Friday, 28 May 2010

Prodigious Pronghorn Population Projections

ResearchBlogging.orgCan the presence of wolves be good for prey species? Intuition seems to suggest the answer is no. After all, wolves eat prey, and being eaten is fairly bad for one's health. Wolves are implicated in a number of natural declines of prey species in a number of systems, especially in closed populations where immigration can't bolster floundering populations. The use of wolf control is a controversial tool which I won't discuss at great length here, but one of the key assumptions is that it increases survival of some age classes of prey.  Before I go much further, I've added some entries to my glossary over to the right, for ecological terms.

One of the most vulnerable age classes are neonatal animals and juveniles, or the year's young. While surviving to second or third year doesn't guarantee you'll survive to old age, if you're a deer, it does mean you're fairly less likely to die. Adults are very good at surviving, since evolution has shaped them to be surviving machines, capable of hyper-vigilance, amazing running speeds, and so on. The young, however, enjoy much less of that sort of protection. There tends to be lower survival for the young of the year, and this can be a major factor governing population fate - if you die young, it's hard to do much breeding.

Re-colonizing wolves provide a wonderful mini-experiment to see what the effects of their presence or absence is on various age groups. Kim Berger and Mary Conner, both at Utah State University at the time, studied the effects of re-colonization on pronghorn neonatal survival. Berger and Conner selected two study sites with wolves, and one that was free of wolves in Wyoming. Coyotes were much less abundant at the sites with wolves than they were at the sites with wolves, as they had demonstrated in a previous study.

Fawns were located through a good old fashion leg-work (they must have good vehicle access out there!), and through watching gravid females until they dropped their fawns. They would capture the neonates, weigh them, age them, record the sex, and assess over-all health in a few categories before fitting them with a breakaway radio collar with a mortality sensor. When the animal dies, the radio signal from the collar changes, allowing the researches to swoop in and look for cause of death. Some things they would look for include
  1. whether the fawn was alive at the time of the attack
  2. tracks, scat, hair, and any caches that might help them identify the predator
  3. signs of starvation or other accidents.
The sort of predator who killed it could be classified based off of previously described characteristics. One thing I've always wondered is if there is a better way to ID the cause of death more adequately. However, kill site characteristics have been frequently used before, and will suffice for here.

Ignoring the factors of survival for a moment, there was much greater fawn mortality among wolf free sites than wolf abundant, with as low as 0% survival in one year. The overwhelming majority of the mortality came from either demonstrable coyote predation, or likely coyote predation in those wolf free sites. Coyote predation was still a major factor in wolf abundant sites, but over-all survival was much higher in all years, with the lowest being 24%, higher than the highest survival in any year in the wolf free site. Thus, the presence of wolves appears to beneficially change the survival rate of pronghorn calves through reducing coyote mortality.  Modelling this forward using other vital rates for pronghorn, they find that the wolf-free sites will gradually suffer total collapse, while the wolf abundant sites are quite stable. Clearly, our earlier intuition was not entirely correct about the effects of having wolves in these populations.

This fits nicely into a framework that I've previously mentioned, which is the "Meso-predator release hypothesis." The idea is that wolves or other apex predators (bears, cougars, etc.) keep middling scavenger-predators (such as fox, coyote, racoon, skunk) suppressed when they're around and healthy. They can suppress them through feeding on common prey, attacking meso-predators directly, and so on. This is a top-down effect, where high trophic levels (animals that do bulk of the eating other things) influence the composition of the lower trophic levels (prey; the things that are eaten).

Within the state of Alaska, it begs the question whether there are significant numbers of meso-predators, and whether they are similarly released in times of intensive management. A entirely separate and important question is whether this release negatively impacts prey populations. To my knowledge, no one has done any sort of extensive study of meso-predators in the state. Our knowledge of background fox and coyote density is not very good, and after asking several people, I begin to suspect there are no density estimates for regions of the state. Given that, I am sure no one has looked at meso-predator densities during periods of IM. Anyone who knows otherwise is encouraged to email me! However, the latter most question is probably best identified - there have been studies of calf mortality after bouts of IM, and I don't anyone's identified a shift in mortality towards meso-predation.

Berger KM, & Conner MM (2008). Recolonizing wolves and mesopredator suppression of coyotes: impacts on pronghorn population dynamics. Ecological applications : a publication of the Ecological Society of America, 18 (3), 599-612 PMID: 18488620

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