Raising a racket: invasive species compete acoustically with native treefrogs

Potential effects of traffic noise on anuran call characteristics in Louisiana, USA during winter

Urban environments expose wildlife to levels of anthropogenic noise they would not experience in rural areas (e.g., traffic noise), and research suggests that many species adjust their acoustic signals for optimal transmission in urban soundscapes. However, our understanding of anuran (order Anura) responses to noise pollution in urban environments of the southeastern United States is limited, particularly for species that can breed during winter. Our goal was to examine how vocal anuran advertisement call characteristics during winter varied with increasing distance from roadways in bottomland hardwoods of Louisiana, USA. We deployed acoustic recording units at two sites (i.e., rural and urban) perpendicular to Interstate 10 at 200-, 400-, and 600-m intervals (i.e., close, middle, and far) from November 2019 to January 2020. We detected Cajun Chorus Frogs (Pseudacris fouquettei) and Cricket Frogs (Acris spp.) at our rural site, and only detected Cricket Frogs at our urban site. At the rural site, Cajun Chorus Frogs produced longer duration notes at the far location compared to the middle location. At the urban site, Cricket Frogs produced higher dominant frequency calls at the close location compared to the far and middle locations and longer duration notes at the far location compared to the close location. We were unable to account for additional factors in our models (e.g., temperature, noise levels), but our results generally align with previous research. Our study provides baseline data for future research to examine the potential effects of traffic noise on winter advertisement calls in locations with similar environmental conditions and species.

Experimental evaluation of how biological invasions and climate change interact to alter the vertical assembly of an amphibian community

While biotic-abiotic interactions are increasingly documented in nature, a process-based understanding of how such interactions influence community assembly is lacking in the ecological literature. Perhaps the most emblematic and pervasive example of such interactions is the synergistic threat to biodiversity posed by climate change and invasive species. Invasive species often out-compete or prey on native species. Despite this long-standing and widespread issue, little is known about how abiotic conditions, such as climate change, will influence the frequency and severity of negative biotic interactions that threaten the persistence of native fauna. Treefrogs are a globally diverse group of amphibians that climb to complete life-cycle processes, such as foraging and reproduction, as well as to evade predators and competitors, resulting in frog communities that are vertically partitioned. Furthermore, treefrogs adjust their vertical position to maintain optimal body temperature and hydration in response to environmental change. Here, utilizing this model group, we designed a novel experiment to determine how extrinsic abiotic and biotic factors (changes to water availability and an introduced predator, respectively) interact with intrinsic biological traits, such as individual physiology and behaviour, to influence treefrogs' vertical niche. Our study found that treefrogs adjusted their vertical niche through displacement behaviours in accordance with abiotic resources. However, biotic interactions resulted in native treefrogs distancing themselves from abiotic resources to avoid the non-native species. Importantly, under altered abiotic conditions, both native species avoided the non-native species 33 $$ 33 $$ %- 70 % $$ 70\% $$ more than they avoided their native counterpart. Additionally, exposure to the non-native species resulted in native species altering their tree climbing behaviours by 56 % - 78 % $$ 56\%\hbox{--} 78\% $$ and becoming more vertically dynamic to avoid the non-native antagonist. Our experiment determined that vertical niche selection and community interactions were most accurately represented by a biotic-abiotic interaction model, rather than a model that considers these factors to operate in an isolated (singular) or even additive manner. Our study provides evidence that native species may be resilient to interacting disturbances via physiological adaptations to local climate and plasticity in space-use behaviours that mediate the impact of the introduced predator.

Invading the soundscape: exploring the effects of invasive species’ calls on acoustic signals of native wildlife

The transmission and reception of sound, both between conspecifics and among individuals of different species, play a crucial role in individual fitness, because correct interpretation of meaning encoded in acoustic signals enables important context-appropriate behaviours, such as predator avoidance, foraging, and mate location and identification. Novel noise introduced into a soundscape can disrupt the processes of receiving and recognising sounds. When species persist in the presence of novel noise, it may mask the production and reception of sounds important to fitness, and can reduce population size, species richness, or relative abundances, and thus influence community structure. In the past, most investigations into the effects of novel noise have focused on noises generated by anthropogenic sources. The few studies that have explored the effects of calls from invasive species suggest native species alter behaviours (particularly their vocal behaviour) in the presence of noise generated by invasive species. These effects may differ from responses to anthropogenic noises, because noises made by invasive species are biotic in origin, and may therefore be more spectrally similar to the calls of native species, and occur at similar times. Thus, in some cases, negative fitness consequences for native species, associated with noises generated by invasive species, may constitute interspecific competition. Possible negative consequences of invasive species calls represent an overlooked, and underappreciated, class of competitive interactions. We are far from understanding the full extent of the effects of invasive species on native ones. Further investigation of the contribution of noise interference to native species’ decline in the presence of invasive species will significantly increase our understanding of an important class of interactions between invasive and native species.

Elucidating mechanisms of invasion success: effects of parasite removal on growth and survival rates of invasive and native frogs

Identifying the mechanisms underlying biological invasions can inform the management of invasive species. The enemy release hypothesis (ERH) suggests that invasive species have a competitive advantage in their introduced range because they leave behind many of their predators and parasites from their native range, allowing them to shift resources from defenses to growth, reproduction, and dispersal. Many studies have demonstrated that invasive species have fewer parasites than their native counterparts, but few studies have tested whether the loss of these natural enemies appears to be a primary driver of the invasion process.To test the ERH, we conducted a mark-recapture study in which we used an anthelmintic drug to successfully reduce parasitic worms in invasive Cuban treefrogs (Osteopilus septentrionalis) and native treefrogs (Hyla spp.) at half of 12 wetlands, marking nearly 4,200 frogs. If the ERH is supported, we would expect that treating for parasitic worms would have a greater benefit to native than invasive hosts.Growth and survival rates of invasive and native treefrogs responded similarly to the anthelmintic treatment, suggesting that the Cuban treefrog's release from parasitic worms does not appear to significantly contribute to its invasiveness in established areas. Instead, it appears that the overall faster rates of growth and maturation, higher survival rates, and larger body sizes of Cuban treefrogs that we observed may contribute to their expansion and proliferation.Synthesis and applications. Although Cuban treefrogs have a lower diversity of parasitic worms in their invasive than native range, this does not appear to significantly contribute to their invasion success in areas where they have been established for more than 20 years. This suggests that any manipulation of parasites in invasive or native hosts would not be an effective method of controlling Cuban treefrogs or reducing their impacts. Further research into other hypotheses is needed to explain the Cuban treefrog's success and help guide management actions to reduce their spread and negative impacts. Our study demonstrates that enemy release may not be a primary driver of invasiveness, highlighting the need for more experimental tests of the enemy release hypothesis to examine its generality.

Continental-scale citizen science data reveal no changes in acoustic responses of a widespread tree frog to an urbanisation gradient

One of the major drivers of global biodiversity declines is habitat loss and modification, often associated with anthropogenic environments. To mitigate biodiversity declines, a comprehensive understanding of how species respond to novel anthropogenic environments is required. Compared to natural habitats, human-modified environments often have increased noise and light pollution, potentially affecting acoustically communicating species, such as frogs. These areas may force animals to modulate or alter their calls to communicate with potential mates, as they compete with anthropogenic noise. Using large-scale citizen science data, coupled with remotely sensed data, we examined how the advertisement calls of the Australian red tree frog (Litoria rubella) varied in response to a gradient consistent with anthropogenic disturbance. After measuring a suite of acoustic properties of L.rubella across its range, we discovered that their advertisement calls showed no response to a disturbance urbanisation gradient. The advertisement calls of the species were highly variable, both at continental and local scales. Our results indicate that acoustic communication in male L.rubella may not be impeded in human-modified habitats as (1) they are a loud species typically heard over background noise and multi-species choruses and (2) their calls are highly variable—potentially serving as a buffer to any acoustic disturbances. Overall, our results provide evidence that some frog species may be acoustically urban tolerant and provide a greater understanding of the responses frogs exhibit to human-mediated environmental change.

Deer browsing alters sound propagation in temperate deciduous forests

The efficacy of animal signals is strongly influenced by the structure of the habitat in which they are propagating. In recent years, the habitat structure of temperate forests has been increasingly subject to modifications from foraging by white-tailed deer (Odocoileus virginianus). Increasing deer numbers and the accompanying browsing have been shown to alter vegetation structure and thus the foraging, roosting, and breeding habitats of many species. However, despite a large body of literature on the effects of vegetation structure on sound propagation, we do not yet know what impact deer browsing may have on acoustic communication. Here we used playback experiments to determine whether sound fidelity and amplitude of white noise, pure tones, and trills differed between deer-browsed and deer-excluded plots. We found that sound fidelity, but not amplitude, differed between habitats, with deer-browsed habitats having greater sound fidelity than deer-excluded habitats. Difference in sound propagation characteristics between the two habitats could alter the efficacy of acoustic communication through plasticity, cultural evolution or local adaptation, in turn influencing vocally-mediated behaviors (e.g. agonistic, parent-offspring, mate selection). Reduced signal degradation suggests vocalizations may retain more information, improving the transfer of information to both intended and unintended receivers. Overall, our results suggest that deer browsing impacts sound propagation in temperate deciduous forest, although much work remains to be done on the potential impacts on communication.

Vertical foraging shifts in Hawaiian forest birds in response to invasive rat removal

Worldwide, native species increasingly contend with the interacting stressors of habitat fragmentation and invasive species, yet their combined effects have rarely been examined. Direct negative effects of invasive omnivores are well documented, but the indirect effects of resource competition or those caused by predator avoidance are unknown. Here we isolated and examined the independent and interactive effects of invasive omnivorous Black rats (Rattus rattus) and forest fragment size on the interactions between avian predators and their arthropod prey. Our study examines whether invasive omnivores and ecosystem fragment size impact: 1) the vertical distribution of arthropod species composition and abundance, and 2) the vertical profile of foraging behaviors of five native and two non-native bird species found in our study system. We predicted that the reduced edge effects and greater structural complexity and canopy height of larger fragments would limit the total and proportional habitat space frequented by rats and thus limit their impact on both arthropod biomass and birds' foraging behavior. We experimentally removed invasive omnivorous Black rats across a 100-fold (0.1 to 12 ha) size gradient of forest fragments on Hawai'i Island, and paired foraging observations of forest passerines with arthropod sampling in the 16 rat-removed and 18 control fragments. Rat removal was associated with shifts in the vertical distribution of arthropod biomass, irrespective of fragment size. Bird foraging behavior mirrored this shift, and the impact of rat removal was greater for birds that primarily eat fruit and insects compared with those that consume nectar. Evidence from this model study system indicates that invasive rats indirectly alter the feeding behavior of native birds, and consequently impact multiple trophic levels. This study suggests that native species can modify their foraging behavior in response to invasive species removal and presumably arrival through behavioral plasticity.