A comparative analysis reveals little evidence for niche conservatism in aquatic macrophytes among four areas on two continents
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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Citations
114 citations
Cites background from "A comparative analysis reveals litt..."
...…related to, for example, ecosystem functioning (Symstad et al., 2003) and resilience (Folke et al., 2004), biogeographical regionalization (Divisek et al., 2016), niche conservatism (Alahuhta et al., 2017), species conservation (Brooks et al., 2008) and ecosystem services (Naidoo et al., 2008)....
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..., 2016), niche conservatism (Alahuhta et al., 2017), species conservation (Brooks et al....
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...Another line of evidence comes from a recent study showing that the niche of aquatic plants is not conserved among different regions (Alahuhta et al., 2017)....
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References
83 citations
67 citations
"A comparative analysis reveals litt..." refers background in this paper
...We emphasise, similarly to Wasof et al. (2013), that the explanatory variables used 507 are directly associated with actual ecological mechanisms, which determine the niche for each of 508 the macrophyte species (Rørslett 1991, Toivonen and Huttunen 1995, Vestergaard and Sand-Jensen 509 2000)....
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65 citations
"A comparative analysis reveals litt..." refers methods in this paper
...2010, Alahuhta et 177 al. 2014, Alahuhta 2015). Macrophytes were sampled or observed by wading, diving, snorkelling or 178 by boat, using rakes and hydroscopes. Survey methods are described in detail for Finland in 179 Alahuhta et al. (2013), for Sweden in Naturvårdsverket (2010), for Minnesota in Alahuhta (2015), 180 and for Wisconsin in Sass et al. (2010). It is important to note that the sampling methods were 181 identical within each area....
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...2010, Alahuhta et 177 al. 2014, Alahuhta 2015). Macrophytes were sampled or observed by wading, diving, snorkelling or 178 by boat, using rakes and hydroscopes. Survey methods are described in detail for Finland in 179 Alahuhta et al. (2013), for Sweden in Naturvårdsverket (2010), for Minnesota in Alahuhta (2015), 180 and for Wisconsin in Sass et al....
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62 citations
"A comparative analysis reveals litt..." refers background in this paper
...For example, wide variation in 89 influential chemical and physical characteristics typically exists within a small geographical area 90 (Elser et al. 2007), and species respond to these major environmental gradients (Bennett et al. 2010, 91 Sharma et al. 2011, Alahuhta and Heino 2013)....
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...In many freshwater systems, local water 85 chemistry and habitat structure contribute equally or more strongly than climate to species’ 86 distributions and community structure at broad spatial extents (Sharma et al. 2011, Alahuhta 2015)....
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62 citations