Coexistence of mutualist thorn-dwelling ants on camelthorn acacia in Namibia

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

2. 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

3. Study site & species • 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 Camelthorn acaciadominated savannah, Kuzikus Wildlife Reserve in Namibian Central Kalahari Gubb, 1988; Curtis & Mannheimer, 2005

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

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

6. Nest site selection & ant coexistence Domatia volume differed across thorns occupied by the three ant species and for unoccupied thorns Mean (+ SE) volume (mm3) of swollen-thorn domatia varied significantly with occupant ant species (linear mixed-effects model, Cataulacusn = 40, Crematogaster sp. n = 58, Tapinoma n= 98 and unoccupied n= 135). Letters indicate significant differences (p < 0.01). Campbell, Fellowes & Cook (2013) Insectes Sociaux

7. 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 (mm2) of swollen-thorn domatia varied significantly with occupant ant species (linear mixed-effects model, Cataulacus n= 40, Crematogaster sp. n = 58, Tapinoman = 98 and unoccupied n= 135). Letters indicate significant differences (p < 0.001). Campbell, Fellowes & Cook (2013) Insectes Sociaux

8. Nest site selection & ant coexistence 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. Cataulacus intrudens ___ Crematogaster sp. …… Tapinoma subtile---- Total number of ant occupants within swollen-thorn domatia were correlated with species identity and thorn volume (ANCOVA, F5, 190 = 41.36, R2 = 0.52, p < 0.00001), 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-15.247) and Tapinoma(y =12.805x-27.157) Campbell, Fellowes & Cook (2013) Insectes Sociaux

9. 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

10. Provision of antiherbivore defence by ants The experimental exclusion of ants led to an increase in some insect herbivores, but not lepidopterans. 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 0.0167 to determine significance. Campbell, Townsend, Fellowes & Cook (In press)African Journal of Ecology

11. Provision of antiherbivore defence by ants 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) Mean (+SE) number of ants present on branch prior to experiment was correlated with response to presence of a G. postica larva (ANOVA, F2, 24=12.2, P<0.001, letters indicate significant differences between groups) Campbell, Townsend, Fellowes & Cook (In press)African Journal of Ecology

12. 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

13. Acknowledgements • Ministry of Environment and Tourism, Namibia • Field assistants - Gisele Herren & Ian Townsend • Ant identification - Bonnie Blaimer, Brian Taylor, Phil Ward & Peter Hawkes Want to know more? • Email: h.campbell@pgr.reading.ac.uk • Blog: www.scienceheather.wordpress.com • Twitter: @scienceheather • Talk & data available: www.figshare.com