Carnivorous plant: Encyclopedia II - Carnivorous plant - Ecology and modelling of carnivory

Carnivorous plant - Ecology and modelling of carnivory

Carnivorous plants are widespread but rather rare: there are only about 600 species, out of about 250,000 flowering plants. They are almost entirely restricted to habitats such as bogs, where soil nutrients are extremely limiting, but where sunlight and water are readily available. Only under such extreme conditions is carnivory favoured to an extent that makes the adaptations obvious.

The archetypal carnivore, the Venus flytrap, grows under quite extreme environmental conditions. The soils in which it grows have nitrate and calcium levels that are almost too low to measure. This poses an obvious problem since nitrogen is essential for protein synthesis and calcium for cell wall stiffening. Soil phosphate and iron levels are also very low, phosphate being essential for nucleic acid synthesis, and iron for chlorophyll synthesis. The soil is often waterlogged, which favours the production of toxic ions such as ammonium, and its pH is an extremely acidic 4 to 5. Ammonium can be used as a source of nitrogen by plants, but its high toxicity means that concentrations high enough to fertilise are also high enough to cause damage.

However, the habitat is warm, sunny, constantly moist, and the plant experiences relatively little competition from low growing Sphagnum moss. This sort of habitat is typical of many carnivorous plants, and carnivores have a popular reputation as bog plants. However, they are also found in very atypical habitats too. Drosophyllum lusitanicum is found around desert edges and Pinguicula valisneriifolia on limestone (calcium rich) cliffs. Any model that attempts to explain carnivory must explain both why carnivores are so often restricted to wet, sunny sites, and how can they can survive away from them.

In all the studied cases, carnivory allows plants to grow and reproduce using animals as a source of nitrogen, phosphorus and (possibly) potassium, when the usual sources in the soil are absent or limiting. However, there is a spectrum of dependency on animal prey. Pygmy sundews are unable to use nitrate from soil because they lack the necessary enzymes (nitrate reductase in particular), so they are almost entirely dependent on animal prey. Common butterworts (Pinguicula vulgaris) can use inorganic sources of nitrogen better than organic sources, but a mixture of both gives better growth than either alone. European bladderworts seem able to use either source equally well. Animal prey makes up for deficiencies in soil nutrients, but to different extents in different plants.

Plants use their leaves to intercept sunlight. The light energy is used to reduce carbon dioxide from the air with electrons from water, to make sugars (and other biomass), and a waste product, oxygen, in the process of photosynthesis. Leaves also respire, in a very similar way to animals, by burning their biomass to generate chemical energy. This energy is temporarily stored in the form of ATP (adenosine triphosphate), which acts as an energy currency for metabolism in all living things. As a waste product, respiration produces carbon dioxide.

For a plant to grow, it must photosynthesise more than it respires. If a plant respires more than it photosynthesises, then it will eventually burn up all its available biomass, and die. The potential for plant growth is net photosynthesis. Net photosynthesis is the total gross gain of biomass by photosynthesis, minus the biomass burnt up by respiration. Understanding carnivory requires a cost-benefit analysis of these factors.

In carnivorous plants, the leaf is not just used to photosynthesise, but also as a trap. Unfortunately, changing the leaf shape to make it a better trap generally makes it less efficient at photosynthesis. For example, pitchers have to be held upright, so that only their opercula directly intercept light. The plant also has to expend extra energy on non-photosynthetic structures like glands, hairs, glue and digestive enzymes. The energy source for these things is ATP, so the plant has to respire more of its biomass away to keep up with the demand for energy. Hence, a carnivorous plant will have both decreased photosynthesis and increased respiration, making the potential for growth small, and the cost of carnivory high.

The benefits of carnivory are the nitrogen and phosphorus harvested from the prey items. Being carnivorous allows the plant to grow better when the soil contains little nitrate or phosphate. In particular, an increased supply of nitrogen and phosphorus makes photosynthesis more efficient, because photosynthesis depends on the plant being able to synthesise very large amounts of the (nitrogen rich) enzyme Rubisco (ribulose-1,5-bis-phosphate carboxylase/oxygenase), which is the most abundant protein on Earth. The returns of carnivory are therefore more effective photosynthesis.

Clearly some sort of trade-off occurs. It is intuitively clear that the Venus flytrap is more carnivorous than Triphyophyllum peltatum: the former is a full time moving snap-trap, the second is a part time, non-moving flypaper. But is the Venus flytrap more carnivorous than a pitcher plant? The energy 'wasted' by the plant in building and fuelling its trap is a suitable measure of the carnivory of the trap.

Using this measure of investment in carnivory, a model can be proposed. Above is a graph of carbon dioxide uptake (potential for growth) against trap respiration (investment in carnivory) for a leaf in a sunny habitat containing no soil nutrients at all. Respiration is a straight line sloping down under the horizontal axis (respiration produces carbon dioxide). Gross photosynthesis is a curved line above the horizontal axis: as investment increases, so too does the photosynthesis of the trap, because the leaf is receiving a better supply of nitrogen and phosphorus. However, this payoff does not last forever. Eventually some other factor (such as light intensity or carbon dioxide concentration) will become more limiting to photosynthesis than nitrogen or phosphorus supply. As a result, increasing the investment will not make the plant grow any better. The net uptake of carbon dioxide, and therefore the plant's potential for growth, must be positive for the plant to survive. There is a broad span of investment where this is the case, and there is also a non-zero optimum. Plants investing more or less than this optimum will be taking up less carbon dioxide than an optimal plant, and hence growing less well. These plants will be at a selective disadvantage. At zero investment the growth is zero, because a non-carnivorous plant cannot survive in a habitat with absolutely no soil borne nutrients. No real habitat is this stressful, so non-carnivores can survive in the same habitats as carnivores. In particular, Sphagnum is able to absorb the tiny amounts of nitrates and phosphates contained in rain very efficiently, and also forms symbioses with diazotrophic cyanobacteria.

In a habitat with abundant soil nutrients but little light (as shown above), the gross photosynthesis curve will be lower and flatter, because light will be more limiting than nutrients. A plant can grow at zero investment in carnivory; however, this is also the optimum investment for a plant, as any investment in traps reduces net photosynthesis (growth) to less than the net photosynthesis of a plant that obtains its nutrients from soil alone.

Carnivorous plants exist between these two extremes: the less limiting light and water are, and the more limiting soil nutrients are, the higher the optimum investment in carnivory, and hence the more obvious the adaptations will be to the casual observer.

The most obvious evidence for this model is that carnivorous plants tend to grow in habitats where water and light are abundant, and where competition is relatively low: the typical bog. Those that do not tend to be even more fastidious in some other way: Drosophyllum lusitanicum grows where there is little water, but it is even more extreme in its requirement for bright light and low disturbance than most other carnivores. Pinguicula valisneriifolia grows on soils with high levels of calcium, but requires strong illumination and lower competition than many butterworts.

In general, carnivorous plants are poor competitors, because they invest too heavily in structures that have no selective advantage in nutrient-rich habitats. They survive because they can put up with nutrient stresses much higher than non-carnivorous plants can: they succeed because other plants fail. Carnivores are to nutrients what cacti are to water. Carnivory only pays off when the nutrient stress is very high and light is abundant. When these conditions are not met, some plants give up carnivory temporarily. Sarracenia spp. produce flat, non-carnivorous leaves (phyllodes) in winter. Light levels are lower than in summer, so light is more limiting than nutrients, and carnivory does not pay. The lack of insects in winter exacerbates the problem. Damage to growing pitcher leaves will prevent them from forming proper pitchers, and again, the plant produces a phyllode instead: the production of an inefficient, damaged trap is not worth the energy.

Many other carnivores shut down in some season: tuberous sundews die back to tubers in the dry season, bladderworts die back to turions in winter, and non-carnivorous leaves are made by most butterworts and Cephalotus in the less favourable seasons. Part-time carnivory in Triphyophyllum peltatum may be due to an unusually high need for potassium at a certain point in the life cycle, just before flowering.

The more carnivorous a plant is, the more conventional its habitat is likely to be. Venus flytraps live in a very stereotypical, and very specialised habitat, whereas less carnivorous plants (Byblis, Pinguicula) are found in more unusual habitats (i.e. those typical for non-carnivores). Byblis and Drosophyllum both come from relatively arid regions, and are both passive flypapers, which is arguably the lowest maintenance trap form. Venus flytraps filter their prey using the teeth around the trap's edge, so that energy is not wasted on prey items that cost more to digest than they pay back. In any evolutionary situation, being as lazy as possible pays, because energy can be devoted to reproduction, and as far as evolution is concerned, short term benefits in reproduction will always outweigh long-term benefits in anything else.

Carnivory very rarely pays: even "carnivorous plants" avoid it when there is too little light, or an easier source of nutrients, and they use as few carnivorous features as are required at a given time or for a given prey item. There are very few habitats stressful enough to make using biomass to make trigger hairs and enzymes worthwhile. Many plants occasionally benefit from animal protein rotting on their leaves, but carnivory obvious enough for the casual observer to notice is rare.

The surprising lack of carnivorous bromeliads is instructive here: bromeliads seem very well preadapted to carnivory; however, only one or two species can be classified as truly carnivorous. Most bromeliads are epiphytes, and most epiphytes grow in partial shade on tree branches. It is noteworthy that Brocchinia reducta is a ground dweller. By their very shape, bromeliads will benefit from increased prey derived nutrient input. In this sense, most bromeliads are probably carnivorous, but their habitats are too dark for more extreme, recognisable carnivory to evolve.

Adapted from the Wikipedia article "Ecology and modelling of carnivory", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki