Understanding how dispersal limitation structures plant species geographic ranges has received increasing attention in recent years, especially in light of climate change where distributional boundaries shift northward in latitude and upwards in elevation to track suitable habitat (Cunze et al., 2013; Dullinger et al., 2004). The general question of ‘Which species will be able to move and track these changing conditions?’ has brought together a wide variety of research groups, but generalizations still remain challenging (Gaston, 2009), in part due to the many unique abiotic and biotic factors that can come into effect at a species range margin (Sexton et al., 2009).
To understand how accessibility will limit geographic range shifts under future climate conditions, it is important to understand how dispersal influences contemporary boundaries and to consider historic evolutionary and biogeographic processes. There is now increasing recognition that many plant species are at disequilibria (of varying degrees) with suitable climate conditions and even species that appear to be locally abundant, may be dispersal limited at larger spatial scales (Cunze et al., 2013; Dullinger et al., 2004). Human induced movement of plants across the globe and the creation of “invasive species” has acted as the major examples for dispersal and range expansion (Sexton et al., 2009; Vitousek et al. 1997), but evidence for dispersal limitation in the absence of obvious barriers for many native species has received mixed results, suggesting it should be studied as a species level problem in conjunction with other range limiting mechanisms (Dullinger et al., 2004; Gaston, 2009).
The lab group that I am a part of has worked primarily with Mimulus spp. (especially M. cardinalis) as a focal study system to investigate a variety of processes across a species geographic range, including range wide demographic surveys, central and marginal studies of maladaptive gene flow, reciprocal transplants and growth chamber studies (see lab website). This research has challenged some of the most basic assumptions of marginal populations that are often taken for granted such as reduced fitness and survivorship in edge populations (Angert, 2009).
Translocations Over the Northern Range Margin
Geographic range distribution of M. cardinalis re-created from Hiesey (1971)
SPECIES DISTRIBUTION MODELS
The use of species distribution models (SDM) or bioclimatic envelope modeling has become wide spread in throughout ecology and conservation biology partly due to the continual advancement of remote sensing technology and readily available climate & environmental spatial layers (Guisan and Thuiller, 2005). With the rapid pace of climate change SDMs have been used to forecast massive geographic range shifts for species northward in latitude and upwards in elevation to track suitable habitat, but determining which species will be able to disperse to new areas and track their climate envelope has remained challenging (Corlett and Westcott, 2013; Loarie et al., 2008).
A major challenge to the SDM approach is that the models are constructed by correlating occurrence records with environmental variables. This approach assumes that contemporary range limits are at an equilibrium with climate and are not strongly influenced by dispersal limitation (Araújo et al., 2005; Guisan and Thuiller, 2005). This could be problematic if SDMs are providing overly optimistic predictions for species ability to migrate with climate change, when in reality they may be adversely affected in their native range if they fail to disperse (Corlett and Westcott, 2013).
Despite extensive use of correlative SDMs and the characterization of environmental gradient driven range margins, the empirical validation of these models through translocations has remained extremely rare (e.g. McLane and Aitken, 2012). Also, while the discussion of assisted migration through translocations has been a debated mitigation strategy, questions of the practicality and feasibility to manage translocations for a moving target have remained unanswered (Corlett & Westcott, 2013; McLane & Aitken, 2012).
Through the second half of this project I plan to build a SDM for the climate niche of Mimulus cardinalis and evaluate the population performance through a series of translocations across a gradient of predicted suitability. The SDM will be developed from surveyed presence and absence records as well as herbarium records with downscaled climate variables from ClimateWNA (Wang et al., 2012). The model algorithm will consist of an ensemble average and MaxEnt predictions to reflect the most widely used SDM methods. M. cardinalis individuals from two northern populations will be transplanted to 10 sites across the modeled predicted suitability gradient, while microsite features will remain standardized across sites. Each of 20 plots per site will have multiple life stages represented so that an estimate of lambda (finite population growth rate) can be regressed against the predicted suitability for each site.
While significant progress has been made towards model selection and development (Elith et al., 2010; Guisan and Thuiller, 2005); this case study will be a critical empirical experimental evaluation of the overall correlative approach to SDMs which has remained absent. If a species is at a climatic equilibrium then the predicted suitability from a SDM should mirror real world population performance, while if a range boundary is driven largely by dispersal limitation then this correlative modeling approach will not be valid, especially in relation to projected range shifts with climate change. By using M. cardinalis as a case study it is possible to integrate correlative SDMs with a more holistic understanding of range limits within this study system.
|M.cardinalis distribution (points) with bioclimatic suitability envelope.|
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