What is the minimum viable population size for rare plant species?

Population size is a key parameter in conservation biology, and the number of individuals necessary for a population to persist over time is known as the ‘minimum viable population (MVP) size’. Calculating the MVP size for rare plant species can guide the planning of conservation actions and help understanding of whether conservation projects have been successful, but it is extremely difficult to determine what this number actually is. In the words of one conservationist, “how much is enough?”. Traditionally, MVP size has been estimated using computer models but, like weather forecasting, environmental variability over time cannot be predicted with certainty. Here, I suggest that a valuable approach may be to observe the biological effects evident in populations of different sizes in the wild. I show that a range of studies of reproductive effort (fruit and seed production) and genetic variability for rare and endangered plant species indicate that decreasing population size has a gradual effect until a critical point, beyond which any further decline in population size has a drastic impact on plant fitness. These ‘tipping points’ or ‘critical thresholds’ are not exactly the same thing as MVP size, but they do give a realistic indication of when populations are at particularly high risk. Usually, these tipping points occur when population size falls below around 50 to 500 individuals (depending on the species). Estimating tipping points for each species can show which populations are likely to respond positively to conservation actions.