Determining Statistically Robust Changes in Ungulate Browsing Pressure as a Basis for Adaptive Wildlife Management

Abstract

Ungulate browsing has a major impact on the composition and structure of forests. Repeatedly conducted, large-scale regeneration inventories can monitor the extent of browsing pressure and its impacts on forest regeneration development. Based on the respective results, the necessity and extent of wildlife management activities such as hunting, fencing, etc., can be identified at a landscape scale. However, such inventories have rarely been integrated into wildlife management decision making. In this article, we evaluate a regeneration inventory method which was carried out in the Bavarian Forest National Park between 2007 and 2018. We predict the browsing impact by calculating browsing probabilities using a logistic mixed effect model. To provide wildlife managers with feedback on their activities, we developed a test which can assess significant changes in browsing probability between different inventory periods. To find the minimum observable browsing probability change, we simulated ungulate browsing based on the data of a potential browsing indicator species (Sorbus aucuparia) in the National Park. Sorbus aucuparia is evenly distributed, commonly found, selectively browsed and meets the ecosystem development objectives in our study area. We were able to verify a browsing probability change down to ±5 percentage points with a sample size of about 1,000 observations per inventory run. In view of the size of the National Park and the annual fluctuations in browsing pressure, this estimation accuracy seems sufficient. In seeking the maximal cost-efficiency, we were able to reduce this sample size in a sensitivity analysis by about two thirds without severe loss of information for wildlife management. Based on our findings, the presented inventory method combined with our evaluation tool has the potential to be a robust and efficient instrument to assess the impact of herbivores that are in the National Park and other regions.

Abrupt height growth setbacks show overbrowsing of tree saplings, which can be reduced by raising deer harvest

“…However, conifer browsing rates are low compared to deciduous trees in our (see Supplementary Fig. S1 ) and in other studies 32 , 73 . We explain these relatively low rates by the fact that many deciduous tree species can compensate browsing by developing a Lammas shoot in summer 24 , 74 .…”

“…We quantified ungulate impact by predicting the annual browsing probability (BP) for every plot and species using a logistic mixed effect regression based on the data of every state (compare to 32 , 63 ). The mixed effect logistic regression accounts for the binary nature of our browsing response variable [true/false] and allows predictions for plots where a small sample size would result in extreme stochastic browsing percentages 32 , 62 . Furthermore, ungulate browsing is known to be spatially and temporally correlated 64 , which we addressed by implementing the random effect.…”

“…Surprisingly, research has largely overlooked less common species like rowan ( Sorbus aucuparia L.), birch ( Betual spp. ), and sycamore maple ( Acer pseudoplatanus L.)—even though these species are rare, in part, because they are selectively browsed by ungulates, making them potentially sensitive indicators of changing browsing patterns 32 . Even if more data about the physiological responses of these species to browsing were available, the existence of ecological feedbacks would make it difficult to extrapolate to the stand or landscape scale.…”

“…However, these thresholds are based on relatively coarse assumptions (e.g., sapling mortality occurs if height growth decreases by more than 25%), have a very limited local validity (Swiss mountain forests), and neglect all growth parameters apart from browsing. Nonetheless, these thresholds are, to the best of our knowledge, the only ones of their kind and are used in the field to size up ungulate browsing damage in European forests 13 , 32 , 37 .…”

Criteria voor het evalueren van bosverjonging in Natura 2000 habitattypen op de Veluwe

“…Duitsland wordt de lijsterbes sterk geprefereerd, en kan daar worden gebruikt als indicator voor de mate van, en veranderingen in, wildvraat (Bödeker et al 2021). Tegelijkertijd is de lijsterbes zeer resistent tegen begrazing (Miller et al 1982).…”