Biodiversity

Biological diversity is composed of all life forms existing on Earth, including ecosystems, animals, plants, fungi, microorganisms, and genetic diversity. According to the Convention on Biological Diversity, signed in 1992 at the Earth Summit in Rio de Janeiro, biodiversity is ‘the variability among living organisms from all sources, including terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species, and of ecosystems’. Thus, biodiversity includes all the ecological and evolutionary processes occurring at gene, species, ecosystem and landscape level.

It has been estimated that there are approximately 8.7 million species of plants and animals, of which only 1.2 million have been identified and described. This means that millions of organisms and biodiversity are yet to be discovered. 

Some countries such as Brazil, Colombia, Indonesia, China, and Mexico are considered the most biodiverse on the planet. The areas with extremely high levels of biodiversity are called ‘hotspots’. In these areas it is common to find endemic species, which have a restricted distribution to a reduced geographic region, and are not found in any other place on earth.   

Freshwater ecosystems, such as springs and wetlands, represent an important biodiversity reservoir and fulfil key ecological functions to provide society with ecosystem services. Fish are the vertebrate group with, by far, the most estimated number of species; from these around half are freshwater species. Mexico holds in its territory 6% of the world’s firewater fish species, or 60% of North American’s; that would be 506 species belonging to 47 families. The majority of these species are concentrated in Central Mexico.

Despite being among the most important biodiversity reservoirs, freshwater ecosystems are also between the most altered and invaded in the world. From all the North American fish extinctions of the 20th century two thirds are associated with introduced species. Freshwater ecosystems are vulnerable due to their geographic isolation and high rates of endemism. As it happens in islands, species inhabiting freshwater bodies are unable to exploit resources outside of them.

The establishment and population growth of invasive species alters native communities and ecosystem functions. Invaders are responsible for structural and compositional changes in ecological communities, through predation, competition, disease transmission, habitat degradation, and others. These changes lead to a disruption in the productivity and nutrient availability cycles within the habitat, which influence trophic structure and population dynamics. In some cases, invaders cause a radical alteration in the species composition of the place they had invaded which may ultimately lead to native species loss and wildlife homogenisation. Indeed, invasive species are the main cause of bird species going extinct and the second main cause of the extinctions of fish and mammals.

Due to the constant biodiversity loss associated to anthropogenic activities, it is necessary to employ tools that are able to measure how biodiversity changes in space and time. 

Methods to measure biodiversity

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Ideally, to evaluate biodiversity at a global or regional scale, it would be necessary to measure the abundance of all organisms in space and time, using taxonomy (e.g., the number of species present), functional traits (certain species characteristics that allows them to perform different functions such as nitrogen fixation), and the interactions among species that can affect their function (e.g., predation, parasitism, competition).

The easiest way to measure biodiversity is to count species number or richness in a given area. There are 1) methods based on the quantification of the number of species present, and 2) methods based on community structure. Among the former, it is possible to calculate species richness (S, which is based only on the number of species present, without taking into account their importance value), or indices such as Margalef’s (which is based on the relationship between S and the total number of individuals observed). Among the latter are found the indices of proportional abundance, for example indices of dominance such as Simpson’s and indices of equity such as Shannon-Wiener’s. 

It must be taken into account that it is also important to quantify sampling effort, which can be measured as the number of collected individuals, or number of samples, traps, days, or any other measure of area or time. This is necessary to estimate the minimum effort that has to be invested sampling, according to the species accumulation curves.