The following text is a response to the research publication ‘Fluctuations in age structure and their variable influence on population growth’ (Hoy, S.R. et al., 2019).
The study states that in the (almost) natural YNP ecosystem no significant influences on wolf population age variability were detected that would considerably skew the population age structure from the stabilizing trend overtime, nor it was discovered that the age structure bore significant influence on population growth rates (the latter of which were associated with territoriality in the wolves and also in tawny owls).
However, I have read other publications which state that wolf population demographics can be skewed under human hunting pressure, although in these studies it has not been determined for how long the effect takes place (whether, it, as well, tends to stabilize over time or if constantly harvested wolf populations experience a constant skew in age structure).
Wolf populations that are hunted by humans tend to become skewed toward slightly younger demographic groups.
While YNP wolves are still impacted by legal harvest outside the park’s boundaries, there might be a certain threshold of harvest rates affecting a population – above which the population suffers a permanently altered age structure due to external effects and, under constant harvest pressure, the stabilizing long-term tendencies might not allow to bounce back to a more varied demographic structure (including a larger number of seniors).
This might be important due to several reasons.
1. It might point at potentially additive effects of legal harvesting on wolf populations even in places where lethal wolf management is considered compensatory;
That is to say, if in more natural populations the age structure fluctuations are not affected in a manner that the altered state becomes permanent and that it can not balance it out over time and if it was proven that in harvested populations the age structure was skewed in a manner that is more lasting and irreversible unless harvesting is stopped, this points at legal harvest as an additive mortality factor.
In populations which are legally harvested above a certain degree (which is greater than that suffered by legally protected wolves in natural preserves that are harvested only if they roam outside of the protected territory), this might become an external effect more permanently altering the demographic structure.
This could lead to either constant destabilization or a whole new ‘demographic default’ (the population tends to ‘stabilize’ toward an age structure that is different from that observed in natural populations).
It might mean that legal harvesting is never fully compensatory (in the sense that it does not alter natural processes) even if it accounts for the population’s ability to bounce back through reproduction – because legal harvesting impacts wolf population aspects (e.g., long-term age structure) that are not impacted under natural conditions, not in the same way.
While age structure fluctuations, as you discovered, do not affect wolf population growth rates, it might have other consequences which I will discuss later.
2. It might also follow that the effect on the wolf population’s age structure by legal harvesting might ‘spill over’ to other potential areas of influence.
That is to say, in legal harvest areas wolf population demographics might become more subject to alteration by other factors, as well, not just the harvest itself.
Perhaps in such areas the variability is not as independent and temporary stable as in the YNP due to causes other than harvest – causes that have been facilitated by harvest (for example, harvest might change the effects that population density has on age structure etc.).
This could lead to different demographic patterns in harvested populations compared to natural populations.
Harvest effects on the wolf population might not even be direct but indirect – working through other mechanisms (density, dispersal, prey availability etc.).
This could also mean that we cannot learn from the ‘old stories’ (historical accounts) of wolf ecology, behaviour and demographic trends because, while these wolves lived in a relatively more untouched ecosystems, they were also subject to hunting by humans.
Some people refer to ‘the old times’ when ‘wolves were carrying away sheep and babies’ as a proof of what might occur if we do not impose ‘modern’ and ‘active’ wolf management measures.
While many of such narratives are simply false and exaggerated, we should also keep in mind that they refer to wolf populations that were possibly highly exploited because, historically, wolf pelts could have been of actual significance in producing winter-proof clothing and also because there were fewer regulations and the monitoring of the hunting and poaching was not as strict as nowadays.
This means that it is not inconceivable that the stories refer to destabilized wolf populations that are constantly attempting to recover and such populations, as known today, are more prone to discovering themselves in conflict situations with humans.
3. If wolf population age structure is not as stable in areas where harvest rates are higher, perhaps there is also a consequence on the population growth rate (which was not observed in the YNP).
It might follow that wolf populations subject to higher legal harvest rates are also different when it comes to age structure fluctuations and their effects on reproductive rates.
Perhaps hunted wolf populations suffer reproductive issues due to altered age structure that YNP population does not experience.
Thus, the argument that ‘wolves will bounce back’ might only work as long as the age structure is not affected to the degree that it cannot revert to a stable state.
4. Even if there is no further effect from age structure fluctuations on wolf population growth rates in legally harvested populations, either, these effects might appear in other areas, for example, in wolf population effects on prey populations.
For example, research by MacNulty, D.R. et al. (2009) shows that senescent individuals have reduced hunting prowess.
They discuss how this predatory senescence might affect prey populations through reduced kill rates or through inducing change in prey’s behaviour.
To quote,
‘Greater vigilance among elk living outside YNP compared to those inside YNP (Creel et al. 2008) may reflect a similar response given that wolves outside the park are often controlled to reduce livestock predation, thus skewing wolf age structure toward younger and therefore more lethal age classes (Sidorovich et al. 2007). Elk can distinguish between high- and low-risk predators, as indicated by their more aggressive response to adult coyotes (Canis latrans) than to juveniles (Gese 1999).’
This indicates that legal harvest might have ‘unnatural’ (and by this I mean – not observed in natural ecosystems or ecosystems with low human impact), additive effects also on prey populations.
While age structure fluctuations are always present, a constantly skewed age structure which does not stabilize overtime, might cause a predator-prey dynamic which is different from natural states and which might bear a more pronounced impact on some prey species or on some prey age-sex groups or populations, or in some areas etc.
In systems where the wolf prey populations are endangered or otherwise in need of protection, this might be of great importance and legal harvest might exacerbate wolf impact on prey numbers or on prey behaviour in ways that would not be observed in unharvested systems.
Reducing wolf legal harvest (or eliminating it) might lead to a less pronounced destabilization of wolf population age structure which, in turn, might lead to reduced kill rates or other effects of release on vulnerable prey populations.
There might be other consequences such as success in holding territory (Cassidy, K.A. et al., 2015) leading to a greater resident population stability (and potentially reduced human-wildlife conflict rates) as well as improved ecosystem service delivery (long-held knowledge of territory and prey movements by senior individuals).
I suggest that the age structure skewing tendencies are studied more thoroughly and over longer periods in harvested wolf populations in order to determine if the age structure evens out in the long run – in the manner it does in more natural environment – and to perhaps evaluate the rates at which it can bounce back (if they resemble the rates in the non-harvested populations or if they are delayed, i.e., the age structure fluctuates reversibly but at a reduced efficiency).
Furthermore, it would be important to study whether age structure fluctuations are unrelated to the population growth in harvested populations similarly as they are not – in non-harvested populations.
I find it quite likely that legal harvesting and poaching might considerably distort population age structure in wolves because, as determined by Hoy, S.R. et al., 2019, territoriality has a stabilizing effect on population dynamics (and it can prevent the age structure fluctuations to bear statistically significant impact on population growth) and harvesting changes the ability in wolves to maintain territories in accordance with their biological and social mechanisms.
Notes by Ieva 