An international team of scientists from Canada, France and the UK have combined their expertise to explore the responses of animal populations to global warming. The results, published in this week’s edition of Science magazine (30th March) are an important breakthrough because they provide a mechanism by which the timing of breeding is linked to the availability of food. Animals which get the timing wrong have to work so hard to gather sufficient food that they must work beyond their sustainable limits - resulting in smaller offspring and in their premature death.
Professor John Speakman, of the University of Aberdeen and the Rowett Research Institute in Aberdeen, who heads the UK group involved in this work said “Now that we know the mechanism that links the responses to the environmental changes, we can start to model the limits at which animals are capable of responding. This will let us know if there is a critical rate of warming beyond which they will be unable to keep up.”
One of the major impacts of global climate change in temperate habitats is that warming results in an advance in the timing of the spring flush in productivity. Many birds and some mammals time their breeding so that it coincides with this spring flush. By advancing the timing of the flush, global warming might cause birds to miss-time their breeding efforts, with potentially disastrous consequences.
The difficulty with studying this phenomenon is that there are so few birds that get the timing wrong, therefore, it is very difficult to study the consequences of mistiming or to establish how the birds manage to get their timing right.
To investigate the impacts of mistiming, the team of scientists studied the work rates and breeding habits of populations of blue tits living on the Mediterranean island of Corsica and in Southern France. In these populations, birds time their breeding to coincide with the leaf flush of the local woodland, dominated by different species of oak. In Southern France, they found two types of woodland adjacent to each other with very different peaks in their spring flush dates. Because there is a substantial transfer of individuals between habitats there is a relatively large population of birds that mistime their breeding – providing a unique opportunity to study this phenomenon in a wild population.
The scientists studied the energy expenditure of the blue tits in relation to the time of their breeding using a method called the doubly-labelled water technique. This revealed that when birds mistimed their breeding, they had to operate at a level around seven to eight times their resting metabolic rates. The greater the extent of the mistiming the greater the relative energy expenditure. In comparison, humans in a typical Western society work only at 1.6 times their resting metabolic rate. Even Tour-de-France cyclists and Everest mountaineers only work at 3 to 4 times their resting rates. Working at 7 to 8 times the resting level is detrimental to birds, and those which bred at the wrong time not only had these very high energy demands, but they produced much smaller offspring which disappeared from the population more rapidly.
These observations provide a critical and quantifiable link between the mistiming of breeding and mortality, which explains how gradual shifts in the timing of the flush linked to global warming are matched by gradual shifts in the timing of breeding. Birds which get the timing wrong are simply obliged to work so hard that they have a much lower probability of surviving, and consequently, they do not pass on the genes which govern the timing of breeding into the next year’s population.
Now that the mechanism underpinning the response of the birds is known and quantified it should be possible to model the impacts of different rates of warming on the future survival prospects of the populations.
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