Analysis of vegetation changes induced by a European Bison herd in the Kraansvlak area (2003-2009)
Oquiñena Valluerca, I.
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Dutch coastal dunes are highly diverse landscapes in terms of biodiversity. 70%of the Dutch flora can be found in this particular landscape and 15% of plant species dwelling in the Dutch coastal dunes can only be found there. Since the 19th Century, human activities have modified the coastal dunes soil chemistry triggering the coastal dunes degradation and threatening the coastal dunes biodiversity. The process of grass and shrub encroachment promotes the dominance of few tall grass and shrub species at the expense of rare species thus, hampering the coastal dunes species diversity. Large grazers have been reported to counteract this degradation process by removing the soil litter layer and bringing about more open vegetation. European Bison (Bison bonasus) is the largest living herbivore in Europe. European Bison are considered intermediate grazers which diet is based on: grasses, herbs and woody vegetation. Therefore, the Bison can be expected to counteract dune degradation by grass and shrub encroachment. As part of a pilot reintroduction project, 3 Bison individuals were introduced in the Kraansvlak area in 2007. Currently, there are 16 individuals dwelling in the Kraansvlak. In this study, the changes in vegetation induced by this Bison herd in a degraded dune coastal area called the Kraansvlak were analyzed. For that purpose, two aerial pictures and GPS data from radio-collared Bison were used. The aerial pictures were classified with the help of DICRANUM extension to Arcview 3.x to obtain an image of the Kraansvlak vegetation structure in 2 different years: 2003 (before the introduction of Bison) and 2009 (two years after the Bison were introduced). With the help of the GPS data, the Kraansvlak herd kernel utilization distribution map was calculated. This map allowed the identification of the most and least utilized areas and three categories were established: high intensity in use, medium intensity in use and low intensity in use. The relation between intensity in use by Bison and vegetation changes was tested using ANOVA statistical test. According to the study results, Bison caused a significant reduction in the cover of trees and shrubs and it is possible that they caused an increase in short grasses, which is an important habitat for the preservation of the dunes' biodiversity. However, this trend couldn't be statistically proven. Furthermore, several ANOVA statistical tests were also undertaken to look for significant differences in vegetation development between the Kraansvlak and a control area located around the Kraansvlak. In the Kraansvlak in the period 2003-2009 a significant average increase in short grasses class was found to occur and management measures implemented at the Kraansvlak seemed to slow down the degradation of the dunes by grass and shrub encroachment. This trend can be mainly ascribed to the introduction of the Bison and other grazers in the Kraansvlak between 2003 and 2009.