Southern-Summer School on Mathematical Biology

Facilitative interactions are defined as the “modification of some component of the abiotic or biotic environment by one organism that enhances colonization, recruitment, and establishment of another” (Princeton Guide to Ecology). Facilitation was first described as a phenomena occurring between plant species, where the presence of the facilitator species enhances the likelihood of recruitment and establishment of the facilitated species. Interestingly, the interaction often changes from facilitation to competition as the facilitated species grows and ends up competing for resources with the facilitator species. Therefore, unraveling and comprehending the underlying mechanisms that promote facilitation between organisms, although challenging, increases our understanding of community assembly and species coexistence.

Recently, Li et al. (2018) described a non-intuitive reciprocal facilitation between ants and cattle. In a grassland in China, bovids (Bos taurus) are the predominant large vertebrate in the area, and ants from the genus Lasius are the predominant below ground invertebrate. Through experiments, the authors show that the presence of cattle increases the density of ant nests in the region. Reciprocally, they show that ants increase cattle food intake, by means of a behavioral response on foraging. In this system, it appears as the underlying mechanism of reciprocal facilitation involves habitat amelioration and resource enhancement.

The authors propose a few mechanisms to explain how these two phylogenetically distant species might facilitate each other in nature. Develop a model that allows you to untangle the underlying mechanisms of facilitation between ants and bovids.

• Based on your model, what changes would you expect in the dynamics of the system (in terms of species densities, community composition and coexistence) if the facilitation were not reciprocal? In other words, if the interaction is neutral for one of the species, does it affect populational patterns found in the model in which facilitation is reciprocal? From that, can you infer which species is most affected by the interaction?

• The authors argue that the interaction is a facultative facilitation. In the absence of cattle, the effects provoked by ants (i.e., enhancement of soil nutrients increasing plant quantity and quality) could feedback negatively on its own population, altering the micro-habit that favors the construction of ant nests. How could this feedback influence long-term population densities and stability of the system?

Reciprocal facilitation between large herbivores and ants in a semi-arid grassland Xiaofei Li, Zhiwei Zhong, Dirk Sanders, Christian Smit, Deli Wang, Petri Nummi, Yu Zhu, Ling Wang, Hui Zhu and Nazim Hassan1 PRSB (2018) https://royalsocietypublishing.org/doi/full/10.1098/rspb.2018.1665

The Princeton Guide to Ecology Levin, Simon A., et al., eds. Princeton University Press (2009)

data from the paper https://datadryad.org/resource/doi:10.5061/dryad.s7423sv