Keeping track of population changes over time is an important part of rare plant management. They may be marsh species that disappear when wetlands are drained for development, or forest species affected by air pollution, or even tiny alpine flowers that can be harmed by such seemingly innocuous pastimes as hiking and camping. In the USA alone, about one quarter of all plant species are under some degree of threat. In many tropical regions, where land conversion is rapidly increasing, that number may be even higher. Government and private organizations often implement conservation plans to protect threatened species, and they may even undertake efforts to increase their numbers.
Monitoring can have different objectives, depending on the nature of threats to a species.
Observational monitoring can be carried out on populations to determine how their sizes are changing over time. Repeated censuses will reveal whether the population sizes are increasing, decreasing, or stable. If observational monitoring shows that, say, four populations of a rare plant are gradually declining, we can conduct an experiment in which two populations are given some treatment such as controlled burning, while the other two populations remain untreated as controls. Experimental monitoring will reveal if the experimental populations are responding positively to these treatments, either through an increase in numbers or in general overall health of the individuals present on the site. Quasi-experimental monitoring refers to monitoring of a population that is being impacted by some human activity, such as increased use of the site for cattle grazing.