Prey diversity as a driver of resource partitioning between river-dwelling fish species

Feeding ecology and reproductive biology of small coastal sharks in Malaysian waters

Small coastal demersal sharks form a major proportion of the sharks landed in Malaysia. However, little is known about their feeding ecology and reproduction. This study sought to elucidate the dietary patterns, role of ontogeny in prey consumption, and reproductive biology of four dominant small demersal shark species in Malaysian waters: the Hasselt's bamboo shark, Chiloscyllium hasseltii; brownbanded bamboo shark, C. punctatum; spadenose shark, Scoliodon laticaudus; and Pacific spadenose shark, S. macrorhynchos. Dietary analyses revealed a high overlap in prey taxa consumed; clear resource partitioning among co-occurring species based on the percentage Prey-specific Index of Relative Importance (%PSIRI), with higher fish %PSIRI for Chiloscyllium hasseltii, higher cephalopod %PSIRI for C. punctatum, and higher crustacean %PSIRI for both Scoliodon species; and an ontogenetic diet shift, seen through changes in prey size. Based on the examination of reproductive organs, the results showed larger sizes at maturity for males compared to females for all four species; no obvious reproductive cycles, based on hepatosomatic and gonadosomatic indices for all species; female bias in the sex ratio of the embryos of Scoliodon species; and increased reproductive output (number of eggs or embryos and size of eggs) with larger female size for C. hasseltii and Scoliodon species. The partitioning of food resources minimizes direct competition for food and supports coexistence within shared coastal habitats. The reproductive strategies of these small coastal sharks appear to be favorable for supporting short-term population productivity; although a reduction in fishing pressure, especially from bottom trawlers, is essential for the long-term sustainable use of these sharks.

Trophic morphology features allow Astyanax endemic species coexistence in a Neotropical river system

Resource partitioning has an essential role in interspecific relations, especially in congener species, which share many morphological traits. In some places, small characids coexist through resource partitioning, which may reduce their interspecific competition. Astyanax species (e.g., Astyanax minor, Astyanax gymnodontus and Astyanax bifasciatus), for example, coexist in different water bodies from the Iguaçu River basin. These species have high phenotypic plasticity and many morphologic specializations that allow them to live in different habitats. Based on evidences that these species modified their feeding habits because of changes in resource availability in Iguaçu River, this study tested two hypotheses: (a) there are differences in head morphology, number of teeth and number of gill rakers among the species of Astyanax; and (b) there are differences in gill arch and gill raker morphology among the species of Astyanax, which may favour their trophic resource partitioning in Iguaçu River. The head morphological traits and quantitative morphological characters were summarized in a principal coordinates analysis (PCoA), and the analysis of similarities (ANOSIM) showed significant differences among species. Gill morphological measurements were analysed through analysis of covariance (ANCOVA), and it also showed significant differences in gill arch and gill raker morphology among species. Therefore, the analysis of ecomorphological traits related to trophic habits revealed some differences that may suggest a tendency of reducing competition for trophic resources in the Iguaçu River basin.

Co-occurring predators increase biotic resistance against an invasive prey

The presence of multiple predators can lead to variation in predator behavior and ultimately altered risk for shared prey. This concept has seldom been accounted for in studies that consider predator-driven biotic resistance from native marine predators against invasive prey. This study compared the prey selection of whelks and rock lobsters when co-occurring and when foraging in isolation. When in isolation, both predators preferred the native mussel Choromytilus meridionalis, regardless of the abundance of alternative prey. However, when co-occurring, predation risk for all prey species, including the invasive mussel Semimytilus algosus, increased. This was largely driven by greater variation in prey selection by rock lobsters in the presence of whelks. This indicates that predatory efforts from co-occurring predators can result in stronger predation pressure on invasive prey than would be recognized if predators were assessed in isolation.

Competition and resource breadth shape niche variation and overlap in multiple trophic dimensions

Competition plays a central role in the maintenance of biodiversity. A backbone of classic niche theory is that local coexistence of competitors is favoured by the contraction or divergence of species' niches. However, this effect should depend on the diversity of resources available in the local environment, particularly when resources vary in multiple ecological dimensions. Here, we investigated how available resource breadth (i.e. prey diversity) and competition together shape multidimensional niche variation (between and within individuals) and interspecific niche overlap in 42 populations of congeneric tropical frog species. We modelled realized niches in two key trophic dimensions (prey size and carbon stable isotopes) and sampled available food resources to quantify two-dimensional resource breadth. We found a 14-fold variation in multidimensional population niche width across populations, most of which was accounted for by within-individual diet variation. This striking variation was predicted by an interaction whereby individual niche breadth increased with resource breadth and decreased with the number of congeneric competitors. These ecological gradients also interact to influence the degree of niche overlap between species, which surprisingly decreased with population total niche width, providing novel insights on how similar species can coexist in local communities. Together, our results emphasize that patterns of exploitation of resources in multiple dimensions are driven by both competitive interactions and extrinsic factors such as local resource breadth.

Seasonal variations in food resource partitioning among four sympatric gudgeon species in the upper Yangtze River

Knowledge of food resource partitioning among sympatric fish species is crucial for understanding the potential mechanisms of species coexistence. Gudgeons (Teleostei: Cyprinidae: Gobioninae) often dominate fish assemblages in the upper Yangtze River. However, little research has been conducted on their trophic interactions. In this paper, seasonal diet and feeding strategy variations of four sympatric gudgeon species, Coreius guichenoti, Coreius heterodon, Rhinogobio ventralis, and Rhinogobio cylindricus, were investigated by analysis of intestinal tract contents, aiming to explore whether food resource partitioning occurred among them. Fish specimens were collected during spring (April-May) and autumn (August-October) in 2010 in Hejiang, a free-flowing stretch of the upper Yangtze River. Coreius guichenoti, C. heterodon, and R. cylindricus showed omnivorous feeding habits, while R. ventralis exhibited an obligate carnivore feeding habit. Diet overlap among the four studied species was high, especially in spring. However, changes in feeding strategies were observed in autumn. Specifically, C. guichenoti and R. cylindricus expanded their dietary niche breadth and consumed detritus, Sinopotamidae or Hydropsychidae as important complementary food resources. In contrast, C. heterodon and R. ventralis reduced their dietary niche breadth and became more specialized on mussels (Limnoperna lacustris). These results confirmed that sympatric fish species can coexist with high diet overlap, and food resource partitioning among these species may also fluctuate with the seasons.

18S rRNA metabarcoding diet analysis of a predatory fish community across seasonal changes in prey availability

Predator-prey relationships are important ecological interactions, affecting biotic community composition and energy flow through a system, and are of interest to ecologists and managers. Morphological diet analysis has been the primary method used to quantify the diets of predators, but emerging molecular techniques using genetic data can provide more accurate estimates of relative diet composition. This study used sequences from the 18S V9 rRNA barcoding region to identify prey items in the gastrointestinal (GI) tracts of predatory fishes. Predator GI samples were taken from the Black River, Cheboygan Co., MI, USA (n = 367 samples, 12 predator species) during periods of high prey availability, including the larval stage of regionally threatened lake sturgeon (Acipenser fulvescens Rafinesque 1817) in late May/early June of 2015 and of relatively lower prey availability in early July of 2015. DNA was extracted and sequenced from 355 samples (96.7%), and prey DNA was identified in 286 of the 355 samples (80.6%). Prey were grouped into 33 ecologically significant taxonomic groups based on the lowest taxonomic level sequences that could be identified using sequences available on GenBank. Changes in the makeup of diet composition, dietary overlap, and predator preference were analyzed comparing the periods of high and low prey abundance. Some predator species exhibited significant seasonal changes in diet composition. Dietary overlap was slightly but significantly higher during the period of high prey abundance; however, there was little change in predator preference. This suggests that change in prey availability was the driving factor in changing predator diet composition and dietary overlap. This study demonstrates the utility of molecular diet analysis and how temporal variability in community composition adds complexity to predator-prey interactions.

Prey diversity as a driver of resource partitioning between river-dwelling fish species

Although food resource partitioning among sympatric species has often been explored in riverine systems, the potential influence of prey diversity on resource partitioning is little known. Using empirical data, we modeled food resource partitioning (assessed as dietary overlap) of coexisting juvenile Atlantic salmon (Salmo salar) and alpine bullhead (Cottus poecilopus). Explanatory variables incorporated into the model were fish abundance, benthic prey diversity and abundance, and several dietary metrics to give a total of seventeen potential explanatory variables. First, a forward stepwise procedure based on the Akaike information criterion was used to select explanatory variables with significant effects on food resource partitioning. Then, linear mixed‐effect models were constructed using the selected explanatory variables and with sampling site as a random factor. Food resource partitioning between salmon and bullhead increased significantly with increasing prey diversity, and the variation in food resource partitioning was best described by the model that included prey diversity as the only explanatory variable. This study provides empirical support for the notion that prey diversity is a key driver of resource partitioning among competing species.