habitat | The Prairie Ecologist

Last week, I attended a conference aimed at creating a statewide conservation plan for monarch butterflies. The meeting was really informative and thought-provoking. I learned a great deal about the ecology and conservation needs of monarchs from Chip Taylor of Monarch Watch and others, and was part of some good discussions about potential strategies to help the species recover. I thought I’d share some of what I learned from those discussions because they helped me better understand the issues surrounding monarch conservation. Any errors in the following are a result of my misunderstanding what smart and knowledgeable people told me, and I apologize in advance.

Monarch butterflies on a summer morning. The Nature Conservancy’s Platte River Prairies, Nebraska

Monarch Migration

The monarch butterfly is a migratory species, but it takes multiple generations to make the migration from parts of North America to Mexico and back. Here in Nebraska, we’re part of the Eastern Population of monarchs, which extends from roughly the east edge of the Rocky Mountains to the east coast. The butterflies in this population leave their Mexico wintering grounds in late February each year and head north. They lay eggs in the southern United States and the monarchs produced by those eggs then head north into the northern half of the U.S. and the southern edge of Canada during May and early June.

During the summer, there are a couple generations of monarchs that mature and lay eggs without migrating. However, in mid-August and September, monarch adults get the urge to migrate and start heading south. Those that survive the trip usually reach the Sierra Madre Mountains of Mexico by early November.

This migratory monarch is fueling up on a native thistle. Thistles are among the top food sources for monarchs and other butterflies. Allowing non-invasive thistles to persist in natural areas is an important part of pollinator conservation efforts.

Threats to Monarchs

There are two categories of threats to Monarchs: 1) Factors we can control, and 2) Factors we can’t. The big factor we can’t control is weather and the way it interacts with migration timing and butterfly survival. Weather can have a tremendous impact on butterflies, and many millions of butterflies are killed by hot weather, storms, or other events. However, since we can’t control the weather, we have to focus on what we can control.

There is a long list of human-induced factors that affect monarch populations. Those include the conversion of grasslands, roadsides, and field edges to row crops (largely facilitated by government policies) as well as farming practices that have nearly eliminated milkweed from farm fields. Pesticide use is another factor, including pesticides used for farming and pesticides used for other purposes, including mosquito control. Logging of forests in the wintering grounds of Mexico is another important issue.

I’d heard that the loss of milkweed from crop fields was a big deal for monarch butterflies, but hadn’t really understood why. At the conference, we heard that research has shown that about four times as many eggs/plant are laid on milkweed plants in crop fields as on milkweed plants in other habitats. (I’m not sure anyone understands why.) In addition, while the eastern population is spread across a huge area of North America, about 50% of the butterflies that reach Mexico are born in the cornbelt of the U.S. – the intensively farmed Midwestern states. Prior to the widespread use of glyphosate-resistant crops, milkweed was a pretty common inhabitant in crop fields throughout this prime breeding area for monarchs. Now that farmers are so much more efficient at weed control, we’ve lost the most productive egg-laying habitat in the country’s most important breeding area for monarchs.

Conservation Strategies

Because monarchs can only be raised on milkweed, getting more milkweed plants in the landscape, especially within the cornbelt states, is a key part of increasing the monarch population. It’s likely that more than a billion additional milkweed plants will be required to stabilize the monarch population. Increasing milkweed populations to that extent will require a wide range of strategies. In addition, protecting and restoring the wildflower-rich grasslands and other natural areas that provide food for adult monarchs, as well as for thousands of bee and other pollinator species, is also vitally important.

It is critically important to increase the number of swamp milkweed (Asclepias incarnata) and other milkweed species available for monarch egg-laying.

One clear strategy is to plant more of monarchs’ favorite milkweed species in gardens, parks, roadsides, nature centers, and many other sites. In the north-central U.S., milkweed species such as common (Asclepias syriaca), showy (A. speciosa), and swamp milkweed (A. incarnata), are known to be favorites, while green antelope horn (A. asperula) is important in more southern states. You can find sources of seeds and plants at Monarch Watch or from the Xerces Society’s Project Milkweed website. Sites like monarchgard.com can help with garden and landscape design ideas.

More milkweed in gardens and landscaping can make a big difference, but an even bigger part of monarch recovery needs to come from a change in the way milkweeds – and the weedy, edge habitats they thrive in – are perceived by the public. Elimination of milkweed from roadsides, field edges, and odd corners and margins of our landscapes happens because we are uncomfortable with the “messiness” of those areas if they aren’t frequently mowed and/or sprayed with herbicides to make them look uniform in height and composition. Allowing milkweed and other wildflowers to thrive in those odds-and-ends habitat areas can have a huge impact on monarchs and other pollinators, along with pheasants, song birds, and many other wildlife species. Reducing mowing frequency and spot-spraying truly invasive plants – instead of broadcast spraying to kill anything that’s not grass – in these habitats saves both money and time as well.

“Sanitized” monoculture roadsides like this offer no benefit to monarch butterflies or other pollinators. We need to change our cultural aesthetic and acknowledge the value and beauty of roadsides that actually offer habitat values to monarch butterflies, pollinators, pheasants, and other wildlife and invertebrate species.

The Role of Prairies?

Last week’s meeting also encouraged me to focus even harder on an additional aspect conservation I’ve already been working on – improving the contributions of native prairies and rangeland to pollinators. The experts at our meeting, along with other sources of information on monarchs, seem to be focused largely on milkweed and the kinds of farm fields, edge habitats and landscaping mentioned above. While all those are very important, I can’t help but think about the value of native prairies – especially in places like Nebraska where we still have millions of acres of grassland.

Many of our restored Platte River Prairies maintain milkweed populations for many years after establishment, even under fire and grazing management. Figuring out what management strategies facilitate survival of milkweed may be a very important part of successful monarch conservation.

A healthy prairie with a diverse wildflower community is invaluable to bees and other pollinators, and also provides nectar resources needed by monarch butterfly adults. If that prairie contains vibrant populations of milkweed species that provide egg-laying habitat to monarchs, that’s even better. Many prairies don’t currently have strong milkweed populations. Some milkweed species are not strong competitors in a tight-knit plant community, and certain grazing and other management practices tend to further discourage milkweeds. Over the next several years, I am hoping to learn more about how to make prairies support stronger milkweed/monarch populations. Hopefully, we and others can help make North American prairies even better contributors to the survival of monarch butterflies.

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 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. 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 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.

The Prairie Ecologist

Essays, photos, and discussion about prairie ecology, restoration, and management

Tag Archives: habitat

Monarch Conservation Strategies

Prairie Word of the Day – Habitat Heterogeneity