The Alpine Cushion Plant Silene acaulis as Foundation Species: A Bug’s-Eye View to Facilitation and Microclimate

Lortie CJ (2012) The Alpine Cushion Plant Silene acaulis as Foundation Species: A Bug's-Eye View to Facilitation and Microclimate. PLoS ONE 7(5): e37223. doi:10.1371/journal.pone.0037223 The Alpine Cushion Plant Silene acaulis as Foundation Species: A Bug's-Eye View to Facilitation and Microclimate Olivia Molenda 0 Anya Reid 0 Christopher J. Lortie 0 Christian Rixen, WSL Institute for Snow and Avalanche Research SLF, Switzerland 0 1 Ecology and Evolutionary Biology, University of Toronto , Toronto, Ontario , Canada , 2 Department of Biology, York University , Toronto, Ontario , Canada Alpine ecosystems are important globally with high levels of endemic and rare species. Given that they will be highly impacted by climate change, understanding biotic factors that maintain diversity is critical. Silene acaulis is a common alpine nurse plant shown to positively influence the diversity and abundance of organisms-predominantly other plant species. The hypothesis that cushion or nurse plants in general are important to multiple trophic levels has been proposed but rarely tested. Alpine arthropod diversity is also largely understudied worldwide, and the plant-arthropod interactions reported are mostly negative, that is, herbivory. Plant and arthropod diversity and abundance were sampled on S. acaulis and at paired adjacent microsites with other non-cushion forming vegetation present on Whistler Mountain, B.C., Canada to examine the relative trophic effects of cushion plants. Plant species richness and abundance but not Simpson's diversity index was higher on cushion microsites relative to other vegetation. Arthropod richness, abundance, and diversity were all higher on cushion microsites relative to other vegetated sites. On a microclimatic scale, S. acaulis ameliorated stressful conditions for plants and invertebrates living inside it, but the highest levels of arthropod diversity were observed on cushions with tall plant growth. Hence, alpine cushion plants can be foundation species not only for other plant species but other trophic levels, and these impacts are expressed through both direct and indirect effects associated with altered environmental conditions and localized productivity. Whilst this case study tests a limited subset of the membership of alpine animal communities, it clearly demonstrates that cushion-forming plant species are an important consideration in understanding resilience to global changes for many organisms in addition to other plants. - Funding: This research was funded by a Natural Sciences and Engineering Research Council of Canada Discovery Grant to CJL and a Natural Sciences and Engineering Research Council of Canada Pollination Initiative grant to CJL. AR was funded by an Ontario Graduate Scholarship. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Facilitation, or positive interactions between organisms that benefit at least one species and are harmful to neither, is relatively common in most plant communities [1,2] and frequent in stressful climates [3,4]. Processes such as facilitation integral to community assembly are important to consider in light of current ecological issues such as global change, biodiversity, and ecosystem sustainability because biotic interactions may change their impacts. In order to better understand community assembly, a critical assessment of the scope of facilitation is thus needed particularly in harsh environments [4]. Positive interactions in general have significant impacts on community organization, dynamics, and productivity [5], but the major advances to date in the facilitation literature have been primarily focused on plantplant interactions and within a given trophic level [4,6]. Plantinvertebrate facilitation studies are extremely rare; existing studies can be categorized as plant-pollinator or plant-ant facilitation and both categories are well established in a variety of climates [7,8,9,10,11,12,13]. For instance, in the journal Arthropod-Plant Interactions, 65% of all articles published described negative impacts of arthropods on plants such as parasitism and herbivory, 24% focused on pollinators, 10% dealt with global concerns such as diversity, climate change, and technological advances, whilst only 1% of all articles published described positive interactions between arthropods and plants (inspection of all abstracts published in this journal up to 2011). A significant component of ecological interactions are thus being overlooked. Communities in an ecological-change context are comprised of plants, microbes, and invertebrates. It is thus critical for the field of facilitation to incorporate other trophic levels. To do so, it must encompass interactions at some of these additional trophic levels and explore whether facilitation is relevant to community assembly and arthropod-plant interactions. We propose that the logical first step in developing novel implications to these theories is to identify and document the positive interactions between taxa including more than one trophic level. In the alpine, facilitation frequently occurs in the form of nurse plants that modify the environment by reducing physical stress or disturbance thereby allowing less tolerant plant species to survive [4,14,15,16,17,18]. Nurse plants frequently increase plant species richness [19] but not always [20], and cushion plants are likely the dominant form for nurse plant species in the alpine [21]. The structure of their canopy is genetically determined and grows as a dense dome that traps heat, moisture, and nutrients providing them with the ability to moderate harsh alpine conditions because it minimizes the negative effects of wind and low temperatures [22]. As a result, cushion plants are commonly classified as ecosystem engineers in the alpine [5,19,21,23]. With increasing habitat loss due to climate change, cushion plants can thus be a critical first step in assessing the responsiveness of a community to change. Cushions consistently increase species richness at the entire plant community level and can similarly increase biomass [24,25]. Less frequently tested, cushion plants can also positively influence other taxa such as ladybird abundance in the Andes [26,27]. As alpine surfaces are released from glaciations, arthropod predators invade and depend on invertebrates that arrive with upward winds [28]. Other invertebrates can only colonize once a plant system is established; as a result, alpine invertebrate communities are determined by the structure of local plant communities [10], and since cushions are fundamental to enhancing plant diversity, it is reasonable to assume that these effects scale up to other trophic levels. Cushion plants are thus the ideal set of species to explore the relative importance of positive plant-arthropod interactions on (...truncated)