Opposing effects of floral visitors and soil conditions on the determinants of competitive outcomes maintain species diversity in heterogeneous landscapes

Abstract

13Theory argues that both soil conditions and aboveground trophic interactions are equally important for 14 determining plant species diversity. However, it remains unexplored how they modify the niche di erences 15 that stabilise species coexistence and the average fitness di erences driving competitive dominance. 16We conducted a field study in Mediterranean annual grasslands to parameterise population models 17 of six competing plant species. Spatially explicit floral visitor assemblages and soil salinity variation . CC-BY 4.0 International license It is made available under a (which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint . http://dx.doi.org/10.1101/170423 doi: bioRxiv preprint first posted online Jul. 31, 2017; Introduction 27Understanding the mechanisms that maintain species diversity is a central aim in ecology. Although 28 species interact with the environment and with many other species in complex ways, ecologists have 29 traditionally assumed that the importance of biotic and abiotic factors in promoting species diversity is 30 highly asymmetrical. Competition driven by soil conditions is commonly considered to be the primary 31 driver of plant coexistence, and therefore it has been extensively explored (Raynaud & Leadley 2004; 32 Tilman 2006; Craine & Dybzinski 2013; Hendriks et al. 2015). For instance, di erences in the species' 33 ability to drawdown limiting resources such as nitrogen and phosphorous is a classic textbook example 34 illustrating the importance of partitioning soil resources for maintaining species diversity (Tilman 1994). 35On top of this, multitrophic interactions such as those occurring between plant and pollinators, 36 pathogens, or mycorrhizae (Fitter 1977; Bastolla et al. 2009; Bagchi et al. 2014; Parker et al. 2015; 37 Bennett et al. 2017), are thought to play a secondary role in structuring plant communities. 38However, recent work has challenged this view. Chesson & Kuang (2008) presented clear evidence that 39 there is no theoretical support that the relative importance of competition driven by soil conditions 40 versus other kinds of multitrophic interactions is asymmetrical. In fact, they argue that these two types 41 of interactions are equally able to either limit or promote diversity. Competition mediated by other 42 trophic levels has been largely studied under the concept of apparent competition (Holt 1977), which 43 specifically describes how species within a trophic level (e.g. plants) can produce indirect competitive 44 e ects on others via shared enemies (e.g. herbivores, predators). These indirect e ects can be of equal 45 magnitude to that of direct e ects resulting from resource competition. The utility of this concept has 46 also been extended to apparent negative and positive interactions mediated by other organisms, such as 47 shared mutualisms (Morris et al. 2004; Carvalheiro et al. 2014). . CC-BY 4.0 International license I...

Plant species dominance increases pollination complementarity and plant reproductive function

“…Importantly, this aligns with recent studies that show plant–pollinator interactions can alter competitive hierarchies among plant species (Pauw , Lanuza et al. ). Extending this concept from our cage experiment to consider a natural system, we suggest that differences in plant species’ competitive abilities to attract pollinators could well contribute to maintaining the skewed species abundance distributions that are common in natural plant communities (McGill et al.…”

Pollination outcomes reveal negative density‐dependence coupled with interspecific facilitation among plants

“…Facilitation can only foster coexistence if rare species benefit disproportionately in relation to the abundant species (Soliveres et al ). Thus, plants’ pollination niches may represent an axis stabilising plant interspecific competition (Benadi & Pauw ; Lanuza et al ; Johnson & Bronstein ). Nevertheless, landscape‐level NDD was only marginally significant for pollen tubes.…”

Coexistence theory as a tool to understand biological invasions in species interaction networks: Implications for the study of novel ecosystems

“…Indeed, accomplishing this type of experiment is a high time‐consuming task but prior work has shown that it is doable (Saavedra et al, ). In case this approach is not feasible for conservation or logistical issues, another possibility is to use information of natural variation in species abundances and their reproductive outputs across landscapes together with the underlying environmental factors accounting for such variation (Bimler, Stouffer, Lai, & Mayfield, ; Lanuza, Bartomeus, & Godoy, ). Finally, a possibility that works either well is to use temporal variation instead of spatial variation.…”