What comes to your mind when you hear the word “extinction?” Perhaps you imagine a
critically endangered Giant Panda gnawing on bamboo or a tiger crouching in the brush
amidst the seemly endless degradation of their habitats. Or perhaps you imagine herds
of dinosaurs and other creatures that we only know from fossils that are millions
of years old. Odds are pretty good that an image of a plant wasn't the first (or the
second, third, or fourth) thing to pop into your head. And yet, there are about as
many plant species as animals species that are considered to be critically endangered
by extinction (and there would probably be many more if the data were available).
And what about the Chicxulub impact, the massive asteroid that collided with the Earth
about 66 million years ago and almost certainly instigated the mass extinction that
drove the non-avian dinosaurs and about 75% of the species on the planet extinct?
Well, it turns out that this impact, at the boundary between the Cretaceous (K) and
Paleogene (Pg) periods, has been implicated in the extinction of approximately half
of all plant species that existed in North America at the time.
There are two roads to extinction. First, a species can go extinct simply because
of bad “luck.” Perhaps a species experiences a few years of poor reproduction and
is then subject to a freak storm that causes high mortality of the few individuals
left. As an analogy, think of gambling at the slot machines, which involves no skill
or predictability in winning or losing. You walk to a machine with a handful of coins
and, more often than not, you walk away a few minutes later empty-handed. The second
road to extinction happens when a species' traits are poorly matched to its environment
and that species' ability to replace itself through reproduction is outpaced by its
mortality rate. A sudden shift in climate, for example, can create a hostile environment
for species that were once well suited to a particular location, and they must either
migrate towards more favorable conditions or face extinction. To go back to the gambling
analogy, a poker player whose skills are poorly matched to others at the table will
rapidly go broke despite the random likelihood of being dealt a good hand on occasion.
So, what happened after the Chicxulub impact on what is now the Yucatán Peninsula
66 million years ago? Were the few winners that survived the mass extinction event
just lucky? Or did they have traits that somehow made them better suited to survive
the decades-long impact winter that followed? And if the latter, which traits were
favored or disfavored? Ecologists studying contemporary extinctions have been asking
these sorts of questions for decades and have devised a number of statistical tools
to disentangle the two roads towards extinction. However, similar approaches have
not been so forthcoming in paleoecology, in which data are often incomplete. In this
issue of PLOS Biology, Blonder and colleagues overcome this barrier by combining modern
ecological approaches with data from a diverse and exceptionally well-preserved set
of fossilized plant leaves from North Dakota (United States) that spanned a 2.2 million-year
period bracketing the K–Pg mass extinction event (Figure 1).
10.1371/journal.pbio.1001948.g001
Figure 1
Fossil leaves record information about plant ecological strategies.
Seen here is a Late Cretaceous specimen from the Hell Creek Formation, morphotype
HC62, taxon “Rhamnus” cleburni. Specimens are housed at the Denver Museum of Nature
and Science in Denver, Colorado. Image credit: Benjamin Blonder.
The primary goal of Blonder and colleagues' work was to determine whether plant species
that went extinct during the K–Pg mass extinction event were non-random with respect
to their traits relative to those that persisted. Taking their cue from studies on
extant plant species and their distributions, Blonder and colleagues measured two
key plant leaf traits—leaf mass per area (LMA, indicating the amount of carbon invested
per leaf area) and leaf minor vein density (VD, indicating the ability to move carbon
and water in and out of leaves)—on hundreds of fossilized plant leaves. These two
traits provide insights into where a given plant species is along the “leaf economics
spectrum.” On the one hand, leaves can be “fast return,” indicated by relatively low
LMA and high VD; these leaves are relatively cheap to make and can allow plants to
take up resources rapidly when conditions are favorable (e.g., warmer and wetter)
but are lost (along with their energetic maintenance costs) when conditions are less
favorable. This strategy is exemplified by many deciduous species that acquire resources
in variable environments. On the other hand, they can be “slow return,” indicated
by higher LMA and lower VD; these are more costly to make and are longer lived. Species
on this end of the spectrum tend to live in less variable environments and are often
evergreen.
Blonder and colleagues estimated the VD and LMA of leaf fossil assemblages starting
around 1.4 million years before the K–Pg boundary and going up to 0.8 million years
after it. They found a more than 10% decrease in the average LMA and a nearly 70%
decrease in its variance across the K–Pg boundary, mostly because species with particularly
high LMA that were abundant in the Cretaceous largely disappeared in the Paleogene.
On the other hand, they found a nearly 30% increase in the VD of the plant assemblage
across the boundary (but no change in variance), and this occurred primarily because
of a loss of species with low VD that lived during the Cretaceous but went extinct
during the K–Pg event. Both of these results are consistent with a hypothesis that
the lower light levels and high climatic variability that resulted from the impact
winter after the bolide impact resulted in a directional shift towards species with
fast strategies (low LMA, high VD) and away from species with slower strategies (i.e.,
angiosperms that are evergreen, such as holly and rhododendrons).
Importantly, Blonder and colleagues were cognizant of a number of potential sampling
biases that might have influenced their results and found that most of their results
were robust to these biases. The one bias that might have influenced their results,
however, was the preservation sites themselves. Cretaceous sites tended to come from
floodplains, whereas Paleogene sites were more likely to come from channels of flowing
water. When they corrected for this potential bias by comparing traits from within
each type of site, the VD results remained significant, but the LMA results were no
longer so because of low sample sizes. In the end, the results showed that changes
in VD were quite strong and those in LMA moderate, showing a clear shift in plant
functional traits indicating that the K–Pg mass extinction was strongly selective,
rather than random, in determining its victims.
The results from Blonder and colleagues indicating that species on the slow-return
side of the leaf economics spectrum disproportionately succumbed to extinction also
suggest some tantalizing, albeit speculative, insights into how the ecosystems in
which these species were embedded may have changed as a result of the mass extinction
event. The shift towards plant communities with a higher proportion of fast-return
species could have led to higher rates of ecosystem functioning (e.g., productivity,
decomposition rates, and water cycling), and this could have fed back to influence
a number of local and global processes. Regardless, by extending concepts and tools
from modern ecological study into the deep past, the study by Blonder and colleagues
provides a deeper understanding of what determines the winners and losers during a
mass extinction and may even provide insights into the ongoing mass extinction that
appears to be resulting from the dramatic and multifaceted global changes caused by
humans.
Blonder B, Royer DL, Johnson KR, Miller I, Enquist BJ, et al. (2014) Plant Ecological
Strategies Shift Across the Cretaceous–Paleogene Boundary.
doi:10.1371/journal.pbio.1001949