Climate change and pollinators

 By Irene Shonle, CSU Extension El Paso County

Plants and pollinators have evolved over the millennia to depend on each other. Pollinators need the nectar and pollen that plants provide, and plants need the pollinators to carry pollen from one plant to another to achieve fertilization and seed set. 

Climate change is having a negative impact on both plants and pollinators as the changing temperatures affect each species differently. In some cases the resulting mismatch in bloom time and pollinator activity can mean that some plants don’t get pollinated, and some bees don’t find the food they need at a critical time.

Plants tend to bloom primarily in response to a combination of increasing temperatures and daylight. This is why we see flowers bloom earlier in years with warm springs, and later in cold springs.  Insects also respond to similar environmental cues, which makes sense, given that mutually dependent species would be expected to be fairly synchronized.

European scientists Holzschuh and Kehrberger found that the European pasque flower (Pulsatilla vulgaris) responds to rising temperatures by flowering earlier each year, whereas one of its major pollinators, a solitary bee species, does not quite keep pace by hatching earlier. 

 

This system may have worked well for ordinary weather vagaries, but the warming of the earth’s climate has caused plant species to bloom an average of a half-day earlier each year. In total, that can result in the growing season of some species now beginning up to a month earlier compared to 45 years ago. Insects aren’t quite keeping pace, although the timing of their emergence is less well understood than with plants, especially for solitary, ground-nesting bees species (as many in Colorado are).  It appears as though insects have a higher threshold temperature for development than those required for plants, so plants are more likely than insects to bloom earlier in response to springtime warming (Forrest and Thomson, Ecological Monographs 2011, Holsuch and Kehrberger, Plos 1 2019).

This has implications for both wild populations of both the plants and pollinators.  If pollination gets missed, early spring ephemeral plants (which are often critical early nectar plants) will reduce in population size. Pollinators that miss the peak bloom of an important plant will be forced to switch to a different species – if another one is even availableavailable (Holzschuh and Kehrberger Plos One 2019).

Wild plums provide early nectar for pollinators

On a home landscape level, we can all help the pollinators by including a variety of flowering plants that bloom from the very earliest days of spring until later in the season when more is in bloom.  Walk around your neighborhood and open space to see what is blooming when, and take notes, especially of plants that you don’t already have in your yard- such as perhaps quince, currants, plums, creeping phlox or violets.  Highly hybridized plants like tulips and daffodils may or may not provide nectar, despite their showy flowers.

Three lobed sumacs (Rhus trilobata) don't have showy flowers, but are great for early pollinators

There are some early plants that aren’t as showy, but still provide nectar and pollen – such as maples, sumacs and even willows.  If an early spring means the pollinators miss one plant, a diversity of plants with slightly different bloom times will mean that the insects can still find food.

We can all do our part to keep pollinator populations as robust as possible – starting with our own backyards!