Problems

The first and biggest problem has more to do with the characteristics of humans than of predators. Biologists are people, and they like to be sure they're getting interesting and reliable results, preferably in the shortest time possible and without getting wet too often. Field studies on predators, such as weasels, do not meet these requirements, for four reasons.

First, weasels are vastly less abundant than rodents, and their populations are often quite unpredictable. The low density and patchy distribution of weasels mean that a field study on any member of their family is more of a gamble than a project on voles. As graduate students, both of us skated on very thin ice when we began our research. King (1975c) trapped only five common weasels in the first 6 months of twice-daily trap rounds every other week (total 4,032 trap rounds), and none of this substantial effort contributed anything to the final analysis. Powell (1979b) trapped more fishers in his first year, when he had no transmitter collars (the order was delayed 8 months by the manufacturer), than in the following 3 years combined when he had more collars than he needed.

Second, predation is not a fixed process that, like a chemical reaction, reliably reappears whenever and wherever certain conditions are met. The interactions between predators and prey are flexible, and the outcomes variable from year to year and place to place.

Third, one cannot deduce anything about predation from watching the predators on their own: One has to study the prey as well, and this, of course, doubles the work. One must count not only the number of adult prey present, but also the number of young born and when. One must count the numbers added to and subtracted from the prey population by all causes, not only predation. And one must document the local and seasonal variation in all these processes. A proposed study on weasels usually involves too much work for one person, and can be tackled only by a team of at least two: one to study the weasels, the other to study the prey. Additional people who study the habitat and other animals will contribute even more to the study. We were each extraordinarily lucky to work in the same study area as someone else working on important prey species and willing to share data with us (Brander 1971; Flowerdew 1972).

Fourth, all known techniques for counting small mammals, the most important prey of weasels, are imprecise, and errors of estimation will increase at compound interest through the series of calculations needed to estimate predation rates. Moreover, a weasel's estimate of the number of small mammals available will include, for example, the nestlings, transients, and trap-shy individuals that are missed by the human observer, and will exclude those that are unavailable to weasels for some reason but present and counted by the human observer. Finally, all weasels know who their neighbors are, and who else might be hunting the same local stock of small mammals.

It is very important to realize the difference between a study of predation and a study of the food habits of predators. Predation is a matter of rates and relative numbers, and its results can be understood only in terms of whole populations, not only of the predator and of its prey, but also of the predator's competitors and enemies, and of its response to changing conditions. A record ofa weasel killing a blackbird is a valid observation of the behavior of that weasel. A list of the prey identified in the stomachs of 1,000 weasels can be a valid estimate of the food habits of the population from which those 1,000 weasels came. But unless the densities of weasels and prey available in each case are known, plus the relative importance of other predators and prey in the same area, neither is a study of predation.

The difference can be illustrated in terms of a familiar analogy: shopping. Clearly, the stock of a certain item of goods in a shop depends not only on the number removed by shoppers but also on its price and replacement rate and on a host of other interactions involving the whole local shopping center. Table 7.1 summarizes some of these interactions, and introduces some useful shorthand terms that cannot be avoided in any discussion of predation.

Table 7.1 The Ecology of Predation Explained in Terms of an Analogy with Shopping

Concept

Predation

Shopping

1.

The prey

Voles

Morningmunch breakfast cereal

2.

The predators

Weasels

Shoppers

3.

The locality

An ecological community

A town

4.

The habitat

A particular field

A particular shop

5.

Prey spectrum

Total local fauna

Total stock of shop

6.

Prey available

Subset of (5) within killing range

Subset of (5) affordable

7.

Prey selected today

Subset of (6) according to the opportunities and needs of the day

8.

Searching time

Time to locate vole

Time to find right shop and right

shelf

9.

Pursuit time and

Ability to catch and kill vole

Ability to find item and pay

killing power

10.

Prey replacement

Reproduction rate of voles plus

Manufacturer's production rate

rate

recruitment rate of population

plus buying rate of shop

11.

Risk factor

Chances of injury to weasel

Chances of overspending

during kill

12.

Penalty for

Death

Loss of face and credit

misjudging (11)

13.

Preference for

Most vulnerable of those

Best quality of those offered

worthwhile

cheap

14.

Functional response

Increase in voles taken per weasel

Increase in items bought per

with increased density/

shopper with increased

availability

opportunity

15.

Numerical response

Increased breeding success of

Increased number of shoppers

weasels with vole density

with increased opportunity

16.

Surplus killing/

Killing above requirements and

Stocking up on specials

caching

storing surplus when

opportunity offers

17.

Alternative prey

Shrews

Grapefruit

18.

Specialist

Weasel willing to search for voles

Shopping around for

rather than eat shrews

Morningmunch

19.

Generalist

Weasel willing to eat shrews

Person of wide tastes

when voles scarce

20.

Impact of predation

Nil

When (14) and (15) much less

Supply exceeds demand

than (10)

Controlling

When (14) and (15) exceed (10)

Demand exceeds supply

The widespread conviction that predation must control prey populations is an old one, common among scientists as well as the general public, but it tends to overlook two important points (White 2001). First, despite appearances, few predators are efficient prey-harvesting machines, not even weasels (Chapter 6). Foraging efficiency is very hard to measure in the wild, but in one study of common weasels hunting rodents in forest at high density, fewer than half of all attempted kills were successful (Jgdrzejwski et al. 1992). Second and consequently, most potential prey animals are not killed by weasels. Far more die for other reasons, especially the very young, without ever meeting a weasel. So, studies of predation impact cannot come to a valid conclusion without working out whether all these other reasons add to or substitute for predation. Not only is that calculation extremely difficult to work out, but also the answer depends on where and when you look, and on the scale of the interactions studied (Powell 2001).

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