Spring flowers have co-evolved with insect pollinators for a long time. The flowers require pollen delivery to set seed. To entice insects to visit and deliver the pollen, they produce food rewards (nectar and pollen). When conditions are right, the insects get fed and the flowers get pollinated. But spring, with its highly variable weather conditions, is always a chancy time for pollination. If something goes awry for either party in a pollination partnership, it can have serious effects on both partners. So, for example, in some years a late freeze might damage blueberry flowers. This could hinder pollination and deprive pollinating insects of food, leading to poor blueberry crops and — perhaps — low reproductive success for the pollinators.
Plants and pollinators rely on environmental signals to time their life cycles and activity patterns. Some species may use day length as the principal trigger, while others use temperature. Some may use both, or other environmental factors. These signals trigger genetically determined responses, such as emergence by insects and blooming by plants. In some cases, the response to the trigger may include evaluation of body condition and the internal resources available. That is reported to happen for bumblebees and is likely to happen for plants as well.
A purple mountain saxifrage is often pollinated by bees. (Photo by Martina Kallenberger)
Day length is very predictable, temperature much less so. The globally warming climate means that a temperature-responsive species in a pollination partnership may shift its timing, but one that is responsive to day length does not. Similarly, temperature-responsive species may have different settings for their responses and therefore respond differently to climate warming. These changes can have serious consequences. If temperature changes induce the flowers to bloom too early (or too late) for their pollinators, or if the insects come out before (or after) the flowers are available, some species will fail to reproduce and will die out. The co-evolved system will break down.
The risks are especially great for flowers that depend mainly on a single kind of insect, as many of our species do. For instance, bees are probably the main pollinators of blueberry flowers, although various flies may visit too. In the absence of bees, the other visitors are likely to be insufficient to achieve good pollination. It could be a similar story for flowers that depend chiefly on certain beetles or flies.
In our area, the situation is complicated by our notorious local variations in microclimate. Conditions differ with elevation, exposure to insolation, proximity to the ocean, the proximity of garden flowers as alternative foods, and so on. So what might happen in one part of a population might differ from that in another and we might observe differences in pollination success and insect abundance. In a few cases, the local populations might be so separated from each other that they can evolve local adaptations, but in most cases, there is likely to be gene flow among the populations, reducing that possibility. A further complication is that even if pollination and insect diets are good, climate change might alter the conditions for plant growth and insect nesting, which are essential for the interactions to continue.
To make specific predictions about the likely fates of our local species in response to climate warming requires specific information about the environmental triggers to which each species responds. That information is not available now.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
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