This study evaluated, for the first time, the dung beetle taxonomic and functional responses along an urbanization gradient in Amazon. Our results highlighted the consistent shift in species composition, and reduction of both taxonomic and functional diversity from forest patches located outside the city towards those located in city core. In addition, forest cover was the main driver of dung beetle responses at the landscape scale, where the increase of forest cover positively affected dung beetle diversities. These results indicate the negative impact of urbanization on dung beetle taxonomic and functional diversity in Amazonian cities. Our findings highlight and reinforce the importance of maintaining forest cover to conserve dung beetles and consequently, their ecological functions (e.g., nutrient cycling, secondary seed dispersion and others) in urban landscapes.
Dung Beetle Taxonomic And Functional Response Along A Preserved-rural-urban Gradient In The Brazilian Amazon
The species richness and abundance of dung beetles were higher in preserved sites than in rural and urban fragments. These results corroborate the findings of Korasaki et al. (2013) and Salomão et al. (2019) in Atlantic Forest, and Correa et al. (2021a,b) in the Brazilian Cerrado, who also found that urbanization negatively affected dung beetle species richness and abundance. Overall, dung beetle assemblages from tropical forests are greatly influenced by three factors: i) vegetation structure (Halffter and Arellano 2002; Gardner et al. 2008a; Costa et al. 2017); ii) availability of mammalian dung resources (Nichols et al. 2009; Bogoni et al. 2019; Raine and Slade 2019) and iii) microhabitat conditions (Larsen 2012; Davis et al. 2013). The low species richness and abundance in forest patches located in the core of urban areas indicate that, although such remnants maintain ecological communities, they are highly impoverished when compared to the patches located in the city surroundings. It is important to consider that forest fragments located outside the city of Juína (preserved forests) are larger than rural fragments. The observed effects of habitat type on urban fragments in this study are in accordance with previous fragmentation studies (Nichols et al. 2007; Filgueiras et al. 2015), in which smaller fragments (which were our urban fragments) retain a lower diversity compared to larger ones (rural fragments and preserved ones). Cities highlight as an important element of the landscape matrix that can present novel scenarios and trends regarding the paradigm of habitat loss and fragmentation (Fahrig et al. 2022). As Amazon region comprises a relative pristine tropical ecosystem when compared to other Neotropical ecosystems (e.g. Cerrado, Atlantic Forest), biodiversity resilience to urbanization and to forest fragmentation are potentially distinct. In order to disentangle the effects of forest fragmentation from urbanization ones, future studies should aim at analyzing patches of different sizes throughout the spatial gradient that comprises the urban core and surroundings.
We found that dung beetle assemblage structure differed among preserved sites, rural and urban fragments. The fragmentation of natural habitats, caused by urbanization, modifies microclimatic factors (abiotic and/or biotic factors), including soil temperature, air pollution and insect trapping by artificial light (McKinney 2008; Chen et al. 2010; Edmondson et al. 2016). In addition, physical changes along the gradient preserved-rural-urban strongly influence available habitats for native species (McKinney 2008). Therefore, the new abiotic conditions imposed on dung beetles by urbanization can directly affect the biology, dispersion, and colonization ability of species (Hanski and Cambefort 1991), modifying the assemblage structure of the dung beetles (Correa et al. 2021a). Thus, animal species that successfully occupy forest fragments in urban landscapes survive under climatic conditions highly contrasting from those observed in conserved forests or in rural landscapes (McKinney 2008; Huang et al. 2009; Grimm et al. 2011). However, the presence of well-defined assemblages in each habitat type highlights the importance of each of them for conserving diversity of dung beetles in the landscape studied. Indeed, urban fragments can provide refuges for dung beetles in the urban matrix (see Korasaki et al. 2013; Salomão et al. 2019; Correa et al. 2021a), while rural fragments may help to maintain biodiversity and their associated ecological functions in agricultural landscapes (Gray et al. 2014). Our results may indicate that, although dung beetle assemblages from Amazon region are highly sensitive to urbanization, there are groups of species that are maintained even in the most urbanized remnants of a city.
Even though we detected changes in dung beetle richness and assemblage structure along preserved-rural-urban gradient, the presence of indicator species in preserved forests and urban fragments suggest that dung beetle species respond differently to urbanization effects (see Salomão et al. 2019; Correa et al. 2021a). In the case of indicator species of preserved forests, they are more susceptible to changes in habitat (McGeoch et al. 2002), and in many cases may be restricted to certain habitat types or conditions (e.g., forest-dependent species; see da Silva et al. 2019). Thus, with the increase in the urbanization these species are the most susceptible for extinction in urban landscapes in the Amazon Forest. In contrast, urban fragments can offer advantages to opportunistic species and species more tolerant to the changes in the environmental dynamics (e.g. matrix-tolerant species; see da Silva et al. 2019), that result in an increase of their populations. It is important to consider that most species were classified as indicators of preserved site. We believe that this reflects the history of the region, which was originally covered by forested sites, thus comprising the environmental scenario in which species adapted to inhabit in Amazon. Since urbanized fragments are structurally less conserved (Salomão et al. 2019), the few indicators of urban fragments in the current study could be related to the expansion of species that benefit from open-habitat conditions that come from the expansion of the arc of deforestation in Amazon region (França et al. 2021; Maldaner et al. 2021).
Until now, there has been very little information on the effects of urbanization on dung beetle functional diversity in Neotropical region (see Correa et al., 2021a,b). We found that functional richness (FRic) was higher in the preserved forests, while functional dispersion (FDis) and functional evenness (FEve) did not differ among habitat types. This lack of difference of FDis and FEve among the habitat types can be an indication that only the identity of traits (FRic) is being influenced by urbanization and not the structure of the functional assemblage. Similar FDis values may indicate a higher dispersion of functional traits in rural and urban fragments corresponding to a gain in the variability of responses to urbanization disturbances among species that contribute in a similar way to the ecosystem function (Laliberté et al. 2010). Similar FEve values suggest that the space of the functional niche is being uniformly occupied by dung beetle species (see Audino et al. 2014; Correa et al. 2021a). Thus, Amazonian dung beetles may show characteristics that allow them to tolerate the environmental conditions created by the urbanization disturbance, where the niche space occupied in rural and urban fragments is really being exploited (Barrágan et al. 2011; Audino et al. 2014; Correa et al. 2019, 2021a). An alternative hypothesis is that our results are context dependent. Since Juína is a relatively small city in Amazon region compared to Manaus – ca. 2,250,000 inhabitants, and Belém – ca. 1,500,000 inhabitatns (IBGE 2021), it is possible that in Amazon metropolitan regions the effects of urbanization on functional diversity may differ. Finally, the lower FRic values found in rural and urban fragments suggest a loss of functionally specialized species in these areas, causing low stability through time and reduction in ecosystem processes provided by dung beetles (Cadotte et al. 2011; Díaz and Cabido 2011; Audino et al. 2014). The decrease of ecosystem services such as nutrient cycling, seed dispersal (Nichols et al. 2008) and reduction of fecal helminth transmission that cause human diseases (Nichols and Goméz 2014) in urban ecosystems can indirectly affect human life quality in the urban landscapes (see Correa et al. 2021a).
Landscape Descriptors As Drivers Of Dung Beetle Responses In A Preserved-rural-urban Gradient
Our results demonstrated that forest cover was the main driver of changes in dung beetle diversities, and the decrease in forest cover led to low dung beetle species richness, abundance, FEve and FRic. Besides, the increase in forest edges restrained dung beetle species richness. In the tropics, the loss of native green areas due to urbanization apparently follows the same trend observed in other human-made matrices, as pasturelands (Alvarado et al. 2018b), plantations (Beiroz et al. 2019), and water cover due to hydroelectric (Storck-Tonon et al. 2020). In our opinion, the message in clear: the loss of tropical forests comes with a linear decrease of biodiversity, which is simplified both taxonomically and functionally. Nonetheless, we need to be careful, because there are clear differences among the ecosystems regarding the negative effects of the loss of forest cover and increase in the amount of edge. For example, in the fragmented landscape of the Atlantic rainforest, located in South America, the amount of edge apparently is the most important driver of biodiversity changes, negatively affecting dung beetle assemblages (Souza et al. 2020). Notwithstanding, in the Northernmost tropical rainforest of Americas, in Mexico, landscape heterogeneity is the most important driver of changes in dung beetle assemblages (Alvarado et al. 2018b; Rivera et al. 2020). In our study, landscape heterogeneity (i.e. Shannon Diversity of land use) did not affect beetle diversity, being landscape composition the most important drivers. Since Amazon region is suffering from a relative recent intense landscape transformation (but see Levis et al. 2017), its communities are still pristine when compared to other tropical ecosystems. Pristine ecosystems are much more sensitive to landscape changes than chronically disturbed ones (Melo et al. 2013). This marked sensitiveness may explain the clear effect of forest cover, a much more direct landscape predictor, instead of other landscape variables.
Precisely, our results highlight the importance of the amount of native forest habitat in Amazonian cities (Rico-Silva et al. 2021; Fragata et al. 2022). According to the habitat amount hypothesis the species richness in a habitat site increases with the amount of habitat in the ‘local landscape’ defined by an appropriate distance around the site, with no distinct effects of the size of the habitat patch in which the site is located. This, in practice and based on our results, suggest that, in order to optimize biodiversity conservation in urban Amazonian landscapes, land-sparing models may be appropriate (Ibañez-Alamo et al. 2020). It has been argued that buildings interspersed with green patches that concentrate biodiversity-supporting vegetation are the best approach for cities. Finally, the development of sustainable initiatives for the conservation of biodiversity in urban landscapes, such as public policies aimed at the maintenance of urban forest fragments, can help to maintain biodiversity within cities (MacGregor-Fors et al. 2016).