biodiversity

It is not unusual today to see news articles as this: ....."A WILDERNESS ECOSYSTEM IN COLLAPSE": Like a "fragile silken web" Alaska's Aleutian Islands ecosystem is quickly unraveling says the L.A. Times . Most visible at the top of the food chain, "water once brimming with seals, otters and king crab" are replaced by other species, like shark, pollock and urchins that temporarily fill the void while the "vast subarctic ecosystem is collapsing." Scientists say that "ecological shifts as sudden and sweeping" as those in the Aleutians "usually can come only from human interference," with global warming, overfishing, and pollution thought to be primary factors.

What makes a community such as it is? part of the answer lies in its' diversity.....

Patterns of species diversity : Why are some ecosystems so rich and others so poor?

I. Many different theories abound which attempt to explain the biodiversity or species richness of any given community:

Group 1. those involving physical geography : with all of these a pattern of increasing biodiversity exists associated with an environmental gradient but not necessarily explained by it directly.....

1. altitude: the higher the altitude of the given land mass the greater the diversity. It may be a case that less land surface exists the higher up the mountain you go, it may be that these communities are more isolated, so species couldn't migrate there to establish themselves and have formed distinct communities along the mountain height, it may be higher UV radiation leads to higher mutation load.

2. latitude: the closer to the equator the greater the diversity. For example tree species:

TRF = 40-110 species /ha.
Deciduous forests of NA = 10-30 species /ha

Boreal forests in Canada = 1-5 species / ha

3. depth; the shallower the depth the greater the diversity. In oceans, the shallowest waters , i.e. coral reefs have the highest diversity. Light can only penetrate 30 meters down --> less photosynthesis --> less oxygen release. less food --> less animals.

Group 2. those indirectly related to geography:
4. productivity:

With respect to latitude, the sun's rays hit the earth most directly at the equator and most tangentially at the poles. The greater interception of light allows greater photosynthesis, thus higher productivity to support a broader trophic base.
With respect to altitude, the temperature drops, moisture drops decreasing the ability again of the primary producers to generate enough calories to support many species.

With respect to the oceans and deeper fresh water bodies, water intercepts wavelengths exponentially. Thus little light energy reaches the depths, and only those alga that can either float to the surface or those in crystal clear reef waters can photosynthesize at maximum rates

5. climatic variability: variability here may be viewed in a number of ways. Some argue that the deciduous forest with it seasonal climate cannot support as many plants nor animals since the growing season is restricted to half the year. This is basically a productivity argument. Where as in the tropics, a 12 month growing season permits more species to coexist.

Others would argue the reverse, that changing conditions may support more species if the alternate climate state could support a better adapted group of species. However this assumes that the environmental change is predictable.

6. age of the environment: from paleoecology studies we know that the more northern and southern land masses have been in transition due to glaciation events. The tropics however have remained relatively stable, and the long period has allowed the evolution of many fine-tuned & coevolved species.

In NA, species are thought to be still migrating from their southern refugia. In Europe, with mountain chains running east-west, lots of species never made it over the mountains with the last glaciation event and went extinct.

7. harshness of the environment: one can assume that the harsher the environment the harder it is to adapt to prevailing conditions, and the lower the diversity. However many argue that harshness is a relative term. If you are adapted ( i.e.. Polar bear) what would be harsh to another critter is the norm for you.

Group 3. arguments involving biotic inputs

8. predation: if a predator can keep the populations of two competing species low enough, then both might survive rather than one excluding the other. Thus communities with robust predator populations should have a higher biodiversity.

9. competition; high competition will lead to narrower niches to avoid competition between species thus leading to many species which are highly specialized.

Even in highly productive environments, is it not possible that a few dominants should exist rather than a mass of species? Here the argument is offered that given all trees, lets say in the tropics, play essentially the same role, then any morphological or physiological differences which reduces competition may be sufficient to allow a large number to exist rather than many of few species. The same argument can be applies to birds or other animals in highly productive environments. However in 'harsh' environments the major force is the abiotic environment not biotic interactions, and in this case the few who can tolerate the extremes survive.

Evidence? yes if one considers deserts & lakes, but no when one considers eutrophication; here competition reduces species diversity

10. succession: species diversity is highest at intermediate stages, where there are combinations of both r & K-selected species. Thus environments that perhaps change more rapidly over time should see more periods of interfacing stages and more biodiversity.

11. biotic heterogeneity; The more complex the environments as a result of high species diversity ( think of the 7 canopy layers of the TRF) the more different niches can be formed by the critters there. Thus diversity begets diversity.

Group 4. those involving local geography or inconsistency:
12. physical disturbances: Areas with intermediate disturbance, such as waves crashing on rocky intertidal shores, storms in tropical and deciduous forests, and so on allow gaps which can be colonized by new species, or at least species different from their neighbors, leading to higher diversity.

13. Isolation: biogeography island theory states the smaller, the further away from the mainland the less the species diversity. This is true not only for real islands, but also areas that are isolated on the mainland. This is especially true for the fragmented areas we are creating today or mountain tops or areas beyond mountains.

14. physical heterogeneity: The more complex the landscape the more species that can survive there as specialization or formation of niches allows coexistence. A park with mountains, meadows, forests is more likely to hold more species than the same area containing only desert land.

Study questions:

1. Go to the text and learn about the island biogeography model. What parameters are involved in predicting species number.

2. Why is it that the largest animals are most likely to go extinct? as with the Pleistocene loss? think through generation time, etc.

3. Marine algae along the west coast of NA do not increase in species richness toward the tropics but peak at about 70 species / 100 Km of coastline around 40 degrees north latitude. ( Gaines & Lubchenco). Along the east coast of NA species richness is highest in tropical areas . Discuss why this is so.