Shrub Invasion Decreases Diversity and Alters Community Stability in Northern Chihuahuan Desert Plant Communities

Department of Biology, University of New Mexico, Albuquerque, New Mexico, United States of America.
PLoS ONE (Impact Factor: 3.23). 06/2008; 3(6):e2332. DOI: 10.1371/journal.pone.0002332
Source: PubMed

ABSTRACT Global climate change is rapidly altering species range distributions and interactions within communities. As ranges expand, invading species change interactions in communities which may reduce stability, a mechanism known to affect biodiversity. In aridland ecosystems worldwide, the range of native shrubs is expanding as they invade and replace native grassland vegetation with significant consequences for biodiversity and ecosystem functioning.
We used two long-term data sets to determine the effects of shrub encroachment by Larrea tridentata on subdominant community composition and stability in formerly native perennial grassland dominated by Bouteloua eriopoda in New Mexico, USA.
Our results indicated that Larrea invasion decreased species richness during the last 100 years. We also found that over shorter temporal scales species-poor subdominant communities in areas invaded by Larrea were less stable (more variable in time) compared to species rich communities in grass-dominated vegetation. Compositional stability increased as cover of Bouteloua increased and decreased as cover of Larrea increased.
Changes in community stability due to altered interspecific interactions may be one mechanism by which biodiversity declines in grasslands following shrub invasion. As global warming increases, shrub encroachment into native grasslands worldwide will continue to alter species interactions and community stability both of which may lead to a decline in biodiversity.

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    • "Artemisia cover ranged between 9% and 30% across the study sites, which leaves between 70% to 91% of these communities beyond the influence of Artemisia facilitation-increased resistance to B. tectorum dominance. We also report the first evidence suggesting that facilitation increases community stability in space at intermediate stress levels, but decreases stability at higher stress levels (Butterfield 2009), adding to growing evidence that disturbance-mediated shifts from competition to facilitation may reduce community stability (Baez & Collins 2008). Our low stress-disturbance communities were analogous to 'low severity' environments described by Butterfield (2009). "
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    ABSTRACT: Questions(1) What combinations of overlapping water and heat stress and herbivory disturbance gradients are associated with shifts in interactions between Artemisia tridentata subsp. wyomingensis (Artemisia) and herbaceous beneficiary species? (2) Do interactions between Artemisia and beneficiaries shift from competition to facilitation with increasing stress-disturbance where facilitation and competition are most frequent and strongest at the highest and lowest levels, respectively? (3) Do such relationships differ for native and non-native beneficiaries? (4) What are the implications of any observed shifts in interactions between community compositional stability in space and susceptibility to invasion?LocationNorth American Artemisia communities.Methods We tested the stress-gradient hypothesis (SGH) in an observational study consisting of 75 sites located along overlapping water and heat stress and disturbance gradients. We used spatial patterns of association among Artemisia and six native and two non-native beneficiary species; including the invasive annual grass Bromus tectorum, representing a diverse array of life history strategies, to infer whether the net outcome of interactions was facilitation or competition. We assessed implications for community stability by examining shifts in community composition in space and resistance to invasion.Results/Conclusions Cattle herbivory, a novel disturbance and selective force, was a significant component of two overlapping stress gradients most strongly associated with observed shifts in interactions. Facilitation and competition were strongest and most frequent at the highest and lowest stress levels along both gradients, respectively. Contrasting ecological optima among native and non-native beneficiaries led to strikingly different patterns of interactions. The four native bunchgrasses with the strongest competitive response abilities exhibited the strongest facilitation at their upper limits of stress tolerance, while the two non-natives exhibited the strongest competition at the highest stress levels, which coincided with their maximum abundance. Artemisia facilitation enhanced stability at intermediate stress levels by providing a refuge for native bunchgrasses, which in turn reduced the magnitude of B. tectorum invasion. However, facilitation was a destabilizing force at the highest stress levels when native bunchgrasses became obligate beneficiaries dependent on facilitation for their persistence. B. tectorum dominated these communities, and the next fire may convert them to annual grasslands.
    Journal of Vegetation Science 07/2015; DOI:10.1111/jvs.12327 · 3.71 Impact Factor
    • "Evidence showed that the increase in total cover was largely due to an increase in grass cover and tall shrubs or trees despite the fact that dwarf shrubs have declined in abundance at many sites. However, unlike in many shrublands and grasslands across the globe where the trajectory is that of an increase in shrubby elements due to climate change and land use (Baez and Collins, 2008; Beaumont et al., 2011; Bestelmeyer et al., 2007; Gibbens et al., 2005; Fredrickson et al., 1998; Midgley et al., 2002, 2008; Okin et al., 2009; Snyder and Tartowski, 2006; Van Auken, 2009), the synthesis of the Fig. 6. Increase in abundance of Aloe ferox in the foreground and general increase in cover of thicket species particularly on the lower slopes in right distance. "
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    ABSTRACT: We synthesize recent literature concerned with the nature, extent and rate of vegetation change in the Albany thicket, Grassland and Nama-karoo biomes of the semi-arid, south-eastern interior of South Africa at a range of spatial and temporal scales in relation to local and global drivers. The change in cover of three main growth forms (grasses, dwarf shrubs [< 1 m] and tall shrubs [> 1 m]) at the community, ecotone and biome levels was analysed using field surveys and repeat photography spanning 27 to > 100 years. There has been an increase in grass and tall shrub cover and a decrease in dwarf shrub cover across the region over time. Vegetation composition and cover has been most stable in the Albany Thicket biome while Azonal habitats across the region have changed the most. The Grassland biome has experienced a significant increase in the cover of grasses and tall shrubs and a significant decline in cover of dwarf shrubs. Total cover and the cover of grasses have increased significantly in the Nama-karoo biome. We interpret these patterns as evidence for a south westwards shift in the Grassland/Nama-karoo biome boundary in the second half of the 20th century. Although there has been relatively little change in the boundaries between the Albany Thicket and adjacent biomes, several species with subtropical biogeographic affinities have increased in cover and abundance over this period in all biomes investigated. Drivers of the observed changes include rainfall amount and seasonality, temperature, land use and elevated CO2 concentrations in the atmosphere.
    South African Journal of Botany 06/2015; DOI:10.1016/j.sajb.2015.06.001 · 0.98 Impact Factor
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    • "patch-effect) across a shrub density and/or cover gradient may generate hump-shaped relationships between diversity and the degree of woody encroachment at the landscape level (Box 1). Interestingly, these aforementioned studies accord with an optimum woody canopy cover of 30%, a level that corresponds to peaks in richness of a range of different taxa, including plants (Soliveres et al. 2014; but see Báez and Collins 2008), birds (Sirami et al. 2009) and mammals (Blaum et al. 2007). It should be noted, however, that in some Australian shrub species, the positive patch-level effect on plant richness and soil fertility holds for woody canopy cover levels of ~50% (Soliveres and Eldridge 2014), which is close to the maximum cover level observed for shrubs in eastern Australia (Daly and Hodgkinson 1996). "
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    ABSTRACT: Since European settlement, there has been a dramatic increase in the density, cover and distribution of woody plants in former grassland and open woodland. There is a widespread belief that shrub encroachment is synonymous with declines in ecosystem functions, and often it is associated with landscape degradation or desertification. Indeed, this decline in ecosystem functioning is considered to be driven largely by the presence of the shrubs themselves. This prevailing paradigm has been the basis for an extensive program of shrub removal, based on the view that it is necessary to reinstate the original open woodland or grassland structure from which shrublands are thought to have been derived. We review existing scientific evidence, particularly focussed on eastern Australia, to question the notion that shrub encroachment leads to declines in ecosystem functions. We then summarise this scientific evidence into two conceptual models aimed at optimising landscape management to maximise the services provided by shrub-encroached areas. The first model seeks to reconcile the apparent conflicts between the patch-and landscape-level effects of shrubs. The second model identifies the ecosystem services derived from different stages of shrub encroachment. We also examined six ecosystem services provided by shrublands (biodiversity, soil C, hydrology, nutrient provision, grass growth and soil fertility) by using published and unpublished data. We demonstrated the following: (1) shrub effects on ecosystems are strongly scale-, species-and environment-dependent and, therefore, no standardised management should be applied to every case; (2) overgrazing dampens the generally positive effect of shrubs, leading to the misleading relationship between encroachment and degradation; (3) woody encroachment per se does not hinder any of the functions or services described above, rather it enhances many of them; (4) no single shrub-encroachment state (including grasslands without shrubs) will maximise all services; rather, the provision of ecosystem goods and services by shrublands requires a mixture of different states; and (5) there has been little rigorous assessment of the long-term effectiveness of removal and no evidence that this improves land condition in most cases. Our review provides the basis for an improved, scientifically based understanding and management of shrublands, so as to balance the competing goals of providing functional habitats, maintaining soil processes and sustaining pastoral livelihoods.
    Australian Journal of Botany 03/2015; 62(7):594-608. DOI:10.1071/BT14137 · 1.36 Impact Factor
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