Why flowering seasons matter

The synchrony problem

A specialist bee that emerges when its plant is flowering has a life. One that emerges two weeks early finds nothing. This coupling — phenological synchrony — has been maintained for millions of years because both organisms respond to broadly similar cues, principally temperature and daylength.

Warming is decoupling them, because plants and insects weight those cues differently. Where a plant responds more strongly to temperature and a bee more strongly to daylength, or vice versa, a warm spring moves one and not the other.

Who is at risk

The specialists are the concern. Melitta tricincta provisions its cells with red bartsia pollen and nothing else. Chelostoma campanularum uses campanula pollen alone. Andrena florea uses white bryony. For these bees a two-week shift is not an inconvenience; it is the difference between reproducing and not.

The gap problem, which is separate

Distinct from mismatch is simple discontinuity. A landscape can have abundant forage in May and August and nothing in between, and a colony that runs out in June does not benefit from what arrives in July.

This is what oilseed rape landscapes do. A vast flow in April and May followed by nothing produces colonies that build enormously and then starve, and it is why beekeepers in arable regions feed in June. The same applies to almond orchards in California, which support two million colonies for three weeks a year and almost nothing afterwards.

What is actually being observed

• Spring flowering in Britain has advanced measurably over recent decades, with the effect strongest in early-flowering species.
• Several bumblebee species have shifted uphill and northward at the trailing edge of their ranges.
• The bilberry bumblebee's retreat to higher ground in Britain is one of the more clearly documented examples.
• Some Arctic and montane bumblebees have no available uphill range remaining.

What helps

Continuity is the practical answer at every scale. A garden that flowers from February to November buffers against shifts in any one species' timing. A farm with hedgerows, margins and a mix of crops does the same. A landscape with only one flowering event, however enormous, does not.