Gregory R. Houseman

PERTURBATIONS ALTER COMMUNITY CONVERGENCE, DIVERGENCE, AND FORMATION OF MULTIPLE COMMUNITY STATES

Environmental perturbations (e.g., disturbance, fertilization) commonly shift communities to a new mean state, but much less is known about their effects on the variability (dispersion) of communities around the mean, particularly when perturbations are combined. Community dispersion may increase or decrease (representing a divergence or convergence among communities) if changing environmental conditions alter species interactions or magnify small initial differences that develop during community assembly. We used data from an experimental study of disturbance and fertilization in a low-productivity grassland to test how these two perturbations affect patterns of species composition and abundance. We found that a one-time biomass reduction decreased community dispersion, which persisted over four growing seasons. Conversely, continuous fertilization increased community dispersion and, when combined with disturbance, led to the formation of three distinct community states. These results illustrate that perturbations can have differing effects on community dispersion. Attention to the variance in community responses to perturbations lends insight into how ecological interactions determine community structure, which may be missed when focusing only on mean responses. Furthermore, multiple perturbations may have complex effects on community dispersion, yielding convergence or divergence patterns that are difficult to predict based on analysis of single factors.

Conyza canadensis suppresses plant diversity in its nonnative ranges but not at home: a transcontinental comparison.

The impact of invasive species across their native and nonnative ranges is poorly quantified and this impedes a complete understanding of biological invasions. We compared the impact of the native North American plant, Conyza canadensis, which is invasive to Eurasia, on species richness at home and in a number of introduced regions through well replicated transcontinental field studies, glasshouse experiments and individual-based models. Our results demonstrated mostly negative relationships between C. canadensis abundance and native species richness in nonnative ranges, but either positive or no relationships in its native North American range. In glasshouse experiments, the total biomass of Conyza was suppressed more by species from its native range than by species from regions where it is nonnative, but the effects of Conyza on other species did not show a consistent biogeographical pattern. Finally, individual-based models led to the exclusion of Conyza from North American scenarios but to high abundances in scenarios with species from the nonnative ranges of Conyza. We illustrate biogeographical differences in the impact of an invader across regional scales and suggest that inherent differences in one specific aspect of competitive ability, tolerance to the effects of other species, may play some role in these differences.

A framework for quantifying the magnitude and variability of community responses to global change drivers

A major challenge in global change ecology is to predict the trajectory and magnitude of community change in response to global change drivers (GCDs). Here, we present a new framework that not only increases the predictive power of individual studies, but also allows for synthesis across GCD studies and ecosystems. First, we suggest that by quantifying community dissimilarity of replicates both among and within treatments, we can infer both the magnitude and predictability of community change, respectively. Second, we demonstrate the utility of integrating rank abundance curves with measures of community dissimilarity to understand the species-level dynamics driving community changes and propose a series of testable hypotheses linking changes in rank abundance curves with shifts in community dissimilarity. Finally, we review six case studies that demonstrate how our new conceptual framework can be applied. Overall, we present a new framework for holistically predicting community responses to GCDs that has broad applicability in this era of unprecedented global change and novel environmental conditions.

Propagule pressure-invasibility relationships: testing the influence of soil fertility and disturbance with Lespedeza cuneata

Although invasion risk is expected to increase with propagule pressure (PP), it is unclear whether PP-invasibility relationships follow an asymptotic or some other non-linear form and whether such relationships vary with underlying environmental conditions. Using manipulations of PP, soil fertility and disturbance, we tested how each influence PP-invasibility relationships for Lespedeza cuneata in a Kansas grassland and use recruitment curve models to determine how safe sites may contribute to plant invasions. After three growing seasons, we found that the PP-invasibility relationships best fit an asymptotic model of invasion reflecting a combination of density-independent and density-dependent processes and that seeds were aggregated within the plant community despite efforts to uniformly sow seeds. Consistent with some models, community invasibility decreased with enhanced soil fertility or reduced levels of disturbance in response to changes in the fraction of safe sites. Our results illustrate that disturbance and soil fertility can be a useful organizing principle for predicting community invasibility, asymptotic models are a reasonable starting point for modeling invasion, and new modeling techniques—coupled with classic experimental approaches—can enhance our understanding of the invasion process.

Targeting Vulnerable Life-Stages of Sericea Lespedeza (Lespedeza cuneata) with Prescribed Burns

Abstract There is growing interest in whether invasive species may be controlled by utilizing management strategies that target vulnerable life stages. We manipulated the timing of fire and measured its effects on sericea lespedeza germination and seedling survival. Although fire strongly decreased germination in the laboratory, fire increased germination under field conditions. Additionally, fire caused small decreases in seedling survival in the field. Therefore, controlled burns are likely to encourage spread of sericea lespedeza and are unlikely to effectively control this invasive species. Although targeting vulnerable life stages is a promising strategy for invasive species control, our results illustrate that system-specific studies may be needed to unravel potentially complex interactions between biotic and abiotic factors before effective control strategies can be devised. Nomenclature: Sericea lespedeza, Lespedeza cuneata (Dum. Cours.) G. Don Management Implications: Sericea lespedeza is a noxious invader capable of suppressing native species in grasslands and prairies in the Great Plains. Currently, repeated herbicide application is the only effective method of controlling sericea lespedeza; however, herbicides are cost prohibitive for many land managers. Fire has been considered as a potential management tool for controlling this weed but it is not currently recommended because adult plants may resprout. However, controlled burns might be useful to control recolonization from seed after adult plants have been killed with herbicides. Our results show that burning seedlings in the growing season immediately following germination slightly reduces sericea lespedeza survivorship but creates a large flush of germination that more than offsets this reduction in survivorship. While it remains possible that other burning treatments may reduce the spread of this noxious weed, the treatments we tested suggest that fire will encourage sericea lespedeza invasion.

Asynchrony among local communities stabilises ecosystem function of metacommunities

Abstract Temporal stability of ecosystem functioning increases the predictability and reliability of ecosystem services, and understanding the drivers of stability across spatial scales is important for land management and policy decisions. We used species‐level abundance data from 62 plant communities across five continents to assess mechanisms of temporal stability across spatial scales. We assessed how asynchrony (i.e. different units responding dissimilarly through time) of species and local communities stabilised metacommunity ecosystem function. Asynchrony of species increased stability of local communities, and asynchrony among local communities enhanced metacommunity stability by a wide range of magnitudes (1–315%); this range was positively correlated with the size of the metacommunity. Additionally, asynchronous responses among local communities were linked with species’ populations fluctuating asynchronously across space, perhaps stemming from physical and/or competitive differences among local communities. Accordingly, we suggest spatial heterogeneity should be a major focus for maintaining the stability of ecosystem services at larger spatial scales.

Does tallgrass prairie restoration enhance the invasion resistance of post-agricultural lands?

AbstractThere is building interest in the use of ecological restoration to enhance the biotic invasion resistance of disturbed lands. However, few studies have rigorously examined the effect of community restoration on biotic invasion resistance under conditions of controlled invader propagule pressure. Results are presented from a field experiment conducted in a post-agricultural grassland in eastern Kansas to explore the interplay of tallgrass prairie restoration and invader propagule pressure in modulating plant invasion. Seed additions of multiple native and non-native species were used to provide a general test of biotic invasion resistance under varied propagule availability. Restoration increased plant diversity, increased above ground productivity, reduced the availability of light, soil moisture and bare soil microsites and strongly suppressed the invasion of all species sown into the experiment, including the highly invasive exotic legume, Lespedeza cuneata. In the absence of restoration, L. cuneata rapidly dominated plots where it had been sown, particularly at the highest propagule pressure. Results of multiple regression modelling suggested that restoration most likely increased community resistance to L. cuneata invasion through changes in functional guild composition rather than through changes in species diversity. Overall our study indicates that restoration of abandoned agricultural land to native tallgrass prairie can enhance invasion resistance in the face of substantial invader propagule pressures by altering community composition to dominance by native species that are efficient in utilizing resources.

Intraspecific seed interactions alter seedling emergence of Lespedeza cuneata under field conditions

There is growing recognition that seeds influence one another prior to, or shortly after, germination. Both interspecific and intraspecific seed–seed interactions have been reported, but for the latter, the evidence is almost exclusively from laboratory and greenhouse studies so that it is unclear whether such intraspecific seed interactions occur under field conditions. We tested how Lespedeza cuneata seed density influenced seedling emergence within a native grassland located in Kansas, USA and whether this response was related to seed leachate. The percentage of L. cuneata seedlings that emerged under field conditions was negatively related to seed density. After confirming this pattern in the greenhouse, we found that at low seed densities, addition of seed leachate had similar effects on seedling emergence as high seed densities in the absence of leachate. Our results provide some of the best evidence to date, that intraspecific seed density can influence seedling emergence in the field and that these effects are likely linked to seed leachate.

The Wichita State University Biological Field Station: Bringing Breadth to Research Along the Precipitation Gradient in Kansas

Biological field stations are expected to have an increasingly important role in ecological research because of mounting environmental concerns and the growing importance of ecological research networks. Kansas has several well-known field stations that occur along an east-west precipitation gradient, but these sites are primarily located in the northern half of the state. In this paper, we describe how the sites comprising the Wichita State University Biological Field Station represent important ecoregions in the southern portion of Kansas, which could be utilized to enhance research networks in the region. For each site, we describe the relevant historical and environmental characteristics and provide some examples of the long-term research projects that are currently underway.

Data and code from: Asynchrony among local communities stabilizes ecosystem function of metacommunities

R code, derived metrics, and limited metadata associated with Wilcox et al. (2017). Asynchrony among local communities stabilizes ecosystem function of metacommunities. Ecology Letters. When using this data or code, please cite the original publication: Wilcox, K.R., A.T. Tredennick, S. Koerner, E. Grman, L. Hallett, M. Avolio, K. La Pierre, G. Houseman, F. Isbell, D. Johnson, J. Alatalo, A. Baldwin, E. Bork, E. Boughton, W. Bowman, A. Britton, J. Cahill, S. Collins, G-Z. Du, A. Eskelinen, L. Gough, A. Jentsch, C. Kern, K. Klanderud, A. Knapp, J. Kreyling, Y. Luo, J. McLaren, P. Megonigal, V. Onipchenko, J. Prevey, J. Price, C. Robinson, O. Sala, M. Smith, N. Soudzilovskaia, L. Souza, D. Tilman, S. White, Z. Xu, L. Yahdjian, Q. Yu, P. Zhang, Y, Zhang. (2017). Asynchrony among local communities stabilizes ecosystem function of metacommunities. Ecology Letters vol(iss):xx-xx. Additionally, please cite the Figshare file set: Wilcox, K.R., A.T. Tredennick, S. Koerner, E. Grman, L. Hallett, M. Avolio, K. La Pierre. (2017). Data and code from: Asynchrony among local communities stabilizes ecosystem function of metacommunities. Figshare. https://dx.doi.org/10.6084/m9.figshare.5384167. R Code The analysis proceeds in several steps, which can be viewed most easily by examining the ~/Wilcox_etal_DerivedData_and_Code/analysis_scripts/main_text_scripts/patches_source_all_scripts.R file. Questions about the code or analysis should be directed to Kevin Wilcox (wilcoxkr@gmail.com) or Andrew Tredennick (atredenn@gmail.com). Derived Data We provide the full set of metrics (e.g., alpha, beta, and gamma stability and diversity) for each of our study sites. The main analysis and all figures in the paper can be reproduced using these metrics. Metrics were calculated from time series of abundance data from 62 grassland sites around the globe, although primarily from North America and Europe. The data is part of the CoRRE Data Base (http://corredata.weebly.com/), and those interested in using proprietary data not included in this fileset are encouraged to contact the CoRRE data base maintainers (http://corredata.weebly.com/contact.html).