Defining Biodiversity
Species
By some measures, worldwide biodiversity may be increasing. At the same time the rate of extinction, due mostly to habitat loss, remains high, especially in tropical areas where deforestation continues at a pace noticeable to even the most unconcerned bystander. You might wonder how these statements can both be true.
Biodiversity, a term introduced by Thomas Lovejoy in 1980, usually refers to the number of recognized species in a particular area. These species are most frequently defined based on obvious distinctive physical features such as feather, hair, and skin coloration, size ratio, and, most importantly, an ability to produce fertile offspring. This way of defining species, however, does not encompass the numerous examples of plants and animals that readily hybridize (the result of a cross between two species), the fact that similar species may look very different, and that some species reproduce asexually.
Hybrids
Defining biodiversity as the number of species in an area is not always straight forward. Take, for instance, two orchid species from Brazil, Epidendrum fulgens and Epidendrum puniceolutem. These two species have a different numbers of chromosomes and live in two different habitats, yet a hybrid between the two species produces fertile plant. Amazingly, they can be recrossed with the original two species. It is not just plants that hybridize. Two small subterranean species of mammal known as tuco-tucos can produce hybrids that are fertile. This makes it difficult to define whether there are still two species or, now, three species.
Cryptic Species
Ants present yet another example where cryptic species, or species that differ more genetically than in physical appearance, also occurs.
One study of ponerine ants was able to identify new species by measuring genetic differences, long before being able to identify any physical differences. Twig ants (Pseudomyrmex), with their long bodies and big eyes, are frequently very difficult to tell apart, but may be very different genetically.
Better Definitions of Biodiversity
Today, biodiversity is used to refer to a species physical and genetic characteristics, as well as its spacial distribution. Using this definition we can see that an acre of agricultural farmland is not as biodiverse as and acre of neotropical rainforest. One of the most amazing things about tropical rainforests is not just the number of different species, but the number of different species present per acre. For example, John Kircher, in his book The New Neotropical Companion, noted that with skill and patience a person might find 25 to 30 species of trees in a five acre section of the Smokey Mountains National Park. Compare this to nearly 300 species of tree in a five acre section of some areas of the Amazon.
How Biodiversity Might Increase
Habitat Fragmentation
Habitat fragmentation can lead to higher levels of biodiversity. For example, when humans cut into large tracts of forest, the result is often a landscape that is more fragmented and has more edges.
There is a well know edge effect where species that specialize on using both forested and non-forested landscapes move in. Many of these species are nonnative and compete well in the edge, but may not compete well in the forest interior. In the short-term, this may lead to increased biodiversity, because the fragmented landscape has allowed more species to live in the same area. Of course, this only works if there is a large enough forest fragment left to support the current native species and new nonnative ones.
Nonnative Species
Human transportation networks move a lot of hitchhiker species around the globe. Some of these nonnative species are able to survive, or even thrive, in the new areas. At least for plants, the nonnative species tend to follow the “rule of ten” pattern, meaning that ten percent of the nonnative species survive and establish viable populations in the new environment (approximately ten percent of these survivors go on to become invasive pests). While these new nonnative species frequently cause the demise of some native species, it usually happens near the edges and may only result in the loss of a few natives.
So short-term additions of nonnative species and small losses of native species means biodiversity may increase. Remember, however, that this may be only a short-term increase and is really only well documented with plants. The loss of some native species, such as keystone species, may lead to a crash of the ecosystem and many species losses. Also potentially devastating to biodiversity is when nonnative species turn into invasive pests. Invasive pest species contribute to a higher rate of native species loss, especially on islands. An example is the brown snake (Boiga irregularis), reportedly introduced accidentally onto the island of Guam. The snake has eaten just about every native bird into extinction. Hawaii, too, has lost nearly half of its species of birds from the introduction of cats, rats and ants.
Warmer and Wetter Earth
As you move north and south from the equator, there is a drop in the average number of species per acre. This drop correlates with a less sunlight in higher latitudes and, thus, lower average temperatures. It’s not hard to see that warmer temperatures allow more species to survive longer, as long as there is enough water. Just look at the number of spiders that survive indoors in temperate zones during the winter. With the warmer weather we have noticed species moving northward into areas that in the past were too cold for them to survive. The northward migration of species can result in increasing number of species in areas that traditionally held fewer. In other words, biodiversity is increasing due to immigration.
Will Biodiversity Continue to Increase?
There are very few studies indicating an an increase in biodiversity, but many indicating it is decreasing. While the Earth’s average measure of biodiversity may increase in some areas, there remain many places where it is decreasing and decreasing fast. Islands in the Pacific continue to be decimated by nonnative species turned invasive. Climate change threatens many of the species in northern latitudes that are unable to adapt to prolonged warmer temperatures. Coral reefs, where as many as 40% of all species of marine fish live, have declined significantly from over fishing and higher ocean temperatures. The forests of the tropics are still being converted into agricultural use, and it is foreseeable that many fragmented forests, grasslands, wetlands, and rivers will not be able to continue to support native biodiversity ultimately leading to decreasing biodiversity worldwide.
Assemblage Time Series Reveal Biodiversity Change but Not Systematic Loss
