By Sachin Medigeshi Harish
I am a PhD student at Concordia University working under the supervision of Dr. Selvadurai Dayanandan. My current research is focused on the regeneration dynamics of forest tree communities in the biodiversity hotspot of the Western Ghats and Sri Lanka. In September (2019), I had the opportunity to participate in the Association for Tropical Biology and Conservation Asia-Pacific chapter meeting in Thulhiriya, Sri Lanka. I am extremely grateful to the QCBS for providing me the financial support needed to attend the conference.
I had the opportunity to present my research during the session about species interaction and community ecology. My talk focused on the Janzen Connell hypothesis—one of the prominent explanations for biotic neighbourhood interactions such as competition and facilitation among plant individuals inter- or intra specifically. This hypothesis assumes that species-specific natural enemies—such as pathogens and herbivores—drive conspecific neighbour interactions between tropical tree communities. According to this hypothesis, individuals will experience a reduction in their probability of survival when surrounded by a high density of conspecific neighbours. This process leads to an increase in space availability for new species recruitment and reduces competitive exclusion.
To test this hypothesis, we analysed the tree survival rates of 5 scrub forest (SF) and 5 deciduous forest (DF) plots, located within the Biligiri Ranganatha Swamy Temple Tiger Reserve (BRT) in the Western Ghats, India. Within a 100 m X 100 m plot all woody trees having 1 cm in diameter at breast height (dbh) were tagged, mapped, measured and identified to species level. To examine neighbourhood effects on the survival of individual saplings (1-4.9 cm dbh), we used generalised linear mixed effect models with binomial errors to model individual survival as a function of conspecific and heterospecific neighbourhood density.
Our results showed that conspecific density had a strong negative effect on the survival of saplings within scrub forest plots. These results may be due to intraspecific competition for below-ground resources and attack by specialist natural enemies. Conversely, heterospecific density had a strong negative effect on the survival of saplings within deciduous forests. This effect was likely driven by interspecific competition for below-ground resources. However, heterospecific positive density dependence on survival in the SF can be attributed to herd immunity effect in which specialised natural enemies have a harder time locating their host tree saplings, when non-susceptible neighbours are in proximity.
The effects of heterospecific neighbours varied more widely for individual species survival than effects of conspecific neighbours, in both scrub and deciduous forest plots. Hence, we concluded that heterospecific density is likely to play a larger role in shaping species’ relative abundance in the dry tropical forest communities of BRT.
The feedback I received after my presentation and sharing of ideas with Prof. Vojtěch Novotný, of the Czech Academy of Sciences and Prof. Savitri Gunatilleke, of the University of Peradeniya was particularly valuable in both the interpretation of my results and my understanding of community assembly of tropical trees. Finally, this meeting provided many exciting networking opportunities that will contribute positively towards the long term development of my career.
I had an additional opportunity to visit the Maramgamuwa restoration field site with Prof. Ranawana, of the University of Peradeniya and my supervisor—Prof. Selvadurai Dayanandan. This field visit gave me valuable insights into the natural forest regeneration processes that occur after clear cutting (Eucalyptus), and the dynamics of tree species which form secondary forests. During my time, I spent an additional two days learning about tropical tree morphology, endemicity and IUCN status of various tropical tree species at Royal Botanical Gardens Peradeniya.
Sachin Medigeshi Harish is a Ph.D. student at Dr. Selvadurai Dayanandan’s laboratory in the Department of Biology, Concordia University, Montreal. Before joining Concordia university, he worked for India biodiversity portal, a citizen science initiative to document biodiversity richness of India.