General conclusions
The accessible area, M , for a species is a function of dispersal opportunities and limitations to dispersal across complex geographic landscapes (Barve et al. 2011; Peterson et al. 2011), and is not solely a consequence of a species’ fundamental niche characteristics. That is, the available environments within M depend on the environmental characteristics of the specific biogeographic area where the species occurs, thus introducing variation that has little or nothing to do with a species’ true fundamental niche. M is a function of the location of coastlines, rivers, mountain ranges, deserts, ocean currents, or any number of geographic factors that constrain the distributional potential of species (Barve et al. 2011). Given that most speciation (at least in vertebrates) is believed to be allopatric in nature (Barraclough and Vogler 2000; Turelli et al. 2001; Coyne and Orr 2004), each node on a phylogenetic tree will involve a species with a distinct M , and thus a different set of associated environments, which makes it likely that the estimated fundamental niche will not be equivalent to the true fundamental niche, and that the estimated fundamental niche of different species in a clade will differ even when their true fundamental niche has been evolutionarily stable.
We present a simple methodology that incorporates knowledge gaps directly, and therefore avoids—at least in largest part—these biases. We see a number of next steps in developing this methodology further—specifically, we would develop nichevol tools to encompass Bayesian estimation approaches and considering alternative evolutionary models. We would also take into account the frequency of occurrence of environments across the accessible area of each species in making conclusions about niche limitation (e.g. Meyer and Pie 2018)—that is, non-occurrence in relatively rare environments should perhaps not be taken as evidence of niche limitation. We are exploring implementation of these next steps in coming applications of this methodology.
In conclusion, the challenge of understanding change in species’ ecological niches across evolutionary history lies in characterizing the entirety of a species’ niche on a branch of a phylogeny. Typically, a single value (e.g., the central tendency, maximum, or minimum) has been taken as a representation of the niche (Graham et al. 2004b; Ackerly et al. 2006; Yesson and Culham 2006; Kozak and Wiens 2010). As noted previously, however, summary values can often miss effects such as specialization or generalization (Evans et al. 2009; Smith and Donoghue 2010; Nyári and Reddy 2013). Furthermore, a single summary value in absence of information regarding the availability of environments to the species replicates and transmits the effects of limitation by M in the form of strong directional biases toward conclusions of greater niche lability.