| BIOMETRICS & PARTRIDGES - SUSSEX STUDY – LONG-TERM INSECT TRENDS KEY FINDINGS
Overall invertebrate numbers (excluding micro-arthropods such as mites and springtails) declined by 67% across the Sussex Study area from 1970 to 2015. The numbers of pollinators in our samples has increased slightly (by 6%) over the 46 years, while agents of biocon- trol (predators and parasi- toids) declined by over 60%. Chick-food invertebrates for farmland birds have declined, with grey partridge chick-food index down by 45% since the beginning of the Sussex Study. All chick-food indices signifi- cantly declined from 2010 to 2015, indicating the on-going need for management to address these declines. Management undertaken to conserve grey partridges on part of the Sussex Study area resulted in significant increases in plant bugs (44%) and leaf beetle and weevil numbers (81%), with the change in numbers of sawfly larvae and caterpillars of butterflies and moths (25%) higher than that on the remainder of the Sussex Study.
Julie Ewald Dick Potts Ryan Burrell Steve Moreby Nicholas Aebischer
Conservation headlands in the managed areas provide chick-food invertebrates such as sawflies. © Peter Thompson/Tom Birkett/GWCT
Two measures of biocontrol can also be calculated from the Sussex Study invertebrate samples, predatory invertebrates (including spiders, harvestmen, earwigs, lacewings, ground beetles, some rove beetles, soldier beetles, ladybird beetles and predatory flies) and parasitoids (insects, including several families of parasitoid wasps, big-headed and tachinid flies which live in a host insect and ultimately kill the host). Despite an increase in biocontrol abundance in the 1990s, both measures (predators and parasitoids) have shown a decline of over 60% since the beginning of the study (see Table 1). Predatory invertebrates recorded in our suction samples declined by over half (56%) in the 1970s alone and have continued to decline throughout this century. Parasitoids have shown similar trends with an additional decline in the 1980s. The changes in five chick-food indices, measuring food resources for grey
partridge, corn bunting, skylark and yellowhammer chicks, as well as the generic chick food index (see Review of 2010), show a similar pattern of overall decline (see Table 1). Declines in these indices were highest in the 1970s, from -30% to -50%. There was no clear pattern from 1980 to 2000 although there was some recovery and stabilisa- tion through these three decades, particularly in the 1990s and 2000s. All indices show significant declines of between -10% and -27% from 2010 to 2015. This underlines the need for more widespread management directed at increasing chick-food invertebrate resources across the whole of the Sussex Study and beyond. Recent management for grey partridge conservation on one farm on the Sussex Study has shown amazing success at turning around the fortunes of grey partridges (see Review of 2014). The management includes increased habitat provision through conservation headlands, beetle banks and undersown spring cereals, as well as estab- lishment of over-winter habitat, feeding and legal predator control. We compared the numbers of the six groups of invertebrates that make up many of the chick-food indices on the area managed for partridges to the remainder of the Sussex Study area. We split the 46 years into pre-management (1970 to 2002) and post-management (2003-2015) (see Figure 2). The long-term declines in the number of chick-food invertebrates from 1970 to
2002 occurred on both the area that went on to be managed and the remainder of the Sussex Study. Aphids, plant bugs, ground and click beetles and spiders and harvest- men declined on both areas, caterpillars declined on the area that went on to be managed and only leaf beetles and weevil abundance did not significantly change. After
36 | GAME & WILDLIFE REVIEW 2016
www.gwct.org.uk
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