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Correlation is not causality?

Posted by MSHeard on 28 Jul 2015 at 12:42 GMT

We are concerned that this paper is too speculative and does not contain any mechanistic or causal links to explain the results.

The authors state that “Aluminium is the most significant environmental contaminant of recent times”, however, this simple statement ignores the fact that the majority of aluminium in the environment is naturally occurring. Further, even though the investigators have shown a weak, but statistically significant correlation between pupal Al and weight – they provide no mechanistic context for this relationship, i.e. what is the exposure pathway, is it cause-effect or simply a correlation? Indeed, beyond measuring aluminium concentrations, the levels of other trace elements (and pollutants) are not measured in the pupae. This is important because the factors that may cause excess exposure to aluminium (geology, aerial deposition from industrial release, low soil pH) are also likely to result in relevant exposure to other trace elements. Hence bees may be subject to exposure to multiple metals and pollutants and not just aluminium alone. Further, there are also no measures taken from unexposed bee colonies i.e. a control or base line pre-exposure. Since the authors also observe no effect on colony growth or ‘health’, eliciting a link between dementia in humans and performance in bees solely to aluminium exposure is wildly speculative and misleading.

Even though speculative, the authors do show the presence of aluminium in pupae. However, this is unsurprising since this metal has been measured in many species in unpolluted areas. Possible exposure routes to Al include atmospheric deposition/contact or ingestion via pollen, nectar (or water). To be confident of the significance of any correlation to bee health you would have to provide evidence that exposure to Al via these routes is a) possible and b) significantly different across the gradient tested. This has not been done. In terms of Al exposure, the most likely exposure route is via plants, however in this case the most likely driver of variability in plant Al would be underlying geology, with local modification due to soil acidification increasing Al (and other metal) bioavailability.

Matthew Heard, Steve Lofts, Dave Spurgeon, NERC Centre for Ecology & Hydrology

No competing interests declared.

RE: Correlation is not causality?

aluman replied to MSHeard on 28 Jul 2015 at 14:57 GMT

It is quite difficult to understand the point being made by Heard et al. at CEH. I am sure that they know that they do not need to lecture me about the biological availability of aluminium.
Our short paper makes just one simple observation. The aluminium content of bumblebee pupae is very high. These data are not speculative, they are absolute.
These were 'unexposed' bee colonies!
As we pointed out the likely source of the aluminium in the pupae was their food, nectar and pollen. If nectar and pollen in this area of southern England are heavily loaded with aluminium then it would be pertinent to ask why? To my knowledge, and CEH might well have the relevant data for this, the soils in this region are unlikely to have high levels of biologically available 'naturally occurring' aluminium. So, establishing the source of the aluminium to the plants and thereafter to the bee pupae would be an interesting follow-up study. Aluminium-based fungicides and herbicides might be likely candidates?

No competing interests declared.