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Ecological Succession

Fair warning, this one is a little "science geekier" than most posts - which is saying quite a bit - but succession is an idea we see across aquatic and terrestrial systems and a concept that the reasonably well informed outdoors-person should have some understanding of.

Oxbow created by 2018 flood events
Evidence of stream movement - the 2018 flood created an isolated pool that is no longer connected to the rest of the stream.

Succession is probably an idea you are familiar with but maybe did not know what it is called or that we even had a name for the community changes that take place over time. In most of Wisconsin, prairies are maintained by fire, grazing, or other disturbances. Without these disturbances, most prairies become "shrubbier" and eventually early succession forests, and then eventually mature forests. Or we have witnessed a place we have known for decades slowly change over time. Most commonly in Wisconsin - and as I talked around but not about in a post on what Wisconsin looked like - is that land that was once farmed but no longer is, changes over time. Try the Wisconsin Historic Aerial Imagery Finder and see for yourself how much our landscape has changed from the 1930's and 1940's when the first aerial images were taken.

As mentioned in the first paragraph - and is even more evident in the video above - succession is not just a concept in terrestrial ecosystems but occurs in aquatic ecosystems. Over time, lakes slowly fill (eutrophication) and will - again over much time - transition to wetlands and eventually terrestrial habitats. Moving water in streams prevents this terrestrialification (I might have coined a new word) but they also change over time. We see the remnants of these changes - oxbow lakes where channels once flowed, erosion of the valleys or canyons that streams have flowed through - all around us. We see less signs of old streams in Wisconsin due to the relatively recent Wisconsin Glaciation. However places like the Lower Kickapoo River in the unglaciated Driftless Area show a great number of oxbows and meanders. Age - the time a stream has been confined in its current valley - is why the Kickapoo River is the self-proclaimed "Crookedest River in the World". In softer sediments, sinuosity is a function of stream age; in harder sediments - like the Grand Canyon - depth of the canyon is the result of age and erodibility.

Image source: By LucasMartinFrey - Own work, CC BY 3.0,

Most of the discovery and subsequent research in how succession works comes from terrestrial environments. Much of the early work was on Lake Michigan sand dunes where early ecologists reasoned that dunes closer to the lake were younger (had been in place for less time) and those farther from the lake are older. We talk about this as a "space for time substitution" - a way to study processes that occur over long periods of time in a more efficient way. Rather than waiting for 150 years for mature forest to develop, we can simply move farther from the lake to find places that were once more like the dunes closer to the lake. We see the same thing with vegetation communities as glaciers retreat.

Glacier Bay National Park
A receding glacier at Glacier Bay National Park, taken from a small airplane.

Streams experience succession as well. One of the papers I have often used in ecology course with students is a paper or two from Sandy (Alexander) Milner from his studies of Glacier Bay, Alaska. At Glacier Bay, they've been able to watch succession happen. And using the space for time substitution concept, they've been able to see succession occurring from now to several hundred years into the past - all in a reasonably small area. They saw succession not only of the landscape but they also saw predictable changes to stream temperatures (they warmed as time went by). These temperature changes lead to the succession of macroinvertebrate communities and fishes.

From Flory and Milner (2000). Paper available at the link.

Here you can see the recession of the glaciers at Glacier Bay National Park and begin to understand how the landscape, the bay, and the streams feeding it have changed over time. We have a nearly 300 year history at this location. Today, we see early successional habitats that are dominated by mosses and grasses and youthful streams that are low in nutrients close to the glaciers. Further from the glaciers, Sitka Spruce and Hemlock trees constitute the climate or final community. The streams within the landscape are a function of the landscape that feeds them. Youthful streams are cold, fully of glacial flour, and rather infertile. Further from the glaciers in both space and time, older streams now host salmon, sculpin, and a few other fishes.

From Flory and Milner (2000). Paper available at the link.

As you can see, early on, only chironomids (midges) are present and later, as the stream warms, mayflies (Ephemeroptera) and stoneflies (Plecoptera) begin to be found. And given a little more time, caddis (Trichoptera) and black flies (Simuliidae) show up. A little later, craneflies (Tipulidae) and others that are more dependent upon organic matter show up. This follows the landscape succession where at first, there are only mosses and grasses. Eventually alders and other shrubby trees take over. And eventually, the spruce and hemlock climax forests take over. These forests are generally 150 years old or older. In this cold landscape, succession occurs slowly, much like it probably did as the glaciers covering most of Wisconsin. Interestingly, salmon play a role in the succession of both the stream and its landscape. Salmon semelparity (reproduce once and die) evolved, we think, because it allowed their offspring a better chance of survival. Not only do the nutrients they bring from the ocean help create a more productive stream for their offspring, vegetation along the stream grows larger and more productive than streams that lack salmon.

I write about Alaska so I can write about what Wisconsin probably looked like 10,000 or so years ago. You may have read my post or seen a host of other stories recently online about how Iowa and Minnesota are not Driftless. Their "Driftless-like" area was glaciated about half a million years ago which means that today, evidence of glaciation is mostly only visible to geologists. Time heals (most) all wounds. And the glaciers were pretty significant "wounds" to the landscape.

Dodge County aerial photo from 1940
An aerial image from Dodge County, WI in 1940. Below is the more current look at that same location.

While we can't travel 10,000 years into the past, Milner's and his colleagues studies in Alaska can shed some insight into what Wisconsin looked like as the glaciers retreated. Certainly the glaciated part of the state was most impacted, however the Driftless would have been much colder and was certainly impacted by the glaciers. Outside of the Driftless, we see a landscape dominated by glacial features - the hills are drumlins and kames. We see lakes and wetlands where large blocks of ice created low spots in the newly created landscape. Once created as the glaciers retreated, these lakes and wetlands begin to go through succession. For lakes and wetlands, that means that they begin to fill with sediments - either those delivered from their watershed or primary productivity that then dies and decomposes. In other words, the succession of lakes and wetlands is their filling of sediments. Eventually lakes become wetlands, though that process is generally quite slow.

recent Dodge County (WI) aerial image
A more recent aerial image of Dodge County, WI

Of course, humans have a way of altering the landscape. The above images from Dodge County near where I grew up show succession. Today, we see more forests than we did in 1940. I wrote about this in much more detail in a post, Wisconsin Before the Europeans. As you can see in the image below, the state was once more forested, much more forested. Of course, much of this land was converted to agricultural purposes. Today, we see a bit less agriculture in the places were the lands and growing seasons make agriculture difficult. So these lands have / are going through succession. That is, they are aging. Former farmlands started growing grasses, then shrubs and early successional trees like birch and aspen, and then into climate communities. The climax community type depends upon the location, its soils, the locations temperature and precipitation, and a host of other factors.

Stepping back 10,000 or so years ago, Wisconsin would have been a lot like Glacier Bay today. Of course, 10,000 years is much longer than the glaciers at Glacier Bay have been retreating. So we see a landscape that is more shaped by humans than glaciers. We see succession all around us - that abandoned property that "went wild", the oxbox and old stream channels on streams, and at the extreme, sites where dams have been removed are filled with sediments and dam removal will lead to a quick change in the landscape and its vegetation. And we "fight" succession all the time through the mowing of our lawns, the planting and harvesting of crops, and the burning of prairies in the fall or spring is to prevent later succession species from taking hold.

Flood effects at stream confluence
A stream confluence after the 2018 floods. This site looks much different today.

Collectively, these things that set back succession are termed disturbances. On the landscape, these are typically fire, grazing, mowing, and cutting of trees. For streams, floods are the most significant form of disturbance. After the 2018 floods, we saw streams that altered their courses. In some cases, oxbows were created as streams were moved in their valleys. In others, their banks were changed and terrestrial areas covered with sediments - as in the photo above. After the disturbance, the community becomes more like an earlier successional "stage" (stages may or may not exist, depending upon your view of how succession works). And slowly over time, the stream and landscapes heal themselves. Or we have gone in and sped up the process.

Restored stream reach
A stream reach that was restored / manipulated / "fixed" / etc. after the 2018 floods.

What we call it when we "restore" or "fix" stream reaches is a post for another day. Habitat improvements is probably the name we most commonly use. Of course that name is not without conflict. What we have done with these streams following the 2018 floods is to reshape and re-vegetate the banks more quickly - and with less sediments moved downstream - than would have happened naturally. Succession is a relatively slow process, we have the ability speed up the process. And that's a post for another week...

Small oxbow pond.
A small oxbox that was once a favorite fishing spot before the 2018 floods moved the steam 100 yars to the east.

To wrap up this post, succession is a natural process but of course our landscape is so dominated by people and our disturbances that there is little of what we might call natural around us. Over time, landscapes age - for lakes and wetlands, they fill in; for streams they cut and meander; and for terrestrial lands, at least for most of Wisconsin, they grow trees and become climax forests. Disturbances set back this succession which means that streams and terrestrial and wetland plant communities look more like they did during an earlier stage. For streams, it means we see straighter streams with fewer fine sediments, but of course these streams begin to age as soon as the disturbance is over.

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