Risk: The Game of Invasive Plants

Controlling the Invasions

Invasive species have become a common buzzword in agricultural, horticultural, and scientific circles within the last few decades, in the wake of increasingly rapid and uncontrollably spreading plant species degrading and invading ecosystems around the world. The rate and breadth of the impacts from invasive plants has led to significant research into controlling and predicting invasive species around the world. Whether motivated by threatened economic livelihoods, undermined aesthetic values, or declining environmental quality, there has been considerable literature and research performed in the attempt to limit invasive species.

Many different tactics and perspectives on how controlling invasives might be done most efficiently have been put forward. The recent study “A Geographic Assessment of the Risk of Naturalization of Non-native Woody Plants in Iowa” published by Widrlechner and Iles (2002) has taken a different approach in predicting relative ‘invasiveness’ of plants, using geographic comparison and analysis to denote greater likelihood of invasive qualities and behaviors in plants. Many factors have been suggested as the supposed linchpin to securing or inhibiting invasive qualities in plants. Other approaches to quantifying and limiting invasiveness in plants have included comparative growth rates in plants, where relative growth rate (RGR), the comparative speed at which plants grow, and associated variables are examined in association with known and suspected invasive plants. Grotkopp et al. (2010) used this approach in their assessment of woody native and nonnative plant species, finding that RGR values for known invasive species were indeed significantly higher than that of native woody species. Another dominant line of thinking has identified life histories of plant species as the key feature enabling invasive qualities and behaviors. Reichard and Hamilton (1997) investigated the role that various life history qualities and facts, ranging different physical mechanics and seasonality factors, can have on plant invasiveness. Taking into account biological and life-history qualities led the authors to approximately 80% predictive accuracy in identifying invasive plants from a selected pool, illustrating the valid role these assessments can play in determining invasive qualities.

Look to the Land

Widrlechner and Iles use geographic clues and trends to explore potentially predictive connections between analogous climates and greater invasiveness by nonnative plants in the Midwest. Motivated by the absence of detailed invasive criteria, the authors set up their research to establish geographic risk assessment as an effective tool in predicting relative invasiveness in plants. Their study focused specifically on naturalized and naturalizing woody invasive species in Iowa, specifically investigating patterns of naturalization and geographic origin of these nonnative woody plants. The authors compiled data comparing the geographic origins of a species with its relative success in naturalizing to Iowa, creating a map highlighting regions that contribute greater proportions of successfully naturalizing woody species. Species sourced from northeastern China and southeastern Europe were especially adept in naturalizing to Iowa’s environment, leading the authors to caution against importation of nonnative woody species from these regions. Their high naturalization success points towards potential in those plants to evolve invasive behaviors when introduced to the Iowa environment.

 Widrlechner and Iles’ take a novel approach to invasive species management by tackling the problem from a preventative stance, predicting and fending off invasions before they occur instead of looking to limiting invasive damage once an invasion has taken root. However, their techniques in creating predictive criteria are undermined by a weak data set. The correlations between geographic origins and invasive potential can illustrate coarse trends in invasive behaviors, but lack sufficient detail and quantified analysis to decidedly contribute to invasive risk analysis. The broad categorization of relative risk, as outlined by Widrlechner and Iles, based on proportional correlations of geographic and climatic similarity seems to counteract the goals they set for themselves in pursuing this project; that is, identifying specific criteria for predicting invasiveness in plants. The sample sizes used by the authors is not particularly exhaustive either, with a selection of only 100 nonnative species covering the global spectrum of origin.

Predicting and Preventing

Despite its flaws, the concept suggested by Widrlechner and Iles is sound and would stand to reduce accidental introduction of aggressive invasive species through the horticultural industry, which has its legacy in creating invasive species problems. Using the concept of analyzing geographic origin and climatic similarities when profiling nonnative plant species has significant potential as a resource for horticulturalists and other parties to remain informed of risks in nonnative plant introduction. A database combining different predictive factors of plants to create a more comprehensive predictive criteria for nonnative plants, woody and otherwise, would benefit the horticulture field as well as other related parties and individuals. The more preventative measures and awareness of risks that can be installed on both global and local scales, the more effective our defenses against invading species will be. 

                Figure 1: Suggested map combining different factors to create cohesive and comprehensive                 predictive criteria for invasive potential in nonnative plants.

Safety In Numbers? Not Likely in Plant Communities

The phrase ‘safety in numbers’ is familiar to you. Almost all of us have heard some variation of this idiom repeated in a biologically oriented course. All things considered, this idea often holds true in nature. In a school fish, a large body of individuals is used to essentially buffer predation to promote individual survival within the bigger group. However, a recent study conducted by Kathryn Yurkonis of the Biology Department at the University of North Dakota and her colleagues revealed that large patches of the same plant species might facilitate invasion in perennial grasslands. Their findings require us to rethink current invasion ecology and its possible effects on prairie conservation projects in Iowa.

What distinguishes a perennial grassland system?

         Perennial grasslands are usually characterized by a blended abundance of grasses and flowering plants called forbs. Historically, these prairies have contained extremely fertile soils in part due to the frequency of fires. These fires also maintained a high level of biodiversity by promoting disturbances that allowed for a variety of species to cohabit a general area. Since the soil is so rich, much of this land has been taken over by agricultural for commercial purposes. According to the Prairie Iowa Network, less than 0.1% original prairie exists in remnant patches across Iowa. Thus it is crucial that we maintain these current historic sites appropriately in conjunction with implementation of restoration projects. Fortunately, Yurkonis et al. provide novel insight as to how these projects can be initially set up to increase invasion resistance. Like stated previously, it all has to do with how species are arranged within a community assemblage.

How can plant communities differ within perennial systems?

Ecological communities can differ in two key ways: species richness, as determined by the number of species within the area; and species evenness, which is dependent upon the abundance of a species in that same area. Effectually, both species richness and evenness affect diversity maintenance. Yurkonis et al. flesh this idea out and stress how arrangement, which is driven by richness and evenness, is the ultimate driving force behind invasion resistance.

Arrangement is usually driven by a combination of external and internal factors. Examples of each may include the varying levels of resource availability amongst patches (called resource heterogeneity) and local competition strategies, respectively (Bolker & Pacala 1997). The subsequent community arrangement determines the invasibility of the area; the term invasibility emerged in the field of invasion ecology to describe the susceptibility of environments to invasion by species from other regions of the world (Davis et al. 2005). Yurkonis et al. revealed how initial species patterns affect invasion resistance, and specifically how large conspecific (that is, same species) patches encourage invasion.

Fig. 1. A comparison of invasion resistance between large conspecific patches and smaller ones. Yurkonis et al. have shown that enlarging the size of same species patches actually increases the abundance of invasive species within that plot.

The Study: Big is Bad!

          Little research has been done to characterize the effect of species patterns on competitive interactions between neighbors in perennial grasslands. That is why the work presented by Yurkonis et al. remains exciting and relevant to prairie restoration and conservation in present-day Iowa. The researchers planted community assemblages that varied in size of conspecific patches, starting with smaller patches that gradually grew in magnitude. All groups were planted at the Iowa State University Horticultural Research Station in Ames, Iowa. This ensured that soil and weather conditions were constant for each assemblage and thus any change in resistance could be attributed to differences in arrangement. Figure 1 (above) demonstrates a visual representation of their findings.To reiterate, the researchers found that invader abundance was greater in plots that were initially planted in large conspecific patches; that is, big patches are bad at keep invading species at bay!

Lessons Learned: How new approaches can aid prairie management projects

These findings do not necessarily reflect what someone might expect to occur, but it suggests a new way to think about prairie dynamics extremely relevant to current restoration and reconstruction efforts. The implications of these sorts of results are grand, and careful consideration should be taken when undergoing these sorts of ventures. It would make no sense to focus on planting large collections of the same plant species within a plot of land set aside for reconstruction, as the community would soon be degraded by invasive species. In order to sufficiently maintain historical prairie, it remains ever important that studies like the one carried out by Yurkonis et al. continue to characterize population dynamics and spatial arrangements.

Perhaps future conservation projects can aim to illustrate the relationship outlined by Yurkonis and her colleagues on a larger, more realistic scale. In doing so, we can only expect to learn more about how to adequately approach initial steps in addition to processes of maintenance required in these sorts of projects.

References

Bolker, B. & Pacala, S.W. “Using moment equations to understand

           stochastically driven spatial pattern formation in ecological systems.”

          Theoretical Population Biology 52 (1997): 179–197.

Davis, Mark A., Ken Thompson, and J. Philip Grime. “Invasiblity: the local

          mechanism driving community assembly and species diversity.”

          Ecography 28.5 (2005): 696-704.

Yurkonis, Kathryn A., Brian J. Wilsey, and Kirk A. Moloney. “Initial species

pattern affects invasion resistance in experimental grassland plots.” Ed.

Amy Symstad. Journal of Vegetation Science (2011).

Measuring restoration success in Iowa’s tallgrass prairies

In recent decades, expansion of agriculture and other human activities has jeopardized Iowa’s native tallgrass prairie habitats, which historically dominated the local landscape. At present, upwards of 90% of the North American land area once covered in tallgrass prairie has since been cultivated (Sampson & Knopf, 1994).

It is no surprise, then, that researchers have awarded much attention to the conservation and restoration of these habitats throughout the state. In a 2005 article in the Journal of Applied Ecology, Dr. Leanne Martin and colleagues explored this issue in depth. The study set out to understand ecological discrepancies between remnant prairies (those having never been cultivated or otherwise anthropogenically manipulated) and reconstructed or restored prairies (in which the natural character of the landscape had been reconciled and native communities re-introduced) using measurements of species diversity as key factors for comparison (Martin et al., 2005).

The authors note from the outset that species diversity can be measured as “evenness” or as “richness,” the former signifying abundance and distribution within species and the latter defining the number of species present in a given area. With regard to these diversity parameters, the study touches on several provocative observations present in the literature. Among the most notable of these is that “restoring spatial components of diversity is rarely recognized as a goal in restoration, even though it is an integral component of ecological systems.” Martin and her colleagues suggest that presently the rehabilitation of community structure and ecosystem process rates (i.e. nutrient transport and primary productivity) are the dominating objectives in restoration efforts. While telling and valuable, these measures of restoration success have thus far limited the potential for quantitative evaluation of success in restoration.

Given this gap between the implementation and assessment of restored habitat, the researchers set out to establish a set of measurable parameters that may be used to gauge restoration success in terms of spatial species diversity—a feature distinguishing this study from others of its kind. These parameters, stemming from several important ecological attributes, include 1) the proportion of native species, 2) ecosystem processes, 3) plant diversity at all spatial scales, and 4) animal and microbial diversity at all spatial scales.

The study zeroed in on the first three of these metrics of restoration success in the context of the largest tallgrass prairie restoration in the United States, located within southern Iowa’s Neal Smith National Wildlife Refuge. Using 400 cm² quadrats, the researchers collected above ground biomass and surface litter from eight prairie plantings at the Neal Smith Refuge during three periods (autumn, early summer, late summer)—a distinction that allowed for sampling of early- and late-growing species. Also sampled, for comparison, were three remnant prairie sites selected for their nearby location and geological similarities to the restored sites.

To facilitate meaningful comparisons, the researchers quantified spatial diversity at the “neighborhood-scale” (quadrat-to-quadrat) and the larger “prairie-scale.” Consistent with the researchers’ expectations and with the conclusions of other studies of this type, restored sites at the neighborhood-scale had substantially lower diversity and richness relative to the remnant sites. In other words, any given plot of a restored prairie contained fewer species overall than plots sampled at a remnant prairie site. Contrarily, the species evenness in the restored sites at the neighborhood-scale was not significantly different from that in remnant sites—an observation that Martin associates with grazing in the restored sites, which has been shown to increase species evenness (McNaughton, 1979).

At the prairie-scale, however, the researchers’ findings were contrary to their expectations: the proportion of species diversity and richness at this level was substantially greater in restored sites than in remnant sites. As the authors suggest, this is likely because while restored sites had fewer species in any one quadrat than in remnant sites, it was often the case that the dominant species changed more frequently between quadrats in restored sites.

So what do these findings mean for the future of tallgrass prairie management? Martin et al. suggest that in order to restore tallgrass prairies effectively to their “remnant” condition, management regimes should put especial emphasis on maintaining high species richness at the neighborhood-scale. This can be accomplished, as the authors recommend, by focusing on “local-scale restoration methodologies,” such as mowing, that allow for maintained density and richness in the rehabilitation process.

References

Martin, L.M., Moloney, K.A., and Wilsey, B.J. 2005. An assessment of grassland restoration success using species diversity components. Journal of Applied Ecology, 42. pp 327-336.

McNaughton, S.J. 1979. Grassland-herbivore dynamics Serengeti, Dynamics of an Ecosystem (eds A.R.E. Sinclair & M. Norton-Griffiths), pp 46-81. University of Chicago Press.

Sampson, F. and Knopf, F. 1994. Prairie Conservation in North America. Bioscience, 44. pp 418-421.

Reconstructing the Past

Many modern conservation efforts are focused on returning the landscape back to its original state before the settlement of pioneers, yet even the rolling prairie that once covered most of Iowa shows only a sliver of the land’s biological history. The contemporary flora and fauna communities of prairie and cropland look very different from those of nearly 20,000 ago. The Late Wisconsinan glacial period lasted roughly from 21,000 to 15,000 years ago and drastically changed Iowa’s landscape through complete ice cover for thousands of years. While plants and animals exist and evolve together in certain locations, Baker et al.’s 1986 article from the Journal of Quaternary Science contends that these communities shift over time. Richard Baker and his colleagues at the University of Iowa worked together with researchers from Washington, North Dakota, and Minnesota to study the biological history of the Conklin Quarry in southeast Iowa to shed light on the ancient Iowan landscape. Their work compares ancient flora and fauna communities to those of the modern day in a study which may suggest a new way of approaching conservation. 

Conklin Quarry: a Picture of the Past

Conklin Quarry in Johnson Co., Iowa, contains a large collection of fossils from 18,000-16,000 year ago of a diverse array of organisms. These fossils include pollen, plants, insects, and small-mammals that were preserved in the deposits. While most sites that had been covered in ice, or full-glacial sites, have some of these fossil types, very few locations match the sheer breadth of samples from each group that Conklin Quarry has. This site’s rich selection of fossils represents the environmental spectrum during the Late Wisconsinan glacial period, a range of dates which is normally rare in fossils sites. In addition to the date range, the quarry has arctic plant remains found 1000km south of the nearest reported Wisconsinan arctic floras. This unique site provides important evidence for understanding both the glacial and postglacial development of flora and fauna today. 

Plant Fossils

In order to map flora communities of 18,000-16,000 years ago, Baker and his lab studied pollen and plant macrofossils. The pollen samples found were mostly degraded, although pine, spruce, and sedge species could be clearly identified. The samples found in Iowa from the Wisconsinan period contained low pollen concentrations, which match low pollen concentrations found in tundra and tree-line areas in eastern Canada today. Spruce and dwarf birch macrofossils were found in abundance, which are native boreal forest species and extend to the northern tree limit today. Many of the less common macrofossils represent arctic-alpine plants found today only in the tundra. Several species found normally grow along water, suggesting a pond was present at one time. Both the pollen and plant macrofossils match species found in eastern Canada presently, suggesting open conditions between the forest limit and the limit of tree species in a tundra environment. 

Insect and Vertebrate Fossils

The remains of the insects found were remarkably well preserved, showing species that could occur today within or near forest/tundra transition zone. Close analogues to the insect communities at the Conklin Quarry only exist today in tree-line environments where temperatures average 10-12°C in July (compared to 23°C Iowa today). The taxa of vertebrate macrofossils are found presently, like the insects, only in tundra environments. Some fossils, such as the singing vole, are found today only near running water, which suggests an ephemeral steam or a wet environment. 

 

Fig.1: Map of North America (1986). All flora and fauna populations represent 1986 conifer and insect populations found in Conklin Quarry site.

Communities Today

While both the plant and animal fossils represent species presently found in tundra, Baker et al. found that these communities shifted over time. The insect and animal species found in the quarry are present today in northern coastal regions in Canada and Alaska, while the plant species (conifers) grow primarily in a large swath across Canada (Fig.1). Between 18,000-16,000 years ago and now, the analogue communities, or flora and fauna that evolve and develop together, have changed. Baker et al.’s research shows that the evolution of communities is dynamic, and that flora and fauna populations dissociated from one another as glaciers retreated. These results suggest that biological communities can evolve, disassociate, and even disappear over time. Instead of focusing just on certain ecosystems, conservation efforts might find understanding the community evolution beneficial. If the relationships between various species change and even disappear over time, perhaps the aim to restore prairies to match their relic counterparts should be reexamined.

Baker, R.B., R. Sanders Rhodes II, T.J. Fresst, G.R. Hallberg, J.A. Janssens. 1986. “A full-glacial biota from southeastern Iowa USA.” Journal of Quaternary Science 1 (2) 91-107.