Biodiversity through Time
Intro:
What is Biodiversity?
- Defined as the variation of life found on any place on Earth.
How do we measure biodiversity?
- One way to measure biodiversity is through species richness which refers to
the count of species in a given area.
Biodiversity through Time:
How do we compare levels of diversity today to those we had in the past?
- Most estimates of prior levels of biodiversity come from looking at various
flora and fauna found in fossil records around the world.
What are some of the problems with comparing biodiversity levels found today to those in the
past?
- Hard to estimate
- Which species were present at exactly what time.
- Past taxa that are still found alive today vs. those that have gone extinct
- Understanding of overall diversity of species still living today vs. those that
have gone extinct.
- Pull of the Recent
- Focusing on species that lived for longer periods of time vs. those that went
extinct within a shorter time period.
Calculating Biodiversity Changes:
- The change in (N) the number of species at a given time is equal to the
number of new taxa originating (S) *(N) minus the number of taxa that
become extinct (E) *(N) in that same time period. If the number of
species originating (S) is greater than that becoming extinct (E) than the
overall species richness and therefore levels of biodiversity are growing.
- Level increasing or decreasing?
Limitations to diversity:
- What controls levels of diversity?
- Density dependent factors
- Intermediate amount of diversity.
- Too high or too low
Number of new species originating at a
given time is highest at intermediate
levels of diversity.
The number of new
species emerging is zero
because the ecosystem
cannot support new
species.
Changes Through Time:
Phanerozoic:
Beginning of this time period
- Mostly Marine animals with hard shells or skeletons
- 4000 marine skeletonized families and 20,000 genera
- 542 millions years ago
- Cambrian and Ordovician mark time periods of rapid increase in diversity
- Paleozoic is an era marked by a plateau in levels of diversity
- Steady increase through the Mesozoic and Cenozoic.
- Diversity is considered to have reached its peak in the late Tertiary
The graph below shows how diversity has been increasing throughout time, also
not that the green lines refer to species that are well known while the grey lines
contain all species found in the fossil record.
Also note that there have been 5 mass extinctions recorded during this era, these
are depicted by yellow arrows on the graph and represent times when levels of
diversity decreased.
Rates of Origination and Extinction:
- Through time the rate of origination of species has been greater than the rate
of extinction.
- Five mass extinctions
- Throughout the history of the Phanerozoic the plants and animals with the
highest rates of origination have also been those with the highest rates of
extinction.
- Specialists vs. Generalists
- Small population sizes and small geographic range vs. large population sizes
and large geographic range
- Levels of Extinction
- Red Queen Hypothesis: Species have to constantly evolve just to stay in place.
Origination and Diversification:
-
Species evolve most rapidly when there are new niches or roles available to
them. Ex: Darwin’s finches,
-
Competitive Displacement- when a new group of species causes the extinction
of a prior group of species. Ex: Angiosperms (flowering plants) replacing
Gymnosperms ( pines, cones.)
- Incumbent Replacement- when extinction of the previous taxon opens up a
niche for a second taxon to occupy.
- Key adaptation- an adaptation that enables an organism to occupy a
substantially new niche, often by using a novel resource or habitat.
Future of Biodiversity:
- Human interruption towards future biodiversity
- Environmental change especially global warming
- Human needs vs. Ecosystem sustainability.
Putting this in Perspective: A Look Back at the Burgess Shale:
- According to Gould, the history of life is instances of extinction followed by massive
differentiation within a small selection of surviving organisms; this is quite different than
the accepted theory of steadily increasing complexity and diversity.
 Revisiting the Iconography of the Cone: “many of our pictures are incarnations
of concepts masquerading as neutral descriptions of nature.”
 Reinforced comfortable views of human inevitability and superiority.
 Problems with typical cone:
1.
2.
3.
4.
5.
6.
 Solving the problem?
According to Gould…invert cone – WHY?


Cambrian explosion: occurred just prior to 530 million years ago.
- proof of his theory – dates to the time of the Burgess Shale organisms;
following a burst of an increase in diversity of species, an extinction occurred,
followed by another period of increasing diversity
- “Too many big stones in the barrel” so new phyla was needed, therefore
inducing an increase in disparity
- Niche fulfillment
So what does this have to do with biodiversity??