Fighting for resources – a native forb competitively excludes a closely related introduced species

Do niche or fitness differences prevent local-scale long-term coexistence between two closely related annual plant species?

Introduced plant species are pervasive features of contemporary ecosystems that can negatively impact biodiversity and ecosystem function. Despite the occurrence of introduced species in many natural systems, a firm understanding of the mechanisms that underlie invasion success is still lacking. Introduced species are expected to be more successful when they either possess functional differences that reduce competition with native species for limiting resources—i.e. stabilizing niche differences—or when they possess functional differences that provide a relative competitive advantage for acquiring limiting resources over native species—i.e. average fitness differences. Niche and fitness differences simultaneously influence the impact that introduced species have on native species as they invade a system.

Competing plants
Native Plectritis congesta (pink) competitively excludes introduced Valerianella locusta (white)in the oak savanna habitat remnants where these species co-occur. Image credit: J. WIlliams.

In their new study published in AoBP, Johnson & Williams investigated whether niche and/or fitness differences prevent local-scale long-term coexistence between two closely related annual plant species. They observed that the native Plectritis congesta and the introduced Valerianella locusta co-occur at community scales but rarely at the scale of direct interactions in the Garry oak savannah ecosystem of south-western British Columbia, Canada. The authors parameterized models of competitor dynamics to quantify niche and average fitness differences between these species with results from an experiment where they manipulated competitor densities and water availability. Their model, corroborated with field data, predicts that P. congesta competitively excludes V. locusta in direct competition across water availability conditions. In quantifying this outcome, the study demonstrates mechanistically how a native species may limit abundance of an introduced invader. It also emphasizes that coexistence mechanisms operate at scales beyond direct interaction and are necessary to maintaining much of the species diversity observed at the community scale.

Researcher highlight

Jens Johnson grew up in the United States and in 2016 moved to Canada to conduct a MSc in Geography at The University of British Columbia with Dr. Jennifer Williams. He is currently a PhD student in the Plant Science program at The University of British Columbia with Dr. Risa Sargent.

The work included in this article was conducted as a central component of Jens’ MSc thesis. During this time, he focused on exploring the effects of species interactions on plant population dynamics in threatened oak savanna habitat remnants on Vancouver Island, British Columbia. At this current point in his academic career, Jens is interested in using a combination of experimental manipulation and observational field data to disentangle effects of land use and local factors (including species interactions) on wild bee populations and on pollination.

William Salter

William (Tam) Salter is a Postdoctoral Research Fellow in the School of Life and Environmental Sciences and Sydney Institute of Agriculture at the University of Sydney. He has a bachelor degree in Ecological Science (Hons) from the University of Edinburgh and a PhD in plant ecophysiology from the University of Sydney. Tam is interested in the identification and elucidation of plant traits that could be useful for ecosystem resilience and future food security under global environmental change. He is also very interested in effective scientific communication.

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