Invasive alien plants infiltrate bird‐mediated shrub nucleation processes in arid savanna

Abstract

Summary 1The cultivation and dissemination of alien ornamental plants increases their potential to invade. More specifically, species with bird-dispersed seeds can potentially infiltrate natural nucleation processes in savannas. 2 To test (i) whether invasion depends on facilitation by host trees, (ii) whether propagule pressure determines invasion probability, and (iii) whether alien host plants are better facilitators of alien fleshy-fruited species than indigenous species, we mapped the distribution of alien fleshy-fruited species planted inside a military base, and compared this with the distribution of alien and native fleshy-fruited species established in the surrounding natural vegetation. 3 Abundance and diversity of fleshy-fruited plant species was much greater beneath tree canopies than in open grassland and, although some native fleshy-fruited plants were found both beneath host trees and in the open, alien fleshy-fruited plants were found only beneath trees. 4 Abundance of fleshy-fruited alien species in the natural savanna was positively correlated with the number of individuals of those species planted in the grounds of the military base, while the species richness of alien fleshy-fruited taxa decreased with distance from the military base, supporting the notion that propagule pressure is a fundamental driver of invasions. 5 There were more fleshy-fruited species beneath native Acacia tortilis than beneath alien Prosopis sp. trees of the equivalent size. Although there were significant differences in native plant assemblages beneath these hosts, the proportion of alien to native fleshyfruited species did not differ with host. 6 Synthesis. Birds facilitate invasion of a semi-arid African savanna by alien fleshyfruited plants, and this process does not require disturbance. Instead, propagule pressure and a few simple biological observations define the probability that a plant will invade, with alien species planted in gardens being a major source of propagules. Some invading species have the potential to transform this savanna by overtopping native trees, leading to ecosystem-level impacts. Likewise, the invasion of the open savanna by alien host trees (such as Prosopis sp.) may change the diversity, abundance and species composition of the fleshy-fruited understorey. These results illustrate the complex interplay between propagule pressure, facilitation, and a range of other factors in biological invasions.

Superiority in competition for light: A crucial attribute defining the impact of the invasive alien tree Schinus molle (Anacardiaceae) in South African savanna

“…molle is dioecious and female trees produce large crops of small bright pink berries arranged in bunches on pendulous stems. These are eaten and dispersed by birds in California (Nilsen and Muller, 1980) and in South Africa (Milton et al, 2007), and young plants generally establish below perch sites, mainly other trees. Most S. molle saplings and trees are thus rooted below the canopies of dominant native trees, as are R. lancea trees, the seeds of which are also bird-dispersed.…”

“…However, the widespread coexistence of the study species over large areas provides a useful natural experiment for gaining insights in this regard. S. molle trees frequently grow in association with indigenous trees where their seeds are dispersed by birds (Milton et al, 2007). If the alien species is able over-top indigenous tree species and to outcompete them for light, then the alien species would become structurally dominant, possibly altering the functioning of the ecosystem.…”

Alien plant invasions in tropical and sub-tropical savannas: patterns, processes and prospects

“…Some species definitely do invade savannas in South Africa: Chromolaena odorata in Hluluwe-Imfolozi (Macdonald 1983), Opuntia stricta in Kruger National Park (Fig. 1b;Foxcroft et al 2004;Foxcroft and Rejmánek 2007), and Prosopis spp., Schinus molle and several other alien trees and shrubs in arid savanna around Kimberley (Milton et al 2007). While most alien plant species that are currently invasive in South Africa arrived in the region in the 1800s, the invasion of grassland and savanna biomes by O. ficus-indica dates back to the 1770s (Henderson Alien plant invasions in savannas 3915 (Eiten 1972(Eiten , 1983Coutinho 1978Coutinho , 1982 In such a habitat, the ground layer receives dappled sun and shade, which permits growth of a wide diversity of grasses and flowering plants.…”

Seed dispersal networks in the Galápagos and the consequences of alien plant invasions

“…the ratio between animal and plant species tends to be lower on islands than on continents [9]. This can magnify the effect of anthropogenic shifts in the assemblage of frugivores, affecting seed dispersal and influencing overall vegetation structure and ecosystem functioning [10]. Knowledge of seed dispersal processes can thus prove crucial to understanding the dynamics of plant invasions and the planning of effective conservation strategies, such as the control or eradication of fleshy fruited weeds [11].…”

“…In recent years, this growing realization has lifted the focus of conservation efforts from species to ecosystems [1,19]. While there is an increasing number of studies documenting different aspects of the dispersal of native and invasive plants by frugivores [10,20], the consequences of the integration of alien plants into seed dispersal networks has been poorly explored [21]. By contrast, several studies have evaluated the impact of alien plants upon pollination networks.…”