The invasive potential of Australian banksias in South African fynbos: A comparison of the reproductive potential ofBanksia ericifoliaandLeucadendron laureolum

Abstract

Several taxa of the Australian Proteaceae have invaded South African fynbos and require costly management programmes to bring under control. Banksia spp. have been introduced only recently to fynbos regions. The invasive potential of Banksia ericifolta (Proteaceae) was investigated by comparing its recruitment potential with that of an indigenous proteaceous shrub, Leucadendron laureolum. Both species are overstorey shrubs that are killed by fire and rely on canopy-stored seeds (serotiny) for recruitment. Eight year old B. ericifolia shrubs produced an average of 16 500 seeds per plant, which is thirty times more than the average of 570 seeds produced by 10 year old L. laureolum shrubs. The seed bank of fi. ericifolia was not only larger than that of L. laureolum (1098 w 525 viable seeds m ~ -projected canopy cover), but also considerably larger than that described for the species in its native environment (200-330 seeds m^^ina9year old stand north of Sydney). Leucadendron lau reoleu m released most of its seed a few days after the cones were burnt, whereas seed release in B. ericifolia was spread over 12 weeks. The seeds of B. ericifolia had lower wingloading and fall rates than L. laureolum and were dispersed over greater distances. The relative seedling growth rates of the two species were very similar (0.03 g per day), but below-ground biomass was greater and proteoid roots were more developed in B. ericifolia seedlings than in L. laureolum after 100 days. Four year old B. ericifolia plants growing in the field had attained over twice the height of indigenous proteoids and accumulated up to 10 times the fresh biomass of L. xanthoconus, a species which is ecologically similar to L. laureolum. The Bioclimatic Prediction System (BIOCLIM) was used to create a bioclimatic profile of B. ericifolia and identify climatically suitable areas in the Cape Province. Results show that its potential distribution covers most fynbos areas in the southwestern Cape. It is concluded that B. ertcifolia has the potential to be highly invasive in fynbos.

Herbivores, but not other insects, are scarce on alien plants

“…However, the spiny habit of these three Hakea species makes them morphologically distinct from Brabejum. Non-invasive, but broadly cultivated and locally naturalized, are the broad-leaved species H. salicifolia (Richardson et al 1987), and a few Banksia species (Honig et al 1992), these being considered here.…”

Predicting the Geography of Species’ Invasions via Ecological Niche Modeling

“…The demonstration of broad predictivity of species' invasive potential opens the door to the possibility of biota-wide risk assessments of invasive potential, as has been partially implemented for Australian banksias possibly invading the South African fynbos biome (Honig et al 1992). Although limited by the availability of occurrence information for entire biotas for source areas, this step can potentially inform efforts to avoid further problems with invasives enormously.…”

“…The basic point is that with ever-increasing scales of human movement, species are finding ever-increasing opportunities to move as well, resulting in many introductions of species to novel landscapes (NAS 2002). Although many colonizations fail (Williamson and Fitter 1996), for reasons of demography or of chance, this increased opportunity for colonization certainly plays a major role in increasing numbers of successful invasions.Several reviews of the natural history and ecology of invasive species have been conducted in the hope of identifying factors key in determining the success of invasions that could serve as the basis for a predictive understanding of which species represent potential invaders (Honig et al 1992;Perrins et al 1992;Scott and Panetta 1993;Lonsdale 1994;Carlton 1996;Rejmánek and Richardson 1996;Williamson 1996Williamson , 1999Reichard and Hamilton 1997;Ricciardi and Rasmussen 1998;Enserink 1999; NAS 2002). Unfortunately, the list of such identified key factors is short, consisting principally of characters associated with weedy habit and prior history of successful invasion.…”

Uses and Requirements of Ecological Niche Models and Related Distributional Models

“…Here, the idea is that-given apparently widespread evolutionary conservatism in ecological niche characteristics-species will often 'obey' the same set of ecological rules on invaded distributional areas as they do on their native distributional areas. As such, the geographic potential of invasive species is often quite predictable, based on their geographic and ecological distributions on their native distributional areas (Beerling et al 1995;Higgins et al 1999;Hinojosa-Díaz et al 2005;Hoffmann 2001;Honig et al 1992;Iguchi et al 2004;Panetta and Dodd 1987;Papes and Peterson 2003;Peterson 2003a;Peterson et al 2003a;Peterson and Robins 2003;Peterson et al 2003b;Peterson and Vieglais 2001;Podger et al 1990;Richardson and McMahon 1992;Robertson et al 2004;Sindel and Michael 1992;Skov 2000;Sutherst et al 1999;Welk et al 2002;Zalba et al 2000), although the factors that make a species invasive are clearly more complex than just niche considerations ).…”

“…Finally, additional evidence comes from studies of invasive species, in which species are transplanted to a distinct geographic and community context. Although it has been suggested based on theoretical musing and limited laboratory experiments that shifting species' interactions would confound any possible predictivity (in this case in the context of anticipating climate change effects on species' distributions) (Davis et al 1998), numerous studies have successfully predicted the invasive distributional potential of species based on native-range ecological characteristics (Beerling et al 1995;Higgins et al 1999;Honig et al 1992;Iguchi et al 2004;Panetta and Dodd 1987;Papes and Peterson 2003;Peterson 2003a;Peterson et al 2003a;Peterson and Robins 2003;Peterson et al 2003b;Peterson and Vieglais 2001;Richardson and McMahon 1992;Scott and Panetta 1993;Skov 2000;Sutherst et al 1999;Zalba et al 2000).…”