Anthropogenic Noise Pollution Research Articles

State of Play in Marine Soundscape Assessments

A soundscape is the recording of all sounds present in an area, creating a holistic view of the acoustic profile in an ecosystem. Studying acoustic parameters of marine soundscapes as a whole has been shown to give an indication of the health status of the location, as well as correlate to which species may be present and using the area. With the rapid innovation of technology, especially data storage and declining cost of equipment, marine soundscape research is fast increasing, and these previous limitations have been switched for computing capacity for data analysis. Here, we perform a systematic assessment of literature of marine soundscape studies, from 1978, when the first soundscape study was reported, until 2021. We identified 200 primary research studies that recorded soundscapes and captured their geographical location, depth, habitat, duration of the study, and number of sites in each study. Using this data, we summarize the state of play in marine soundscapes studies, and identify knowledge gaps in the spatial coverage, depth profiles, habitat representation and study duration. Spatially, studies are biased towards the northern hemisphere. They are also more prevalent in more easily accessible ecosystems, in order from most to least studied, in coastal (38%), pelagic (20%), tropical coral reef (17%), rocky reef (7%), polar (5.5%), seagrass meadows, oyster reef and kelp/algal forest (<5% each) areas, with zones of cold-water coral the least studied (0.3%). Continuing the trend of accessibility, studies also tended to focus on shallow ecosystems. Most recordings (68%) were conducted in the upper 50 m, with 13% in 50-200 m depths, and only 0.6% at a depth >4000 m. With anthropogenic noise and other pollution sources increasing globally, these gaps in research should be further addressed, especially as they pertain to vulnerable ecosystems, many of which are affected by global climate change and anthropogenic influences. It is crucial that marine soundscape studies continue to be developed and pursued, to establish baselines for healthy ecosystems and/or document recovery following management actions.

Natural cues for invertebrate and fish hearing: Particle motion measurements on coral reefs

Coral reef soundscapes are increasingly studied for their ecological uses by invertebrates and fishes, their applications for monitoring habitat quality, and to investigate effects of anthropogenic noise pollution. Few examinations of aquatic soundscapes have reported particle motion levels and variability, despite their relevance to invertebrates and fishes. We quantified ambient particle acceleration from orthogonal hydrophone arrays over several months at four coral reef sites in the U.S. Virgin Islands, which varied in benthic habitat and fish communities. Temporal trends of particle acceleration were similar to those of sound pressure, and the strength of diel trends in both metrics significantly correlated with percent coral cover. Low frequency (<100 Hz) and high magnitude empirical particle acceleration levels diverged from plane wave approximations. Particle acceleration levels were also determined for boat and example fish sounds. Comparisons with particle acceleration derived audiograms suggest greatest capacity of invertebrates and fishes to detect soundscape components below 100 Hz, and potentially poorer detectability of soundscape cues by some invertebrates compared to fishes. Based on our results, we discuss research questions in soundscape ecology for which reporting of particle motion is essential versus those for which sound pressure may suffice.

Acoustic modulation of reproductive hormones in the blacktail shiner, Cyprinella venusta, a soniferous cyprinid

Animal communication networks consist of multisensory signals and often include an acoustic element. Many freshwater fishes, including the blacktail shiner, Cyprinella venusta , produce specific vocalizations during courtship and agonistic behaviours. Similar to other vocal taxa, fish communication networks are vulnerable to noise pollution. The dominant frequencies of anthropogenic noise often overlap spectrally with animal acoustic signals. To investigate the role of acoustic signalling in breeding soniferous fishes in a physiological context, we conducted a series of acoustic playback trials. We measured waterborne levels of oestradiol (E2) and prostaglandin F2 α (PGF2 α ) in gravid females exposed to courtship calls, and levels of 11-ketotestosterone (11KT) in males exposed to agonistic calls. In a separate series of trials, we exposed individuals to calls masked with bridge traffic noise. Hormone levels were quantified using enzyme-linked immunosorbent assays (ELISA) and analysed using repeated measures ANOVAs. Gravid females exhibited a significant drop in E2 ( P = 0.023) and an increase in PGF2 α during courtship calls ( P = 0.01). Our research suggests that growl signals contribute to the onset of spawning behaviours in female C. venusta and this response may be dampened by anthropogenic noise pollution. • We examined effects of anthropogenic noise on acoustic signalling in C. venusta. • Males produce growls (courtship/spawning) and knocks (male–male aggression). • Unmasked growls modulated levels of reproductive hormones in gravid females. • Males did not respond hormonally to unimodal stimuli. • Fish exposed to masked vocalizations did not exhibit hormonal changes.

Anthropogenic Noise and Light Pollution Interact with Modulating Factors to Affect Sleep Behaviour in a Free-Living Bird

Anthropogenic Noise and Light Pollution Interact with Modulating Factors to Affect Sleep Behaviour in a Free-Living Bird

Auditory evoked potential in stranded melon-headed whales (Peponocephala electra): With severe hearing loss and possibly caused by anthropogenic noise pollution

Highly concentrated live mass stranding events of dolphins and whales happened in the eastern coast of China between June and October 2021. The current study adopted the non-invasive auditory evoked-potential technique to investigate the hearing threshold of a stranded melon headed whale (Peponocephala electra) at a frequency range of between 9.5 and 181 kHz. It was found that, at the frequency range of from 10 to 100 kHz, hearing thresholds for the animal were between 20 and 65 dB higher than those of its phylogenetically closest species (Pygmy killer whale). The severe hearing loss in the melon headed whale was probably caused by transient intense anthropogenic sonar or chronic shipping noise exposures. The hearing loss could have been the cause for the observed temporal and spatial clustered stranding events. Therefore, there is need for noise mitigation strategies to reduce noise exposure levels for marine mammals in the coastal areas of China.

Noise elicits the Lombard effect in midshipman mating vocalizations

Anthropogenic noise pollution is an emerging global threat to fish populations. Among a suite of deleterious effects, noise can potentially impede reproductive success in some fishes by masking their mate advertisement vocalizations. Using the plainfin midshipman fish (Porichthys notatus), a marine toadfish that produces a distinctive “hum” during courtship, we investigated how noise affects male vocalizations and spawning success in the wild. We recorded nesting males for three days and measured the frequency, amplitude, and duration of their vocalizations before, during, and after exposure to artificial noise (a c. 118-Hz tone). We also counted eggs in nests exposed to 10 days of artificial noise versus control nests that were not exposed to artificial noise. Males exposed to noise reduced the number of vocalizations they produced, reduced the frequency of their vocalizations, and increased the amplitude of their mating hum (Lombard effect). However, chronic noise exposure did not clearly affect spawning success, suggesting that the Lombard effect allowed males to sustain clear advertisement signals when competing with the relatively weak artificial noise source.

Low-amplitude noise elicits the Lombard effect in plainfin midshipman mating vocalizations in the wild

Anthropogenic noise pollution is an emerging global threat to fish populations. Among a suite of deleterious effects, noise can potentially impede reproductive success in some fishes by masking their mate advertisement vocalizations. Using the plainfin midshipman fish, Porichthys notatus , a marine toadfish that produces a distinctive ‘hum’ during courtship, we investigated how noise affects male vocalizations and spawning success in the wild. We recorded nesting males for 3 days and measured the frequency (i.e. pitch), amplitude and duration of their vocalizations before, during and after exposure to artificial noise (∼118 Hz tone). We also counted eggs in nests exposed to 10 days of artificial noise versus control nests that were not exposed to artificial noise. Males exposed to noise produced fewer vocalizations, reduced the frequency of vocalizations and increased the amplitude of their mating hum (Lombard effect). However, chronic artificial noise exposure did not significantly affect spawning success, suggesting that the Lombard effect allowed males to sustain clear advertisement signals when competing with a relatively weak artificial noise source. Future studies are needed to determine whether such vocal adjustments incur costs for males, and how common anthropogenic noises, such as boat engines, affect spawning and reproductive success. • Artificial noise elicited an amplitude increase in mate advertisement vocalizations. • Artificial noise elicited peak frequency decreases in three different vocalizations. • All vocalizations occurred less often in the presence of artificial noise. • Exposure to artificial noise for 10 days did not appear to affect spawning success.

Potential for acoustic masking due to shipping noise in the European lobster (Homarus gammarus)

Marine traffic is the most pervasive underwater anthropogenic noise pollution which can mask acoustic communication in marine mammals and fish, but its effect in marine invertebrates remains unknown. Here, we performed an at sea experiment to study the potential of shipping noise to mask and alter lobster acoustic communication. We used hydrophones to record buzzing sounds and accelerometers to detect lobster carapace vibrations (i.e. the buzzing sounds' sources). We demonstrated that male individuals produced carapace vibrations under various ambient noise conditions, including heavy shipping noise. However, while the associated waterborne buzzing sounds could be recorded under natural ambient noise levels, they were masked by shipping noise. Additionally, lobsters significantly increased their call rates in presence of shipping noise, suggesting a vocal compensation due to the reduction of intraspecific communication. This study reports for the first time the potential acoustic masking of lobster acoustic communication by chronic anthropogenic noise pollution, which could affect ecologically important behaviors.

Assessments and Control of Anthropogenic Noise Pollution in Students’ Residential Areas in Ogbomoso

Increased rate of noise-associated risk factors such as speech interference and reduction in productivity, necessitated that control and regulation measures be put in place, to contain anthropogenic noise pollution in the students’ hostels. Therefore, this study assessed the various anthropogenic sources of noise pollution in students’ hostels and developed a Sound Level Monitor and Control (SLMC) device. 1250 undergraduate students across 5 students’ residential zones were sampled for demographics and investigations were conducted into respondents’ perceived medical history, identification of noise sources, and evaluation of hearing loss. Effects of noise levels were evaluated using 100 respondents’ rooms per zone following standard procedures, considering Sound-System-Only (SSO), Generators-Only (GO), and combination of Sound-System-and-Generator (SSG), loud-conversations, etc., as sources of noise. However, a noise control device incorporated with a circuit breaker was developed. The respondents were 51.2% male and 48.8% female, with 58% in the age range 18 – 27 years. The medical history showed that 1.2 and 6.4% had a hearing problem in short and long times, respectively, while 43.6% affirmed that SSO was a major noise pollution causal factor. SSO, GO, loud conversations, traffic, and grinding machines were identified as the prominent sources of anthropogenic induced noise. The minimum average SL result gave a value of 62.8400dB for both ventilated and unventilated rooms, which is 14% above 55dB threshold value recommended by the National Environmental Standards and Regulations Enforcement Agency. The developed SLMC device gave notification at the SL above 55dB for 15 seconds before disconnecting the sound system if not regulated.

Modelling collective navigation via non-local communication.

Collective migration occurs throughout the animal kingdom, and demands both the interpretation of navigational cues and the perception of other individuals within the group. Navigational cues orient individuals towards a destination, while it has been demonstrated that communication between individuals enhances navigation through a reduction in orientation error. We develop a mathematical model of collective navigation that synthesizes navigational cues and perception of other individuals. Crucially, this approach incorporates uncertainty inherent to cue interpretation and perception in the decision making process, which can arise due to noisy environments. We demonstrate that collective navigation is more efficient than individual navigation, provided a threshold number of other individuals are perceptible. This benefit is even more pronounced in low navigation information environments. In navigation ‘blindspots’, where no information is available, navigation is enhanced through a relay that connects individuals in information-poor regions to individuals in information-rich regions. As an expository case study, we apply our framework to minke whale migration in the northeast Atlantic Ocean, and quantify the decrease in navigation ability due to anthropogenic noise pollution.

Measuring Sound at a Cold-Water Coral Reef to Assess the Impact of COVID-19 on Noise Pollution

This study compares the noise levels at the cold-water coral Tisler reef, before and after the closure of the border between Norway and Sweden, which occurred as a direct result of the COVID-19 pandemic. The Tisler reef is a marine protected area located under a ferry “highway” that connects Norway and Sweden. Cold-water coral reefs are recognised as being important hotspots of both biodiversity and biomass, they function as breeding and nursing grounds for commercially important fish and are essential in providing ecosystem functions. Whilst studies have shown that fishery, ocean warming, and acidification threaten them, the effects of noise pollution on cold-water coral reefs remains unstudied. To study the severity of noise pollution at the Tisler reef, a long-term acoustic recorder was deployed from 29 January 2020 until 26 May 2020. From 15 March COVID-19 lockdown measures stopped passenger vessel traffic between Norway and Sweden. This study found that the overall noise levels were significantly lower after border closure, due to reduced ferry traffic, wind speeds, and sea level height. When comparing the median hourly noise levels of before vs. after border closure, this study measured a significant reduction in the 63–125 Hz 1/3 octave band noise levels of 8.94 ± 0.88 (MAD) dB during the day (07:00:00–19:59:59) and 1.94 ± 0.11 (MAD) dB during the night (20:00:00–06:59:59). Since there was no ferry traffic during the night, the drop in noise levels at night was likely driven by seasonal changes, i.e., the reduction in wind speed and sea level height when transitioning from winter to spring. Taking into account this seasonal effect, it can be deduced that the COVID-19 border closure reduced the noise levels in the 63–125 Hz 1/3 octave bands at the Tisler reef by 7.0 ± 0.99 (MAD) dB during the day. While the contribution of, and changes in biological, weather-related and geophysical sound sources remain to be assessed in more detail, understanding the extent of anthropogenic noise pollution at the Tisler cold-water coral reef is critical to guide effective management to ensure the long-term health and conservation of its ecosystem functions.

Perception of the acoustic environment during COVID-19 lockdown in Argentina

The COVID-19 pandemic has significantly modified the behavior of societies. The application of isolation measures during the crisis resulted in changes in the acoustic environment. The aim of this work was to characterize the perception of the acoustic environment during the COVID-19 lockdown of people residing in Argentina in 2020. A descriptive cross-sectional correlational study was carried out. A virtual survey was conducted from April 14 to 26, 2020, and was answered mainly by social network users. During this period, Argentina was in a strict lockdown. The sample was finally composed of 1371 people between 18 and 79 years old. It was observed that most of the participants preferred the new acoustic environment. Mainly in the larger cities, before the isolation, mechanical sounds predominated, accompanied by the perception of irritation. Confinement brought a decrease in mechanical sounds and an increase in biological sounds, associated with feelings of tranquility and happiness. The time window opened by the lockdown offered an interesting scenario to assess the effect of anthropogenic noise pollution on the urban environment. This result offers a subjective approach, which contributes to understanding the link between individuals and communities with the environment.

Spatio-temporal changes of road traffic noise pollution at ecoregional scale

Noise pollution is a pervasive factor that increasingly threatens natural resources and human health worldwide. In particular, large-scale changes in road networks have driven shifts in the acoustic environment of rural landscapes during the past few decades. Using sampling plots from the Spanish Landscape Monitoring System (SISPARES), 16 km2 each, we modelled the spatio-temporal changes in road traffic noise pollution in Ecoregion 1 of Spain (approximately 66,000 km2). We selected a study period that was characterised by significant changes in the size of the road network and the vehicle fleet (i.e. between 1995 and 2014) and used standard and validated acoustic computation methods for environmental noise modelling (i.e. European Directive, 2002/49/EC) within sampling plots. We then applied a multiple linear regression to expand noise modelling throughout the whole of Ecoregion 1. Our results showed that the noise level increased by 1.7 dB(A) in average per decade in approximately 65% of the territory, decreased by 1.3 dB(A) per decade in about 33%, and remained unchanged in 2%. This suggests that road traffic noise pollution levels may not grow homogeneously in large geographical areas, maybe due to the concentration of large fast traffic flows on modern motorways connecting towns. Our research exemplifies how landscape monitoring systems such as cost-effective approaches may play an important role when assessing spatio-temporal patterns and the impact of anthropogenic noise pollution at large geographical scales, and even more so in a global context of constricted resources and limited availability of historical data on traffic and environmental noise monitoring.

A meta‐analysis of the influence of anthropogenic noise on terrestrial wildlife communication strategies

Abstract Human‐caused noise pollution dominates the soundscape of modern ecosystems, from urban centres to national parks. Although wildlife can generally alter their communication to accommodate many types of natural noise (e.g. wind, wave action, heterospecific communication), noise pollution from anthropogenic sources pushes the limits of wildlife communication flexibility by causing loud, low‐pitched and near‐continuous interference. Because responses to noise pollution are variable and taxa specific, multi‐species risk assessments and mitigation are not currently possible. We conducted a meta‐analysis to synthesize noise pollution effects on terrestrial wildlife communication. Specifically, we assessed: (a) the impacts of noise pollution on modulation of call rate, duration, amplitude and frequency (including peak, minimum and maximum frequency); and (b) the literature on anthropogenic noise pollution by region, taxa, study design and disturbance type. Terrestrial wildlife (results driven by avian studies) generally respond to noise pollution by calling with higher minimum frequencies, while they generally do not alter the amplitude, maximum frequency, peak frequency, duration and rate of calling. The literature on noise pollution research is biased towards birds, population‐level studies, urban noise sources and study systems in North America. Synthesis and applications. Our study reveals the ways in which wildlife can alter their signals to contend with anthropogenic noise, and discusses the potential fitness and management consequences of these signal alterations. This information, combined with an identification of current research needs, will allow researchers and managers to better develop noise pollution risk assessment protocols and prioritize mitigation efforts to reduce anthropogenic noise.

Higher-pitched bird song towards the coast supports a role for selection in ocean noise avoidance

ABSTRACT Effective communication in birds is often hampered by background noise, with much research focusing on the effect of anthropogenic noise on passerine bird song. Continuous low-pitch natural noise can drive changes in both spectral and temporal patterning of bird vocalisations, but the extent to which these effects may also affect birds that lack vocal learning is not well understood. We used a gradient of exposure to natural low-frequency noise to assess whether it exerts selective pressure on innate vocalisations. We tested whether three species of Pogoniulus tinkerbirds adapt their song when exposed to continuous low-frequency noise from ocean surf. We show that dominant frequency increases the closer birds are to the coast in at least two species, indicating that ocean surf sound may apply a selective pressure on songs. Tinkerbirds adapt their songs by increasing dominant frequency to avoid masking by ambient noise, therefore improving long-range communication. Our study provides evidence that natural ambient noise affects vocalisations in birds whose songs develop innately. We believe that our results can also be extrapolated in the context of anthropogenic noise pollution, hence providing a baseline for the study of the effects of low-frequency ambient noise on birds that lack vocal learning.

Do the Conventions on the Law of the Sea and Biological Diversity adequately protect marine biota from anthropogenic underwater noise pollution?

Anthropogenic underwater noise pollution (AUNP) generated by, inter alia, commercial shipping, military exercises, the use of sonar and seismic surveys has increased dramatically since the early 1950s. This has caused or contributed to the death and suffering of marine biota. International and domestic law must adequately regulate AUNP in order for this transboundary and transjurisdictional form of pollution to be addressed. This article examines the two most comprehensive multilateral international conventions regulating the world's oceans and biodiversity – the Convention on the Law of the Sea and the Convention on Biological Diversity – to address the question of whether these two conventions adequately protect marine biota from AUNP. It is argued that the existing regimes established under these conventions are inadequate because they do not: sufficiently recognise AUNP as a form of pollution; provide comprehensive and binding direction as to practical measures to prevent, mitigate or eliminate AUNP; or provide adequate enforcement regimes. To remedy these inadequacies, this article concludes by outlining a number of non-exhaustive law reform recommendations.

Underwater noise pollution in China’s Yangtze River critically endangers Yangtze finless porpoises (Neophocaena asiaeorientalis asiaeorientalis)

Underwater sound plays an important role in some critical life functions of many aquatic animals. Underwater noise pollution has received relatively more attention in ocean systems. However, little attention has been paid to freshwater systems, such as the Yangtze River which is the habitat of critically endangered Yangtze finless porpoises (Neophocaena asiaeorientalis asiaeorientalis). In 2012, the underwater noise levels in 25 sites along the middle and lower sections of the Yangtze River were measured. The root mean square sound pressure level (SPL) and unweighted sound exposure level (SEL) at each site ranged between 105 ± 2.4 (median ± quartile deviation) and 150 ± 5.5 dB. Obvious spatial and temporal variations in the SPL were detected among the 25 sites. The SPL and SEL in the middle section of the Yangtze River were smaller (approximately 15 dB) and fluctuated more compared to those in the lower section. The power spectrum in the mainstem was site specific. However, all the spectra levels were higher than the audiogram of Yangtze finless porpoises. Majority of the sites had an averaged cumulative unweighted SEL (72%) and cumulative weighted SEL (68%) that surpassed the underwater acoustic thresholds for onset of hearing temporal threshold shifts for finless porpoise. Porpoise bio-sonars were detected in 89% of sonar monitoring sites indicating that noise pollution in the Yangtze River greatly threatened porpoise survival. In 8% of the sites, the averaged cumulative weighted SEL exceeded that of underwater acoustic thresholds causing non-recoverable permanent threshold shifts of finless porpoises auditory system whereas it was less than 1 dB below the underwater acoustic thresholds in other 8% of the sites. These sites urgently needed noise mitigation and management strategies. These results will facilitate the evaluation of the impacts of anthropogenic noise pollution on local finless porpoises and give further guidelines on its effective conservation.

Singing under glass: rapid effects of anthropogenic habitat modification on song and response behaviours in an isolated house sparrow Passer domesticus population

Anthropogenic noise pollution and the introduction of novel infrastructure can impose strong selective pressures on avian communication by affecting the efficacy with which acoustic signals are transmitted and received. Many species have now been shown to sing at higher frequencies in noisy urban environments. However, few studies have investigated the effects of signal modification on the response behaviours of receivers, and fewer still have been able to indicate the timescale over which these changes in pitch have occurred. We compare vocal communication between house sparrows Passer domesticus that reside within the world's largest, single‐span glasshouse (completed in the year 2000), and house sparrows directly outside this glasshouse, in open farmland. The glasshouse contrasts both acoustically and physically with the external environment, low frequency background noise being significantly louder inside than outside. We show that minimum song frequency was significantly higher inside the glasshouse than in surrounding farm habitat. Using song playback, we also found that birds within the glasshouse reacted more strongly to playbacks from the glasshouse habitat than they did to playback of song from farm birds outside. The degree of difference in frequency is similar to that shown for other bird species between urban and rural environments, demonstrating that such behavioural differences may arise over a relatively short time period (14 yr in this case).

Seismic noise influences brood size dynamics in a subterranean insect with biparental care

Anthropogenic noise pollution is known to alter the behaviour of acoustically sensitive animals. Many animals also sense vibrations through solid substrates and use substrate-borne vibrations in conspecific communication. The effects of substrate-borne noise pollution, however, remain largely unknown. Here, we investigate the potential for seismic (soil-borne) noise to alter the reproductive behaviour of the burying beetle Nicrophorus marginatus, a species that breeds below the soil surface on vertebrate carcasses and provides biparental care to offspring. Nicrophorus marginatus beetles produce sound using stridulatory structures on the elytra and abdomen, but no ears have been identified in these beetles, suggesting that stridulation might function to produce substrate-borne signals. We examined the timing of stridulation during reproduction, measured neural responses of beetles to substrate-borne vibrations, and measured beetle reproduction in the presence and absence of seismic noise. We found that parental beetles stridulate throughout carcass preparation and the burial process and confirmed that adult beetles are sensitive to low-frequency seismic vibrations. Variables related to brood size were affected in treatments with seismic noise, with burying beetles producing smaller broods with lower total mass than those in control treatments, providing support for the hypothesis that substrate-borne noise may impose fitness costs for soil-dwelling animals. The precise mechanisms leading to reduced brood size remain unknown but may relate to disruption of seismic communication or inaccurate assessment of resource size. Additional investigations are required to understand the degree to which human-generated seismic noise in natural settings influences other edaphic species, and whether these behavioural impacts lead to shifts in edaphic community structure or function.

The effect of anthropogenic noise and weather conditions on male calls in the bladder grasshopper Bullacris unicolor

ABSTRACT Acoustic communication in animals relies upon specific contexts and environments for effective signal transmission. Increasing anthropogenic noise pollution and different weather conditions can disrupt acoustic communication. In this study, we investigated call parameter differences in the bladder grasshopper Bullacris unicolor inhabiting two sites in close proximity to each other that differed in their noise levels. Calling activity was monitored via passive acoustic recorders. Weather conditions, including wind speed, temperature and humidity, were also recorded. We found that the interval between successive calls increased with higher noise levels at both sites, and the peak frequency became lower. The total number of calls detected also decreased with anthropogenic noise, but this relationship was only evident at the noisier site. In addition, grasshoppers shifted the timing of their calls to later in the night at the noisier site, possibly to take advantage of relatively lower noise levels. We also found that weather conditions, particularly temperature, had a significant influence on call parameters. Further studies are thus needed to disentangle the effects of anthropogenic noise and environmental variables on calling activity in this species. Our results lend support to the growing concern regarding the effects of noise pollution on animal acoustic signalling systems and also highlight the complexity of factors which affect sound signalling in natural environments.