Aquatic top predator prefers terrestrial prey in an intermittent stream

Ecosystems are often trophically entangled, sharing subsidies of energy and nutrients across habitat boundaries (e.g., forest to stream, stream to forest). In freshwater habitats surrounded by riparian vegetation, aquatic consumers may benefit from pulsed inputs of terrestrial organic matter, such as leaf litter and drowning organisms. This is especially pronounced in intermittent streams where the boundary between terrestrial and aquatic habitats is seasonally blurred. Over the past 10 years, we observed an aquatic insect predator Abedus indentatus (Hemiptera: Belostomatidae) largely feeding on terrestrial organisms in intermittent streams at Pinnacles National Park, California. To expand on these observations, we developed a behavioral assay where predators were presented with a range of prey comprising 11 species of various sizes from both aquatic and terrestrial habitats. We found that predator behavior was non-random and often influenced by prey characteristics. In particular, we found that predators tended to prefer large, terrestrial prey. Additionally, we found a positive relationship between size and strike probability among aquatic prey; however, this relationship did not hold for terrestrial prey items, indicating that certain size breakpoints might overcome increased levels of prey defense. Our results indicate that A. indentatus might favor prey items with weaker defenses within aquatic environments. However, other factors (i.e., prey size) might mediate foraging behavior. Because aquatic macroinvertebrate predators in intermittent streams depend on the terrestrial-aquatic interface, local predator dynamics may not be fully understood without considering fluxes of prey crossing ecosystem boundaries.

Methods
In order to detect patterns in prey preference with respect to size and origin (aquatic or terrestrial), we used a “prey-drop” assay similar to that of Hazarika and Goswami (2012) and Velasco and Millan (1998). This design allowed us to observe predator behaviors when simultaneously presented with a suite of prey representing a range of nutritional values and defensive capabilities. We gathered forty four specimens of Abedus indentatus from Chalone Creek in Pinnacles National Park, California, United States (36.46584, -121.15532) on May 28 and June 21, 2022. The land that now encompasses Pinnacles is contained within the ancestral territories of the Amah Mutsun and Chalone peoples, whose modern descendants maintain their eco-cultural heritage at the site. To capture our specimens, we conducted serial edge sampling and pool sweeps (3 each) using a 5 mm mesh net. Each sampling event covered roughly 100 m along the stream channel. Individuals were transported in 20 cm by 10 cm plastic containers with mesh tops for oxygenation and a handful of clean leaves and sticks from their home stream. They were housed under indirect natural light and fed commercially sourced mealworms (Tenibrio molitor) for 5-8 days. Food was withheld for 48 hours prior to testing to ensure that all specimens were hungry. Aquatic prey was collected on open space in San Geronimo, California, United States (Lat 38.01363, Long -122.65289) using serial edge sampling and pool sweeps, while terrestrial prey was captured via aerial netting, sweep-netting tall grass, and beating coast live oak. 

Each treatment’s prey set consisted of one of each of the following: a smaller giant water bug Belostoma flumineum (wild caught, mean length 18.5 mm ± 2.06), a roach Blaptica dubia (commercially obtained, mean length 15.1 mm ± 1.7), a water boatman Corixidae sp. (wild caught, mean length 6.8 mm ± 1.33), a cucumber beetle Diabrotica undecimpunctata (wild caught, mean length 5.6 mm ± 0.91), a diving beetle Dytiscidae sp. (wild caught, mean length 5.5 mm ± 0.5), an earwig Forficula auricularia (wild caught, mean length 12.2 mm ± 1.66), a larva and an adult of the damselfly Ischnura sp. (wild caught: adult mean length 27.5 mm ± 2.10; larva mean length 10.7 mm ± 1.73), a backswimmer Notonecta sp. (wild caught, mean length 11.4 mm ± 1.2), and two size classes of the cricket Acheta domestica (commercially obtained, mean length 16.7 mm ± 1.41 for large specimens, mean length 8.2 mm ± 1.16 for small specimens). These prey items represent a range of sizes, as well as gradients of agility and physiological defense. Roughly half of the potential prey choices are primarily aquatic (B. flumineum, Corixidae sp., Dytiscidae sp., nymphal Ischnura, and Notonecta) while the remainder are terrestrial (Blaptica dubia, D. undecimpunctata, large and small Acheta, adult Ischnura, and Forficula). All prey types offered except for Blaptica were observed on riparian margins at Pinnacles during preliminary field observations, or were directly sampled in long-term benthic data (Apgar et al. unpublished data).   

Each individual A. indentatus was tested in a 30 by 20 by 20 cm acrylic observational arena fitted with a diagonal wooden perch. Water was sourced from San Geronimo Creek (38.01035, -122.65382). At the beginning of each assay, one A. indentatus was placed into the test arena alone and allowed to acclimate for 10-15 minutes. Following this acclimation period, a full set of prey items was dropped simultaneously and haphazardly into the observational arena with the A. indentatus (Figure 1 E). Each A. indentatus was observed over a 2-hour period which started at the time of the prey drop. Usually, three assays were conducted simultaneously, with the observer waiting until the first prey item from a treatment had been captured before moving on to the next to minimize inaccuracies related to human observational error. Captured prey items were taken from predators after 5 minutes using forceps to prevent them from feeding on only one prey item for the duration of the observational period. This allowed for higher resolution data on prey preference order. We quantified prey preference in two ways: 1) the binary response (attack or no attack) of each A. indentatus to each prey item, and 2) the order in which prey items were captured in each trial. For each assay, we also recorded the number of successful and failed predation attempts; the start, duration, and end of each predation event; and the time at which we removed a captured prey item.

We used Plackett-Luce models to determine the worth of a given prey item when compared to other prey within a set. 

Finally, to test if the size of a given prey item influenced the probability of selection, we calculated prey mean length by measuring 10 individuals of each prey species. We fitted logistic regressions for both aquatic and terrestrial prey items. All calculations were completed in R statistical software (R Core Team 2023) using an ɑ of 0.05.