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Journal ArticleDOI

A comparative analysis reveals little evidence for niche conservatism in aquatic macrophytes among four areas on two continents

TL;DR: The results suggest that niche shifts, rather than different environmental conditions, were responsible for variable responses of aquatic macrophytes to local ecological variables, which highlights the need to study niche conservatism using local-scale data to better understand whether species' niches are conserved.
Abstract: One of the most intriguing questions in current ecology is the extent to which the ecological niches of species are conserved in space and time. Niche conservatism has mostly been studied using coarse-scale data of species' distributions, although it is at the local habitat scales where species' responses to ecological variables primarily take place. We investigated the extent to which niches of aquatic macrophytes are conserved among four study regions (i.e. Finland, Sweden and the US states of Minnesota and Wisconsin) on two continents (i.e. Europe and North America) using data for 11 species common to all the four study areas. We studied how ecological variables (i.e. local, climate and spatial variables) explain variation in the distributions of these common species in the four areas using species distribution modelling. In addition, we examined whether species' niche parameters vary among the study regions. Our results revealed large variation in both species' responses to the studied ecological variables and in species' niche parameters among the areas. We found little evidence for niche conservatism in aquatic macrophytes, though local environmental conditions among the studied areas were largely similar. This suggests that niche shifts, rather than different environmental conditions, were responsible for variable responses of aquatic macrophytes to local ecological variables. Local habitat niches of aquatic macrophytes are mainly driven by variations in local environmental conditions, whereas their climate niches are more or less conserved among regions. This highlights the need to study niche conservatism using local-scale data to better understand whether species' niches are conserved, because different niches (e.g. local versus climate) operating at various scales may show different degrees of conservatism. The extent to which species' niches are truly conserved has wide practical implications, including for instance, predicting changes in species' distributions in response to global change.

Summary (2 min read)

INTRODUCTION

  • Different niche concepts exist (e.g., Hutchinson 1957) , including fundamental, realised and existing fundamental niches (see review by Chase and Leibold 2003) .
  • Fine-grained data enables evaluating effects of the Eltonian noise hypothesis (Soberón and Nakamura 2009) .
  • The authors study examines whether or not niches of aquatic macrophytes are conserved between different geographical areas.

Study areas and macrophyte species

  • These four distinct study areas show a clear east-west orientation.
  • In addition to these major climatic differences, both Finland and Minnesota have harsher climate conditions than Sweden and Wisconsin.
  • In addition, many aquatic macrophytes are known to have efficient dispersal abilities, and often aggressively colonize new habitats (Santamaria 2002) .
  • Species' prevalence varied among the areas and was often similar between geographically neighbouring study areas (Table S2 ).

Explanatory variables

  • Explanatory data consisted of lake-specific local, climate and spatial variables (Table S1 ).
  • In Finland, water chemistry comprised of median values of 1-m surface water samples taken during the growing season (June-September) over the period 2000-2008.
  • Spatial variables originated from db-MEMs, were orthogonal (linearly independent) and were obtained from spectral decomposition of a truncated distance matrix of the spatial relationships among sampling locations.
  • The authors used geographic coordinates of lake centres to calculate Euclidean distances between lakes, and only positive eigenvectors were employed in additional analyses.
  • These spatial eigenvectors are specific for each study region, thus cannot be directly compared across the different regions.

Statistical analyses

  • The authors used two methods to test for differences in mean environmental conditions and heterogeneity of environmental conditions among the study areas.
  • These analyses were done separately for standardized values of "local", "climate" and "combined local-climate" variable groups.
  • Significance of among-study area differences was tested through permutation of least-squares residuals.
  • In the ordinal approach, the authors evaluated whether local and climate niches vary among the same 11 species across the study areas using Outlying Mean Index analysis (OMI, Dolédec et al. 2000) .

Response of macrophyte species to ecological variables in different study areas

  • Contrary to their expectations, the same macrophyte species responded differently to ecological variables in the four study areas (Table 1 ).
  • Only 3 (Ceratophyllum demersum, Phalaris arundinacea and Phragmites australis) of the 11 species studied were primarily affected by the same major ecological gradient over all the four study areas based on the variation partitioning procedure.
  • These results emphasise that local variables dominate over climate constraints in affecting the distributions of aquatic macrophyte species at regional extents.
  • The importance of local and climate variables on the studied species' distributions varied strongly across study areas.
  • Spatial variables with both large and small eigenvalues (SV1-SV20), indicating broad-and fine-scale variation in spatial structure, were the most influential for the studied species in Finland, Sweden and Minnesota.

Niche parameters: differences among species and among areas

  • Niche positions did not remain relatively similar in different study areas, and niche breaths were not especially wide for all species in all study areas (Table 3 ).
  • The niche positions were correlated among study areas, but correlations found for niche breadths among the study areas were low (Table 4 ).
  • In general, niche positions within each continent were positively correlated; however, correlations were negative between the continents.
  • For the niche breadths, species' values for Finland and Sweden were weakly positively correlated based on local, climate or combined localclimate conditions.
  • Other relationships varied incongruently among the study areas.

DISCUSSION

  • Species' niches and dispersal-related processes have recently been considered when studying niche conservatism in relation to their geographic distributions (Soberón 2007 , Godsoe 2010 , Peterson 2011) .
  • Only 3 of the 11 species studied were primarily affected by the same pure component across all the study areas based on the variation partitioning procedure.
  • The influence of alkalinity on macrophytes is related to the use of bicarbonate (HCO3 -) as a source of carbon for submerged species, directly influencing photosynthesis, growth and long-term survival (Rørslett 1991, Vestergaard and Sand-Jensen 2000) .
  • Previous niche conservatism studies have mostly examined shifts in climate niches (Hawkins et al. 2014 , Wasof et al. 2015) ; however, the authors found that climate variables contributed less than local variables to the distribution of aquatic macrophytes.
  • Spatial processes were, quite unexpectedly, also rather important for many macrophyte species.

Niche shifts or conservatism?

  • The authors found surprisingly little evidence for niche conservatism in the distributions of the 11 macrophyte species among the four study areas.
  • This finding suggests that the same species may have a wide niche breadth and/or non-marginal niche position in one area and a narrow niche breadth or marginal niche position in another area relative to the niches of the other species studied.
  • Thus, different responses of the same species to local environmental conditions may not be due to differences in environmental heterogeneity among the study areas, but rather, are more likely due to region-specific niche shifts in aquatic macrophytes in relation to local environmental conditions.
  • Alternatively, the environmental conditions across the study area are taken into account, and the environments available to the species are used differently between various regions.
  • More research is needed, however, to confirm assumptions because phenotypically plastic species traits causing incongruent results across their study areas can theoretically still be evolutionarily conserved.

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Figures (5)

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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
1, 2*
, 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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4
5
1
University of Oulu, Department of Geography, FI-90014 University of Oulu, Finland,
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2
Finnish Environment Institute, Freshwater Centre, FI-90014 Oulu, Finland
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3
Swedish University of Agricultural Sciences, Department of Aquatic Sciences and Assessment,
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SWE-75007 Uppsala, Sweden
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4
University of Minnesota Duluth, Natural Resources Research Institute, 5013 Miller Trunk
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Highway, Duluth, MN 55811, USA
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5
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
25
of species are conserved in space and time. Niche conservatism has mostly been studied using
26
coarse-scale data of species distributions, although it is at the local habitat scales where species
27
responses to ecological variables primarily take place. We investigated the extent to which niches of
28
aquatic macrophytes are conserved among four study regions (i.e., Finland, Sweden and the US
29
states of Minnesota and Wisconsin) on two continents (i.e., Europe and North America) using data
30
for 11 species common to all the four study areas. We studied how ecological variables (i.e., local,
31
climate and spatial variables) explain variation in the distributions of these common species in the
32
four areas using species distribution modelling. In addition, we examined whether species niche
33
parameters vary among the study regions. Our results revealed large variation in both species
34
responses to the studied ecological variables and in species’ niche parameters among the areas. We
35
found little evidence for niche conservatism in aquatic macrophytes, though local environmental
36
conditions among the studied areas were largely similar. This suggests that niche shifts, rather than
37
different environmental conditions, were responsible for variable responses of aquatic macrophytes
38
to local ecological variables. Local habitat niches of aquatic macrophytes are mainly driven by
39
variations in local environmental conditions, whereas their climate niches are more or less
40
conserved among regions. This highlights the need to study niche conservatism using local-scale
41
data to better understand whether species niches are conserved, because different niches (e.g., local
42
vs. climate) operating at various scales may show different degrees of conservatism. The extent to
43
which species niches are truly conserved has wide practical implications, including for instance,
44
predicting changes in species’ distributions in response to global change.
45
46
47

3
INTRODUCTION
48
49
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.
53
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
94
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-
96
environment relationships are relatively similar for the same species across different study areas.
97
98
Recently, dispersal has also been included in the set of important processes affecting the
99
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
101
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
118
and spatial) explain variation in the distributions of common aquatic macrophyte species in four
119

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Frequently Asked Questions (1)
Q1. What are the contributions in this paper?

Alahuhta et al. this paper, Frauke Ecke, Lucinda B. Johnson, Laura Sass and Jani Heino studied macrophytes among four areas on two continents.