Biogeography of the Californian Trapdoor Spider

Basic Ecology of the Californian Trapdoor Spider

Californian trapdoor spiders are found in a wide range of habitats and climates,  ranging from hot, dry cresote bush scrub to cool montane red fir forests, and wet coast redwood forests.  They can be found in elevations from from sea level to 3080m.

Trapdoor Spider Burrows

Species construct tubular silk-lined burrows in the ground and prey nocturnarally upon invertebrates which wander within reach of the entrance.  This entrance is well camouflaged and is closed off by a collapsible silken collar.

Species prefer ground surfaces that are sheltered, relatively stable, sloping,  and either north-facing or otherwise shaded. 

Burrows are seldom found in soil that is subject to erosion, but rather in soil bound together by roots and/or sheltered under roots or rocks.

General Distribution of the California Trapdoor Spider

Aliatypus is a mygalomorph genus consisting of eleven described species (Coyle 1994).  Ten live exclusively in California and one species exists in Arizona. These species are:

• A. aquilonius
• A. californicus
• A. erebus
• A. gnomus
• A. gulosus
• A. isolatus
• A. janus
• A. plutonis
• A. thompsoni
• A. trophonius
• A. torridus

Californian trapdoor spiders are found in a wide range of habitats and climates, ranging from hot, dry cresote bush scrub to cool montane red fir forests and wet coast redwood forests (Coyle 1974). 

They can be found spanning elevations from sea level to 3080m. 

With the exception of A. isolatus (which is found in Arizona), all of the spiders are found exclusively within the state of California.  A. aquillonius  is found in Nothern California, A. erebus, A. trophonius, A. gnomus, A. californicus,  and  A. janus  are found in Central California,  and  A. thompsoni, A. gulosus, A. torridus,  and A. plutonis  are found in Southern California. 

Of these eleven, three occur in disjunct populations.  A. erebus  and A.californica  are separated into into coastal and Sierran populations by the Central Valley.

Dispersal and Barriers to Gene Flow to the Californian  Trapdoor Spider

The overwhelming barrier to dispersal and hence gene in the genus Aliatypus is climate. Temperature ranges of too hot or too cold restrict movement among these spiders. More importantly, environments with low humidity form oftentime insurmountable barriers.

Under natural conditions, aquatic raftings, short distance spiderling dispersal, and male wandering are the only important means of dipsersal.

Furthermore, speciation within this lineage, has been also facilitated by low vagility. These species experience no aerial dispersal and are burrow bound. Analysis of other taxon experience these conditions yields similar results (ex. salamander genus Batrachoceps ).

Intrageneric speciation

Aliatypus is monophyletic, leading to the hypothesis by Coyle that the shifting climatic conditions of the California gave rise to this genus.

As the California climate became drier, barriers to migration and dispersal arose, fragmenting populations of this genus.

Pollen analysis shows that 17,000 to 23,000 years ago, favourable woodland habitat extended continously from current A. isolatus localities to the present range of A. janus.

These sister species were formed when the interglacial expansion of the Great Basin and Sonaran deserts disjuncted their widespread common ancesteral population. A. californicus and A. gnomus, A. janus and A. aquilonius, and A. erebus and A. trophonius are also hypothesized to be sister species that were formed when large populations were severed by the formation of more arid habitat during climatic changes in California.

Minor adaptive radiation is evident in this genus and has resulted in a set of species adapted to a wide range of habitat types and climates.  

A. plutonis, A. torridus, A. gulosus, and A. thompsonis have invaded particularly arid habitats. A. janus and A. isolatus are adapted to higher elevation, cold winter habitats. Body size and burrow depth are varied adaptations in this genus.

Interspecific variation and speciation

There are three species of Aliatypus which experience disjunct populations: A. erebus, A. Californicus and A. isolatus.

For the former two species, the central valley of California, a semi-arid grassland, appears to be a strong barrier to gene flow between the isolated coastal and Sierran populations.

It is thought that during the Pleistocene, dispersal across the Central Valley occurred through woodland habitats. In 1974, discoveries of isolates of both A. californicus in Sutter Buttes supports the hypothesis that this area in the Central Valley was a corridor allowing gene flow.

Within the A. isolatus populations, barriers from grassland and desert habitat which arose all over the southwest after the most recent glacial period isolated the mesic, montane populations.

Even within contigous populations, slow gene movement and peripheral habitats have contributed to variant populations. A. thompsoni, A. californicus, and A. janus show morphological and genetic variation within contiguous populations. Low d microhabitat differences have been proposed to account for this phenomenon.

Synopty

Aliatypus  species are usually syntopica (living side by side) with one or more species of burrowing mygalomorph spiders.  These tend to be species which are potentially their most important competitors. 

However, differences in habitat and microhabitat preferences tend to exclude competition.  Antrodiaetus pacificus,  for example, mostly prefers more humid and cooler habitats than most Aliatypus  species.   

Bothriocyrtum californicum  inhabits dry, south-facing, gently sloping hillsides or level terrain, areas where Aliatypus  species tend to be absent.

Synopty with other species of Aliatypus  is relatively rare.  In areas where there is a broad overlapping of ranges, differences in microhabitat preferences account for the lack of synopty. 

For example, although A. california  and A. erebus  overlap considerably, A. erebus  is more tolerant of drier and colder environments and therefore differs in its microhabitat.

In both cases, where there is an overlapping of habitat preference, one species tends to be more common than the other.  Aptostichus  species tend to be more common  in drier, more southern areas that harbor bother genera. 

A. gulosus  and A. thompsoni   have been found living together in ravines in Chattsworth and Eaton Canyon Park.  However, data collections shows that A. thompsoni  is more abundant.

Conclusion

This genus is still remarkably underresearched and many questions still lie unanswered or need further study.  The primary research, F. Coyle, also points out that many demes of this genus are under threat or have already been destroyed as a result of urban, agricultural and other anthropogenic pressures. 

In one of the early notations of this genus of spiders (Hutchinson 1904, pp. 83), it was already pointed out that agriculture, particularly as a result of the plow, presented a “formidable enemy” to the survival of the Californian

References

Coyle, F.A., 1974. Systematics of the trapdoor spider genus Aliatypus (Araneae: Antrodiaetidae). Psyche 81, 431-500.

Coyle, F.A. 1994. Cladistic analysis of the species of the trapdoor spider genus Aliatypus (Araneae, Antrodieatidae). Journal of Arachnology 22:218-224.

Coyle, F.A. and W.R. Icenogle. 1994. Natural history of the California trapdoor spider genus Aliatypus (Araneae, Antrodiaetidae). Journal of Arachnology 22:225-255.

C. E. Hutchinson.  1904.  A Trapdoor Spider.  Scientific American July 30, 1904

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