Trophic cascade within and across ecosystems: the role of anti-predatory defenses, predator type, and detritus quality

Species in one ecosystem can indirectly affect multiple biodiversity
components and ecosystem functions of adjacent ecosystems. The magnitude
of these cross-ecosystem effects depends on the attributes of the
organisms involved in the interactions, including traits of the predator,
prey and basal resource. However, it is unclear how predators with
cross-ecosystem habitat interact with predators with single-ecosystem
habitat to affect their shared ecosystem. Also, unknown is how such
complex top-down effects may be mediated by the anti-predatory traits of
prey and quality of the basal resource. We used the aquatic invertebrate
food webs in tank bromeliads as a model system to investigate these
questions. We manipulated the presence of a strictly aquatic predator
(damselfly larvae) and a predator with both terrestrial and aquatic
habitats (spider) and examined effects on survival of prey (detritivores
grouped by anti-predator defense), detrital decomposition (of two plant
species differing in litter quality), nitrogen flux and host plant growth.
To evaluate the direct and indirect effects of each predator type on
multiple detritivore groups and ultimately on multiple ecosystem
processes, we used piecewise structural equation models. For each response
variable, we isolated the contribution of different detritivore groups to
overall effects by comparing alternate model formulations. Alone,
damselfly larvae and spiders each directly decreased survival of
detritivores and caused multiple indirect negative effects on detritus
decomposition, nutrient cycling, and host plant growth. However, when
predators co-occurred, the spider caused a negative non-consumptive effect
on the damselfly larva, diminishing the net direct and indirect top-down
effects on the aquatic detritivore community and ecosystem functioning.
Both detritivore traits and detritus quality modulated the strength and
mechanism of these trophic cascades. Predator interference was mediated by
undefended or partially defended detritivores as detritivores with
anti-predatory defenses evaded consumption by damselfly larvae but not
spiders. Predators and detritivores affected ecosystem decomposition and
nutrient cycling only in the presence of high-quality detritus, as the
low-quality detritus was consumed more by microbes than invertebrates. The
complex responses of this system to predators from both recipient and
adjacent ecosystems highlight the critical role of maintaining
biodiversity components across multiple ecosystems.