The fluctuating world of a tundra predator guild: bottom‐up constraints overrule top‐down species interactions in winter

Abstract

Global warming is predicted to change ecosystem functioning and structure in Arctic ecosystems by strengthening top-down species interactions, i.e. predation pressure on small herbivores and interference between predators. Yet, previous research is biased towards the summer season. Due to greater abiotic constraints, Arctic ecosystem characteristics might be more pronounced in winter. Here we test the hypothesis that top-down species interactions prevail over bottom-up effects in Scandinavian mountain tundra (Northern Sweden) where effects of climate warming have been observed and top-down interactions are expected to strengthen. But we test this 'a priori' hypothesis in winter and throughout the 3-4 yr rodent cycle, which imposes additional pulsed resource constraints. We used snowtracking data recorded in 12 winters (2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) to analyse the spatial patterns of a tundra predator guild (arctic fox Vulpes lagopus, red fox Vulpes vulpes, wolverine Gulo gulo) and small prey (ptarmigan, Lagopus spp). The a priori top-down hypothesis was then tested through structural equation modelling, for each phase of the rodent cycle. There was weak support for this hypothesis, with top-down effects only discerned on arctic fox (weakly, by wolverine) and ptarmigan (by arctic fox) at intermediate and high rodent availability respectively. Overall, bottom-up constraints appeared more influential on the winter community structure. Cold specialist predators (arctic fox and wolverine) showed variable landscape associations, while the boreal predator (red fox) appeared strongly dependent on productive habitats and ptarmigan abundance. Thus, we suggest that the unpredictability of food resources determines the winter ecology of the cold specialist predators, while the boreal predator relies on resource-rich habitats. The constraints imposed by winters and temporary resource lows should therefore counteract productivity-driven ecosystem change and have a stabilising effect on community structure. Hence, the interplay between summer and winter conditions should determine the rate of Arctic ecosystem change in the context of global warming.

Transient top‐down and bottom‐up effects of resources pulsed to multiple trophic levels

“…Moreover, adding midge carcasses to heathlands near Mývatn increased grass cover, plant biomass, detritivore and herbivore densities, and abundances of several predator groups (e.g., predaceous beetles and parasitoids) (Hoekman et al 2011, Gratton et al 2017). These results are also consistent with how pulsed resources influence food webs in other nutrient‐poor ecosystems, which are generally controlled by bottom‐up forces (Sanchez‐Pinero and Polis 2000, Schwinning and Sala 2004, Stoessel et al 2019).…”

An innovative use of orthophotos – possibilities to assess plant productivity from colour infrared aerial orthophotos

“…) and in modelling species interactions within the terrestrial arctic predator guild (Stoessel et al. ) in mountain tundra ecosystems. But also in biotope mapping in urban and rural landscapes, rel‐NDVI ortho has shown to be useful to automatically separate vegetation covered land from non‐vegetation, which is a highly desirable separation, even without the attempt to translate rel‐NDVI ortho values into an estimation of primary productivity (Akbari et al.…”

“…We have applied rel-NDVI ortho in a number of recent studies where it has been shown to be useful in modelling lemming abundance on a 5-30 m scale (Le Vaillant et al 2018) and in modelling species interactions within the terrestrial arctic predator guild (Stoessel et al 2018) in mountain tundra ecosystems. But also in biotope mapping in urban and rural landscapes, rel-NDVI ortho has shown to be useful to automatically separate vegetation covered land from non-vegetation, which is a highly desirable separation, even without the attempt to translate rel-NDVI ortho values into an estimation of primary productivity (Akbari et al 2003;Grafius et al 2016;Sk anes and Stoessel 2016).…”

Multispecies integrated population model reveals bottom-up dynamics in a seabird predator-prey system

“…Population dynamics models have been used to understand the effect of interspecific interactions and environment on species demography. However, these models are in general not demographically structured (Stenseth et al 2015, Pacoureau et al 2019a, Stoessel et al 2019) or only partially (Millon et al 2014, Saunders et al 2018, Pacoureau et al 2019b). Unstructured approaches consider individuals as being equivalent but differences in size, age and ontogenic stages exist within a population and may be of importance in the context of interspecific interactions.…”