Wessels, Saskia Cornelie (2008):
The contribution of sheep zoochory to the conservation and restoration of target plant communities in isolated sand ecosystems.
Darmstadt, Technische Universität, TU Darmstadt, [Online-Edition: urn:nbn:de:tuda-tuprints-9482],
[Ph.D. Thesis]

Abstract

The dispersal of seeds is crucial for the survival of many plant species. Fragmentation of the European cultural landscape and changes in land management, e.g. decrease of livestock numbers and lower mobility of livestock between different areas, have severely limited spatial dispersal processes, and thereby made a major contribution to the reduction of species richness of cultural landscapes. Reintroduction of extensive grazing regimes involving large domestic herbivores is often claimed to restore the connectivity between separated populations via endo- and epizoochorous dispersal of seeds. This thesis aimed to investigate sheep-induced seed dispersal over long distances and its contribution to the recruitment of especially target species. The use of a sheep grazing regime which involves consecutive grazing of well-developed target communities, as a seed source, followed by less-developed sinks, could increase the quality of dispersed seeds (more target species). If recruitment takes place successfully, spatially isolated populations can be functionally re-connected or new sites colonised. The study was carried out in three inland sand ecosystems, relatively isolated from each other in the landscape (Darmstadt region, Germany). The two larger areas “Ehemaliger August-Euler-Flugplatz von Darmstadt” and “Griesheimer Düne und Eichwäldchen” are both nature protection areas, grazed by sheep since 1999/2000. Animals walk a 3-km distance when transferring from one area to the other. Recently, for restoration purposes an abandoned agricultural field “Streitgewann” was also included in the grazing system. “Streitgewann” is situated between the first two areas and could function as a “stepping stone”. Dispersal and post-dispersal processes within and between the above-mentioned areas were analysed. As seed sources mostly well-developed target communities Koelerion glaucae (Kg), Armerio-Festucetum trachyphyllae (AF) and Allio-Stipetum capillatae (AS) were used. All investigations employed three or four tamed male sheep. First of all, species compositions of grazing sources and epizoochorously dispersed seeds (both intra- and inter-area transfers) were determined. The quantities and species composition of seeds detected in sheep coats before and after walking from one area to another were compared to test for potential seed losses. Additionally, the impact of seed surface structure, seed mass and plant seed-releasing height on attachment was tested. Naturally attached seeds of Stipa capillata and Agrimonia procera were used to study retention times and detachment rates during different herbivore activities (long-distance walking, grazing). In total, 56 seed species were detected in investigated sheep coats. Half of the species in the three vegetation types were target species. In sheep coats target species were slightly over-represented; among them were seven Red List species. Quantitatively, most transported seeds belonged to target species, whereas almost no graminoid competitors were detected. No significant numbers of epizoochorously transported seeds dropped out of the sheep coats during 3-km inter-area transfers, whereas many seeds of S. capillata and A. procera detached in the sink area. Longer duration of the grazing period and grooming behaviour probably stimulate seed detachment. Furthermore, it was shown that attachment is determined by seed mass and seed morphological appendages. Seeds without well-developed seed structures are probably negatively affected by seed mass, whereas seeds with structures (e.g. hooks) seem to have an equal attachment probability, regardless of mass. The fate of epizoochorously dispersed seeds was studied for 14 seed species. Most of them were endangered target species of inland sand ecosystems, for which epizoochorous dispersal was already shown in our previous investigations. Three 10 m x 10 m sand fields were established at a former agricultural field. Per field two sheep were present for 24 h; seeds were experimentally attached to their coats. The impact of sheep trampling on seedling emergence was also tested for two threatened species: Jurinea cyanoides (EU Fauna-Flora-Habitat directive species) and Koeleria glauca. Sheep trampling patterns, seed detachment rates and spatial dispersal patterns of the three largest-seeded species (Stipa capillata, Medicago minima and Cynoglossum officinale) were studied. Also, seedling emergence and survival of all included species were recorded over an 8-month period. The relatively brief sheep presence at the sand fields resulted in high detachment rates. Seedlings of almost all species emerged (up to 40 % of original seed number). Analysing the patterns of sheep trampling, seed shadow, and seedling emergence by use of Spatial Analysis by Distance Indices (SADIE), showed non-random distributions. Generally, a positive correlation between sheep abundance and the number of relocated seeds and emerged seedlings was found, whereas individual species showed a variable trampling response. The seedling emergence of J. cyanoides increased after trampling, whereas K. glauca showed no effect. Estimation of the potential contribution of herbivores to plant species dispersal via the endozoochorous pathway has mostly been based on results of seedling emergence tests. To check the reliability of such estimates, a comparison of viable seed contents in sheep dung by use of two methods was conducted. Dung was collected after a 6-day grazing period in one of the target communities (Allio-Stipetum). Following the Ter Heerdt method (TH), samples were concentrated and kept under controlled conditions (glasshouse or climate room, duration experiment 5 months). In the second seedling emergence test (common garden method, CG) unconcentrated samples were kept outdoors (15 months). Qualitative and quantitative similarities between the two methods were calculated by use of Sørenson similarity index (QS). Detected seeds showed a highly similar species composition (QS = 0.81) and content (QS = 0.69). Some more seedlings emerged via TH, and especially several small-seeded winter annuals (Veronica arvensis, Arenaria serpyllifolia, Arabis glabra) showed a higher emergence. More species and higher quantities of hard-coated seed species of Fabaceae and Cistaceae emerged outdoors (CG). In relation to each other, both methods offer a valuable means of estimating the potential seed input via dung deposition. If the main research interest is the species composition, CG should be used. TH, on the other hand, better estimates the overall viable seed content and uses less space and time. Finally, the impact of endozoochorously transported seeds, the deposition of dung, disturbance by removal of the moss layer (Hypnum cupressiforme), and dung removal by the dung beetle Typhaeus typhoeus (Linnaeus, 1758) on fine-scale species richness was investigated (factorial experiment).. Sheep dung was collected after the same source paddocks used for epizoochory investigations were grazed (Kg, AF, AS). The species composition and viable seed content of dung-embedded seeds were analysed by use of seedling emergence tests. Treatments were established in the field in stands having different productivity (Kg < semi-ruderalised AF < ruderalised AF). From the three dung types (referring to vegetation types), viable seeds of 59 taxa were found, whereas under field conditions, 23 dung-borne taxa were detected (60 % target species). Quantitatively, most seedlings belonged to target species (e.g. Helianthemum nummularium, previously absent in the sink area), whereas competing graminoids emerged only within more productive AF plots. Despite a lower seedling emergence, dung deposition did promote species richness in Kg and semi-ruderalised AF plots. The impact of dung deposition can be modified when dung beetles transport dung pellets into larvae chambers, situated below-ground (> 40 cm). As a result, several disturbance species, e.g. Oenothera biennis and Polygonum aviculare, emerged at high rates from the dung beetle-disturbed topsoil. Disturbance of the moss layer promoted seedling emergence from dung and soil seed bank in semi-ruderalised AF plots, and the emergence of especially herbs resulted in increased fine-scale species richness. The vegetation of the most productive AF plots responded differently to the two treatments (disturbance of moss layer and dung deposition) in comparison with semi-ruderalised AF plots. Phytodiversity and plant density were initially suppressed in both treatments and additionally dung deposition decreased the abundance of graminoid competitors Carex hirta and Cynodon dactylon. Nevertheless, the reduced competition and soil fertilising effect of dung presumably promoted those same competitors in a later stage. Increased competition probably limited species diversity. The effect of herbivore-induced disturbance and endozoochorously transported seeds on fine-scale species richness probably depends at least partly on the productivity of the established vegetation. The present study has shown that a regime of sheep grazing, the use of well-developed target communities as seed sources and inter-area transfers between isolated habitats offers a good management measure for conservation and restoration of open inland sand ecosystems in fragmented landscapes. A large sheep flock (around 600 animals) and the presence of a high proportion of habitat-typical species in the selected grazing sources promote epi- and endozoochorous dispersal of millions of seeds of target species over long distances. Additionally, herbivore activity can increase the availability of micro-sites. By altering main determinants of recruitment (especially seed and micro-site availability), sheep play an important role in the establishment of target species and promote species-richness, especially in early and mid-successional stages.

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