Effects of disturbance on marginal populations: human trampling on Ascophyllum nodosum assemblages at its southern distribution limit

Abstract

The negative effects of disturbances on the long-term persistence of populations may be more severe in marginal populations, i.e. those occurring at the boundaries of geographical and ecological distribution ranges. Human trampling is an important source of anthropogenic disturbance for intertidal assemblages. Effects of human trampling on intertidal rocky shore assemblages dominated by Ascophyllum nodosum (L.) Le Jol. at its southern distributional limit in Europe were tested experimentally. Four different intensities of trampling disturbances were applied to experimental areas, over a period of 5 mo. Percentage cover of A. nodosum and associated organisms was quantified during the disturbance period and for a recovery period of 1 yr. The 2 highest trampling intensities drastically reduced the cover of A. nodosum. Trampling also negatively affected Fucus vesiculosus and understorey species, while indirectly allowing colonisation by ephemeral green algae. One year after the end of disturbance, A. nodosum assemblages in the highest disturbance intensity areas were still significantly different from assemblages in the control and lowest disturbance intensity areas. These results were mainly driven by a switch in the dominant canopy species, from A. nodosum to F. vesiculosus. Results obtained in this study show that disturbance by human trampling negatively affects A. nodosum assemblages and that the effects are dependent on trampling intensity. These findings highlight the risk of local extinction imposed on these populations by anthropogenic disturbance, and the consequent need for protection measures for such sensitive populations. KEY WORDS: Ascophyllum nodosum · Trampling disturbance · Distributional range limits · Macro-algal communities · Resilience · Portuguese biogeography Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 378: [81][82][83][84][85][86][87][88][89][90][91][92] 2009 ents and space that may enhance the density of local populations or the appearance of new colonists (Grime 1973, Connell 1978, Connell et al. 1997). These contrasting effects give rise to different post-disturbance assemblages, in terms of composition and dominance of species (Airoldi 1998).In rocky intertidal areas, human activities due to recreational and harvesting uses represent one important source of disturbance that directly or indirectly affects marine organisms (Keough & Quinn 1998, Brown & Taylor 1999, Schiel & Taylor 1999, Milazzo et al. 2002a, Thompson et al. 2002. Among them, trampling has been the main focus of experimental studies showing that species composition, percentage cover, diversity and biomass of macroalgal assemblages can be significantly affected by increasing trampling intensity (Keough & Quinn 1998, Milazzo et al. 2002b.Assemblages may develop the ability to persist under natural cyclic disturbance events, but their degree of resistance and resilience to episodic noncyclic events, such as those resulting, for example, from anthropogenic pressur...

Benthic assemblages of rock pools in northern Portugal: seasonal and between-pool variability

“…trampling) disturbances, which can damage the existing bioconstructions and reduce the density of new recruits (Dubois et al 2002(Dubois et al , 2006. On the studied shore, mechanical impacts from waves and trampling have clear seasonal patterns related to the occurrence of extreme storms, mostly in winter (Dias et al 2002) and human frequentation of the shore, which is particularly intense in summer (Araújo et al 2009). As extreme waves could directly cause seasonal differences in the abundance of S. alveolata that are opposite to the present patterns and trampling might drastically affect intertidal assemblages on emergent rock rather than in rock pools, both mechanisms are unlikely to provide convincing explanations to our findings.…”

Spatial variability in intertidal macroalgal assemblages on the North Portuguese coast: consistence between species and functional group approaches

“…Moreover, harvesting and trampling could be important processes acting at this spatial scale. Many of the studied locations such as Foz, Aguda, Amorosa and Viana are exposed to an intense seasonal trampling due to recreational activities in summer, and Araújo et al (2009) found negative effects of trampling on macroalgal assemblages. Harvesting of invertebrates such as sea urchins is noticeable in many locations such as Viana or Belinho (personal observation).…”

Marginal populations under pressure: spatial and temporal heterogeneity of Ascophyllum nodosum and associated assemblages affected by human trampling in Portugal

“…A previous experimental study on the Portuguese population of Ascophyllum nodosum showed that high intensities of human trampling can negatively affect the mean abundance of this species, the cooccurring fucoid Fucus vesiculosus and understorey species, while enhancing the cover of ephemeral green algae (Araújo et al 2009). However, how changes in the intensity of disturbance affect spatial and temporal patterns of the same population and associated organisms has never been experimentally investigated.…”

“…Intense trampling associated with such activities can drastically affect patterns of distribution, abundance and diversity of macroalgal assemblages on rocky shores (Keough & Quinn 1998, Araújo et al 2009). …”

“…Previous studies have indicated a variety of abiotic and biological factors and processes as responsible for variations in the patterns of A. nodosum and associated assemblages at several locations within its geographical range (Jenkins et al 1999, Cervin et al 2005, Svensson et al 2009). In contrast, experimental studies on the effects of disturbance on marginal populations are rare (but see Araújo et al 2009).…”