Sunday, 21 December 2014

Extinction of Pollinating Insects and the History of Agricultural Improvement

There were numerous papers of interest in 12 December issue of Science. Amongst them was one on the history of the disappearance of pollinating insects from the British countryside by Jeff Ollerton, Hilary Erenler, Mike Edwards and Robin Crockett of the University of Northampton*. They used 494,117 records held by the Bees, Wasps and Ants Recording Society (BWARS) and defined locally extinct species as those not recorded for at least 20 years, despite extensive efforts by naturalists. Twenty-three bee and flower-visiting wasp species have become extinct in Britain since the mid-19th century including ones that were widespread. 

The analysis shows clearly that the most rapid phase of local extinction began in the 1920s, suggesting that changes agricultural practices that were or had been taking place were responsible. The authors suggest the import of guano in the latter half of the 19th century hit bee and wasp populations in two ways, firstly by increasing grass growth at the expense of wild flowers and, secondly, by obviating the need for strict rotational cropping. Rotational cropping à la ‘Turnip’ Townshend involved a fallow year—good for nectar-rich ‘weeds’—and a legume year—good for long-tongued bees.

From Science 12 December 2014
The proportion of permanent grassland also increased dramatically during the late 19th and early 20th century. Even before the massive effort of the 2nd World War to make Britain self-sufficient in food, lessons from the 1st World War were being applied in the 1920s and 30s to increase productivity on British farms. The authors point out that the Haber process (always called the Haber-Bosch process in my day and a favourite question in chemistry exams) allowed the industrial manufacture of nitrogen fertilisers and led to the further decline of wild flowers.

One statement, I was somewhat surprised about is: …beginning in the 1920s, before the agricultural intensification prompted by the Second World War, often cited as the most important driver of biodiversity loss in Britain. I think those who were aware of the history of agricultural development in Britain would have known that the changes were already well under way in the 1920s and 1930s, not just the 1940s. 

The chapter ‘Grassland Research’** by Frank Raymond (1922-2012) describes the changes that occurred in grassland from the 1800s to the 1970s. The records used by Ollerton and his colleagues begin during the agricultural depression of the last quarter of the 19th century when the productivity of British grasslands deteriorated until the 1914-18 War. So pollinating insects would never have had it so good. A great deal of land was ploughed up for arable farming during the War. After the war, the drive to adapt the system of ley farming (a period of grass-growing in a rotation of crops) to the establishment of more persistent and productive grasses really took off. This drive was led by Sir George Stapledon FRS and his Welsh Plant Breeding Station. Because of the close links maintained with leading farmers the new practices and varieties of grass spread quickly since productivity was so markedly increased. However, this is where there appears to be a dichotomy between grassland productivity and the results of the study on pollinating insects. There was another Depression in the 1930s and the improvements that were available could not be adopted. Stapledon’s survey of 1937/38 showed only a small percentage of grassland was fully productive, huge areas had reverted to scrub and much of the land went untenanted. It could be argued that this should also have been a boom time for pollinating insects. However, according to the results obtained, extinction continued apace. One explanation is that there is a delay between the introduction of new practice and extinction. Indeed, the paper includes this cautionary note:

Our study adds to a debate on the rates and causes of regional and country-wide extinctions of British biodiversity (including invertebrates, vertebrates, and plants) and the limitations imposed by data quality. The available data for bee and flower-visiting wasp extinctions within Britain show that there are deep historical roots to this loss in pollinator diversity that correlate with transformations of land management related to changes in agricultural policy and practice, a conclusion also drawn by these other studies. Agriculture accounts for 70% of British land use, strongly suggesting that this relationship is causal, though the exact drivers of extinctions are clearly multifactorial and complex. For example, for some species there may have been a mismatch in the timing of extinctions in relation to specific agricultural changes (an “extinction debt”) that we cannot currently identify.

Notwithstanding the outstanding questions on timing, and ignoring the major changes in arable farming and the loss of hedgerows, there is no doubt that the system we ended up with in Britain—intensive grassland production, geared to the output of milk and meat and based on rye grass monocultures, must have had an effect on the abundance of pollinating insects and on the survival of species that depend on plants that were eliminated by such systems. I had that same sinking feeling in the pit of the stomach when I looked at our fields at the Hannah as I do when looking at agricultural monocultures throughout the world—oil palm, rubber, tea, coffee, wheat, maize etc. etc. My question to my former and late colleagues, Malcolm Castle and David Reid, was always whether we could incorporate more traditional grassland plants. The answer in the 1980s was ‘No’ as they explained why. Would the answer in 2014 still be ‘No’ and if so, are the present conservation measures on farms sufficient not only to prevent further loss of biodiversity but also to enable the recolonisation by pollinating species that have become locally extinct over the past 100 years or so?

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*Extinctions of aculeate pollinators in Britain and the role of large-scale agricultural changes. 
Ollerton et al. Science 346, 1360 (2014); DOI: 10.1126/science.1257259 

**Raymond, WF. 1981. Grassland Research. In Agricultural Research 1931-1981, ed Cooke GW. pp 311-323. London: Agricultural Research Council.