Diet composition of Guanapo River guppies from high predation (HP; N = 21) and low predation (LP; N = 21) sites during the dry season (data from [28]).

<p>Proportions of the 3 food items are estimated marginal means (±SE) calculated by the MANCOVA (population of origin as the fixed effect and fish length as a covariate); data reported have been back-transformed.</p

Correlation between guppy standard length and a) proportion of invertebrates and b) proportion of detritus in guppies diet from the Aripo River.

<p>Wet season (filled diamonds) and dry season (open circles) fish are shown. Regression line shows the only significant relationship, which is between proportion of invertebrates in diet and guppy length during the wet season.</p

Contrasting Population and Diet Influences on Gut Length of an Omnivorous Tropical Fish, the Trinidadian Guppy (<i>Poecilia reticulata</i>)

<div><p>Phenotypic plasticity is advantageous for organisms that live in variable environments. The digestive system is particularly plastic, responding to changes in diet. Gut length is the result of a trade-off between maximum nutrient absorption and minimum cost for its maintenance and it can be influenced by diet and by evolutionary history. We assessed variation in gut length of Trinidadian guppies (<i>Poecilia reticulata</i>) as a function of diet, season, ontogeny, and local adaptation. Populations of guppies adapted to different predation levels have evolved different life history traits and have different diets. We sampled guppies from sites with low (LP) and high predation (HP) pressure in the Aripo and Guanapo Rivers in Trinidad. We collected fish during both the dry and wet season and assessed their diet and gut length. During the dry season, guppies from HP sites fed mostly on invertebrates, while guppies in the LP sites fed mainly on detritus. During the wet season, the diet of LP and HP populations became very similar. We did not find strong evidence of an ontogenetic diet shift. Gut length was negatively correlated with the proportion of invertebrates in diet across fish from all sites, supporting the hypothesis that guppy digestive systems adapt in length to changes in diet. Population of origin also had an effect on gut length, as HP and LP fish maintained different gut lengths even in the wet season, when their diets were very similar and individuals in both types of populations fed mostly on detritus. Thus, both environment and population of origin influenced guppies gut length, but population of origin seemed to have a stronger effect. Our study also showed that, even in omnivorous fish, gut length adapted to different diets, being more evident when the magnitude of difference between animal and plant material in the diet was very large.</p></div

Mean guppies gut length for HP and LP guppies during the dry and the wet season in the Aripo River.

<p>Only data from guppies between 14 and 20 mm are included in this graph. Guppies from the HP (high predation) population are shown in filled circles, while guppies from the LP (low predation) population are in open triangles. Bars represent ±1 SE.</p

Mean proportion of invertebrates in diets vs. mean relative gut length.

<p>Only guppies between 14 and 20 mm were included. Each data point represents one site (Aripo HP and LP for both dry and wet season, and Guanapo HP and LP from the dry season). Relative gut length was calculated as the gut length divided by fish length. An average value was assigned for the proportion of invertebrates for each site, which was the estimated marginal mean obtained from the diet analysis (data for the Guanapo fish are from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136079#pone.0136079.ref028" target="_blank">28</a>]).</p

Univariate ANCOVA results on the effects of population of origin and season on the proportion of invertebrates and detritus in the diets of guppies from the Aripo River in Trinidad.

<p>SS = sums of squares; df = degrees of freedom; MS = mean squares. All F-ratios are based on type-III sums of squares.</p

Season effect on the diet of HP and LP guppies from the Aripo River.

<p>Percent diet composition is shown for the dry (a) and the wet (b) seasons. Data showed here represent the estimated marginal means calculated by the MANCOVA on arcsin transformed data. Estimated marginal means and standard errors have been back-transformed for the graphical representation. Food categories analyzed are invertebrates, in dark grey and amorphous detritus. LP: low predation; HP: high predation. Bars represent ±1 SE.</p

Stable Isotopes of C and N Reveal Habitat Dependent Dietary Overlap between Native and Introduced Turtles <i>Pseudemys rubriventris</i> and <i>Trachemys scripta</i>

<div><p>Habitat degradation and species introductions are two of the leading causes of species declines on a global scale. Invasive species negatively impact native species through predation and competition for limited resources. The impacts of invasive species may be increased in habitats where habitat degradation is higher due to reductions of prey abundance and distribution. Using stable isotope analyses and extensive measurements of resource availability we determined how resource availability impacts the long term carbon and nitrogen assimilation of the invasive red-eared slider turtle (<i>Trachemys scripta elegans</i>) and a native, threatened species, the red-bellied turtle (<i>Pseudemys rubriventris</i>) at two different freshwater wetland complexes in Pennsylvania, USA. At a larger wetland complex with greater vegetative species richness and diversity, our stable isotope analyses showed dietary niche partitioning between species, whereas analyses from a smaller wetland complex with lower vegetative species richness and diversity showed significant dietary niche overlap. Determining the potential for competition between these two turtle species is important to understanding the ecological impacts of red-eared slider turtles in wetland habitats. In smaller wetlands with increased potential for competition between native turtles and invasive red-eared slider turtles we expect that when shared resources become limited, red-eared slider turtles will negatively impact native turtle species leading to long term population declines. Protection of intact wetland complexes and the reduction of introduced species populations are paramount to preserving populations of native species.</p></div

Appendix A. Macroinvertebrate community biomass.

Macroinvertebrate community biomass

Mean (±1SE) visitation by guppies, killifish and crabs in all three stream reaches during day and night.

<p>Total daily mean was based on 12 hr diel periods and weighted to compensate for fewer night observations.</p><p>NA = not applicable (guppies not present in upstream reach).</p