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Density- and distance-dependent seedling survival in a ballistically dispersed subtropical tree species Philenoptera sutherlandii

Published online by Cambridge University Press:  01 January 2008

S. Boudreau  and
M. J. Lawes
S. Boudreau*
Affiliation:
School of Biological and Conservation Sciences, Forest Biodiversity Research Unit, University of KwaZulu-Natal, P/Bag X01, Scottsville, 3209, South Africa
M. J. Lawes
Affiliation:
School of Biological and Conservation Sciences, Forest Biodiversity Research Unit, University of KwaZulu-Natal, P/Bag X01, Scottsville, 3209, South Africa
*
1Corresponding author. Current address: Northern Research Chair on Disturbance Ecology, Centre d’études nordiques, Département de Biologie, Université Laval, Sainte-Foy, Qc, G1K 7P4, Canada. Email: stephane.boudreau@bio.ulaval.ca
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Abstract:

We examine the density- and distance-dependent seedling survival of Philenoptera sutherlandii, a common pod-bearing and dehiscent legume (Fabaceae) in Ongoye Forest, South Africa. Short-range ballistic dispersal causes seed to fall beneath the parent tree, where density- or distance-dependent mortality effects are expected to be concentrated. One hundred and eighty marked seedlings were monitored in a 0.5-ha plot containing 30 adults. Our survival data do not support the escape hypothesis. Predation levels declined with increasing seedling density (positive density-dependent survival), but seedling survival after 15 mo was not distance-dependent. Nevertheless, a unimodal (hump-shaped) recruitment curve, typically associated with decreasing seedling density and increasing seedling survival with distance, was observed. In the context of ballistic dispersal, this recruitment curve may indicate a hump-shaped dispersal kernel with predator satiation at high seedling densities near a parent tree. This recruitment curve likely arises because generalized insect seedling predators while attracted to the adult trees also tend to forage farther away. Short dispersal distances, in turn generate the high densities needed to satiate seed and seedling predators. Predator satiation results in long-term survival rates in P. sutherlandii similar to more widely dispersed and less common tree species.

Keywords

ballistic dispersalescape hypothesisFabaceaeinsect predationJanzen–Connell modelmortalitypredator satiation
Type
Research Article
Information
Journal of Tropical Ecology , Volume 24 , Issue 1 , January 2008 , pp. 1 - 8
Copyright
Copyright © Cambridge University Press 2008

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