Summary

1
Shrub encroachment has been widely observed in savanna regions. This study analysed the causes of shrub encroachment in the lowveld savanna of north-eastern Swaziland, southern Africa, and highlighted management regimes that can be used to reduce or prevent it.

2
The rates and dynamics of shrub encroachment were quantified for the period 1947–90 using aerial photographs, and for 1997 using a ground survey. Five similar areas with different land-use histories were compared to investigate the relative importance of fire, herbivory, rainfall, soil type and shrub density in driving shrub dynamics.

3
In the study area as a whole, shrub cover increased from a mean of 2% in 1947 to 31% in 1990. Dichrostachys cinerea accounted for most of the increase in cover, contributing 81% to total shrub cover during 1997. Shrub cover was strongly correlated with shrub density and weakly negatively correlated with tree cover.

4
Shrub encroachment varied across land-use fence lines. The key determinants of shrub dynamics were grazing, through its negative effect on fire frequency and an interaction between drought frequency and high shrub cover. Browsing pressure had a significant but minor impact on dynamics, while soil type had no significant effect. High grazing pressures through their effect on fire frequency were critical throughout the study period in promoting shrub encroachment, while the interaction between drought and high shrub cover produced declines in the later stages. Browsing had an impact on encroachment only in the early stages.

5
Frequent fires, facilitated by low grazing pressures, were capable of preventing shrub encroachment. When coupled with drought, frequent fires could reduce high shrub densities.

6
As cover and density were strongly correlated, it can be inferred from the negative correlation between change in cover (density) and initial cover (density) that the rate of shrub encroachment was cover (density)-dependent and that there was a shrub equilibrium of 40% cover, approximating to 2400 plants ha−1. Shrub population growth was driven by events (fire, drought) as well as continuous agents (density dependence, mean browsing and grazing pressure).

7
Bush encroachment can be reversed by a combination of management (frequent fires) and climatic events (drought). The implications for savanna management are discussed.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2664.2001.00567.x

Dynamics of shrub encroachment in an African savanna: relative influences of fire, herbivory, rainfall and density dependence

Despite a long tradition in plant ecology of studies of patch dynamics, recent developments of models for large scale dynamics in source-sink and metapopulations have largely focused on animals. In contrast to mobile unitary animals, many plants resist extinction. even under conditions where only a part of the life cycle can be maintained. This model of remnant population dynamics adds to the two commonly recognized source-sink and metapopulations dynamics. A review of the literature suggests that all three types of dynamics are common in plants. Regional dynamics are related to life-cycle characteristics determining dispersal ability and longevity of life cycle stages. Short-lived or highly habitat specialized plants with good dispersal tend to build up metapopulations, i.e. systems of local populations and non-occupied but potentially suitable sites, interconnected with dispersal. and in a continuous flux of local colonization and extinction. At the other extreme, long-lived plants with clonal propagation. or plants with extensive seed banks, tend to build up remnant population systems, in which many local populations persist over periods long enough to bridge unfavourable phases of successional development, intervening periods of favourable conditions. Source-sink populations are a special case of metapopulations, in the sense that they comprise both persistent refuge populations and ephemeral populations maintained by dispersal. It is suggested that a concept of local population inertia in remnant population systems, scales to higher level phenomena of vegetation inertia, and to community stabilization (through enhanced recovery after perturbations). Such an inertia may contribute to explain cases of exceptionally high species diversity, and lack of pronounced mass extinctions of plants in the fossil record.

https://www.jstor.org/stable/pdfplus/3546063.pdf

Regional dynamics of plants : a review of evidence for remnant, source-sink and metapopulations

It has been suggested that the habitats which a species occupies can be divided into sources and sinks, depending on whether or not local reproduction is sufficient to balance mortality. Source populations are those where reproduction exceeds mortality, surplus individuals dispersing to sink populations where mortality exceeds local reproduction. Sink populations would not be viable in the absence of immigration. A difference equation model is constructed to show that sources and sinks cannot be identified from a simple comparison of the demographic rates between populations, as measured by the numbers of births and deaths. Viable populations may appear to be non-viable simply because the dispersal of individuals into them depresses fecundity or increases mortality as a result of density-dependence. The consequence is that local recruitment appears insufficient to balance local mortality. Viable populations that appear as sinks, as a result of the dispersal of individuals into them, are termed here as 'pseudo-sinks'. They will clearly be difficult to distinguish from genuine sinks on the basis of a simple comparison of the numbers of births and deaths in different populations. Examples of source and genuine sink populations and the data required to establish them are discussed.

http://www.jstor.org/stable/pdfplus/5833.pdf

Sources, sinks and pseudo-sinks

The complete demographies of 16 populations of Cirsium vulgare were followed in a replicated experiment. The experiment was a factorial combination of two intensities of sheep grazing in each of three seasons - winter (grazed or ungrazed), spring (grazed or ungrazed), and summer (light or heavy grazing) - giving eight treatments in two blocks. For 6 years from 1987 to 1992 the population sizes of C. vulgare were monitored in each of the 16 paddocks. After 1989 grazing in spring or winter or increased grazing in summer all increased population sizes. Population sizes fluctuated widely between years. The effects of the grazing treatments and plant sizes on the transitions between nine life-history stages were determined (...)

Demography of Cirsium vulgare in a grazing experiment.

I discuss two classes of models that predict patterns of dispersal across heterogeneous environments; the source-sink and balanced dispersal models. Data collected during a long-term study of small mammals in an experimentally fragmented landscape support the predictions of the balanced dispersal model. In addition, a literature review indicates that for 28 species, few empirical studies have enough detailed demographic data to distinguish between assumptions of the two models and to test their predictions. The review indicates both models are poorly supported by empirical studies and many of the studies cited as evidence for source-sink dynamics cannot distinguish a sink habitat from a habitat with a low carrying capacity. I conclude that a continuum should exist between species vagility and the dispersal patterns seen in heterogeneous landscapes. Species with high vagility and the ability to assess habitat quality should tend towards balanced dispersal. In species with low vagility or where dispersal is controlled by abiotic factors (wind, or water currents), source-sink dynamics will be more common. Investigators must collect detailed demographic and movement data if they hope to test these models and further our understanding of the role of spatial dynamics in ecology.

Testing models of source-sink dynamics and balanced dispersal

The major aim of this paper is to investigate what factors are responsible for determining the abundance of Sorghum intrans F. Muell. ex Benth. in the wet-dry tropics of northern Australia. The role of both density-dependent and density-independent processes are examined. In addition, we specifically investigate how variation in weather conditions from year to year, local variations in edaphic conditions, fire and ant predation can be expected to affect plant density. In doing so we provide an example of how a theoretical and an empirical approach together may be used to examine the various influences that affect the population dynamics of a plant

M. H. Andrew

Papers

Modelling the population dynamics of an annual plant Sorghum intrans in the wet-dry tropics