Functional traits and habitat types rather than elevation predispose an
African cycad to pre-dispersal seed predations
Abstract
The genus Encephalartos is entirely endemic to Africa. Like most cycad
species, the genus is at risk of extinction. One of the threats
jeopardizing the future of the genus is reproduction failure. Our
objective is to investigate what predisposes Encephalartos species to
reproduction failure. We collected functional traits of 430 individuals
of Encephalartos villosus, including pre-dispersal seed predation and
habitat types and elevation in Origi Gorge Nature Reserve in South
Africa. Then, we analysed our data by fitting a structural equation
model (SEM). Surprisingly, elevation does not predict pre-dispersal seed
predations, adding to the inconsistent effects of elevation on seed
predation. However, there was evidence of more predated seeds on plants
with more leaves, fitting the patterns of resource concentration
hypothesis which predicts more insect herbivores, particularly
specialist herbivores, where food resources (e.g., leaves, flowers,
fruits, and seeds) are abundant. We also found that more predated seeds
are in open habitats, perhaps mirroring the specialist feeding behaviour
of the weevil Antliarhinus zamiae feeding on the seeds of Encephalartos
spp. Furthermore, taller plants tend to bear more predated seeds,
potentially because taller plants are easy located by the weevil.
Finally, large canopy correlates negatively with predated seeds,
mirroring our finding of increased seed predation in open habitats. Our
SEM explains 67% of the variations in pre-dispersal seed predations,
suggesting that this metamodel provides insights into the predisposition
of cycad seed to predations. Since open habitats correlate with more
seed predation, we suggest that anthropogenic activities that contribute
to open forest must be avoided if we are to limit seed predations.