Patterns in Biodiversity IV

Community Ecology
I. Introduction
II. Multi-species Interactions within a Trophic Level
III. Multi-species interactions across Trophic Levels
IV. Succession
V. Biodiversity: Patterns and Processes
A. The Species-Area Relationship
B. The Latitudinal trend in Diversity
1. The Pattern
2. Potential Causes
3. Cradle or Museum?
The extent of tropical habitats was much larger in the past. The high diversity in the
tropics, today, may simply be a consequence of these old species area relationships, or
the longer ecological stability in these areas. In essence, the tropics may be diverse
today because they are a ‘museum’ – preserving old species that are hanging on in
these last remnants of their formerly broad habitats. Or, the tropics may still be a
source of diversity – a cradle - producing new species at a faster rate than others. So,
are the tropics a cradle or a museum? Jablonski et al. 2006 examined these
alternatives and a third: the hypothesis that the tropics are a cradle, but that the
diversity they produce doesn’t necessarily stay in the tropics – it moves out to the
temperate zone…perhaps as a consequence of lower competition. This is the “out of
the tropics” model. They analyzed the abundance and current distribution of bivalve
genera first seen in the Pleistocene, Pliocene, and Miocene. In each geologic period,
more new taxa were seen in the tropics than in the temperate zone. In addition, for the tropical taxa originating
during each geologic period, the majority of taxa NOW exhibited a shift to temperate regions. So, the tropics
generated more bivalve taxa during each epoch (it’s a cradle), but the current distribution of those taxa was
temperate (out of the tropics model). This may also explain the earlier inconsistency between the “time since
divergence” of tropical and temperate species. Temperate species may have a more recent ancestor, but it may
have been a tropical ancestor.
C. Geologic Time
1. Patterns in diversity
- The general pattern in taxon diversity is an increase through geologic time. Although
there have been dramatic declines in diversity during periods of mass extinctions, and
although there is probably a strong sampling effect involved (older species are less
likely to have a fossil preserved up to the present)
- for higher taxa, we often see a pattern of “innovation, radiation, and contraction”. A
new group evolves; for fossils, this necessarily means the innovation of a new morphology (as we see nothing
else). We then see the radiation of a groups of species with this new trait. This often correlates with a decrease
in diversity of another (typically more primitive) group. Then, we may see a contraction in the number of taxa
within the group – either as a result of competition within the group or as a function of competition from a more
advanced, recently evolved group.
- radiations can occur for three reasons, all associated with the “colonization of a new adaptive zone”… a new
adaptive zone represents a new way to use a resource, or a way to use a resource not already in use. This can
occur through an evolutionary innovation (change in the population), through the colonization of a new place
that has open niche space (change in location), or through the extinction of dominant competitors (temporal
change in the environment).
2. The importance of ecological interactions to geologic patterns in diversity
- Competition is discussed, above – causing competitive contraction within a group, or the competitive
displacement of one group by another.
- predation has also been important as a selective pressure favoring major new morphological innovations – like
hard body coverings. Prior to the Cambrian, multicellular animals had no hard parts. Worms were prey to
stinging cnidarians (polyps, jellyfish). This predation pressure meant that the evolution of hard parts was strongly
selected for – resulting in the colonization of a new adaptive zone – “predator-free space”. Arthropods and
mollusks radiate dramatically, and the Cambrian explosion was the result – in part due to the
increased likelihood of fossilizing hard parts.
- mutualisms have also had important effects in spurring diversity. The evolution of
eukaryotes by endosymbiosis is one example, the evolution of endosymbiotic fungi in plants is
another, and the evolutiuonary ‘exploitation’ of insects by plants is another… the radiation of
flowing plants correlates with the radiation of the major groups of insect pollinators –
hymenopterans (bees, wasps, ants), dipteral (flies), Lepidoptera (moths and butterflies), and
beetles (coleoptera).
Study Questions:
1. What is the “out-of-the tropics” hypothesis, what data supports it, and how might this explain the evidence
contradictory to the “evolutionary speed” hypothesis?
2. What common pattern do we see in association with the evolution of a novel morphology?
3. What are three reasons for adaptive radiations?
4. Provide examples of how competition, predation, and mutualisms have been important in the evolution of
new groups. Be specific.