Habitat destruction, pollution, and overharvesting, as well as climate change and invasive species, have led to conspicuous reductions in biological diversity. Globally, increasing numbers of species are under threat, populations of vulnerable taxa are declining, and ecosystem function is changing as a result. Although these large-scale patterns emerge from processes that are based on local community structure, as yet there is no comprehensive analysis of how temporal change in ecological assemblages contributes to this global picture.
An international team of researchers re-examined 100 world-wide monitoring studies and were surprised to discover that, over decades, the number of species in many places has not changed much -- or has increased. But the researchers did discover something changing rapidly: which species were living in the places being studied. Almost 80 percent of the communities the team examined showed substantial changes in species composition, averaging about 10 percent change per decade -- significantly higher than the rate of change predicted by any models. This shows that a rapid global turnover of species is happening, resulting in novel biological communities. One potential driver for this is the intensification of trade and transport, combined with the rapid increase in invasions of exotic taxa.
For this study the colleagues gathered all data sets representing more than 6.1 million species occurrence records from 100 individual time series. The study incorporated data for some 36 000 species including mammals, birds, fish, invertebrates, and plants. The geographical distribution is global, and includes main ecosystems such as marine, freshwater, and terrestrial biomes. The time interval ranges from 1874 to the present, although most data series were obtained for the past four decades.
I am fairly certain that most of the studies included in this meta-analysis were not studies that used DNA-based identification systems (not even the more recent ones) although I would strongly recommend doing that at some point in the future. The last decade has shown how many species there might be out there which we haven't discovered let alone described yet. Any such study should take into account that there is a considerable number of species that goes undetected either because it wasn't considered as different (e.g. cryptic lineages) or too small and therefore not interesting enough to be part of a study. It would be very interesting to start such time series utilizing e.g. metabarcoding and see if the patterns observed the present study remain.
A word of caution - the results of this study are by no means an 'all-clear' signal as biodiversity change may be as large a concern as biodiversity loss. The authors thankfully make this very clear:
Our core result—that assemblages are undergoing biodiversity change but not systematic biodiversity loss—does not negate previous findings that many taxa are at risk, or that key habitats and ecosystems are under grave threat. Neither is it inconsistent with an unfolding mass extinction, which occurs at a global scale and over a much longer temporal scale. The changing composition of communities that we document may be driven by many factors, including ongoing climate change and the expanding distributions of invasive and anthrophilic species. The absence of systematic change in temporal α diversity we report here is not a cause for complacency, but rather highlights the need to address changes in assemblage composition, which have been widespread over at least the past 40 years.