May 2022 International SAT Reading Test (Q 42-52)

In the early 1990s, Gary Polis, a professor at the
University of California at Davis who studied how
food webs were regulated, began to question the
notion that ecosystems are self-contained when the
⁵ facts from his otvn research on oceanic island
ecosystems off Baja California did not conform to the
prevailing theory. For example, oceanic island
ecosystems are sharply separated from each other
and from the mainland by large distances and a
¹⁰ seemingly impermeable saltwater barrier. These
inhospitably arid islands, being mostly covered by
Opuntia cactus, had low primary productivity.
Herbivorous insect species were accordingly rare.
The islands nevertheless supported extraordinarily
¹⁵ high abundances of spider predators; more so on
smaller than on larger islands. This didn't make
sense in light of reigning ecological principles. Larger
island and intact mainland ecosystems should be
more likely to support longer food chains and greater
²⁰ abundances because they are more productive and
provide more living space.

Polis and colleagues discovered that island
ecosystems are not self-contained, self-supporting
entities and that the internal working of an island
²⁵ ecosystem closely involves the size of the island and
what flows across its boundary. Instead of
concentrating solely within the island ecosystem,
they focused on the flow of nutrients and materials
across the island boundary. This led to the discovery
³⁰ that the boundary of the island—the shoreline—was
not impermeable. Dead algae and drowned animal
carcasses that washed up onto the shore from oceanic
drift provided important subsidies for the island
economies. The shoreline sustained high abundances
³⁵ of detritivorous insect species that consumed the
algae and scavenged the decomposing carcasses. The
detritivorous insects became an abundant resource
that sustained the predatory spiders and scorpions.
The detritus subsidy from the ocean propped up the
⁴⁰ island economy.

The smaller islands supported more predators
because of their physical properties. Smaller islands
have a higher ratio of perimeter to area, meaning that
they have more shoreline relative to their overall area
⁴⁵ than do larger islands. This allows consumers from
all over the small island to access the subsidy. By
contrast, individuals living in the middle of the larger
islands have a lower likelihood of encountering the
subsidy. That is, the heterogeneous spatial
⁵⁰ arrangement of species and resources across the
islands mattered considerably. This challenged
another classical view: that resources and species are
evenly—homogeneously—arranged in space within
self-contained ecosystems.

⁵⁵ The abnormally high abundance of predators led
to a feedback in which the abundant predator trophic
level controlled the abundance of the island's
herbivorous insects, thereby further contributing to
their rarity on the islands. This lessens the insect
⁶⁰ damage to plants. Hence the effects of the subsidy
reverberate throughout the entire island food web.

The lesson here is that two very different kinds of
ecosystems can be inextricably linked through
resource flows across their boundaries. The amount
⁶⁵ of subsidy provided and its attendant
ecosystem-wide effects depend on the spatial
arrangement of the donor and recipient ecosystems
and on the species interactions within both of them.
If, for example, marine production is altered by
⁷⁰ environmental impacts, or from species imbalances
in the marine food chain due to, say, overfishing, then
the amounts of algae and animal carcasses that
subsidize the island economies become altered too.
Shut off the marine subsidy completely and the
⁷⁵ island ecosystem Is likely to collapse to a barren
desert.