The East African lakes: specialised and saturated communities?
Explosive speciation in cichlids has resulted in distinctively structured cichlid-Cichlidogyrus communities in Eastern Africa. Meta-communities in Lake Tanganyika (LT ) and the Lake Victoria region (LV ) are more specialised, and less nested and connected than those inferred for species elsewhere (Fig. 3; Appendix S1). Few additional interactions were predicted (Fig. 5b) suggesting that most are already known for the sampled species. This saturation of interactions might be expected as new ecological opportunities, an important determinant of interactions at the global scale, arise less in comparatively stable lake ecosystems such as LT (Salzburgeret al. 2014). Newcomers to these systems might find little opportunity to radiate (e.g. Koch et al. 2007). Furthermore, simulations suggest that specialisation in a network can be a by-product of adaptive radiation (Maynard et al. 2018).
Specialisation was particularly high for LT . Notably, host repertoires were frequently not estimated for these parasite species as many infect only single host species. These restricted host repertoires (Fig. 5b) might also be an explanation for the poor performance of NLP for the LT network as the input variables are by design identical for duplicate interactions. The algorithms performed well for LVlikely because of the higher nestedness (Fig. 3). For instance, only one recorded monogenean species infects deepwater cichlids from theLT tribe Bathybatini (Kmentová et al. 2016) whereas co-infections for 36 host species in the LV community are reported (Fig. 2b, Appendix S1.2). Species of LV might be more interconnected than LT due the younger age of the host radiation (0.4 M years; see, Salzburger et al. 2014), which left the network less saturated. The Lake Victoria region has also experienced rapid ecosystem degradation over the past century including the introduction of a novel predator, Lates niloticus (L.), and eutrophication causing the decline and extinction of many native fishes (Marshall 2018). Environmental disturbance might have promoted the expansion of host repertoires as new ecological opportunities arise and parasites increase their change of survival through ecological fitting (Brooks et al. 2019). For instance, Cichlidogyrus sp. ‘nyanza’ is one of the few species with a host repertoire more determined by ecological parameters than the host evolution at least at the recent evolutionary scale (Fig. 4), although this pattern might emerge from the comparatively low genetic differentiation of theLV radiation (Salzburger et al. 2014). In conclusion, cichlid-Cichlidogyrus communities in the East African lakes appear more stable and saturated than those elsewhere. Yet the younger age and environmental degradation might make LV a less stable system than LT . Limitations of the dataset call for more extensive sampling of host-parasite communities in these lakes to better understand these interactions. Other lake systems, e.g. Lake Malawi could be of particular interest for future comparative studies mirroring the research conducted on the cichlid radiations in this region (e.g. Duponchelle et al. 2008).