Why are carnivorous plants?

Carnivorous plants have long fascinated scientists, and were described by Charles Darwin in the book Insectivorous plants (Darwin, 1875). Carnivorous plants typically attract, capture and digest animal prey by modified leaves called traps. No carnivorous plant is able to capture prey by its flower. Givnish et al. (1984) proposed that a plant must fulfil two basic requirements to be considered as carnivorous. First, it must be able to absorb nutrients from dead prey, and thereby obtain some increment to fitness in terms of increased growth, pollen production or seed set. Secondly, the plant must have some adaptation or resource allocation whose primary result is the active attraction, capture and/or digestion of prey. The first is needed to differentiate carnivory from defensive adaptation that immobilizes or kills animal enemies without leading to substantial nutrient absorption and thus increased plant survival. The second is required because many plants can passively profit by absorbing some nutrients from dead animals decomposing in the soil or on leaf surfaces. A plant must have at least one adaptation (i.e. active attraction, capture and digestion) in combination with nutrient absorption to be qualified as carnivorous, because many genera of carnivorous plants lack some of these attributes.

A long-standing problem in evolutionary biology, i.e. an explanation for the ecological conditions under which botanical carnivory is likely to evolve repeatedly, was resolved by Givnish et al. (1984). Several comprehensive reviews of the rise of carnivorous plants have been published over the past decade, all focusing on trade-offs among physiological and morphological traits. A new paper in Annals of Botany reviews the cost–benefit model for the evolution of botanical carnivory in view of new data on the molecular biology of trap leaves and highlights the importance of energetic costs of active trapping mechanisms. It also address the similarities between carnivory and plant defence mechanisms and the role of jasmonate signalling in carnivory and extends the intepretation of the cost–benefit model to alternative nutrient sequestration strategies in carnivorous plants.

Andrej Pavlovič and Michaela Saganová. A novel insight into the cost–benefit model for the evolution of botanical carnivory. Annals of Botany 06 May 2015 doi: 10.1093/aob/mcv050
The cost–benefit model for the evolution of botanical carnivory provides a conceptual framework for interpreting a wide range of comparative and experimental studies on carnivorous plants. This model assumes that the modified leaves called traps represent a significant cost for the plant, and this cost is outweighed by the benefits from increased nutrient uptake from prey, in terms of enhancing the rate of photosynthesis per unit leaf mass or area (AN) in the microsites inhabited by carnivorous plants. This review summarizes results from the classical interpretation of the cost–benefit model for evolution of botanical carnivory and highlights the costs and benefits of active trapping mechanisms, including water pumping, electrical signalling and accumulation of jasmonates. Novel alternative sequestration strategies (utilization of leaf litter and faeces) in carnivorous plants are also discussed in the context of the cost–benefit model. Traps of carnivorous plants have lower AN than leaves, and the leaves have higher AN after feeding. Prey digestion, water pumping and electrical signalling represent a significant carbon cost (as an increased rate of respiration, RD) for carnivorous plants. On the other hand, jasmonate accumulation during the digestive period and reprogramming of gene expression from growth and photosynthesis to prey digestion optimizes enzyme production in comparison with constitutive secretion. This inducibility may have evolved as a cost-saving strategy beneficial for carnivorous plants. The similarities between plant defence mechanisms and botanical carnivory are highlighted.