Ingestion and dispersal: direct and indirect effects of frugivores on seed viability and germination of Corema album (Empetraceae)

Calviño-Cancela, M., 2004. Ingestion and dispersal: direct and indirect effects of frugivores on seed viability and germination of Corema album (Empetraceae). Acta Oecologica, 26(1), pp.55-64.

The effect of gulls, blackbirds and rabbits on the viability and germination of Corema album seeds are compared. Frugivores can affect seed viability and germination (1) directly, through the effect of ingestion and (2) indirectly, dispersing seeds to different sites with different conditions. These two major factors in the quality of a seed disperser are not necessarily concordant in direction and magnitude. Gulls and blackbirds have similar direct effects, being much better than those of rabbits, due to the low probability of germination of seeds within rabbit pellets. Seed germination occurs mainly in the open ground, particularly in the sparse scrub, and is very low under vegetation cover. This pattern becomes crucial determining the indirect effects of seed dispersers that will depend on their capacity to carry seeds to the most suitable sites for germination. Gulls and rabbits disperse most of seeds to open ground, exerting a positive indirect effect on germination, whereas blackbirds disperse seeds mainly under shrubs, thus exerting a negative indirect effect. Direct and indirect effects on seed germination are concordant for gulls but discordant for blackbirds and rabbits. Gulls were the best dispersers; the overall probability of germination for a seed dispersed by gulls was 17.59%. The quality of blackbirds and rabbits was relatively low (3.49% and 1.17%, respectively). Frugivores seem to be essential for germination of C. album seeds, not as much for their direct effects but for their ability to carry seeds to suitable sites.

Fruit consumers and seed dispersers of the rare shrub Corema album, Empetraceae, in coastal sand dunes

Calviño-Cancela, M., 2005. Fruit consumers and seed dispersers of the rare shrub Corema album, Empetraceae, in coastal sand dunes. Rev Ecol (Terre Vie), 60, pp.97-106.

Avec des goélands, des lapins, des mammifères carnivores, des petits lézards et des passériformes, le cortège des frugivores qui consomment les fruits de Corema album (Empetraceae), un arbuste rare, est remarquable par sa diversité et parce que quelques espèces sont des disséminateurs de graines inhabituels. Les merles et les goélands jouent un rôle important dans les îles Cíes. Les goélands sont aussi des disséminateurs importants dans la baie de Trece, mais ne semblent pas jouer un rôle important à São Jacinto où les mammifères carnivores jouent le rôle prédominant. Dans les îles Cíes, les observations des visites alimentaires ont montré que le nombre des visites aux plantes par les disséminateurs est le meilleur indice de leur importance quantitative, suivi du nombre de fruits manipulés par visite, alors que l’efficacité de la manipulation des fruits n’a eu qu’un effet mineur. Le nombre des fruits manipulés par visite, mais non la proportion de fruits avalés entiers, s’est avéré significativement
lié â la taille des oiseaux.

Seed and microsite limitations of recruitment and the impacts of post-dispersal seed predation at the within population level

Calviño-Cancela, M., 2007. Seed and microsite limitations of recruitment and the impacts of post-dispersal seed predation at the within population level. Plant Ecology, 192(1), pp.35-44.

Seed predation may cause important seed losses in plant populations, but its impact on the dynamics of populations will depend on the degree of seed or microsite limitations for recruitment. Seed losses will only affect recruitment if it is seed limited. The nature of recruitment limitation (seeds or microsites) is usually ascribed to whole plant populations but it may vary within populations among microhabitats and habitats. Thus, the potential impact of seed predation will also vary within the population, being highest where recruitment is seed limited. The impact to the whole population will depend on the spatial concordance between the intensity of seed predation and that of seed limitation. Recruitment limitations (with seed addition experiments), seed predation (with seed removal experiments), and the dynamics of seed availability in the soil (with soil samples taken both after seed dispersal and before the following dispersal event) of the shrub Corema album (Empetraceae) were investigated in dunes in NW Spain, at microhabitats ‘open ground’, ‘underneath C. album’, and ‘underneath C. album’ at two habitats, sparse and dense scrub. The nature of recruitment limitation (seeds vs. microsites) varied within the population. It was seed limited in the microhabitat ‘open ground’ and microsite limited under shrub cover. The spatial patterns of seedling recruitment were unrelated to seed availability but strongly affected by germination requirements. The spatial discordance between seed availability and recruitment implies a crucial constraint for processes affecting seed availability (seed predation but also e.g., dispersal) to impact recruitment. They will not affect its spatial pattern but only its quantity as long as they act in those sites selected by seeds to germinate. Seed predation was highest underneath mother plants and lowest in open ground. Thus, its potential impact is low, as it is centred where recruitment is not seed limited. This study shows that the analysis of seed predation in relation to recruitment limitations at smaller spatial scales within the population provides more insight to understand its impact.

Simplifying methods to assess site suitability for plant recruitment

Calvino-Cancela, M., 2011. Simplifying methods to assess site suitability for plant recruitment. Plant Ecology, 212(8), pp.1375-1383.

Few studies link seed dispersal with its demographic consequences, or provide reliable estimates of seed dispersal effectiveness. One reason is the complexity of measuring the suitability for plant recruitment of seed arrival sites. In this study, I compare three methods that differ in the effort required to measure site suitability for seedling recruitment. All are based on the proportion of seeds that become seedlings (seedling-to-seed ratios). Method I is the most detailed and labour intensive. The fate of seeds was followed throughout the different steps of the recruitment process, from fruit removal until seedling emergence, including both seeds dispersed by different animals and undispersed seeds. Method II is based on seed addition experiments. Seeds were sowed in plots, and seedlings emerging were counted in the following two seasons. In Method III, average seed input during dispersal was measured with soil seed bank samples taken in pre- and post-dispersal periods, and seedling emergence estimated with samples of three seasons. Method II provided results similar to those of Method I, which, conversely, provides more insight in the actual processes driving recruitment. Method III, however, systematically underestimated site suitability (seedling-to-seed ratios) by about 50% as compared to the other methods in all microhabitats studied. Relative instead of absolute indices of site suitability were, however, reliable with this method. Method II and III are significantly less costly and could be good alternatives to Method I for some purposes, simplifying future studies on the demographic consequences of seed dispersal and the effectiveness of dispersers.

Spatial patterns of seed dispersal and seedling recruitment in Corema album (Empetraceae): the importance of unspecialized dispersers for regeneration

Calviño-Cancela, M. Spatial patterns of seed dispersal and seedling recruitment in Corema album (Empetraceae): the importance of unspecialized dispersers for regeneration. Journal of Ecology 90, 775–784 (2002). DOI: 10.1046/j.1365-2745.2002.00711.x

  • Spatial patterns of seed dispersal and seedling recruitment of Corema album were examined among and within habitats to determine the quantitative importance of different dispersers in each type of habitat, and their effectiveness in carrying seeds to suitable habitats for seedling recruitment.
  • Gulls, blackbirds and rabbits were, respectively, the main dispersers (45%, 40% and 15% of Corema album seeds). Within habitats, blackbirds disperse seeds mainly to female Corema album shrubs, while gulls and rabbits disperse seeds mainly to open ground.
  • The quantitative role of dispersers varies among habitats because of their habitat preferences, causing the spatial pattern of seed rain to differ.
  • Open ground has the highest density of seedlings and the highest seedling-to-seed ratios. Regeneration is more active in the pioneer scrub than in the mature scrub and the herbaceous vegetation.
  • Gulls, rather than specialist frugivores, are the most effective dispersers in carrying seeds to suitable sites for recruitment.

Effectiveness of a varied assemblage of seed dispersers of a fleshy-fruited plant

Calviño-Cancela, M. & Martín-Herrero, J. Effectiveness of a varied assemblage of seed dispersers of a fleshy-fruited plant. Ecology 90, 3503–3515 (2009).

Disperser effectiveness is the number of new plants resulting from the activity of one disperser relative to other dispersers or to nondispersed seeds. Effectiveness remains largely uninvestigated due to the complexity of its measurement. We measured the effectiveness of seed dispersers (Larus michahellis, Turdus merula, and Oryctolagus cuniculus) of the shrub Corema album (Empetraceae) using a simulation model of the recruitment process that was parameterized with field data of seed dispersal, predation, and seedling emergence and validated with independent data on seedling density. The model allows tracking the fate of seeds dispersed by each animal and estimating, for the first time, disperser effectiveness as seedlings per square meter contributed by each disperser. It also allows quantifying the relative importance of different recruitment processes in determining the quantity and spatial distribution of recruitment. Larus michahellis was the most effective disperser in two of the three habitats studied,contributing 3–125 times more than the other two species, whose lower effectiveness depended
mostly on deposition patterns (T. merula) or deleterious effects on seedling emergence (O.cuniculus). The dependence of the plant on each disperser differed between habitats and was the greatest in sparse scrub, where recruitment depended almost exclusively on gulls (90%). Quantity and quality of dispersal were not correlated; quality was a better predictor of disperser effectiveness. Seedling emergence was the most crucial process in determining both the spatial pattern of recruitment among microhabitats (99.8% of variance explained) and the quantity of recruitment within microhabitats (43–83%). A sensitivity analysis showed that increasing seed dispersal improved the recruitment for all dispersers when there was no competition for fruits. However, with limited fruits, increased dispersal of lower quality dispersers reduced overall recruitment. Our results show important differences in effectiveness among dispersers and illustrate the different influences of the components of effectiveness, which varied depending not only on the disperser but also on the circumstances (e.g., type of habitat).