Recently, the Laikipia savannas in Kenya have been invaded by Pheidole megacephala ("big-headed ants", or "BHA"), with interesting and alarming consequences for local conservation and for ecosystem resilience. The BHA invasion has caused large changes to the tree community in Laikipia's black-cotton savannas, which are mainly comprised of the "whistling thorn" Old-World acacia tree (Vachellia drepanolobium)— in fact, >95% of the savanna woody cover is provided by this single tree species, which also provides nitrogen inputs and microhabitats for savanna fauna. BHA exterminate many of the native mutualist ant species that tend and defend these trees, and ultimately increase the mortality of untended whistling thorn trees. While recent research in the system has shown that the lack of ant defenders leaves these invaded tree communities open to damage by mega-herbivores, we investigated the effects of the invasion from an ecophysiological/entomological perspective: “Are big-headed ants directly affecting the physiology of whistling thorn trees, and is the net photosynthetic rate changing after invasion? Further, do these highly aggressive invasive ants provide any defense against invertebrate herbivores and parasites?”. We tested these questions at Ol Pejeta Conservancy in Laikipia, at which we had identified invaded and non-invaded whistling-thorn tree communities within 1 km of each other. In June-August of 2015 and 2016, we used a LI-6400 Portable Photosynthesis System (Li-Cor) and various field techniques to measure changes to net photosynthesis and transpiration of new leaf growth, as well as leaf damage by invertebrates, density of invertebrate larval parasites, and overall new growth. We also used live insect baits to measure the aggression of invasive and native ant species towards invertebrates in a field behavioral study.
Results/Conclusions
Despite their strong aggression towards native ground-dwelling insects, BHA provided virtually no defense against invertebrate herbivores, while native ant species were effective repellants (ANOVA, F[2,32]=5.99, p<0.01). Further, invaded acacias had higher leaflet damage caused by invertebrates (ANOVA, F[2,57]=23.8784, p<0.001) and reduced overall canopy growth (F[2,57]=10.3, p<0.001). Finally, invaded acacia canopies had reduced net photosynthesis in new growth (F[2,32]=9.15, p<0.0001). These results suggest that acacia communities are unable to sustain regrowth and photosynthesis upregulation when faced with sustained absence of mutualists, contrary to previous ecophysiological experiment results in the system. Further, the loss of a key functional group of insects may result in large changes to the ecological landscape, which exemplifies scenarios theorized by P. Vitousek (1990) regarding biological invasions and their effects on ecosystem functions and processes.