Biology Reference
In-Depth Information
contain greater biodiversity than do any of their smaller, constituent subregions.
My backyard in Honolulu is not very diverse; Honolulu is somewhat more
diverse; the island of Oahu is yet more diverse; the entire chain of Hawaiian
Islands is still more diverse; the Pacific Basin is yet more diverse; and the entire
world is the most diverse. Different processes are involved in generating diversity
at different geographic scales (Sax and Gaines, 2003), and this can potentially
confuse discussion of biodiversity. In speaking of recent concerns for biodiversity
protection, we are speaking of preserving diversity at the largest scale - that is,
ensuring that the sum total of diversity on the entire planet is not diminished.
Conceptually, this is a simple matter of ensuring that species extinction does not
occur. Hawaii has many species unique to that archipelago. If we artificially
inflate species numbers by importing alien species that cause the extinction of
Hawaii's unique species, we may have boosted species numbers within Hawaii
but at the cost of the global total. Replacement of globally unique elements by
artificial inflation of regional species numbers with widespread aliens is not a
service to biodiversity, but rather the converse: it decreases biological diversity.
And indeed, introduced species are among the major drivers of biotic homogeni-
zation, the process by which formerly distinct biotas are beginning to look more
and more alike (McKinney and Lockwood, 1999).
One also frequently hears the argument that species movements are “natural”
and that concern about alien species is, therefore, unjustified. This claim too is spe-
cious. For the term “natural” to apply in any scientifically meaningful way it must
refer to a phenomenon occurring at background ecological temporal (the rate at
which a phenomenon occurs) and spatial (geographic) scales. As I have mentioned
earlier, introduction rates in Hawaii are now approximately one million times as
frequent as the natural, background rate. Similar high rate increases have been
measured for other regions too (Ricciardi, 2007). The geographical reach of species
transport by humans also extends far beyond what the organisms could have
achieved under natural processes. To give just one example, there is no way that
chameleons - ponderous arboreal lizards native to Africa and western Asia - could
possibly have colonized places as remote as Hawaii or California under their own
power. The geographical barriers that helped give rise to the tremendous and
regionally unique biological diversity across Earth are proving ineffectual in the
face of human modes of transport such as ships and planes. Moreover, the number
of species and individuals moved during each introduction event is often now much
larger than could have occurred under natural conditions (Ricciardi, 2007). For
example, a single load of ballast water may dump millions of individuals of hun-
dreds of species, a form of dispersal unparalleled in pre-human history. In short,
there is nothing remotely natural about the tempo and extent of modern biological
mixing by human action.
Another variant of this argument is to posit that because humans are a part of the
natural world, anything we do is also natural and, hence, no cause for worry. Under
this reasoning, our transport of alien species is natural and we shouldn't be overly
concerned with it. Of course, by that same logic, genocide, torture, and slavery are
natural too. I doubt that most readers would find these other human actions
