Revisiting invasion on tussocks: Relating environmental variables to invasion Martin Genova1, Conall Gaffney2, Anson Pang3 1University of California, Santa Cruz, 2University of California, Berkeley 3University of California, Los Angeles ABSTRACT Classic invasion theory predicts that higher species diversity increases the resistance of a community to invasion by non-native species. However, studies that challenge this theory propose that environmental variables that covary with native diversity play an important role in determining invasibility in communities. One way that environmental variables can act to determine community invasibility is by facilitating propagule pressure of non-native species. Here, we investigate how environmental variables, through the mechanism of propagule pressure, directly act to influence the invasion success of non-native plant species on riparian tussock communities in the South Fork Eel River (Mendocino County, CA). We found that distance to upstream tussocks influenced the number of non-native species on tussocks, independent of native species richness. The results of this study support the idea that environmental variables interplay with native diversity to determine community invasibility. Keywords: invasion, tussocks, propagule pressure, biotic resistance, diversity/invasibility hypothesis INTRODUCTION invasion need to be investigated in order to develop optimal management strategies As humans have spread species beyond across different systems. their native range, the impact of their One influential theory that has predicted introductions have negative economic and the invasibility of plant communities is the ecological implications (Pejchar & Mooney diversity/invasibility hypothesis, first 2009). Understanding the factors that developed by Elton (1958). The hypothesis influence the establishment of invasive posits that more diverse plant communities species in natural communities can help are less likely to be invaded and established predict the eXtent of invasion in natural invaders will have reduced success. This systems (Fridley et al. 2007) This theory has been supported by theoretical understanding can better inform and mathematical models (Case 1990, management strategies that maintain MacArthur 1970), and microcosm field biological diversity and ecosystem functions eXperiments where native species diversity (Fridley et al. 2007, Levine et al. 2003). is eXperimentally manipulated (Knops et al Therefore, mechanisms that influence CEC Research | https://doi.org/10.21973/N3WM1Z Spring 2019 1/9 1999, Levine 2000, Symstad 2000). For strength of the diversity/invasibility eXample, the performance of an invasive hypothesis and its effect on shaping plant annual weed, narrowleaf hawksbeard communities (Levine 2000, Von Holle 2005). (Crepis tectorum), was found to be One mechanism influenced by negatively correlated with native species environmental variables that can determine richness in eXperimental Minnesota the success of invaders is propagule grassland plots (Naeem et al. 2000). These pressure (Colautti et al. 2006, Simberloff findings implicate that maintaining biodiversity 2009, Von Holle and Simberloff 2005). In in natural communities is necessary to the conteXt of plants, propagules can be reduce invader establishment and success. vegetative cones or seeds that can be While the diversity/invasibility hypothesis carried by biotic (e.g. herbivores) or abiotic has been well supported, the pertinence of (e.g. wind, water) vectors. Propagule the hypothesis to what actually occurs in pressure is a measurement that incorporates plant communities is debated among propagule size and the temporal and spatial ecologists (Tilman 1999, Enserink 1999). patterns of propagule arrival to predict the There have been several observational likelihood that a species establishes in a studies that have refuted the applicability of new community (Simberloff 2009). The the diversity/invasibility hypothesis to Theory of Island Biogeography (Macarthur natural communities, often finding invader & Wilson 1967) correlates propagule success to be positively correlated with pressure with environmental variables such native species richness (Keeley et al. 2003, as the size of an island and the distance to Planty-Tabacchi et al. 1995, Wiser et al. mainland to determine the rate at which 1998). However, critics of these species establish. The same general idea observational studies point out that the can be applied to plant invaders, in which mechanisms driving non-native success are environmental variables can influence the in reality environmental variables that likelihood that propagules will arrive in new covary with native species richness, making habitat patches (Simberloff 2009). it appear that diversity increases non-native However, the way that propagule pressure invasion success (Levine & D’Antonio 1999, acts to facilitate invasion success can vary Stohlgren 1999). Additionally, there has among ecosystems (e.g. Von Holle & been a growing body of eXperimental Simberloff 2005). Therefore, investigating evidence that environmental variables the environmental variables that can act to determine invader success more than increase propagule pressure in a given plant native species richness (Luo et al. 2018, system can help ecologists predict the Towers & Dwyer 2018, Von Holle 2005). For success of invasion. eXample, the abundance of a non-native In this study, our aim was to elucidate the invasive, tropical whiteweed (Ageratum environmental variables that affect the conyzoides), was found to be more strongly invasibility of tussocks on the South Fork Eel correlated with the organic matter and River in Northern California. Tussocks are nitrogen content in the soil than with native numerous and discrete units of habitat that species cover (Luo et al. 2018). With this in allow us to investigate how environmental mind, taking environmental variables into variables act to facilitate the establishment account is necessary to determine the of native and non-native species. Levine CEC Research | https://doi.org/10.21973/N3WM1Z Spring 2019 2/9 (2000) found that the number of non-native METHODS species on tussocks along a 7 km stretch of the South Fork Eel River increased with 2.1 Study System native species diversity at the community level (along the river). However, when We conducted our research on South Fork native species richness was manipulated at Eel River in Angelo Coast Range Reserve the neighborhood level (within tussocks), (Mendocino County, California) May 7–11, he found that the opposite was true. Levine 2019. The focus of our research was on (2000) suggested that the positive tussocks, which are small, grassy islands relationship between non-native and native formed in the river by torrent sedge (Carex species richness on tussocks at the nudata) (Levine 2000). Singular C. nudata community level could be due to physical are perennial plants that grow in clumps factors that covary with native species richness. and catch organic matter in the river, The objective of our study was to creating habitat for other plants. We investigate environmental variables that surveyed tussocks in 14 groups (Figure 1) in drive the observed positive native/non- which groups were defined as clumps of at native species richness relationship at the least nine individual tussocks. All tussocks community level. The results of this study within a group were within 5 m from can shed light on the importance of native another tussock within the group. diversity on invader success on tussocks. 2.2 Field Survey Using propagule pressure as a mechanistic framework driving invader establishment on Within each tussock group, we randomly tussocks, we hypothesized that (1) tussocks selected nine tussocks to measure several closer to the front of groups of tussocks environmental variables that could drive that (2) had a larger area, (3) were propagule pressure on tussocks. To see how eXperiencing a lower stream velocity, (4) propagules from upstream influence were closer to upstream tussocks, and (5) tussock native and non-native species were closer to the stream shore would have richness, we looked at (1) the location along a greater number of both native and non- group of tussocks, (2) tussock area, and (3) native species richness. We believe these the stream velocity for each tussock. patterns would occur because such tussocks Propagules from other tussocks was are likely to receive more seeds carried investigated by looking at (4) the distance along the stream or arriving from shore, to upstream tussocks. The source of and thus eXperience a higher propagule propagules from the shore was eXamined as pressure. In this study, we also wanted to the (5) distance to stream shore. The answer whether environmental variables location along a tussock group was directly influence invasion success on calculated as a percentage that each tussocks independent of native species tussock was along the total length of the richness. The answer to this question would group (Figure 2). For distance to upstream allow us to disentangle the environmental tussocks, we measured the distance a variables that could covary with native tussock was from the two closest upstream diversity yet directly influence the invasibility tussocks that were within 45 degrees from of tussocks. CEC Research | https://doi.org/10.21973/N3WM1Z Spring 2019 3/9 a vertical line drawn directly upriver (Figure obstruction 3 m upstream from a tussock, 2). Stream velocity was measured by