Tag Archives: biodiversity

Habitat Heterogeneity in One Photo

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I was featured on a podcast episode that came out today. I’ve enjoyed being a guest on quite a few podcasts, but I think this one might have produced the best synthesis of many of my thoughts on prairie management. If you’re interested in listening to it, check out the Wild Ag Podcast on your favorite app or click here.

A variety of habitat conditions across one swath of grassland at The Nature Conservancy’s Platte River Prairies in Nebraska back in 2023. Click on the photo if you want to see a bigger version.

Speaking of prairie management, as I was preparing a presentation on the topic last week, I came across an aerial photo I’d taken a couple years ago at our Platte River Prairies. The image does a great job of illustrating what habitat heterogeneity can look like across one management unit.

Variation in available habitat conditions (habitat heterogeneity) can be created by many factors. Some of those factors are unrelated to our actions as land managers. Topography, soil texture, and soil moisture, for example, all have big influences on what plant species can grow in a particular place. The same conditions can also drive the height and density of plants – high/dry/south facing slopes will have much shorter/sparser vegetation than low-lying/moist valleys, for example.

In the photo shown above, that influence can be seen in at least a couple ways. First, in the foreground of the photo, there are yellow stripes created by concentrations of blooming perennial sunflowers and goldenrods. Those plants are spread throughout the site, but are most lush and abundantly flowering in soils laid down by old river channels hundreds/thousands of years ago when this part of the prairie was part of the active Platte River.

Similarly, in the top part of the photo, you can see a broad slough that looks slightly darker than the surrounding vegetation. That slough is also an old river channel and has not only different soils, but is also lower in elevation and closer to groundwater, which helps determine both the plant species growing there and the height and vigor of those plants.

The management history of this site also plays a big role in the available habitat today. The top portion of the photo is unplowed (remnant) prairie, though it had some years of chronic overgrazing that decreased its plant diversity before The Nature Conservancy acquired it. We’ve replenished some of that lost plant diversity by overseeding missing species, but the plant community is still not what it was. On the other hand, it does have a very strong native sedge community and a pretty nice set of early season wildflowers that aren’t in the portion of prairie shown at the bottom of the photo.

That area shown in the lower portion of the photo was in row crops for years before being planted (not by us) back to grassland with a few wildflower species. Since then, it has increased in plant diversity, but no botanist would mistake it for an unplowed remnant prairie. It provides excellent habitat for many animal species, but has obvious differences from the adjacent remnant site.

Finally, the most dramatic variation in vegetation structure at the time this photo was taken was driven by recent management actions taken by Platte River Prairies manager Cody Miller and others. The entire prairie shown in the photo had cattle on it during the year of the photo (and in prior years). However, the grazing pressure across the site was very uneven, driven by prescribed burning.

In 2023, a spring fire was conducted in the area shown in the foreground, and a late July burn took place near the top right of the photo. When the 2023 growing season started, cattle focused most of their grazing in the recent spring burn, keeping that vegetation short and allowing plants elsewhere to grow tall. After the smaller August burn took place, cattle shifted some of their grazing there, reducing grazing pressure on the spring burn – which is partly why the sunflowers are blooming so abundantly. If you’re interested, you can read more about that summer fire and see more photos of it here and here.

Meanwhile, most of the remainder of the site had lots of tall vegetation and relatively thick thatch (accumulated dry vegetation from previous years’ growth) because it hadn’t been burned recently and had recovered from previous grazing bouts.

This mid-September photo shows part of the remnant prairie, with unburned grassland on the left and the summer burn on the right. The two areas provide very different habitat conditions for plants and animals, each of which is valuable.

As a whole, this management unit (roughly 500 acres, with more prairie across the creek to the north – out of frame to the right) provided a wide variety of habitat conditions for the plants and animals living in it back in 2023. Last year, in 2024, we burned yet another patch, which shifted grazing pressure to a new portion of the site and allowed the 2023 burned areas to start growing tall again. New management treatments in 2025 will continue to shift things around, while maintaining the same kind of habitat variety – just in different places.

If you missed it, I talked much more about habitat heterogeneity and why it’s important in this recent post. Our primary objective for the Platte River Prairies is to sustain high ecological resilience, which relies heavily upon species diversity (animals, plants, and more). We’re working under the assumption that providing a constantly-shifting mosaic of habitat types is the best way to support that species diversity.

There’s a lot of science that backs up that shifting mosaic assumption, but we try to test it whenever we can. Plant diversity has been very stable over the last 20 years or so on the sites I’m able to monitor closely. Habitat use by wildlife, including insects, is harder to quantify, but what we’ve seen has been positive. Birds species appear in different places each year, but all the species we’d expect to see always show up. Similarly, regal fritillary butterflies and other insect species seem to be doing well, but follow their favorite habitat conditions around the site. The plains pocket mouse – a species of concern in this part of the world – maintains surprisingly consistent populations, regardless of our management actions.

We’re always looking for researchers who’d like to help us look more closely at any aspect of the system – let me know if you’re interested! In the meantime, we’ll keep experimenting and learning the best we can.

Habitat Heterogeneity Through Time and Space

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Habitat heterogeneity is a fun jargon term within the fields of ecology and land stewardship.  It’s a good one to know if you’re at a party trying to impress an ecologist.  Although, to be honest, if you’re trying to impress an ecologist, going to a party is probably not the best way to start.  You’d be better off meeting them “accidentally” on a hiking trail and bending down to closely examine a set of tracks or a fly feeding on a flower.  Ecologists are impressed when people notice things in nature.

Regardless of whether you’re trying to impress an ecologist, habitat heterogeneity is an important concept.  Most commonly, it is used to describe the variety of habitat conditions available in a particular location.  It’s generally good to have numerous habitat types represented across a site that each provide something a little different in terms of food or shelter for animals or growing conditions for plants.  That kind of variety supports the habitat needs of a broad diversity of species.

This prairie has at least four different habitat types available, including variation in the height and density of vegetation, as well as what species of plants are blooming.

It’s not hard to imagine why lots of different habitat conditions would lead to a high number of species thriving in a prairie.  However, if you really want to impress an ecologist, you’ll also consider the time elements, not just the spatial elements of habitat heterogeneity.  In other words, it’s not just important to vary the kinds of available habitat across space, it’s also important for each of those habitat patches to change through time.

Good farmers and gardeners know the value of rotating crops.  Rotation can help soil productivity because each crop uses soil resources differently.  Shifting the location of crops from year to year also helps reduce impacts from diseases and insect herbivores that can build up over time when their favorite food source is available year after year. 

Many of the same concepts apply very well to prairie and other natural land stewardship work.  When habitat conditions remain static in a given location, bad things can happen to the inhabitants of that patch.  Disease organisms (pathogens), parasites, and herbivores can build up high populations.  Predators can learn to be very effective at finding and catching prey.  Within plant communities, the plants best adapted to the current conditions will thrive at the expense of others.  If those conditions don’t change, the plants less adapted to those will struggle to survive.

Here are two specific examples to illustrate why variation through time matters so much:

Years ago, a wildlife refuge manager told me about a situation he’d encountered early in his career.  He was charged with managing wetlands for ducks.  Ducks like nesting in dense vegetation, so he was told to make sure all the wetlands in his site had thick cover around them to maximize the number of duck nests.  After a few years of this, though, he realized that nesting success was actually very low, even though he was providing the habitat ducks supposedly wanted. 

What he thinks happened is this: The consistent dense vegetation allowed the vole population around wetlands to grow.  That higher population attracted predators like bull snakes to feed on the voles.  Unfortunately, those predators also enjoyed eating duck eggs, which worked out poorly for those ducks. 

Bull snakes are just one of many predators that like feeding on both voles and duck eggs.

Regardless of whether the manager’s hypothesis was right or not, when he started varying the habitat structure from year to year, nesting success went up.  He managed the site so that each year, some of his wetlands were surrounded by dense cover and others weren’t.  Every nesting season, he’d change which wetlands had thick vegetation around them.  Even though there was less overall nesting cover each year, ducks did better because nests suffered from lower rates of predation.

The second example comes from my own experience.  Canada milkvetch (Astragalus canadensis) can be frustrating to harvest seeds from because the pods are often empty by the time we get to them.  Those empty pods are easily identified because of the exit holes left by larvae of a small weevil (Tychius liljebladi) that feed on the seeds and then burrow out of the pod when they’re done.  After leaving, the larvae dig into the soil beneath the plant, where they overwinter.  The next summer, they emerge as adults and will lay eggs on the flowers in mid-summer to continue their life cycle. 

Holes left by weevil larvae usually indicate there’s no point picking seed from a particular patch of Canada milkvetch plants.

I’ve seen populations of Canda milkvetch go many years in a row without ever producing seeds because of the recurrence of those weevils.  That’s not good for the longevity of a population.  However, we’ve found that if we can break that annual pattern, we can get a year of strong seed production, which is good for both us and the plants.  The easiest way to break the cycle is to prevent the plants from flowering for a year.  If the weevils emerge from the soil and don’t see flowers to lay eggs on, they’ll fly off in search of flowers elsewhere.

The following year, when Canada milkvetch blooms again, a few weevils might travel from other places to breed on the flowers, but there aren’t high numbers of them popping out of the soil right beneath the plants.  Usually, that results in low rates of seed predation.  Even delaying flowering by a few weeks can help.  In years where milkvetch plants are mowed or grazed in a way that makes them bloom later in the season, they tend to produce lots of seed because the weevils were done breeding by the time the flowers appeared.

Following the crop rotation model, the best way to vary habitat conditions through time is to create a kind of ‘shifting mosaic’ of habitat patches across a site.  The ‘mosaic’ part of that phrase represents the variety of available habitats.  Those can include patches with tall, thatchy vegetation as well as others with short plants and exposed bare ground – and still other patches where that tall and short structure is intermixed.  It’s also helpful to have patches differ in the plant species that are blooming or otherwise thriving.  That might include some patches where annual plants are temporarily flourishing instead of the usually dominant perennials.  Having all those patches arrayed near each other allows animals to move between them to look for food, escape predators, seek out shade or sunlight, and anything else they need.

The ‘shifting’ aspect of the mosaic, though, is also crucial.  Each habitat type should appear in a different place each year, or on some similar time scale.  That can be created by simply mowing or burning different patches within a site at varying times of the year.  You might end up with a patch that was burned in the early spring, another that was mowed in mid-summer, and another that hasn’t been mowed or burned for a while.  That idled patch could then be burned the following year and the previous year’s burned patch could be mowed sometime during the growing season.  Or whatever. Basically, you want to shake things up each year so you’re applying management in different places over time and always providing a good mix of habitat options.

Grazing can be used the same way, except that there’s a lot more flexibility in how it’s applied.  Patch-burn grazing is a terrific way to combine fire and grazing to create heterogeneity across time and space.  I’ve written about the ‘open gate rotational grazing’ approach we’ve been experimenting with.  That’s another potential option.  In both those examples, each part of a prairie is grazed for a full season or more and then allowed to rest for a couple years.  The grazing and rest cycles are staggered across the site so there are always patches in each stage of that cycle available for the plants and animals that thrive best in those conditions.

Patch-burn grazing is just one of many ways to create a shifting mosaic of habitats. Here, cattle are grazing a recently burned patch while mostly ignoring an adjacent unburned patch. Not shown in the photo are other nearby areas with more patchy habitat structure. Each of those provides different cover, foraging opportunities, and growing conditions for animals and/or plants.

Those grazing approaches, though, are only a couple examples among an infinite array of choices.  You can vary the stocking rate, timing, and duration of grazing to achieve lots of different results.  You can combine grazing with fire and/or mowing to create even more variation.  There’s lots of room for creativity, but more importantly, to observe, learn, and adapt over time.

Regardless of the stewardship options available to you, the key is to look for ways to provide a good variety of habitat options for animals and growing conditions for plants.  More to the point of this post, it’s important to make sure those habitat options shift around in location to help prevent anyone from winning too consistently at the cost of others. 

Or, if you’re still trying to impress that ecologist, you might say that you’re trying to foster both spatial and temporal heterogeneity.  If that doesn’t work, you might try talking about how fascinating parasitoid-host relationships are.  That always gets me going.