Geographic shifts in the effects of habitat size on trophic structure and decomposition

Abstract

Habitat size is known to affect community structure and ecosystem function, but few studies have examined the underlying mechanisms over sufficient size gradients or in enough geographic contexts to determine their generality. Our goal in this study was to determine if the relationship between habitat size and leaf decomposition varied across geographic sites, and which factors may be driving the differences. We conducted replicated observations in a coastal forest in Brazil, and in rainforests in Costa Rica and Puerto Rico. We used leaf litter decomposition and macroinvertebrate composition in bromeliad phytotelmata of varying sizes to determine the relationships between habitat size, trophic structure and decomposition over a wide geographical range. We experimentally disentangled the effects of site and litter quality by quantifying invertebrate control of decomposition of a native and a transplanted litter type within one site. We found that the relationship between bromeliad size and decomposition rates differed among study sites. In rainforests in Costa Rica and Puerto Rico, decomposition was strongly linked to macroinvertebrate trophic structure, which varies with bromeliad size, driving strong bromeliad size‐decomposition relationships. However, in Brazil there was no relationship between bromeliad size and decomposition. Our manipulative experiment suggests that within coastal forest in Brazil, the poor quality of native litter resulted in little invertebrate control of decomposition. Furthermore, the key detritivore in this site builds a predator‐resistant case, which likely prevented effects of bromeliad size on trophic structure from being transmitted to decomposition even when litter quality was increased. We conclude that differences in both leaf litter quality and macroinvertebrate traits among sites determine the link between decomposition and macroinvertebrates, and consequently the decomposition‐bromeliad size relationship. These results show that the response of decomposition to habitat size is context‐dependent, and depends on which component of the food web is the main driver of the function.

Biogeographic context dependence of trophic cascade strength in bromeliad food webs

“…A similar pattern was observed in in situ decomposition with litter bags in unmanipulated bromeliads, using the same three litter species in the same three sites: detritivores processed litter faster in Costa Rica and Puerto Rico than in Brazil (LeCraw et al. ). The weak influence of macroinvertebrates on decomposition in the Brazil site may be due to low quality of the dominant Myrtaceae leaf litter.…”

“…A recent experiment in the same Brazilian site as the current study has shown that the low‐quality Myrtaceae leaf litter native to the Brazilian restinga forest inhibited decomposition by macroinvertebrates, whereas higher quality Melastomataceae litter imported from the nearby Atlantic Rainforest in Brazil was decomposed faster by macroinvertebrates (LeCraw et al. ). Together, these studies suggest that the low quality of leaf litter at the Brazilian site impedes macroinvertebrate consumption of detritus.…”

“…We demonstrated that the Myrtaceae litter decomposed more slowly than Melastomataceae litter when placed in mesh bags in bromeliads in our Brazil study site (LeCraw et al. ).…”

“…Although we did not attempt to disentangle the effects of litter identity from site environment here, a companion study does isolate litter identity effects by holding site and invertebrate community constant (LeCraw et al. ).…”

Leaf litter traits drive community structure and functioning in a natural aquatic microcosm

“…In addition, our results provide novel evidence that these effects of litter species can be understood in terms of the physicochemical traits of the litter. Such changes in litter composition are very relevant for bromeliads and other phytotelmata, which are widespread over several vegetation types (LeCraw et al., ; Srivastava et al., ). Although these results were obtained using bromeliads as natural microecosystems, they might not be restricted to these systems.…”