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).
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.
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.
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.
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…
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…