Small populations of fig trees offer a keystone food resource and conservation benefits for declining insectivorous birds

Abstract

Novel restoration approaches are required to provide food and habitat for declining bird populations, particularly as pressures increase from growing human populations and climate change. Fig (Ficus) species support many frugivores but there is a gap in our knowledge about the importance of these insect-pollinated plants to insectivores. We tested the influences of fig-population size and the number of fig-wasp-producing fruit per tree on avian-insectivore visitation to fig trees in eastern Australia over a three-year period. Eighty-four bird species visited fig trees in our study; two thirds (55) of these species were insectivores. More individual insectivores (1686) than frugivores (1051) visited fig trees (p < 0.0001). More insectivore species visited individual fig trees in small, fragmented populations (<16 fig trees) than in large populations (>50 fig trees; p = 0.016). We showed that figs provide insectivores with an important, year-round, food source. We showed that this occurred in a dry, temperate ecosystem and in a mesic, sub-tropical ecosystem. Insectivore visitation was significantly correlated with the number of ripening fig syconia and the number of emerging fig wasps but not with abundances of other insects in fig trees. Temporal resource partitioning between insectivores and frugivores was identified, with insectivores foraging as fig syconia were ripening, and frugivores foraging after syconia had fully ripened. Ficus species are very likely to provide similar keystone resources for avian insectivores throughout tropical, subtropical and temperate regions globally. This study revises our understanding of the role played by Ficus trees in supporting avian-insectivore populations.