6. Spatial and temporal monitoring to understand declines
in and threats to insular arthropods
Evidence is accumulating globally that terrestrial insect abundance and
biomass are in decline across multiple regions, habitats and taxa
(Hallmann et al., 2017; Wagner, Grames, Forister, Berenbaum, & Stopak,
2021), although the historical scarcity of highly-standardised long-term
arthropod monitoring programs leaves uncertainty of the nature, scope,
and taxonomic and geographic variation of the problem. The establishment
of such programs has been limited by the time and specialised expertise
required to process samples across whole arthropod communities. It is
difficult to evaluate which of the potential drivers of long-term
declines are most responsible, and thus most important to address,
without extensive data with minimal biases (Cardoso & Leather 2019; van
Klink et al., 2020, 2022). Thus, HTS combined with methods to measure
abundance provide a route to the practical and comparable long-term
monitoring of communities globally.
Islands should be particular priorities for monitoring given their
relative biodiversity value, high human impact, and utility as
harbingers of more general global change (Fernández-Palacios et al.
2021). However, they are underrepresented in global initiatives for
biodiversity monitoring and biodiversity indicator frameworks, prompting
calls for coordinated surveying and monitoring of island biotas (Borges
et al., 2018). Two facets of island biota provide cause for concern in
the context of potential decline in abundance and biomass. First,
general decreases in arthropod abundance are likely to exacerbate
extinction risk at the level of individual species, due to already
geographically limited range sizes (Manes et al., 2021; Veron, Mouchet,
et al., 2019). Second, island biotas are inherently at risk from species
invasion and decreased local abundance of native species could increase
this threat (Bellard et al., 2017; Russell & Kueffer, 2019; Borges et
al., 2020). Understanding island arthropod decline in the (broadly
common) absence of historical data (but see e.g. Colom, Traveset,
Carreras, & Stefanescu, 2021; Theng et al., 2020) can, to some extent,
be addressed by sampling across gradients for suspected drivers of
decline, such as climate (Ferreira et al., 2016) and disturbance
(Cardoso, Rigal, Fattorini, Terzopoulou, & Borges, 2013). Given the
critical focus on abundance and biomass, multiplex barcoding can be
used, and total abundance partitioned to individual species for the
identification of more nuanced abundance changes across species.
Alternatively, PCR free metagenomic approaches allow estimating the
abundance of different arthropod species (Ji et al. 2020), and wocDNA
metabarcoding has been demonstrated to reveal relative abundances of
species, when coupled with specimen counts (Lim et al. 2021). Although
still in early development, CNNs and DL (see above) hold promise for
photographing and archiving samples prior to wocDNA metabarcoding, for
future abundance estimation (Arribas et al., 2022).
The high throughput and efficiency of HTS barcoding approaches represent
a viable long-term solution for monitoring and documenting change within
island arthropod communities, and in the context of an iGON, these can
be integrated within existing frameworks (e.g. Borges et al., 2018).
Suggestions for a coordinated approach to inventory and temporal
monitoring, through spatially extensive inventory with a subset of sites
subject to temporal sampling (Arribas et al., 2021a), can provide needed
baseline data for conservation planning. Range size is frequently used
in conservation planning, within which species with small ranges and
often declining abundances are given higher priority. Indeed, range
restriction and population trends are integral to the International
Union for Conservation of Nature (IUCN) criteria to identify and
classify species threatened with global extinction. While downstream
bottlenecks of the red-listing process are now being addressed by
semi-automated systems (Cazalis et al. 2022), upstream the common data
deficiency for arthropods within islands limits effective red listing by
the IUCN. Strategically designed spatial and temporal HTS barcode
sampling networks can provide species records at scales appropriate for
both IUCN needs, and the needs of local conservation and management
agencies and stakeholders. Such scales go as far as the microhabitat
level, as threat might be influenced by it. As an example, classical
sampling of Madeiran spiders suggests ground associated species are at
greater risk of local extinction than those from canopy microhabitats
(Cardoso, Crespo, Silva, Borges, & Boieiro, 2017; Crespo, Silva,
EnguĂdanos, Cardoso, & Arnedo, 2021). This parallels more general
inferences for greater climate change impacts for forest floor arthropod
species compared to canopy species in Puerto Rico (Lister & Garcia,
2018). More involved implementations of stratigraphically structured
sampling and HTS barcoding with abundance data (i.e. multiplex barcoding
or wocDNA barcoding with artificial intelligence for image recognition)
also has the potential to simultaneously contribute local species
records, basic niche information (stratigraphic distribution), and local
abundance.