Ecology. Michael Begon

Figure 3.27 Enrichment commonly leads to a switch from oxygen to (anaerobic) alternatives as a resource for respiration. (a) The proportion of microbial peptides in communities occupying the pitchers of Sarracenia purpurea that were either controls or enriched, originating from microbes with different respiratory modes. (b) The percentage of taxa that were dormant (metabolically inactive) in control and nitrogen‐enriched plots in saltmarshes over four years. Bold lines are medians, boxes represent 25–75 percentiles and whiskers show ranges, with outliers also shown.

      Source: (a) After Northrop et al. (2017). (b) After Kearns et al. (2016).

Similarly, but on a larger scale, enrichment of salt marshes in Massachusetts, USA, led to soil microbial communities in which a much increased proportion of the taxa was dormant, that is, metabolically inactive (Figure 3.27b), but among those that were active, there was a major shift from aerobic to (in this case) obligatorily anaerobic taxa, many using sulphate or sulphur rather than oxygen as their respiratory resource. Clearly the prevalence of dormancy and the presence of facultative anaerobes mean that communities can switch rapidly from a widespread reliance on oxygen to the use of alternative resources for respiration.

      APPLICATION 3.5 Permafrost, methanogenic anaerobic respiration and global warming

      As the earth warms (see Section 22.2) regions of permafrost near the poles (where the soil remains frozen, year‐round, for at least two consecutive years, see Section 1.5) are thawing. This is leading to a transition in these regions initially to ‘palsa’ habitats – mounds in the landscape supporting lichens and low shrubs – then to partly thawed bogs dominated by mosses (Sphagnum spp.), and then to fully thawed mires dominated by sedges (e.g. Eriophorum spp). This transition itself has potential implications for global warming, since it involves a shift from CO₂‐emitting palsas to mires and fens that take up CO₂ but emit methane, a more potent greenhouse gas. High‐methane emitting fen habitats contribute seven times as much greenhouse impact as palsa, per unit area (McCalley et al., 2014). Our understanding of the roles played by the microbial communities of the soils in these habitats remains poor. But this is likely to be crucial if we wish to predict the trajectory of the positive feedback loop through which warming leads to thawing, leading to methane emission, more warming, more thawing, and so on. (In Section 17.3 we discuss permafrost as an example of an ecosystem that, on thawing, can pass a ‘tipping point’, shifting it from one regime to another.)

Microbes that produce methane as a respiratory by‐product are Archaea, not bacteria. Most are hydrogenotrophic, using hydrogen as an electron acceptor. However, there is another smaller, but important group that are acetoclastic, cleaving acetate into methane and CO₂, and the methane produced by the two groups can be distinguished by characteristic isotopic signatures. Over a natural gradient of thawing in northern Sweden, methane emissions were greater from fully thawed mires than from partly thawed bogs, but were also more dominated by acetoclastic methanogens (Figure 3.28). Crucially, this shifting balance was associated in turn with variation in the ratio of methane‐to‐CO₂ production from anaerobic respiration (much higher from mires than from bogs) with consequences in turn for the models currently being used to predict future climate change, which typically assume the fraction of anaerobically metabolised carbon that becomes methane to be fixed (McCalley et al., 2014). Results like those in Figure 3.28 therefore throw doubt on the validity of this simplifying assumption and press the case for further work on the dynamics of anaerobic resource use in these rapidly changing systems.

Figure 3.28 Methane production increases when permafrost thaws, and its microbial origins change. (a) Emissions of methane (CH₄), over time, at sites in northern Sweden at various stages of thawing from permafrost, as indicated. Bars are SEs. (b) The isotopic signatures of those methane emissions, δ¹³C‐CH₄, measured as the relative difference in the ratio of ¹³C to ¹²C in the methane, compared with an international standard material, expressed as parts per thousand. Bars are SEs. (c) The composition of the microbial community in each case as inferred from those isotopic signatures, subdivided into bacteria and Archaea further subdivided into hydrogenotrophic and acetoclastic methanogens, and ‘others’.

      Source: After McCalley et al. (2014).

3.7 Organisms as food resources

      predators, grazers and parasites

‘True’ predators predictably kill their prey. Examples include a mountain lion consuming a rabbit but also consumers that we may not refer to as predators in everyday speech: a water flea consuming phytoplankton cells, a squirrel eating an acorn (both herbivorous predators), and even a pitcher plant drowning a mosquito. Grazing can also be regarded as a type of predation, but the food (prey) organism is not killed. Only part of the prey is taken, leaving the remainder to live on with the potential to reproduce or regenerate. Also, grazers feed on (or from) many prey during their lifetime. Cattle and sheep are grazers of plants, but blood‐sucking flies, for example, are carnivorous grazers. True predation and grazing are discussed in detail in Chapter 9. Parasitism, too, is a form of predation in which the consumer usually does not kill its food organism, but unlike a grazer, a parasite feeds from only one or a very few host organisms in its lifetime. Chapter 12 is devoted to parasitism.

      specialists and generalists

An important distinction amongst animal consumers is whether they are specialised or generalised in their diet. Generalists (polyphagous species) take a wide variety of prey species, though they very often have clear preferences and a rank order of what they will choose when there are alternatives available. Some specialists consume only particular parts of their prey though they range over a number of species. This is most common among herbivores because, as we saw in Figure 3.26 and shall see again in Figure 3.29, different parts of plants are quite different in their composition. Thus, many birds specialise in eating seeds though they are seldom restricted to a particular species. Other specialists, however, may feed on only a narrow range of