Volume 50, Issue 1 p. 63-77
Original Article

Source-sink dynamics within a plant population: the impact of substrate and herbivory on palm demography

Eric J. Berry

Corresponding Author

Eric J. Berry

Biology Department, Saint Anselm College, 100 Saint Anselm Drive, 03102 Manchester, NH, USA

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David L. Gorchov

David L. Gorchov

Department of Botany, Miami University, 45056 Oxford, OH, USA

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Bryan A. Endress

Bryan A. Endress

Conservation and Research for Endangered Species, San Diego Zoo, 92027 Escondido, CA, USA

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Martin Henry H. Stevens

Martin Henry H. Stevens

Department of Botany, Miami University, 45056 Oxford, OH, USA

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First published: 21 November 2007
Citations: 27

Abstract

Site factors have frequently been shown to affect survival, growth, and reproduction in plant populations. The source-sink concept proposed by Pulliam is one way of integrating this spatial demographic variation into population models. Source-sink models describe a population where propagules from “source” habitats sustain less productive “sink” areas. We adapted this concept to model the population dynamics of the understory palm Chamaedorea radicalis on two substrates, rock outcrops and forest floor. In our model, sources and sinks correspond to fine-scale demographic structure within the population, rather than spatially discrete subpopulations as described in the Pulliam model. We constructed a stage-structured population matrix model that integrates the site-specific demography of individuals across two habitats types that are linked by migration. We then parameterized this model with field data from C. radicalis. To address whether observed differences in palm demography between rock outcrops and the forest floor were due to natural variation between microsites or due to differences in browsing intensity from free range livestock, we parameterized separate models based on the substrate-specific demography of protected, non-browsed palms and of palms exposed to burro browse. Results showed that herbivory reduced survival and fecundity on the forest floor, which in the absence of seed migration resulted in a projected decline of forest floor palms (sinks). However with seed dispersal, palms persisted and total population growth (both substrates) was projected to be positive, indicating that seed dispersal from non-browsed palms on rock outcrops (sources) was sufficient to sustain C. radicalis on the forest floor.