Sources of biodiversity

Biodiversity is one of life’s great perks.  Whether your interests run to gardening, woodworking, wine tasting, or hiking, the variety of life forms all around us enriches and sustains our lives as humans.  The variety in our diets, in the landscapes that surround us, in the medicines that heal us and in the works of art that inspire us are all brought about through biodiversity.  Life would be possible without variety but it would be esthetically flat and likely without comfort.  I would like to offer a couple of thoughts about where biodiversity comes from and to suggest that some aspects of its riotous and overwhelming distinctiveness  can be predicted. I will, for the moment, ignore the mechanisms of how diversity arises – that is, evolution – and concentrate on the end results of its billions of years of operation.

There are three general conclusions that we can make about diversity, each is predictive in its own way, although only the second of the three is useful in the study of ecosystems.  They are:

1) The McShea-Brandon Rule[1]:  Diversity tends to increase gradually over long periods of time.

2) Wallace’s Rule: Diversity is concentrated in the most productive ecosystems, and

3) Haldane’s Rule[2]: Diversity is concentrated in particular groups of organisms.

Diversity increases with time

The first of these, which McShea  and Brandon  call Biology’s First Law, comes to us primarily from the fossil record.  The process by which species branch into new species is slow (or at least infrequent) and the fossil  record provides the only observation tool that allows us to consider the long term.  In general, the marine fossil record, shows a gradual increase in the number of life forms over a 500 million year span.  This record is punctuated by five mass extinctions or rapid declines in diversity, the most notable of which occurred at the end of the Permian period about halfway through the fossil record[3].   Each time the  fossil  record recovers with increasing diversity to compensate for these losses and adds even more forms of life  than existed before the mass extinction.

This pattern of increasing diversity provides little comfort in our present environmental crisis, one which many describe as a mass extinction of our own design.  The variety of life lost in the last couple of hundred years may take hundreds of thousands of years to repair through evolution.  Nonetheless, as a general rule, diversity increases gradually and then disappears suddenly.  It has done so for so long that we can expect this pattern to be repeated.

Diversity increases with energy

Wallace’s rule , the observation that more species are found towards the equator, is named after Alfred Russell Wallace , the co-discoverer of the principle of natural selection.  From a predictive science standpoint, the relationship between latitude and diversity is a greater discovery than the ever present property of natural selection.  Currie[4] has demonstrated that whether measured as solar radiation or evapotranspiration (the amount of water taken up into the atmosphere as a result of the sun’s energy), the input of energy into an ecosystem is well correlated with diversity. It explains 80-90% of the variation in the number of plants or animals over wide geographic ranges.  This makes the energy–diversity relationship one of the fundamental theories in ecology.  What is intriguing about this idea is that more opportunities for species to survive are created where there is more food.  These opportunities or “niches” as they are called in ecology either contribute to species formation or they help maintain species that have either evolved locally or arrived from elsewhere.

Diversity favours certain body plans over others

At first glance, Haldane’s rule appears to be an example of Natural Selection: that individuals well adapted to their environment will have many offspring and will pass on their genes better than most.  In fact, our 3rd rule may actually be an exception to natural selection.  It is unlikely that the body plan for a given group of organisms can consistently produce better adapted individuals over a long period of evolutionary time and across a wide diversity of environments.  The specific advantage of one individual over another should depend on the details of the environment and the individuals involved – not the general design of each individual. Still, we have many examples of groups that have done very well indeed.  Table 1 takes a look at how rapidly different groups of organisms divide into new branches and ultimately into new species.  Taxonomy groups species into a series of increasingly general categories from species and families to orders and classes.  Here, I show the average number of orders per class, the average number of families per order and the average number of species per family.  In a sense, these values represent the number of surviving branches from speciation events at different points in evolution.  According to Haldane’s Rule, a diverse group of organisms should maintain a high rate of branching at different levels of taxonomy.  This may reflect a greater probability to split into new species and/or a greater probability of surviving and adapting to new environments and opportunities.

Table 1: Major groups of plants and animals in declining order of number of species. Values in red represent a top ten rate of branching at a given taxonomic level compared to other groups.  Data from  Smithsonian (2010)[5].

As it turns out, Haldane’s rule doesn’t hold up in a precise, quantitative sense.  There is no correlation between the branching rates of classes, orders or families, not even of their rank order. You simply cannot predict how often a family splits into species from the number of times its class splits into orders.  In coarser terms, however, it is clear from Table 1 that the top ten rates of division are common at all taxonomic levels for diverse groups.  Rarely will one of the less diverse groups even crack the top ten.  Haldane’s rule holds true across long periods of evolution and contrasting environments.  It is as if the body armour and multiple appendages of insects and crabs give them a head start in adapting to new situations.  By the same token, evergreen trees  (conifers)– though common – cannot match the sheer diversity of their flowering cousins. This is surprising.  In the auto industry, the overall brand reputation of a car company has a larger impact than the specific features of a given model in determining sales.  But there is nothing in Natural Selection that would lead us to a similar prediction for groups of organisms.

These three rules allow us some grasp on when, where and what to expect from biodiversity.  That so much of the wonder of nature is left unpredicted is a challenge for science.  It does not, however, take away from the beauty of biodiversity.  My own hope is that we will learn to predict more of the patterns of life on earth while continuing to delight in its radiant quirkiness.