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.