Differential habitat use and recruitment facilitate coexistence in a community with intraguild predation.

Theory predicts that species engaged in intraguild predation (IGP) can only coexist under limited conditions, yet IGP is common in nature. Habitat complexity can promote coexistence by reducing encounter rates, but little information is known about the contribution of differential habitat use. We hypothesized that differential use of alternative habitats promotes coexistence of an intraguild (IG) predator and prey. We evaluated predictions of this hypothesis with an experimental introduction of an IG predator fish into four natural stream communities that previously contained only the IG prey fish. We monitored the development of this IGP over the course of four years to determine how each species used alternative stream habitats. The introduced species preferred pool habitats while the resident species was more evenly distributed across pools and riffles. The density of the resident decreased in the pool habitat preferred by the invader, accompanied by a local increase in the mean of the resident size distribution. Selective predation by the invader on hatchling residents appears to impact the residents' demographic response. The continued recruitment of resident juveniles in riffles, where the introduced species is rare, facilitated the persistence of the resident. This differential use of habitats was not accompanied by a change in the resident's growth rates in either habitat. Our results showed that differential habitat selection and recruitment promoted persistence during an invasion involving IGP, which helps to bridge the gap between theory and observation in explaining coexistence in IGP systems.

Fish introductions and light modulate food web fluxes in tropical streams: a whole-ecosystem experimental approach.

Decades of ecological study have demonstrated the importance of top-down and bottom-up controls on food webs, yet few studies within this context have quantified the magnitude of energy and material fluxes at the whole-ecosystem scale. We examined top-down and bottom-up effects on food web fluxes using a field experiment that manipulated the presence of a consumer, the Trinidadian guppy Poecilia reticulata, and the production of basal resources by thinning the riparian forest canopy to increase incident light. To gauge the effects of these reach-scale manipulations on food web fluxes, we used a nitrogen (15 N) stable isotope tracer to compare basal resource treatments (thinned canopy vs. control) and consumer treatments (guppy introduction vs. control). The thinned canopy stream had higher primary production than the natural canopy control, leading to increased N fluxes to invertebrates that feed on benthic biofilms (grazers), fine benthic organic matter (collector-gatherers), and organic particles suspended in the water column (filter feeders). Stream reaches with guppies also had higher primary productivity and higher N fluxes to grazers and filter feeders. In contrast, N fluxes to collector-gatherers were reduced in guppy introduction reaches relative to upstream controls. N fluxes to leaf-shredding invertebrates, predatory invertebrates, and the other fish species present (Hart's killifish, Anablepsoides hartii) did not differ across light or guppy treatments, suggesting that effects on detritus-based linkages and upper trophic levels were not as strong. Effect sizes of guppy and canopy treatments on N flux rates were similar for most taxa, though guppy effects were the strongest for filter feeding invertebrates while canopy effects were the strongest for collector-gatherer invertebrates. Combined, these results extend previous knowledge about top-down and bottom-up controls on ecosystems by providing experimental, reach-scale evidence that both pathways can act simultaneously and have equally strong influence on nutrient fluxes from inorganic pools through primary consumers.

Experimental evaluation of predation as a facilitator of invasion success in a stream fish.

Predator-prey relationships in poikilotherms are often size dependent, such as when adults of two interacting species are capable of eating juveniles of the other species. Such bi-directional predation can be important during the establishment and spread of an invading species, but its role remains poorly understood. Using a combination of laboratory and mesocosm experiments and field introductions, we demonstrate that guppies, Poecilia reticulata, prey on juvenile killifish, Rivulus hartii, and thereby facilitate their establishment in the habitat of a potential predator. Laboratory studies found that mature guppies can consume larval Rivulus, and experimental stream studies showed that guppies reduced the number of Rivulus surviving from eggs. Growth trials found that interspecific competition, while significant, cannot account for the declines in the survival of juvenile Rivulus seen in field surveys. Finally, a field experiment, in which guppies were introduced into previously guppy-free stream reaches, resulted in a marked reduction in the abundance of juvenile Rivulus relative to guppy-free controls. Together, these results indicate that reducing the native Rivulus population represents an important.mechanism promoting guppy invasion success.

Annotated list and key to the stream fishes of Trinidad & Tobago.

Based on historical and museum records and recent extensive collecting we compiled a checklist of 77 fish species reported from the streams of Trinidad and Tobago. A key with photographs is provided to aid in identifications, as well as brief notes on habitat, diet, reproduction, maximum size, local common names and distribution.