I have been following the landscape of fear and the general trophic cascade debate, more specifically, regarding the Isle Royale simple wolf-moose-balsam fir system as well as the Yellowstone National Park complex system in which wolf-elk-aspen/willow relations have been studied.
The latest publication on the matter that I read was the paper, ‘Diel predator activity drives a dynamic landscape of fear’ in which, as far as I understand, the authors have determined that no such strict landscape of fear (LOF) results the from predator-prey interactions that could be considered as temporarily and spatially constant.
Personally, I disagree with their findings. More precisely, not with their findings but with their interpretations. For example, temporal restriction of landscape of fear does not eliminate the concept itself. It simply becomes more complex and dynamic.
Their findings were, however, interesting.
Namely, that the prey appear to learn the predator diel activity patterns and adjust their behaviour so that they can feed in high risk areas during low risk times (e.g., elk feed more at night while the wolves are most active during dawn and dusk hours).
Which is why, as concluded in the study, herbivore fitness is not as much affected (they do not starve) as it would be under circumstance of complete exclusion from certain preferred but high risk areas (e.g., open areas and riparian areas).
Lately I have also begun studying plant sciences which led me to some ideas that could, to a degree, explain why even on occasion of limited LOF (not complete exclusion but temporal, dynamic exclusion) the effects in the plant community might still be significant.
(Comments by the trophic cascade and LOF researcher Robert Beschta have downplayed this potential compensation.
Still, I will offer my theory here.)
As I was reading the discussion in the publication, I started wondering whether this ‘diel-dynamic landscape of fear’ might still result in positive effects on vegetation communities and growth through some additional mechanisms that are related to plant productivity which not homogeneous during the 24 hours, either, but rather bound to its own diel cycle.
From what I have read (and truly, I have not yet read that much to this day which is why I am reluctant to give more specific examples) – plants grow the most during the night when they probably utilize the carbohydrates produced during the daylight hours.
Meanwhile, photosynthesis (energy production for growth and the respective storage of the energy) is restricted to daylight hours when light is available.
However, during day (peak heat hours), many plant species close part of their stomata in order to reduce evapotranspiration rates.
This means that photosynthesis might be most pronounced during morning and evening hours (i.e., the crepuscular hours which, in the particular study, are most related to predator activity although not in all systems wolves are most active during this time).
It could be that plants invest more intensely in their productivity during hours when herbivory becomes, coincidentally, more restricted.
If, for example, during this period the plant is not disturbed (herbivore grazing can also cause shading as well as reduce the leaf area), it might inadvertently contribute to the plant productivity by ‘leaving the plant in peace’ precisely when it is ‘working the hardest’ to provide for itself.
Thus – even if the landscape of fear operated only during crepuscular hours in certain areas (herbivores were excluded from these areas during dawn and dusk but not during other times of day), its impact on plant productivity might still be important because it affected herbivory during the exact diel hours that were especially significant for plant metabolism.
Also, if plants grow most during night, this might be the time when the carbohydrates stored in the roots become transported to the aboveground parts where they become most accessible to herbivores.
Thus, herbivores might benefit from feeding at night and this might compensate for lost the foraging hours during high risk predation periods.
That could explain why herbivores do not suffer fitness consequences as great as could be possible if they were excluded from important foraging grounds.
On another note, I have read research about phenolic compounds in plants. The research has been conducted for the purposes of agriculture (Gori, A. et al., 2020).
In this paper, the authors concluded that the highest polyphenolic concentrations in the plant species that they investigated, were observed in July at 1 pm (month and time).
Polyphenolic compounds, in this case, were sought after for harvesting purposes because they are considered health-promoting when consumed in moderate amounts by humans.
At the same time, the polyphenolic compounds are actually plant defense chemicals (secondary metabolites) and they might negatively impact the digestibility and the quality of plants consumed by herbivores (reduce grazing/browsing pressure) or they might even deter certain herbivores.
It might be possible that while predators exclude herbivores from some places during some periods of potential activity, this becomes offset by differences in plant quality accessible to the prey during the lower risk hours (e.g., nighttime or winter).
It could partly balance out some of the limitations of access to foraging grounds which is why the general fitness of the herbivores might not decrease substantially and might be compensated for.
I am not offering these examples as true explanations.
I am offering them as illustrations on how plant diel activity might prove to be an curious factor that could be considered in order to follow up with the effects of predator activity throughout the entire trophic chain.
References
Kohl, M.T., Stahler, D.R., Metz, M.C., Forester, J.D., Kauffman, M.J., Varley, N., White, P.J., Smith, D.W. and MacNulty, D.R. (2018), Diel predator activity drives a dynamic landscape of fear. Ecol Monogr, 88: 638-652. https://doi.org/10.1002/ecm.1313
Notes by Ieva 