Agriculture Reference
In-Depth Information
Box 9.1
Ecosystem
The concept of ecosystems emerged early in the twentieth century as ecologists
began to grapple with the complex interactions defining the relationships
between the biota and the abiotic environment. The concept is appealing in its
generality, applying equally well from a pond to an ocean, or from a woodlot
to a forest biome, or for that matter to the planet as a whole. At the heart of the
ecosystem concept is the recognition that flows of energy and materials
through the system sustain the interactions among its biotic and abiotic
components. Ultimately all ecosystems depend either on the thermal energy
and material flows associated with deep-sea vents or, most commonly, on the
solar energy that is captured by photosynthetic organisms such as plants and
phytoplankton - the primary producers. Other organisms in ecosystems
function as consumers of primary producers or decomposers breaking down
organic matter. In contrast to the emphasis of evolutionary biology on the
diversity and adaptation of organisms, ecosystem science has focused more
on the overall structure and nature of the flows of materials and energy through
the system than on the particular organisms in the system. A contemporary
challenge in ecosystem science is to understand the relationship between
biodiversity and ecosystem function.
Kikuzawa and Kudo 1995) studied two evergreen species and a deciduous species
associated with these snowbed habitats in which snowmelt occurred from early
June through early August (see Fig.
9.5
). In both evergreen species, leaf longevity
declined with a longer favorable period, whereas in the deciduous species leaf lon-
gevity increased with the length of the favorable period. Leaf longevity of the
deciduous species is restricted by the length of the favorable period; thus, leaf lon-
gevity necessarily is reduced in shorter favorable periods. In contrast, evergreen
species can prolong leaf longevity beyond winter, thus compensating for the
decrease in photosynthesis resulting from a shortened favorable period by
prolonging leaf longevity and exploiting subsequent snow-free periods. By
changing favorable period length in Kikuzawa's (1991) model with other param-
eters held constant, Kikuzawa and Kudo (1995) simulated this pattern of decreasing
versus increasing leaf longevity in evergreen and deciduous species, respectively,
with a longer snow-free period (Fig.
9.5
). Because leaf longevity is only one
element in the suite of foliar traits affecting production potential, this snowbed
community also affords an example of how the deciduous species adjust to ensure
payback on leaf construction costs when the favorable period is short. Unable to
extend their leaf longevity, plants growing in places subject to shorter snow-free
periods instead increased their photosynthetic rates by increasing investment for
photosynthetic machinery and decreasing costs such as defense. In three deciduous
species in this snowbed habitat, leaf mass per area (LMA) decreased and foliar
