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A comparative analysis reveals little evidence for niche conservatism in aquatic
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macrophytes among four areas on two continents
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Janne Alahuhta
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, Frauke Ecke
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, Lucinda B. Johnson
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, Laura Sass
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and Jani Heino
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University of Oulu, Department of Geography, FI-90014 University of Oulu, Finland,
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Finnish Environment Institute, Freshwater Centre, FI-90014 Oulu, Finland
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Swedish University of Agricultural Sciences, Department of Aquatic Sciences and Assessment,
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SWE-75007 Uppsala, Sweden
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University of Minnesota Duluth, Natural Resources Research Institute, 5013 Miller Trunk
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Highway, Duluth, MN 55811, USA
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Illinois Natural History Survey, Prairie Research Institute, University of Illinois, 1816 South Oak
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Street, Champaign, IL 61820, USA
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Finnish Environment Institute, Natural Environment Centre, Biodiversity, FI-90014 Oulu, Finland
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*Correspondence: Janne Alahuhta, University of Oulu, Department of Geography, P.O. Box 3000,
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FI-90014, University of Oulu, Finland.
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E-mail: janne.alahuhta@oulu.fi
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SUMMARY
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One of the most intriguing questions in current ecology is the extent to which the ecological niches
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of species are conserved in space and time. Niche conservatism has mostly been studied using
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coarse-scale data of species’ distributions, although it is at the local habitat scales where species’
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responses to ecological variables primarily take place. We investigated the extent to which niches of
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aquatic macrophytes are conserved among four study regions (i.e., Finland, Sweden and the US
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states of Minnesota and Wisconsin) on two continents (i.e., Europe and North America) using data
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for 11 species common to all the four study areas. We studied how ecological variables (i.e., local,
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climate and spatial variables) explain variation in the distributions of these common species in the
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four areas using species distribution modelling. In addition, we examined whether species’ niche
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parameters vary among the study regions. Our results revealed large variation in both species’
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responses to the studied ecological variables and in species’ niche parameters among the areas. We
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found little evidence for niche conservatism in aquatic macrophytes, though local environmental
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conditions among the studied areas were largely similar. This suggests that niche shifts, rather than
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different environmental conditions, were responsible for variable responses of aquatic macrophytes
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to local ecological variables. Local habitat niches of aquatic macrophytes are mainly driven by
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variations in local environmental conditions, whereas their climate niches are more or less
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conserved among regions. This highlights the need to study niche conservatism using local-scale
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data to better understand whether species’ niches are conserved, because different niches (e.g., local
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vs. climate) operating at various scales may show different degrees of conservatism. The extent to
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which species’ niches are truly conserved has wide practical implications, including for instance,
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predicting changes in species’ distributions in response to global change.
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INTRODUCTION
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The extent to which ecological niches of species are conserved in space and time has important
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implications for a wide variety of biogeographical, ecological and evolutionary questions (Wiens
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and Graham 2005, Pearman et al. 2008, Warren et al. 2008). These questions range from ecological
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specialization to predicting changes in species’ distributions under global change (Wiens et al.
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2010, Peterson 2011). For example, the reliability of predictions provided by species distribution
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modelling is questionable if niche shifts have truly taken place in different areas (Bennett et al.
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2010, Wiens et al. 2005, Wiens et al. 2010). The concept of niche conservatism assumes that a
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niche of a species remains unchanged or changes only slowly over hundreds to millions of years
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(Wiens and Graham 2005, Pearman et al. 2008). Furthermore, it assumes environmentally
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unsuitable conditions can limit a species’ geographic range when niche conservatism prevails
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(Wiens et al. 2010). Although this concept was coined relatively recently (Peterson et al. 1999), the
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idea of ecological niche dates back to the early 20th century (Grinnell 1917, Elton 1927). Different
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niche concepts exist (e.g., Hutchinson 1957), including fundamental, realised and existing
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fundamental niches (see review by Chase and Leibold 2003). A fundamental niche describes the
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environmental conditions and resources that a species is potentially able to use, whereas the realised
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niche represents the part of the fundamental niche that the species actually occupies as a result of
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biotic interactions (e.g., predation, competition and parasitism). The existing fundamental niche is
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the portion of the fundamental niche that is represented across the area accessible to the species
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(Peterson et al. 2011).
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Niche conservatism, in relation to the geographical distributions of species, is typically studied
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using coarse grid-based data of species’ ranges at broad spatial extents (Hawkins et al. 2014, Wasof
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et al. 2015), while niche shifts in space and time are less frequently investigated using fine-grained
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data (i.e., samples from local ecosystems) at broad spatial extents (but see Bennett et al. 2010,
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Valdujo et al. 2013; Wasof et al. 2013). The ready availability of spatial data at medium to coarse
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scales provides the opportunity to examine niche conservatism with respect to climatic variation
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(Broennimann et al. 2007, Hawkins et al. 2014, Wasof et al. 2015); however, local habitat niche
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studies require fine scale data not available across large regions. Fine-grained data enables
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evaluating effects of the Eltonian noise hypothesis (Soberón and Nakamura 2009). This hypothesis
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predicts that ecological interactions and species’ effects on resources define individual distributions
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at fine spatial scales, whereas coarse-scaled abiotic factors structure distributions at broader scales.
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Local-scale habitat variables can strongly affect species’ niche shifts even at broad spatial scales,
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because it is the local habitat to which species respond in the first place (e.g. Wasof et al. 2013).
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The phenomenon is exemplified by water acidity-related niches of freshwater diatoms which are
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conserved across continents (Bennett et al. 2010). In many freshwater systems, local water
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chemistry and habitat structure contribute equally or more strongly than climate to species’
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distributions and community structure at broad spatial extents (Sharma et al. 2011, Alahuhta 2015).
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These local habitat variables are also essential in determining species’ niche parameters, because
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ecological gradients across freshwater ecosystems are often strong. For example, wide variation in
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influential chemical and physical characteristics typically exists within a small geographical area
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(Elser et al. 2007), and species respond to these major environmental gradients (Bennett et al. 2010,
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Sharma et al. 2011, Alahuhta and Heino 2013). Typically, researchers are interested in knowing
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how species’ niche positions and niche breadths vary in relation to local habitat variables
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(Boulangeat et al. 2012, Wasof et al. 2013, Heino and Grönroos 2014), and potential niche shifts
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can be observed from differences in these habitat niche parameters for the same species across
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different areas (Ackerly 2003). Species niches are probably conserved if the distribution-
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environment relationships are relatively similar for the same species across different study areas.
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Recently, dispersal has also been included in the set of important processes affecting the
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relationship between realised niches and geographical distributions (Soberón 2007, Soberón and
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Nakamura 2009, Godsoe 2010, Peterson 2011). Dispersal is incorporated in the concept of spatial
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processes, which depending on their dispersal abilities and possible geographical barriers allows
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species to track variation in suitable habitats (Heino and de Mendoza, 2016). Species disperse
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among suitable habitats that are structured as a network of habitat patches, varying in area, degree
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of isolation and quality, surrounded by unsuitable habitats in the landscape (Hanski 1998, Leibold et
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al. 2004). Spatial processes can constrain species’ responses to environmental variability, thus
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relating directly to niche conservatism (Wiens et al. 2010). Spatial processes are especially
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important in lake systems, because adaptation to living in water leads to the formation of well-
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delimited populations surrounded by an inhospitable terrestrial matrix (Dahlgren and Ehrlén 2005,
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Hortal et al. 2014). Lake macrophytes are a particularly suitable organismal group for the study of
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spatial variability because these plants cannot actively move and have strong spatial structure even
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within lake habitats (Nilsson et al. 2010, Arthaud et al. 2013). To date, it has been assumed that the
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ecological niches of aquatic macrophytes remain unchanged in space (Chambers et al. 2008),
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although no actual study has investigated whether or not the niches of aquatic macrophytes are
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conserved at broad spatial extents.
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Our study examines whether or not niches of aquatic macrophytes are conserved between different
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geographical areas. Our primary aim is to investigate how ecological variables (i.e., local, climate
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and spatial) explain variation in the distributions of common aquatic macrophyte species in four
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