Whither invasions under climate change and different management scenarios?

Peruvian pepper tree under the spotlight

Global change complicates the formulation of sustainable strategies for dealing with invasive species. Much work at the C·I·B is examining the many ways in which climate change and other facets of global change could influence biological invasions in South Africa. Since different facets of global change potentially influence a wide array of drivers of biological invasions, it is informative to seek insights by studying alien species about which a lot is known.

South Africa is the world capital for invasions of alien trees. One species about which a lot of basic ecological knowledge has been gathered recently is the Peruvian pepper tree (Schinus molle; Anacardiaceae). It is a long-lived and drought-tolerant evergreen dioecious tree native to the arid central region of South America. The species was introduced to South Africa in about 1850, and since 1950 was widely planted, especially as a shade tree at picnic sites along major roads. In the last two decades it has become highly invasive in semi-arid savannas and is now listed as a major invader. Thanks mainly to the work of former C·I·B student Donald Iponga towards his PhD thesis, a lot is known about the ecology of this tree in South Africa, including:

This information paved the way for a multi-author study that set out to explore the potential distribution of this invasive species under scenarios of climate change and different management interventions. Such information is needed to formulate effective long-term intervention strategies.

Predicted environmental suitability for the invasive tree Schinus molle

Predicted environmental suitability for the invasive tree Schinus molle modelled using locations of planted and naturalized individuals via boosted regression trees. Maps show the current predicted suitability for planted (a) and naturalized (b) individuals in each cell, and in panel (c) areas suitable only for planted (red), only for naturalized individuals (blue), and areas suitable for both (black). Reproduced from Richardson et al. (2010; Ecography; 33, 1049-1061).

This study, led by C·I·B core team member Dave Richardson, four other C·I·B-affiliated authors and two international collaborators, combined niche modelling and fine-scale process-based modelling to define regions at high risk of invasion and to simulate likely dynamics at the landscape scale. Localities of planted and naturalized trees were mapped along 5380 km of roads — a transect that effectively samples a large part of western South Africa. Correlative modelling was used to generate profiles of present and future environmental conditions characterizing its planted and naturalized ranges. A cellular-automata simulation model was developed to study the dynamics of S. molle under future climates and different management scenarios.

The overall potential range of S. molle in the region was predicted to shrink progressively with predicted climate change. Some of the potential range of S. molle defined based on current conditions (including areas where it is currently highly invasive, e.g. around Kimberley) is likely to become less favourable. However, the species could persist where it is well established long after conditions for recruitment have deteriorated. Some areas where the species is not widely naturalized now (notably the fynbos biome) are likely to become more favourable. This modelling approach allows for the delineation of areas likely to be invaded in future by considering a range of factors at different scales that mediate the interplay of climatic variables and other drivers that define the dimensions of human intervention such as distance from planted trees and the density of planted plants, both of which affect propagule pressure.

Read the paper:

Richardson, D.M., Iponga, D.M., Roura-Pascual, N., Krug, R., Thuiller, W., Milton, S.J. & Hughes, G.O. (2010). Accommodating scenarios of climate change and management in modelling the distribution of the invasive tree Schinus molle in South Africa. Ecography 33: 1049-1061.

For further details, e-mail Dave Richardson or Donald Iponga.