DOI: 10.5061/dryad.8fp98
title: Data from: Variation in the impact of non-native seaweeds along gradients of habitat degradation: a meta-analysis and an experimental test

description: Biological invasions are acknowledged among the main drivers of global
 changes in biodiversity. Despite compelling evidence of species
 interactions being strongly regulated by environmental conditions, there
 is a dearth of studies investigating how the effects of non-native species
 vary among areas exposed to different anthropogenic pressures. Focusing on
 marine macroalgae, we performed a meta-analysis to test whether and how
 the direction and magnitude of their effects on resident communities and
 species varies in relation to cumulative anthropogenic impact levels. The
 relationship between human impact levels and non-native species impact
 intensity emerged only for a reduced subset of the response variables
 examined. Yet, there was a trend for the effects of non-native species on
 community biomass and abundance and on species abundance to become less
 negative at heavily impacted sites. By contrast, the magnitude of negative
 effects of seaweed on community evenness tended to increase with human
 impact levels. The hypothesis of decreasing severity of invader’ impacts
 along a gradient of habitat degradation was also tested experimentally at
 a regional scale by comparing the effects of the removal of non-native
 alga, Caulerpa cylindracea, on resident assemblages among rocky reefs
 exposed to different anthropogenic pressures. Assemblages at urban and
 pristine site did not differ when invaded, but did so when C. cylindracea
 was removed. Our results suggest that, despite the generally weak
 relationship between human impacts levels and non-native species impacts,
 more negative impacts can be expected in less stressful environments (i.e.
 less degraded or pristine sites), where competitive interactions are
 presumably the driving force structuring resident communities.
 Implementing strategies for controlling the establishment of non-native
 seaweeds should be, thus, considered a priority for preserving
 biodiversity in relatively pristine areas. On the other hand, control of
 invaders at degraded sites could be warranted to lessen their role as
 propagule sources.
description: Data from: Variation in
 the impact of non-native seaweeds along gradients of habitat degradation:
 a meta-analysis and an experimental testBiological invasions are
 acknowledged among the main drivers of global changes in biodiversity.
 Despite compelling evidence of species interactions being strongly
 regulated by environmental conditions, there is a dearth of studies
 investigating how the effects of non-native species vary among areas
 exposed to different anthropogenic pressures. Focusing on marine
 macroalgae, we performed a meta-analysis to test whether and how the
 direction and magnitude of their effects on resident communities and
 species varies in relation to cumulative anthropogenic impact levels. The
 relationship between human impact levels and non-native species impact
 intensity emerged only for a reduced subset of the response variables
 examined. Yet, there was a trend for the effects of non-native species on
 community biomass and abundance and on species abundance to become less
 negative at heavily impacted sites. By contrast, the magnitude of negative
 effects of seaweed on community evenness tended to increase with human
 impact levels. The hypothesis of decreasing severity of invader’ impacts
 along a gradient of habitat degradation was also tested experimentally at
 a regional scale by comparing the effects of the removal of non-native
 alga, Caulerpa cylindracea, on resident assemblages among rocky reefs
 exposed to different anthropogenic pressures. Assemblages at urban and
 pristine site did not differ when invaded, but did so when C. cylindracea
 was removed. Our results suggest that, despite the generally weak
 relationship between human impacts levels and non-native species impacts,
 more negative impacts can be expected in less stressful environments
 (i.e., less degraded or pristine sites), where competitive interactions
 are presumably the driving force structuring resident communities.
 Implementing strategies for controlling the establishment of non-native
 seaweeds should be, thus, considered a priority for preserving
 biodiversity in relatively pristine areas. On the other hand, control of
 invaders at degraded sites could be warranted to lessen their role as
 propagule sources. Study ID corresponds to the article identification
 number reported in Appendix 2 Region: study region; WesMed = Western
 Mediterranean Sea, GNS = Great North Sea Type: trophic level of target
 organism Habitat: IRB = intertidal rocky bottom, ISB = intertidal soft
 bottom, MATTE = Posidonia oceanica dead matte, OYSTBED = oyster beds, SRB
 = subtidal rocky bottom, SSB =subtidal soft bottom Invader: non-native
 macroalgae Data source: Ninv = non invaded, Inv = invaded
 Halpern_filtered_5_5: values of Halpern index averaged with a 5x5 square
 kernel around study sitesDataset.pdf