters…?”



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.

DSC_0410For 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.”


Author: Paul Weihe

Associate Professor of Biology at Central College, traditional author (Textbook of Limnology, Cole & Weihe, 5th ed.; Waveland Press), and now...blogger!

4 thoughts on ““Size Matters…?””

  1. One of my concerns is that the push to improve Iowa water quality, justified though it is, will overwhelmingly drive wetland policies in Iowa over the next few decades. Given the tiny number of CREP wetlands that have been built so far, compared to the numbers of wetlands that researchers say are needed to reach Iowa’s nutrient reduction goals, that concern still seems theoretical. But maybe not for long. (For those unfamiliar with Farm Bill alphabet soup, CREP wetlands are Conservation Reserve Enhancement Program wetlands, and they are designed for nutrient removal.)

    There are many questions. Will all future Iowa wetland funding be focused on nutrient-removal wetlands, at the expense of other kinds of wetlands? Even now there are political efforts to grab every possible dollar of state conservation funding to spend on farm pollution, per IWILL. Will Iowa allow major enlargements of farm drainage systems in return for new nutrient-removal wetlands, which could eliminate a lot of shallow-sheetwater shorebird habitat? Will the anti-regulatory movement in general make it easier to destroy irreplaceable original wetlands, with dubious mitigation wetlands supposedly replacing them?

    It’s not that CREP wetlands are bad. I have a mini-CREP myself and have seen how it is used by wildlife. But as this “Size matters” post points out well, there are many important wetland functions. A laser-beam focus on nutrient removal is not compatible with all of them.

    Also, some wetland research has focused on how small a nutrient-removal wetland can be and still function. That research is needed and justified, given the strong desire of many Iowa farmers and landowners to take as little land out of rowcrop production as possible. But a keep-it-really-teensy wetland goal is not compatible with the needs of many kinds of wildlife. And as this post points out, some small wetland restorations turn out to be bad and/or neglected. Without regular good management, many little “wetlands” can become cottonwood/reedgrass plantations very quickly.

    Liked by 1 person

  2. Thanks for yet another thoughtful and eloquent comment. (I really appreciate your participation in this project!).
    I agree of course with all of your comments. I’ll have to walk a fine line to get a message across (“wetlands improve water quality”) without a listener/reader hearing “so go ahead and send them anything, in any amount.” I do believe we can accomplish multiple goals with wetlands, but that will require real commitment.


  3. Thank you for describing that major attitudinal challenge. You are so right. A wetland should not be regarded as a get-out-of-doing-infield-conservation-for-free card.

    As you have pointed out before, small wetlands are increasingly being restored close to lakes so the wetlands can act partly as buffers. By processing farm nutrient pollution, the wetlands keep the lakes cleaner. But if the farmland that drains into the wetlands consists of partly-bare often-tilled degraded eroding soil, those wetlands will have limited lifespans and effectiveness. It is frustrating to see an algae-filled buffer wetland that would be so much healthier if the upstream farmland had cover crops and adequate grassed filter strips. And the farmland would be much healthier too.

    Story County recently finished an evaluation of all the watersheds in the county. One result is a map that shows the exact sites where good farm conservation would be most effective in protecting water. Of course certain other watersheds in other counties have similar maps. Iowa needs to reach the point where all landowners and farmers are looking at those maps and acting on them, with the help of adequate conservation funding.


    1. Of course, I couldn’t agree more with your sentiment. If my wanderings and musings have any central theme, it’s that the water and land are connected. I’m glad to hear that dedicated and thoughtful workers have given us the tools to care of our environment. I surely hope we choose to do so!


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