Monday set a record low temperature in Tucson for this date: 17 degrees Fahrenheit at the airport. And this just after we learn that most of us younger than 28 have never been alive for a month that was globally colder-than-average! (I say most because this consistent streak of hotter-than-average months started in April 1985, so my first two months were pre-heat wave.) What are we to make of it? What does it mean for future biodiversity?

My housemate's adorable puppy was fascinated with the ice on our run this morning! She is only about eight months old, so this is her first winter, and maybe the coldest she has ever been.
My housemate’s adorable puppy was fascinated with the ice on our run this morning! She is only about eight months old, so this is her first winter, and maybe the coldest she has ever been.

First, we have to unpack variability in time and space versus trends over larger temporal and spatial scales. Variability is pretty important for maintaining biodiversity. This is obvious if you look at large scale variability: Tucson may have had a low of 17 degrees, but  Greer, Arizona, near Sunrise Ski Resort is expected to have a low of 0 degrees Fahrenheit tonight. Pine trees grow thick between the chairlifts there, as opposed to our saguaro forests in Tucson.

On a finer scale, if you garden, you may know that some of your plants prefer one side of your house or another, to get sun or shade or different soil. In time, you might know that some years are colder or drier than others. This affects what wildflowers come up.

And you know that even within a season,  some days you need your puffy jacket and some days just a sweater. Some days are powder at the ski resorts, some days are sunny.

So it may be no surprise that temperatures averaging out over the globe or even continent may be warmer-than-usual, while individual valleys, in which cold air sinks in an inversion, might experience colder-than-average months. And that the cold days within a season come on different dates, so because Tucson often freezes for only a few days every winter, it had never done so on January 14 specifically.

But what does this mean for biodiversity? And how might that diversity be affected if we have a general trend of warming, or a general trend of increased variability? Dr. Peter Adler and his colleagues gave this question a thought back in 2006. First, they summarize a key reason environmental variability (in this case, how the environment changes in time) can help to stabilize biological communities:

” ‘Storage effect’ theory derives the conditions under which climate variability will have stabilizing or destabilizing effects on species coexistence (10). The temporal storage effect… requires that three conditions be met. To satisfy condition 1, species must have long lifespans to buffer their populations against unfavorable years. For condition 2, species must differ in their response to climate variation. These species specific responses to climate cause each species to experience relatively more intraspecific competition during its favorable years and more interspecific competition during its unfavorable years. Condition 3 requires that the effect of competition on each species must be more severe in years favorable for that species than in unfavorable years. When condition 2 is present, intraspecific competition will be more severe than interspecific competition. As a result, climate variability gives species an advantage when they become rare—the signature of stabilizing coexistence mechanisms. The size of this advantage when a species is rare (i.e., the strength of the storage effect) can be quantified by comparing species’ average low-density growth rates in variable vs. constant environments, in the presence of competitors (11, 12).”

(As a side note, I would be very grateful if you would leave a comment oh how clear that explanation of the storage effect is! Our lab constantly struggles with how to clearly explain the concept.)

The authors then present some evidence that interannual climatic variability is important to maintaining the diversity of grasses in a Kansas prairie. As a second side note, I’m not convinced their Figure 2 really demonstrates a condition for the storage effect. Please leave a comment whether you think it does, and why it might not! Don’t be fooled into thinking they correlated strictly individual grass blade growth rates with density, though – there is a statistical model fitted to a spatial data set used to generate these plots, as described in the Methods section at the very end.

So how might a diversity maintenance mechanism like the storage effect change with an increased greenhouse effect?  That depends largely on how the species in the community respond to the increase in temperature, or new species invading, or increase in variability. It also depends on how all the other species they interact with respond to those changes. One thing we know: it’s complicated.

Just this afternoon, Dr. David Inouye gave a talk to University of Arizona’s Ecology and Evolutionary Biology Department on the variability of interacting populations of alpine wildflowers. He also mentioned the many observations on mammals and insects that have changed their phenology (seasonal timing) or the altitude at which they are found. These rich data sets may provide fertile ground for further understanding how variability will change with the increased greenhouse effect, and how that may affect biodiversity.

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