I have started this topic in an attempt to understand why the great majority of countrymen I know take it as axiomatic that good conservation requires some predator control whereas most urban greens seem to take the opposite view. I have, in fact, discussed this with a few camp owners in Africa and it is their assessment that around 75% of wildlife photo-tourists fall into the latter belief category. I can certainly sympathise - even agree - that in, some parts of Africa, wildlife reserves are sufficiently large to allow nature to sort itself out with minimal human interference. In fact, it is to such areas that many of are drawn for their sense of remoteness as well as for their wildlife. However, as I have expressed elsewhere in these debates, I remain uncertain that, even in such places, elephants can be allowed free rein because of their potential to damage ecosystems (unless one takes an impossibly long term view). The greater human population densities in many parts of Africa demand, in my view, a different and more interventionist approach to wildlife conservation and good examples of this are seen, for example, in South Africa. I am planning a trip next year to learn more about private game ranches there, including examples of those focusing on hunting and on ecotourism. I shall be interested in stocking densities, species mixes and relative economics. If I manage to learn anything that might be useful to readers here, I'll report back. In the meantime, I thought it might be instructive (just as much for me as for other readers) to ruminate upon the opposing views on predator control that are so polarised in England.
I will start the debate by discussing matters in a theoretical way and consider various hypotheses that are used to explain predator/prey relationships. I will then move on to consider detailed numbers that I have calculated (and which are in considerable need of refinement). To do this, I made use of internet-gleaned information on numbers and weights of various predator and prey species followed by calculations based upon my nutritional experience to determine what weight of prey that predators must consume to sustain their numbers. In theory, one can then work out how many prey items (depending upon prey species) are needed per predator/year.
1) Predator numbers are controlled by prey.
This is often thought to be self evident. In fact, it only applies to specialist predators. A prime example often given relates to the lynx/snowshoe hare, the numbers of which cycle reciprocally. Hares have a high reproductive rate and their numbers increase rapidly until they begin to damage the potential of their food supply, at which point numbers crash. At some point following the crash, the lynx population suffers, allowing hare numbers to increase in line with their recovering food resource.
2) Generalist predators and the predator trap.
Many predators are not dependent upon a prime prey target and can take what's available (eg the fox). If the population of a particular prey species crashes for any reason, the predator can readily switch to alternative prey. Due to the failure of the predator population to crash following the crash in the prey species, the population of the latter is held at a low level because numbers cannot re-build in the continuing presence of stable levels of the predator.
It should be noted that under both scenarios 1) and 2) it is not the predators that are initially responsible for the crashes in prey numbers. However, it should equally be appreciated that prey cycles are not necessarily driven by food limitation. Disease may also result in cycling (eg voles and mountain hares). Cycling populations are generally found in prey species with high reproductive potential. A crash of a major prey resource can also push currently available alternative prey into the predator trap through over-exploitation by predators until such time as the predator alternatives are all lowered and the predators themselves become compromised.
3) Intra-guild competition.
Within the guild of predators, one species can compromise another by one or both of two means. Species can deliberately set out to kill other species or otherwise drive them from their territories (eg big cats and canids in Africa). Alternatively, one predator species can capture so much preferred prey of another species that the latter declines in numbers as a result of food resource limitation. It is thought that intra-guild competition occurs between foxes and stoats and, certainly, between foxes and ground nesting raptors. It is more debatable as to whether badger/hedgehog interaction can be as intra-guild competition. Certainly, badgers kill most hedgehogs they encounter, but, though the latter are themselves earthworm and nest predators, it seems more likely that badgers regard them as desirable food items rather than as potential competitors.
4) Mass-dependent predation risk.
This hypothesis posits that predators can impact adversely on prey, not only by killing them directly, but also by influencing adversely their foraging behaviour such that they are more likely to succumb when adverse environmental conditions occur. (as example, see "Mass-dependent predation risk as a mechanism for house sparrow declines?" authored by R. McLeod et al, Biology Letters, 2006.)
5) Density dependent mortality.
This concept proposes that a prey animal has to die for some reason. If predation doesn't get it, starvation, disease or something else will. The prey population as a whole won't be adversely affected because one type of death will compensated for by another. It applies most to species with high reproductive potential (usually prey species) which are liable to run out of food or become diseased if numbers are not kept in check by predation. However, it is possible to over-simplify. Consider, for example, egg predation, using partridges as an example. Partridges are capable of laying, perhaps, 30 eggs sequentially. However, they will generally stop laying and become broody after having produced a clutch of a dozen or so. Thus, if a predator goes off with a few or even the whole of the first clutch, it won't be too damaging to the partridge population. Chick hatching date will be later, but this will not necessarily be a problem, given the vagaries of the English weather. If, however, nest predation occurs well into the incubation period, it is likely that the partridge won't produce a second clutch. Of course, the predator may take the mother, plus all eggs or chicks in which case no compensation at the individual level will be possible. Generally, English ground nesting bird species do not display density dependent mortality because they exist in areas where natural food supply is adversely impacted by man's influence and where they are already probably caught in predation traps. However, voles probably do to the extent that, even with a good local food resource, high densities can lead to deaths through disease.
From this preamble, it ought to be clear that predator/prey ratios are very complex and multifactorial. Without very detailed research, it is impossible for one side or the other of the debate to claim either that predators are or are not not responsible for sustained declines in prey populations. One must remember that correlation does not necessarily prove causation. It was for this reason that I thought to apply my nutritional knowledge to the problem. I quickly determined that the results were full of guesswork. Nevertheless, I thought them worth reporting because, as far as I know, they represent a novel approach and, further, that others may be able to contribute by challenging or refining the numbers.
Introduction and methods:
I have only looked at some mammalian and avian predators (ignoring hedgehogs, rats, corvids, owls and greater spotted woodpeckers, for example. I have considered a range of prey species, but haven't included the full range (pigeons, doves, corvids)
For mammals, I used estimates of population numbers gleaned from a report by the Tracking Mammals Partnership (www.jncc.defra.gov.uk/pdf/pub05_ukmammals_speciesstatusText_final.pdf. Mean body weights for males/females combined were taken from various sources. The report only goes up to 2002. Since then, badger numbers have increased and those of rabbits decreased (but I have ignored this). I think the numbers I used related to adults only and didn't include young of the year. I assume they related to the standing spring population. My conclusions ignore young stock, and the total annual mass of prey numbers arising from multi-clutching and multi-littering.
For birds, I used various sources, but mainly emanating from the BTO where possible. They quote numbers of breeding pairs. I have doubled these and made an allowance for subdults to give an estimated number for each raptor species I have examined. Since I haven't made any allowance for subadult mammals, I can be accused of inconsistency.
In calculating predator appetites, I started with the likely daily dry matter intakes, expressed as percentages of body weights, required to satisfy energy requirements for maintenance, cost of free living and reproduction. I then divided the numbers derived by 0.3 to convert them to fresh matter (assuming prey are 70% water) and then by 0.8 (assuming 20% waste). I multiplied by 365 to give a number that represented the number of times/annum that a predator would consume its own body weight. (These worked out at 33 for cats, foxes and badgers, 45 for small mustelids, 60 for buzzards and 68 for kestrels and sparrow hawks.) The species differences are explicable by the facts that smaller species have higher metabolic rates than larger ones and that birds have higher rates than mammals.
By multiplyng the species population number by its mean body weight and then by the multiplier calculated as described above, I was able the calculate the tonnage of food eaten annually by each of the selected predators. Thus, as an example, 200,000 foxes of mean weight of 7.0 kg, eating dry matter at the rate of 2.5% of their body weight per day (hence 33 times their weight as fresh food/annum) would collectively eat 46200 tonnes of food/annum. In the case of the fox, clearly not all that they eat is prey so I've guessed that it is only 70% and arrived at an annual prey consumption of 32000 tonnes. For badgers (because of earthworms and agricultural crop consumption), I have guessed that only 20% is prey (13200 tonnes). I have combined stoats, weasels and mink as small mustelids and arrived at a total annual prey consumption of 5130 tonnes. Equivalent tonnages for buzzards are 12000, for sparrow hawks 2244 and for kestrels 2516. This gives an annual consumption figure of all prey for the predators considered so far of 67090 tonnes.
By multiplying my prey numbers by their weights, I arrived at the following figures:
Rabbits, 43200 tonnes. Brown hares, 2700 tonnes. Rats, 1110 tonnes. Voles (all species), 1080 tonnes. Mice (all species) 600 tonnes and shrews, 66 tonnes. This totals 48756 tonnes.
I then made a totally wild guess at 100 million small birds of average weight 25 g to give me a further 2500 tonnes of prey. Next, I calculated the weight of pheasant and partridges released, subtracted the 40% likely to have been shot, and came up with a figure of 43000 tonnes. To the initial 48756, I can thus add a further 45500 tonnes to give a total of 94256 tonnes.
Finally, I gave my attention to cats. There are reliable data to suggest that domesticated cats bring back 84 million prey items to their their homes annually. Unfortunately, this doesn't give any information about what is killed and not brought back. However, one can say that the weight of prey items (approx 2100 tonnes and split in number at two thirds mammals, one third birds) brought home, assuming it is eaten, only represents 2.25% of their dietary needs. It is far more worrying to consider feral cats (up to 20% of the total cat population according to WildCRU). I have guesstimated a figure of one million (well below 20%). I have also taken account of the fact, maybe half, mainly in urban and suburban areas, probably receive some or all of their dietary needs from non prey items. Nevertheless, shockingly, this still leaves a tonnage of 74250 prey items consumed by 500000 feral cats. If my figures are close to being correct, cats-mainly feral - are consuming more weight of prey than all other predators combined.
I have probably greatly underestimated the tonnage of smaller prey items that are available to predators because I have worked on a standing population number and not accounted for successive generations over the year. On the other hand, roughly half of my prey weight was made up of rabbits and hares, accessible to fewer, larger predator species which, nevertheless, also take from the pool of smaller prey species. Furthermore, I strongly suspect that the (24 million 2002) population figure I have used for rabbits may be less after the emergence of haemorrhagic viral disease. Possibly ironically, my figures suggest that release of game birds is assisting predators, despite keepers' attempts at control. I have certainly underestimated badger effects because their numbers have doubled since 2002 and I haven't allowed for the fact that subadults should have been added to the adult ones.
I don't know whether anything useful has come out of this exercise. I hope, however, that it will provide the framework for a vigourous debate.