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In this video, we will explore one of the most basic questions in ecology:
Why are species present in some areas and absent in others?
What, for example, might explain the restricted geographical range of the
3-horned octopotomus?
Answering this question requires a downlink with a satellite stationed in
geosynchronous orbit above the planet Zed in the Andromeda Galaxy.
Zed is the home of the squishy but gentle 3-horned octopotomus, or 3HOP as she
is known to her friends, an aquatic beast with a specialized diet of crystal-shelled mud slugs.
The distribution of 3HOPs on Zed is restricted to the headwater lakes and
streams above the Great Escarpment, a watery realm it shares with other
freshwater critters, including the golden catfish.
The aquatic community below the falls is a bit different. The crystal-shelled mud
slugs and golden catfish are there, but so too is the roach-crab which, like the
3HOP, has an affinity for the slugs.
And, lurking in the deeper pools and eddies, skulks the vampire eel,
a voracious predator on anything lacking a protective shell.
Ah, so you're thinking, maybe the 3HOP cannot survive below the falls because it
is easy prey for the eel, or is outcompeted by roach-crabs for the mud slugs?
Well done. But good scientists try to construct a complete list of alternative
explanations when trying to understand a pattern or process. They call this their
list of multiple working hypotheses, and there are other plausible reasons
why 3HOPs may be absent below the falls.
3HOPs, for example, may not be able to tolerate the warmer water temperatures
at lower elevations, or, given that 3HOPs breathe thru their skin, it might not be
water temperature per se, but the lower levels of dissolved oxygen carried by
warmer waters that the 3HOPs cannot tolerate.
For example, if 3HOPs require dissolved oxygen levels of 8 milligrams per liter or
higher, they could not survive in water temperatures above 27 degrees Celsius.
And, to be complete, we should consider the possibility that 3HOPs could survive
and reproduce below the falls, even with the eels and roach-crabs, if the
octopotomi could only get there safely. The falls, more specifically, the rocks at
the bottom, might pose a lethal barrier to any 3HOP swept downstream.
This more complete list maps nicely onto the 3 broad categories earthly ecologists
have identified as the primary factors limiting the distribution of species on
that blue-green planet. One of these over-arching factors is biotic; that is,
limitations imposed by other organisms, including predators, interspecific
competitors, parasites, pathogens, and prey.
Abiotic limitations in contrast, are physical factors like temperature, or rainfall, or dissolved
oxygen, non-living components of the environment that might nonetheless
prevent a species from living in that region.
And lastly, a species might be absent from an area not because it couldn't
survive and reproduce there, but because it simply can't, or hasn't, gotten there.
Ecologists call these "dispersal barriers."
Back on planet Zed, vampire eels and roach-crabs represent biotic factors,
water temperature and dissolved oxygen abiotic factors, and the potentially lethal
falls a dispersal barrier that might restrict 3HOPs to the upland lakes and streams.
Before discussing how we might disentangle these potentially competing
hypotheses, let's look at a few earthly examples.
The importance of predation as a biotic factor influencing the distribution of a
species is evidenced by the restriction of many colonial, ground-nesting seabirds to
islands devoid of foxes or bobcats or weasels. A case in point, the world's
largest nesting colony of California gulls, over 30,000 of them, used to breed on
Negit Island in Mono Lake high in the Sierra Nevada Mountains of California.
When water diversions by Los Angeles lowered the lake level, a land-bridge
formed linking the island to the shore, permitting coyotes to reach the island,
and forcing the surviving gulls to nest elsewhere.
A more subtle biotic limitation appears to explain the altitudinal distribution of
honeycreepers, a unique family of birds found only in the Hawaiian archipelago.
Prior to the arrival of humans, there were over 50 species of honeycreepers
inhabiting the islands. The rats, pigs, goats, and cats we brought in, along with
the pineapple and coffee plantations, have wiped out more than half.
Only 17 species of honeycreepers survive, and most are critically endangered.
Many of the remaining honeycreepers are restricted to the highest elevations
on the big island of Hawaii. Why?
Because mosquitoes were accidentally introduced to Hawaii in 1826.
Mosquitoes are vectors not only for human malaria, but for avian malaria as well,
a disease that is devastating for the honeycreepers, as they lack evolved defenses against the parasite.
The mosquitoes, being tropical, are restricted to the lowlands on the island, providing a
mountain-top refuge for the native birds. Sadly, human-induced global warming is
enabling the mosquitoes to survive at higher elevations, further threatening the honeycreepers.
Temperature is one of several important abiotic factors that can limit the
distribution of an organism. For example, you don't find many lizards in
the northern United States, and none in Alaska, because lizards are ectotherms
and need warm temperatures to survive and reproduce. Thus, if you want to find
lizards in North America, head to the Sonoran Desert!
While hunting lizards in the Sonoran Desert, you will likely be impressed by
the majestic saguaro cactus, a species found nowhere else on the planet.
The northern limits of the saguaro, much like lizards, is determined by temperature.
While the cacti can tolerate sub-zero temperatures for a day, a freeze lasting
longer than 24 hours is lethal.
The last factor, barriers to dispersal, can either be geographical or temporal.
Hawaii, the most isolated island chain in the world, has only two native mammals;
a bat and a seal, species that could fly or swim to the archipelago.
The islands are not inherently hostile to other species of mammals, they just couldn't
get there, as we tragically learned when, along with the rats and cats and pigs, we
imported mongooses to the islands to control the rats.
The mongooses, unfortunately, prefer to eat honeycreepers.
Time too can be an obstacle. The eruption of Krakatau in 1883 left the
island barren. It took 3 years for ferns and a few flowering plants to colonize, a
decade for coconut trees, and almost 15 years before the first birds arrived and
started breeding.
Let's turn our attention back to Zed. How could we determine whether biotic
factors, abiotic factors, or dispersal barriers keep 3HOPs from the lowlands?
One approach would be to set up a series of artificial ponds which, conveniently,
can be constructed using cattle tanks. You could then systematically
manipulate the conditions in the tanks, circulating warm water thru some,
others with cold, some with high levels of dissolved oxygen, others low.
Then, again systematically, stock some of the tanks with vampire eels, some with
roach-crabs, some with both, and some with neither. Seed the tanks with mud
slugs, add some 3HOPs, and record what happens. You would, of course, want
replicates of each treatment.
While time prohibits an examination of all possible results, an example or two
should illustrate the power of our experimental design. For example,
results showing that 3HOPs could not survive and reproduce in warm but
artificially oxygenated water, with or without vampire eels or roach-crabs,
would suggest that water temperature prevents octopotomi from colonizing the
lowlands of Zed. Alternatively, if 3HOPs do fine in warm-water, low oxygen tanks,
but cannot survive in artificial ponds inhabited by vampire eels, the factor
limiting 3HOP distribution would likely be predation. Factors, however, might also interact.
What evidence from our experiments, for example, might suggest
that water temperature and competition from roach-crabs together restrict
3HOPs to Zed's high-elevation lakes and streams?
Factors limiting the distribution of an organism can act more subtly by
affecting that organism's population size. And population size, in turn, can
influence the likelihood of extinction. Now that you are armed with a more
thoughtful understanding of the biotic and abiotic factors impacting both the
distribution and abundance of a species, let's apply that knowledge as we go
hunting for the Loch Ness Monster.