Avian top-down effects on insects and their host plants under human-driven global change

Insectivorous bird populations are declining globally, as are the insects upon which they depend. Furthermore, many of the plants on which those herbivorous insects depend are being displaced by the spread of agriculture and invasion by exotic species. We discuss the consequences of these declines for changes in trophic control of herbivorous insects by insectivorous birds, and the indirect effects on host plants. We first briefly review the evidence for and causes of bird and insect decline, and the current evidence for trophic control by insectivorous birds. We then hypothesize how trophic control may change under three scenarios: reduced bird populations alone, invasion by exotic insect species and conversion of native habitat to agriculture. We hypothesize that trophic control will decrease under all three scenarios, resulting in higher abundance of herbivorous insects and more frequent outbreaks, higher chronic levels of herbivory and reduced primary productivity. Because birds often specialize to some degree on certain insect species and forage preferentially in certain plant species, reduced trophic control may in turn reduce plant diversity in more native vegetation. Similarly, reduced trophic control in agriculture will require greater reliance on pesticides and, with it, the negative consequences of increased pesticide use.