You’ve seen posters and read bumper stickers. You’ve watched news videos and opened colorful solicitations for money. Cries of “Save the bees!” and “Protect our pollinators!” are ubiquitous, having replaced pleas for pandas, whales, and spotted owls. But wait. How can you save a bee?
The truth is, you can’t. Not really. If you catch an endangered rusty-patched bumble bee and put it on your sun porch with flowers and sugar syrup, you’ve gained nothing. If you snag a monarch butterfly and give it an entire greenhouse full of the finest milkweed, you’ll receive nothing for your efforts except a large invoice.
We simply cannot save individual insects. We can’t even save large aggregations. The one and only thing we can protect is a pollinator’s habitat. If pollinators have the space they need, and that place is filled with their natural foods, building materials, housing choices, and water, then and only then can we begin to save them.
Connectivity is Key
Note that I said “begin.” That’s because even a perfect environment containing all the desired components must be connected to others like it. Small populations must interact with other populations in order to share genes and maintain diversity.
Populations that are cut off from each other can spiral into oblivion, something the biologists call an extinction vortex. In simplistic terms, the smaller a population becomes, the more it inbreeds, and the more it inbreeds, the faster the gene pool shrinks. As the gene pool shrinks, it offers fewer genetic choices to future generations, meaning the offspring are less likely to have the traits that would allow them to survive unusual circumstances such as pathogens, droughts, or predators.
In one study, researcher Penelope Whitehorn monitored populations of native bumble bees, Bombus muscorum, on nine islands off the coast of Scotland that were isolated from mainland populations.1 She found the island populations to be much more susceptible to a parasite called Crithidia bombi that lives in the bee’s gut. In addition, the inbred populations were much more likely to produce infertile males, which further weakened the breeding population.
Weaknesses such as susceptibility to parasites or infertility are much more likely to show up when a population is small. Although so-called bad genes occur in large populations, too, most bad genes are recessive, so they express themselves only in the homozygous state. That is, an individual must have received the same recessive gene from each parent.
Of course, the genetics of haplodiploid creatures such as bees operate a bit differently. Since males have only one set of chromosomes, a so-called lethal gene would not be passed to progeny through the male. Instead a male with a lethal gene would simply die. But in fact, most genes that affect the fitness of bees are not lethal. For instance, genes affecting foraging ability or cold tolerance can easily be transferred to female offspring through both parents.
Although double recessives are possible in a large population, they are statistically rare. However, in a small inbreeding population, a bad gene may become quite common, so offspring are much more likely to get a pair of them. This condition, called inbreeding depression, can cause a population to fail quickly. Like water circling a drain, the population gets smaller and weaker until it just disappears.
Habitat fragmentation hastens extinction
Biologists have been studying habitat fragmentation for years, mostly in the form of island biogeography.2 After monitoring island populations, studying their fossil records, and comparing them with similar populations from other islands, a number of principles became clear. Chief among these is large islands will host a greater number of species than small islands, and islands that are close together will have more species than islands farther apart. Area and distance are key to species richness.
Large islands have lots of resources and many variations of habitat. Think of Australia as a large island, and you can see lots of opportunity for a variety of species to thrive. Habitats are extremely varied and the land mass is so large, the chances of inbreeding are small.
Now think of an island of ten acres in the middle of the Atlantic. Surely, it could support some forms of life, but the opportunities are limited. The number and types of habitat are small, food sources are limited, inbreeding is high, and if something goes terribly wrong — like a hurricane — the whole thing could collapse.
In contrast, let’s put a similar little island a quarter mile off the Australian coast. The insects and birds living on that island are close enough to interact with populations on the mainland. With occasional cross breeding with those “foreign” populations, the gene pool is strengthened. Even after a catastrophic event such as a storm, populations can be boosted with input from the mainland.
Slicing and dicing the continent
Back here in North America, we too have a big chunk of land with lots of habitat to choose from and plenty of resources. Islands, in the traditional sense, are rare, and most of the ones we have are close to a large land mass. So what’s the problem?
The problem is simple. We have segmented our continent into millions of small parcels that are disconnected from one another. This division, called habitat fragmentation, produces biological islands. The landscape is still physically connected, but travel between the different pieces is impossible for many species.
In nature, many features can cause a patch of land to be a biological island. Think of mountain ranges, deserts, rivers, and lava flows. All of these things can stop species from moving in or out. But the number of those natural barriers doesn’t hold a candle to all the artificial barriers we have built and the sheer number of biological islands we have created. The impenetrable walls we have built leave wildlife locked into wastelands of limited resources and restricted genetics.
Have you ever seen an aerial photo of New York City’s Central Park? It’s a perfect biological island, a green oasis surrounded by multiple roads and immense skyscrapers, which are further surrounded by expanses of water. Even now, some creatures can get in and out by themselves, including certain birds and insects, but most cannot.
Unfortunately, a habitat fragment doesn’t need to be nearly as ….