Showing papers by "Steven I. Higgins published in 1996"

Modeling invasive plant spread: the role of plant-environment interactions and model structure.

Abstract: Alien plants invade many ecosystems worldwide and often have substantial negative effects on ecosystem structure and functioning. Our ability to quantitatively predict these impacts is, in part, limited by the absence of suitable plant-spread models and by inadequate parameter estimates for such models. This paper explores the effects of model, plant, and environmental attributes on predicted rates and patterns of spread of alien pine trees (Pinus spp.) in South African fynbos (a mediterranean-type shrubland). A factorial experimental design was used to: (1) compare the predictions of a simple reaction-diffusion model and a spatially explicit, individual-based simulation model; (2) investigate the sensitivity of predicted rates and patterns of spread to parameter values; and (3) quantify the effects of the simulation model's spatial grain on its predictions. The results show that the spatial simulation model places greater emphasis on interactions among ecological processes than does the reaction-diffusion model. This ensures that the predictions of the two models differ substantially for some factor combinations. The most important factor in the model is dispersal ability. Fire frequency, fecundity, and age of reproductive maturity are less important, while adult mortality has little effect on the model's predictions. The simulation model's predictions are sensitive to the model's spatial grain. This suggests that simulation models that use matrices as a spatial framework should ensure that the spatial grain of the model is compatible with the spatial processes being modeled. We conclude that parameter estimation and model development must be integrated pro- cedures. This will ensure that the model's structure is compatible with the biological pro- cesses being modeled. Failure to do so may result in spurious predictions.

Current and future threats to plant biodiversity on the Cape Peninsula, South Africa

Abstract: The biodiversity of the Cape Peninsula (49127 ha in extent) has been considerably affected by various factors since European settlement in 1652. Urbanization and agriculture have transformed 37% of the original area of natural vegetation. Lowland vegetation types have been worst affected, with almost half of the transformation occurring in one of the 15 recognized vegetation types. Vegetation at high altitudes has been little affected by urbanization and agriculture, but alien trees and shrubs are now threatening biodiversity in these areas. Of the area not affected by urbanization and agriculture 10.7% is currently under dense stands (>25% canopy cover) of alien plants and another 32.9% is lightly invaded. Dense stands of Acacia cyclops, the most widespread invader, cover 2510 ha, 76% of the total area under dense alien stands. This paper assesses the impacts of these factors on aspects of the plant biodiversity of the area, namely, the distribution of major vegetation types and of endemic, rare and threatened plant taxa and of taxa in the Proteaceae (a prominent element in almost all communities, taken as an indicator of overall plant biodiversity).

The Nyl River floodplain, South Africa, as a functional unit of the landscape: A review of current information

Abstract: Summary The Nyl River floodplain is one of southern Africa's conservation, eco-tourism and agricultural assets. Its location in a water-stressed region means that the biota of the Nyl River floodplain must compete for water resources with other water users. In order to compete, however, the system must clearly articulate its dependence on water and its economic and conservation value. A review of the system's structure and a statement of our present understanding of how the system functions is provided. This information is used to qualitatively assess the system's dependence on run-off from its catchment areas. Negative impacts on the system's biota, particularly the aquatic plants and waterbirds, are predicted if run-off is reduced. It is concluded, however, that a quantitative model of the system's functioning is required for the resolution of water resource disputes and for the effective management of the system. It is anticipated that interdisciplinary studies will be required to provide this quantitative model of the Nyl River floodplain and of similar systems. Resume La plaine inondable de la riviere Nyl est un des atouts de la conservation, de l'ecotourisme et de l'agriculture sudafricains. Sa situation dans une region si pauvre en eau signifie que les communautes vivantes de la plaine inondable de la riviere Nyl sont en competition pour les ressources en eau avec les autres utilisateurs d'eau. Pour assurer cette competition, le bassin hydrographique doit clairement articuler sa dependance vis a vis de l'eau, sa valeur economique et son importance pour la conservation. Nous presentons ici une revue de la structure du systeme ainsi que l'etat de nos connaissances sur le fontionnement de ce systeme. Cette information est utilisee pour evaluer qualitativement la dependance du bassin vis a vis de l'ecoulement a partir de ses zones de captage. Si le captage est diminue, on prevoit des effets negatifs sur les communautes vivantes du bassin, specialement sur les plantes aquatiques et les oiseaux d'eau. On conclut cependant qu'il faut etablir un modele quantitatif du fonctionnement du bassin pour resoudre les disputes au sujet de l'eau et pour gerer efficacement le bassin hydrographique. On prevoit que des recherches interdisciplinaires seront necessaires pour dresser ce modele quantitatif de la plaine inondable de la riviere Nyl et de bassins hydrographiques comparables.