As I have been reading on plant secondary metabolites and other antiherbivory strategies in plants (a nice review can be found in the publication by Iason, G., 2007), I have begun wondering whether the interactions between different taxa and plants could result in a certain herbivory regulation mechanism where some herbivory agents induce changes in the vegetation that limit other herbivores thus reducing herbivory on a specific plant or in a specific plant community ensuring the potential for regeneration if not by all plants then by the vegetation system itself.
I believe this mechanism could have worked in undisturbed systems where the trophic webs were intact and overabundance of specific herbivores with the consequent overgrazing/overbrowsing was not as common.
Also, I believe that the mechanism has been imperiled through the practice of clear-cutting which creates vast areas of disturbed vegetation that during regrowth produce at the same time saplings that are attractive to herbivores and PSM that lower the forage quality.
I suspect that not only overpopulation by ungulates due to the disappearance of large predators and due to changes in land management practices has altered the natural plant-herbivore guild interactions.
I also think that the herbivore species have been forced to adapt to new forage quality (or, indeed, non-quality) and that they have evolved endurance to consume relatively unpalatable plants as long as they are have not surpassed toxicity thresholds that result in the death of the animal or short-term / long-term fitness reduction (e.g., digestion upset / lower body mass) that the animal can note and mind modifying its subsequent behavior to avoid the unpalatable forage.
The new concentrations of plant chemicals might have disrupted the (large mammalian) herbivores’ natural ‘appreciation’ for the taste of the forage and the increased tolerance toward less palatable species might have caused a situation where large ungulates (but also perhaps smaller herbivores) overgraze and overbrowse because they are not as ‘picky’ as they used to be and they do not stop consumption upon the earlier levels of ‘disagreeability’ of the taste of the forage.
However, I think that while the ecosystems were still intact with respect to the food webs, activities by invertebrate as well as vertebrate herbivores were not limited just by predation and the subsequent top-down population control, nor merely by the bottom-up mechanism of forage abundance.
I think they were also regulated through the mechanisms of forage quality with herbivores avoiding plants that had been foraged on by other herbivores due to altered plant chemicals and forage quality.
These mechanisms could have worked on a very small scale (individual plant or even individual plant’s parts, e.g., leaves, one branch/twig etc.) but also on a larger scale (from clonal plants that communicate through runners to groves of trees that warn one another on predation risk – the famous example being the acacias).
This could be described as interspecific herding where the herding is not really achieved by predators but by other herbivores inducing certain levels of damage and preventing further damage.
I suppose these mechanisms could still be relevant in smaller taxa such as invertebrates that are likely less adaptive to continuous exposure to increased plant chemical defense levels.
(However, there are studies that indicate at increased herbivory by invertebrates following large herbivore (red deer, moose) browsing due to regeneration of foliage and twig shoots.)
I think that large herbivores might be ‘out of the picture’ so to speak at least in most ecosystems because the sustained exposure to chemically altered plants (through human disturbance or overpopulation) has limited the large herbivores’ ability to detect unpalatability at concentrations as low as resulting from much smaller herbivore activities (although invertebrates may compensate for what they lack in size with numbers but their impact would still have to surpass that of the ‘regular anomaly’ in order to ‘herd’ large ungulates).
Meanwhile, the effects on plant community scale might also have changed because due to the scarcity of vegetation and the overbrowsing/overgrazing by unregulated, overabundant ungulates, the smaller taxa might have been forced to include suboptimal plants or plant parts in their diet that otherwise would have not suffered grazing/browsing pressure in systems where the more palatable plants had been rendered unpalatable directing the invertebrate herbivore (assuming a high state of mobility) toward other plant community.
The reactions in small mammal herbivores as well as stationary invertebrates might be different from those in more mobile and less philopatric species.
It might be curious to discover whether such interspecific, intraguild herding exists and whether it has the potential of imposing self-regulation to some degree within the herbivore guild outside of the top-down controls by large predators.
I would also be interested to learn on what scales plants react to disturbance and which plants might prevent further herbivory in the entire community or grove (rather than preventing or, more precisely, reducing further herbivory only on an individual plant level).
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I have now read quite a lot of how one herbivore species can actually facilitate other herbivore species (frequently, large herbivores facilitating invertebrates) through inducing regeneration of plant organs (high nitrogen in recent growth).
However, I wonder how this affects other parts of the plant.
I do believe that some basic herbivory can actually enhance plant growth because I find it very likely plants would have adapted to herbivory using it to their benefit.
Still, e.g., if the shoots get grazed/browsed frequently and the plant has to invest its resources in vegetative regeneration, this could lead to reduced investments in reproductive effort or reallocation of resources from roots to shoots/leaves etc.
Thus, while some species might be facilitated (species that use resprouted leaves or shoots), other species that rely on, for example, flowers, seeds or roots could be disadvantaged.
Currently, I am not able to link these processes to regulating mechanisms (this subnote was directed at resource redistribution) but I suspect the link is there and I hope to find it.
It is not know how it translates back into the herbivore guild that feeds on shoots and leaves (both mammals and invertebrates).
One mechanism that I can see is the lack of reproduction from either roots or seeds that lead to reduced population of the plant species, i.e., diminished resources in the year to come for the herbivores that induced resource allocation away from roots and seeds and into the current year’s vegetative regrowth.
Notes by Ieva 