Parameterization and validation of an ungulate‐pasture model

Abstract

Ungulate grazing and trampling strongly affect pastures and ecosystems throughout the world. Ecological population models are used for studying these systems and determining the guidelines for sustainable and economically viable management. However, the effect of trampling and other resource wastage is either not taken into account or quantified with data in earlier models. Also, the ability of models to describe the herbivore impact on pastures is usually not validated. We used a detailed model and data to study the level of winter‐ and summertime lichen wastage by reindeer and the effects of wastage on population sizes and management. We also validated the model with respect to its ability of predicting changes in lichen biomass and compared the actual management in herding districts with model results. The modeling efficiency value (0.75) and visual comparison between the model predictions and data showed that the model was able to describe the changes in lichen pastures caused by reindeer grazing and trampling. At the current lichen biomass levels in the northernmost Finland, the lichen wastage varied from 0 to 1 times the lichen intake during winter and from 6 to 10 times the intake during summer. With a higher value for wastage, reindeer numbers and net revenues were lower in the economically optimal solutions. Higher wastage also favored the use of supplementary feeding in the optimal steady state. Actual reindeer numbers in the districts were higher than in the optimal steady‐state solutions for the model in 18 herding districts out of 20. Synthesis and applications. We show that a complex model can be used for analyzing ungulate‐pasture dynamics and sustainable management if the model is parameterized and validated for the system. Wastage levels caused by trampling and other causes should be quantified with data as they strongly affect the results and management recommendations. Summertime lichen wastage caused by reindeer is higher than expected, which suggests that seasonal pasture rotation should be used to prevent the heavy trampling of winter lichen pastures during summer. In the present situation, reindeer numbers in northernmost Finland are in most cases higher than in the management solutions given by the model.

Soft resilience: moisture-dependent lichen elasticity buffer herbivore trampling in cold alpine-tundra ecosystems

“…Humidity moderate the negative trampling effects via conferring increasing resilience to trampling damage and volume loss. Even though this relationship is crucially important for modeling and predicting potential trampling loss, it is not well studied, and generally modeled as a simple, fixed factor (Moxnes et al 2001;Tahvonen et al 2014;Pekkarinen et al 2017). But such effects will depend strongly on local climate conditions, and only be important during dry weather conditions.…”

“…less exposed ecosystems presumably more resilient to trampling effects. These have been nonlinear wastage factor functions with 'relative loss' values ranging from 0.5 to 4.5 depending on density of lichen and season (Moxnes et al 2001;Tahvonen et al 2014;Pekkarinen et al 2017). Heggenes et al (2017) experimentally documented that reindeer trampling can be a major factor, potentially generating considerably more trampling volume loss of lichen than lichen volume eaten by reindeer, if trampling occurs during dry weather.…”

“…Unfortunately, effects of trampling are understudied relative to grazing, even though some studies suggest trampling per se may be a major ecological factor (e.g. Cumming and Cumming 2003;Ludvikova et al 2014;Pekkarinen et al 2017).…”

Recent changes in mountain birch forest structure and understory vegetation depend on the seasonal timing of reindeer grazing

“…Notably, lichen biomass had stayed unchanged in dry habitats and declined in semi-dry habitats during the past 12 years despite lower reindeer numbers prior to 2015 than prior to 2003. The higher lichen biomass in winter than summer ranges supports that pasture rotation promotes lichen abundances (Pekkarinen et al, 2017), but yet, combined pasture rotation and reduced reindeer numbers had not enabled the lichen biomass to increase in time. It is possible that climate warming affects lichens through yet unidentified interactions.…”

Predation costs and compensations in reindeer husbandry

“…An economic-ecological context is required when studying sustainable and economically viable reindeer herding (Pekkarinen 2018), as the profitability of reindeer herding highly depends on winter pasture conditions (Pape and Löffler 2012), herding systems (Kumpula et al 2014, Pekkarinen et al 2017, and on the predation pressure caused by large carnivores (Hobbs et al 2012, Kumpula et al 2017). However, the few studies that combine reindeer-pasture dynamics with the economics of reindeer herding do not take into account the effects of predation.…”