Interactions between weather, ungulates and geese create opportunities for wood-pasture cycles

Abstract

Managing nature reserves to achieve coexistence of ungulate species and vegetation heterogeneity is challenging in small reserves. A central question is whether spatial heterogeneity is maintained or restored through occasional natural disturbances. This is the premise of the wood-pasture hypothesis, which asserts that natural disturbances, such as winter severity, are sufficiently significant to sustain the long-term coexistence of ungulate species and provide opportunities for the establishment of shrubs and trees, thereby enhancing vegetation heterogeneity. Testing the wood-pasture hypothesis under field conditions necessitates long-term time series of both vegetation and herbivore dynamics. Therefore, we employed a modeling approach to test this hypothesis. The study area was the Oostvaardersplassen, a nature reserve in the Netherlands characterized by rich and productive clay soil. This area hosts an assemblage of Heck cattle, Konik horses, and red deer. Additionally, large numbers of geese frequent this reserve, competing for the same food resources. The Oostvaardersplassen is one of the few areas in Europe where neither the populations of coexisting herbivore species nor the vegetation have been managed and its development was monitored for several decades. This provides unique insights into natural herbivore-vegetation interactions that can be applied to the management of other nature reserves. The results indicate that weather variability and the presence of both geese and large ungulates are essential factors in creating a heterogeneous landscape. The general pattern is that weather variability induces fluctuations in ungulate numbers, and geese reduce the numbers of ungulates. Coexistence of the three ungulate species remains possible regardless of weather variability and the presence of geese. However, the presence of the largest ungulate, cattle, is threatened when weather is highly variable and geese numbers are high. The vegetation is likely evolving towards a predominantly open landscape with occasional opportunities for the establishment of spiny shrub species. Nature reserves managed with an assemblage of ungulate species, which numbers are controlled by food supply, winter severity and competition, should thus focus on the substantial presence of a few species ("biomassality," e.g., for migratory birds) rather than achieving the highest species diversity in a small area ("biodiversity").