Introduction
Our planet is experiencing a
massive decline in biodiversity, which is largely due to human
activities, and which could ultimately lead to the sixth extinction of
animal and plant species on the Earth.1, 2 This global
change in the environments can affect ecosystem functioning and lead to
significant disruptions of ecosystems, which may threaten the human
sources of livelihood and the current way of life. Loss of biodiversity
is a global concern and may lead to a variety of possible adverse
consequences for the human population.3 The reasons
underlying such loss of biodiversity are complex and have been suggested
to be largely linked to the consequences of growing urbanisation and
industrialization, climate change, increasing pollution, and increasing
utilization of chemicals, which have impact on the environment and
microorganisms with which humans coevolve.4, 5
Biodiversity was defined by von Hertzen et al. 1 as
“The variability among living organisms from all sources,
including, inter alia, 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 ”. Haahtela6 also suggested that micro-organisms play a key role
in the link between biodiversity-related environmental changes and human
health. Loss of biodiversity and disappearance of natural habitats may
reduce the diversity of environmental microbiota, i.e., the biodiversity
of the outer layer.6 According to Haahtela6, humans are protected by two nested layers of
biodiversity, i.e. microbiota of the outer layer and of the inner layer.
The outer layer is dependent on the environment we live in (including
soil, natural waters, plants, and animals); and the inner layer inhabits
the human body (including gut, skin, and airways) and is dependent on
colonization from the outer layer. Furthermore, the diversity and
composition of human microbiota (i.e., inner layer) can also be
influenced by environmental exposures (i.e., outer
layer).7-9
In 2011, von Hertzen et al. 1 proposed that loss of
biodiversity also leads to immune system dysfunction and increases the
risk of chronic inflammatory diseases, including asthma and allergies,
chronic obstructive pulmonary disease, type 1 diabetes, obesity and
inflammatory bowel diseases, and could therefore have important public
health implications. The biodiversity hypothesis proposed by von Hertzen
et al. 1 is consistent with the observed declining
trends of biodiversity indices, such as Waterbird Population Status
Index (WPSI) and Living Planet Index (LPI), and with increasing trends
in the prevalence of asthma and allergic rhinitis since the
1970’s.5 Rapidly declining biodiversity may be a
contributing factor to another global megatrend, the rapidly increasing
prevalence of allergies and other chronic inflammatory diseases among
urban people.5 Increasing evidence suggests that the
diversity of human microbiota influences the risk of asthma and
allergies.10 Changes in the development of microbiota,
evidenced by low gut and airways microbiota diversity in infancy, has
been associated with the development of atopy and asthma later in
life.10, 11 The biodiversity hypothesis has stimulated
substantial research on the role of biodiversity for the risk of
developing asthma and allergic diseases, but the results have so far
been inconsistent.10, 11 This heterogeneity in results
may be related to different definitions and measures of biodiversity,
timing and duration of exposure, as well as differences in duration of
the follow-ups. Therefore, this systematic review and meta-analysis aims
to summarize the current knowledge on the role of biodiversity in the
development of asthma, wheezing, and allergic sensitization.