bioRxiv preprint doi: https://doi.org/10.1101/740282; this version posted January 11, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 1 Ecotone formation through ecological niche construction: the role of 2 biodiversity and species interactions 3 Kevin Liautaud*, Matthieu Barbier, Michel Loreau 4 Centre for Biodiversity Theory and Modelling, Theoretical and Experimental Ecology Station, UMR 5 5321, CNRS and Paul Sabatier University, 09200 Moulis, France 6 *Correspondence : [email protected], ORCID : https://orcid.org/0000-0001-6164-8415 7 Abstract 8 Rapid changes in species composition, also known as ecotones, can result from various 9 causes including rapid changes in environmental conditions, or physiological thresholds. The 10 possibility that ecotones arise from ecological niche construction by ecosystem engineers has 11 received little attention. In this study, we investigate how the diversity of ecosystem engi- 12 neers, and their interactions, can give rise to ecotones. We build a spatially explicit dynamical 13 model that couples a multispecies community and its abiotic environment. We use numerical 14 simulations and analytical techniques to determine the biotic and abiotic conditions under 15 which ecotone emergence is expected to occur, and the role of biodiversity therein. We show 16 that the diversity of ecosystem engineers can lead to indirect interactions through the modi- 17 fication of their shared environment. These interactions, which can be either competitive or 18 mutualistic, can lead to the emergence of discrete communities in space, separated by sharp 19 ecotones where a high species turnover is observed. Considering biodiversity is thus critical 20 when studying the influence of species-environment interactions on the emergence of ecotones. 21 This is especially true for the wide range of species that have small to moderate effects on 22 their environment. Our work highlights new mechanisms by which biodiversity loss could 23 cause significant changes in spatial community patterns in changing environments. 24 Keywords: Biodiversity, Community patterns, Ecological niche construction, Ecosystem en- 25 gineers, Ecotones, Species interactions 1 bioRxiv preprint doi: https://doi.org/10.1101/740282; this version posted January 11, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 26 1 Introduction 27 Whether species composition changes gradually, or forms discrete zones along environmental gra- 28 dients has been the subject of a long-standing debate in ecology (Clements, 1916; Gleason, 1926; 29 Braun-Blanquet, 1928; Hedberg, 1955; McIntosh, 1967). Observational studies have found both 30 gradual (Whittaker, 1956; Vazquez G. and Givnish, 1998; Ellison et al., 2010; Lieberman et al., 31 1996) and discrete patterns (Kitayama, 1992; Hemp, 2006; Tuomisto and Ruokolainen, 1994; 32 Kessler, 2000). Rapid changes in community composition along gradients, also termed ecotones 33 (Kent et al., 1997), have been observed in a wide range of ecosystems, such as alpine treelines 34 (Germino et al., 2002), tropical mountain forests (Martin et al., 2007) and coastal environments 35 (Sternberg et al., 2007; Walker et al., 2003). Hereafter, a transition will be termed "rapid" when 36 its scale is much smaller than the spatial scale of the landscape, even though the transitional area 37 may show mixing of species. 38 While rapid changes can be blurred by species dispersal (Liautaud et al., 2019) or stochasticity 39 in nature, it is important to understand the theoretical conditions under which rapid community 40 changes can emerge. These rapid changes in species composition can coincide with rapid changes 41 in environmental conditions, such as the frost line (Kitayama and Mueller-Dombois, 1992) or 42 discontinuities in edaphic conditions (Tuomisto and Ruokolainen, 1994; Kessler, 2000). In these 43 cases, it is often assumed that changes in abiotic conditions are responsible for the change in species 44 composition (McIntosh, 1967; Kent et al., 1997). This assumption is supported in many cases, but 45 it may obscure the possibility that, in other settings, the two boundaries emerge together from the 46 influence of species on their abiotic environment. The mechanisms that can lead to such transitions 47 are poorly known, and in particular the respective contributions of species-environment feedbacks 48 and interspecific interactions. 49 Species that are able to modify their abiotic environment are often called "ecosystem engineers" 50 (Jones et al., 2010). Classical examples range from beavers that impact water flow and habitat 51 heterogeneity (Wright et al., 2002), to cushion alpine plants that buffer extreme temperatures 52 and increase soil moisture (Badano et al., 2006). Ecological niche construction is a particular 53 case in which engineers modify the environment to their own benefits (Kylafis and Loreau, 2008, 54 2011), creating a feedback with the environment (an example in which engineers can instead create 55 succession is presented in the Appendix). This ecological process should be distinguished from 2 bioRxiv preprint doi: https://doi.org/10.1101/740282; this version posted January 11, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 56 the related concept of niche construction in evolutionary theory in which we would also expect 57 species traits to evolve over time (Odling-Smee et al., 2003, 1996). Examples of ecological niche 58 construction range from plant-water feedbacks in arid environment (Dekker et al., 2007) to increases 59 in nutrient inputs by trees in tropical ecosystems (De longe et al., 2008). Such feedbacks can govern 60 species distributions (Wilson and Agnew, 1992), particularly under harsh environmental conditions 61 (K´efiet al., 2007; Gilad et al., 2004; Meron et al., 2004; von Hardenberg et al., 2001), and lead 62 to the emergence of ecotones (Bearup and Blasius, 2017; Jiang and DeAngelis, 2013). Classical 63 studies on ecosystem engineers, however, have generally focused on the effects of a particular 64 species having strong effects on the abiotic environment (Jones et al., 2010; Bouma et al., 2010; 65 Prugh and Brashares, 2012). But many more species have small or moderate impacts on their 66 environment. Such species, which are often neglected individually, might substantially affect their 67 environment when aggregated. Furthermore, previous studies have scarcely explored what types 68 of interactions can arise between multiple species that engineer their shared environment. We 69 thus propose to focus on the role of diversity and species interactions in the emergence of ecotones 70 through ecological niche construction. 71 Biodiversity can have two main effects on the emergence of species-environment feedbacks : a 72 cumulative effect of species number, and a heterogeneity effect due to variations in species' pref- 73 erences and engineering ability. Cumulative effects are similar to complementarity in biodiversity- 74 ecosystem functioning relationships (Loreau and Hector, 2001; Hooper et al.). The fact that species 75 coexist with weak or no competition implies the existence of different niches, i.e. other factors be- 76 yond the environmental preference modelled here. This cumulative effect arises when there is 77 no single identifiable engineer, but where community acts collectively to create an ecotone. A 78 potential example is the occurrence of ecotones between mangroves and hardwood forests, where 79 several mangrove tree species can modify water salinity in synergy (Sternberg et al., 2007). In 80 contrast, the heterogeneity effect of biodiversity arises when there are differences in species' pre- 81 ferred environmental states. We investigate the effect of these differences on emergent competition 82 or facilitation between ecosystem engineers, and how this could play a role in ecotone emergence. 83 In this study, we build a theoretical model that couples the dynamics of a community and 84 of its abiotic environment to assess the role of ecosystem engineers and of their diversity in the 85 emergence of ecotones in space. In our model, ecotones are represented by abrupt changes, in- 3 bioRxiv preprint doi: https://doi.org/10.1101/740282; this version posted January 11, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 86 cluding discontinuities. In the presence of multiple interacting species, we show that ecological 87 niche construction can lead to the emergence of indirect interspecific interactions -which can be 88 either positive or negative - through environmental modifications. Similarly, we show that even 89 species with different preferences can act synergistically as a single community. We then assess 90 the consequences of these different interaction types for community patterns in space, and identify