4.2 | Niche dynamics within the Mygalomorphae
That niche evolution has occurred in both directions several times
across the mygalomorph adaptive landscape (Fig. 1, 2) indicates that the
‘optimal’ niche changes depending on environmental conditions due to
trade-offs in niche dynamics (Winemiller et al., 2015). Aspects that
show patterns of variation across the adaptive landscape include
prey-capture area and method, predator defense, microhabitat, and
microclimate regulation.
If we consider the two extremes of the mygalomorph adaptive landscape,
we see strategies that vary across all four of the dimensions mentioned
above. Mygalomorph spiders rely heavily on substrate-borne vibrations to
detect prey, and their silken constructions (and the objects directly
attached to them) determine the size of their foraging area (Coyle,
1986; Main, 1982). Opportunistic, web-building taxa have extensive
prey-capture areas because they detect prey across the entire capture
web, which also helps to slow/entangle prey, decreasing the spider’s
need to physically restrain it (Coyle, 1986, 1995). Web-building taxa
construct no clearly-defensive structures except for the web itself and
tend to escape disturbance by retreating up fissures in the substrate
(JDW, pers. obs.), thus taking advantage of the complex microhabitats in
which they live, which must have adequate crevices under rocks, in or
around vegetation or under embankments for retreat construction (Coyle,
1995; Eberhard & Hazzi, 2013; Raven, 1983). As these spiders generally
do not burrow, they probably have less ability to regulate the
microclimate of their retreat and less protection against natural
disasters such as floods, although the retreats of some species will
follow natural crevices deep into embankments or under rocks, which may
serve a similar regulatory function to a burrow and explain the
occurrence of some opportunistic, web-building taxa in quite arid
environments (e.g., Cethegus in Australia, Raven, 1983;Euagrus in North and Central America, Coyle, 1988).
At the other end of the spectrum are burrowing and/or nesting taxa that
modify their entrance with a trapdoor. Observations suggest that some
trapdoor spiders will not strike at prey unless it touches the burrow
entrance or comes within millimeters of it, indicating a comparatively
tiny foraging area (Bond & Coyle, 1995; Coyle et al., 1992). Within
this tiny foraging area, they rely entirely on physicality and the
element of surprise to restrain prey, and this probably explains
adaptations such as the strong lateral spines found in many species with
trapdoors or other entrance modifications. Further evidence that a
trapdoor entrance reduces foraging area is provided by the multitude of
modifications that trapdoor-building species construct to extend their
sensory radius, including radiating silk- or twig-lines (Main, 1957; Rix
et al., 2017), soil tabs (Coyle & Icenogle, 1994), and foliage
‘moustaches’ (Rix et al., 2017) among others (Coyle, 1986). Open burrows
and/or burrows with other types of modification besides a trapdoor
probably increase the prey-capture radius relative to a trapdoor
entrance, as evidenced by Coyle (1986), who demonstrated that
collar-building Antrodiaetus enjoy a larger prey-capture area
than trapdoor-building Aliatypus (both family Antrodiaetidae),
primarily because strikes in the ‘dorsal sector’ are restricted in the
latter by the trapdoor hinge. Regarding predator/parasite defense, the
burrow is a double-edged sword, providing both camouflage and a means of
protection, but also limiting avenues of escape. Certain fungi, buthid
scorpions, pompilid wasps and acrocerid flies are known to specialize on
burrowing mygalomorph spiders (Kurczewski et al., 2021; Pérez-Miles &
Perafán, 2017), and predators such as centipedes (MGR, pers. obs.) and
even other araneophagic spiders may target them (Dippenaar-Schoeman,
2002). This has led to the evolution of myriad defensive strategies in
burrowing taxa, including secondary escape shafts (Harvey et al., 2018),
false bottoms (Main, 1985), spherical pellets used to block the entrance
(Leroy & Leroy, 2005), phragmotic abdomens (Rix et al., 2018),
urticating setae (Bertani & Guadanucci, 2013), and of course, entrance
modifications which camouflage the burrow and can be held closed against
intruders. Finally, the construction of a burrow allows access to
relatively bare habitats without natural crevices, and may also allow
greater regulation of the microclimate in the burrow (primarily
temperature and humidity), and resistance to natural disasters like
droughts and floods (Cloudsley-Thompson, 1983; Coyle, 1986). This
regulatory function may be further increased by modifications that allow
the burrow entrance to be closed, for example a trapdoor, which may
explain why, in families containing both trapdoor-builders and species
that utilize a more open entrance type, the trapdoor-builders are often
those that have spread into arid environments (e.g., in the Australian
Idiopidae, Rix et al., 2017, and the North American Antrodiaetidae,
Coyle, 1986). Although, there are also burrowing species with an open
entrance that have adapted and radiated in arid environments (e.g., the
theraphosid genus Aphonopelma , Hamilton et al., 2011, and the
anamid genus Aname , Rix et al., 2021), and direct experiments on
a trapdoor-building lycosid found that the trapdoor provides negligible
difference to conditions at the bottom of the burrow, indicating that it
may primarily serve other functions such as predator defense or flood
avoidance (Steves et al., 2021).
The evolution of nest retreats deserves specific discussion. Our results
indicate that nests have always evolved from burrowing,
trapdoor-building ancestors. As nests are short and presumably less
well-insulated than a burrow, these taxa probably lose some degree of
microclimate regulation, which explains why most nest-building taxa
occur in mesic environments (e.g., Migidae, Griswold & Ledford, 2001,Sason , Raven, 1986). However, Coyle (1986) points out a likely
benefit of nesting, which is that the spider can sense prey over the
entire exposed surface of the nest, expanding the foraging area relative
to a burrow. Many nests have two trapdoor entrances, one at each end,
and this probably allows greater exploitation of this expanded
prey-capture area and provides a second escape route from predators.
Nests also allow the exploitation of new microhabitats, as they are
often constructed off the ground, on tree trunks or cave walls (Decae et
al., 2021; Griswold & Ledford, 2001; Raven, 1986). In this way,
evolution from a burrow to a nest represents an evolutionary pathway
with similar trade-offs to the opportunistic, web-building niche: the
sacrifice of microclimate regulation for an expanded foraging area and
exploitation of a different microhabitat.
Patterns of niche trade-offs in the Mygalomorphae are clearly complex
and cannot be explained with reference to a single environmental
variable. Climate and weather, environmental complexity and niche
availability, and the abundance of predators and prey probably all play
a role in determining the success of a particular behavioral niche in an
environment. Furthermore, microhabitat differences mean that in optimal
conditions, species inhabiting different niches often occur together,
for example in sub-tropical eastern Australia many areas exist where
several burrowing (e.g., Idiopidae, Anamidae), nesting (Barychelidae,
Migidae) and opportunistic (Euagridae, Hexathelidae and Atracidae) taxa
occur in direct sympatry. In general, burrowing taxa probably have the
highest resilience to environmental extremes and are also able to
exploit relatively bare microhabitats. In contrast, web-building and
nest-building taxa probably require milder environmental conditions but
allow the spider to expand its foraging area and exploit new
microhabitats: existing spaces under logs, embankments and foliage for
opportunists, and hard substrates off the ground for nest-builders.