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Review
. 2011 Jul;14(7):709-22.
doi: 10.1111/j.1461-0248.2011.01630.x. Epub 2011 May 19.

Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis

David J Eldridge  1 , Matthew A Bowker, Fernando T Maestre, Erin Roger, James F Reynolds, Walter G Whitford
Affiliations
Review

Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis

David J Eldridge et al. Ecol Lett. 2011 Jul.

Abstract

Encroachment of woody plants into grasslands has generated considerable interest among ecologists. Syntheses of encroachment effects on ecosystem processes have been limited in extent and confined largely to pastoral land uses or particular geographical regions. We used univariate analyses, meta-analysis and structural equation modelling to test the propositions that (1) shrub encroachment does not necessarily lead to declines in ecosystem functions and (2) shrub traits influence the functional outcome of encroachment. Analyses of 43 ecosystem attributes from 244 case studies worldwide showed that some attributes consistently increased with encroachment (e.g. soil C, N), and others declined (e.g. grass cover, pH), but most exhibited variable responses. Traits of shrubs were associated with significant, though weak, structural and functional outcomes of encroachment. Our review revealed that encroachment had mixed effects on ecosystem structure and functioning at global scales, and that shrub traits influence the functional outcome of encroachment. Thus, a simple designation of encroachment as a process leading to functionally, structurally or contextually degraded ecosystems is not supported by a critical analysis of existing literature. Our results highlight that the commonly established link between shrub encroachment and degradation is not universal.

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Figures

Figure 1
Figure 1
Regressions of rainfall with the response ratio (lnRR) of shrub cover, ant richness, vertebrate richness and vascular plant richness. Details of the models fitted are given within each panel.
Figure 2
Figure 2
Regressions of rainfall with the response ratio (lnRR) of litter cover, aboveground carbon, soil bulk density (0-15 cm depth), soil conductivity (0-15 cm depth) and total soil nitrogen (N, 30-45 cm depth). Details of the models fitted are given within each panel.
Figure 3
Figure 3
Final structural equation models depicting effects of plant traits upon the surrogate of ecosystem structure (Ln RR statistic). Boxes indicate measured variables entered in the model. Hexagons indicate composite variables used either to model multi-level categorical predictors, or to pool effects of a group of otherwise conceptually related predictors. Arrows represent “paths”, i.e. hypothesized effects that one variable has on another. Numbers adjacent to paths are path coefficients, and are an effect size statistic directly analogous to regression weights. When P < 0.10, the associated bootstrap probability value is presented in superscript. In the case of paths leading from predictors to the response variable, the path widths are scaled proportionally to the path coefficient. Paths leading into composites are “loadings”; multiplying them by a path coefficient leaving a composite yields the effects of individual indicators on the response variable. Although not shown to simplify graphs, all predictors are freely allowed to covary.
Figure 4
Figure 4
Final structural equation models depicting effects of plant traits upon the surrogates of ecosystem function (Ln RR function). The remainder of the legend is as in Fig. 3.
Figure 5
Figure 5
Potential outcomes of shrub encroachment in former grasslands in relation to three factors: 1) human use preference, 2) the trait assemblage of encroaching shrubs, and 3) abiotic contingencies (e.g., average annual rainfall). These outcomes are separated according to the utilitarian (U; e.g. grazing, recreation), ecosystem functional (F; e.g., soil physio-chemical variables related to nutrient cycling and C storage), and community structural (S; e.g. cover, biodiversity and spatial pattern) perspectives. Four possible trait assemblages (identified by the SEM analyses) can conceivably result in: 1) enhancement of both functional and structural components, 2) reduction of both components, 3) enhancement of one component and reduction of the other, in addition to various combinations that are neutral for either soil function or community structure. The function component may be further contingent upon abiotic gradients such as aridity due to the apparent effects of moisture availability on variables related to C cycling. Thus, for a single preferred human use such as biodiversity conservation there are a four different encroachment outcomes ranging from purely positive for utility (+), function (+) and structure (+) to purely negative for all three, as well as multiple mixed responses. In the case of the human use preference recreation, the encroachment outcome in terms of utility will be mixed (m) depending on whether the preferences of recreationalists align with environments that are shrub-encroached or shrub-free.
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