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.