Imagine a contest that pits a major insect taxon against a popular class of vertebrates. The competition: which are the more important predators of insects, ants or birds. This interclass Olympic event was held as part of an National Science Foundation-funded study of top-down control of arthropod (e.g. insects, spiders) populations in shade-coffee farms in Chiapas, Mexico, undertaken by scientists from the Smithsonian Migratory Bird Center (National Zoo) and the University of Michigan.
Ecologists' vision of what is important is often clouded by their love and interest in a particular type of organism. Ants and birds have often been hypothesized to be important predators shaping arthropod assemblages, particularly in tropical forests. Surprisingly, no studies have systematically compared the impact that birds and ants have on their prey base.
But how to measure “importance”? Rather than count all of the prey captured by each ant and bird, researchers measured the impact on arthropod numbers through a simple exclosure experiment.
Birds and ants were excluded from the branches of 20 individual trees of a common canopy species (Inga) in the dry season and then again in the rainy season. Birds were kept away by covering the branches with monofilament netting. Ants were removed from branches that were then protected with tangle-foot (a sticky substance used as a protection against walking or crawling insects).
With the playing field now level, the contest was on. Birds caused a 57% reduction in arthropod abundance—and a 77% reduction in those greater than 5 mm in length. Interestingly, the effect of predation was greater in the rainy season, when arthropod abundance was higher and bird abundance lower (because of the absence of migrants). Maybe the fewer resident birds use more resources while they are rearing young. The reduction was widely distributed across major orders of arthropods. By far the hardest hit group was roaches, which experienced an astounding 92% reduction in abundance.
On the other hand, ants caused no overall reduction in the size of the arthropod population. Both small decreases and increases were found in a few individual orders of arthropods, but even where a significant effect was detected, it was generally specific to a season. The overall lack of ant predation is all the more surprising since this species of Inga is an “ant-plant” that offers sustenance to ants in its extra-floral nectaries. It has been proposed that defense against insect pests is one of the advantages that Inga gains from attracting foraging ants.
The results of this experiment may be more than a game. Shaded tropical agroecosystems produce many millions of dollar worth of globally-traded commodities (particularly coffee and cocoa) each year. Birds are clearly an important natural insecticide protecting these crops against pests.
Another result of the study falls into the realm of the serendipity of science. Focusing on one of the preferred prey of birds, scientists noticed the unmistakable design of a multi-limbed goddess on the nymphs of roaches (see photo). The presence of the roach goddess thus far defies simple scientific explanation.