Download Human Appropriation of Net Primary Production as An

Helmut Haberl
Human Appropriation of Net Primary
Production as An Environmentallndicator:
The human appropriation of net primary production (NPP)
significantly alters the energy flow of ecosystems. The
NPP-appropriation, defined as the difference between the
NPP of the hypothetical undisturbed vegetation and the
amount of biomass currently available in ecological cycles,
is investigated for the 99 political districts of Austria (1990).
Calculations are based on data for land-use, forestry, yield,
and climate. Total aboveground NPP of the actual vegetation was found to be 7% less than that of the potential
natural vegetation. Additionally, 34% of potential production
is harvested, resulting in a total reduction of ecologically
available aboveground NPP of 41%. SinGethis could have
significant ecological effects, e.g. on biodiversity, it is of
potential interest for strategies of sustainable development,
indicators for stresses on the environment, and the
environmental effects of increased utilization of biomass.
This article relies on a spatially highly resolved study on the
societal NPP appropriation in Austria. Austria is an industrialized country with medium population density (area 83 000 km2,
population 7.8 million). Forests cover about 45% of its area. This
is a rather high percentage for Central European standards due
to the mountainous landscape.
MATERIALS
AND METHODS
In this article, NPP appropriation is defmed as the difference between the NPP of the potential natural vegetation (NPPo, i.e. the
vegetation that would prevail if human interference were absent)
and the amount of biomass currently available in ecological cycles (NPPJ. Two processes contribute to NPP appropriation
(NPP.): i) changes in the averageproductivity (NPP per unit area
and year) of ecosystems, e.g. the construction of a road in a
forested ecosystem; and ii) harvest. 1f the NPP of the actual veg-
etationis denotedas NPPact and harvestas NPPh,total NPP appropriation can be calculated with the formula:
NPP. = NPPo - NPP, withNPP,
INTRODUCTION
The production of biomass by green plants is the main energetic
basis of lire on earth and provides the primary input für most
food chains of all types (herbivory, carnivory, detritivory). While
hunters and gatherers dwelled upon the products of photosynthesis much like any other kind of animal species, thus, reaching only very small densities, the cultural evolution of humanity has seena tremendous intensification ofbiomass use (1). This
could only be achieved by a transformation of natural ecosystems into managed Olles with an increasing number of ecosystem variables hefig controlled. While agriculture allowed the
harvest of more biomass per unit area, it did not generally increase the average productivity in terms of total energy or carbon fIXation. Additionally, ever-increasing areas are used für the
construction of buildings, loads, etc. and thus their primary productivity is reduced.
While ecologists were concerned about the possible ex.haustion of the world's biomass resources already very early (2), it
was the seminal paper of Vitousek et al. (3) that opened the dOOf
für a broader discussion of the problem. In the meantime, it is
estimated that the human appropriation of the products of photosynthesis amounts to between 25 and 39% of the global terrestrial net primary production (NPP) (3, 4). This result inspired
Meadows et al. to worry about the possible consequencesof the
human world biomass use, which can be expected as a result of
the projected doubling of the world population and economy
within the next 20 or 30 years (5) and was broadly noticed in
the discussion on sustainable development (6, 7). This discussion revealed that NPP is an important limiting resource für the
future development of humanity and current levels of NPP appropriation already appeal to be considerable (3). Moreover, current strategies of sustainable development in the energy sector
partly rely on the substitution of fossil fuels by biomass in order to reduce CO2 emissions-a strategy that would further increase NPP-appropriation.
Ambio Vol. 26 No. 3, May 1997
= NPPact- NPPh
All calculations were perfonned on the spatiallevel of communities (Austria consists of approximately 2350 communities).
Since there are few reliable data on the subterraneanprimary production of forest ecosystem~(it was significantly underestimated
in previous research (8-10)), this article focuses on aboveground
net primary production (ANPP) which can be assessed with
greater accuracy. (see Haberl (11) für an in-depth description of
all applied methods).
The ANPP of the potential natural vegetation in Austria was
estimated with two independent methods: i) The average productivity of different types of natural vegetation was assessed
on the basis of the literature available (11), applying regression
analyses on the relation between mean annual temperature, precipitation and net productivity in forest ecosystems using data
compi1ed by Cannel (12) and the climate data of Walter and
Lieth (13); ii) The so-called Miami modell ofLieth (14) was used
with a correction für Lieth's assumption on subterranean NPP.
Data on mean annual precipitation and lang-tenn temperature
averages were obtained from the Austrian association of meteorology (15) and modified to reflect elevation. The area of each
community was distributed over six elevation classes (less than
600 m, 600-1300 m, 1300-1700 m, 1700-2200 m, 2200-2600
m, more than 2600 m) assessedby a geographic infonnation system (Loibl, pers. comm.) which served to detennine the potential natural vegetation. The data used für the calculation of
ANPPoand ANPPact in natural ecosystems(and actual forests,
as described below) are given in Table 1.
The productivity of the actual vegetation was calculated by
using land-use data as assessedby the Austrian Central Statistical Office (16). The productivity of agricultural areas (crops and
meadows) was estimated by using harvest factors of the fonn
NPP = H x F, where H is the commercial harvest and F an appropriate factor für total or aboveground productivity. Harvest
@RoyalSwedishAcademyofSciences1997
143
factors were taken from the literature (14,
17-20).The productivityof grazingland was
I
calculated from averageproductivity estimates,dependingon elevation,basedon the
literature. The productivity of forests was:
j
estimatedby two independentmethods,fIrSt'
by harvestfactors from the literature (11),
usingthe Austrianforestinventory(21), and
secondby assumingthe averageproductivity by forest type, modified by elevation
class(Table 1).
Harvestwascalculatedfrom Austrian agricultural and forestry statistics(22, 23). Subterraneanparts of
cropslike potatoes(about 1% of total harvest)were countedas
"above-ground".Agricultura1biomasswasconvertedto dry mass
and calorific value using standardtables on nutritive value of
the materialsunderconsideration(24, 25), wood was treatedin
the samewar on the basisof tableson species-specific
dry-matter contentand calorific value.
RESULTS
Both calculation methods für the ANPPo of the hypothetical undisturbed vegetation of Austria led to almost the same result.
While Lieth's Miami model predicts the ANPP to be 1445
PI yr-1 (74 mill. t yr-l dry matter, DM), the assumption of average productivities dependent on elevation gave an estimate of
1501 PI yr-1 (77.6 mill. t yr-1 DM). Since Lieth's model is based
on rather old productivity data which tended to underestimate
productivity (26), the higher value is believed to be more reliahle and taken as a reference point. The average productivity of
the Austrian vegetation is 0.93 kg m-2 yr-1 or 17.9 MI m-2 yr-l.
Human activities, above all agriculture and construction, have
significantly lowered the productivity of the vegetation in Austria. ANPPaclwas estimated to be 1396 PI yr-1 which is 105 PI
yr-1 (7.6%) lower than the ANPPo. In terms of dry matter, the
difference is smaller (ANPP acl= 74.2 mill. t yr-1 DM), since the
calorific value of forest biomass is higher than that of most cultivated herbaceous plants. About 50 PI yr-1 of this reduction is
due to construction. The difference between the two independent methods used to estimate primary production of forests (elevation classes and forest inventory) was only about 2%, with
forest inventories giving a slightly higher value. The value reported above is based on elevation classes. It should be mentioned that these estimates are conservative, since the statistical
data on land-use für construction are believed to be lower than
actual values and many of the underlying assumptions (11) were
made with great caution to avoid a possible overestimation of
NPP appropriation.
Together with a straightforward estimation of harvest (512
PI yr-1 or 27.6 mill. t yr-1 DM) ANPP appropriation (ANPP.)
can be assessedto amount to 617 PI yr-1 or 41.1 % of potential
aboveground production in Austria (Fig. 1).
My tentative calculations on total NPP and its appropriation
show that the difference betweenNPPoand NPPact should be
much higher than that für aboveground NPP, because the
belowground productivity of forests is considerably higher than
that of annual crops; even in cases where aboveground productivity is similar or crops are more productive. Total NPP appropriation, however, is smal1er,since mainly aboveground biomass
is harvested. As these results are rather uncertain, they will not
be discussed hefe in detail (11).
As Figure 2 shows, there is more NPP appropriation in districts with high ANPPo than in districts with low ANPPo. Obviously, fertile regions are more intensively cultivated and a higher
share of their net primary production is harvested. Settlements
and roads are also preferably situated in low, fertile regions.
Thus, thehigher the level of ANPPo, the higher the proportion
of appropriated ANPP. Natural fertility explains 50% of the variance of ANPPjANPPo, this relation being significant at p <
0.001 (chi square).
DISCUSSION
The reliability of the results can be assumed to be high für
aboveground NPP. A previous study (27), which used much simpler calculation methods and a much narrower data base, yielded
rather similar results. The results on average productivity of
ANPPoand ANPPacl are weIl in line with the comparabledata
in recent large-scale productivity studies (14, 28-30). The appropriation of ANPP in Austria is higher than the estimates für
global NPP appropriation, ranging from 25 to 39% (3,4). Nevertheless, it can be assumed that values für other highly industrialized western and Central European countries may be even
higher: They typically have less than the Austrian 45% of forests with rather low NPP appropriation per m2. Furthermore,
20
-'>.
'I
E
15
~
10
00Z
c(
:
I
5
~:::::~~~~
EE
-ANPPact
ANPPO
-ANPPt
Regional subdivision of Auslria (99 polilical
dislricls)
ANPPo
Figure 2. NPP appropriation greatly reduces the spatial differences
between the energy input of different terrestrial ecosystem types. While
ANPPact
Figure 1. Appropriation of aboveground net primary production (AN PP)
in Austria 1990. Of the 1501 PJ yr-1 which would be available in natural
ecosystems if human interference were absent, only 884 PJ yr-1 can
actually serve as energy input of all heterotrophic food chains.
144
ANPPo and ANPPact fall within a range between 8 and 22 MJ m-2 yr-', the
amount of energy actually remaining serving as input for the
heterotrophic food chains is much more evenly distributed and ranges
from about 7 to 15 MJ m-2 yr-l. ANPPoand ANPPactare the aboveground
NPP of the potential vegetation and the actual vegetation, respectively.
@RoyalSwedishAcademyof Sciences1997
Arnbio Vol. 26 No. 3. Mav 1997
12.6%of the Austrianterritory is above1800m elevation,where
no NPP appropriationwas assumedto occur. In other countries
agriculturalareas,roadsandbuildingsarelikely to covera much
higher percentageof the surface.
Theseresultsraise the questionof which effectshumanNPP
appropriationmay haveon naturalecosystems.
Obviously,it significantly altersthe energyflow of natural ecosystemsand thus
may be seenasan indicator für the intensity of humaninterventions into natural ecosystemprocesses(27). But what do we
know aboutits likely effectson the structureand functioning of
ecosystems?
If we follow the argumentsof Hutchinsonin bis famouspaper "Homageto SantaRosalia,or why are there so many kinds
of animals?" (31), we may suspectthat areduction of energy
flow is likely to causeareduction of the length of food chains.
His argumentwas that sincelessthan 10% of the energyavailahle at the level n of a food chain can be gatheredat the level
n + 1, food chainscannotbe very lang.
Although it appearsto be clear that the length of food chains
is ultimately constrainedby the amountof energy available,it
may weIl be that in many casesother factors-body size,stability of food chains,etc.-may be more important. Until now,
empirical studiesfailed to produceunequivocalresults on this
matter.While Briand and Cohenfound no correlationbetween
energyflow andfood chainlengthin an analysisof 38 food wehs
(32), Yodzis could show that ectothermfood chainsare significantly longerthanendothermfood chains(33). Sinceectotherms
convertfood to secondaryproductionmuchmoreefficiently than
endothermsthis result supportsthe energytheory of food chain
length regulation. In addition, only recently an experimental
study hagshownthe importanceof energyavailability on food
chain length (34). In model calculations,Oksanenshowedthat
the amountof energyavailableper unit areagreatly influences
food chain structure,as rar as vertebralesare concemed:According to the resultsof his models,big grazerswill dominate
ecosystemswith an averageabovegroundproductivity below
approximately 12 MJ m-2yr-l, while complex food chains in
wbich the herbivoresareregulatedby carnivores,andhavemuch
lower populationdensity,prevail in richer habitats(35).
If it is true that a reductionof energyflow reducesthe length
of food chains,then a secondassertionof Hutchinson(31) may
also prove correct,namely that the amountof energyavailable
exertsan importantinfluenceon speciesdiversity.In the last two
decadesthis ideaexperienceda renaissance
asthe so-calledspecies-energytheory of biodiversity (4, 36-38). In short, the species-energytheorypredictsthat the numberof specieswhich can
inhabit a certain environmentincreaseswith the amountof energy available;conversely,the numberof specieswill decrease,
if energy flow is reduced (Fig. 3). The rationale behind this
E2
E1
E energy flow (e.g. NPP)
Figure 3. Species-energy curves demonstrate the relation between
energy flow (E) and species richness (N). If energy flow is reduced,
species-energy theory predicts a reduction of species richness.
Ambio Val. 26 No. 3. May 1997
theory is that in habitats with abundant resources rivaling species will be ahle to specialize with respect to more gradients and
thus can avoid extinction due to Gauses principle of competition exclusion (36). While in poor habitats there are few,
generalistic species, in resource-rich habitats many specialists
prevail. ODe reason für this are the "costs of cpmmonness", i.e.
negative effects of high population densities, e.g. parasitism,
pests, specialized predators (38).
The species-energy theory has been shown to be an extension
of species-areatheory (37), which relies on the theory of islandbiogeography by MacArthur and Wilson (39). The core of this
theory is that the number of species on an island is a steady state
between immigration (and speciation) and extinction, and the
bigger the island, the greater a number of species it is ahle to
support. Species-energy theory claims that this assertion can be
explained by the fact that ceteris paribus bigger islands provide
more energy, and predicts that among islands of the same size
more productive ones will support a higher number of species
(4). The species-energy theory is not only ahle to explain the
gradient of species diversity from the poles to the equator, hut
has also been empirically tested and verified (38, 40--42).
Even if the species-energy theory may be an innovative approach in biodiversity research, it is currently not generally accepted. For example, it cannot explain the "paradox of enrichment" (43); i.e. the observation that nutrient-rich (and thus more
productive) habitats may have lower species diversity than less
fertilized ones, a phenomenon Tilman has explained with a
microeconomic model (44). Since such a model fails to explain
the big biogeographical biodiversity gradient from the poles to
the equator, however, it cannot claim to be the unique theory of
biodiversity. In general, it is likely that the explanation of
biodiversity patterns requires more than ODescientific approach.
As rar as the Austrian data are concerned, my results are consistent with the species-energy theory. If the properties of the
species-energy curve ofWright (4) are assumed, species-energy
theory predicts that in Austria between 5 and 13% of the species should have gone extinct up to now. Actual surveys show
that 8% of the bird species, 7-14% of the reptiles (but no amphibians) have gone extinct in Austria (45, 46). However, since
this may just be coincidental, it is not a very strong argument in
favor of the species-energy theory. But it does ascertain that the
theory does not contradict the data. Further research, based on
the data now available, which have a high spatial resolution, will
be directed towards an attempt to explain biodiversity patterns
with variations of available energy (ANPPJ.
CONCLUSIONS REGARDING SUST AINABLE
DEVELOPMENT
Even if the effect of NPP appropriationon biodiversity is yet
unproven,it is obviousthat the currentlevel of NPP appropriation constitutesa significantinterventioninto the naturalenergy
flow of ecosystems.The potential effects this interferencemay
have,and which arecurrentlynot weIl understood,demandthat
NPP appropriationshould be regardedas an important indicator für pressureson the environment.Sincethe NPP of a natural ecosystemappearsto be an insurmountablelimit (globally
asweIl asat the locallevel), this indicator shouldbe considered
asa coreparameterfür sustainabledevelopment(47).
Many strategiesfür sustainabledevelopmentin the energysector seekto promotethe substitutionof fossil fuels with biomass
(48). Most of these strategies,however, imply an increaseof
biomassharvestand thus are likely to contributeto an increase
of NPP appropriation,e.g. an increaseof firewood combustion
für heatingpurposes,and thus could threatenbiodiversity.As a
consequence,
target conflicts may exist betweenCO2-reduction
and the conservationof biodiversity, which are both important
aspectsof sustainabledevelopment.Moreover,the calculations
@RoyalSwedishAcademyofSciences1997
145
presentedabove show that the idea of a simple substitutionof
fossil fuels with biomasswould not work, at least in Austria,
simply becausethe total energyinput of 1680PI yr-1(including
biomass für nutrition) of Austria already is higher than the
ANPPact
(1396PI yr-1 (49).
Which strategiesmay be envisagedto avoidthis potentialconflict? One possible part of a solution is the cascadeuse of
biomass.This meansthat wastebiomassshouldbe usedfür energy generationinsteadof harvestingmore biomass.This would
permit an increaseof biomassuse für energygenerationwithout augmentingNPP appropriation.Oneexampleis the production of biogas from wet biomass waste, e.g. animal manure.
Potentials für the generationof additional energy from used
biomassshouldbe systematicallyinvestigated.
Anotherstrategy,which hasthe advantageof alleviatingother
ecologicalproblemsasweIl, is basedon the observationthat the
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50. Für help, advice,anddiscussions1amindebtedto W. Bittermann,M. Fischer-Kowaiski,
B. Hammer,W. Hüttler, W. Loibl, H. Payer,H. Schandl,V. Winiwarter, H. ZangerlWeisz,andtwo anonymousreferees.
51. First submitted28 November 1995.Acceptedfür publication after revision 19 June
1996.
Helmut Haberl, PhD, is currently with the Austrian Institute
of Applied Ecological Research and at the Interdisciplinary
Institute of Research and Continuing Education (IFF),
Department of Social Ecology. He works on energy and the
environment, environmental indicators, societal metabolism
and the colonization of nature, and sustainable
development. His address: IFF, Social Ecology,
P.O. Box 232, Seidengasse 13, A-1070 Vienna, Austria,
e-mail: [email protected]
@ Royal Swedish Academy of Sciences 1997
Arnbio Vol. 26 No. 3. May 1997