My Track Record

In the course I’m currently teaching, we’ve been reading the paper Bee pollination improves crop quality, shelf life and commercial value by Bjorn K. Klatt et al.

As explained nicely by Erik Stokstad, the paper documents how strawberries benefit from pollination by bees, as opposed to pollination by wind or self-fertilization. It turns out that, on average, bee-pollinated strawberries are larger than others and also have fewer odd shapes, better color, and superior firmness. This detailed look at strawberry quality is a useful extension of past studies showing that insect pollination often boosts crop quantity (i.e., yield).

In making their case for the agricultural importance of bees, Klatt et al. say that bee pollination accounts for at least $1.44 billion of the value of the $2.90 billion strawberry market in the European Union (EU). While I accept the take-home message that bees add a lot of value, I sure wish the authors had explained their calculations better.

The $1.44 billion estimate is the sum of two factors: $1.12 billion in market value of the fresh berries, and another $0.32 billion corresponding to improved shelf life. Let’s consider each of these in turn.

The figure of $1.12 billion is introduced in this section of the Results:

Bee pollination resulted in strawberry fruits with the highest commercial value (figure 1a). On average, bee pollination increased the commercial value per fruit by 38.6% compared with wind pollination and by 54.3% compared with self-pollination. Fruits resulting from wind pollination had a 25.5% higher market value than self-pollinated fruits. Pollination treatments were stronger than differences between varieties and thus had a main effect across all varieties (see table 2 for AICc and likelihood values). Our results suggest that altogether, bee pollination contributed 1.12 billion US$ to a total of 2.90 billion US$ made with commercial selling of 1.5 million tonnes of strawberries in the EU in 2009 [1]—but so far without consideration of the monetary value provided by enhanced shelf life (see below).

Figure 1 indicates that the mean value of 1000 wind-pollinated berries was ~$13.80 and the mean value of 1000 bee-pollinated berries was ~$22.40. The value of the wind-pollinated berries thus represents a ~38.6% DECREASE in value relative to the bee-pollinated berries; alternatively, the value of the bee-pollinated berries is a 62.9% INCREASE over the value of the wind-pollinated ones. A 62.9% increase takes us from $1.78 billion (the hypothetical value of strawberries only pollinated by wind) up to $2.9 billion, giving us the reported $1.12 billion boost from bees. The authors’ mention of a 38.6% increase when they meant a 38.6% decrease is not exactly a big deal, but initially made their math baffling to me and my students.

Perhaps more significantly, the reported market values reflect the classification of strawberries into commercial grades by the first author. Ideally, the first author would have rated the berries while “blinded,” i.e., without knowing which ones came from which treatments (bees, wind, or self). The paper doesn’t mention blinding, so I fear that there was the potential for a pro-bee bias.

Now for the $0.32 billion due to improved shelf life:

Bee pollination strongly impacted the shelf life of strawberries by improving their firmness (figure 2a). The firmness values of each treatment and variety were related to shelf life, measured as the number of days until 50% of fruits had been lost owing to surface and fungal decay (see the electronic supplementary material, S3). Higher firmness resulting from bee pollination potentially elongated the shelf life of strawberry fruits by about 12 h compared with wind pollination, and by more than 26 h compared with self-pollination. After 4 days in storage, only 29.4% of the wind-pollinated fruits and none self-pollinated fruit were still marketable, whereas, at the same time, 40.4% of the bee-pollinated fruits remained in a marketable condition. Thus, bee pollination accounted for a decrease of at least 11.0% in fruit losses during storage. These findings suggest that the value for bee pollination calculated in section 3a(i) has to be increased to accommodate this impact on the shelf life of strawberries. Hence, pollination benefits on the shelf life of strawberries potentially added another 0.32 billion US$ to the commercial value of strawberry pollination.

Here it’s clear that the authors got $0.32 billion by multiplying 11% by $2.9 billion. What’s less clear is the meaning of “storage” (did the unspecified storage conditions simulate those typically used by strawberry farmers/distributors/vendors?) and the reason(s) why a duration of 4 days was used in this calculation (is this a typical time between harvesting and consumers’ purchases?).

Such details aside, here’s a more general question relevant to both components of the $1.44 billion estimate. To what extent are commercial strawberries pollinated by bees in the wild?

The calculations assumes that the study site — a field in Germany — is representative of most or all commercial strawberry farms in Europe. The study site was intentionally set up near well-established bee hives and nests; are most or all European strawberry farms situated similarly? Perhaps the answer is obvious to people with relevant expertise, but the paper doesn’t say. It’s worth noting that if only half of commercial strawberry fields enjoy bee pollination, the estimates of bees’ economic impact would need to be cut in half.

Considering that the paper trumpets a billion-dollar claim in its abstract, more information on the calculations underlying that claim would have been appropriate. At least that’s how I see it — comments from real ecologists (Jeremy?), as well as others, are welcome!