Latitudinal effects on the ants of sandy soils in coastal southwestern Australia

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Ants are ectothermic, and thus likely to be affected by solar radiation. Given constant substrates, how do ant communities vary with changes in latitude, within the mediterranean-type climate in Australia?

Here I compare the ant communities - as assessed by pitfall trapping and faunal inventories - between the Perth Metropolitan area (latitude 32 degrees South, https://en.wikipedia.org/wiki/Perth) of Western Australia and Fitzgerald River National Park (34 degrees South, https://en.wikipedia.org/wiki/Fitzgerald_River_National_Park) on the southern coast of the same state.

These coastal study areas differ in latitude by two degrees, with a corresponding difference in solar radiation.

Pitfall trapping was conducted by Rossbach and Majer (1982) in the northern study area (https://espace.curtin.edu.au/bitstream/handle/20.500.11937/18075/19045_downloaded_stream_137.pdf?sequence=2), and by me (in 1982) in the southern study area. I thank J D Majer, R D Wooller, and Julianne Waldock for their generous help in this study.

The regional faunal inventories are those of Heterick (2009, https://museum.wa.gov.au/sites/default/files/1.%20Heterick.pdf).

Nutrient-poor, siliceous sands and partly calcareous sands were sampled in both study areas. The vegetation was Banksia over species-rich heath on the siliceous sands and phyllodinous Acacia on the partly calcareous sands, in both study areas.

The total number of ant individuals trapped did not differ greatly according to substrate in either study area. However, the number seemed overall at least 50% greater in the northern than in the southern study area. This suggests that ants were more abundant, or more active, or both, under the greater solar radiation of the northern study area.

The results:

PITFALL TRAPPING

The ants can be divided into the following groups, with respect to the latitudinal distinction as well as the distinction in substrates.

Indiscriminate (although the species differ in most cases):

• Rhytidoponera metallica var. inornata (https://www.inaturalist.org/taxa/201975-Rhytidoponera-metallica)
• Monomorium as genus
• Crematogaster as genus
• Meranoplus as genus (but see below for differences shown by faunal inventories at a regional scale)
• Iridomyrmex as genus
• Camponotus as genus

Trapped in northern area only (* on both siliceous and partly calcareous sands):

• possibly Rhytidoponera violacea* (https://www.inaturalist.org/taxa/1271343-Rhytidoponera-violacea)
• Aphaenogaster sp. (https://www.inaturalist.org/taxa/68931-Aphaenogaster)
• Cardiocondyla 'nuda'* (https://www.antwiki.org/wiki/Cardiocondyla_nuda)
• Pheidole variabilis latigena* (https://bie.ala.org.au/species/urn:lsid:biodiversity.org.au:afd.taxon:289c46cb-323e-42bc-8a7f-22b350fbcb4e)
• Iridomyrmex agilis* (https://www.inaturalist.org/taxa/202004-Iridomyrmex-agilis)
• Notoncus gilberti* (https://www.inaturalist.org/taxa/967271-Notoncus-gilberti)

Trapped in southern area only:

• Pheidole sp. JDM 399 (see https://www.inaturalist.org/observations?place_id=6827&taxon_id=47343&view=species)
• Chelaner sp. or spp. (see https://www.inaturalist.org/taxa/1286771-Chelaner-kiliani)
• Pachycondyla (Brachyponera) lutea (https://www.inaturalist.org/taxa/967177-Brachyponera-lutea)
• Tapinoma sp. (see https://www.inaturalist.org/observations?place_id=6744&taxon_id=47341&view=species)

Restricted to siliceous sands:

none, although Iridomyrmex conifer seems absent from calcareous sands in the Perth Metropolitan Area (https://www.inaturalist.org/taxa/896426-Iridomyrmex-conifer)

Restricted to partly calcareous sands:

• possibly Ochetellus glaber, at a local rather than geographical scale (https://www.inaturalist.org/taxa/367056-Ochetellus-glaber)

Partly calcareous sand tends to be more unstable than siliceous sand, possibly militating against those ants that nest in the ground.

The above results are rather nebulous, and difficult to interpret.

However, a clearer result emerges from Appendix 2 of Heterick (2009, https://museum.wa.gov.au/sites/default/files/1.%20Heterick.pdf), who has listed all the ant species on the more northerly Swan Coastal Plain and the more southerly Esperance Plains. These are the regions within which the Perth Metropolitan Area and Fitzgerald River National Park lie.

REGIONAL FAUNAL INVENTORIES

The more northerly area has a total of 218 species of ants, whereas the more southerly area has only 156 species.

This difference is noticeable in the following genera in particular, which contain far more species in the northerly area (Swan Coastal Plain) than in the southerly area (Esperance Plains):

• Cerapachys (9 spp. vs 1 sp.),
• Tetramorium (6 spp. vs 1 sp.),
• Melophorus (10 spp. vs 4 spp.),
• Meranoplus (9 spp. vs 3 spp.), and
• Podomyrma (5 spp. vs 1 sp.).

The latitudinal difference in the faunal inventories applies not only to Melophorus, known to be thermophilic and fast-moving, and to Podomyrma, one of the few genera in southwestern Australia specialised for arboreal activity and nesting, but also to at least one genus lacking any obvious relationship to solar radiation. Meranoplus is slow-moving, relying on armour and thanatosis rather than flight from potential predators.

In summary, there is some evidence of a latitudinal effect on ants in southwestern Western Australia.

Posted on March 20, 2022 09:34 PM by milewski