Geese are overlooked dispersal vectors for vascular plants in archipelago environments

Dirk Hattermann, Markus Bernhardt-Römermann, Annette Otte & Rolf Lutz Eckstein

Species rich vegetation of a rocky shore with geese droppings in the foreground. Photo credit: Dirk Hattermann.

Since plants do not move, they have to adapt to changing environmental conditions on the spot or disperse their seeds by means of wind, water or animals to escape unfavourable conditions. Seed dispersal is especially important in archipelagos, which consist of a mixture of plant communities on a mosaic of islands, separated by seawater.

Seed dispersal in the gut of geese may play a major role for island plant distribution, because geese are common on islands. Since geese droppings in previous studies contained only seeds from a few plant species, their impact on local vegetation was considered to be low. Based on studies performed on dabbling ducks, it has long been assumed that also geese mainly disperse aquatic and wetland plants.

This paper addressed the potential of the Greylag Goose (Anser anser) for internal dispersal of plants in three archipelagos along the Baltic coast (Stockholm, Västervik, Blekinge) to answer the following questions:

(i) What proportion of the island species is dispersed by geese?

(ii) Which functional characteristics are typical for plants dispersed by geese?

(iii) Which plant communities are likely to benefit from internal dispersal by geese?

We first recorded the presence of higher plants in nine island habitats on 108 islands during the growing seasons in 2015 and 2016. This island species pool comprised 428 plant species.

Secondly, goose droppings were collected on vegetation-free rocks along the rocky shores of 45 islands from June to August 2016. The dried droppings from each island were crumbled, thinly spread on top of sterile potting soil in germination trays, and these placed randomly in a temperature-controlled greenhouse. All seedlings emerging from the droppings were identified and counted. All vascular plant species germinating from goose droppings represent the goose-dispersed species pool.

Finally, we compared goose-dispersed plants and all occurring species per island with respect to plant characteristics. The relation between goose-dispersed plants and plant species per island is shown in the diagram. Averages larger than zero indicate higher values or higher proportions of a plant characteristic in goose-dispersed plants than in the island species pool and vice versa.

Geese dispersed viable dispersal units (“diaspores”: seeds, fruits) of 97 plant species, which represents at least 22% of the island species pool. Geese dispersed a higher proportion of grass-like plants and less woody species; short-lived plants were also significantly overrepresented. On average, seed volume of goose-dispersed species was about one order of magnitude smaller than that of the island species pool. About 51% of all goose-dispersed species were found in droppings from at least two archipelagos. Geese dispersal may benefit plants species of rocky shores, but species of all island habitats were found in the droppings. The relative importance of geese as dispersal vectors may increase under ongoing land-use changes and cessation of grazing networks. Owing to differences in size, behaviour and food preferences, though, geese may not compensate the effects of other grazers, such as cows or sheep for plant dispersal or community structure.

Differences between the goose-dispersed species and all species occurring on an island for whole plant characteristics (growth habit, life form, life span, and reproduction by seeds) and dispersal characteristics (diaspore size and production of fleshy fruits). Differences are expressed as mean “log response ratios” and confidence intervals. Response ratios larger than zero indicate higher values of a plant characteristic in goose-dispersed plants than in the island species pool. (Figure 1 from the original paper.)

This is a plain language summary for the paper of Hattermann et al. published in the Journal of Vegetation Science.