Tag Archives: conservation

Why Grassland Birds are Poor Indicators of Prairie Quality

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I presented this argument to a Nebraska symposium on grassland birds in 2008 and managed to escape relatively unscathed.  Now I’m testing my luck with a wider audience.  At least no one can throw things at me through the computer…

Let me start by saying that I’m a big fan of birds.  I really enjoyed working on my graduate research, which focused on grassland birds and their vulnerability to prairie fragmentation.  I also think birds are generally pretty and interesting. However, the truth is that prairie birds make up only a tiny percentage of the species in prairies (most of which are invertebrates, followed by plants).

Grassland birds make up a tiny percentage of the species living in a prairie - the vast majority of which are invertebrates and plants.

However, grassland birds are often held up as indicators of whether or not a prairie – or a prairie landscape – is “healthy” or “high quality.”  A common refrain in prairie conservation goes something like this; “If we have our full complement of grassland birds in this prairie and/or landscape, it’s a good bet that all the other species are also doing well.”  Unfortunately, while prairie birds are relatively easy to study and monitor, they may not do a good job of reflecting how the rest of the prairie is doing.  Let’s look at some of the most important attributes of prairies and some of their major threats – and consider how well birds correlate with them.

Species Needs – Survival, Reproduction, and Dispersal.

First and foremost, species have to survive and reproduce in order to persist in a prairie.  This applies to every species, from large vertebrates to tiny invertebrates and the entire suite of plants.  It’s important for us to know that grassland birds are surviving and reproducing, but can they tell us whether other species are doing the same?  You could argue that because they eat insects, grassland birds could have an impact on the survival of some insect species.  That’s true to a point, but grassland birds are generalist feeders – they tend to eat whatever insects are easiest to catch at any particular time – so while the abundance of grassland birds might impact the overall abundance of insects, you can’t really tie the presence of a particular grassland bird species to the survival of a particular insect species (or vice versa).  In other words, the plight of a rare leaf hopper or butterfly species is unlikely to be correlated with grassland birds.  Nor are grassland birds good predictors of plant species survival – the presence of meadowlarks or Henslow’s sparrows tell us nothing about whether or not compass plant or leadplant is thriving.  Grassland birds require certain habitat structure types (short vegetation, tall/dense vegetation, etc.) but they don’t much care whether that vegetation consists of smooth brome and sweet clover or a large diversity of native plants.

Henslow's sparrows are a bird of conservation concern and their presence in a prairie can be seen as a conservation success. However, although they tend to require fairly large prairies, and can indicate the presence of certain vegetation structure types, they don't indicate whether or not a prairie has a diverse plant or insect community.

In addition to basic survival, animal and plant species need to be able to move around the landscape in order to recolonize places where they have disappeared, and to maintain genetic interaction between populations (important for genetic diversity).  In landscapes where prairies exist as isolated remnants, moving between prairies becomes very difficult.  Corridors of prairie vegetation between prairies become important in those landscapes, and prairies near each other provide better opportunities for interaction within species than do more isolated prairies.  Because most grassland birds fly south at the end of each season and return the next year, (and the ones that don’t can still fly long distances between prairies) they don’t rely on those physical connections between prairies like most other animals and plants do.  Since grassland birds are pretty unique in terms of long-distance flying ability, they are a poor indicator of conditions that affect less mobile species.

Ecological Services and Ecological Function – Pollination, Seed Dispersal, etc.

Apart from the needs of individual species, prairies rely on certain processes to keep everything humming along.  Pollination and seed dispersal are two good examples.  Both affect the viability of prairie plant species, and neither has much to do with grassland birds.  Pollination primarily relies on plant diversity and bees – the most important pollinator group in prairies – and both plant diversity and bees are pretty disconnected from grassland birds.  We don’t know much about the role of prairie birds as seed dispersers, but it’s a good bet that they do very little seed dispersal during the summer when they’re primarily eating insects.  The role of migrating grassland birds as seed dispersers would be an interesting thing to study – but our use of grassland birds as indicators of prairie quality is always based on their presence during the breeding season.  If you were going to measure whether or not pollination and seed dispersal were functioning adequately, you’d likely evaluate the diversity of plants, the abundance of bees, and some of the potential obstacles to seed dispersal (tree lines, isolation of prairies, etc.), but I don’t think measuring grassland birds would tell you much.

Resilience – Redundancy and the Ability to Withstand Stresses and Invasive Species.

One way to think about the resilience of a prairie is as a measure of how well the prairie can bounce back from stresses.  For example, species diversity adds resilience to a prairie because when many species are present – especially when they overlap in the roles they play – the loss of an individual species can be a relatively minor blow.  If there are dozens of bee species pollinating flowers in a prairie, a disease that wipes out one or two species will probably not have a huge impact on seed production.  A diversity of plant species can also help to dampen the impacts of an event such as a severe drought or intensive grazing that temporarily weakens the vigor and growth of dominant plant species.  When there are lots of plant species present, the weakening of some leads to increased growth and abundance of others.  This helps maintain a stable supply of food for herbivores, and also helps prevent encroachment by invasive species that might otherwise take advantage of the weakened plant community.

Grassland birds may help bolster the resilience of a prairie in some ways.  They might, for example, help suppress an outbreak of grasshoppers by focusing their feeding on that easy-to-find prey species, and thus limit its abundance.  However, as discussed earlier, they don’t have much to do with pollination, nor would they help provide food for herbivores during a drought.

If you want pollination you need bees, not birds.

Threats to Prairies – Habitat Fragmentation, Invasive Species, Broadcast Herbicide Use, and Chronic Overgrazing.

One of the best arguments for grassland birds as an indicator of prairie health is that they are vulnerable to the loss and fragmentation of grassland habitat.  This is true.  A diverse and successfully reproducing community of grassland birds requires relatively large and unfragmented grassland.  In addition, because some grassland breeding birds need short vegetation and others need tall/dense vegetation, a diversity of birds can indicate a diversity of available habitat structure  – and that’s important to many other wildlife species as well.  Greater Prairie Chickens are often promoted as particularly good indicators because they are a single species that needs both large grasslands and a diversity of habitat structure.

However, there are a couple of other things to consider.  First, while some grassland bird species need large prairies, we don’t really know whether the minimum area required by grassland bird species is larger or smaller than that required by plant or insect species.  We know that many prairie plant species have survived for a very long time in tiny isolated prairies.  But because individual plants of many species can live almost indefinitely, due to their ability to generate new plants through rhizomes and other asexual means, the populations of plants in those tiny prairies could be in a death spiral due to the lack of genetic interaction with other populations.  You could make the argument that plants need much larger prairies than birds (thousands of acres, perhaps?) in order to maintain genetic fitness.  We simply don’t know.  And we know even less about the prairie size needs of insects.

Second, while grassland birds do require fairly large grasslands, most don’t actually require PRAIRIES.  Prairie chickens and many other grassland bird species have benefitted greatly from Conservation Reserve Program (CRP) fields that have added large numbers of acres of switchgrass, brome, and low-diversity grasslands to agricultural landscapes.  However, those same CRP fields have done very little for native wildflower populations or pollinator insects (or other insects that rely on diverse communities of native plants) so the increase in grassland birds in landscapes with CRP doesn’t really tell us much about the health of most other prairie species.

This smooth brome-dominated grassland is of very little to most prairie species, but would provide relatively good habitat for some grassland bird species (except for the trees in the background).

Because grassland birds can live comfortably in grasslands made up of a few native grasses, or even non-native grasses, they are poor indicators of the impacts of most invasive species – a major threat to prairies.  Similarly, broadcast herbicide use that greatly reduces the number of plant species in a prairie has little impact on the grassland birds nesting there.  Finally, a prairie that is being overgrazed would certainly have different bird species than one that is not being grazed at all, but a prairie that was chronically overgrazed for decades – and then managed well again – would have a pretty low number of prairie wildflower species but a very nice-looking grassland bird community.

One threat that grassland birds can be an excellent indicator for is tree encroachment on prairies.  Most grassland birds avoid nesting anywhere near even a solitary tree, let alone a grove of them, so a prairie with few birds could indicate a tree problem.  On the other hand, it’s pretty hard to miss a bunch of trees growing in a prairie…

Summary

Here’s the real point.  Grassland birds are an important component of prairies.  A prairie without all of its appropriate prairie bird species, or in which those species are not successfully raising broods, is missing something valuable.  Improving grassland bird success in prairie landscapes is an important and worthwhile objective.  At the same time, however, a prairie that has a full complement of successful grassland bird species doesn’t necessarily have diverse plant and insect communities, functioning ecological processes, or a low risk of invasive species or other threats.  In other words, grassland birds are an important component of high quality prairies, but their presence and/or success doesn’t necessarily mean a prairie is high quality.

Now that I’ve spent 1,500 words bashing prairie birds (I really do like birds…) the relevant question is, “What SHOULD we use as indicators of prairie conservation success?”  I wish I had a simple answer.  Part of the answer, of course, depends on how you visualize prairie quality (see my earlier post on this subject) because evaluation needs to reflect objectives.  But if our vision of a high-quality prairie includes species diversity, habitat heterogeneity, and other complexities, our evaluation methods will have to be complex as well.  Figuring out how to “take the pulse” of prairies may be the most important conservation challenge we face, because without that information we can’t design effective conservation strategies.

While we still have a lot to learn about how to take that pulse, it’s clear that we’ll have to do more than just count birds…

How Should We Be Preparing Prairies for Climate Change?

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When I wrote my recent book on prairie management, I included a very short section at the end on climate change.  Essentially, my advice to prairie managers was that managing prairies for biological diversity would help them be resilient enough to absorb climate change impacts.  I still think that’s good advice, but it leaves out some other options.

One of the purposes of this blog is to allow me to expand upon the ideas from my book, so I’m taking this opportunity to do that with the issue of climate change.  In this case, I asked for help from John Shuey, Director of Conservation Science for the Indiana Chapter of The Nature Conservancy.  John is a good friend and someone I have tremendous respect for, and I can always count on him to cut through the fog and address issues directly.

What I like most about his ideas is that they are all things we can actually DO RIGHT NOW.  One of the frustrating things about climate change is that it’s hard to design strategies when we don’t really know what climate conditions will be like in the future.  But John suggests strategies that would be good ideas regardless of what the climate does.  It feels good to have a map to follow, and this one points in the right direction – even if we don’t know exactly where we’re going.

Prairie Ecologist:

“What do you see as the major threats to prairie and savannah conservation from climate change in your state?”

Shuey:

“Well, most models predict three key changes in Indiana’s climate change future.  It will likely be hotter, with a slight increase in precipitation, and there will be more frequent severe weather events such as tornados, straight-line winds and ice storms.   The increase in precipitation will be during the dormant season with predicted decreases during the summer.  All that basically translates into three perceived threats: increased drought stress, increased fire frequency and intensity, and increased severe weather damage such as flash flooding and blow downs.”

Cardinal flower and other moist-soil-dependent plants may be particularly sensitive to increases in drought stress.

 

Prairie Ecologist:

 

“The central United States went through the Xerothermic Period between 8,000-5,000 BP, which was considerably warmer and drier than our present climate.  Does the fact that our prairies have already survived that period give us hope for the next phase of climate change?”

Shuey:

“Based on paleobotanical data, expanses of grassland and savanna dominated much of Indiana during the Xerothermic Period, and wooded communities increased in abundance as climates became cooler and moister.  Species compositions in today’s prairies will undoubtedly shift in response to climate change, but appropriate native species should be present at many sites to moderate those changes.  Some species will increase in abundance and others will decrease – even disappear from sites altogether.  The key for biodiversity conservation is to design strategies that will allow those changes to happen while minimizing species loss and preserving ecological functions.  For example, we can help to ensure that the full range of habitat conditions will persist in our conservation areas by designing restoration projects now that meet the future needs of species most at risk from climate change.”

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Prairie Ecologist:

“Fortunately, prairies are pretty resilient communities, but we’ve put a lot of stress on them already – especially in landscapes where prairies are small and isolated from each other.   How does that habitat fragmentation affect our conservation options?”

Shuey:

“The concept that ‘species will have to adjust northward’ as climate changes is very problematic in a landscape that is among the most developed in the world.   Even if it is theoretically possible for species to respond to warming trends by moving northward, fragmented landscapes like those in Indiana will not permit much movement between conservation sites.

Because of that fragmentation, we need to do as much as we can to make habitat patches as internally resilient as possible.  This can be done by maximizing the both the size and physical variation (e.g. slope, aspect, and soil moisture) of our natural areas.  We can increase the size of, and even reconnect, fragmented habitats through restoration of adjacent areas where that’s feasible.  Larger habitats can hold larger populations of species, which gives them a better chance of survival.  In addition, larger sites usually provide more variation in topography and hydrologic gradients, which can increase the chance that species will find the conditions they need to survive somewhere in the conservation area.  For example, shady microhabitats on north facing slopes may partially mitigate the impacts of regional increases of the evapotranspiration rates (a.k.a. drought stress).  Some of the rare species found on these slopes today may not make it through the changes, but those microclimates will likely still be loaded with locally rare species in the future.  Sadly, some of those ‘rare’ species may be abundant today but restricted to narrow ecological creases decades from now.

It’s also important for natural areas and to contain multiple examples of each habitat type, especially those most at risk from climate change – e.g. things like wet prairies and other moist habitats.  This accomplishes two things; it maximizes habitats that are likely to mitigate drought impacts, and it creates a repeating mosaic of ecological gradients that is more likely to support metapopulations (multiple populations interacting with and supporting each other) of species pushed to the brink.  As we design conservation areas and engage with private landowners in priority landscapes, we need to preserve as many examples of each habitat type as we can within regional landscapes. ”

Prairies that include a range of habitat types (wet to dry, shady to sunny, etc.) provide species more opportunities to find appropriate habitat as climatic conditions change. (Griffith Prairie - Prairie Plains Resource Institute)

Prairie Ecologist:

“Talk more about wet prairies and other natural areas that rely on the proximity to groundwater or other hydrological features for their survival.  Increasing drought stress sounds like a big deal for those sites…?”

Shuey:

“First, it’s important to know that many of our wetland systems in Indiana, such as bogs and fens, functioned though the Xerothermic Period in essentially the same manner as they do today.  These sites are literally the source of the pollen records used to re-create paleoclimates such as the Xerothermic.  It seems likely that their water budgets were reduced, and wetlands were probably smaller relative to their presettlement extent in Indiana, but they still survived.

It will be very important to protect groundwater and surface water inputs to natural areas wherever possible.  Groundwater diversion, especially for irrigation, is already a concern at some of our most important sites, and needs to be addressed.  If we really do get more precipitation during the dormant season, that might help recharge surface aquifers.  However, increased droughts may counteract that, so we will need to help develop policies that help moderate surface and groundwater depletions and encourage wetland restoration and protection.

River flows will probably become more flashy because of increased storm intensity.  The prevalence of channelized streams across the Midwest means that most run-off from big rains is lost quickly downstream.  This creates unstable streambeds and increases non-point source pollution in rivers.  It also means that most of that water is not captured in wetlands where it can provide habitat and help contribute to groundwater recharge.  Implementing the increased use of two-stage ditches may be one way to help moderate flood damage while still preserving a more natural stream flow regime.

Finally, restoration of areas adjacent to wetlands and low prairies provides opportunities to improve hydrologic conditions in two ways.  First, wetland restorations in formerly cropped areas can complement the hydroperiod of nearby natural wetlands.  New wetlands can be designed to stay wet longer – or dry up sooner – depending upon what may be missing (or predicted) in existing sites.  That full range of hydroperiod conditions is particularly important for successful breeding by reptile and amphibian populations.  Second, restoring portions of the landscape surrounding small natural wetlands can help buffer them from the impacts of diversion ditches and other hydrologic alterations.”

Prior to converting this Platte River cropfield to prairie, we tried to restore the kind of hydrologic gradients appropriate to a river floodplain wet prairie. Not only did that increase the diversity and resilience of that restoration, it also provided complementary habitats to the existing remnant prairies adjacent to it. (The Nature Conservancy - Nebraska)

Prairie Ecologist:

“Are there other things we need to be thinking about relative to climate change?”

Shuey:

“Three things come to mind.  First, invasive species will be moving into new areas as they, too, adjust to the changing climate.  Unfortunately, invasives are more likely to be able to move around fragmented landscapes than many of our native species, and we need to be prepared for that.  I think that the struggle to manage native grasslands will intensify in the future, and that we can never let our guard down.

Second, we tend to focus on the losers when we discuss climate change.  It’s important to remember that there will be interesting winners as well.  For example, in southern Indiana we are focusing heavily on small glade and barrens habitats surrounded by dense forest.  I expect these glades and barrens – which, structurally, are just prairies that grow on very thin soils and bedrock – to thrive!   We are aggressively restoring these habitats to their pre-fire suppressed condition so that they will be poised to take advantage of future harsh growing season droughts.

And finally, it’s important to remember that the predicted changes are PREDICTIONS.  We have to be flexible in our strategies as we move forward.   My guess is that I understand perhaps half of the future impacts to our sites – enough that we can take good ‘no regrets’ actions (productive strategies regardless of climate change) for the future.  But we’ll need to continue adapting strategies as we learn more.  If we are still following my current prescriptions 10 or 20 years from now we’re probably not paying attention to either changes on the ground or model refinements.”