I have been reading curious research regarding, e.g., diel activity patters by large herbivores and omnivores and how these patterns sometimes do not follow the current predation risk exposure.
For example, wild boar that have not even evolved adaptations with respect to night vision (tapetum lucidum), still retain nocturnal activity patterns in areas where large predators have been extinct.
This nocturnal activity is frequently explained by retained adaptations to the species co-evolution with predators (reactions to ‘predators past’).
However, in many areas wild boar would, for example, demonstrate increased activity during bright moonlight nights despite predator presence in the region (which suggests that they are not as ‘scaredy’ as assumed).
Nocturnal / crepuscular predators should not affect diurnal feeding, either.
It has been reasoned that the absence of diurnal feeding might be due to human disturbance and hunting and, therefore, wild boar rather select the risk that is posed by large predators than the disturbance and risk by humans thus altering their activity patterns and choosing to be active at night.
(Coincidentally, wild boar are poorly adapted to tolerating high temperatures that might limit daytime activity in the warmer months.)
However, human hunters do not really persecute wild boar during midday and hunting by humans is rather restricted to twilight hours.
Why would wild boar not utilize, under such conditions, daytime hours that are suited for their vision (albeit which they do not rely on extensively), e.g., during late autumn, winter and early spring when thermoregulation is not an issue?
I have also read other publications where it has been stated that predator presence may alter prey species (elk) behaviour causing them to reduce their foraging effort, this does not always translate to adverse fitness effects.
These publications prompted me to wonder whether there might be other factors at play.
One such mechanism that I came up with was that differential quality of plants throughout the diel cycle.
I will attempt to add to this post if I find publications that support or oppose my theory.
But the theory is as follows – plants might not exhibit the same quality during different hours of the day and night and feeding during specific time of the plant circadian rhythm (for example, in cases where herbivores/omnivores retain nocturnal behaviour despite current absence of threat during crepuscular and diurnal hours or where herbivores respond to predation risk by altering and reducing their foraging time but no health effects can be detected) might result in differential quality forage uptake that might somewhat compensate for loss of diurnal activity or reduced foraging duration.
As I have stated in other posts, I am a beginner botany and I am not yet able to present descriptions of mechanisms that might induce differences in forage quality on a circadian basis.
However, I am aware that plants grow at night and sometimes more extensively than during day – using up the energy resources produced during daytime photosynthesis and stored in below-ground biomass (roots, rhizomes).
For example, study by Michael, T.P. et al., 2008 discusses genes that are responsible for phasing important processes in plant circadian activity.
I quote, ‘In plants, growth rates of the embryonic stem (hypocotyl) are maximal at different times of day, depending on external photoperiod and the internal circadian clock.’ (Michael, T.P et al., 2008)
The study also refers to the preconditions to evolutionary adaptations to low-light and darkness situations (why and how plants can induce growth while, theoretically, due to lack of light, they could remain inactive).
Many plants have to being early development (as seedlings or regenerating vegetative organs during spring) in low-light conditions (below soil or in early spring when photoperiod is yet short, e.g., in northern regions).
Thus, growth cannot be limited to daylight hours because it would seriously reduce the plant’s developmental potential.
Similarly, if plants have suffered damage (by herbivores or other disturbance), they should recover fast (regrow lost or damaged organs) in order to achieve maximum photosynthesis during day.
This would be best achieved by regrowing these organs during dark hours.
But if plants were more active regarding aboveground biomass accumulation (stem elongation, leaf production etc.) during certain times of day and night, would this not alter the nutrient flows and distribution between different plant organs?
For example, if plants produced energy through photosynthesis during day to store it on root level, the maximum carbohydrate content in belowground organs might be achieved by evening.
If plants then, during night, invested this energy into aboveground biomass regeneration or development, the carbohydrate stores belowground might be somewhat depleted by morning.
Meanwhile, nutrient and energy flow into aboveground organs might be higher during peak growth / regeneration periods and this might be the best time to forage on, e.g., stems, leaves etc.
Regardless of whether these rhythms contribute to diel activity patterns and forage quality in the specific cases that I mentioned, it might be very interesting to follow these changes in plant activity and how they affect forage quality and if they indirectly modify herbivore behaviour (on occasion the herbivores have detected these differences).
Not all herbivores aim for the same nutrients (not predominantly) and thus the effects would be herbivore species-specific according to the nutrient acquisition goal in the herbivore species and the nutrient location in the plant organism during the particular time of day.
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These processes should also be regarded from the plant’s perspective.
For example, if the peak photosynthesizing occurs during morning and evening hours (in seasons when daytime is characterized by high temperatures and (partial) closure of stomata and in species where stomata are regulated in the first place), this might be the time most crucial to retain the maximum of plant’s green organs as well as to reduce overshadowing (a factor relevant to large herbivores and especially large herding herbivores).
Thus, the dawn and dusk predator activity could particularly benefit plants as they can produce carbohydrates and store them efficiently while the herbivores avoid predation risk.
Meanwhile, increased productivity during peak hours could lead to overall plant vigour which benefits the herbivores, as well, because the plants can regrow browsed parts and attain greater biomass.
Inability to graze and browse during certain hours could be crucial to maintain healthy vegetation communities that sustain healthy herbivore communities.
Grazing/browsing during daytime in the warm season is less likely due to thermoregulation but it could be beneficial if it occurs because a slight shade might improve plant performance due to reduced necessity to regulate moisture loss that inhibits gas uptake.
There could be other aspects that I currently cannot discern and understand.
For example, foraging on roots at night could either reduce plant nighttime growth (roots are unable to provide nutrients and soil water for aboveground organs) or instead stimulate it (sensing damage, the plant might attempt to translocate nutrients to aboveground organs).
This would all be very exciting to study.
Notes by Ieva 