Observed Responses: Phenological Changes?

Recent climate changes, with particular large shifts in spring temperatures around temperate regions have resulted in clear observation of phenological responses by a large range of species. Many organisms have responded by advancing the timing of their annual seasonal activities, including the timing of flowering in plants, reproduction by amphibians, the emergence of butterflies and insects, and the timing of when birds nest and migrate.

Phenological Stages of Trees

Warmer spring temperatures since 1970s have resulted in advances of tree phenology by approximately 8 days in the northern hemisphere, causing an earlier peak in the abundance of insects. Due to the earlier availability of food sources, many species of birds have advanced their breeding and egg laying date. Crick et al 1997 examined the dates of 65 bird species’ first eggs laid between 1971 to 1995 and found that there was a significant advance in the egg laying dates of 20 species by an average of 8.8 days.  For species that shifted their nesting dates earlier, they generally stand a better chance in survival as it allows them to secure their food supply earlier than competitors, but also due to the prolonged period before reaching winter season, the survival of newborns are enhanced.

Birds Breeding in the UK

Likewise, another major shift observed in recent years has been the changes in arrival time of migrant bird species to Europe and the British Isles. One particular example is the Swallow, where records suggests their arrival time advances by approximately 1.6 to 1.8 days per every 1 degrees rise in temperature. Similarly, the arrival time of Sand Martins (Riparia riparia) closely correlates with recent rise in temperatures, with a steady earlier trend in arrival observed since 1960s – as shown in the graph below. 

Graph showing mean Sand Martin (Riparia riparia) arrival time at 3 Welsh bird  observatories: Skokholm, Bardsey, Skomer.
Note: The slightly late arrival trend in the 1970s correlates with a period of cooler spring temperature recorded in the 70s

The Sand Martin!

Conversely, species that have not responded in shifting their annual cycle earlier are at a higher risk of extinction, as they become unsynchronized with the phenology of their food sources. Long distance migrant species are in particularly vulnerable in adapting to climate change in this sense.  Some species are hampered by the fact that the timing of their spring migration depends on endogenous rhythms and environmental stimuli not related to climate conditions, like day length. Similarly, sometimes temperatures in breeding and wintering regions are changing at a different pace, which can also prevent bird species from responding sufficiently.

One prime example of a bird species being adversely affected from climate change is the long distance migrant bird, the Pied Flycatcher  (Ficedula hypoleuca). Pied Flycatchers normally spend their winters in dry tropical forests in West Africa, and migrate to European temperate forests during breeding seasons. Records suggests that between 1980-2000, they have significantly advanced their mean egg-laying date by approximately 10 days, but have not made significant changes to their arrival timing on their breeding grounds during spring. 

A proper adaptation is hampered because their decision for spring migration is triggered by day length that is not related to changes in climate conditions and spring temperatures in Europe (their breeding area). Hence, by being unsynchronized and arriving at an inappropriate time to fully exploit their food supply, they will lose out as their competitors arrive at an earlier timing to optimally exploit the food source available. Thus, many have suggested that this factor, as a result of climate change, may have partly led to the decline of the Pied Flycatcher in Europe during the past few decades.

Migration route of Pied Flycatcher (Red Region: Breeding
 Areas, Green Region: Where they spend their winter)

The Pied Flycatcher!

Again, this has shown how there are both winners and losers in response to a warmer climate. So far, we have clear evidence that climate change, more specifically temperature, has a large impact on phenology of species, including their reproduction, distribution, survival and abundance. As such, with the availability of historical data, phonological events can be seen as a climate indicator and an important element in monitoring species responses to climate change at the regional, national and international scale.  Since climate change has also significantly affected rainfall distribution and pattern, in the future, one of the possibilities for further research can be to examine how rainfall might affect the phenology of biodiversity around the world.  

What are your opinions?