Live Fast and Specialize, Live Slow and Generalize
Pace of life…specialization…climate change…how do these things help us predict extinction?
Often in the course of our days, we ponder the pace of our lives. Thoughts like today is going by so slowly or this past year has been a blur pervade our psyches at a deep level. Our awareness of time is as existentially unclear as it is inconsistent, leaving us wondering where the years went as the present only oozes by. Often, when we’re most engaged with our lives’ activities, we lose complete awareness of time, only punctuating it with passing glances at the clock.
In contrast to this rather blurry conception of the pace of life, biology basically has it nailed down. In the context of generational progression, biologists think about the speed of life histories, punctuating it not with passing glances at a clock or calendar, but with breeding. In nature, where success is measured in how extensively genes are passed onto the next generation, reproduction is key. And depending on what sort of life an organism breeds, it may breed frequently or rarely, producing many offspring or just one.
It is in this variation that life history comes up. We could define an organism’s life history by its breeding biology, and we’d find that this definition is useful. We could look specifically at a bird’s fecundity, an observation that would inform our understanding of how quickly a population would increase. Some species, called k-strategists, reproduce infrequently, spending a long time to raise a small number of offspring. Examples of this would be humans, birds of prey, or elephants. Other species, called r-strategists, reproduce much more frequently, spending almost no time to produce many offspring. Many insects, like mayflies, are examples of this strategy. Interestingly, these two categories carry with them some general traits: k-strategists generally live longer with a low mortality rate in the population, while r-strategists generally die young with a high mortality rate in the population.
Through this dichotomy, we begin to understand what a biologist means when he or she says slow life history. If an organism’s life history is defined by reproduction — which, in the eyes of natural selection, it might as well be — then this dichotomy represents very simply a slow or a fast life history. K-strategists, living a long time and producing few offspring, could be said to live slowly. R-strategists, on the other hand, produce many offspring and die young, and thus could be said to live fast.
Cool. So a slow life history is often a k-strategist, while a fast life history is often an r-strategist. Got it.
Unfortunately, as is often the case, the Platonic ideal of dichotomies, of black-and-white perspective, is often useful only in theory. In reality, life presents us with a wide range of fine variation. Some species show minor tendencies toward an r-strategy, while others tend toward k-strategy. Others still show an amalgam of tendencies from both sides of the dichotomy, blurring the line entirely.
In birds, this line is indeed almost as blurry as our understanding of time’s passage. But while dichotomizing isn’t useful in this case, establishing varying degrees is. Some species may have a very fast life history, while others may live at a more moderate pace. A whole spectrum of “life speeds” could be quantified, and, once quantified, new species could be assigned to a category by degree, e.g. Species A is fast, Species B is Moderate, Species C is Moderately Slow, etc.
In this time of climate change, this life history information becomes of paramount important to understanding how species will cope with change. Species with slow life histories, for example, may not be able to adapt to change as quickly because their generations pass by slowly. Elephants, for example, are declining precipitously in part because they do not reproduce frequently enough to regenerate their population. Similarly, as their generations go by slowly, it takes more time for a species with a slow life history to adapt to change. Why? Because new generations with new genetic variations that may help cope with new challenges come about at a slow rate. Species with fast life histories, on the other hand, can adapt fast. Their generations come by frequently, and given that each new generation is often an overproduction, there is plenty of variation to be had. Species with fast life history can adapt more easily to decadal or even year-to-year changes than can species with slow life histories.
But when we look at birds, this isn’t even close to the number of factors that warrant study.
A bird’s life history alone will tell you very little in a void. In some cases, it may actually be adaptive to live slowly and reproduce rarely. In which such cases, you ask? Take high arctic birds of prey. In some years, food — like lemmings — is abundant, and this prey buffet makes it easy to raise many chicks. But in other years, lemmings may be all but nonexistent, making it more advantageous to breed sparingly, as any offspring may starve. A paucity of resources is only one potential circumstance in which a slow life history is advantageous, but for now, let’s move on.
In birds, when pondering the effects of climate change and development, one other factor stands tall as an excellent informant: specialization.
Climate change and human development into natural areas have many unexpected consequences. One such consequence is the homogenization of biodiversity, where certain species die off in the presence of humans or under the influence of climate change, leaving only a severely limited range of species in their wake. Often, what this comes down to is how specialized certain species are. In other words, how limited the range of conditions a species can cope with is. The broader this range, the more tolerant to change a species will be. But if a species is highly specialized — like, say, a plant that can only exist under the dense shade of an old-growth, temperate forest — any disturbances are going to hurt (badly). If humans enter such an area to fragment and degrade the forest with logging, it becomes likely that the pristine forest interior our hypothetical plant requires won’t exist. And suddenly, that species of plant doesn’t exist in the entire area. This phenomenon explains something peculiar to suburbs. If you’re bird-savvy, you’ll notice that while suburbs often harbor plenty of greenspace, the species range is unvarying. Robins, cardinals, goldfinches, Red-winged Blackbirds, House Sparrows…the list is consistent across much of North America. This is because of the habitat disruption inherent to suburban development; only a limited range of species are tolerant enough to survive in that artificial environment.
Sometimes, this limited range of tolerated conditions doesn’t focus on habitat characteristics. Sometimes, it’s as simple as a temperature range. Mosquitoes in general are known to only tolerate certain temperature ranges throughout the day. In this case, however, climate change is working in their favor: as global average temperatures increase, warmer temperatures expand up altitudes, closing in on mountaintops. It is extremely common for mountaintops (and high altitude areas in general) to harbor unique, isolated floras and faunas. Often, because of mosquitoes’ limited temperature tolerance, these species evolve in the absence of mosquitoes...and more importantly, in the absence of pathogens they carry. As warmer temperatures close in on mountaintops, so too will mosquitoes and the disease they carry.
But that’s a story for another time. For now, know that specialization is a significant ecological factor. When combined with life history information, we attain a predictive ability that is indeed very powerful:
We can predict extinction risk.
A study published early in 2014 tried this power out for size, debuting its utility among bird species in the Czech Republic. And here, we find the fascinating intertwinement of the above-explained factors. Let’s dive in.
In essence, this study was rectifying a long lasting misunderstanding that specialized species will be more direly affected by climate change, and thus will be more at risk of extinction. As we’ve seen, studying only one factor in a void often gets us nowhere, even if it is vaguely heuristic.
It turns out that when we factor in life history — i.e. how quickly or slowly a bird species lives, reproduces, and dies — specialization isn’t always inherently bad in the face of climate change. Similarly, generalism — the opposite of specialization — can be disadvantageous in certain life histories.
Looking first a specialized species, we see that species with a limited tolerance of change are less likely to adapt…unless they have a fast life history. If a species reproduces often and produces many offspring, it can do two things. First, it can adapt more rapidly and more easily to change. Second, it can compensate more easily for population declines.
Generalists, on the other hand, are often given credit for their ecological preparedness for climate change. With their wide range of tolerance, gradual changes in temperature, in weather patterns, in species composition, could even provide advantages. Just look at the classic examples of crows or gulls, whose generalist omnivory allow them to excel in areas of extreme human impact.
So how does life history factor into these considerations? Picture a generalist — say a crow — that can feed on a wide range of foods in a wide range of places. With this wide range of options for nourishment, the crow has an unusual freedom to select those foods that are both accessible and nutritious. They can discriminate the best foods from their many options.
If our hypothetical crow is feeding discriminately, why would they utilize a fast life history and breed indiscriminately? In the words of our muse study,
“[Generalist] species are able to allocate more energy to future reproduction and could wait for optimal environmental conditions before breeding, leading to more favourable population status in the long term.” — Koleček et al. 2014
Seeing as generalists can wait for optimal conditions, it might even be detrimental not to wait, and to breed at the wrong time.
With environmental change becoming the norm in this time of climate change, generalists with a fast life history may be hit as hard as specialists with a slow life history. Regardless of the species, raising offspring is a costly exercise. If a generalist spends all the time and energy needed to breed constantly, it may not live long to produce all the offspring it could produce given its many tolerated resources. Put another way, a long-lived generalist who breeds here and there will ultimately be more successful at passing on its genes than a generalist in a hurry.
So what does this all mean?
As a biology-obsessed mind, I like to think of these things as little secrets that nature is only just now sharing with us. Like those little, trifling secrets shared in high-school lunch-table cliques, the natural world shares with us its wonders, but only when we persistently prod in the right way.
Here, natural selection is revealing the following maxim:
If you’re a specialist in this day and age, live fast and prosper. If you’re a generalist, live slowly and prosper.
Those species that deviate from these restrictions are indeed at a heightened risk of extinction (and there are many). We must remember, however, that all species have their tipping point; this is no reason for celebration that climate change really isn’t that bad for nature. Ecosystems are crumbling around the world. The function of this study is not to show that many species of birds are less vulnerable than we thought. Instead, it aims to inform our understanding of why some species decline faster than others, and ultimately, why some ecosystems crumble faster than others.
It is remarkable indeed that with this conceptual formula — and others like them — we can tease out the reasons for extinction. Even more powerful, it allows us to predict the future and act accordingly, preventing what atrocities human influence may inflict upon the natural world.
So while in an existential sense our lives’ pace may be uncomfortably ambiguous, in nature, this sort of life history is exceedingly important. What may be more important, though, is the understanding that in ecology, one factor is never enough. Life history alone will not predict a species’ extinction risk to climate change, just as level of specialization, climatic niche, or migratory strategy won’t.
It is an amalgam of factors, like these, that one requires in understanding the natural world — and our influence upon it.
So go, fellow questioners, and leave no stone unturned. There is no detail too small, no observation too trite, no question too useless, in the context of understanding our planet. The richer our vocabulary of details, the richer our knowledge of nature becomes.
Happy questioning, friends.
After all, what nobler endeavor is there than to understand this:
Still curious? Read on! Here’s the study that inspired this essay:
Koleček, J., Albrecht, T. and Reif, J. (2014), Predictors of extinction risk of passerine birds in a Central European country. Animal Conservation, 17: 498–506. doi: 10.1111/acv.12117