Anecdota

Laughter is the Best Medicine

Global forest network cracks the case of tropical biodiversity


So the big question here is why are
there so many species in the tropics? The flipside of that is why are there so few
species relative to the tropics in the temperate zone? So we’re at Washington
University’s Tyson Research Center located about 25 minutes outside of St.
Louis and just on the edge of the Ozark eco-region. Today, we’re going to visit one
of a large number of forest ecology plots that are coordinated through the
Smithsonian Institution in a global network known as the Smithsonian Forest
Global Earth Observatory — Forest Geo for short. Our plot here is 25 hectares. It’s about 47 football fields, where we have every
stem mapped, measured, tagged, and identified — and then our collaborators
have done this at 23 other forest plots around the world. The big question is why
are there so many species in the tropics and fewer species in the temperate zone,
which really give rise to one of the most ubiquitous and awe-inspiring, but also daunting, biological patterns on earth, and that’s the latitudinal
gradient and species diversity. With respect to a particular hypothesis
that’s been around since at least the early 1970s — that there may be
specialized interactions among tree species and their natural enemies like
seed predators, pathogens, herbivores (both above and below ground) that could
explain why any one given species in the forest is disallowed from building up
such high local densities that it essentially competitively excludes other
species and causes the erosion of diversity. A lot of these processes are
thought to be strongest or operate effectively or efficiently at the
earliest life stages within forests — the seed, seedling and sapling stages. The
Smithsonian Forest Geo network really provides a unique opportunity to test
these hypotheses because all of the census protocols — tree census protocols — are standardized across the network. So that’s what we really took advantage
of in our analysis. We were looking for those patterns where the adult trees are
having a negative effect on their offspring. So here’s Carya Tomentosa
and Mockernut Hickory, and you can see there are areas of high adult density so
that the size of the circle is related to the size of the tree; you can see
cluster of adults here and not many young — the young tend to be over here,
away from the adults. So this is one of those species that shows the strong
effect of adults on young, and so it’s not advantageous as a young sapling to
be growing up underneath a large parent individual to the same species, because
you’re going to be attacked by the same enemies that are attacking that parent.
What it allows is space for other species to come in, and so if this
interaction is really strong, you get all these other species coming in and higher
species diversity. The second major question that we were interested in
addressing was does the strength of this process, this negative effect of
adults on their own offspring, is it stronger for common or for rare species
and temperate and tropical ecosystems? And so it really begs this question why.
Why are there more species in the tropics, but then why are there so many
more rare species? How are they able to persist? One of the things that we know
from a variety of studies is that many different ecosystem functions or
services like carbon sequestration from forests, or water purification, or the
effects of plants on maintaining the diversity of animals, can really depend
on the amount of plant species diversity or tree species diversity within a
forest. And so given this link between species diversity in a forest
and a variety of important ecosystem services that humans depend on, any
process that’s involved in maintaining diversity in a forest then immediately
has many important and broad applications for environmental
sustainability.

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