forests also host a huge proportion of the world’s terrestrial arthropod (and thus
insect) diversity, the most diverse taxonomic group among all eukaryotic organisms.
Although estimates are still fraught with uncertainty, recent models predict tropical
arthropod diversity to range between 2.5 and 3.7 million species (medians), of which
approximately 70% remain scientifically undescribed (Hamilton et al. 2010). Among
arthropods, beetles in turn represent the most important taxonomic group (Hamilton
et al. 2010; Stork et al. 2015). Despite uncertainties in arthropod species estimates,
several field studies provided concrete numbers on area-related patterns in species
richness. In tropical rain forests in Panama (San Lorenzo), for example, Basset et al.
(2012) collected 6144 arthropod species from 0.48 ha. The authors extrapolated this
finding to larger areas, and concluded that the whole 6000-ha forest reserve sustains
about 25,000 arthropod species.
As suggested by maps illustrating global biodiversity patterns, there is a strong
latitudinal diversity gradient (Barthlott et al. 2007), with a continuous decline in
species richness across different taxonomical groups from the tropics to the polar
zone. Many angiosperm families, for example, are restricted to the tropics (Sherratt
and Wilkinson 2009), a pattern that also applies to other taxonomic groups such as
arthropods, amphibians, or fishes. As a consequence, forest area-related species
richness decreases from tropical to subtropical, to temperate and boreal forest
ecosystems. Observable relationships between the latitudinal decline in plant diversity and (e.g.) insect diversity suggest that the latitudinal gradient in insect species
richness could be a direct function of plant diversity, which increases sevenfold from
temperate to tropical forests (Novotny et al. 2006).
3.2 Drivers of Biodiversity Loss
There are six major drivers of biodiversity loss acting across forest types: deforestation, overexploitation, failure of protection, climate change, pollution, and invasive
species (also cf. MEA 2005 and Mazor et al. 2018). According to IUCN analyses
(2019), the importance of globally acting drivers of biodiversity loss decreases in the
order: ‘Biological resource use’ (particularly wood harvest and logging), ‘Agriculture’, ‘Natural system modifications’, ‘Residential and commercial development’
(particularly development of urban areas), ‘Pollution’, and ‘Climate change’
(Maxwell et al. 2016; IUCN 2019).
Significant losses of forest area have been documented for the tropics in particular. In the years 1990–2005, the net loss of natural tropical forest area was estimated
to 135 million hectares (with the highest total losses (i.e. across biomes) found in
South America (62 million ha) and Africa (61 million ha); Table 1; FAO 2015,
Keenan et al. 2015). In contrast, a net increase of forest area was observed in
temperate forests during this period (41 million ha), mainly attributable to the
establishment of plantations or reforestation projects or the natural expansion of
forest area. The most important driver of global forest diversity loss is deforestation,
mostly attributable to the conversion of forests to agricultural area by slash-and-burn
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A. Fichtner and W. Härdtle
insect) diversity, the most diverse taxonomic group among all eukaryotic organisms.
Although estimates are still fraught with uncertainty, recent models predict tropical
arthropod diversity to range between 2.5 and 3.7 million species (medians), of which
approximately 70% remain scientifically undescribed (Hamilton et al. 2010). Among
arthropods, beetles in turn represent the most important taxonomic group (Hamilton
et al. 2010; Stork et al. 2015). Despite uncertainties in arthropod species estimates,
several field studies provided concrete numbers on area-related patterns in species
richness. In tropical rain forests in Panama (San Lorenzo), for example, Basset et al.
(2012) collected 6144 arthropod species from 0.48 ha. The authors extrapolated this
finding to larger areas, and concluded that the whole 6000-ha forest reserve sustains
about 25,000 arthropod species.
As suggested by maps illustrating global biodiversity patterns, there is a strong
latitudinal diversity gradient (Barthlott et al. 2007), with a continuous decline in
species richness across different taxonomical groups from the tropics to the polar
zone. Many angiosperm families, for example, are restricted to the tropics (Sherratt
and Wilkinson 2009), a pattern that also applies to other taxonomic groups such as
arthropods, amphibians, or fishes. As a consequence, forest area-related species
richness decreases from tropical to subtropical, to temperate and boreal forest
ecosystems. Observable relationships between the latitudinal decline in plant diversity and (e.g.) insect diversity suggest that the latitudinal gradient in insect species
richness could be a direct function of plant diversity, which increases sevenfold from
temperate to tropical forests (Novotny et al. 2006).
3.2 Drivers of Biodiversity Loss
There are six major drivers of biodiversity loss acting across forest types: deforestation, overexploitation, failure of protection, climate change, pollution, and invasive
species (also cf. MEA 2005 and Mazor et al. 2018). According to IUCN analyses
(2019), the importance of globally acting drivers of biodiversity loss decreases in the
order: ‘Biological resource use’ (particularly wood harvest and logging), ‘Agriculture’, ‘Natural system modifications’, ‘Residential and commercial development’
(particularly development of urban areas), ‘Pollution’, and ‘Climate change’
(Maxwell et al. 2016; IUCN 2019).
Significant losses of forest area have been documented for the tropics in particular. In the years 1990–2005, the net loss of natural tropical forest area was estimated
to 135 million hectares (with the highest total losses (i.e. across biomes) found in
South America (62 million ha) and Africa (61 million ha); Table 1; FAO 2015,
Keenan et al. 2015). In contrast, a net increase of forest area was observed in
temperate forests during this period (41 million ha), mainly attributable to the
establishment of plantations or reforestation projects or the natural expansion of
forest area. The most important driver of global forest diversity loss is deforestation,
mostly attributable to the conversion of forests to agricultural area by slash-and-burn
388
A. Fichtner and W. Härdtle