We found that opportunistic observations by iNaturalist users recorded more species than structured surveys cumulatively at most sites and, on average, more species per year at half the monitoring sites. In addition, iNaturalist recorded a different subset of species, with fewer than half the species observed opportunistically by iNaturalist users being recorded by structured surveys. iNaturalist likely recorded more species in this study, at least in part, due to the substantially greater sampling effort, with the iNaturalist observations being acquired from more than 1200 observation events between 2017 and 2019 (i.e., dives where at least one species was recorded and submitted to iNaturalist) compared to only 71 structured surveys done over the same period. The high number of species recorded by iNaturalist clearly demonstrates the considerable potential of opportunistic observations as an effective tool for documenting species richness. Tiralongo et al. (2021) similarly noted the efficiency of using opportunistic observations to record fish biodiversity, with considerably more species recorded during underwater photography competitions in the Mediterranean than various standardised survey techniques.
Sydney has a large community of predominantly local divers and underwater camera ownership is prevalent, and this likely helped the region accumulate such a substantial number of observations in the relatively short 3-year period since the Australasian Fishes project commenced. The fact that the study region is also dominated by local divers who often revisit the same sites frequently may mean that many contributors have a high degree of familiarity with local species and actively seek out rare or cryptic species. The high number of submissions in Sydney may have resulted in more species being recorded than in less populated areas of Australia or those with less active diving, snorkelling, or fishing communities. This bias towards areas of high population density in opportunistic databases and other citizen science initiatives has been shown previously and discussed extensively (Szabo et al. 2007; Tiago et al. 2017; Callaghan et al. 2019). Despite this, we consider the success of Australasian Fishes in Sydney within a relatively short time period to indicate the potential of iNaturalist in regions with less diving, snorkelling or fishing, given sufficient time and promotional effort to grow the project.
Losey et al. (2012) found that the species richness derived from opportunistic observation of ladybugs was similarly greater than the combined richness of several structured professional taxonomic surveys. However, in that case the difference was attributed to not only the greater number of opportunistic samples but also to a greater geographic spread. A greater spread of sampling effort is likely to have also influenced species richness in this study, but at a localised site scale. That is, the structured surveys were constrained to standardised transects at a consistent depth, with only one 50 m stretch of reef generally sampled at each dive site. In addition, a similar area is sampled on repeat surveys with transects commencing from a consistent GPS coordinate. In contrast, a recreational diver could easily cover several hundred meters of reef on a single dive, and the depths and area covered would vary among different divers and visits. Further, iNaturalist observations come from a range of different types of contributors, including snorkelers and fishers, and these groups may observe species that are less frequently encountered by scuba divers. Snorkelers, for example, will likely encounter more species that inhabit shallower waters, which may be under-represented in the structured surveys which were done by Scuba diving only. Consequently, most of a site is likely to be covered by the combined efforts of many iNaturalist contributors, which in this dataset included hundreds of visits to some sites. Although fishers have the potential to contribute unique observations of species, which are attracted to bait but may avoid divers or snorkelers, this is unlikely to have occurred in this study as only a very small proportion of observations (8 of 7600 photos) were contributed by fishers, all of which were of species also observed in-situ by divers or snorkelers.
It is important to note that the structured surveys used by Reef Life Survey are not specifically designed to measure species richness, rather, it is a global scale survey with effort primarily directed at sampling many sites with a consistent methodology, instead of sampling individual sites intensively (Edgar and Stuart-Smith 2014). It is also worth highlighting that the structured surveys were considerably more efficient at recording species with approximately five times as many species recorded per dive. This is likely due to the structured surveys recording all species observed within the sampling parameters while iNaturalist users are highly selective about what they photograph and contribute. Importantly, the use of a consistent methodology by RLS and similar structured survey approaches allows for robust comparison of trends through time and across sites, on a global scale. In addition, RLS gathers a suite of information, which is not readily obtainable from iNaturalist photographs such as the relative abundance of species, the size of species, as well as documenting the habitat composition using photo-quadrats. Comparison of iNaturalist, or similar opportunistic observations, to a more intensive structured survey program that is designed to specifically capture biodiversity would be a valuable future research direction. Such a comparison would help better understand how much sampling effort is required to capture similar amounts of biodiversity using structured and unstructured approaches.
The fact that fewer than half of the species recorded at all sites between 2017 and 2019 were present in both datasets demonstrates a considerable difference in the species recorded by the opportunistic observers and structured surveys. In part, this is likely to result from the greater overall species richness recorded by iNaturalist at most sites, which is also reflected by the large proportion of species that were unique to iNaturalist at each site. The large number of species unique to iNaturalist suggests that users are photographing and contributing species that are not readily captured by conventional structured surveys. Several cryptic species such as Weedy Seadragon (Phyllopteryx taeniolatus), White’s Seahorse (Hippocampus whitei), Sydney Pygmy Pipehorse (Idiotropiscis lumnitzeri), and Dwarf Lionfish (Dendrochirus brachypterus) were recorded frequently by iNaturalist but rarely present in the RLS dataset. In addition, some rare or low abundance species were also recorded more by iNaturalist including Port Jackson Sharks (Heterodontus portusjacksoni), Smooth Stingray (Bathytoshia brevicaudata), Three Bar Porcupinefish (Dicotylichthys punctulatus), and Comb Wrasse (Coris picta). In contrast, the two species more frequently detected by RLS, the Girdled Parma (Parma unifasciata) and Clark’s Threefin (Trinorfolkia clarkei), are commonly encountered on Sydney’s rocky reefs. A similar result was reported by Tiralongo et al. (2020) who found that underwater photographers were effective at finding rare, small and cryptic fish species while Snäll et al. (2011) found that rare and uncommon bird species were essentially missed by structured surveys but captured by opportunistic citizen records.
Many iNaturalist contributors are likely to spend a substantial part of their dive searching for rare or cryptic species, simply for the challenge and reward of photographing species that are difficult to find. They may also be more likely to contribute photographs of these species to iNaturalist as their perceived value as a biodiversity observation may be greater due to their rarity. In contrast, rare or less abundant species are likely to be missing from the RLS dataset simply due to the reduced sampling effort and consequently a decreased probability of encounter during surveys. Further, although RLS includes a specific method for cryptic species, including looking in caves and overhangs along the transect, a consequence of using standardised transects means that observers are not free to ‘roam’ the dive site searching for certain species. The tendency of opportunistic observers to seek out rare species can be considered as a bias, however as noted by others, the fact that species are recorded that are often missed by structured surveys can equally be viewed as one of the key benefits of such methods (Snäll et al. 2011; Kamp et al. 2016).
In addition to rare species being favoured over common ones, there is potential for bias towards interesting species and away from less remarkable ones (Isaac and Pocock 2015; Prudic et al. 2018; Caley et al. 2020). Indeed, many of the species recorded more frequently by iNaturalist in this study, are also arguably very ‘photogenic’ such as seahorses and other syngnathids or ‘charismatic’ such as sharks and rays. There is also the potential for iNaturalist observations to be skewed towards species, which are more readily photographed, with many of the species more commonly recorded by iNaturalist in this study being benthic or slow-moving species. A recent traits analysis for birds found evidence that large-bodied species and those that occur in large flocks are over-represented in iNaturalist compared to the semi-structured eBird checklists, potentially as they are easier to find and photograph (Callaghan et al. 2021). A similar quantitative assessment of which fish traits affect the likelihood of a species being represented in opportunistic databases such as iNaturalist, although beyond the scope of this study, deserves further exploration as it influences how opportunistic photographs can be utilised for future research and biodiversity monitoring.
Action to conserve biodiversity, such as determining locations for protection, often relies on species occurrence data to identify biodiversity hotspots or areas that contain rare or endangered species. This is particularly important for rare or cryptic species, which can require substantial time and effort to find using conventional structured surveys. The high species richness and rare species recorded by iNaturalist in this study clearly demonstrates the enormous potential of platforms such as iNaturalist as a tool for documenting biodiversity and species conservation. Importantly, a large proportion of the observations were submitted over a relatively short 3-year period, following the launch and active promotion of the Australasian Fishes project, demonstrating the potential to gather large numbers of biodiversity observations through opportunistic observation platforms such as iNaturalist. This is largely the result of the relative ease of gathering and contributing iNaturalist observations, where essentially the only requirement is a photograph, compared to the high level of training and dedication required to gain the knowledge and skills required to do structured surveys. This means that large numbers of people can easily contribute to platforms such as iNaturalist, since the barriers to participation are low, resulting in substantial sampling effort due to greater ‘people power’.
In addition to having a large recreational diving community, the rapid growth of the Australasian Fishes iNaturalist project may be attributable, at least in part, to the various marine citizen science projects that preceded it in Australia (e.g., RLS, Redmap). These have potentially helped establish a highly engaged diving community, which is willing to contribute to citizen science initiatives. The ability to replicate the success of Australasian Fishes or similar citizen science initiatives may also be limited in lower socioeconomic countries where there is less time and money for expensive activities (Haklay 2013; Walker et al. 2021) like scuba diving and underwater photography. However, iNaturalist is a global platform with high levels of engagement world-wide, and substantial numbers of fish photographs have been contributed for many geographic areas including lower socioeconomic areas such as South-east Asia, Central America and The Caribbean. Importantly, for many of these regions there is often limited monitoring of marine environments by scientists due to a lack of funding, however, they are popular destinations for scuba diving tourists. As such, there is considerable potential to supplement structured survey data in undersampled regions by recruiting tourists (Schaffer and Tham 2020; Callaghan et al. 2021). The relative ease of contributing observations means that platforms such as iNaturalist may be particularly well suited to documenting biodiversity in areas dominated by tourism diving where potential participants are unlikely to have the time or local species knowledge to do more complex surveys (Hermoso et al. 2021). However, given the considerable differences in the experience and motivations between tourist and local divers (Hermoso et al. 2021), it is difficult to know for certain how the results of our study, in a region with a highly active community of local divers would translate to areas dominated by tourist divers. In areas where recreational diving or snorkelling is minimal, including by tourists, it may be possible to gather opportunistic observations by engaging with other users of the marine environment such as commercial or subsistence fishers (Fulton et al. 2019). Expanding the current study to regions dominated by tourist divers, or those used by recreational, commercial or subsistence fishers would be an important future research direction and further exploration of the differences in experience, knowledge and motivation to participate in citizen science would be a valuable addition.
The lack of standardised methods for gathering observations, and the subsequent variability in effort and numbers of observations, is clearly one of the main limitations of opportunistic observation databases. For example, almost two-thirds the iNaturalist observation events (e.g., dives) in this study had three or less fish species yet it is considered likely that in many of these cases more fish were photographed but not submitted. Further, there is likely to also be many additional observation events where users didn’t submit any photographs to iNaturalist as they didn’t record any species or photographs which they considered worth submitting. If some users are only submitting ‘interesting’ observations or ‘good’ photographs, then simply encouraging existing users to share all their observations may improve the representation of more common species. Alternatively, more data could be gathered by capitalising on incidental data (Callaghan et al. 2021) as common species may often be captured in the background of photographs, and this is an area that deserves further exploration.
Ultimately, the greater number of species recorded by iNaturalist than structured surveys does not mean that opportunistic observations are a better way of measuring species richness or monitoring biodiversity. Indeed, relying on opportunistic observations alone for biodiversity conservation decision making could be highly problematic due to the biases of this method. For example, the increase in species recorded with greater observation effort could potentially result in more popular sites being protected, such as those with greater accessibility, instead of more biodiverse ones (Nelson et al. 1990; Reddy and Dávalos 2003). Our results from Camp Cove illustrate this point, as iNaturalist recorded the second lowest number of species at this site, hypothetically making it a low priority for protection, however it had the second most species based on the structured surveys. The low iNaturalist species count in this case was likely due to Camp Cove being a less popular dive site with both the lowest number of iNaturalist sampling events and the least photographs submitted. As has been suggested and demonstrated by others (Fithian et al. 2015; Giraud et al. 2016; Soroye et al. 2018; Rapacciuolo et al. 2021), integrating opportunistic citizen science observations with structured survey data from more traditional sources (e.g., government monitoring and university research) will help ensure that both common and rarer species are well represented in biodiversity monitoring. It is worth noting however, that combining data sources may not always be the best approach, and where there are sufficient structured surveys it may be more efficient and reliable to use these data alone, especially if there is considerable and unknown bias in the opportunistic observations (Simmonds et al. 2020).