Role of donkeys and goats as “ecosystem engineers” under scrutiny

The albino donkey, after which Asinara Island was probably named. Photo: CC BY-SA 2.0 via Wikimedia Commons
The albino donkey, after which Asinara Island was probably named. Photo: CC BY-SA 2.0 via Wikimedia Commons

The important role of roaming donkeys and goats in dispersing seeds in a semi-natural island ecosystem in the Mediterranean has been determined by researchers.

“Grazing animals are ecosystem engineers influencing the landscape dynamics and a potential conservation measure,” said Julia Tabea Treitler and her colleagues, writing in the journal BMC Ecology.

The study team from the University of Hildesheim in Germany wanted to learn more about how donkeys and goats dispersed seeds on the island of Asinara, northwest of Sardinia.

The island is a national park and covers 51.9 square kilometres. A total of 709 plant species have been recorded there, including 35 species endemic to Sardinia and other Western Mediterranean islands.

Its plant diversity arises from its mountainous terrain as well as coastal zones, with several different soil types.

The island is home to a variety of grazing animals such as horses, donkeys, goats, wild boars and mouflons, which were introduced centuries ago. The island’s white Asinara donkeys are particularly well known.

In the Mediterranean region, goats are a productive way of using areas dominated by scrublands. However, they can cause decline and deterioration of vegetation if not adequately managed.

On Asinara, goats and donkeys roam wild throughout the island, with goats considered to cause a decline in biodiversity. There are about 330 donkeys and 1400 goats in all.

Goats are intermittently captured to reduce their number under the park’s management plan.

“Up to now, there are hardly any studies investigating the dispersal capacity of grazing animals in a complete Mediterranean island ecosystem,” the researchers said, adding that knowledge about the importance of donkeys and goats for plant dispersal was needed to evaluate and produce viable management plans.

For the study, the dung of donkeys and goats was collected fortnightly from the end of March until mid-August 2014 from eight sampling areas. In all, the study team ended up with 87 samples of donkey dung and 88 from goats.

The collected dung was dried and stored at room temperature until it was processed. Each sample of 9 grams was crumbled and prepared for germination in sterile soil. They were kept moist for six months.

All seedlings were counted and identified. They were removed as soon as possible after germination to reduce competitive effects.

Asinara Island, viewed from Stintino beach. Photo: Markus Braun, public domain, via Wikimedia Commons
Asinara Island, viewed from Stintino beach. Photo: Markus Braun, public domain, via Wikimedia Commons

A total of 618 seedlings of 90 plant species belonging to 17 plant families germinated in the dung samples of donkeys.

A total of 2395 seedlings of 72 plant species and 23 corresponding plant families germinated from the goat samples.

“Out of the whole species pool of Asinara island (709 plant species), donkeys dispersed 11.7% and goats 9.7% of the occurring plant species.”

The average number of germinated seedlings in the dung of donkeys and goats did not show significant differences. However, there was a greater variability in the dung of goats.

Sedges and rushes featured prominently among the seedlings grown in the dung from both donkeys and goats.

However, the samples of goats exhibited a high number of shrubs and leguminous forbs while donkeys dispersed a high number of seeds of grasses and also of leguminous forbs.

Significantly more seedlings of forbs and sedges and rushes germinated in the dung of goats than in the dung of donkeys. In addition, there was a considerable trend of more shrubs that potentially can be dispersed by goats than by donkeys.

“In contrast, the dung of donkeys contained significantly more viable seeds of grasses than the dung of goats.”

Donkeys and goats showed significant differences in the dispersal of plants with different growth height. The dung of goats exhibited significantly more seedlings of greater growth height (80–100cm). However, donkeys’ dung contained significantly more viable seeds of plants with small growth heights, in the height classes 10–20cm and 20–40cm.

They found considerable seasonal variations in the number of germinated seedlings.

In general, goats dispersef a higher number of seeds/spores of all plant types except grasses.

The authors noted that plant diversity depended on grazing intensity, though some studies have shown that low intensity of grazing tended to foster a low species diversity in the vegetation in Mediterranean semi-arid habitats.

“This might go back to the long history of grazing in the Mediterranean, thus wild and domestic grazers have shaped vast parts of the vegetation.

“Our results point out differences in the dispersal capacity of donkeys and goats and thus indicate complementing seed input in a semi-natural island ecosystem.

“This corroborates the importance of the dispersal activities of both animal species.

“Removing an animal species completely from the island might lead to considerable changes in the vegetation dynamics of the island.

“Besides affecting the composition and structure of the vegetation, this abandonment might also change light conditions as well as physical and chemical characteristics of the soil.”

Supporting the importance of traditional land management, temporary seasonal removal of goats might be an option to integrate the benefits of goats to maintain and enrich the island ecosystem.

The researchers said their findings emphasized the importance of donkeys and goats for the vegetation dynamics of Asinara.

“Nevertheless, besides the positive effects of goats, stocking rates and the extent of the degradation through them should be taken into consideration (e.g. over-grazing, soil damage and preventing regeneration).”

The study team comprised Treitler, Tim Drissen, Robin Stadtmann, Stefan Zerbe and Jasmin Mantilla-Contreras. All are with the University of Hildesheim except for Zerbe, who is with the Free University of Bozen-Bolzano.

Complementing endozoochorous seed dispersal patterns by donkeys and goats in a semi-natural island ecosystem
Julia Tabea Treitler, Tim Drissen, Robin Stadtmann, Stefan Zerbe and Jasmin Mantilla-Contreras
BMC Ecology 2017 17:42

The study, published under a Creative Commons License, can be read here

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