Unimodal relationship between small-scale barnacle recruitment and the density of pre-existing barnacle adults

Recruitment is a key demographic process for population persistence. This paper focuses on barnacle (Semibalanus balanoides) recruitment. In rocky intertidal habitats from the Gulf of St. Lawrence coast of Nova Scotia (Canada), ice scour is common during the winter. At the onset of intertidal barnacle recruitment in early May (after sea ice has fully melted), mostly only adult barnacles and bare substrate are visible at high elevations in wave-exposed habitats. We conducted a multiannual study to investigate if small-scale barnacle recruitment could be predicted from the density of pre-existing adult barnacles. In a year that exhibited a wide adult density range (ca. 0–130 individuals dm−2), the relationship between adult density and recruit density (scaled to the available area for recruitment, which excluded adult barnacles) was unimodal. In years that exhibited a lower adult density range (ca. 0–40/50 individuals dm−2), the relationship between adult and recruit density was positive and resembled the lower half of the unimodal relationship. Overall, adult barnacle density was able to explain 26–40% of the observed variation in recruit density. The unimodal adult–recruit relationship is consistent with previously documented intraspecific interactions. Between low and intermediate adult densities, the positive nature of the relationship relates to the previously documented fact that settlement-seeking larvae are chemically and visually attracted to adults, which might be important for local population persistence. Between intermediate and high adult densities, where population persistence may be less compromised and the abundant adults may limit recruit growth and survival, the negative nature of the relationship suggests that adult barnacles at increasingly high densities stimulate larvae to settle elsewhere. The unimodal pattern may be especially common on shores with moderate rates of larval supply to the shore, because high rates of larval supply may swamp the coast with settlers, decoupling recruit density from local adult abundance.

Unimodal relationship between adult and recruit density in barnacle populations

Recruitment is a key demographic step for population persistence. This paper focuses on understanding barnacle (Semibalanus balanoides) recruitment. In rocky intertidal habitats from the Gulf of St. Lawrence coast of Nova Scotia (Canada), winter ice scour is common. At the onset of intertidal barnacle recruitment in the spring, mostly only adult barnacles and bare substrate are visible at high elevations on the shore. We conducted a multiannual study to investigate if small-scale barnacle recruitment could be predicted from the density of pre-existing adult barnacles. In a year that exhibited a wide adult density range (ca. 0–130 individuals dm-2), the relationship between adult density and recruit density (referred to the available area for recruitment, which excluded adult barnacles) was unimodal. In years that exhibited a lower adult density range (ca. 0–40/50 individuals dm-2), the relationship between adult and recruit density was positive but simply resembling the lower half of the unimodal relationship. Overall, adult barnacle density was able to explain 26–40 % of the observed variation in recruit density. The unimodal adult–recruit relationship is consistent with previously documented intraspecific interactions. Between low and intermediate adult densities, the positive nature of the relationship relates to the ability of adult barnacles to attract settlement-seeking larvae, likely enhancing local population persistence where it is most needed. Between intermediate and high adult densities, where population persistence may be less compromised and the abundant adults may be detrimental to recruit growth, the negative nature of the relationship suggests that larvae are stimulated to settle elsewhere. This unimodal pattern may be particularly common on shores with moderate rates of larval supply to the shore, as high larval supply rates may swamp the coast with settlers, decoupling recruit density from local adult abundance.

Unimodal relationship between adult and recruit density in barnacle populations

Recruitment is a key demographic step for population persistence. This paper focuses on understanding barnacle (Semibalanus balanoides) recruitment. In rocky intertidal habitats from the Gulf of St. Lawrence coast of Nova Scotia (Canada), winter ice scour is common. At the onset of intertidal barnacle recruitment in the spring, mostly only adult barnacles and bare substrate are visible at high elevations on the shore. We conducted a multiannual study to investigate if small-scale barnacle recruitment could be predicted from the density of pre-existing adult barnacles. In a year that exhibited a wide adult density range (ca. 0–130 individuals dm-2), the relationship between adult density and recruit density (referred to the available area for recruitment, which excluded adult barnacles) was unimodal. In years that exhibited a lower adult density range (ca. 0–40/50 individuals dm-2), the relationship between adult and recruit density was positive but simply resembling the lower half of the unimodal relationship. Overall, adult barnacle density was able to explain 26–40 % of the observed variation in recruit density. The unimodal adult–recruit relationship is consistent with previously documented intraspecific interactions. Between low and intermediate adult densities, the positive nature of the relationship relates to the ability of adult barnacles to attract settlement-seeking larvae, likely enhancing local population persistence where it is most needed. Between intermediate and high adult densities, where population persistence may be less compromised and the abundant adults may be detrimental to recruit growth, the negative nature of the relationship suggests that larvae are stimulated to settle elsewhere. This unimodal pattern may be particularly common on shores with moderate rates of larval supply to the shore, as high larval supply rates may swamp the coast with settlers, decoupling recruit density from local adult abundance.