Functional traits of avian frugivores have shifted following species extinction and introduction in the Hawaiian Islands

The extinction and introduction of species can alter ecological processes
owing to the loss or gain of species roles. In vertebrate-dependent seed
dispersal, mutualisms between frugivores and fruiting plants depend, in
part, on matching of functional traits. High species turnover of
frugivores has occurred on the Hawaiian Islands, owing to both the loss of
native frugivores and the introduction of a new suite of frugivores. How
this turnover has altered the functional traits of frugivores and the
potential impacts on seed dispersal remain unclear. We investigated how
avian frugivore traits differed between historic and modern assemblages of
the Hawaiian Islands. We also tested how traits shifted within foraging
guilds (ground versus arboreal) to distinguish potential impacts on plants
within low versus high forest strata. Compared to historic frugivores, the
modern assemblage is smaller in gape width and body mass in both ground
and arboreal guilds. Wing shape did not significantly change between
assemblages. From results, we postulate that changes in the frugivore
community have likely altered seed dispersal processes by reducing (1) the
size of seeds consumed, (2) frugivory rates per animal, and (3) seed
dispersal distances. Owing to seed size placing strong constraints on
consumption, we reviewed recent studies on frugivory by modern birds in
the Hawaiian Islands and compared the size of seeds consumed versus seeds
available. We found that larger-seeded plants (>8.1mm seed width)
were not consumed by modern birds and were more likely to be of
conservation risk compared to smaller-seeded plants. Consequently,
dispersal limitation may threaten Hawaiian plant communities, with
larger-seeded plants at greatest risk of extinction. Broadly, we show that
extensive turnover within assemblages may lead to significant changes in
functional traits, with potential knock-on effects for mutualistic
interactions and communities.