Content sample (ENVR 100 p. 2/3)
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Ok, let's move on to the Principles 3 and 4 of biodiversity.
Principle 3: Biodiversity increases resilience
Biodiversity is key to ecosystem function and the production of goods and services. Ecosystem processes depend on the presence and abundance of organisms with particular functional traits. Each species is linked to other species, so when one disappears the other species that depend on it may disappear.
The rivet hypothesis indicates that the more rivets you take out of the plane, the more vulnerable things become.
A rivet is like a screw. Would you fly on a plane with one rivet missing? It may only be the rivet holding the arm of the seat to the chair on its own, and expendable; but, if you start removing lots of rivets, what happens?
It becomes less and less likely that that plane will fly as intended. Likewise with species in an ecosystem.
Because each species is linked to other species, removal of one may change the whole system. Removal of many will certainly change the whole system; and the more species are removed from an ecosystem, the less stable that ecosystem becomes. There is a growing understanding of the importance of functional biodiversity in preventing ecosystems from tipping into undesired, disturbed states.
This means that apparent redundancy is required to maintain an ecosystem’s resilience. Ecosystems that depend on single or a few species for critical functions are vulnerable to disturbance; and they are at a greater risk of tipping into states that are no longer capable of providing the food and habitat for the rest of the species in the system, or for providing humans with the ecosystem goods and services we depend upon.
So this brings us to our biodiversity principle #3. Biodiversity increases resilience. This is true at all levels—from genetic diversity to species diversity to ecosystem diversity.
“All species fit together in an intricate, interdependent, self-sustaining whole. Rips in the biological fabric tend to run. Gaps cause things to fall apart in unexpected ways.” Meadows, 1989
Here is one example of this is how biodiversity increases resilience. The three species shown above are linked together in the rainforest ecosystem.
Remember that changes in biodiversity can reduce ecosystem goods and services and alter ecosystem processes.
Brazil nut trees (above, lower left) grow in primary rainforest in the Amazon where they depend upon a variety of large-bodied bees (see upper right image), the only pollinators capable of pollinating the tree's flower.
Brazil nuts also depend on Agoutis—the big, cute rodent (shown above, top left). Agoutis are the only animals strong enough to break open the hard shells of the brazil nut seed. While agoutis eats some seeds, they also scatter the seeds around the forest by burying caches far away from the parent tree.
Without the agouti for dispersal and the large-bodied bees bee for pollination, the brazil nut trees don’t produce many brazil nuts and cannot spread; hence, they don’t grow well on plantations. In Brazil, it is illegal to cut down brazil nut trees, but when forest is clear-cut for agriculture or development, we get single trees surrounded by acres and acres of non-forested ecosystem. These trees may look alive, individually, but are actually a member of the living dead…not part of a functioning ecosystem, unable to make fruit or reproduce or spread.
What is rare about this situation is we actually know these connections; but remember the rivet hypothesis and remember that we don’t know all the connections in even the most well-studied ecosystem.
Changes in diversity can also directly reduce ecosystem goods and services, which directly affect human welfare. For example, consider the gastric brooding frog, below.
Principle 4: Biodiversity is declining fast
Species extinction has always occurred and would certainly occur without human actions. On average, species are on earth for 1-10 million years, so species extinction is natural. But species are being lost now at an alarming rate. The fossil record shows the background extinction rate for marine life is 0.1 to 1 extinctions-per-million specie, per year and for mammals it is 0.2 to 0.5 extinctions-per-million species, per year.
Today, the rate of extinction of species is estimated to be 100 to 1000 times more than what could be considered natural.
The Rockstrom paper indicated that it is difficult to imagine a planetary boundary for biodiversity loss. Although we know that a rich mix of species is vital for resilient ecosystems, we don’t know how much and what kinds of biodiversity can be lost before this resilience is eroded. Rockstrom indicates that a preliminary estimate for a planetary boundary might be 10 times the background level—and we are currently at 100 to 1000 times the background level.
The Great Extinctions of the Distant Pasts...and our threat today
Great extinctions have determined the composition and diversity of life on earth. About 97% of species that have ever existed are extinct. This image (below) shows Biodiversity over time with the Y axis in thousands of genera. (Genera is the plural of genus.) The X axis shows millions of years ago so the 0 point is now and the 50 indicates 50 million years ago.
Explore the great extinctions on this image.
It is important to understand that the rates of biodiversity loss we are now experiencing is similar to previous great extinctions. We are in the middle of a sixth great extinction event—this time, it isn’t asteroids or volcanoes but human impacts that are responsible.