Spatial heterogeneity is essential for the persistence of many inherently unstable systems such as predator-prey and parasitoid-host interactions. Since biological interactions themselves can create heterogeneity in space, the heterogeneity necessary for the persistence of an unstable system could be the result of local interactions involving elements of the unstable system itself. Here we report on a predatory ladybird beetle (Azya orbigera) whose natural history suggests that the beetle requires the patchy distribution of the mutualism between its prey, the green coffee scale (Coccus viridis), and the arboreal ant, Azteca instabilis in a coffee plantation in Chiapas, Mexico. While A. orbigera adults suffer from constant attacks by the ants, the larvae have waxy filaments that render them immune to these attacks and are thus able to forage freely in prey-rich ant-tended areas. Furthermore, ladybeetle larvae indirectly gain enemy-free space by living in ant-patrolled areas. This natural history suggests that the ladybeetle needs habitats with ants for the survival of larvae and habitats without ants for the survival of the adult stages. All these interactions, however, are affected or disrupted by behavioral changes that ants suffer in the presence of their parasitoid, the phorid fly Pseudacteon sp. Given that ants drastically reduce their activity level in the presence of the parasitoid and thus fail to protect their mutualistic partner against ladybeetle predation, we hypothesized that the spatial pattern of the phorid fly would greatly influence the distribution and persistence of this predatory ladybeetle.
Results/Conclusions
Through a four-year survey in a 45-ha plot in the coffee plantation we determined that just like phorids do, ladybeetle adults show a positive correlation with ant nest cluster size. The greater phorid presence in bigger clusters presumably allows ladybeetle adults to oviposit and prey upon ant-tended resources. Ladybeetle larvae, on the contrary, did not show this correlation; phorid-induced low activity periods may break down the enemy-free space normally provided by the ants. Thus, the persistence of this ladybeetle appears to depend not simply on the existence of habitats with ants and areas without ants but in a particular spatial aggregation of these habitats. Based on the natural history of the system, we constructed a spatially explicit model and showed that the spatial distribution of ant nests found in the farm may be caused by these local interactions and that this particular pattern is indeed key for the persistence of the ladybeetle populations.
