Summary Savanna ecosystems show codominance of trees and grasses, and the mechanisms involved in their coexistence remain unresolved. We investigated the possible influence of small‐scale heterogeneities and disturbances in determining tree spacing and tree‐grass coexistence in semi‐arid savannas, using a spatially explicit, grid‐based simulation model. We added factors such as seed clumping, locally improved moisture conditions, vegetation clearing and combinations of all three of these variables to a previously published model. We also examined the influence of changing the size and spatiotemporal correlation among individual heterogeneities. Increased seed availability in localized clumps, especially in combination with the other heterogeneity types, had the strongest impact on the long‐term tree‐grass coexistence in the model. Localized deposition of tree seeds in herbivore dung and underground seed caches built by seed‐collecting rodents may act as a determinant of the distribution of trees in savannas. Spatially autocorrelated small‐scale heterogeneities led to a more stable coexistence of trees and grasses than uncorrelated heterogeneities did, and at high levels of autocorrelation runs with all three variables led to long‐term coexistence in up to 60% of the rainfall scenarios tested. The size of individual heterogeneities, also played a significant role in determining whether trees would establish as lone individuals or in clumps. In simulations that included small‐scale heterogeneities, the number of isolated trees scattered throughout the landscape increased, whereas large heterogeneity patches and high spatiotemporal autocorrelation tended to promote tree clustering. Simulations based on rainfall data from the southern Kalahari produced a realistic density and spatial distribution for trees in this semi‐arid savanna for a realistic range of small‐scale heterogeneities. Small‐scale disturbances and heterogeneities, at least those that furnish better establishment conditions for tree seedlings, therefore act to increase the range of environmental conditions under which trees and grasses can coexist as savanna.

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.

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Dryland nomadic bird species, as a proportion of all bird species in a biome in southern Africa, are highest in the arid grassland and arid and semi‐arid Karoo in South Africa. Nomadic birds, of which the most widespread species is the greybacked finchlark Eremopterix verticalis (Smith), are most frequently observed in the north‐central and north western Nama Karoo. The species richness of nomadic species is inversely correlated with species richness of all bird species in the Karoo. Since the distribution of nomadic birds is in areas where rainfall is patchy, low (<250 mm per year) and aseasonal, this supports the idea that fewer species are able to cope with resources that are patchy in time and space, and that there has been selection for nomadism in the species that are able to use patchy environments. Species richness and abundance of nomadic birds is negatively correlated with rainfall amount but positively correlated with the coefficient of variation of the rainfall and with rainfall in autumn. The frequency of nomadic birds is inversely correlated with altitude range; nomadic species are most often recorded in structurally simple habitats (shrubland and grassland) on open plains. Most nomadic bird species in the Karoo are granivorous. Perennial desert grasses are important components of the habitat and diet of small nomadic granivores, and also provide nest sites and nest material. Nomadic birds can breed throughout the year, without a clearly defined ‘season’ in both the Succulent and Nama Karoo. Average clutch sizes do not differ significantly between resident and all nomadic species in the arid and semi‐arid Karoo. Nomadism is an evolutionary stable strategy for individual species only when extremes in environmental conditions are frequent enough, and unpredictable enough, to maintain movements to high resource patches or to maintain dispersal away from low resource patches. If high rainfall years are too regular or infrequent, or peaks in fluctuations of resources in the environment too low, or rainfall patches are randomly distributed, nomadism would not be maintained as part of the individual behaviour pattern.

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