Ant mutualist mediated reverse Janzen-Connell hypothesis in New Guinean ant plants

Shafia ZAHRA

Doctoral thesis

Abstract:

In this thesis, I investigate the presence of reverse Janzen-Connell effects in ant-plant communities in Papua New Guinea. Janzen-Connell effects help to explain species diversity maintenance by focusing on natural enemies' role in controlling plant distributions. It is postulated that spillover of enemies near adult plants reduces seedling survival, promoting coexistence and diversity. Conversely, reverse Janzen-Connell effects occur when mutualists enhance survival near conspecific adults, potentially creating larger recruitment zones leading in the extreme to species monodominance. I use a combination of observation and ant exclusion experiments, as well as spatial analysis to explore reverse Janzen-Connell effects. Ant-plant interactions, particularly myrmecophytic ones, show moderate species specificity, a key assumption of classic Janzen-Connell effects. Mutualistic ants enhance plant viability by eliminating herbivores and pathogens, reducing damage, and optimizing photosynthesis. I also explore the specificity of myrmecophytic systems compared to other ant-plant interactions across environments using a data collation approach. I speculate that reverse Janzen-Connell effects can be observed in ant-plant mutualisms, ectomycorrhizal fungi, and plants. I also found through a global meta-analysis that myrmecophytic networks are the most specialized, with specialisation being driven by network type rather than environmental factors. Thus, myrmecophytes are a suitable system to explore the presence of reverse Janzen-Connell effects. Spatial analysis showed the presence of reverse Janzen-Connell effects in plants in seedlings of Meliaceae family and less strongly in adults. In the ant exclusion experiments, conspecific saplings exhibited higher survival and growth, but this was not related to the treatment itself meaning the plants are potentially benefiting from root fungi from adult conspecifics. Saplings with ant patrolling had better survival, although caution is needed due to small sample size. Lastly, I found that ant inhabitation positively influenced growth in older ant-plants, but this was not linked to local ant-plant density, and leaf loss from herbivory was lower when saplings were near adult conspecifics, possibly due to protective volatiles. Taken together this suggests some weak evidence for reverse JanzenConnell effects in the lowland rainforest of Papua New Guinea.

Language used: English

Date on which the thesis was submitted / produced: 20. 5. 2024

Thesis defence

• Supervisor: Tom Maurice Fayle, Ph.D.

UNIVERSITY OF SOUTH BOHEMIA IN ČESKÉ BUDĚJOVICE

Faculty of Science

Doctoral programme / field:
Biology / Entomology