When and howdo prey animals die, vs. preadator species?

July 4, 2007 12:28 PM

How do prey animals die versus predator animals?

What are the causes of mortality for prey animals in their natural environment? For predator animals?

Basically, I'm looking for aggregate breakdowns of causes of death (by percentage), and for prey animals especially death consequent to predation. Ratio of death by predation / death by all causes would be sufficient, especially if combined with age (age range) at time of death.

Especially useful would be statistics for closely related but distinct species or sub-populations.

Less useful but helpful would be comparisons of histograms of age at mortality contrasting several predator and prey species/populations, e.g, 30% of mice die before age one month, 20% age one month to one year, 20% one year to 18 months, 30% 18 months or greater.

Bonus if prey population is mice.

Underlying questions, do histograms of age at mortality differ for prey species and predator species generally? Where do primates, including humans in their natural environment, compare? Do sub-populations within a species/population show significantly different age-at-mortality histograms?

posted by orthogonality to Science & Nature (12 answers total)

I'm confused by your underlying questions: most species are not readily split into "predator" and "prey" for a start. Unless you better define your question I doubt you are going to find the kind of definitive resource you're looking for. If you search on scirus.com and google scholar you should get about a million hits for white papers that you can narrow down by location etc. and use to refine your questions.

A few points to keep in mind:
species have different strategies for reproductive success; mice have umpty million offspring, most of whom die. Primates have very few offspring, a much higher percentage of whom survive. Looking at rates of recruitment to the adult breeding population might be more what you're after, although it's hard to tell from your question.

It would be probably be easier to find quantative data for taxa other than mice, birds in particular are often the subject of this kind of study. Search for survivorship, mortality, nesting success etc. There are also a lot of studies on game species.

You can expect to see significantly different age-at-mortality histograms not only amongst different regions but from year to year at the same location as conditions change. You will have to read and see if the conditions were atypical or not (El Nino, drought, fire, lemming invasion, etc).

Finally remember that comparing analysis from very different studies is not at all likely to give you a meaningful conclusion.
posted by fshgrl at 1:19 PM on July 4, 2007

Response by poster: fshgrl writes "I'm confused by your underlying questions: most species are not readily split into 'predator' and 'prey'"

Ok, good point; death by predation rates by, and death overall by age will suffice. (And yes, I'm aware of of r- and K-selected species.)

"You can expect to see significantly different age-at-mortality histograms not only amongst different regions but from year to year at the same location as conditions change. You will have to read and see if the conditions were atypical or not (El Nino, drought, fire, lemming invasion, etc)."

Good point, and sizes of predator and prey population affect one another (Lotka-Volterra).

Birds sound like a good bet, and thanks for the scurus.com reference.
posted by orthogonality at 1:43 PM on July 4, 2007

Predator/prey relationships aren't really static enough to give numbers like that. They're affected by a number of factors that you'd need to have the answers to if you wanted to create a model, such as: Which species are involved? What diseases/parasites exist in the population? How many individuals can the habitat support? What additional predator and prey species are present?

There are too many causes of mortality other than predation; the numbers you're looking for can change dramatically even for the same predator/prey associations and any data you get would be strongly tied to the location/environment the data was collected in.

All of that is to say that fshgrl is right about extrapolating data from different studies. Having said that, though, you might find it helpful to contact the Department of Fish and Game or a university that has zoology/wildlife programs; they might be able to point you to some data you could use.
posted by stefanie at 2:01 PM on July 4, 2007

This is a very interesting question, but you need more boundaries on it. Or to make them clearer-- I'm a little confused. You're really interested in death from predators vs. other causes? Or is it what's killing predators vs. prey? You're interested in mice specifically but the broader question as well?

Causes of death in the wild are difficult to document-- you need to be able to observe mortality for lots of individuals, and that is a really tough thing to do. Birds are a good place to start looking for trends because birds are very well studied compared to other groups, and there's a lot of data on them. Be wary of generalizations to all predators and prey from a few datasets though.

You might look into papers on trophic position to start, since what is predator and what is prey is heavily dependent on context. Are you interested only in top predators or in mesopredators too? There's a lot of omnivory (where a predator eats from several levels of the food web at once) in nature.
posted by Tehanu at 3:29 PM on July 4, 2007

Best answer: I can't answer your question but I'll try...

The wolf and moose populations on Isle Royal in lake Superior have been studied since 1958. The isolation of the island creates a somewhat controlled environment that allows researches to study a simplified version of a predator prey relationship.

Basically the fir trees are eaten by the moose and the moose are eaten by the wolves.

What are the causes of mortality for prey animals in their natural environment?
(From this)

At least 45% of the animals examined had been killed by wolves (Canis lupus). Calves and yearlings comprised 29.3% and 3.5%, respectively, of the wolf-kills, but were underrepresented in relation to their occurrence in the population. A pronounced selectivity for "old" animals was demonstrated with age classes 12-17 years comprising 29.3% of the wolf-kills. As a group, the wolf-kills also showed a significant preponderance of females. Age distribution of moose dying of unknown causes (34.9% of the total sample) differed significantly from that of wolf-kills. Young adults comprised 24.2% of the unknown mortality remains, while "middle-aged" (7-11 years) and old animals accounted for 37.9% and 12.4%. Moose dying of unknown causes showed a highly significant preponderance of males, but age distribution between sexes did not differ statistically. Age distribution among winter wolf-killed moose showed a significant increase of younger animals in the kill during the latter half of the period 1959-69. The role of wolf predation in the regulation of this population is discussed.

...for predator animals?

(From this)

...High mortality among pups seemed to be the point of population control. Socio-economic factors may have controlled the size of the large pack. Availability of food during the period of parturition and rearing probably was critical to survival of young...

(And this)

Being killed by a wolf is a common cause of death among wolves; especially in places where humans are an unimportant cause of death. In a typical year on Isle Royale, one to three wolves are killed each year by other wolves. A typical alpha wolf may kill 2 to 4 wolves in his or her life time. (The entire wolf population is ranges between 15-50)

(Other data about the relationship.)
posted by 517 at 5:38 PM on July 4, 2007

Response by poster: Oh yeah, 517's answer is what I wanted! Now, can I get more like that? Please?
posted by orthogonality at 9:46 PM on July 4, 2007

There are hundreds of scientific papers that deal with this kind of thing, a search on Google Scholar for something like this "causes (death, mortality) deer" will pull up some relevant literature. However, literature on mice will be hard to sift though because so many lab studies use mice - try "Apodemus" instead.

As already mentioned, simple comparative statistics across species on this matter will be hard to find because there are so many caveats (mortality will depend on age, maternal care quality, weather etc) which make cross study comparisons almost meaningless. Quantifying the role of predators as a factor of mortality is notoriously difficult in natural populations and requires highly detailed data. Figures will be more reliable for large beasts than smaller ones (mouse sized things are hard to study, they are often eaten whole/ torn apart and are usually not individually marked like the subjects of many ecological studies). Studies that look specifically at causes of mortality in small mammals are very rare.

You should also look at literature on senescence/aging. Many of these will take a statistical approach to estimating mortality causes. For example, they might compare death rates in captivity (where animals are protected from predation) to those in the wild (where they are not). A good start would be papers by Ricklefs or by Promislow. Adding "body size" and/or "comparative" into your search terms should pull up some papers where the effects of body size on mortality rates are considered. There's a good one by Gaillard comparing mortality amongst large hervivorous mammals.
posted by jonesor at 4:42 AM on July 5, 2007

orthogonality: "Oh yeah, 517's answer is what I wanted! Now, can I get more like that? Please?"

Unfortunately, no - that's why the Isle Royale study is so interesting. It is essentially a closed system: Moose and wolves migrated across ice to the island, and they're basically trapped there now. Small chance of any new migration on or off the island, big enough ecosystem to support both species, but small enough to study easily. The long-term study aspect is also a big feature - this is an aggregate of decades of research, not a short-term snapshot like many other studies. It's pretty unique.
posted by caution live frogs at 5:41 AM on July 5, 2007

Keep in mind there will probably be trophic shifts as the organism ages. For example, a marine invertebrate's eggs or larvae are eaten by different "predators" than the adults.
posted by nekton at 8:22 AM on July 5, 2007

Humans are "predator" animals for many species on the planet, but we are "prey" to many, many micro-predator bacteria, fungi, and protists that kill and (sometimes) eat us. I assume you're dealing only with macro-predators?
posted by meehawl at 11:54 AM on July 5, 2007

Response by poster: meehawl writes "I assume you're dealing only with macro-predators?"

Yeah, and thanks fir clarifying that for me.
posted by orthogonality at 3:46 PM on July 5, 2007

Nervous about piping up here because I don't have any science credentials or education and have an inflated sense of speculation based on personal experience and reading.

Neurochemical research and lifespan in rats.

Your question interests me in regard to pathological narcissists (predators) and the people they vampire for narcissistic supply/attention (prey). It's my anecdotal observation that pathological narcissists live longer lives than those they prey upon.

Among the psychologists I've casually discussed/joked about this with, a few have said that when they heard a person lived to be in their 90's or more, they assumed the person were narcissistic and had seen their narcissistic suppliers into the grave.

As far as I know, in the human brain, the immune system part of the brain is enveloped by the limbic system, seat of emotions. I would speculate that people/animals in emotional/survival distress suffer weakened immune systems and as a result lead shorter lives.

The only scientist I know who has researched emotions and health as interdependent is Candace Pert, in her Molecules of Emotion. If you wrote her, [e-mail], she might know of relevant books or scientists.
posted by nickyskye at 5:10 PM on July 22, 2007