Why are eucalypt trees so poor for ants?

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In this Post, I put together two aspects of Australian natural history that may not have been fully linked by naturalists previously. These are:

• the paucity and scarcity of ants on eucalypt trees, and
• the richness and abundance of short-billed birds competing for the same resource-category, viz. carbohydrate-rich exudates, on the same trees.

Australia has a remarkable incidence of ants (Formicidae), in terms of diversity, abundance, and the presence of archaic lineages such as Myrmeciinae (https://www.antwiki.org/wiki/Australian_ants and https://en.wikipedia.org/wiki/List_of_ants_of_Australia and https://en.wikipedia.org/wiki/Myrmeciinae).

Australia is also known for its diversity and abundance of indigenous eucalypts (https://en.wikipedia.org/wiki/Eucalypt#:~:text=Eucalypt%20is%20a%20descriptive%20name,%2C%20Allosyncarpia%2C%20Eucalyptopsis%20and%20Arillastrum.), particularly Eucalyptus (https://en.wikipedia.org/wiki/Eucalyptus), Corymbia (https://en.wikipedia.org/wiki/Corymbia), and Angophora (https://en.wikipedia.org/wiki/Angophora).

It is therefore puzzling that ants tend not to occur on eucalypts, even where they nest in diversity and abundance on the ground beneath the trees (https://espace.curtin.edu.au/bitstream/handle/20.500.11937/38928/19100_downloaded_stream_192.pdf?sequence=2).

In southwestern Western Australia for example, there are about 500 species of ants in 12 subfamilies and 61 genera. However, only a few species qualify as arboreal (https://museum.wa.gov.au/sites/default/files/1.%20Heterick.pdf). Examples are:

• Podomyrma spp. (https://www.antwiki.org/wiki/Podomyrma and https://www.antwiki.org/wiki/Podomyrma_adelaidae), and
• Tetraponera punctulata (https://www.brisbaneinsects.com/brisbane_ants/Pseudomyrmecinae.htm).

Furthermore, eucalypts are relatively unattractive to ants even when planted on other continents (e.g. https://www.nature.com/articles/s41598-018-20823-1 and https://www.researchgate.net/publication/323266274_Homogenization_and_impoverishment_of_taxonomic_and_functional_diversity_of_ants_in_Eucalyptus_plantations and https://www.scielo.br/j/bn/a/tWRkqTJNR5jHYR77dvgrG7M/?lang=en and https://onlinelibrary.wiley.com/doi/epdf/10.1111/aec.13060 and https://www.scielo.br/j/bn/a/tWRkqTJNR5jHYR77dvgrG7M/?format=pdf&lang=en).

The relative lack of ants in the most typical of Australian trees is puzzling because:

• eucalypts are an affluent source of carbohydrate-rich exudates, which are a major category of food for arboreal ants worldwide,
• eucalypts tend to have hollow trunks and branches (https://www.environment.nsw.gov.au/resources/nature/Factsheet5TreeHollows.pdf), providing cavities for nesting by insects,
• although many species of eucalypts have smooth bark, many others have fissured/rough bark providing safe passage for ants (https://hunterlandcare.org.au/eucalypt-bark-types/), and
• genera of ants particularly associated with trees on all other vegetated continents, such as Camponotus (https://en.wikipedia.org/wiki/Carpenter_ant) and Crematogaster (https://en.wikipedia.org/wiki/Crematogaster), are indigenous over most of Australia but largely terrestrial here.

My explanation for the poverty of eucalypt trees for ants is basically as follows:

the carbohydrate-rich exudates mediated by sap-sucking hemipteran insects tend to take solid forms on eucalypts, less suitable for ants than for certain distinctively Australian birds (https://www.birdlife.org.au/australian-birdlife/detail/exuding-abundance).

In particular, lerp insects produce starchy - as opposed to sugary - structures called lerps (https://en.wikipedia.org/wiki/Lerp_(biology)). These are reliable enough in Australia to support specialised, short-beaked birds such as:

• weebills (https://en.wikipedia.org/wiki/Weebill#Diet), which are extremely small-bodied,
• pardalotes (https://en.wikipedia.org/wiki/Pardalote#Behaviour_and_ecology), which are small-bodied, and
• certain meliphagids (https://en.wikipedia.org/wiki/Honeyeater) such as Melithreptus (https://en.wikipedia.org/wiki/Melithreptus), which are relatively large-bodied.

Sap-sucking hemipterans, in their quest for protein, routinely excrete superfluous carbohydrates. These are available, in various forms, to other animals. A watery form is honeydew (https://en.wikipedia.org/wiki/Honeydew_(secretion)), while a solid form is lerp: starchy shells built by the insects (https://en.wikipedia.org/wiki/Psyllidae#/media/File:Eucalyptus_Lerp_psyllid.jpg and https://en.wikipedia.org/wiki/Lerp_(biology)).

Australia is the continent poorest in aphids (https://en.wikipedia.org/wiki/Aphid) and possibly mealybugs (https://en.wikipedia.org/wiki/Mealybug and https://www.agric.wa.gov.au/control-methods/aphids-mealybugs-and-scales?page=0%2C1). The place of these sap-suckers is instead largely taken by other hemipterans.

A particularly relevant group is Psylloidea: Aphalaridae: e.g. Cardiaspina (https://australian.museum/learn/species-identification/ask-an-expert/what-do-psyllids-look-like/ and https://en.wikipedia.org/wiki/Psylloidea and https://en.wikipedia.org/wiki/Psyllidae and https://en.wikipedia.org/wiki/Aphalaridae and https://en.wikipedia.org/wiki/Cardiaspina).

Another member of Aphalaridae, namely Eucalyptolyma maideni, produces lerp in the form of large (1 cm long), complex structures (https://www.inaturalist.org/observations?place_id=any&taxon_id=362773 and https://www.jungledragon.com/specie/21476/spotted_gum_lerp_psyllid.html).

The relative lack of honeydew applies not only to eucalypts but also to other trees indigenous to Australia (https://www.eucalyptus.com.br/artigos/22_Insect+Pests+Eucalyptus+Australia.pdf and https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0007/390283/Psyllids-Insect-Pests-of-Eucalypts.pdf and https://www.eucalyptus.com.br/artigos/22_Insect+Pests+Eucalyptus+Australia.pdf and https://www.annualreviews.org/doi/abs/10.1146/annurev.en.36.010191.003225?journalCode=ento and https://www.proquest.com/openview/e1ec5d4391f6a1625c1453f4f8431838/1?pq-origsite=gscholar&cbl=33543).

Aphids tend to spare the energetic costs of protecting themselves from predators, by relying on ants to defend their soft, vulnerable bodies (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1685857/ and https://www.researchgate.net/publication/338536831_Ant-Hemiptera_Associations). This relationship is mutualistic, with honeydew serving as the currency paid to the ants by the aphids.

By contrast, lerp insects (Psylloidea) tend to be armoured: they invest in shells which are more difficult for ants to handle, to transport, and to digest than is honeydew. This tips the competitive advantage towards relatively large-bodied consumers such as small birds with suitably-adapted beaks.

One of the reasons why this explanation has not previously been offered (e.g. see page 8 of https://espace.curtin.edu.au/bitstream/handle/20.500.11937/38928/19100_downloaded_stream_192.pdf?sequence=2) is that it places certain birds in the same guild (https://en.wikipedia.org/wiki/Guild_(ecology)) as certain ants - despite the phylogenetic divide.

When birds are considered in the context of ants, it is normal to think of them as predators rather than competitors. This is partly because even the smallest of the bird species invoked here is far more massive than any individual ant. However, the comparison may make more sense when one bears in mind that colonial insects are partly clonal, the collection of 'individuals' being analogous to the individual body of a vertebrate.

Posted on Μάρτιος 23, 2022 1107 ΜΜ by milewski