Tag Archives: diversity

Prairie Word of the Day – Habitat Heterogeneity

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Do you know what time it is?  It’s time for another PRAIRIE WORD OF THE DAY!

Today’s Prairie Word of the Day (fine, it’s actually two words) is:

Habitat Heterogeneity

Heterogeneity is really just a longer word for Diversity, which is another way of saying “lots of different things”.  So why use the word “heterogeneity” instead of just saying “lots of different things”?  Well, for one thing, using big words makes a person sound smart, and when you’re a prairie ecologist and no one really understands what it is you do for a living, it’s good to at least sound smart.

More importantly, there’s a nice alliterative (another big word!) feel to the phrase Habitat Heterogeneity, which happens to be one of the most important phrases in prairie ecology.  In fact, I would argue that the foremost objective of any prairie manager should be to create habitat heterogeneity within the prairie(s) they manage.

Habitat heterogeneity simply means diversity or variety in habitat types.  Habitat homogeneity is the opposite – a lot of habitat that is all the same.

Every organism in a prairie has its own unique habitat requirements, so the number of different habitat types in a particular prairie is correlated with number of species that can live there.   Let’s use prairie birds as an example.  Birds such as upland sandpipers like to nest in large patches of relatively short-stature grassland.  Around here, a big hay meadow is great habitat for them, especially if it was cut fairly late in the previous year and is still short in stature when the subsequent breeding season starts.  On the other hand, Henslow’s sparrows want to nest in prairie habitats with relatively tall and dense vegetation.  It would be highly unusual to find Henslow’s sparrows and upland sandpipers nesting in the same patch of prairie because their habitat preferences are very different.  So, if you want both species in your prairie, you have to provide both short and tall/dense habitat.  Other birds have their own unique habitat requirements, including nearly bare ground (e.g., horned lark), relatively short, but with abundant thatch (grasshopper sparrow), tall with lots of tall/weedy wildflowers (dickcissels), tall and nearly impenetrably dense vegetation (sedge wrens), and many others.  Only if your prairie provides all those different habitat conditions will you attract all those different bird species.

Dickcissels prefer

Dickcissels prefer habitat with lots of tall wildflowers or weeds.  They often weave their nest into the stems of a tall clump of vegetation.

The same diversity of habitat preferences exists in other groups of prairie species as well.  Among small mammals, for example, voles tend to prefer habitats with abundant thatch, while pocket mice are more often found where bare ground is abundant – and there are many others.  Insects and other invertebrates have the same kind of diversity in habitat preferences

Scale is important.  While a bird, mammal, or insect might have a broad preference for a certain kind of habitat structure, it is likely to need some heterogeneity within that habitat too.  A mouse, for example, might prefer patches of prairie with fairly sparse vegetation, but it is likely to need a few clumps of vegetation dense enough to hide in when predators are near.  Insects and reptiles are ectothermic (cold blooded) and need to regulate temperature, so while a snake might like to hide in tall dense so it can bite your ankle as you walk by (I’M KIDDING!), it also needs some places to bask in the sun.  All of this means that habitat heterogeneity is important any many different scales.  Heterogeneity at a fairly large scale (acres) helps provide places for many different animals to live in a prairie, but heterogeneity within the home range or territory of each animal (square meters, or even centimeters) can be important too.

Some habitat heterogeneity occurs simply because soil texture, nutrients, and moisture, along with topography all vary across a landscape.  A prairie is likely to have areas of more productive soils where vegetation grows tall and thick, and less productive soils where vegetation is more sparse, for example.  In addition, water will pool in some areas of a prairie more than others because of topography, altering the habitat for both plants and animals.  However, despite this “naturally occurring” heterogeneity, it’s still important for prairie managers to look for ways to provide more.

This landscape at The Nature Conservancy's Broken Kettle Grasslands in northwest Iowa shows the kind of natural heterogeneity that occurs in many landscapes.

This landscape at The Nature Conservancy’s Broken Kettle Grasslands in northwest Iowa shows the kind of natural heterogeneity that occurs in many landscapes.  Topography, soil variability, and other factors create a diverse set of conditions for plant growth and habitat structure.  Land management can add to that heterogeneity and improve it even more.

Prescribed fire and haying/mowing do a great job of altering habitat structure at a fairly large scale (acres).  By applying those treatments in different parts of a large prairie each year (and varying the timing of each from year to year), a manager can create a shifting mosaic of habitat patches that supports a wide diversity of animals.  However, both fire and mowing are pretty non-selective – most or all of the vegetation within a burned or mowed area gets the same treatment.  Leaving unmowed patches of grass here and there and varying the height of the mower as it moves across the site can help leave more heterogeneity behind.  Designing prescribed fires so that not all vegetation burns (e.g., mowing around some patches ahead of time, burning on days with lower temperatures or higher humidity, etc.) can also help with habitat heterogeneity – though those kinds of fire might also be less effective at killing trees or accomplishing other objectives.

In prairies where livestock grazing (cattle or bison, for example) is feasible, it is much easier to create small scale heterogeneity because grazers pick and choose which plants, and how much of each plant, to eat at any one time.  By controlling grazing intensity, and varying it across both time and space, managers can create prairie patches that are ungrazed, almost completely grazed, and in various stages of partial grazing – with a mixture of tall vegetation and nearly bare ground.  The uneven application of “fertilizer” from the rear ends of grazers contributes even more to habitat heterogeneity by temporarily altering soil productivity in lots of little spots across the prairie.

These cattle at Konza Prairie in Kansas

These cattle at Konza Prairie in Kansas have created a nice example of small scale habitat heterogeneity by grazing many of the grasses short while leaving leadplant, purple prairie clover, and other wildflowers ungrazed.

Whether it’s fire, mowing, grazing, herbicides or various combinations of those, creating habitat heterogeneity may the most important job of a prairie manager.  We still have a lot to learn about how the scale and configuration of habitat patches affect wildlife and insect populations.  What we do know, however, is that the prairies thrive when they have a lot of different types of habitat.  …When they have habitat heterogeneity.

And that, folks, is your Prairie Word of the Day.

Contrasting Approaches to Prairie Management: Leopold, Land Health and Cabbages.

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“A Land Ethic” is the concluding essay in Aldo Leopold’s 1949 book, A Sand County Almanac, and is the most powerful and relevant piece of conservation writing I’ve ever read.   Leopold’s essay spells out the changes we need to make in the way we view our relationship with the land, and it is both impressive and frustrating that nearly everything in that essay still reads true today.  If anyone reading this blog post has never read A Sand County Almanac, please stop reading this, go pick up a copy of the book, and read it.

I’ll wait…

Instilling a land ethic in my kids is one of my highest priorities as a parent.

Instilling a land ethic in my kids is one of my highest priorities as a parent.

One of my favorite parts of “A Land Ethic” is the section titled “Land Health and the A-B Cleavage”.  I’m always blown away by how neatly Leopold synthesizes the field of conservation into the first three sentences of that section:

A land ethic, then, reflects the existence of an ecological conscience, and this in turn reflects a conviction of individual responsibility for the health of the land. Health is the capacity of the land for self-renewal. Conservation is our effort to understand and preserve this capacity.

The very next paragraph introduces an issue I’ve been wrestling with for a long time.

Conservationists are notorious for their dissensions. Superficially these seem to add up to mere confusion, but a more careful scrutiny reveals a single plane of cleavage common to many specialized fields. In each field one group (A) regards the land as soil, and its function as commodity-production; another group (B) regards the land as a biota, and its function as something broader.

Leopold goes on to describe how some in his field of forestry are “quite content to grow trees like cabbages” while others have a less agronomic and more ecological approach.  He also points out that the fields of wildlife management and agriculture each have their own parallel examples of this dichotomy.  These contrasting views of the world still exist in various forms, from the way farmers think about their fields to the way people in cities value natural resources.

I see Leopold’s A-B cleavage very clearly in prairie management.  One example can be found in the way prairies are used for livestock production.  Many livestock producers see prairies primarily as grass farms, and the worth of a plant is determined by its palatability and nutritional value to livestock.  Broadcast herbicide application is commonly used in an attempt to remove “weeds” and leave only the grasses cattle or other livestock most like to eat.  Optimization of grass production also manifests itself in grazing strategies; particularly systems that move cattle through multiple paddocks with the primary goal of increasing the production and dominance of grass.  Grass farmers are exasperated by the perennial reoccurrence of “weeds”, despite their best efforts to prevent them. They don’t see that those weeds fill ecological roles grasses can’t, including roles that improve the overall health and productivity of the land.

A grass farmer's nightmare, these ragweed plants and other "weeds" play important ecological roles and fill in open spaces created by intensive grazing or drought.

A grass farmer’s nightmare, these ragweed plants and other “weeds” play important ecological roles, including filling open spaces created by intensive grazing or drought.  Removing them with herbicide application only results in more “weeds”, which fill the same spaces again.  Perennial grasses easily outcompete these opportunistic plants under light or no grazing pressure.  In fact, a year after this photo was taken, this site was dominated by big bluestem.

Other ranchers, however, see prairies as much more than grass farms.  Instead, they recognize that their livelihood relies on an extraordinarily diverse community of organisms above and below ground.  To these ranchers, the only weeds are those that reduce the diversity of the prairie community and make the land less healthy by Leopold’s definition of health: the capacity of the land for self-renewal.  Livestock producers on this side of the cleavage understand that they and their livestock depend upon soil, plants, pollinators, predators, pathogens, and the interactions between them.  A plant’s importance is not judged only by whether or not a cow will eat it, but also by the roles it fills in the larger community.  Those contributions include the communities of microbes living in the plant’s root system, pollen and nectar for pollinators, and food for herbivores, which in turn support predators that regulate populations of their prey.  Ranchers with an ecological perspective gain as much appreciation from watching bees buzzing from plant to plant as they do from watching the weight gains of their cattle, even though the latter is what allows them to pay their taxes and keep their land.

Just as there is a broad range of perspectives among livestock producers, there are wide contrasts among those who manage prairies for recreation and conservation purposes as well.  Some prairie managers have a single species or narrow set of species as their primary management target.  Those targets are include game species such as ring-necked pheasants, white-tailed deer or ducks, but may also be butterflies or a particular class of plants or grassland birds.  Narrowly-focused managers tend to manage their prairies in much the same way each year so as to provide a consistent set of conditions that best suit their preferred species.  Over time, prairie species not favored by that kind of repetitive management regime diminish in number or disappear, decreasing the overall diversity of the prairie community.  Since ecological resilience (today’s term for Leopold’s “land health”) relies heavily on species diversity, the loss of species from the community weakens its ability to support others, including the managers’ favorites.  Ironically, as the community weakens, managers tend to become even more agronomic in their approach by adding food plots, controlling predators, or increasing herbicide use to suppress plants taking advantage of a less diverse community.

Prairie managers with a broader ecological perspective start with the premise that a diverse prairie community is of utmost importance.  They manage in ways that prevent any particular species or group from becoming too dominant at the expense of others.  These managers don’t judge success by the abundance of one species or another, they focus on processes such as pollination, predator/prey interactions, and other indicators of community diversity and ecological resilience.  Variation in the species composition of a prairie between years is a cause for celebration rather than trepidation.  Year-to-year variation means that many different species (especially insects and animals) experience success over time, rather than just the few who thrive under more stagnant conditions.  Managing for diversity is not a simple process, and requires a careful eye and flexible approach, but the result usually produces strong and stable populations of all species – including those desired by more narrowly-focused managers.

Prairies managed for overall diversity have strong ecological processes that support all species - including game animals, songbirds, insects, plants and livestock. Prairies managed for overall diversity have strong ecological processes that support all species - including game animals, songbirds, insects, plants and livestock.

Prairies managed for overall diversity have strong ecological processes that support all species – including game animals, songbirds, insects, plants and livestock.

Whether we are managing for livestock, game species, butterflies, or flowers, the approach we take is critically important.  There is a strong temptation to continually maximize what we think are the enabling conditions for the species we’re most interested in, but that approach ignores the broader complex system those species depend upon.  The grass that cattle eat relies on productive soil, and that soil’s productivity is supported by a diversity of plants, microbes, pollinators and other prairie community members.  Similarly, the fate of butterflies, pheasants, and deer all depend upon the roles played by their fellow citizens of the prairie.

Trying to deal with all of that complexity may seem daunting, but take another look at Leopold’s definition of land health and conservation: Health is the capacity of the land for self-renewal. Conservation is our effort to understand and preserve this capacity.  Note that Leopold doesn’t say conservation entails fully grasping and perfectly managing the complexities of the land.  Instead, he defines conservation as the effort to do so.   I like that.  We don’t have to know everything to be successful.  Instead, conservation is an adaptive process of learning, incorporating new knowledge into our actions, and then trying again.  That means that conservation is accessible to anyone who appreciates the broad complexity of natural systems and attempts to work within them, rather than against them.

…And isn’t that more fun than just growing cabbages?

Disclaimer:  The author of this post intends no disrespect to those who grow cabbages.  Instead he was simply referring to a quote from earlier in the post to reinforce a point and attempt to end a long and heavy essay with something a little lighter.  Cabbage growing is a perfectly legitimate and important activity, especially when done with appropriate consideration for the broader ecological context of soil and other ecosystem processes required to sustain long-term production…  …Ok, I’ll just stop now.

Previous related posts you might be interested in:

Calendar Prairies

Are Botanists Ruining Prairies?