One of the gravest issues that many natural and semi-natural habitats are facing nowadays is related to eutrophication, i.e., overabundance of nutrients resulting in shifting dominance relationships between plant species, altered vegetation composition and the subsequent changes in any associated fauna, fungi etc.
My post has not been written with the purpose of justifying or endorsing any harmful practices that lead to habitat eutrophication.
However, I have been wondering whether all of the habitats that we have in our time come to associate with low nutrient levels, have been as nutrient deficient through history and whether there might have been plant ecotypes/genotypes lost that used to be more flexible under different nutrient level conditions (while the current ecotypes/genotypes have adapted to nutrient poor habitats and therefore cannot adapt to the sudden increase in nutrient input caused by leaching, atmospheric deposits etc.).
Nutrient levels depend on several factors some of which are fixed and have been fixed for millennia or even longer periods of time.
Such factors are the deposits that have formed bedrock and subsoil.
Also, water regime is of great importance as there are habitats where nutrients ‘travel’ while there are other habitats (e.g., ombrotrophic habitats) where nutrient input is very low.
However, nutrients become recycled on a more dynamic level, as well.
They can be recycled through the creation and decomposition of litter as well as by herbivores who consume living biomass and return nutrients in the form of urine and faecal matter.
Thus, for example, changes in saprotroph density or herbivore density (and guild composition) can result in altered nutrient uptake / input rates.
Human management has been one of such factors of influence on nutrient dynamics.
The traditional pastures, coppices etc. have been replaced by intensive agricultural practices.
As a result, many plant (and associated animal, particularly, invertebrate) species have been lost because these species are not competitive in nutrient abundant conditions and they become dominated by grasses, nitrophilous plants etc.
I do not argue that there are habitats which are naturally nutrient poor.
These habitats would have their specific plant communities and the plants would have adapted to the low nutrient conditions.
There are, supposedly, fewer habitats which are naturally rich in nutrients (e.g., deciduous forests where soils are formed from the leaf litter as well as fast-decomposing woody debris).
However, many plants demonstrate a tendency to grow in varied conditions (within certain limitations, of course).
I wonder if there has not been a tendency for the plants to adapt to the traditional farming and forestry practices in a manner that eliminates ecotypes and genotypes which would have otherwise functioned more efficiently under both low nutrient and moderate/high (respective to historical not modern levels) nutrient conditions.
Many species have become abundant precisely due to the historical land use practices (creating openings in forests, maintaining grasslands and sparse woody groves, e.g., coppices).
These species are threatened because the traditional practices are gone (habitat loss) and because their former habitats have been impacted by eutrophication (due to pollution but also due to changes in farming/forestry methods).
I wonder if this contrast has been so devastating because, prior to the onset of industrial agriculture and mass-scale forestry, the traditional farming had actually lowered the nutrient content in natural but mainly semi-natural habitats.
Traditional farming and forestry frequently removed biomass (which contained nutrients) from the respective habitats without returning them or returning them in a reduced manner (e.g., reaping hay for livestock which does not graze on the grasslands, cutting copses and removing the wood material, grazing livestock on pastures at low densities while eliminating other herbivores and thus limiting the organic fertilizer rates).
Additionally, many plants are not capable of ensuring nutrient storage in roots and rhizomes (or other winter storage organs) if the aboveground biomass has been removed (harvested) prior to its senescence.
These (and other) practices might have resulted in gradually decreasing nutrient levels in semi-wild habitats.
As traditional farming/forestry has been around for thousands of years and many plant / animal species have adapted to these practices (indeed, have evolved with them), it is possible that these species also adapted to lower and lower nutrient levels.
The sudden boost in nutrients experienced in 19th-20th-21st century would have been enormous.
But it is also possible that before human traditional land management, at least some plant species were adapted to slight eutrophication (if eutrophication is defined as the nutrient rate above the level observed in the past centuries or millennia while human land use was already in effect).
As the nutrients slowly disappeared from the habitats, the genotypes and ecotypes that could compete under increased nutrient conditions, were also lost.
This could have impacted natural habitats, as well (e.g., through timber removal, through cutting browse for livestock, through overhunting animals that return nutrients in the habitats they have fed in or who transport them from other habitats that might have been higher or lower in nutrients).
For example, large mammals (bison, auroch etc.) would have deposited considerable quantities of nutrients to the habitats they were extirpated from.
For a while, livestock probably returned nutrients to these habitats as they were grazed in forests, natural meadows etc. but this would not have been as effective as receiving input from native herbivores that grazed and browsed all year round and adjusted their numbers to the forage availability, and that dispersed only ‘natural and semi-natural’ nutrients (while livestock such as pigs had access to human food, too).
It is also possible that the loss of micronutrients (macronutrients were likelier returned than micronutrients) as well as the loss fungal/microorganism interactions (due to, for example, removal of timber or due to very reduced levels of plant litter over the winter period) affected competition, dominance and nutrient processing in plant species changing their responses to certain conditions.
It would be interesting to study pre-historic vegetation communities (as revealed by macrofossils, pollen deposits, molecular technologies etc.) paying attention to the plant abundance in different putative nutrient conditions (for example, whether some forb species that are currently considered only associated with low nutrient settings would show up in richer deciduous habitats etc.) in order to determine whether there existed genotypes/ecotypes that were once adapted to survive both in low and moderate level nutrient conditions.
Notes by Ieva 