Cumulatively, these ubiquitous small-predator
trophic cascades can have enormous impacts on a
wide variety of ecological functions, including food
production. For example, arthropod pests are responsible for 8 to 15% of the losses in most major
food crops. Without natural biological control,
this value could increase up to 37% (57). In the
United States alone, the value of pest control by
native predators is estimated at $4.5 billion annually (58).
Nutrient cycling and decomposition
The diversity of invertebrate communities, particularly their functional diversity, can have
dramatic impacts on decomposition rates and
nutrient cycling (59–61). Declines in mobile species that move nutrients long distances have
been shown to greatly affect patterns of nutrient
distribution and cycling (62). Among large animals, Pleistocene extinctions are thought to have
changed influx of the major limiting nutrient,
phosphorus, in the Amazon by ~98%, with implications persisting today (3).
Water quality
Defaunation can also affect water quality and
dynamics of freshwater systems. For instance,
global declines in amphibian populations increase algae and fine detritus biomass, reduce
nitrogen uptake, and greatly reduce whole-stream respiration (Fig. 5E) (63). Large animals,
including ungulates, hippos, and crocodiles,
prevent formation of anoxic zones through
agitation and affect water movement through
trampling (64).
Human health
Defaunation will affect human health in many
other ways via reductions in ecosystem goods
and services (65), including pharmaceutical com-
pounds, livestock species, biocontrol agents, food
resources, and disease regulation. Between 23
and 36% of all birds, mammals, and amphibians
used for food or medicine are now threatened
with extinction (14). In many parts of the world,
wild-animal food sources are a critical part of the
diet, particularly for the poor. One recent study
in Madagascar suggested that loss of wildlife as a
food source will increase anemia by 30%, leading
to increased mortality, morbidity, and learning
difficulties (66). However, although some level of
bushmeat extraction may be a sustainable ser-
vice, current levels are clearly untenable (67); ver-
tebrate populations used for food are estimated
to have declined by at least 15% since 1970 (14). As
previously detailed, food production may decline
because of reduced pollination, seed dispersal,
and insect predation. For example, loss of pest
control from ongoing bat declines in North Amer-
ica are predicted to cause more than $22 billion
in lost agricultural productivity (68). Defaunation
can also affect disease transmission in myriad
ways, including by changing the abundance, be-
havior, and competence of hosts (69). Several
studies demonstrate increases in disease preva-
lence after defaunation (41, 42, 70). However, the
impacts of defaunation on disease are far from
straightforward (71), and few major human patho-
gens seem to fit the criteria that would make
such a relationship pervasive (71). More work is
Cont
B-R B-A C F
FP
Fi Di C-D
C-W AMF Ph Tr
AD TP NP SL
He
GI RS T
Exc
A
B
C FP
AD He
NP
SL
Di
Fi
C-W
AMF
Large wildlife
removal
Cascades to other consumers
Functions and services
Plant-animal interactions
30
0
0.5
1.0
0
-0.5
0.5
0
-5
0
1
-4
Ef
ec
t
si
ze
(I
n
(E/C)
Efe
ct
siz
e
(I
n(
E/C)
Efe
c
t
si
ze
(I
n
(E/C)
N
u
mb
e
r
d
u
ng
pil
es
Fig. 4. Results of experimental manipulation simulating differential
defaunation. As a model of the pervasive ecosystem effects of defaunation,
in just one site (the Kenya Long Term Exclosure Experiment), the effects of
selective large-wildlife (species >15 kg) removal drive strong cascading consequences on other taxa, on interactions, and on ecosystem services (81).
(A) In this experiment, large wildlife are effectively removed by fences, as
evidenced by mean difference in dung abundance (T1 SE) between control
and exclosure plots. (B) This removal leads to changes in the abundance or
diversity of other consumer groups. Effects were positive for most of these
small-bodied consumers—including birds (B-R, bird species richness; B-A,
granivorous bird abundance), Coleoptera (C), fleas (F), geckos (G), insect
biomass (I), rodents (R), and snakes (S)—but negative for ticks (T). (C)
Experimental defaunation also affects plant-animal interactions, notably
altering the mutualism between ants and the dominant tree, Acacia
drepanolobium and driving changes in fruit production (FP), ant defense by
some species ( AD), herbivory of shoots (He), thorn production ( TP), nectary
production (NP), and spine length (SL). (D) Large-wildlife removal also
causes major effects on ecosystem functions and services, including changes
to fire intensity (Fi), cattle production in both dry (C-D) and wet (C-W)
seasons, disease prevalence (D), infectivity of arbuscular mycorrhizae fungi
(AMF), photosynthetic rates (Ph), and transpiration rates ( TR). Data in (B) to
(D) are effect size [ln(exclosure metric/control metric)] after large-wildlife
removal. Although this experiment includes multiple treatments, these
results represent effects of full exclosure treatments; details on treatments
and metrics are provided in table S3. [Photo credits: T. Palmer, H. Young,
R. Sensenig, and L. Basson]
