Seasonal variation in the strength of interference competition among headwater stream predators

Abstract

Vertebrate communities in headwater streams are assumed to be regulated through competitive and predatory interactions. Although documented predation is rare, studies regularly report competitive dominance by fish that, as larger competitors reliant on aquatic habitat, exclude semi‐aquatic salamanders to marginal stream habitat. However, it is unclear whether fish interact with stream‐breeding salamanders through indirect effects such as competition for resources (e.g. food or cover) or fear (i.e. threat of predation) nor is it known whether these interactions are consistent through time. This study used a novel caging approach to determine if competitive outcomes between a headwater fish and salamanders were regulated primarily through resource depletion (exploitative competition) or behavioural avoidance (interference competition). We paired banded sculpin (Cottus carolinae) and larval red salamanders (Pseudotriton ruber) of similar body size in independent flow through mesocosms with intra‐ and inter‐specific pairs allowed to interact physically or non‐physically. The experiment was repeated in the autumn and in the spring when stream salamander larvae begin to transform into terrestrial juveniles. Banded sculpin negatively influenced growth of red salamanders regardless of whether they were allowed to physically interact, suggesting interference competition and behavioural avoidance. This asymmetrical effect was strongest in the spring when salamanders underwent metamorphosis at higher rates in the presence of fish. However, in the autumn, the effects were more balanced between the two species with salamanders impacting fish through exploitative competition. By studying the temporal relationships between two competitors and using a caging method novel to competition studies, we established that the outcomes of competition are dependent on season and may vary in type relative to the timing of life‐history events. For this community, these results suggest that outcomes of competition are highly dependent on season and could indicate a biotic mechanism maintaining headwater salamander distributions through source–sink dynamics. Our results also suggest that, in this species interaction, it may be unwarranted to assume that the outcomes of competition at one time represent the complex relationships regulating community interactions.

Investigating Ex Situ Competitive Interactions in a Novel Fish Community with Implications for the Introduction of Sunfish (Lepomis spp.) in Headwater Streams

“…were excluded from physically interacting with the C. tennesseensis , we observed a slight improvement of individual performance, suggesting that competition between C. tennesseensis and Lepomis spp. follows both exploitative and interference pathways (Cecala et al, 2020). Despite expectations that more aggressive L. cyanellus would have a stronger negative effect, we did not observe differences between the effects of the two species, which may be a result of using small and similarly sized individuals.…”

“…Mesocosms were 60 3 120 3 60 cm fiberglass tanks outfitted with a recirculating pump system. Each stream mesocosm drained into its own sump, creating independent replicates and base flows and temperatures similar to local streams and prior experiments (13.7±0.18C; 40.5 L/min; Ennen et al, 2016; Cecala et al, 2020). Mesocosms were set up to replicate a small stream pool environment.…”

“…We used package lme4 to perform our model and package emmeans to conduct post hoc evaluations using the Tukey adjustment for multiple comparisons (Bates et al, 2015;Lenth, 2019). For intraspecific treatments, many studies randomly select an individual to represent the treatment to minimize differences in variances among treatments (e.g., Cecala et al, 2020), yet when we observed our results, each intraspecific pair had an individual that greatly improved their body condition relative to their partner. Therefore, we included both individuals in our models.…”

Should landscape variation in population status be assessed with individual‐ or population‐level indicators?

“…The density of the most terrestrial species, southern two‐lined salamander, was positively associated with larger, more northwest‐facing streams, which are associated with cooler terrestrial conditions than more southeast aspects (MacHattie and McCormack 1961). Conversely, the density of the most aquatic species, spotted dusky salamander, declined with drainage area, which is generally associated with increased flow rates and increases in exploitative and interference competition with other aquatic predators (Vannote et al 1980, Hairston 1986, Creed 2006, Cecala et al 2020). The direction and sensitivity of population responses to landscape, interspecific, and intraspecific variation across life stages seems linked to the extent to which a species relies on aquatic habitat post‐metamorphosis.…”

Clay Models and eDNA are Useful Tools for Identifying Predators of Salamanders

“…These salamanders are highly abundant in aquatic and terrestrial ecosystems and constitute vital links for energy and nutrient flow between these habitats (Davic and Welsh 2004). Plethodontid salamanders are generalist predators and are themselves preyed upon by large invertebrates (Jung et al 2000) and vertebrates such as birds (Fenster and Fenster 1996, Wilson and Simon 1985), snakes (Jobe et al 2019), mammals (Hart et al 2019, Llewellyn and Uhler 1952), fish (Cecala et al 2020), and other salamanders (Bock and Fauth 1992). In addition to their ecological role, plethodontid salamanders are notorious for toxic/adhesive skin secretions adapted for predator deterrence, and some species have bright coloration that functions in aposematism or mimicry (Brandon and Huheey 1975, Brandon et al 1979, Brodie et al 1974).…”