Comparing an exotic shrub's impact with that of a native life form analogue: Baccharis halimifolia vs Tamarix gallica in Mediterranean salt marsh communities

Abstract

Nomenclature Tison & de Foucault (2014) for vascular plants, Julve AbstractQuestions: To what extent may impact vary according to the invaded community, region or sites? Do impact thresholds exist, above which native species richness and/or cover declines rapidly? Does impact following invasion by the exotic shrub Baccharis halimifolia differ from that of a native shrub (Tamarix gallica) during natural successions?Location: Mediterranean coast, SE France.Methods: Vegetation was sampled on 120 4-m² quadrats, half of which were in areas invaded by B. halimifolia and the other half in nearby non-invaded areas, in two distinct community types nested in three sites in each of two regions. Mixed models were built to explain variations in impacts on native vegetation according to community types, sites and regions. Next, 85 additional plots variously invaded by B. halimifolia were used to assess the nature of the relationships between B. halimifolia cover and impact in Juncus communities. Linear regressions were compared to polynomial regressions, and when relevant, regression trees were built to identify threshold values. Finally, we compared the vegetation from 90 plots either dominated by B. halimifolia, by T. gallica (a native shrub) or without a shrub layer (control) using dissimilarity indices in wet meadows.Results: The magnitude of B. halimifolia impacts depended mainly on community type, with no differences between regions. Community structure (species richness, Shannon's diversity) declined linearly with increasing cover of B. halimifolia, with the most rapid decline in annual species. Native species cover fitted better with a cubic regression, with an impact threshold of around 86% cover of B. halimifolia. Impact of B. halimifolia on community diversity was higher than that of the native shrub T. gallica. While the magnitude of species composition changes was comparable for both shrubs, the higher nestedness component of dissimilarity found for B. halimifolia may indicate a higher short-term impact.Conclusion: Improved understanding of the impacts of exotic shrubs on community structure and composition can be gained via comparisons with the impacts caused by native shrubs. Higher impacts of the exotic B. halimifolia on Mediterranean salt marsh vegetation are likely explained by a denser canopy, shorter juvenile period and more frequent recruitment.

Managing for biodiversity: impact and action thresholds for invasive plants in natural ecosystems

“…Fried and Panetta (2016) found that native species' responses to invasion were complex and, in general, non-linear across a gradient of B. halimifolia cover. Species richness declined linearly with increasing B. halimifolia cover (indicating a non-threshold relationship), whilst the abundance of native perennial forbs and graminoids declined significantly only when B. halimifolia exceeded 80%, i.e.…”

“…Several other studies have also provided evidence for negative impact thresholds for a variety of invasive plants, including the shrub Baccharis halimifolia in Mediterranean saltmarshes (Fried and Panetta 2016), the monocarpic perennial Heracleum mantegazzianum in northern European grasslands (Thiele et al 2010b), the vine Delairea odorata in northern Californian coastal scrub and riparian communities (Alvarez and Cushman 2002), and the scrambling herbs Tradescantia fluminensis, Plectranthus ciliatus and Asparagus scandens in New Zealand temperate forests (McAlpine et al 2015). Fried and Panetta (2016) found that native species' responses to invasion were complex and, in general, non-linear across a gradient of B. halimifolia cover.…”

“…The upper limit of this range will be determined largely by two factors, the first being ecological and the second economic. Where the biodiversity value of the asset being managed is very high, there will be a need to protect the life forms (or species) that are most sensitive to the presence of the invader, whether the relevant damage function is linear or non-linear (Fried and Panetta 2016). The second determinant will be the cost of control, which is expected to increase with the cover of the targeted species (Fig.…”

“…For widespread and dominant invasive plants with demonstrably negative effects on native ecosystems, there is growing evidence that ecosystem responses are non-linear, such that they occur only once a particular level of invasive plant abundance has been exceeded; that is, a negative impact threshold relationship (Alvarez and Cushman 2002, Gooden et al 2009, Thiele et al 2010b, McAlpine et al 2015, Fried and Panetta 2016. In this paper we describe a mechanistic framework for such relationships and explore the role of thresholds in triggering invasive plant control.…”

A prioritization process for invasive alien plant species incorporating the requirements of EU Regulation no. 1143/2014

“…Negative environmental impacts of alien plants are indeed typically density‐dependent (Richardson et al ., , ; Bímová et al ., ; Staska et al ., ; Fried & Panetta, ); the formation of dense populations occurs more with perennial than annual plants, especially tall perennials (Hejda et al ., ). There are, however, exceptions for alien species hybridizing with native species, which may pose a high risk even at low densities (Daehler & Strong, ; Huxel, ; Wolf et al ., ).…”