Does invasion by armored catfish shift trophic ecology of native fishes? Evidence from stable isotope analysis.

Popular as aquarium fish, armored catfishes from South America (Pterygoplichthys spp.) have been introduced and become invasive in tropical and subtropical regions worldwide. These ecosystem engineers can deplete basal resources (e.g., periphyton and detritus), with potential negative effects for native fauna. We studied the trophic ecology of fishes in the Usumacinta River Basin, Guatemala, where Pterygoplichthys is now widespread and locally abundant. We analyzed stable isotopes (δ13 C, δ15 N) in fish tissues and basal resources to assess the potential impact of Pterygoplichthys on the trophic ecology of six co-occurring native fishes that feed at a similar trophic level (Astyanax aeneus, Dorosoma petenense, Thorichthys pasionis, Oscura heterospila, Poecilia mexicana, and Gambusia sexradiata). The study was conducted during the dry season in the La Pasion (LPR; high invasion) and San Pedro (SPR; low invasion) rivers. We compared isotopic spaces occupied by native fish and Pterygoplichthys, estimated isotopic overlap, and evaluated the trophic displacement of native species. We also evaluated the relationships of environmental factors, including the relative biomass of the invasive catfish, with δ13 C and δ15 N signatures. Except for P. mexicana, native species had lower isotopic overlap with the catfish in LPR. Native fish isotopic spaces were compressed and shifted toward higher trophic positions in LPR relative to SPR. Benthic food resources were important for Pterygoplichthys in both rivers, and water-column resources had greater relative importance (RI) for native species in LPR. Native fish δ13 C was significantly associated with Pterygoplichthys biomass, conductivity, and water flow velocity; and water depth and sedimentation had a significant association with native fish δ15 N. Findings provide evidence that invasive Pterygoplichthys, along with environmental factors, impact the trophic ecology of native fish in the Usumacinta Basin. Additional field research conducted over longer time periods and mesocosm experiments that account for fish assemblage and environmental variation could elucidate Pterygoplichthys impacts via food resource depletion or habitat alteration.

Populares como peces de acuario, los plecóstomos de América del Sur (Pterygoplichthys spp.) han sido introducidos y se han vuelto invasivos en regiones tropicales y subtropicales de todo el mundo. Estos ingenieros ecosistémicos pueden agotar los recursos basales (p. ej., perifiton y detritos), con posibles efectos negativos para la fauna nativa. En esta investigación estudiamos la ecología trófica de los peces en la cuenca del río Usumacinta, Guatemala, donde Pterygoplichthys se ha dispersado y es localmente abundante. Analizamos isótopos estables (δ13 C, δ15 N) en tejidos de peces y recursos basales para evaluar el impacto potencial de Pterygoplichthys en la ecología trófica de seis peces nativos que se alimentan a un nivel trófico similar (Astyanax aeneus, Dorosoma petenense, Thorichthys pasionis, Oscura heterospila, Poecilia mexicana y Gambusia sexradiata). El estudio se realizó en la temporada seca en los ríos La Pasión (LPR; alta invasión) y San Pedro (SPR; baja invasión). Comparamos el espacio isotópico ocupado por peces nativos y Pterygoplichthys, estimamos la superposición isotópica y evaluamos el desplazamiento trófico de las especies nativas. También evaluamos las relaciones de los factores ambientales, incluida la biomasa relativa del plecóstomo, con las señales isotópicas δ13 C y δ15 N. Con excepción de P. mexicana, las especies nativas tuvieron una menor superposición isotópica con el plecóstomo en LPR. En peces nativos se observó compresión y desplazamiento de espacios isotópicos hacia posiciones tróficas más altas en LPR en relación con SPR. Los recursos alimenticios bentónicos fueron importantes para Pterygoplichthys en ambos ríos, y los recursos de la columna de agua tuvieron mayor importancia relativa para las especies nativas en LPR. Los valores de δ13 C en peces nativos están significativamente asociados con la biomasa de Pterygoplichthys, la conductividad y la velocidad del flujo de agua; la profundidad del río y la sedimentación tuvieron una asociación significativa con los valores de δ15 N de peces nativos. Los hallazgos proporcionan evidencia de que Pterygoplichthys, junto con factores ambientales, impactan la ecología trófica de los peces nativos en la cuenca del Usumacinta. Investigación de campo adicional durante períodos de tiempo prolongados y experimentos de mesocosmos que integren dinámicas de ensamblaje de peces y variación ambiental podrían dilucidar los impactos de Pterygoplichthys a través del agotamiento de los recursos alimentarios o la alteración del hábitat.

Incorporating indirect pathways in body size-trophic position relationships.

Body size, trophic position (TP), and trophic niche width are important elements of food webs; however, there is still debate regarding their interrelationships. Most studies have tested these correlations using datasets restricted to carnivores and bivariate models that disregard potential indirect effects of other factors, their interactions, and phylogeny. We analyzed relationships among TP, consumer size, maximum food item size, food item size variation (a proxy for trophic niche width), and two other traits (gut length and mouth width) using confirmatory path analysis of an extensive dataset for freshwater fishes that encompass both carnivorous and non-carnivorous species. Consumer size was associated with maximum food size, food size variation, mouth width, and gut length, all of which mediated indirect relationships between body size and TP. Mouth gape was associated with maximum food size, and consumers that fed on larger food items had higher TP. Consumers with relatively long guts generally fed on small and homogeneous food items near the base of the food web. Models were consistent whether or not accounting for phylogeny, but varied according to trophic guilds. However, the body size of both carnivorous and non-carnivorous was not directly associated with TP. Therefore, the incorporation of functional traits and their intermediate pathways is critical for understanding size-based trophic relationships of animals that encompass diverse feeding strategies. Our results caution approaches that rely on body size as a surrogate for TP, especially in systems where plants and detritus are consumed directly by a significant number of animals, such as in most freshwater ecosystems.

Is There a Relationship between Fish Cannibalism and Latitude or Species Richness?

Cannibalism has been commonly observed in fish from northern and alpine regions and less frequently reported for subtropical and tropical fish in more diverse communities. Assuming all else being equal, cannibalism should be more common in communities with lower species richness because the probability of encountering conspecific versus heterospecific prey would be higher. A global dataset was compiled to determine if cannibalism occurrence is associated with species richness and latitude. Cannibalism occurrence, local species richness and latitude were recorded for 4,100 populations of 2,314 teleost fish species. Relationships between cannibalism, species richness and latitude were evaluated using generalized linear mixed models. Species richness was an important predictor of cannibalism, with occurrences more frequently reported for assemblages containing fewer species. Cannibalism was positively related with latitude for both marine and freshwater ecosystems in the Northern Hemisphere, but not in the Southern Hemisphere. The regression slope for the relationship was steeper for freshwater than marine fishes. In general, cannibalism is more frequent in communities with lower species richness, and the relationship between cannibalism and latitude is stronger in the Northern Hemisphere. In the Southern Hemisphere, weaker latitudinal gradients of fish species richness may account for the weak relationship between cannibalism and latitude. Cannibalism may be more common in freshwater than marine systems because freshwater habitats tend to be smaller and more closed to dispersal. Cannibalism should have greatest potential to influence fish population dynamics in freshwater systems at high northern latitudes.