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EXPERIMENTAL VERIFICATION OF ECOLOGICAL NICHE MODELING

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Ecology, 87(10), 2006, pp. 2433–2439Ó 2006 by the Ecological Society of AmericaEXPERIMENTAL VERIFICATION OF ECOLOGICAL NICHE MODELING INA HETEROGENEOUS ENVIRONMENTJESSICA W. WRIGHT,1KENDI F. DAVIES,2JENNIFER A. LAU,3ANDREW C. MCCALL,4AND JOHN K. MCKAY5Center for Population Biology, One Shields Avenue, University of California, Davis, California 95618 USAAbstract. The current range of ecological habitats occupied by a species reflects acombination of the ecological tolerance of the species, dispersal limitation, and competition.Whether the current distribution of a species accurately reflects its niche has importantconsequences for the role of ecological niche modeling in predicting changes in species ranges asthe result of biological invasions and climate change. We employed a detailed data set of speciesoccurrence and spatial variation in biotic and abiotic attributes to model the niche of a nativeCalifornia annual plant, Collinsia sparsiflora. We tested the robustness of our model for both therealized and fundamental niche by planting seeds collected from four populations, representingtwo ecotypes, into plots that fully represented the five-dimensional niche space described by ourmodel. The model successfully predicted which habitats allowed for C. sparsiflora persistence,but only for one of the two source ecotypes. Our results show that substantial niche divergencehas occurred in our sample of four study populations, illustrating the importance of adequatelysampling and describing within-species variation in niche modeling.Key words: ecological niche modeling; edaphic variation; GAM models; local adaptation; nichedifferentiation; serpentine soils; transplant experiments.INTRODUCTIONThe niche of a species is the set of environmentalvariables that determine the geographic distribution ofthat species, either in the presence of biotic interactionsincluding competition (the realized niche), or withoutthese biotic interactions (the fundamental niche) (Hutch-inson 1959, Pulliam 2000, Holt 2003). Ecological nichemodels use various mathematical techniques to relatethe occurrence of species to environmental data (Guisanand Zimmerman 2000). Niche modeling has receivedincreased attention recently because it has importantimplications for conservation and management efforts(Kearney and Porter 2004, Chefaoui et al. 2005), thespread of invasive species (Peterson and Vieglais 2001,Peterson 2003) and the response of species to globalclimate change (Ackerly 2003, Peterson et al. 2004).However, niche models often ignore genetic variation inhabitat use and the evolutionary potential for niches todiverge among populations of a species (Peterson andHolt 2003). Spatial variation in the edaphic and bioticvariables that comprise a species’ niche can produceselection, ultimately resulting in adaptation to localenvironmental conditions (provided that sufficient ge-netic variation exists). Such local adaptation is welldocumented and can produce divergence among pop-ulations of a single species with respect to nichedimensions. Finally, local adaptation, or within speciesniche divergence, ha s played an important role intheoretical models for the evolution of species niches(Turesson 1922, Clausen 1962, Holt 2003), speciation(Darwin 1859, Maynard Smith 1966), and the main-tenance of genetic variation (Levene 1953, Felsenstein1981).There are several approaches to evaluating the abilityof a niche model to predict the potential niche of aspecies (Guisan and Zimmerman 2000). Here we use anexperimental technique that we have not found appliedin the niche modeling literature—to test the modeldirectly by planting individuals into well-describedhabitats. If the model represents a meaningful character-ization of the realized niche, then it should also predicttransplant performance as a function of the measuredenvironmental variables.Our approach proceeded in three steps. First wecollected fine-scale, spatial, environmental data, whichwe used to construct a model describing the currentniche of C. sparsiflora. Second, we tested this model byplanting seeds into well-characterized environments.Finally, we expand our exploration of the model byManuscript received 2 September 2005; revised 23 March2006; accepted 11 April 2006; final version received 5 May 2006.Corresponding Editor: D. P. C. Peters.1Present address: USDA Forest Service, Pacific SouthwestResearch Station, Institute of Forest Genetics, 1100 WestChiles Road, Davis, California 95618 USA;E-mail: [email protected] address: Department of Ecology and Evolu-tionary Biology, University of Colorado, Boulder, CO 80309USA.3Present address: Department of Plant Biology, Univer-sity of Minnesota, 1445 Gortner Avenue, St. Paul, Minnesota55108 USA.4Present address: Biology Department, Denison Univer-sity, Granville, Ohio 43023 USA.5Present address: Bioagricultural Sciences and PestManagement, C129 Plant Sciences Building, Fort Collins,Colorado 80523 USA.2433REPORTS(1) comparing the performance of plants collected fromdifferent habitats to determine how intraspecific varia-tion may influence the applicability of ecological nichemodels and (2) by considering plants growing with andwithout competition, thus providing insight on therelationship between the fundamental and realizedniche.METHODSStudy systemIn the North Coast range of California, habitat types(chaparral, grassland, oak woodland) and soil types(valley sediment, volcanics, serpentine) can change in amatter of meters (Kruckeberg 1984, Stebbins and Hrusa1995). The McLaughlin University of California Natu-ral Reserve is located in the North Coast range of north-central California. It is principally an open oak wood-land, with serpentine chaparral, serpentine meadows,and non-serpentine meadows and woodlands (moreinformation available online)6.The native California annual plant, Collinsia sparsi-flora (Scrophulariaceae s.l.) occurs on both serpentineand non-serpentine soils at the McLaughlin Reserve.Previous work has shown that there are serpentine andnon-serpentine ecotypes occurring in the two soil typesat the reserve (Wright et al. 2006).Environmental dataIn 2001, a 600 3 550 m habitat sampling grid wasestablished at the McLaughlin Reserve, with grid pointsevery 50 m. Within the grid, six smaller grids (gridlets)were established, with gridlet points every 10 m (100 350 m; Fig. 1). While the grid placement was random, theplacement of the gridlets was selected so as to sampleareas of


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