Two alternatives to the stress‐gradient hypothesis at the edge of life: the collapse of facilitation and the switch from facilitation to competition

New evidence demonstrates that facilitation plays a crucial role even at the edge of life in Maritime Antarctica. These findings are interpreted as support for the stress‐gradient hypothesis (SGH) – a dominant theory in plant community ecology that predicts that the frequency of facilitation directly increases with stress. A recent development to this theory, however, proposed that facilitation often collapses at the extreme end of stress and physical disturbance gradients. In this paper, we clarify the current debate on the importance of plant interactions at the edge of life by illustrating the necessity of separating the two alternatives to the SGH, namely the collapse of facilitation, and the switch from facilitation to competition occurring in water‐stressed ecosystems. These two different alternatives to the SGH are currently often amalgamated with each other, which has led to confusion in recent literature. We propose that the collapse of facilitation is generally due to a decrease in the effect of the nurse plant species, whilst the switch from facilitation to competition is driven by environmental conditions and strategy of the response species. A clear separation between those two alternatives is particularly crucial for predicting the role of plant–plant interactions in mediating species responses to global change.     

Promotion of mid-successional seedling recruitment and establishment by Juniperus virginiana in a coastal environment

Positive interactions between Juniperus virginianaand woody seedlings may influence trends in primary succession along theAtlantic Coast of eastern North America. Woody species richness was greaterbeneath isolated J. virginiana trees than in more exposed,grass-covered dune sites on a Virginia, USA barrier island. Fleshy fruitedseedswere more abundant in the seed bank beneath J. virginianathan in adjacent exposed sites, suggesting that J.virginiana may be utilized by passerine birds. Photosyntheticallyactive radiation was reduced below J. virginiana, and soiltemperature fluctuations were moderated during the growing season. In addition,moisture content, organic matter, and chlorides were higher for soils underJ. virginiana than in exposed sites. For plantedPrunus serotina and Sassafras albidumseedlings, mortality was lower beneath J. virginiana ascompared with the exposed treatment; however, no S.albidumseedlings survived beyond mid-August. The effect of J.virginiana on the recruitment and distribution of mid-successionalwoody seedlings in coastal environments may be passive, through the non-randomdistribution of fleshy seeds by perching birds, or active, through increasedseedling survival due to J. virginiana initiatedalterations in microclimate and edaphic factors.     

Implications of the spatial pattern of Vigur's Eyebright (Euphrasia vigursii) for heathland management

Euphrasia vigursii is a rare, endemic plant of heathland in South West England. Its numbers have declined markedly in recent years. The great majority of plants in modern times has been found on Lydford High Down, and this site has recently undergone a change in management to encourage a greater cover of dwarf-shrubs, and this may have consequences for the E. vigursii population. The shrub-grass mosaic structure was quantified using line transects and mapped. Associations with other species were investigated with 0.25 m² samples. Relationships with Ulex gallii dwarf-shrubs were examined by detailed mapping within four 100 m² sampling areas and second-order spatial analysis with Ripley's K/L and Wiegand-Moloney's O-ring statistics. Height and canopy dimensions of the dwarf-shrubs were also recorded. Individuals of E. vigursii were normally found in areas with 35–50% gorse cover, and with the shrubs about 30 cm tall. At finer scales, within the subpopulations, E. vigursii plants were found in patches of up to 0.9 m in diameter, and were positively associated with Euphrasia anglica. E. vigursii plants were located around the edges of U. gallii shrubs more than expected by chance. A parasitic relationship is possible, or the dwarf-shrubs might have a facilitative effect on the arrival, establishment and/or growth of E. vigursii. Any significant increase in dwarf-shrub cover from current levels might cause a further reduction in the population size of E. vigursii. The population has declined on Lydford High Down from an estimated 21,000 individuals in 2002 to less than 500 in 2004. Areas where small populations of E. vigursii have recently disappeared do show an increased cover and height of dwarf-shrubs, compared to areas where the population has survived. While increasing the cover of dwarf-shrubs on Dartmoor may be a laudable objective in general terms, it may not be the most appropriate way to manage populations of E. vigursii on Lydford High Down. Often conservation priorities are set to meet large-scale, habitat-orientated objectives, but fail to recognise the fine-scale needs of rare populations that may need somewhat different management regimes.     

Competition and facilitation between the alien mussel Mytilus galloprovincialis and indigenous species: Moderation by wave action

As the alien species that most dominates space along the South African coast, the Mediterranean mussel Mytilus galloprovincialis has radically altered community composition on invaded shores. We experimentally assessed interspecific interactions between this invasive species and dominant indigenous species in conjunction with considering how wave action moderates such interactions. The density of both M. galloprovincialis and the limpet Scutellastra granularis increased with wave action. Conversely, the tube-building polycheate Gunnarea capensis was negatively affected by wave exposure, being most abundant on sheltered shores. The influence of wave action on the indigenous mussel Aulacomya ater, however, remains unclear. M. galloprovincialis outcompeted both G. capensis and A. ater at moderate to high exposure levels, whereas it had both positive and negative effects on S. granularis. It outcompeted adult limpets on primary rock space on semi-exposed and exposed shores, reducing densities of this portion of the population. However, recruitment of S. granularis was facilitated by M. galloprovincialis, as greater numbers recruited to the secondary substratum offered by mussel shells. Again this interaction intensified with wave action. Due to the extremely high density of recruits on secondary space, the net effect of M. galloprovincialis on S. granularis was positive. Thus, wave action not only influences the abundance of individual species, but also mediates both positive and negative interspecific interactions in rocky shore communities, including the impact of alien species such as M. galloprovincialis.