Recently I read the PhD Thesis ‘Demography and life history of the Eurasian beaver, Castor fiber‘ by Campbell, R.D. (2009).
In the chapter ‘Size matters: growth trajectories and compensatory growth in the Eurasian beaver’ it was established that beavers would invest in attaining greater body size (but not body weight) which results in a competitive advantage (increased probability to acquire a dominant breeding position).
However, the increase in body size is not compensated by a simultaneous increase in body weight which suggests that larger individuals are not necessarily in a good body condition (body weight : body size is a better predictor of individual fitness than body size alone).
Therefore, investment in body size (reproductive chances) could infer a higher mortality risk due to poorer body condition but due to also, for example, increased foraging activity (in order to acquire a sufficient amount of resources) that can lead to exposure to predators and genetic damage.
It is important to invest in body size early on during development because this is the only life stage when size can be increased (while weight might also ‘catch up’).
Those who do not acquire a breeding position during the first 2 – 4 years of their life are probably very unlikely to ever become breeders.
However, survival is higher in beavers of average size which can also lead to lower reproductive success over lifetime in large-sized individuals (fewer years for reproduction).
I was wondering if this trade-off where beavers invest in size growth at the expense of overall fitness is something that has always befallen the species.
My reasoning is based on the potential interaction of eutrophication and lack of micronutrients.
Riparian systems (especially, habitats surrounding beaver wetlands due to their high retention of decomposing organic matter and riparian overflows as well as beaver-felled trees on the shore) are normally rich in nutrients.
However, I believe that even these habitats suffer from eutrophication due to human activity.
Increased amounts of nitrogen and phosphorus become available to plants (macronutrients) while many of the other nutrients (micronutrients) might remain at the former levels (apart from industrial pollution sites where some of these nutrients can accumulate at toxic rates).
Thus far, I have not encountered any publications that discuss changes in growth patterns in plants that have supranormal access to nitrogen and phosphorus but average or below average access to micronutrients.
However, I wonder if this – just like in beavers – might result in increased growth rates regarding the size of the plants at the expense of the overall condition (micronutrient availability and their respective contribution to plant functioning).
If plants absorb macronutrients, these nutrients might become building blocks (especially, in combination with sufficient carbon sources) allowing plants to speed up the growth in size.
Meanwhile, if the micronutrient absorption is not equally increased, the plant might suffer some type of dysfunctions and its nutritional value for herbivores might become lower.
The plant, too, would offer basic nutrients (carbon, nitrogen, phosphorus) but not micronutrients that are important in ensuring proper functionality in the organism and that might participate in otherwise compensatory processes where increased growth is balanced out with an altogether higher primary and secondary metabolism rates reducing the degree of trade-off between size and fitness.
It might be that some herbivores (such as beavers) have become subject to these consequences of euthrophication developing maladaptive life strategies.
For example, it is apparently beneficial for the beaver to reach a large body size early in its life in order to secure breeding position after dispersal (because if this has not been achieved, mortality is probably very high in lone beavers and most of them would die within a year before ‘getting a second chance’).
Investing in body size has likely been a strategy for as long as the beavers have been around.
However, due to eutrophication and possible changes in plant nutrient contents (a lot of biomass vs. nutritional on many levels), the strategy might have become maladaptive.
Beavers might be able to grow larger by consuming these nitrogen-carbon rich plants but the secondary metabolism processes might be impeded negatively affecting the overall body condition (the organism cannot catch up with the growth rate because additional nutrients are needed).
Perhaps beavers might even become too large (relative to the limits of their organism) because the availability of protein is so high.
There might be some mechanisms that become disrupted thereby not signalling to the organism that growth should be reduced because further growth would impair other organism functions.
For example, in our societies, we also consume food that is nutritionally devoid while it offers plenty of macronutrients leading to obesity and poor health.
Our bodies are not fully satiated because regardless of the amount of food consumed, we still have not satisfied our nutritional demands.
Beavers apparently do not suffer from obesity (because their body weight does not increase) but their organism during the growth stage might react differently to nutrient overavailability by directing these compounds to achieve a greater body size.
The lack of satiety might, similarly, cease to inform the individual that they should stop foraging and the excessive foraging might lead to exhaustion which is not felt but which is reflected in body condition.
I am not certain if it is possible that macronutrients become invested in size growth withouth the coincidental fat depositions.
But I am rather certain that changes in the nutritional quality of vegetation (especially, in typically nutrient rich habitats where the species have evolved to be able to consume micronutrients at higher rates as well as where organic carbon is present to bind nutrients long enough for plants to be able to absorb euthrophic resources) should affect herbivores and that herbivores would have a difficulty to follow their own body condition because it is very likely that they have evolved to track body growth (caused by macronutrient availability) but not body condition (reliant also on micronutrients).
In the oligotrophic and mesotrophic habitats, animals would have had little need to distinguish between increase in size and overall health because, historically, prior to euthropication, both would have been correlated and larger size would have been more important because it benefits the individual socially.
Therefore, I believe that herbivorous species might not have evolved an ability to detect a problem if the macronutrients are available (growth and basic maintenance occurs) but micronutrients are deficient (secondary metabolism is impeded).
Larger, social animals might even become misled by their social and reproductive success thus further failing to timely notice their deteriorating condition.
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I have mentioned phosphorus as a readily available macronutrient due to euthropication but today I have read that phosphorus is not accessible to plants in acidic environment with high organic content because in its organic forms, it is not assimilated easily.
Thus, phosphorus could actually be a limiting resource in beaver wetlands where beavers forage – but perhaps not in other habitats that affect other herbivores.
Notes by Ieva 