Coexistence of mutualist thorn-dwelling ants on camelthorn acacia in Namibia Heather Campbell, Mark Fellowes & James Cook School of Biological Sciences, University of

Ant-plant mutualisms Ant–plant mutualisms widespread (>100 genera of tropical plants & 5/12 ant subfamilies Ants defend plants from herbivores & receive food (extrafloral nectar and/or food bodies) Myrmecophytes = specialised plants with structures modified for ants, known as domatia Single host plant can associate with multiple ant species throughout its lifetime or across geographic range but usually only a single ant species at a time Different ant species may vary their provision of antiherbivore defence

Gubb, 1988; Curtis & Mannheimer, 2005 Study site & species Camelthorn acacia dominated savannah, Kuzikus Wildlife Reserve in Namibian Central Kalahari Camelthorn acacia, Vachellia erioloba, is a keystone species & most widespread tree in Namibia Under threat from harvesting & land use changes Possesses hollow swollen-thorn domatia inhabited by ants, but species & nesting patterns are unidentified

Nest site selection & ant coexistence We found four ant species from different genera Unidentified Crematogaster sp. Cataulacus intrudensTapinoma subtileTetraponera ambigua Campbell, Fellowes & Cook (In prep.) In contrast to other African ant plants, many V. erioloba trees (95 % in our survey) were simultaneously co-occupied by multiple ant species

Co-occupancy of myrmecophyte plants by multiple ant species Provision of antiherbivore defence Do domatia characteristics on host plants a) influence patterns of ant distributions? b) enable/limit ant species coexistence? Does each ant species a) provide antiherbivore defence? b) differ in their effectiveness? Nest site selection & coexistence

Nest site selection & ant coexistence Mean (+ SE) volume (mm 3 ) of swollen-thorn domatia varied significantly with occupant ant species (linear mixed-effects model, Cataulacus n = 40, Crematogaster sp. n = 58, Tapinoma n = 98 and unoccupied n = 135). Letters indicate significant differences (p < 0.01). Domatia volume differed across thorns occupied by the three ant species and for unoccupied thorns Campbell, Fellowes & Cook (2013) Insectes Sociaux

Nest site selection & ant coexistence Nest entrance hole size differed between the three ant species occupying thorns and unoccupied thorns Mean (+ SE) area of entrance hole (mm 2 ) of swollen-thorn domatia varied significantly with occupant ant species (linear mixed-effects model, Cataulacus n = 40, Crematogaster sp. n = 58, Tapinoma n = 98 and unoccupied n = 135). Letters indicate significant differences (p < 0.001). Campbell, Fellowes & Cook (2013) Insectes Sociaux

Number of ants correlated with species and thorn volume, including interaction in predicting numbers of individuals (also true for proportion of ants that were immatures). Each species has different nest requirements and different colony response in investment in different life stages. Nest site selection & ant coexistence Total number of ant occupants within swollen-thorn domatia were correlated with species identity and thorn volume (ANCOVA, F 5, 190 = 41.36, R 2 = 0.52, p < ), as well as interaction term (p < 0.05), resulting in different slopes for each species in predicting the response in numbers of ants to thorn volume; Cataulacus (y = 3.206x-4.658), Crematogaster sp. (y =5.845x ) and Tapinoma (y =12.805x ) Campbell, Fellowes & Cook (2013) Insectes Sociaux Cataulacus intrudens __  _ Crematogaster sp. …  … Tapinoma subtile --  --

Provision of antiherbivore defence by ants Herbivore damage was higher on branches without ants Mean (+SE) percentage of leaflets damaged per branch. Black bars are branches with ants experimentally excluded and white bars represent branches with patrolling ants. Campbell, Townsend, Fellowes & Cook (In press) African Journal of Ecology

Provision of antiherbivore defence by ants Explanatory variableDevianceFP Coleoptera Treatment <0.01 Trial <0.05 Treatment x Trial Hemiptera Treatment < Trial <0.01 Treatment x Trial Lepidoptera Treatment Trial Treatment x Trial ANODEV table for generalised linear model with quasi-Poisson errors. The minimal adequate model was found by elimination of non-significant terms, beginning from the maximal model. As three separate tests were conducted, we used a Bonferroni- corrected α of to determine significance. Campbell, Townsend, Fellowes & Cook (In press) African Journal of Ecology The experimental exclusion of ants led to an increase in some insect herbivores, but not lepidopterans.

Provision of antiherbivore defence by ants Mean (+SE) number of ants present on branch prior to experiment was correlated with response to presence of a G. postica larva (ANOVA, F 2, 24 =12.2, P<0.001, letters indicate significant differences between groups) Campbell, Townsend, Fellowes & Cook (In press) African Journal of Ecology Across all ant species, larvae of G. postica usually ignored (64% of time) Aggressive behaviour only by Crematogaster approx. half of the time Successful eviction correlated with number of ants & tending of scales/treehoppers (P <0.001)

Conclusion Most V. erioloba trees are simultaneously & stably co-occupied by multiple ant species – great opportunity to study coexistence & mutualism Each ant species occupies nests with different domatia morphology - nest size & availability influences ant colony (different responses in investment in immatures according to availability of nest space) Presence of ants reduces herbivore damage by some types of insect herbivores but not all (effective against hemipterans and coleopterans, but not lepidopterans) Only Crematogaster ants show aggression to herbivores but success depends on high numbers of ants & tending of scales/treehoppers

Acknowledgements Want to know more?  Ministry of Environment and Tourism, Namibia  Field assistants - Gisele Herren & Ian Townsend  Ant identification - Bonnie Blaimer, Brian Taylor, Phil Ward & Peter Hawkes Blog: Talk & data available: