Are insects and other invertebrates in decline in Australia?

As we reflect on the 19th and 20th centuries as the ages of scientific exploration and discovery, the 21st century will probably go down in history as the age of extinction with the realisation that the sixth mass extinction event, and probably a new geological Epoch – the Anthropocene, is well and truly upon us. During the past two centuries, considerable effort was devoted to the discovery of plant and animal species (Taxonomy Decadal Plan Working Group 2018), from field collection of samples, curation and identification of material to taxonomic description and documentation (Bebber et al. 2007; Bebber et al. 2010), as well as quantifying global species richness by estimating the overall number of species (e.g. Erwin 1982; May 1988; Joppa et al. 2011; Pimm et al. 2014; Stork et al. 2015). However, while we may never know the exact number of species, there is now a concerted effort to catalogue extinctions and quantify the rate and extent of loss of species in the recent past (~200 years), present and future (e.g. Pimm and Raven 2000; Thomas et al. 2004; Dunn 2005; Costello et al. 2013; Pimm et al. 2014; Régnier et al. 2015; Woinarski et al. 2015; Geyle et al. 2018; J. Woinarski et al. unpublished data). Although extinction is a natural process in the evolution of life on Earth, the current global rate of extinction is estimated to be several orders of magnitude higher than the normal background rate (Pimm et al. 2014; Régnier et al. 2015). It is against this background of extinction that the seminal review paper by Sánchez-Bayo and Wyckhuysb (2019) is most timely. These authors analysed 73 long-term studies, mostly in Europe and North America, documenting changes in insect richness and/or abundance in both terrestrial and aquatic ecosystems in recent decades and suggested that insects are threatened worldwide (but see Simmons et al. 2019). Sánchez-Bayo and Wyckhuysb (2019) estimated that the proportion of insect species in decline (41%) to be twice as high as that of vertebrates and that about a third (31%) of all insects in the countries monitored are threatened with extinction. Their findings parallel similar trends reporting catastrophic declines in insect biomass in Germany (Hallmann et al. 2017) and Puerto Rico (Lister and Garcia 2018). It is not yet certain if declines in insect biodiversity are truly global, but the reports from the Northern Hemisphere (Hallmann et al. 2017; Vogel 2017; Lister and Garcia 2018; Sánchez-Bayo and Wyckhuysb 2019) are cause for concern given the widespread nature of the four key threatening processes identified by Sánchez-Bayo and Wyckhuysb (2019), namely, (1) habitat loss and fragmentation (conversion of natural habitats for agriculture, urbanisation and industrialisation); (2) pollution from synthetic pesticides (insecticides, herbicides and fungicides) and fertilisers as a result of intensive agriculture; (3) invasive species (feral animals, tramp ants and weeds) and pathogens; and (4) global climate change. Thus, from an Australian and, more broadly, Southern Hemisphere perspective, there are three key questions that need to be addressed: (1) are Australian insects and other invertebrates in rapid decline and to what extent are they threatened with extinction? (2) If they are in decline, are the drivers or key threatening processes similar to those in the Northern Hemisphere? And (3) given those threats, what can be done to rectify the situation and reverse the decline? To address the first question, there is a critical need for evidence on the extent of change of insect biodiversity, especially decline and extinction, in Australia. That evidence is best obtained by analysing datasets acquired through rigorous monitoring programs based on standardised replicated sampling, which is the standard scientific method to determine whether the observed differences are significantly different to reject the null hypothesis (H0: no change) and accept the alternative hypotheses (H1: an increase, or H2: a decrease). Monitoring is the process of measuring temporal changes in components of biodiversity, such as species abundance, species richness and species composition, or changes in biological attributes of species such as distribution (geographic range size/boundaries), phenology (adult flight period), biomass or other ‘endangered phenomena’ (i.e. a spectacular aspect of the life history involving a large number of individuals that is threatened with demise; Brower and Malcolm 1991). Importantly, monitoring is used to gauge how a site or locationmight be significantly changing over time from a predetermined standard (baseline) and provides the early warning system for detrimental environmental change (Stork et al. 1996; Braby and Williams 2016). However, unlike several long-term projects in the Northern Hemisphere initiated in the 1970s, such as the ButterflyMonitoring Scheme in Europe (van Swaay et al. 2006; van Swaay et al. 2008; van Swaay et al. 2015) and similar monitoring studies undertaken on butterflies in California, USA (Forister et al. 2010; Forister et al. 2011; Forister et al. 2016), there are few monitoring programs for terrestrial invertebrates in Australia (Braby andWilliams 2016; Rix et al. 2019). This contrasts markedly with aquatic invertebrates for which a national program has long been established to assess river health (Yen and Butcher 1997). Poor taxonomic resolution and general lack of data for insects and other invertebrates are also major impediments – the taxonomy and spatial distribution of most species in Australia are simply too poorly known to draw inferences on the extent of change. Another problem concerns application of the IUCNRed List Criteria to assess changes in conservation status (e.g. fromNear Threatened to Threatened or from Threatened to Extinct). Conservation effort is largely directed at listed threatened species, but the information available for putatively declining insect species is often too fragmentary to meet the evidentiary standards required for listing such species as threatened