Top-down and bottom-up effects as causes of the mid-elevational peak in Eastern Himalayan arthropod abundance
Arthropod abundance is greater at the mid-elevations (≅2000m) in the eastern Himalayas than above or below. This elevation marks the lowest point along the elevational gradient that freezes over in the winter. Although arthropods are generally more abundant, ants are essentially absent at mid-elevation. Here, I test two hypotheses for greater arthropod abundance at mid-elevations than lower down. First, plants at mid-elevations may invest less in defense mechanisms and more in growth due to the short growing season, leading to greater resource availability for arthropods. Second, insectivorous ants, notably the highly voracious Asian weaver ant Oecophylla smaragdina only found at low-elevations, may be responsible for the relatively low abundance of arthropods at low elevations. I studied physical defense characteristics of 20 common tree species and leaf damage incurred due to insect herbivores on 100 randomly selected trees at a low- and mid-elevational site. To assess the impact of weaver ants on arthropod communities at the low-elevation site, I compared arthropod abundance and rates of leaf damage on paired trees differing in presence/absence of weaver ants.
Contrary to the prediction of the plant defense hypothesis, I found higher levels of leaf damage on trees at low-elevations and no difference in frequency of physical plant defense traits at low and mid-elevations. This suggests that plant defense may actually be higher at mid-elevations. On the other hand, I found a significant effect of weaver ants on the arthropod community, as trees with weaver ants had fewer large arthropods and significantly lower levels of leaf damage. These results suggest that arthropod predation by weaver ants is one cause of the low arthropod abundance at low elevations in Eastern Himalayas. My result affirms that ants can have major impacts on ecosystems and community structure. In the future I plan to study the influence of weaver ants on arthropod communities at lower elevations in more detail through ant-exclusion experiments and diet analyses using next-generation sequencing methods.