Question

1. As described in Canadian Research 53.1, Ngai and Srivastava investigated the effects of predators on nutrient cycling by studying the detritus-based insect community in bromeliads in a Costa Rica rainforest. Discuss Ngai and Srivastava’s experiment and their findings, including the following:
   1. Explain why a single bromeliad plant can be considered an ecosystem.
   2. State the hypothesis that Ngai and Srivastava tested, briefly discuss their methods, and include a graph and brief explanation of their results.
   3. Explain Ngai and Srivastava’s statement, “[Our experiment] demonstrates that predation can have counterintuitive effects of nutrient cycling.”

Answer 1

a.  The Bromeliaceae family provides a diverse array of ecosystem services. The most important services include the maintenance of taxonomic and genetic diversity, provisioning of chemical and pharmaceutical products, food and fiber, traditional knowledge, aesthetic appreciation, cultural heritage, climate control, disease control, and water storage. Bromeliads support high biodiversity by providing resources and serving as microhabitats for other species. Birds and mammals feed on bromeliads or consume the water that they retain.

b.  The availability of nutrients in ecosystems is
determined by resource supply and re-
cycling rates and affects important eco-
system properties (1–3). The relative roles of
abiotic supply and food web configuration in
determining resource-processing rates remain
contentious and poorly understood. Under anthro-
pogenic pressure, ecosystems are predicted to lose
predators disproportionately, affecting ecosystem
processes (4). Current ecological theory predicts
that predator loss will affect nutrient cycling by
changing prey abundance (density-mediated
effects, as in a trophic cascade) (5) or prey foraging
efficiency (trait-mediated effects) (6). These
changes can further affect nutrient cycling by
altering the species composition or size structure
of the prey community. In this study, we examined
the effects of predators on nutrient cycling by
using the detritus-based insect community in
bromeliads. We demonstrate that predation can
have counterintuitive effects on nutrient cycling.
Leaves of tank-forming bromeliads (e.g.,
Vriesea and Guzmania genera) are tightly
interlocking, forming wells that collect water
and leaf litter and provide habitat for aquatic
insect larvae. The detritus not only supports
the insect community but also provides a
source of nutrients for the bromeliad. A natural
gradient also exists in predation where the ma-
jor predator, a damself ly larva (Mecistogaster
modesta), becomes moreabundant as the plant
grows. Although it has been hypothesized that
aquatic insects increase nutrient flux to the bro-
meliad, this relation- ship has never been
documented. First, we ran fertiliza-
tion experiments to deter- mine whether nitrogen
(N) or phosphorus (P) limit the productivity
of the plant and insect Components of this eco-
system (7). Both tissue nutrient ratios and fertilization experiments showed that N, rather than P, primarily limits productivity of bromeliads and can limit insect
productivity [Supporting Online Material (SOM)
text and tables S1 and S2], so we focused on the
effects of trophic structure on N cycling. Leaf
detritus enriched in 15N was used to trace the
movement of N through the food web in bro-
meliads containing either no insects, detritivores
only, or detritivores and predators.
The presence of detritivores alone did not
affect the amount of N entering bromeliads from
the enriched detritus (Fig. 1A). However, in the
presence of both detritivores and predators, there
was a significant enrichment in 15N in bromeliad
leaves compared with plants containing detri-
tivores alone, indicating that the presence of
predators increased the flow of N from litter to
bromeliads. This is surprising given that previous
studies, consistent with the predictions of density-
or trait-mediated effects, have shown that preda-
tors decrease litter decomposition by reducing
detritivore abundance (8) or by decreasing the
foraging rate (9) of detritivorous arthropods.
We hypothesize that the detritivorous insects,
which pupate relatively rapidly, constitute a loss
of litter-derived N for bromeliads when they
emerge. A survey indicated that detritivorous in-
sects generally have higher N:P ratios than those
found in typical litter (Fig. 1B), suggesting that,
as leaf litter is consumed, the insects will preferentially retain N in their body tissues and
release P. Predation by longer-lived damselfly
larvae converts the mobile pool of N contained
in detritivores into fecal pellets that can be de-
composed by microbes or leached to release N in
a form available to the bromeliad. Thus, insects
facilitate nutrient uptake by the plant, but only if
both predators and detritivores are present.
These results emphasize the importance of the
temporal and spatial scales of dispersal for nu-
trient flux. The emergence of adult insects means
that, although detritivores increase resource flux
over larval time scales by releasing nutrients from
litter, these insects act as a nutrient sink for
bromeliads over their entire life span. The faster
emergence rate of detritivores compared with that
of predators allows predation to reduce the loss of
N from the bromeliad. Although we use insects
in bromeliads to examine biotic effects on nu-
trient cycling, our results can give insights into
other systems where mobility differs between
trophic levels. Some trophic interactions, for in-
stance, involve migratory and nonmigratory spe-
cies or species that undergo ontogenetic niche
shifts. This mechanism may also apply if the
prey species has a very different range size than
its predator. Given the increased extinction risk
of higher trophic levels, understanding the
mechanisms whereby predators drive important
ecosystem processes is critical in predicting an-
thropogenic impacts on natural systems.
References and Notes
1. R. Aerts, F. S. Chapin, Adv. Ecol. Res. 30, 1 (2000

(A) d15N in new bromeliad leaves for plants containing no insects,detritivore insects only, or detritivore and predatory insects (mean ± SEM).
Bonferroni-corrected t test (detritivores alone versus control,z = 0.478 and P =0.63; detritivores plus predators versus detritivores alone, z = 2.36 and P =
0.018). (B) Comparison of N:P ratios (by atom) for detritivore larvae and for leaf litter (mean ± SEM). Larger detritivores (chironomid A, scirtids, and
tipulids) have N:P ratios higher than that of leaf litter [F1, 20 = 5.05, P = 0.04 for linear contrast following significant analysis of variance (F4,20 = 3.66,
P = 0.02)]. Chironomid B is a smaller detritivore that accounts for only a small proportion of detritivore biomass in bromeliads.

c.  Conventional ecological theory predicts that predators affect nutrient cycling by decreasing the abundance or activity of prey. By using a predator-detritivore-detritus food chain in bromeliads, we show that predators can increase nutrient cycling by a previously undescribed, but broadly applicable, mechanism: reducing nutrient export by prey emigration. Contrary to expectations, predation on detritivores increases detrital nitrogen uptake by bromeliads. Predation reduces detritivore emergence and hence export of nitrogen from the system. Detritivores therefore benefit their host plant, but only when predators are present. More generally, our results show that predator loss or extinction can dramatically and unexpectedly affect ecosystem functioning.

Leaves of tank-forming bromeliads (e.g., Vriesea and Guzmania genera) are tightly interlocking, forming wells that collect water and leaf litter and provide habitat for aquatic insect larvae. The detritus not only supports the insect community but also provides a source of nutrients for the bromeliad. A natural gradient also exists in predation where the major predator, a damselfly larva (Mecistogaster modesta), becomes more abundant as the plant grows. Although it has been hypothesized that aquatic insects increase nutrient flux to the bromeliad, this relationship has never been documented.