Chapter 9: Species Dynamics

Both biotic and abiotic factors influence population dynamics, and can be either density-dependent or density-independent. Dr. Brown gives four possibilities:

1) Weather (density-independent) can cause changes in population density,

2) Competition, predation, and other biotic interactions (density-dependent) can cause changes in population density,

3) Abiotic and biotic factors can work together; while weather can cause changes in population densities, not enough refuges can create population to either survive or collapse,

4) Certain processes classically described as either stochastic or deterministic do not always hold true. For example, seasons are very predictable, and other biotic interactions might be more stochastic. A simple change in initial conditions can make a seemingly predictable processes appear random.

Island Biogeography

The theory of island biogeography is beautiful in its simplicity: as an island gets farther away from a source (mainland), the rate of colonization decreases. As an island gets bigger and is able to hold more species, the rate of extinction decreases. I really like these figures, and when I first learned this in my biogeography class, I would spend hours playing with different scenarios and figuring out what would happen. I highly recommend it! I also found it great that this is one of the few times that looking at the variation (or exceptions) in a system the model has gotten better. I feel in Macroecology we don’t like to look at the nitty-gritty too much because we’re too busy squinting at the overall picture. Here, it’s by looking at the nitty-gritty the bigger picture becomes more clear.

Extinction

It’s really important to realize that extinction doesn’t occur when the last individual dies, but when a population cannot not sustain itself. For example, if there is skewed sex ratio that makes it difficult – if not impossible – to find a mate, a population might go extinct. Dr. Brown also discusses Allee effects that increase the probability of extinction by reducing birth rates and increasing death rates. These fluctuations are “demographic stochasticity.”

Extinction in the Fossil Record

We previously learned about Sepkoski’s curve, or the curve that shows diversity through time. Imbedded in that curve are the 5 mass extinctions and, although hidden, background extinctions. The latter influenced Van Valen to propose the “Red Queen Hypothesis,” in which species “run” (evolve) as fast as they can to essentially stay in the same place. This is also known as the arms race, where species evolve to alleviate competition or predation (biotic interactions). The mass extinctions, on the other hand, were most likely caused by a huge environmental change (i.e., meteorite landing on the Yucatan peninsula 65 million years ago). Dr. Brown also mentions humans as cause of environmental change. I am torn about this idea. On one hand, we are causing undeniable change to the planet; on the other hand, where is the line drawn for humans to just be “environmental engineers” versus not part of the environment?

Predictability

It appears that extinction risk can be assessed. There are certain predictors, such as having a specialized diet, which can lead to a greater chance of extinction. There are other predictors, like large body size, that also can lead to a greater chance for prediction. Reasons for larger body sized species to be at risk for extinction have previously been mentioned throughout the book: smaller population sizes (could be accentuated by Allee effects), slower generation times, higher infant mortality, and generally slower to adapt to a changing environment.

Speciation

Using the definition of species given previously in the chapter (operationally useful biological entities that are easy to identify and that are useful for biological and ecological studies), Dr. Brown states that for an ancestral population to speciate, there must be ecological and genetic differentiation. A really particularly enlightening sentence for me was, based on the prediction that after every extinction event there have been a series of speciation events, that “some of the differences among [the species] may have originated long after the speciation and may be at best only indirectly related to the speciation process.” Now that would be a cool hypothesis to test! Another (equally exciting) hypothesis to test is the radiation of clades from the same environments to see how the process occurs and if it’s conserved across clades.

Colonization and Range Shifts

For colonization to occur, it requires dispersal (getting to the new space) and establishment (having and maintaining offspring). Dr. Brown shows evidence that this process is probably non-random. For instance, species can be classified as good or poor dispersers, and so their chance of colonizing increases or decreases, respectively. An amazing area of research currently is the studying the success of exotic species in native habitats.

Regulation of Species Diversity

Dr. Brown suggests that the pattern of diversity is given by: amount of fragmentation, rate of dispersal, and how well a species does on fragments. A great example of this is with butterflies that might have populations across the landscape in little patches. In order for these to be maintained, each patch needs to be large enough, dispersal needs to be frequent to maintain genetic diversity and to maintain “sink” populations, and, without dispersal due to a barrier such as a stream, populations of butterflies might differentiate and ultimately speciate. On the continental scale, however, the process is a bit different. There is little dispersal, but a high rate of endemism (i.e., species parcel the habitat into finer niches rather than dispersing).

There’s a lot to discuss, but I have to admit, I got really excited when reading the part about extinction, and using it as an excuse to incorporate a paper that started me on the path to studying macro-level processes:

Sepkoski, J.J., McKinney, F.K., and Lidgard, S. Competitive displacement among post-Paleozoic cyclostome and cheilostome bryozoans.

This paper is really cool because it attempts to assess competition in the fossil record. I’ll give you a minute to think about that.

I am not going to say much about this paper because a) I’ll give a summary during class, and b) I am really curious to see what you all get out of it and don’t want to bias you in any way.

This paper will be fun to discuss the merits of the methodology as well as difficulties in assessing ecology and/or ecological function of organisms in the fossil record, and what other processes could be acting that would produce the same diversity curve.