Introduction
Environmental conditions are ever-changing, temporally and spatially.
Trying to survive amid a change in conditions within a local habitat
requires species to adapt and develop specialised mechanisms. In many
cases, however, the changes are to such an extent that it is better to
leave altogether, be it for one or more generations, or altogether. In
these cases, many species have adapted to disperse. Dispersal can be
defined as the movement of an individual from site of birth to site of
reproduction. If better sites can be reached via dispersal, this drives
evolution as organisms escape unfavourable conditions (Matthysen, 2012).
Dispersal can be divided into three phases: emigration, inter-patch
movement and immigration. During emigration, the individual can obtain
clues from both biotic and abiotic factors in the local sub-habitat.
Using these clues, the individual can then either disperse or not. If
they do, they enter the transfer phase or movement through the habitat,
where many species require clues from the environment in order to choose
a new location to settle in (Clobert et al. , 2001, 2012). The
successful settlement into a new sub-habitat is the immigrant phase.
Many species disperse passively, and so are unable to make informed
choices about the final sub-habitat in which they find themselves.
Dispersal and emigration can reduce the likelihood of competition with
kin, and mitigate against drift and inbreeding (Bengtsson, 1978; Wolff,
Lundy and Baccus, 1988; Perrin and Mazalov, 1999; Ronce, 2007; Hidalgo,
De Casas and Muñoz, 2016). However, environmental variability is
arguably the most important driver for dispersal evolution. Many species
have offspring that either can or cannot disperse. Experimental research
has demonstrated that the ratio of dispersing to non-dispersing
offspring changes in response to the environment; For example, Sinervo
et al. demonstrated the interaction between maternal environmental
conditions, and the resulting offspring dispersal ratio in lizards
(Sinervo et al. , 2006). Dispersal plasticity is taxonomically
widespread and similar results are seen in animals, insects and plants
(Harrison, 1980; Fox and Mousseau, 1998; Dingle, 2006; King and Roff,
2010; Steiner et al. , 2012; Arendt, 2015; Duckworth, Belloni and
Anderson, 2015). The ability to alter offspring dispersal ratio is a
selective advantage when persisting in highly variable environments
(Arendt, 2015).
Within a habitat, there are often several sub-habitats. These
sub-habitats can differ in biotic and abiotic factors. Environmental
variability is determined by how much and how frequently these factors
fluctuate. When environmental conditions within sub-habitats are
constant, but the sub-habitats differ in quality, a non-dispersal
strategy is optimal. This is because offspring dispersing from the
native sub-habitat will encounter lower quality sub-habitats more often
than higher quality sub-habitats (Hastings, 1983). However, in
bet-hedging scenarios, if the environment fluctuates, with sites of
differing quality across time, but statistically the same overall
quality, then producing some dispersing offspring is optimal (Harper,
1977; Den Boer, 1981; Venable and Brown, 1993; Baskin and Baskin, 1998;
Starrfelt and Kokko, 2012). If the environmental conditions in
sub-habitats fluctuate and their quality is overall statistically
different, it is possible that dispersal rates are sub-habitat specific
(McPeek and Holt, 1992). But when sub-habitats are of more or less equal
quality, yet differ in variability, what dispersal strategies will
evolve? Will dispersal be adaptive, and if so, will the dispersal rates
be habitat specific (Seale and Nakayama, 2019)?
In mountainous habitats, the sub-habitat at the top of the mountain can
be considerably different from the sub-habitat at the bottom of the
mountain. If neither sub-habitat is of substantially higher quality, and
environmental variability is different within each, is sensing and
site-specific dispersal adaptive? Here, we use the term “site-specific
dispersal” to describe altering the ratio of dispersing to
non-dispersing offspring produced by an individual, in response to the
environment that they experience during their lifetime. An example of
such site-specific dispersal is given by the plant Aethionema
arabicum. Ae. arabicum is an annual which grows along the
steep stony slopes of the Anatolian Mountains, at a range of 0-3000
meters above sea level (Bhattacharya et al. , 2019). Ae.
arabicum exhibits fruit and seed dispersal dimorphism and is able to
undergo site-specific dispersal between sub-habitats of similar quality
but different variability by sensing its location through temperature
(Fig. 1) (Lenser et al. , 2016; Mohammadin et al. , 2017;
Arshad et al. , 2019; Seale and Nakayama, 2019).
Mountainous habitats, including the Anatolian Mountains, can be roughly
divided into two sub-habitats: high elevation, and low elevation
(Velchev, 1984; Mohammadin et al. , 2017). Neither sub-habitat is
favourable, neither are optimal. The higher elevation is dry, exposed,
and rocky, making the abiotic conditions unfavourable, however, there is
little to no competition. At low elevation, the environment is
overcrowded, shaded and highly competitive, providing many biotic
stresses. However, the constant availability of water supply and
nutrients (Atalay, 2006). One sub-habitat is environmentally variable in
terms of abiotic stresses but with few competition stresses, while the
other is environmentally constant but much more over-crowded, making
them of both low quality and differing in variability.