Natural experiments are great because they tell us what is happening in real life. We aren’t manipulating anything to see what could, should or would have been there. This experiment done by Doak and Morris is a long-term natural experiment that without manipulations tells us what is there, what was there, and what might be there in the future.
It seems that most climate change researchers today agree that as the Earth’s climate warms that species will shift their ranges to compensate for that warming. Typically they will move either towards the poles or to higher elevations. However, as Doak and Morris note in their paper, anywhere from one half to one quarter of species show no net range shift in response to climate change. So, why do some species move to cooler areas and why do some species stay? I doubt the species that stay are investing in A/C and fans to keep them cool...so why is this happening?
According to Doak and Morris, compensatory changes in demographic rates are buffering southern populations of two tundra plants against the effects of climate warming. To confirm these suspicions a field study was conducted over 6 years in which two very different species of tundra plants 1) Moss campion (Silene acaulis) and 2) Apline bistort (Polygonum viviparum) were monitored across their latitudinal ranges and various vital rates were recorded (growth, reproduction and survival). It is ideal to use these tundra plants as indicators of biotic climate change response as they are adapted to life in colder environments. Figure 1: Photos of the study species ->
The image below shows you the range limits for the field sites (the red line below Niwot Ridge is the southern limit). The graphs labelled b through e show you respectively the mean length of snow free period, mean July temperature, mean of the first principal component of snow free period and mean July temperature for each region and the mean total precipitation during the snow-free period for each region.
Figure 2 (below): Map of field sites and climate patterns
The results of this 6 year study are surprising and have important conclusions regarding climate change and range shifts.
-Mean annual survival of one or more size classes is lowest in southernmost populations of BOTH species; all size classes of Moss Campion, especially the smallest size class and especially low survival for the intermediate size classes in Alpine bistort.
-Both species also showed lower probabilities of a propagule recruiting in the southernmost populations.
The above two conditions are what you would expect to see in a species undergoing a northward expansion. However, as seen below they found trends that were the exact opposite of what they expected.
-Growth rate of surviving plants in the southernmost populations were always equal to or higher than the northern populations. Small Moss campion plants were particularly higher and all sizes of Alpine bistort were higher.
-Two different rates of reproduction were measured: fruits per cm2 for Moss campion and probability of inflorescence production for Alpine bistort. These rates were equal to or higher in the southernmost populations for both species.
These estimates were integrated into projection matrices which concluded that the southernmost populations for both species did not have the lowest rates of population growth.
So let’s put some of what these guys found together. How does this all relate back to compensatory changes and how does this allow some species to escape range expansion? Well, since survival increases with size for both species, plants in Southern populations are quickly moved out of the high mortality size classes because of the higher growth rate. In addition, if vital rates are improving with increased temperature the negative effects on other vital rates are probably buffered by these improvements. This results in stable low altitude range limits even with increasing temperatures and would be a great explanation as to who some species escape shifting their ranges.
Ok. It makes sense now, right? But...what contributes to these compensatory patterns in mean vital rates? Well, the main thing is latitudinal differences in climate which becomes evident by comparing the relationship between mean vital rates and yearly climate variables.
What other predictors did the authors find to explain year to year variation in survival, growth and reproduction? Snow free period and mean July temperature- which importantly are both higher in the south. In the south for example, the optimum snow free periods for survival of small and medium moss Campion are higher than the northern sites.
While reading this paper I thought many times about how the magnitude of temperature increase would affect these vital rates, and towards the end of the paper, the authors finally address this. With warming from low to medium temperatures, most vital rates increase. Across moderate temperatures the results are not consistent, rates increase, decrease and remain the same. But from moderate to high temperatures, most rates decrease. This means that with continued warming this compensatory changes thing will NOT continue to work, which is not surprising. The plants can only compensate for so much before they begin to lose the battle.
So, is this beneficial in the end? Is compensating for the affects of warming a good idea to avoid range shits? Not really, no! These plants may think they are clever, but as the authors point out, climate change will eventually win the race. Why? These plants all have tipping points. And once the temperature increases to a point where the plants can no longer compensate, there will be a RAPID decline in the species. Species that do not compensate for climate change and shift their geographical ranges decline, but in a gradual way, which actually may be more advantageous then this compensatory changes stuff. Source: Doak, D.F and W.F. Morris. 2010. Demographic compensation and tipping points in climate-induced range shifts. Nature. 467: 959-962.
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