The presence of natural dynamics is of prime importance to sustain communties of species adapted to unstable environmental conditions. Because of socio-economic motives, natural dynamics are often counteracted or suppressed by human interference. This is particularly true for aeolic dynamics (sand-drift) in coastal dune ecosystems. These dynamics are however essential for the preservation of geomorphological processes, and thus for the conservation of rare species inherent to this habitat.
When sand-drift ceases, a natural succession occurs from white dunes over grey dunes and short dune grasslands to rough herbage with dwarf shrub. In the Flemish coastal dunes, this decrease in dynamics through time is reflected in a spatial transition from dynamic to more stabilized dunes.
Marram grass (Ammophila arenaria) is the dominant species in an association called the Ammophiletum arenariae, which thrives on well-developed, dynamic white dunes. This species is therefore mainly found in coastal foredunes, but also in large, somewhat more inland dunes with sufficient sand-drift and as less vital relicts in more stabilized (grey) dunes. Previous research has shown that the decline of marram grass stands under decreasing wind erosion can be attributed to infection of the roots by pathogenic nematodes (Heterodera arenaria and Pratylenchus spp.).
In this research, the arthropods associated with A. arenaria are investigated in three larger zones within the Flemish nature reserve Westhoek: the coastal foredunes, the central mobile dunes and the slightly more fixated inner dunes. Within each zone, characterized by its own microclimate, several marram grass stands of varying vitality are sampled. The arthropods from each of the grass samples are identified to the species level and their numbers determined, as well as the biomass of the grass. The roots from each sample are treated to reveal the nematode infection rate, which can be used as a measure of the plant’s vitality. The structure of the arthropod community is then compared between grass stands of different vitality and from the different microclimatological zones. Shifts in life history traits are investigated, as well as changes in trophic interactions.
Potted tillers of marram grass from different source populations are randomly placed throughout the landscape. Vegetation structure and percentage Ammophiletum cover are determined within a set of nested spatial scales around each pot, as well as the distance from the pot to surrounding marram grass stands. The presence of a selected number of specialist arthropod species on these introduced stands is then assessed on regular occasions. This way, scale-dependent effects of landscape context as well as the effect of plant origin on the dispersal and colonization capacity of these species can be examined.
Clones of A. arenaria are subjected to two different treatments in the laboratory. One population is grown on regular dune sand while another population receives root infecting nematodes, that cause the decline of the plant in time. Some of the marram grass specialist herbivores are reared on either population. After a series of generations, the effects of plant quality on the herbivore population and possible feedbacks to the plant are examined. Herbivore populations (either above- or belowground) might be structured bottom-up by the grass and/or the herbivores could be exerting top-down control on the plant population. To inquire into the possibility of top-down regulation of the herbivore species, the effects of parasitoids on their population dynamics are considered as well.
As the dynamic dune ecosystem is endangered on a global scale, the inherent specialist arthropods are by definition rare. In case of a decrease of aeolic dynamics, the first ecotope to perish is the most dynamic, namely the moving sand dune, characterized by the Ammophiletum arenariae association. Marram grass specialists are thus expected to be among the most vulnerable organisms in the dune ecosystem. Understanding how their communities are structured, how well they are able to disperse and how their life histories are adapted to their never resting environment, can only be beneficial to their conservation for the future.
arthropods, marram grass, dynamic dunes, life history, community structure, dispersal, plant-herbivore interactions, parasitic nematodes
created:2011-12-14 14:18:59 UTC, source:web