Pruning of host plant neighbours as defence against enemy ant invasions

Crematogaster ant partners of Macaranga protected by "wax barriers" prune less than their congeners

Walter Federle, Ulrich Maschwitz, Berthold Hoelldobler

Research output: Contribution to journalArticle

31 Citations (Scopus)

Abstract

Many ant partners of tropical ant-plants prune the leaves and shoot tips of other plants growing around their hosts. According to the hypothesis proposed by Davidson et al. (Ecology 69:801-808), this specialized behaviour not only protects the host plants against over-growth, but it also conveys a direct benefit to the ant colony as it removes contact points to the neighbouring vegetation where invasions of enemy ants could occur. Here we test this hypothesis by comparing pruning intensity in five closely related Crematogaster (subgenus Decacrema) plant-ant species (and one species of Technomyrmex) that differ in their exposure to competition by other ants. Pruning intensity was quantified by measuring the area loss of paper tape pieces wrapped around the stems of Macaranga host plants. All Crematogaster (Decacrema) ants tested but not Technomyrmex sp. pruned, but the intensity of the behaviour varied strongly between and within species. Pruning was significantly weaker in the three tested Crematogaster species inhabiting Macaranga host plants with a slippery, waxy stem surface, which functions as a mechanical barrier protecting the specific ant partners against generalist competitors. Pruning was generally stronger on more densely ant-populated trees. Even though the number of ants per twig length was lower in associations of ants with glaucous Macaranga hosts, only part of the variation of pruning activity could be explained by "ant density". When corrected for ant density, "wax-running" Crematogaster (Decacrema) ants still pruned more weakly than their congeners inhabiting non-glaucous Macaranga hosts. Pruning is obviously most important when an ant-plant is potentially accessible to intruders, but less necessary when the ant colony is isolated by a protective wax barrier. Our results support the hypothesis that "selfish" defence against invasions is the major selective pressure that has led to the development and maintenance of pruning behaviour in weakly competitive plant-ants.

Original languageEnglish (US)
Pages (from-to)264-270
Number of pages7
JournalOecologia
Volume132
Issue number2
DOIs
StatePublished - 2002
Externally publishedYes

Fingerprint

Macaranga
Crematogaster
prunes
pruning
wax
waxes
host plant
ant
Formicidae
host plants
Technomyrmex
defence
stem
stems

Keywords

  • Allelopathy
  • Ant competition
  • Ant-plant
  • Mutualism
  • Myrmecophytism

ASJC Scopus subject areas

  • Ecology

Cite this

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title = "Pruning of host plant neighbours as defence against enemy ant invasions: Crematogaster ant partners of Macaranga protected by {"}wax barriers{"} prune less than their congeners",
abstract = "Many ant partners of tropical ant-plants prune the leaves and shoot tips of other plants growing around their hosts. According to the hypothesis proposed by Davidson et al. (Ecology 69:801-808), this specialized behaviour not only protects the host plants against over-growth, but it also conveys a direct benefit to the ant colony as it removes contact points to the neighbouring vegetation where invasions of enemy ants could occur. Here we test this hypothesis by comparing pruning intensity in five closely related Crematogaster (subgenus Decacrema) plant-ant species (and one species of Technomyrmex) that differ in their exposure to competition by other ants. Pruning intensity was quantified by measuring the area loss of paper tape pieces wrapped around the stems of Macaranga host plants. All Crematogaster (Decacrema) ants tested but not Technomyrmex sp. pruned, but the intensity of the behaviour varied strongly between and within species. Pruning was significantly weaker in the three tested Crematogaster species inhabiting Macaranga host plants with a slippery, waxy stem surface, which functions as a mechanical barrier protecting the specific ant partners against generalist competitors. Pruning was generally stronger on more densely ant-populated trees. Even though the number of ants per twig length was lower in associations of ants with glaucous Macaranga hosts, only part of the variation of pruning activity could be explained by {"}ant density{"}. When corrected for ant density, {"}wax-running{"} Crematogaster (Decacrema) ants still pruned more weakly than their congeners inhabiting non-glaucous Macaranga hosts. Pruning is obviously most important when an ant-plant is potentially accessible to intruders, but less necessary when the ant colony is isolated by a protective wax barrier. Our results support the hypothesis that {"}selfish{"} defence against invasions is the major selective pressure that has led to the development and maintenance of pruning behaviour in weakly competitive plant-ants.",
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