An email exchange with my ecology prof

[cam1936]

This may or may not interest anyone. I'm taking a basic ecology course as a easy option at the UofC. We're doing population ecology and hunting came up in discussion. Not much was said in class but here is an email exchange I had with him afterwards. It may not make sense without context so here is a picture of the equation in question.

r= per capita rate of population growth under ideal conditions
n= number of indivduals
k= carrying capacity of land
t= time interval
tau= is a time lag to compensate for how current population growth depends on past poulation size (because it takes time for an animal to reach maturity)

My email:


Dr. Cartar,


I have a question, or rather clarification, of one of the topics in today's class. Regarding human harvest of animals (hunting) it was concluded that removing animals contributes cumulatively to animals that would have died anyways of natural causes. I don't quite get this, using the logistic equation for population growth with a time lag it looks to me that depending when this harvest was to take place that it would simply dampen the curve closer towards the carrying capacity. That is a harvest was to take place at any time where N(T-Tau)<K there would be no negative impact on the population. It also seems to me that for population curves where Tau is long enough to send the population far over K (and subsequently crash e.g. Pribilof islands) that a human induced harvest during the right times would dampen the curve and avoid the crash.

I'm not trying to get into the ethics of humans tinkering with an ecosystem. It just seems to me that a human harvest can have a negligible, or even a beneficial, effect on a population so long as certain conditions are met (e.g. when N(T-Tau)<K or when populations are stable and Tau is short enough that a drop in population at one point in time will not cause a crash later).



Thanks,

******** ******

His reply:

Hi ******


If it was you who raised this issue in lecture today, then thanks! I didn't want to leave the impression with the class that mortality was typically compensatory. In fact, tons of studies find otherwise (additive). You even suggested that populations might respond favourably to hunting (overcompensation). I attach a recent paper on the topic that tests the ideas in willow ptarmigan (a bird), but the real interesting bits for you are in the intro, where the hypotheses are introduced and set in context with the literature (attached). This study found a combination of additive and compensatory, depending on hunting levels (compensatory when small quotas, additive when large).


You're right that if a population is growing logistically with a big tau, then arresting the population before they hit K (& crash) might be worthwhile. But since most populations don't appear to be so lagged (the tau of lag?), I think this is a relatively contrived circumstance.


Have a look at this paper, and if you're interested, do a literature search for support for the overcompensation hypothesis (where hunting "helps" a population persist). The pickings are likely slim.


Thanks for your email!


Cheers, Ralph


I can email someone the study he sent me if you want it.



What are you opinions?

 

[Gatehouse]

In BC, hunitng doesn't seem to have much of an impact on many game animals. (There are exceptions) Bad winters and predators are what does them in,and if the herd is over the carrying capacity for the area, the animals starve en masse. You can't stockpile game, so may as well hunt them to keep them under carrying capacity. This is the way most of BC's elk and deer populations are managed by the biologists.

 

[beretta boy]

Examine the current situation with snow geese and the attempts to lower their population with increased bag limits, extended seasons, and contrary to what all the ecologists hold sacred, spring hunting seasons. Their population has seemingly exceeded the carrying capacity of their breeding range (eating themselves out of house & home) Seems we haven't been harvesting near enough of them over the past century. Managers are hoping that hunting will be beneficial to help stabilize their numbers before a crash.

Seems to me that if, say the Eastern Bluebird, weighed maybe half a pound, tasted good when roasted or barbequed and held well for a pointing dog ... then there would probably be a lot more of them around. If it weren't for hunters and their conservation efforts (and $ ) then a lot more species would be in trouble. Whitetailed deer, turkey and wood duck all come to mind as conservation success stories. All are heavily hunted, but not without restriction. Many suggest there are now likely more whitetail now than existed prior to North American colonization. Certainly unrestricted hunting, like the "management" of the late 1800's to "control" the plains bison herds can have a long term negative impact on populations.

 

[TimC]

It right, over here we dont hunt enough, are seasons are long and with no bag limits but hunters are few. The problem is down south where the weather is mild, (21 deg C Today) we have an overpopulation of Herding deer (mainly Fallow) and Muntjac. Even though we have had severe winters 3 years on the trot it seems to have had little effect. The parcelling up of land into small plots for sale ensures that woodland doesnt get managed properly and neither does the deer living there.
In Scotland it was so bad that about 4 or 5 years ago the Deer Commission For Scotland helicoptered marksmen in for a cull and billed the landowners. This is despite the fact that other public activivties had curtailed the stalking seasons (no law of tresspass in Scotland)http://www.heraldscotland.com/sport...r-cull-will-threaten-gamekeepers-jobs-1.89526
http://www.timesonline.co.uk/tol/news/uk/article1053890.ece

 

[G P]

Here is a paper on ptarmigan ecology, possibly the one you mentioned.

http://www.k-state.edu/bsanderc/2011jae.pdf

 

[Paatz]

Is it not geese that are hugely overpopulated and impacting the north?
And we had a deer cull in Kilarney this year. Too many in town.