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, as the leaf receives a better supply of nitrogen and phosphorus. eventually another 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 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 take 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. such habitats do not exist, so for example, "sphagnum" absorbs the tiny amounts of nitrates and phosphates in rain very efficiently and also forms symbioses with diazotrophic cyanobacteria.