2.3 Food chains and food webs
This section includes two graphs. Figure 2 has the standard numerical values on its axes, in this case years from 1830 to 1930 on the horizontal axis and number of lynx furs traded, from zero to 60 000, on the vertical axis. Figure 1 does not have any numbers, although similarly the horizontal axis represents time and the vertical axis represents the number of animals; the arrow by the label shows the direction in which the values are increasing. A graph of this type is often referred to as a 'sketch graph', since it is the shape of the graph, in this instance the general changes in the numbers of animals as time passes, that matters. The precise numbers and the exact amount of time varies depending on the species of animal being studied. Sketch graphs are often used in science to show general trends or to illustrate the general situation.
A particular feeding sequence, of the type described in LoM in a real-life situation, comprises a food chain in which particular primary producers and primary and secondary consumers are identified.
Question: Using information from LoM Chapter 5 and the TV programme 'Meat Eaters', draw food chains ending in (a) the stoat and (b) the African hunting dog (see course S182_5). Include the trophic levels and use the terms primary producer, primary consumer and secondary consumer.
Such food chains reveal the relationships between species. As I've mentioned, they not only reflect the movement of the constituents of food (the example I gave earlier was calcium) moving from one organism to another, but there is also flow of energy. You now know that loss of energy is inevitable between levels; for example, for an impala to gain 1 kg in weight it needs to consume 10 kg of bush vegetation and then only 10% of the impala's mass is assimilated by its predator, the African hunting dog.
These food chains suggest, for example, that a large population of rabbits will flourish only if there is good growth of grass in the area around the warren.
The population of rabbits will be cut back by the stoats, in the gruesome manner you saw in the programme 'Meat Eaters' at 102.12, but obviously if the stoats kill all of the rabbits, there will be no food for them. If the rabbit population in an area crashes because of disease, the reduced grazing allows the vegetation to grow taller but the local population of stoats may also crash.
Figure 1 is a simplified view of changes in predator/prey numbers in any one area over a period of time. Early in the period of study, a decline in the number of predators (the red line), perhaps as a consequence of lack of hunting success, allows numbers of prey (blue line) to increase. But prey eventually become sufficiently numerous to support an increase in the population of predators. Their hunting success on such a grand scale may eventually lead to a fall in numbers of prey (the blue line falls after a peak) which, after a lag period, in turn leads to a fall in predator numbers, after which the cyclical pattern may be repeated.
A good example of a fluctuation of this type can be seen in records of the Canadian lynx population. Records of skins of trapped animals (numbers of skins, for fur, are related to population size) sold to the Hudson's Bay Company show that over a period of 100 years or so there were peaks in the lynx population every nine or ten years (see Figure 2). These changes appear to be related to the size of the population of the snowshoe hare, which is a very significant prey item for this carnivore. Number of snowshoe hares is in turn influenced by shortages of grass upon which they feed, perhaps after especially hard winters.
Of course, changes in ecosystems over time normally reflect shifts in a number of dependent factors. Most primary consumers, for example, eat more than one type of plant - the koala is a notable exception. Most carnivores prey on more than one species and, given the likely unreliability of supply of any one prey species suggested in Figure 1, the reason is obvious. Any over-specialised consumer - perhaps a population of stoats feeding exclusively on rabbits - would pay a high price for their narrowness of diet if prey numbers collapsed. So, a population of primary consumers is likely to be part of more than one food chain, such that different chains interconnect, forming a food web. These more varied and complicated relationships suggest that omnivorous feeding - the subject of much of the remainder of this course - is likely to be a widespread habit, evident once we start to draw up food webs.
Figure 3 shows a food web for one part of the Arctic tundra, with primary consumers that range from diminutive lemmings to very sizeable caribou.
Question: LoM Chapter 5 and the programme 'Meat Eaters' describe the feeding habits of two mammals that inhabit the tundra - the grey wolf and the arctic fox. LoM Chapter 6, and especially the programme 'The Opportunists', very vividly illustrate feeding of grizzly bears. How fully are the feeding habits of these three mammals illustrated in Figure 3?
Surprisingly, the arctic fox doesn't feature in Figure 3, though it is a typical inhabitant of the tundra. It has a wide variety of prey, including lemmings and gulls [pp. 128-129], but also eats fruit, along with carrion of marine mammals, as David Attenborough (DA) mentions. If gulls are classed as secondary consumers, as in Figure 3, the arctic fox could be classified as a tertiary consumer, though an extra tier on the diagram might start to over-complicate it. (Incidentally, wolverines, medium-sized carnivores that are members of the weasel family [p. 178], are known to prey on caribou, often as carrion - another link not apparent in Figure 3.) LoM p. 131 describes wolves as preying on North American bison, which are animals of the prairies rather than tundra; those represented in Figure 3 feed on the less-fearsome caribou. But these wolves are also obliged to prey upon smaller mammals when caribou migrate to their summer feeding grounds. In 'The Opportunists' you saw the highly omnivorous grizzly bear feeding on roots, grass, the carcass of a whale, deer, salmon, clams and berries - a much more wide-ranging diet than implied by Figure 3.
What this example illustrates is that no single, simple food web can do justice to the complexities of an ecosystem. Even with a modest number of species 'boxes', a complete representation of feeding habits can swiftly lead to a confusing multitude of criss-crossing lines! Given that many mammals - and especially omnivores - have very wide-ranging diets, food webs in such cases will be far from the clear representations that they purport to be. Feeding habits change with time too - salmon are only periodically available to grizzlies, as very often are caribou to wolves. Feeding relationships displayed for one location of an ecosystem may not hold true for another. Neither do food webs reflect the energy transferred as organisms die and enter another food chain of breakdown and decomposition. The energy transfer at these stages is into scavengers - fly larvae and burying beetles are familiar examples - and into bacteria and fungi. For such reasons, some ecologists are distinctly lukewarm about the benefits of trying to encapsulate complexity in 'simple to read' food webs. Nevertheless, the drawing up of food webs and the tracing of energy flows underpins a good deal of modern ecological research, combining precise measurements with the approximations and simplifications that are inevitably part of any branch of biological science.
You've encountered different types of diagram in Section 2. Inevitably they are simplifications of the real thing - and they leave out details and assumed irrelevancies. Sometimes diagrams try to capture the essential features of three-dimensional 'reality' or perhaps a vertical section of a leaf or the structure of a human eye, as in previous units in this series. Flow charts of the type you drew for yourself earlier express an idea, in the form of a relationship between different components. They encapsulate the key information that might otherwise take many hundreds of words to describe - why not attempt to use one to summarise an idea or a section of text at this point? Other diagrams convey some form of relationship between two different quantitative variables. All such diagrams inevitably simplify and sometimes distort, or represent things out of context. The familiar adage of 'those who simplify, simply lie' adds a useful note of caution, but getting the most out of diagrams and drawing up your own - useful for consolidating your own learning - is an important part of study.