These recent posts are because I want to be thinking about these topics for my two exams in the morning. I begin here with part two because part one was in February. It was called “Causes of biodiversity”.
This post will focus on the “Maintenance of Biodiversity” or “Como mantener densidad?”

How do we quantify diversity? In fact biologists cannot exactly agree on this question either. There are plenty of formulas for species density and proportion and all of them function, yet some of them are better for particular questions than others. The Shannon index for example accounts for plants and animals in a habitat and returns a numerical value. However, this number means nothing by itself. It must be used as a comparison between the same habitat over time or between two or more different habitats.

The scale of diversity is also a question that must be considered. Do we look at the diversity of species in a habitat? This habitat can be the amazon basin or lake chabot regional park and these would both be alpha diversity. Gamma diversity would be looking at an entire geographic region, such as an old growth forest to a secondary forest to grassland in which species can pass between the habitats without traversing difficult barriers. Beta diversity measures how species compositions change between two habitats, such as a forest and grassland. Gamma encompasses Betas and alphas, and Beta compares two alphas.

Now that I have hopefully shown that this is can get complicated, I will try to explain the ideas people have come up with to explain how one species does not simply take over and drive the others to extinction. In fact, this is called the competitive exclusion principle. If two organisms are placed in a controlled environment and compete for the same resource, one of them will ultimately out compete and “exclude” the other. However, it is built upon five primary principles which are refuted by these hypotheses.

Species exist in an ecosystem in a niche, or ecological role. They include predation, eating leaves, getting sunlight by being the largest tree, or being shade tolerant and living in the understory of a forest. Even predation can be diverse. There is a species of insectivorous bird for eating insects where ever they find them, but also birds that specialize in eating insects from the ground, from mid-story branches, from canopy leaves, etc. These specialized species will be better at their roles than generalists, which keep the generalists from being numerous. Bird foraging can be restricted to different parts of a single tree! The number of niches can increase as resources increase and diversify and as specialization increases. This is called “Niche Diversification”.

Compensatory Mortality states that species will compete and exhaust each other, and rare species are actually favored in an environment.

The Dispersal limitation hypothesis is related to the ability of trees and shrubs to produce successful seedlings. The further away the seed is from the parent tree, the lower its mortality will be.There is a phenomenon called the “seed shadow” which is the first three to five meters from the parent tree. Only in the tropics will mortality be 100% if the seed lands within the seed shadow. This decrease species density. Your species will not be immediately next to you but 20 other species of tree, shrub, or low growing palm will root around you. Diversity is maintained because a single tree cannot establish a monogrowth in the forest.

This concludes the equilibrium theories, where things don’t really change. Non-equilibium explanations depend on changing conditions such as time and space in the maintenance of biodiversity. They are:

Intermediate Disturbance hypothesis: Diversity is highest in an environment that is disturbed frequently. Disturbances are tree fall gaps, landslides, floods. In the temperate zone they can include forest fires. These disturbances are events that allow species that specialize in growing fast in these sunny environments. This allows for species that do well in sunny places and the understory to thrive.

Stochastic Process: Time is very important for this idea. Imagine that all species are after the sames resources. All Species! Exclusion is happening, however, new species are being formed to compete as well. Therefore, speciation is happening at the same rate as extinctions.

Unified Neutral Theory: All species are considered equal. All species, regardless of class or domain are the same and compete for the same resources and are equally effective at competing for those resources. That they are all after the same resources and are equally competitive for them this means that they will never out-compete each other, and species are balanced.
This is a weird one, yet I am told it is in fact an excellent theory to explain many phenomenon in the natural world…

These are only some of the ideas behind what keeps biodiversity going. Most likely some combination of all of these are the whole picture. There are many ideas on what generate biodiversity, and they really work best when combined together. Biodiversity is complicated, like most concepts relating to the natural world, and new ideas are proposed all the time to help add one piece of this puzzle together.