Exploring the effects of competition and predation on the success of biological invasion through mathematical modeling.

Biological invasions are a major cause of species extinction and biodiversity loss. Exotic predators are the type of introduced species that have the greatest negative impact, causing the extinction of hundreds of native species. Despite this, they continue to be intentionally introduced by humans. Understanding the causes that determine the success of these invasions is a challenge within the field of invasion biology. Mathematical models play a crucial role in understanding and predicting the behavior of exotic species in different ecosystems. This study examines the effect of predation and competition on the invasion success of an exotic generalist predator in a native predator-prey system. Considering that the exotic predator both consumes the native prey and competes with the native predator, it is necessary to study the interplay between predation and competition, as one of these interspecific interactions may either counteract or contribute to the impact of the other on the success of a biological invasion. Through a mathematical model, represented by a system of ordinary differential equations, it is possible to describe four different scenarios upon the arrival of the exotic predator in a native predator-prey system. The conditions for each of these scenarios are described analytically and numerically. The numerical simulations are performed considering the American mink (Mustela vison), an invasive generalist predator. The results highlight the importance of considering the interplay between interspecific interactions for understanding biological invasion success.

Concurrent dilution and amplification effects in an intraguild predation eco-epidemiological model.

The dilution and amplification effects are important concepts in the field of zoonotic diseases. While the dilution effect predicts that pathogen prevalence is negatively correlated with increased species diversity, the opposite trend is observed when the amplification effect occurs. Understanding how interspecific interactions such as predation and competition within a community influence disease transmission is highly relevant. We explore the conditions under which the dilution and amplification effects arise, using compartmental models that integrate ecological and epidemiological interactions. We formulate an intraguild predation model where each species is divided into two compartments: susceptible and infected individuals. We obtained that increasing predation increases the disease transmission potential of the predator and the density of infected individuals, but decreases the disease transmission potential of the prey, as well as their density. Also, we found that interspecific competition always helps to decrease the number of infected individuals in the population of the two species. Therefore, dilution and amplification effects can be observed simultaneously but depending on different types of cological interactions.

Speech Perception and Dichotic Listening Are Associated With Hearing Thresholds and Cognition, Respectively, in Unaided Presbycusis.

Presbycusis or age-related hearing loss is a prevalent condition in the elderly population, which affects oral communication, especially in background noise, and has been associated with social isolation, depression, and cognitive decline. However, the mechanisms that relate hearing loss with cognition are complex and still elusive. Importantly, recent studies show that the use of hearing aids in presbycusis, which is its standard management, can induce neuroplasticity and modify performance in cognitive tests. As the majority of the previous studies on audition and cognition obtained their results from a mixed sample of subjects, including presbycusis individuals fitted and not fitted with hearing aids, here, we revisited the associations between hearing loss and cognition in a controlled sample of unaided presbycusis. We performed a cross-sectional study in 116 non-demented Chilean volunteers aged ≥65 years from the Auditory and Dementia study cohort. Specifically, we explored associations between bilateral sensorineural hearing loss, suprathreshold auditory brain stem responses, auditory processing (AP), and cognition with a comprehensive neuropsychological examination. The AP assessment included speech perception in noise (SIN), dichotic listening (dichotic digits and staggered spondaic words), and temporal processing [frequency pattern (FP) and gap-in-noise detection]. The neuropsychological evaluations included attention, memory, language, processing speed, executive function, and visuospatial abilities. We performed an exploratory factor analysis that yielded four composite factors, namely, hearing loss, auditory nerve, midbrain, and cognition. These four factors were used for generalized multiple linear regression models. We found significant models showing that hearing loss is associated with bilateral SIN performance, while dichotic listening was associated with cognition. We concluded that the comprehension of the auditory message in unaided presbycusis is a complex process that relies on audition and cognition. In unaided presbycusis with mild hearing loss (<40 dB HL), speech perception of monosyllabic words in background noise is associated with hearing levels, while cognition is associated with dichotic listening and FP.

Geographic variation in the matching between call characteristics and tympanic sensitivity in the Weeping lizard.

Effective communication requires a match among signal characteristics, environmental conditions, and receptor tuning and decoding. The degree of matching, however, can vary, among others due to different selective pressures affecting the communication components. For evolutionary novelties, strong selective pressures are likely to act upon the signal and receptor to promote a tight match among them. We test this prediction by exploring the coupling between the acoustic signals and auditory sensitivity in Liolaemus chiliensis, the Weeping lizard, the only one of more than 285 Liolaemus species that vocalizes. Individuals emit distress calls that convey information of predation risk to conspecifics, which may respond with antipredator behaviors upon hearing calls. Specifically, we explored the match between spectral characteristics of the distress calls and the tympanic sensitivities of two populations separated by more than 700 km, for which previous data suggested variation in their distress calls. We found that populations differed in signal and receptor characteristics and that this signal variation was explained by population differences in body size. No precise match occurred between the communication components studied, and populations differed in the degree of such correspondence. We suggest that this difference in matching between populations relates to evolutionary processes affecting the Weeping lizard distress calls.

Persistence and size of seasonal populations on a consumer-resource relationship depends on the allocation strategy toward life-history functions.

The long-term ecological dynamics of a population inhabiting a seasonal environment is analyzed using a semi-discrete or impulsive system to represent the consumer-resource interaction. The resource corresponds to an incoming energy flow for consumers that is allocated to reproduction as well as to maintenance in each non-reproductive season. The energy invested in these life-history functions is used in reproductive events, determining the size of the offspring in each reproductive season. Two long-term dynamic patterns are found, resulting in either the persistence or the extinction of the population of consumers. In addition, our model indicates that only one energy allocation strategy provides an optimal combination between individual consumption and long-term population size. The current study contributes to the understanding of how the individual-level and the population-level are interrelated, exhibiting the importance of incorporating phenotypic traits in population dynamics.

The olivocochlear reflex strength in awake chinchillas is relevant for behavioural performance during visual selective attention with auditory distractors.

The auditory efferent system comprises descending projections from the cerebral cortex to subcortical nuclei, reaching the cochlear receptor through olivocochlear fibres. One of the functions attributed to this corticofugal system is to suppress irrelevant sounds during selective attention to visual stimuli. Medial olivocochlear neurons can also be activated by sounds through a brainstem reflex circuit. Whether the individual variability of this reflex is related to the cognitive capacity to suppress auditory stimuli is still controversial. Here we propose that the individual strength per animal of the olivocochlear reflex is correlated with the ability to suppress auditory distractors during visual attention in awake chinchillas. The olivocochlear reflex was elicited with a contralateral broad-band noise at ~ 60 dB and ipsilateral distortion product otoacoustic emissions were obtained at different frequencies (1-8 kHz). Fourteen chinchillas were evaluated in a behavioural protocol of visual attention with broad-band noise and chinchilla vocalizations as auditory distractors. Results show that the behavioural performance was affected by both distractors and that the magnitudes of the olivocochlear reflex evaluated at multiple frequencies were relevant for behavioural performance during visual discrimination with auditory distractors. These results stress the ecological relevance of the olivocochlear system for suppressing natural distractors.

The acoustic adaptation hypothesis in a widely distributed South American frog: Southernmost signals propagate better.

Animal communication occurs in environments that affect the properties of signals as they propagate from senders to receivers. We studied the geographic variation of the advertisement calls of male Pleurodema thaul individuals from eight localities in Chile. Furthermore, by means of signal propagation experiments, we tested the hypothesis that local calls are better transmitted and less degraded than foreign calls (i.e. acoustic adaptation hypothesis). Overall, the advertisement calls varied greatly along the distribution of P. thaul in Chile, and it was possible to discriminate localities grouped into northern, central and southern stocks. Propagation distance affected signal amplitude and spectral degradation in all localities, but temporal degradation was only affected by propagation distance in one out of seven localities. Call origin affected signal amplitude in five out of seven localities and affected spectral and temporal degradation in six out of seven localities. In addition, in northern localities, local calls degraded more than foreign calls, and in southern localities the opposite was observed. The lack of a strict optimal relationship between signal characteristics and environment indicates partial concordance with the acoustic adaptation hypothesis. Inter-population differences in selectivity for call patterns may compensate for such environmental constraints on acoustic communication.

Music Training and Education Slow the Deterioration of Music Perception Produced by Presbycusis in the Elderly.

The perception of music depends on the normal function of the peripheral and central auditory system. Aged subjects without hearing loss have altered music perception, including pitch and temporal features. Presbycusis or age-related hearing loss is a frequent condition in elderly people, produced by neurodegenerative processes that affect the cochlear receptor cells and brain circuits involved in auditory perception. Clinically, presbycusis patients have bilateral high-frequency hearing loss and deteriorated speech intelligibility. Music impairments in presbycusis subjects can be attributed to the normal aging processes and to presbycusis neuropathological changes. However, whether presbycusis further impairs music perception remains controversial. Here, we developed a computerized version of the Montreal battery of evaluation of amusia (MBEA) and assessed music perception in 175 Chilean adults aged between 18 and 90 years without hearing complaints and in symptomatic presbycusis patients. We give normative data for MBEA performance in a Latin-American population, showing age and educational effects. In addition, we found that symptomatic presbycusis was the most relevant factor determining global MBEA accuracy in aged subjects. Moreover, we show that melodic impairments in presbycusis individuals were diminished by music training, while the performance in temporal tasks were affected by the educational level and music training. We conclude that music training and education are important factors as they can slow the deterioration of music perception produced by age-related hearing loss.

Vocal responses of austral forest frogs to amplitude and degradation patterns of advertisement calls.

Degradation phenomena affecting animal acoustic signals may provide cues to assess the distance of emitters. Recognition of degraded signals has been extensively demonstrated in birds, and recently studies have also reported detection of degraded patterns in anurans that call at or above ground level. In the current study we explore the vocal responses of the syntopic burrowing male frogs Eupsophus emiliopugini and E. calcaratus from the South American temperate forest to synthetic conspecific calls differing in amplitude and emulating degraded and non-degraded signal patterns. The results show a strong dependence of vocal responses on signal amplitude, and a general lack of differential responses to signals with different pulse amplitude modulation depths in E. emiliopugini and no effect of relative amplitude of harmonics in E. calcaratus. Such limited discrimination of signal degradation patterns from non-degraded signals is likely related to the burrowing habits of these species. Shelters amplify outgoing and incoming conspecific vocalizations, but do not counteract signal degradation to an extent comparable to calling strategies used by other frogs. The limited detection abilities and resultant response permissiveness to degraded calls in these syntopic burrowing species would be advantageous for animals communicating in circumstances in which signal alteration prevails.

Individual and sex distinctiveness in bark calls of domestic chinchillas elicited in a distress context.

Animals obtain information about their social environment by means of communication signals, which provide relevant subtle cues for individual recognition. An important requisite for this process is the existence of larger between- than within-emitter signal variation. Acoustic signals are complex traits susceptible of variation in their spectral and temporal components, implying that signal distinctiveness can result from differences in single or various acoustic components. In this study, domestic chinchillas were induced to vocalize in a distress context to describe the acoustic characteristics of the bark calls, and to determine features that denote the potential value of this vocalization for individual and/or sexual recognition. The results demonstrate that the variation in spectral and temporal components of the bark calls of chinchillas elicited under a distress context is larger between than within individuals, suggesting the potential of these signals for distinctiveness between individual signalers, although the potential of this call type for sex distinctiveness is quite limited. These results combined with previous studies on auditory capabilities of chinchillas contribute to position this rodent as a valuable model species for studying auditory-vocal interactions.

Contrasting Propagation of Natural Calls of Two Anuran Species from the South American Temperate Forest.

The acoustic adaptation hypothesis predicts that sound communication signals have an optimal relationship with animals' native environments. However, species sharing a habitat produce signals stratified in the spectral domain and exhibit different temporal patterns resulting in acoustic niche partitioning. The diversity generated is likely to affect differently the characteristics of propagating signals. We recorded at various distances from the sound source calls of the frogs Eupsophus calcaratus and E. emiliopugini in the austral temperate forest where they communicate and breed syntopically. The calls of E. calcaratus have higher frequency components and lower amplitude relative to calls of E. emiliopugini, and the acoustic active space for the signals of E. calcaratus is restricted relative to E. emiliopugini. The signals of both species experience similar attenuation patterns, but calls of E. calcaratus are affected by spectral degradation to a larger extent, with linear decreases in spectral cross-correlation and in the amplitude ratio between the first two harmonics. The calls of E. emiliopugini are affected by temporal degradation as a linear decrease in amplitude modulation depth of their pulsed structure. Further studies are needed to assess the relative importance of selective and phylogenetic factors on the divergent propagation patterns reported.

Thermoregulatory capacities and torpor in the South American marsupial, Dromiciops gliroides.

During periods of adverse conditions small endotherms depend on a continuous supply of food and energy to maintain body temperature. Thus, rapid and reversible phenotypic modifications at different organizational levels are key for an efficient use of resources and survival. In this study, we provide a quantitative description of thermoregulatory capacities and energy-saving strategies in the Chilean marsupial Dromiciops gliroides. In particular, we evaluated the effect of thermal acclimation on basal metabolic rate (BMR), thermal conductance (C) and torpor patterns, as well as the presence of non-shivering thermogenesis (NST) as a rewarming mechanism in this marsupial. Non-significant effects of thermal acclimation were observed in BMR, C and body mass, but cold-acclimated individuals exhibited significantly longer torpor bouts. Also, minimum body temperature during torpor, inter-bout body temperature and arousal rewarming rate were lower in cold-acclimated animals. Furthermore, we found that D. gliroides did not display NST in response to Norepinephrine. Hence, despite the high regulation of torpor of other species, D. gliroides shows low flexibility in the ability to adjust energy expenditure and insulation properties, and (as in other marsupials) NST do not seems to be important as thermoregulatory mechanism.

Ample active acoustic space of a frog from the South American temperate forest.

The efficiency of acoustic communication depends on the power generated by the sound source, the attributes of the environment across which signals propagate, the environmental noise and the sensitivity of the intended receivers. Eupsophus emiliopugini, an anuran from the temperate austral forest communicates by means of an advertisement call of moderate intensity within the range for anurans. To estimate the range over which these frogs communicate effectively, we conducted measurements of call sound levels and of auditory thresholds to pure tones and to synthetic conspecific calls. The results show that E. emiliopugini produces advertisement calls of about 84 dB SPL at 0.25 m from the caller. The signals are affected by attenuation as they propagate, reaching average values of about 47 dB SPL at 8 m from the sound source. Midbrain multi-unit recordings show quite sensitive audiograms within the anuran range, with thresholds of about 44 dB SPL for synthetic imitations of conspecific calls, which would allow communication at distances beyond 8 m. This is an extended range as compared to E. calcaratus, a related syntopic species for which a previous study has shown to be restricted to active acoustic spaces shorter than 2 m. The comparison reveals divergent strategies for related taxa communicating amid the same environment.

Severe constraints for sound communication in a frog from the South American temperate forest.

The efficiency of acoustic communication depends on the power generated by the sound source, the quality of the environment across which signals propagate, the environmental noise and the sensitivity of the intended receivers. Eupsophus calcaratus, an anuran from the temperate austral forest, communicates by means of an advertisement call of weak intensity in a sound-attenuating environment. To estimate the range over which these frogs communicate effectively, we conducted measurements of sound level and degradation patterns of propagating advertisement calls in the field, and measurements of auditory thresholds to pure tones and to natural calls in laboratory conditions. The results show that E. calcaratus produces weak advertisement calls of about 72 dB sound pressure level (SPL) at 0.25 m from the caller. The signals are affected by attenuation and degradation patterns as they propagate in their native environment, reaching average values of 61 and 51 dB SPL at 1 and 2 m from the sound source, respectively. Midbrain multi-unit recordings show a relatively low auditory sensitivity, with thresholds of about 58 dB SPL for conspecific calls, which are likely to restrict communication to distances shorter than 2 m, a remarkably short range as compared to other anurans.