What do animals actually do in poor environments? Compete more or facilitate for each other? Isabel Barrio and her co-workers studied this in herbivores in the harsh alpine tundra, resulting in teh new Early View paper: Extending the stress-gradient hypothesis – is competition among animals less common in harsh environments? Here’s Isabel’s own background story to the study:
Many summers in the alpine tundra have inspired our study. Alpine environments are strongly seasonal and are characterized by harsh environmental conditions, but they can host a surprisingly large numbers of herbivores. Annual net primary productivity in alpine ecosystems is generally low, so negative interactions (i.e. competition for resources) are usually expected to dominate among herbivores. However, the stress gradient hypothesis (SGH) developed by plant ecologists leads to the opposite prediction; namely, that in stressful environments, positive interactions (i.e. facilitation) would be the most common type of interaction. But, are plants and animals so different in this respect? Although facilitation among animals has been described in some highly productive ecosystems, such as tropical savannas, no theoretical framework exists that relates the balance between positive and negative interactions along environmental gradients. In our paper “Extending the stress-gradient hypothesis – is competition among animals less common in harsh environments?” we evaluate how the stress-gradient hypothesis might apply to terrestrial herbivores.
We considered alpine herbivores for developing our framework, because most examples of the SGH come from alpine plant communities. According to the SGH, and given that stress for herbivorous animals in these environments can be equated to inverse productivity gradients, we wondered if positive interactions would prevail in alpine environments because of their low productivity. We reviewed the available examples on interactions among alpine herbivores and found very few experimental studies on this topic. Interestingly, they were biased towards reporting on significant competitive interactions. Despite this bias, we found no evidence of competition being the dominant interaction type in low productivity alpine environments, which directly challenges the dominant view among animal ecologists. Although we did not find strong support for the SGH either, we argue that specifically designed experiments can help investigate the applicability of this framework to terrestrial vertebrates. Extending the SGH through clear predictions can provide a solid starting point for understanding the role of positive and negative interactions in structuring terrestrial animal communities.