In the quest of designing grassland communities resistant to invasions during ecological restoration

The post provided by Florencia A. Yannelli

Controlled conditions in greenhouse experiments enable us to test a plethora of hypotheses in community or invasion ecology by reducing the effect confounding factors. Photo credit: Florencia Yannelli.

This post refers to the article Seed density is more effective than multi‐trait limiting similarity in controlling grassland resistance against plant invasions in mesocosms by Yannelli et al. published in Applied Vegetation Science: https://onlinelibrary.wiley.com/doi/abs/10.1111/avsc.12373

Semi-natural grasslands are biodiversity hotspots in Central Europe but they are highly threatened by land-use change and impacts of human activity. This urgency calls for a usage of any ‘available’ area (e.g. roadsides, abandoned mines or set-aside fields) for grassland restoration and for increasing the connectivity between remaining patches. Yet, these areas are exposed to frequent disturbance and are susceptible to the colonization by invasive alien plant species. In this context, the focus of our study was to design seed mixtures that suppress invasive species during grassland restoration due to a high similarity of native plants in the community and the invader. Following the limiting similarity hypothesis, we expected that invasive species will be unsuccessful to establish in communities where native plants share similar niches with the invader, as judged by species functional traits.

This greenhouse experiment was related to my PhD project but also devised as an exercise for the Training School ‘Controlling common ragweed by vegetation management’ within the COST-Action SMARTER (Sustainable management of Ambrosia artemisiifolia in Europe). We established the experiment before the summer school but collected all data and discussed the preliminary results together with the participants. The idea was to create realistic communities, so we used grassland species that commonly occur in our study area, southern Germany, and two target invasive species that are present in urban disturbed areas, i.e. common ragweed (Ambrosia artemisiifolia) and giant goldenrod (Solidago gigantea). We depicted the niche of all study species based on ten functional traits related to establishment success, persistence and competitive ability, and extracted these properties from two trait databases. Using trait information on native and invasive species, we designed specific seed mixtures to suppress each invasive species. We created a greenhouse experiment to test the resistance of two communities in response to the two invasive species, respectively, and set up a control that consisted of invasive species monocultures. This procedure allowed us to check for more general patterns explaining the potential biotic resistance of the two native communities.

Participants of the Training School ‘Controlling common ragweed by vegetation management’ part of the COST-Action SMARTER performing final measurements in July 2015. Photo credit: Johannes Kollmann.

The design of resistant communities to target invasive species – making use of math!

The novelty of our paper’s method was the use of a system of linear equations to design the native plant communities to suppress invasive species, as suggested by Laughlin (2014). This offers the advantage of a multi-trait approach to maximize the similarity between the native community and the invasive species, but also of having communities composed of native species with different abundances. As a result, the two communities were mostly different in terms of species dominating the seed mixture and their seed size. That is, while the seed mixture designed for ragweed was dominated by more large-seeded species such as Centaurea scabiosa, the one for goldenrod was dominated by the smaller-seeded Achillea millefolium.

Is the limiting similarity hypothesis a good predictor of early stage biotic resistance in grassland communities?

We found that the community designed to be resistant against goldenrod invasion was actually the one that presented a higher suppression effect on both invasive species. Although this result could be interpreted as partial support for the limiting similarity hypothesis in the context of our experiment, we found that plant density was a better predictor of resistance (Fig. 1). This can be explained by the fact that most species included in the community designed for goldenrod had smaller seeds, which resulted in a high density of individuals and reflected in the measured leaf area index. This result is consistent with another study, where we also found that higher sowing density increases the chances of native species establishment, resulting in biotic resistance due to fewer resources for invading species (Yannelli et al. 2017a).

Another explanation for our results is related to the dominance of Achillea millefolium in the successful community. This species has a fast germination and shows an early growth form in form of multi-leaves rosettes that quickly covered the available space. Results from another study showed that this species is closely related to both invasive species, which suggests more functional similarity that might have not been captured by our trait selection (Yannelli et al. 2017b).

Fig. 1. Distinct decrease in the total aboveground biomass measured for the two invasive species ragweed and goldenrod (i.e. Ambrosia artemisiifolia and Solidago gigantea, respectively) when grown under competition with the community designed for goldenrod (SG), compared to the other community and the control. Figures from Yannelli et al. 2018; photo credit Florencia Yannelli.

Some limitations that could explain our lack of unequivocal support for the limiting similarity hypothesis are related to the pool of species included in the design of our communities and the use of traits obtained from databases. In practice, the species we choose to include for a restoration project might not be the most suitable in terms of matching similarity between the communities and the invasive species. Also while traits from databases are measured for adult plants, our experiment only covered the establishment phase and thus might not best portray resource acquisition or competitive ability during an early successional stage.

To sum up, we did not find clear support for the limiting similarity hypothesis in the context of our experiment. Instead, from our results at this early stage, increased seed density and fast vegetative growth seem to be better predictors of biotic resistance of grassland communities.

References

  • Laughlin, D.C. (2014). Applying trait-based models to achieve functional targets for theory-driven ecological restoration. Ecology Letters, 17, 771-784. https://doi.org/10.1111/ele.12288
  • Yannelli, F.A., Hughes, P., & Kollmann, J. (2017a). Preventing plant invasions at early stages of revegetation: the role of limiting similarity in seed size and seed density. Ecological Engineering, 100, 286-290. https://doi.org/10.1016/j.ecoleng.2016.12.001
  • Yannelli, F.A., Koch, C., Jeschke, J.M., & Kollmann, J. (2017b). Limiting similarity and Darwin’s naturalization hypothesis: understanding the drivers of biotic resistance against invasive plant species. Oecologia, 183, 775–784. https://doi.org/10.1007/s00442-016-3798-8

Florencia Yannelli is a community ecologist, with a strong interest in invasion and restoration ecology.  While this work was carried out during her doctoral studies at the Chair of Restoration ecology, at the Technical University of Munich, she is currently a postdoc at the Centre for Invasion Biology (C·I·B), Stellenbosch University.

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