Mysteries of the Grasslands


[ The NRS Transect 25:02 (Autumn/Winter 2007 ]

Painters and photographers can’t resist the golden, summertime hills at the NRS’s Sedgwick Reserve in Santa Barbara County. Visitors to the area, whether or not they are artistically inclined, find inspiration in its steeply contoured hills studded with magnificent oaks and cloaked mid-year in tawny-colored grasses. In fact, throughout much of California, such flaxen, velvety hills stand as one of the state’s most striking aesthetic features. Moreover, many rural residents depend upon this same landscape for their agriculture-based lifestyles. And, beyond human enjoyment and sustenance, the grasslands are home to hundreds of animal and plant species.

It surprises many people to learn that the grasslands that color California’s golden hills are not native to the state. Ecologists know they represent one of the most dramatic and extensive plant invasions in recorded history. They estimate that, over the last 150 years, more than 9 million hectares (~22,240,000 acres), perhaps one-quarter of the state, has been converted from grasslands in which native perennials were important to those dominated by exotic annual grasses. Today native grasses cover less than 1 percent of their original range.

Though it’s difficult to reconstruct the composition of California’s grasslands before the arrival of European settlers in the 1700s, most scientists believe that they were dominated by bunchgrasses, especially in the wetter lowlands. These long-lived perennials (the average life span for some species is 200 years) sank deep roots, allowing them to remain green throughout the state’s long, dry, Mediterranean summers. Wildflowers and herbs thrived between the grasses, forming a complex mosaic that supported vast herds of grazing animals, going back to prehistoric times. In the drier inland communities, like the Carrizo Plain or maybe much of the Central Valley, the landscape was probably dominated by annual flowers with bunchgrasses widely scattered or in small patches.

This ecosystem began to change with the arrival of the first European settlers. Along with their cattle, horses, and sheep, the newcomers brought feed grasses from their Old World homes. These annuals, which included such species as common (slender) wild oat (Avena barbata) and ripgut brome (Bromus diandrus), flourished in California’s Mediterranean climate. They were also well adapted to plowing, intensive livestock grazing, and other human-created disturbances. Moreover, unlike native perennials, annuals complete their life cycles in a single season, producing ample quantities of seed before dying back and leaving behind a rich seed bank for future years.

Over the last few decades, there has been a great deal of scientific interest in the California grassland ecosystems, as well as attempts to preserve and restore remnant native stands. Several NRS reserves have proven valuable for these efforts: Sedgwick Reserve, Hastings Natural History Reservation in Monterey County, Jepson Prairie Reserve in Solano County, and McLaughlin Natural History Reserve, which spans Napa, Lake, and Yolo Counties. Taken together, these sites form a north-south transect across a large portion of the state. The anticipated addition to the UC Natural Reserve System of a new reserve in Santa Clara County will provide yet-another link in this chain of protected research sites.

Investigating a Paradox

As a graduate student at Iowa State in the 1990s, Eric Seabloom was impressed with two things about the California grasslands. First, they had undergone a radical transformation in a relatively short period. “There had been a lot of invasions in the Midwest prairies,” he explains, “but the invaders had been similar species, so you had exotic perennial grasses invading native perennial grasses. But in California, there’s been a complete type switch in the communities. It’s one of those cases where a totally novel set of species has come in and taken over, sort of the equivalent of converting an area from grasslands to shrublands, or grasslands to trees. That makes it intriguing from an ecological perspective.”

The second thing that seemed odd to Seabloom was the common assumption that this conversion had happened because the invasive species were better competitors. This seemed counterintuitive to him. How could species that die every year outcompete those that lived for hundreds of years? His experience had taught him that unless a system was highly disturbed, the perennials should be the better competitors. Seabloom soon had a chance to explore these questions in greater detail when he came to UC Santa Barbara to do postdoctoral work with Jim Reichman at the National Center for Ecological Analysis and Synthesis (NCEAS).

The two led a team that began developing an extensive series of research plots at the Sedgwick Reserve to test different factors affecting the dynamics of California’s grassland ecosystems. In a series of field experiments, they addressed a number of key questions:

  • Were the annuals or the perennials superior resource competitors?
  • Were the native perennials, as some suspected, “recruitment limited,” because they didn’t produce enough seeds?
  • Could the perennials and exotics successfully invade each other’s stands?
  • And what impact did human disturbance have on this ongoing competition?

These experiments, conducted over almost a decade from the late 1990s to 2006, revealed a number of interesting insights. First, as Seabloom suspected, the perennials proved to be better competitors than the annuals. In almost every plot, perennial-dominated communities reduced levels of light at the soil surface, soil water, and extractable soil nitrates to significantly lower levels than those of the annual-dominated communities. And when annuals grew within stands dominanted by perennials, their per capita seed production was only one-half to one-third of what it would have been in areas dominated by their own kind.

Finally, in experiments designed to test “mutual invasibility,” the researchers found that adding seeds allowed perennial species to invade annual stands, but the reverse was not true: adding the seeds of annuals to plots dominated by perennials did not lead to an abundance of annuals or a significant decrease in perennials.

The experiments also revealed a few factors that might explain the dominance of the annuals. First, the perennials were recruitment limited; they produced fewer seeds than the annuals (about 20 times fewer seeds per year on a per-area basis) and the seeds had lower germination rates. Also, the perennials responded less well to simulated disturbances. When researchers added pocket gophers, nitrogen (similar to rates in urbanized areas), burning, tilling, and mowing (to simulate grazing), almost every treatment increased the abundance of annual species. The only exception was mowing—the perennials decreased initially, but then increased once the mowing stopped.

Seabloom’s work supported his hypothesis that the success of the annuals was due largely to the recruitment limitation of the perennials and to continuing human disturbance, at least at Sedgwick, where his test plots occupied fields that had long been used for agriculture. He theorizes that more than a century of intense grazing and periodic drought conditions had reduced the population of native grasses to such low levels that they no longer provided a sufficient number of seeds. The small populations, combined with low rates of seed production, establishment, and dispersal, made it difficult for the perennials to reestablish themselves. His work also demonstrated that no treatment completely eliminated either the annuals or the perennials. 

These findings led to a tantalizing idea. If the problem in reestablishing native grasslands is seed limitation, shouldn’t it be possible to restore viable populations of native perennials simply by adding seeds? Seabloom, now on the faculty at Oregon State University, is currently testing that possibility. He has expanded his research to include plots at Hastings and McLaughlin Reserves, where he is adding native seeds to stands of annual grasses to see if his Sedgwick results can be repeated.

Many people remain doubtful. Mark Stromberg, resident manager at Hastings, has been investigating the grasslands for decades. “Resource managers tried this strategy back in the fifties,” he notes, “and it was a total failure. Of all the natives they planted, none survived. At Sedgwick, Eric [Seabloom] was planting seeds in what was an old barley field. It had been farmed and kept weed-free for 50 years, either with tillage or herbicide, so there was no annual seed bank at all. And who knows what the microbial community is like? At Hastings, we’ve found that the microbial community in former agricultural fields still hasn’t recovered 70 years later.”

Nevertheless, Seabloom provides strong evidence, although from Sedgwick only, that native species are better at using resources than the exotics. So the question remains, why are 9 million hectares of California grasslands now dominated by annual species? Did historical droughts force this changeover? Or are such human disturbances as plowing and overgrazing at fault? Could changes in the soil microbes encourage this result? Or might the transformation have been due to a combination of all these factors? None of these explanations seems adequate to explain the situation, but recently a promising new line of inquiry has come to the fore.

Aphids and Yellow Dwarfs

While Seabloom was conducting his experiments at the Sedgwick Reserve, Elizabeth Borer was examining arthropod predator and herbivore responses to arthropod responses to nitrogen addition in the grasslands. One day in the field, she found herself staring at an aphid and pondering, “Why do you matter? You’re an ineffective herbivore. Why should I be interested in you?” Then it dawned on Borer. An aphid is like a mosquito: no one dies from a mosquito bite, but mosquitoes are a vector for pathogens that can make whatever they bite very, very sick.

Could aphid-vectored diseases change the competitive balance in these grasslands? The idea was intriguing. As Borer explains: “Eric [Seabloom]’s work had shown that the natives are better at taking up resources, and that if you use relatively simple mathematical models and carry them out to the future, you would basically predict the decline of the exotic annuals. His work has empirically shown that the native perennials are better competitors for shared resources. So, theoretically, the annuals should disappear. But they don’t.”

Borer, who is now both Seabloom’s faculty colleague at Oregon State University and his spouse, took a different approach to the grassland questions. Her focus was on parasitoids and pathogens. Spurred by her encounter with the aphid, she began investigating the role diseases might play in the grasslands. She found that Carolyn Malmström at Michigan State University had done a lot of work on aphids and the spread of viruses in California grasslands. In particular, Malmström had focused on Barley Yellow Dwarf Virus (BYDV), a suite of five related viruses that were well known as agricultural pests. 

“A vast amount is known about BYDV,” Borer explains. “It’s one of the most widespread pathogens on the globe, and it infects all grasses, including barley, oats, wheat, corn, rye — any of the grains that we eat. So there’s a huge amount known about its effects on crops, and there’s a lot of work going on, trying to develop resistant breeds of crops. That makes it a really nice pathogen to study in ecology, because so much is known from the plant pathology end about its transmission, effects, biochemistry, and epidemiology. Carolyn [Malmström] was the one who really planted that possibility in my mind.”

Borer set up a meeting with Seabloom, who had amassed a huge amount of empirical data and knowledge of the literature, and a modeler, Parviez Hosseini, to discuss her ideas and rough out a mathematical model for the grasslands that incorporated BYDV. Using Seabloom’s and Malmström’s published and unpublished data, Hosseini ran the model and found strong indications that the pathogen could reverse the competitive abilities of the natives and exotics.

For Borer, these results confirmed the importance of viruses in ecosystems, and she is currently working on developing the model more fully. “I’ve been doing a lot of work around aphid responses to annuals and perennials,” she explains, “and how aphids respond to nutrient changes. We’re really trying to look at aphid population dynamics and what role that might play. Once we have those data, Parviez [Hosseini] is going to add in aphid population dynamics to the model. Currently we have a fixed pathogen vector in the model, so he’ll be looking at how aphid preferences and reproduction might change the results we’ve been getting. It looks to me like they will just make the argument stronger, but we’ll have to see as we get more data.”

So does the virus explain the dominance of the invasive annual grasses in California? “It isn’t the whole explanation,” admits Borer. “It’s a contender as a hypothesis, but I don’t think any of the factors provides a complete explanation for the invasion. I don’t think any one factor could explain that, but I do think that viruses have played an underestimated role.”

Developing the Tools

Carolyn Malmström lives in Michigantoday, but her roots are in California. A member of a fifth-generation California family, she spent much of her childhood exploring and playing in the hills above Oakland. The California grasslands are, you might say, in her blood. Much likeBorer, her interest in aphids and theirimpact on the grasslands began while she was investigating a related topic. As a graduate student at Stanford, she worked on the long-term Jasper Ridge Global Change Experiment, which began in 1997 and continues today. She wasstudying the impact on grasslands of elevated CO2 levels, which are known to cause global warming. While she did her fieldwork, she kept wondering if the group was missing any major piece of the picture in terms of plant response to CO2. And one day it struck her: almost all the studies had been done on healthy plants, but not all plants in the world are healthy.

“One reason ecologists have not paid a lot of attention to viruses,” notes Malmström, “is that they’re subtle, particularly in nature. They don’t make the plants turn blue. They’re not in-your-face. Many factors shape vegetation and make them look scruffy. If the damage is subtle, it could be wind damage, frost damage, poor nutrients, or maybe cows grazed there recently. It could be very easy to walk through a natural ecosystem and be unaware of virus presence, particularly if you’re focused on other factors.”

At the time, Malmström had no background in pathogens, so she went to the library, where she began learning about BYDV and the closely related Cereal Yellow Dwarf Viruses (CYDV). She was especially intrigued to learn that the viruses interrupted phloem1 transport in plants, blocking the movement of sugars, because she already knew that high densities of CO2 also influenced sugar transport.

[1 The products of photosynthesis, mostly sugars, are distributed throughout a plant by specialized conducting tissue known as the phloem. The phloem differs from the other type of plant transport tissue, the xylem, in that its conduits are living cells (the xylem is comprised of dead cells), the compounds it transports are organic (the xylem transports water and minerals), and movement within the phloem is bidirectional, up or down (the xylem transports up only).]

Malmström thought she might be on to something, but before she could go any further, she needed to learn how to work with the viruses. The first challenge was finding ways to identify infected plants. Assays available at the time were largely serological techniques that had been developed for crops, which are chemically very simple. Plants in nature are more difficult to analyze because the concentration of viruses tends to be lower and the plants themselves have a lot of secondary compounds that confuse most assays.

Over time, Malmström and her laboratory colleagues developed a compact molecular technique for quickly determining which native and exotic grasses were infected and which groups of viruses were involved. “It was a complex process,” she recalls. “A large fraction of plant viruses, including those we study, are RNA-based, and RNA is always a little bit trickier to work with, because it degrades faster. We finally came up with a multiplex, reverse-transcription polymerase chain reaction test (RTPCR) where you extract all the RNA from the plant and viruses, convert the RNA to DNA, and then amplify it in a thermocycler. It’s sort of like what you see on CSI.” Eventually, Malmström produced a set of inexpensive primers,2 or tests, for detecting a variety of BYDVs and CYVDs at one time, enabling her to identify where viruses occurred in natural ecosystems.

[2 A “primer” in this context is a short sequence of RNA or DNA from which DNA replication can initiate, allowing researchers to determine what microorganisms are present.]

Earlier, while working at UC Berkeley as a President’s Postdoctoral Fellow, Malmström had conducted a series of experiments demonstrating the dramatic impact the viruses had on native grasses. Using seeds collected from around the state, including the NRS’s Jepson Prairie Reserve, she grew a large variety of native grasses in a carefully controlled plot at UC Davis, deliberately infected them with a wild virus, and monitored the infected plants’ growth for one to three years.

Perhaps because she works with pathogens, Malmström has a disconcerting enthusiasm for events that most people would consider bad news. “It was really neat to see!” she exclaims. “Many of the test populations were extremely dwarfed. Seed production dropped, and the native grasses had much higher mortality over the long term. These viruses are not benign in nature. They were having a significant impact.”

Amplifying the Aphids

Aphids cover surprisingly large distances. Each year in February and March, large flights of aphids move up and down California’s Central Valley. Walk through a stand of annual grasses in the spring, and you can find yourself covered in aphids. All of this activity intrigued Malmström and got her to thinking about how the aphids used the two distinctly different types of grasses — the exotic annuals with their substantial seed banks and tight stand structures versus the long-lived perennials that were widely spaced and produced few seeds.

“When these annual grasses invaded, they must have changed the epidemiology of the environment,” Malmström explains, “because they certainly change how the aphids, who spread the virus, behave. That led me to come up with some potential hypotheses and really big-picture, wild ideas about how these viruses might have affected the invasion of these grasses in the first place. That’s a really tough question that may be impossible to prove definitively. It’s hard to go back and prove past events, but you certainly can collect information and see if it’s consistent or not with your ideas.”

To explore how the exotic grasses might change aphid behavior, Malmström conducted a set of studies in which she grew native grasses and exotic grasses together and separately. Her subsequent tests revealed that native grasses, when grown alone, had some aphids, but exotic grasses, also grown alone, were loaded with aphids. And when she ran taste-tests, offering caged aphids both native and exotic grasses, the aphids always preferred the exotic grasses. Next, she looked at aphid reproduction. Again the results were clear: the female aphids on the exotic plants produced more young than those on the natives.

The results of these experiments strongly supported the idea that the rapid growth of aphid populations on the exotic plants affected the native grasses. As Malmström explains, “The exotic grasses have a tendency to amplify an aphid population. The interesting thing, though, is that exotic grasses in California are not green all year round. They go brown before the native plants do, so the aphids that build up on them have to go somewhere else. One of the places they go is to the native grasses that are still green.”

Malmström’s studies showed that the amplification of the aphids in exotic stands could have major consequences for nearby native plants. When she grew native grasses by themselves, about one-third of the native plants became infected. When she grew natives and exotics together, twice as many natives became infected. She attributes some of that difference to changes in the aphid population and behavior, but the net effect is that native grasses growing next to exotic grasses have a greatly increased chance of becoming infected. And while the native annuals begin each year with new, disease-free plants, the long-lived perennials carry their infections throughout their lives.

To Malmström, these circumstances constitute an excellent example of what ecologists call “apparent competition” — where the relationship of the exotic annual grasses with aphids and viruses negatively affects the native perennial bunchgrasses by increasing their rate of infection. As she notes: “Apparent competition is an indirect process related to a third factor. In this case, [the third factor] could be a pathogen. So if the exotic grasses make a pathogen more prevalent and that has a negative consequence on the natives, that triangular relationship means that the presence of the exotics could hurt the natives indirectly. Our hypothesis was that these viruses helped the exotic grasses become established in California in the first place by having a negative influence on the natives. It’s consistent with the experimental data we have so far, but we know nothing about the history of the grasslands and viruses.”

Tracking the Family Tree

Not much is known about the history of viruses in plants. BYDV was first identified in 1951, but its history before that time remains a mystery. How long had it been present in California? How had it changed over time? To try to answer these questions, Malmström and her colleagues analyzed grass samples in herbaria at UC Davis and UC Berkeley to see if they could find any evidence of infection in some of the older samples.

To Malmström’s delight, their search was successful. “By looking through the collections at both Davis and Berkeley, we found evidence of nine distinct viruses in preserved grasses that dated back as far as 1917. Those are some of the oldest viruses people have pulled out of grasses or plants, and they’re certainly the oldest in North America.”

These nonagenarian viruses look much like modern BYDVs. By conducting a phylogenetic analysis, Malmström’s research team was able to estimate how fast the viruses had changed over time. She says, “We have fairly strong evidence that the viruses have been present in California for some time, perhaps from the middle of the 1700s at least.”

In fact, the group’s phylogenetic work revealed insights that ranged far beyond California and track well with historical developments. “The patterns show lots of evidence of global movement more recently,” Malmström continues, “and a really interesting connection between old viruses in California and modern infections in Australia. Our evidence suggests that the viruses may have gone from California to Australia in the 1890s, which is when ships had gotten fast enough that horticultural pests were surviving the trans-Pacific trip, but also before horticultural restraints had been put in place, which happened in 1912. We know Australia didn’t have these viruses beforehand, because they didn’t have the right aphids.”

Practical Applications

The change in BYDV dynamics and the increased incidence of disease in native grasses provide important missing clues to explain the dominance of the invasive annual grasses. The chances of ever reversing this overwhelming invasion now appear to be slight, for doing so would entail altering the disease vector. Farmers have learned to time the planting of their crops to avoid aphids and reduce BYDV infections, but it is not possible to take this same approach with the restoration of native grasses. For the present, those who seek to restore native grasslands must be content with slowing the rate of infection in their plots by carefully weeding out the annuals.

Malmström notes that this weeding is crucial when the native plants are young: “It’s really important for restoration ecologists, when they’re growing-up their grasses for plug plantings, to keep aphids off them at that stage, because it’s easy for the native stock to get infected and then transferred out onto the landscape.”

And if anything, the struggle between invasive species and natives is getting more complex. “Today we have additional exotic grasses, like medusa head and goat grass,” Malmström says, “and they have even more nasty ways of changing plant relationships. We’ve focused on annuals, like wild oats and brome, which aphids really like. These newer ones — not even the cows like them very much. They pose a significant threat to rangeland sustainability. But that’s probably not virus-related.”

Hastings Reserve Manager Mark Stromberg includes a list of successful restoration projects in California Grasslands: Ecology and Management, a book he co-edited (with Jeffrey Corbin and Carla D’Antonio), which was recently published by the University of California Press (see sidebar, page 6). He also provides perspective on current efforts to preserve native California grasslands:

Homeowners and small landowners are learning the value of native grasslands. That’s the hope for California. Restoring a whole county park or a whole ranch is really pushing the envelope in terms of scale. We might be able to do some things, but it’s very expensive getting seeds started, monitoring, weeding. At the local scale, homeowners can monitor their plantings. It’s very personal and reflective. People get a lot of personal, almost spiritual enjoyment out of restoration. They can handle the weeds and try different kinds of flowers and grasses. There are a lot of success stories at that level, up to fifty acres, and that’s the future. —JB

For more information, contact:
Elizabeth Borer
Email: borer@science.oregonstate.edu
Carolyn Malmström
Email: carolynm@msu.edu
Eric Seabloom
Email: seabloom@science.oregonstate.edu
Mark Stromberg
Email: stromberg@berkeley.edu
References
Borer, E. T., P. R. Hosseini, E. W. Seabloom, and A. P. Dobson. 2007. Pathogen-induced reversal of native dominance in a grassland community. PNAS3 104:13, pp. 5473-78.

Malmström, C. M., C. C. Hughes, L. A. Newton, and C. J. Stoner. 2005. Virus infection in remnant native bunchgrass from invaded California grasslands. New Phytologist (2005) 168, pp. 217-30.

Seabloom, E. W., W. S. Harpole, O. J. Reichman, and D. Tilman. 2003. Invasion, competitive dominance, and resource use by exotic and native California grassland species. PNAS 100:23, pp. 13384-89.

[3 Previously, Proceedings of the National Academy of Sciences.] Mysteries of the Grasslands

Painters and photographers can’t resist the golden, summertime hills at the NRS’s Sedgwick Reserve in Santa Barbara County. Visitors to the area, whether or not they are artistically inclined, find inspiration in its steeply contoured hills studded with magnificent oaks and cloaked mid-year in tawny-colored grasses. In fact, throughout much of California, such flaxen, velvety hills stand as one of the state’s most striking aesthetic features. Moreover, many rural residents depend upon this same landscape for their agriculture-based lifestyles. And, beyond human enjoyment and sustenance, the grasslands are home to hundreds of animal and plant species.

It surprises many people to learn that the grasslands that color California’s golden hills are not native to the state. Ecologists know they represent one of the most dramatic and extensive plant invasions in recorded history. They estimate that, over the last 150 years, more than 9 million hectares (~22,240,000 acres), perhaps one-quarter of the state, has been converted from grasslands in which native perennials were important to those dominated by exotic annual grasses. Today native grasses cover less than 1 percent of their original range.

Though it’s difficult to reconstruct the composition of California’s grasslands before the arrival of European settlers in the 1700s, most scientists believe that they were dominated by bunchgrasses, especially in the wetter lowlands. These long-lived perennials (the average life span for some species is 200 years) sank deep roots, allowing them to remain green throughout the state’s long, dry, Mediterranean summers. Wildflowers and herbs thrived between the grasses, forming a complex mosaic that supported vast herds of grazing animals, going back to prehistoric times. In the drier inland communities, like the Carrizo Plain or maybe much of the Central Valley, the landscape was probably dominated by annual flowers with bunchgrasses widely scattered or in small patches.

This ecosystem began to change with the arrival of the first European settlers. Along with their cattle, horses, and sheep, the newcomers brought feed grasses from their Old World homes. These annuals, which included such species as common (slender) wild oat (Avena barbata) and ripgut brome (Bromus diandrus), flourished in California’s Mediterranean climate. They were also well adapted to plowing, intensive livestock grazing, and other human-created disturbances. Moreover, unlike native perennials, annuals complete their life cycles in a single season, producing ample quantities of seed before dying back and leaving behind a rich seed bank for future years.

Over the last few decades, there has been a great deal of scientific interest in the California grassland ecosystems, as well as attempts to preserve and restore remnant native stands. Several NRS reserves have proven valuable for these efforts: Sedgwick Reserve, Hastings Natural History Reservation in Monterey County, Jepson Prairie Reserve in Solano County, and McLaughlin Natural History Reserve, which spans Napa, Lake, and Yolo Counties. Taken together, these sites form a north-south transect across a large portion of the state. The anticipated addition to the UC Natural Reserve System of a new reserve in Santa Clara County will provide yet-another link in this chain of protected research sites.

Investigating a Paradox

As a graduate student at Iowa State in the 1990s, Eric Seabloom was impressed with two things about the California grasslands. First, they had undergone a radical transformation in a relatively short period. “There had been a lot of invasions in the Midwest prairies,” he explains, “but the invaders had been similar species, so you had exotic perennial grasses invading native perennial grasses. But in California, there’s been a complete type switch in the communities. It’s one of those cases where a totally novel set of species has come in and taken over, sort of the equivalent of converting an area from grasslands to shrublands, or grasslands to trees. That makes it intriguing from an ecological perspective.”

The second thing that seemed odd to Seabloom was the common assumption that this conversion had happened because the invasive species were better competitors. This seemed counterintuitive to him. How could species that die every year outcompete those that lived for hundreds of years? His experience had taught him that unless a system was highly disturbed, the perennials should be the better competitors. Seabloom soon had a chance to explore these questions in greater detail when he came to UC Santa Barbara to do postdoctoral work with Jim Reichman at the National Center for Ecological Analysis and Synthesis (NCEAS).

The two led a team that began developing an extensive series of research plots at the Sedgwick Reserve to test different factors affecting the dynamics of California’s grassland ecosystems. In a series of field experiments, they addressed a number of key questions:

  • Were the annuals or the perennials superior resource competitors?
  • Were the native perennials, as some suspected, “recruitment limited,” because they didn’t produce enough seeds?
  • Could the perennials and exotics successfully invade each other’s stands?
  • And what impact did human disturbance have on this ongoing competition?

These experiments, conducted over almost a decade from the late 1990s to 2006, revealed a number of interesting insights. First, as Seabloom suspected, the perennials proved to be better competitors than the annuals. In almost every plot, perennial-dominated communities reduced levels of light at the soil surface, soil water, and extractable soil nitrates to significantly lower levels than those of the annual-dominated communities. And when annuals grew within stands dominanted by perennials, their per capita seed production was only one-half to one-third of what it would have been in areas dominated by their own kind.

Finally, in experiments designed to test “mutual invasibility,” the researchers found that adding seeds allowed perennial species to invade annual stands, but the reverse was not true: adding the seeds of annuals to plots dominated by perennials did not lead to an abundance of annuals or a significant decrease in perennials.

The experiments also revealed a few factors that might explain the dominance of the annuals. First, the perennials were recruitment limited; they produced fewer seeds than the annuals (about 20 times fewer seeds per year on a per-area basis) and the seeds had lower germination rates. Also, the perennials responded less well to simulated disturbances. When researchers added pocket gophers, nitrogen (similar to rates in urbanized areas), burning, tilling, and mowing (to simulate grazing), almost every treatment increased the abundance of annual species. The only exception was mowing—the perennials decreased initially, but then increased once the mowing stopped.

Seabloom’s work supported his hypothesis that the success of the annuals was due largely to the recruitment limitation of the perennials and to continuing human disturbance, at least at Sedgwick, where his test plots occupied fields that had long been used for agriculture. He theorizes that more than a century of intense grazing and periodic drought conditions had reduced the population of native grasses to such low levels that they no longer provided a sufficient number of seeds. The small populations, combined with low rates of seed production, establishment, and dispersal, made it difficult for the perennials to reestablish themselves. His work also demonstrated that no treatment completely eliminated either the annuals or the perennials. 

These findings led to a tantalizing idea. If the problem in reestablishing native grasslands is seed limitation, shouldn’t it be possible to restore viable populations of native perennials simply by adding seeds? Seabloom, now on the faculty at Oregon State University, is currently testing that possibility. He has expanded his research to include plots at Hastings and McLaughlin Reserves, where he is adding native seeds to stands of annual grasses to see if his Sedgwick results can be repeated.

Many people remain doubtful. Mark Stromberg, resident manager at Hastings, has been investigating the grasslands for decades. “Resource managers tried this strategy back in the fifties,” he notes, “and it was a total failure. Of all the natives they planted, none survived. At Sedgwick, Eric [Seabloom] was planting seeds in what was an old barley field. It had been farmed and kept weed-free for 50 years, either with tillage or herbicide, so there was no annual seed bank at all. And who knows what the microbial community is like? At Hastings, we’ve found that the microbial community in former agricultural fields still hasn’t recovered 70 years later.”

Nevertheless, Seabloom provides strong evidence, although from Sedgwick only, that native species are better at using resources than the exotics. So the question remains, why are 9 million hectares of California grasslands now dominated by annual species? Did historical droughts force this changeover? Or are such human disturbances as plowing and overgrazing at fault? Could changes in the soil microbes encourage this result? Or might the transformation have been due to a combination of all these factors? None of these explanations seems adequate to explain the situation, but recently a promising new line of inquiry has come to the fore.

Aphids and Yellow Dwarfs

While Seabloom was conducting his experiments at the Sedgwick Reserve, Elizabeth Borer was examining arthropod predator and herbivore responses to arthropod responses to nitrogen addition in the grasslands. One day in the field, she found herself staring at an aphid and pondering, “Why do you matter? You’re an ineffective herbivore. Why should I be interested in you?” Then it dawned on Borer. An aphid is like a mosquito: no one dies from a mosquito bite, but mosquitoes are a vector for pathogens that can make whatever they bite very, very sick.

Could aphid-vectored diseases change the competitive balance in these grasslands? The idea was intriguing. As Borer explains: “Eric [Seabloom]’s work had shown that the natives are better at taking up resources, and that if you use relatively simple mathematical models and carry them out to the future, you would basically predict the decline of the exotic annuals. His work has empirically shown that the native perennials are better competitors for shared resources. So, theoretically, the annuals should disappear. But they don’t.”

Borer, who is now both Seabloom’s faculty colleague at Oregon State University and his spouse, took a different approach to the grassland questions. Her focus was on parasitoids and pathogens. Spurred by her encounter with the aphid, she began investigating the role diseases might play in the grasslands. She found that Carolyn Malmström at Michigan State University had done a lot of work on aphids and the spread of viruses in California grasslands. In particular, Malmström had focused on Barley Yellow Dwarf Virus (BYDV), a suite of five related viruses that were well known as agricultural pests. 

“A vast amount is known about BYDV,” Borer explains. “It’s one of the most widespread pathogens on the globe, and it infects all grasses, including barley, oats, wheat, corn, rye — any of the grains that we eat. So there’s a huge amount known about its effects on crops, and there’s a lot of work going on, trying to develop resistant breeds of crops. That makes it a really nice pathogen to study in ecology, because so much is known from the plant pathology end about its transmission, effects, biochemistry, and epidemiology. Carolyn [Malmström] was the one who really planted that possibility in my mind.”

Borer set up a meeting with Seabloom, who had amassed a huge amount of empirical data and knowledge of the literature, and a modeler, Parviez Hosseini, to discuss her ideas and rough out a mathematical model for the grasslands that incorporated BYDV. Using Seabloom’s and Malmström’s published and unpublished data, Hosseini ran the model and found strong indications that the pathogen could reverse the competitive abilities of the natives and exotics.

For Borer, these results confirmed the importance of viruses in ecosystems, and she is currently working on developing the model more fully. “I’ve been doing a lot of work around aphid responses to annuals and perennials,” she explains, “and how aphids respond to nutrient changes. We’re really trying to look at aphid population dynamics and what role that might play. Once we have those data, Parviez [Hosseini] is going to add in aphid population dynamics to the model. Currently we have a fixed pathogen vector in the model, so he’ll be looking at how aphid preferences and reproduction might change the results we’ve been getting. It looks to me like they will just make the argument stronger, but we’ll have to see as we get more data.”

So does the virus explain the dominance of the invasive annual grasses in California? “It isn’t the whole explanation,” admits Borer. “It’s a contender as a hypothesis, but I don’t think any of the factors provides a complete explanation for the invasion. I don’t think any one factor could explain that, but I do think that viruses have played an underestimated role.”

Developing the Tools

Carolyn Malmström lives in Michigantoday, but her roots are in California. A member of a fifth-generation California family, she spent much of her childhood exploring and playing in the hills above Oakland. The California grasslands are, you might say, in her blood. Much likeBorer, her interest in aphids and theirimpact on the grasslands began while she was investigating a related topic. As a graduate student at Stanford, she worked on the long-term Jasper Ridge Global Change Experiment, which began in 1997 and continues today. She wasstudying the impact on grasslands of elevated CO2 levels, which are known to cause global warming. While she did her fieldwork, she kept wondering if the group was missing any major piece of the picture in terms of plant response to CO2. And one day it struck her: almost all the studies had been done on healthy plants, but not all plants in the world are healthy.

“One reason ecologists have not paid a lot of attention to viruses,” notes Malmström, “is that they’re subtle, particularly in nature. They don’t make the plants turn blue. They’re not in-your-face. Many factors shape vegetation and make them look scruffy. If the damage is subtle, it could be wind damage, frost damage, poor nutrients, or maybe cows grazed there recently. It could be very easy to walk through a natural ecosystem and be unaware of virus presence, particularly if you’re focused on other factors.”

At the time, Malmström had no background in pathogens, so she went to the library, where she began learning about BYDV and the closely related Cereal Yellow Dwarf Viruses (CYDV). She was especially intrigued to learn that the viruses interrupted phloem1 transport in plants, blocking the movement of sugars, because she already knew that high densities of CO2 also influenced sugar transport.

[1 The products of photosynthesis, mostly sugars, are distributed throughout a plant by specialized conducting tissue known as the phloem. The phloem differs from the other type of plant transport tissue, the xylem, in that its conduits are living cells (the xylem is comprised of dead cells), the compounds it transports are organic (the xylem transports water and minerals), and movement within the phloem is bidirectional, up or down (the xylem transports up only).]

Malmström thought she might be on to something, but before she could go any further, she needed to learn how to work with the viruses. The first challenge was finding ways to identify infected plants. Assays available at the time were largely serological techniques that had been developed for crops, which are chemically very simple. Plants in nature are more difficult to analyze because the concentration of viruses tends to be lower and the plants themselves have a lot of secondary compounds that confuse most assays.

Over time, Malmström and her laboratory colleagues developed a compact molecular technique for quickly determining which native and exotic grasses were infected and which groups of viruses were involved. “It was a complex process,” she recalls. “A large fraction of plant viruses, including those we study, are RNA-based, and RNA is always a little bit trickier to work with, because it degrades faster. We finally came up with a multiplex, reverse-transcription polymerase chain reaction test (RTPCR) where you extract all the RNA from the plant and viruses, convert the RNA to DNA, and then amplify it in a thermocycler. It’s sort of like what you see on CSI.” Eventually, Malmström produced a set of inexpensive primers,2 or tests, for detecting a variety of BYDVs and CYVDs at one time, enabling her to identify where viruses occurred in natural ecosystems.

[2 A “primer” in this context is a short sequence of RNA or DNA from which DNA replication can initiate, allowing researchers to determine what microorganisms are present.]

Earlier, while working at UC Berkeley as a President’s Postdoctoral Fellow, Malmström had conducted a series of experiments demonstrating the dramatic impact the viruses had on native grasses. Using seeds collected from around the state, including the NRS’s Jepson Prairie Reserve, she grew a large variety of native grasses in a carefully controlled plot at UC Davis, deliberately infected them with a wild virus, and monitored the infected plants’ growth for one to three years.

Perhaps because she works with pathogens, Malmström has a disconcerting enthusiasm for events that most people would consider bad news. “It was really neat to see!” she exclaims. “Many of the test populations were extremely dwarfed. Seed production dropped, and the native grasses had much higher mortality over the long term. These viruses are not benign in nature. They were having a significant impact.”

Amplifying the Aphids

Aphids cover surprisingly large distances. Each year in February and March, large flights of aphids move up and down California’s Central Valley. Walk through a stand of annual grasses in the spring, and you can find yourself covered in aphids. All of this activity intrigued Malmström and got her to thinking about how the aphids used the two distinctly different types of grasses — the exotic annuals with their substantial seed banks and tight stand structures versus the long-lived perennials that were widely spaced and produced few seeds.

“When these annual grasses invaded, they must have changed the epidemiology of the environment,” Malmström explains, “because they certainly change how the aphids, who spread the virus, behave. That led me to come up with some potential hypotheses and really big-picture, wild ideas about how these viruses might have affected the invasion of these grasses in the first place. That’s a really tough question that may be impossible to prove definitively. It’s hard to go back and prove past events, but you certainly can collect information and see if it’s consistent or not with your ideas.”

To explore how the exotic grasses might change aphid behavior, Malmström conducted a set of studies in which she grew native grasses and exotic grasses together and separately. Her subsequent tests revealed that native grasses, when grown alone, had some aphids, but exotic grasses, also grown alone, were loaded with aphids. And when she ran taste-tests, offering caged aphids both native and exotic grasses, the aphids always preferred the exotic grasses. Next, she looked at aphid reproduction. Again the results were clear: the female aphids on the exotic plants produced more young than those on the natives.

The results of these experiments strongly supported the idea that the rapid growth of aphid populations on the exotic plants affected the native grasses. As Malmström explains, “The exotic grasses have a tendency to amplify an aphid population. The interesting thing, though, is that exotic grasses in California are not green all year round. They go brown before the native plants do, so the aphids that build up on them have to go somewhere else. One of the places they go is to the native grasses that are still green.”

Malmström’s studies showed that the amplification of the aphids in exotic stands could have major consequences for nearby native plants. When she grew native grasses by themselves, about one-third of the native plants became infected. When she grew natives and exotics together, twice as many natives became infected. She attributes some of that difference to changes in the aphid population and behavior, but the net effect is that native grasses growing next to exotic grasses have a greatly increased chance of becoming infected. And while the native annuals begin each year with new, disease-free plants, the long-lived perennials carry their infections throughout their lives.

To Malmström, these circumstances constitute an excellent example of what ecologists call “apparent competition” — where the relationship of the exotic annual grasses with aphids and viruses negatively affects the native perennial bunchgrasses by increasing their rate of infection. As she notes: “Apparent competition is an indirect process related to a third factor. In this case, [the third factor] could be a pathogen. So if the exotic grasses make a pathogen more prevalent and that has a negative consequence on the natives, that triangular relationship means that the presence of the exotics could hurt the natives indirectly. Our hypothesis was that these viruses helped the exotic grasses become established in California in the first place by having a negative influence on the natives. It’s consistent with the experimental data we have so far, but we know nothing about the history of the grasslands and viruses.”

Tracking the Family Tree

Not much is known about the history of viruses in plants. BYDV was first identified in 1951, but its history before that time remains a mystery. How long had it been present in California? How had it changed over time? To try to answer these questions, Malmström and her colleagues analyzed grass samples in herbaria at UC Davis and UC Berkeley to see if they could find any evidence of infection in some of the older samples.

To Malmström’s delight, their search was successful. “By looking through the collections at both Davis and Berkeley, we found evidence of nine distinct viruses in preserved grasses that dated back as far as 1917. Those are some of the oldest viruses people have pulled out of grasses or plants, and they’re certainly the oldest in North America.”

These nonagenarian viruses look much like modern BYDVs. By conducting a phylogenetic analysis, Malmström’s research team was able to estimate how fast the viruses had changed over time. She says, “We have fairly strong evidence that the viruses have been present in California for some time, perhaps from the middle of the 1700s at least.”

In fact, the group’s phylogenetic work revealed insights that ranged far beyond California and track well with historical developments. “The patterns show lots of evidence of global movement more recently,” Malmström continues, “and a really interesting connection between old viruses in California and modern infections in Australia. Our evidence suggests that the viruses may have gone from California to Australia in the 1890s, which is when ships had gotten fast enough that horticultural pests were surviving the trans-Pacific trip, but also before horticultural restraints had been put in place, which happened in 1912. We know Australia didn’t have these viruses beforehand, because they didn’t have the right aphids.”

Practical Applications

The change in BYDV dynamics and the increased incidence of disease in native grasses provide important missing clues to explain the dominance of the invasive annual grasses. The chances of ever reversing this overwhelming invasion now appear to be slight, for doing so would entail altering the disease vector. Farmers have learned to time the planting of their crops to avoid aphids and reduce BYDV infections, but it is not possible to take this same approach with the restoration of native grasses. For the present, those who seek to restore native grasslands must be content with slowing the rate of infection in their plots by carefully weeding out the annuals.

Malmström notes that this weeding is crucial when the native plants are young: “It’s really important for restoration ecologists, when they’re growing-up their grasses for plug plantings, to keep aphids off them at that stage, because it’s easy for the native stock to get infected and then transferred out onto the landscape.”

And if anything, the struggle between invasive species and natives is getting more complex. “Today we have additional exotic grasses, like medusa head and goat grass,” Malmström says, “and they have even more nasty ways of changing plant relationships. We’ve focused on annuals, like wild oats and brome, which aphids really like. These newer ones — not even the cows like them very much. They pose a significant threat to rangeland sustainability. But that’s probably not virus-related.”

Hastings Reserve Manager Mark Stromberg includes a list of successful restoration projects in California Grasslands: Ecology and Management, a book he co-edited (with Jeffrey Corbin and Carla D’Antonio), which was recently published by the University of California Press (see sidebar, page 6). He also provides perspective on current efforts to preserve native California grasslands:

Homeowners and small landowners are learning the value of native grasslands. That’s the hope for California. Restoring a whole county park or a whole ranch is really pushing the envelope in terms of scale. We might be able to do some things, but it’s very expensive getting seeds started, monitoring, weeding. At the local scale, homeowners can monitor their plantings. It’s very personal and reflective. People get a lot of personal, almost spiritual enjoyment out of restoration. They can handle the weeds and try different kinds of flowers and grasses. There are a lot of success stories at that level, up to fifty acres, and that’s the future. —JB

For more information, contact:
Elizabeth Borer
Email: borer@science.oregonstate.edu
Carolyn Malmström
Email: carolynm@msu.edu
Eric Seabloom
Email: seabloom@science.oregonstate.edu
Mark Stromberg
Email: stromberg@berkeley.edu

References
Borer, E. T., P. R. Hosseini, E. W. Seabloom, and A. P. Dobson. 2007. Pathogen-induced reversal of native dominance in a grassland community. PNAS3 104:13, pp. 5473-78.

Malmström, C. M., C. C. Hughes, L. A. Newton, and C. J. Stoner. 2005. Virus infection in remnant native bunchgrass from invaded California grasslands. New Phytologist (2005) 168, pp. 217-30.

Seabloom, E. W., W. S. Harpole, O. J. Reichman, and D. Tilman. 2003. Invasion, competitive dominance, and resource use by exotic and native California grassland species. PNAS 100:23, pp. 13384-89.

[3 Previously, Proceedings of the National Academy of Sciences.]



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