Seasonality effect on ant (Hymenoptera: Formicidae) activity in an ecotonal environment in the state of Piauí, Brazil

This study aimed to compare the activity of ants during the rainy and dry seasons in a transitional environment (ecotone) at the interface of Cerrado and Caatinga biomes situated in the state of Piauí, where studies of local ant fauna are still incipient. The research was carried out at Floriano, state of Piauí, Brazil. Ants were sampled during the rainy season in December 2018 and during the dry season in June 2019. Three parallel transects were established at least 100 m apart. At each point, three sampling methods were used, with attractive baits placed on the trees and ground, and pitfall traps on the ground. Thirty-one ant species belonging to 14 genera and five subfamilies were collected, with 27 recorded in the rainy season and 17 in the dry season. The only species reported with the three collection methods and the two seasons was Camponotus crassus Mayr, 1862. Seven ant species sampled in this study are new reports for the state of Piauí. There was a significant difference in the average number of ants and the composition of the assemblages between the rainy and dry seasons. We demonstrate that the activity of ants is influenced by seasonality, corroborating our hypotheses. This is only the second study on ant diversity in the state of Piauí, and more studies are necessary to understand the processes and factors which regulate the activity patterns of ants in ecotonal situations like those observed in this region.


INTRODUCTION
Ants are eusocial insects prominent in terrestrial ecosystems because of their wide geographical distribution, abundance, and species richness (Hölldobler & Wilson, 1990;Alonso & Agosti, 2000;Vasconcelos et al., 2001;Jory & Feitosa, 2020). Brazil has the most remarkable diversity of ants in the Americas, and one of the most extensive globally, registering more than half of the species described for the Neotropical Region, which corresponds approximately to 1,500 species distributed in 112 genera (Bolton, 2021).
In the state of Piauí, northeastern Brazil, 152 ant species distributed in 53 genera and nine subfamilies were reported in 24 sites sampled (Jory & Feitosa, 2020). This state requires prioritization in terms of diversity studies, since it presents an ex-tensive transition zone between three Brazilian biomes, the Atlantic Forest, Cerrado, and Caatinga (Jory & Feitosa, 2020), while only basic information is available on the ant fauna in this region.
Transition zones or ecotones are areas where different biomes meet, with marked environmental gradients (Kark & Rensburg, 2006) that can occur on multiple spatial and temporal levels. They are related to the strength of interactions between adjacent ecological systems, which can vary from wide interfaces between biomes to small-scale local transitions (Holland, 1988;Gosz, 1993). Ecotones with large territorial extensions may present different characteristics from the central areas of the biomes in aspects such as rainfall variations and oscillations of environmental variables (Smith et al., 1997). In addition, these differences can have relevant effects in some animal groups when associated with other factors such as genetic isolation, resulting in phenotypic differentiation and speciation (Smith et al., 1997). Studies in ecotone areas are still lacking, particularly in transition areas with the Caatinga biome (exceptions are Silva et al., 2017 andNeves et al., 2013). The conservation of the diversity of these transitional environments is extremely important, as they are characterized by a mosaic of vegetation and certainly influence the species living in the surrounding environments (Neves et al., 2013). Understanding the factors responsible for the coexistence, biology, and variations in the activities of ants according to the characteristics of these environments is essential for understanding local and regional ant diversity.
The Cerrado is considered the largest Brazilian biome, with a high species richness that probably represents 33% of the country's biological diversity (Aguiar et al., 2004;Klink & Machado, 2005). Due to its favorable topographic conditions, soil types, and climate with well-defined seasons, this biome has been deforested on an accelerated scale essentially for livestock and industrial agriculture (Aguiar et al., 2008). In response to this growing exploitation by human activities, the diversity of ants in this biome has mainly been studied to understanding the disturbances caused to natural environments, such as in Eucalyptus monoculture (Majer, 1996;Marinho et al., 2002;Silva & Delabie, 2014); otherwise, their use as bioindicators of preserved and degraded lands was highlighted (Brandão et al., 2011;Tibcherani et al., 2018).
In contrast to the Cerrado, the Caatinga, a predominant biome in the Northeast region of Brazil, is considered a poor biome in species and therefore has less studied when compared to other phytogeographic regions of the country (Leal, 2003). However, this biome constitutes an important biological heritage because it houses many endemic animals and plants (MMA, 2017;Leal, 2003). Studies related to ant diversity have already been carried out in this biome, with an emphasis on environmental monitoring (Leal, 2003;Nunes et al., 2011;Freire et al., 2012). Knowledge on other biological aspects related to diversity yet is still scarce, and the effects of seasonality on the coexistence and activity of ant species are poorly understood (Ulysséa & Brandão, 2013).
Despite the increase in studies in recent years on ant fauna in the Caatinga and Cerrado (for example, Neves et al., 2013), several questions about the structure of the communities remain. Due to the shortage of research on ants in the state of Piauí and the growing threats to diversity resulting from the loss of habitats (Divieso et al., 2020), there is a need to understand the structure of the community of these organisms. In this study, we sought to answer the following questions: (i) Which ant species occur in a transitional environment between the Caatinga and Cerrado biomes in the state of Piauí? (ii) Does ant activity differ according to seasonality? We hypothesized that the ant activity is more significant during the rainy season, when factors that usually determine the species richness and community composition (food and nesting resources, environmental heterogeneity, and abiotic factors) are more readily available compared to the dry season. We expect that ant arboreal and soil predators have a marked activity during the rainy season, while generalist and opportunistic species dominate otherwise.

Study area
The study was carried out at the Fazenda Experimental Novo Recreio (collecting permit ICMBio/SISBIO #64030-1), in a transitional area between the Caatinga and Cerrado biomes in the municipality of Floriano, Piauí (06°46′01″S; 43°01′22″W; Fig. 1). The regionals climate is characterized as semi-arid tropical, with temperatures varying between 21℃ and 38℃, with a six-month dry period (May to October) and a rainy period between November and April (INMET, 2019).

Ant sampling
Ants were collected in December 2018, during the rainy season, and in June 2019 during the dry season (at the same points in both seasons). Three parallel transects were established, at least 100 m apart, with a distance of at least 50 m from the edge. We defined 10 sample points in each of these transects, at intervals of 20 m. At each point, the ants were collected using three different methods: (i) a pitfall trap in the soil (pitfall trap). A 500 mL disposable plastic cup with a diameter of 10 cm and a depth of 19 cm was used as pitfall trap, which was half-filled with a solution composed of water and neutral detergent. The pitfalls remained active in the field for 48 h; (ii) a sardine in comestible oil bait disposed on the ground (soil bait), and (iii) a sardine in comestible oil bait on the nearest bush or tree (tree bait). In the two last cases, tubes (Falcon type) containing small pieces of sardine in comestible oil were used for the baits, which remained active for two hours. We consider the three samples obtained with the different methods as a composite sample at each sampling point. These methods are commonly used and recommended for sampling ants (Delabie et al., 2021).
The biological material resulting from the collections was examined, separating the ants from the other arthropods, and the specimens were conditioned in alcohol 70%. Subsequently, they were sent to the Myrmecology Laboratory of the Cocoa Research Center (CEPEC/ CEPLAC) at Ilhéus, Bahia, Brazil. The ants were mounted and identified at the highest specific level possible using identification keys (Kempf, 1951;Cuezzo, 2000;Schmidt & Shattuck, 2014;Bolton, 2021) and comparison with the reference collection at the Myrmecology Laboratory of the Cocoa Research Center (CPDC, Delabie et al., 2020). Vouchers were deposited in the CPDC collection.

Statistical analysis
We used a paired t-test to assess whether the average number of ants recorded in the samples differed between the two seasons. To determine whether there was a difference in the composition of ant assemblies according to seasonality, an analysis of similarities (ANOSIM) was performed using Bray-Curtis dissimilarity distance, followed by similarity percentage analysis (SIMPER) to determine which species contributed the most to the dissimilarity between the seasons. We used non-metric multidimensional scaling (NMDS) to illustrate the relationship between the variables, using the Jaccard similarity index as the association measure. For this, we considered the presence/absence of each ant species per sampling points. Statistical analyses were performed using PAST 4.02 (Hammer et al., 2001) and R 3.6.1 (R Core Team, 2019), using the package "vegan" (Oksanen et al., 2019).
We recorded three species that occurred only during the dry period: Acromyrmex rugosus (Fr. Smith, 1858), Ca. atriceps, and Gnamptogenys moelleri (Forel, 1912), while 14 species (45.2% of the total) occurred exclusively in the rainy season. Among these, we must emphasize the Atta opaciceps Borgmeier, 1939, B. heeri, Cephalotes pavonii (Latreille, 1809, and Cyphomyrmex transversus Emery, 1894, as unique species representing their respective genera in the study. The ant species D. quadriceps, Ectatomma muticum Mayr, 1870 and Odontomachus bauri Emery, 1892 are also as unique species representing their respective genera in the study, however they were recorded in both seasons (Table 1).
The pitfall trap was the most efficient sampling method, with 29 species collected in the study (93.5%).
Cr. pavonii and Pheidole (group Diligens) sp.06 were only collected using bait on the trees (solely during the rainy period). None of the species occurred exclusively at baits on the ground. Camponotus crassus was the only species recorded in the three collection methods used, and in both seasons studied (Table 1).

DISCUSSION
Our results demonstrate that both the number and occurrence of ant species were significantly different between the two seasons studied, corroborating our hypotheses. The low similarity between the ant assemblages in the different seasons likely indicates the effect of seasonality on ant activities in the ecotone. In dry tropical systems, previous studies have shown that environmental heterogeneity, resource availability and abiotic factors determine species richness and the composition of the ant assemblages (Leal et al., 2008;Fagundes et al., 2009;Gomes et al., 2010;Neves et al., 2013).
Studies on environmental heterogeneity and seasonal analysis (Coelho & Ribeiro, 2006;Passos et al., 2016) demonstrated more significant ant activity in the rainy season, with the genera Camponotus and Pheidole being the most diverse. The subfamily Myrmicinae was dominant in our study, both in the number of species and in the number of genera. The predominance of Myrmicinae is a general pattern in ant studies in natural environments of the Neotropical Region (Hölldobler & Wilson, 1990;Santos et al., 2017;Silva et al., 2017;Koch et al., 2019), and the state of Piauí is not an exception (Jory & Feitosa, 2020). This subfamily represents about 50% of the diversity of known ant species (Bolton, 2021), and presents flexible feeding habits, behavioral dominance, and varied reproduction strategies (Fernández, 2003;Silvestre et al., 2003;Hamidi et al., 2017), as well as extreme adaptability to control large ecological niches in the Neotropical region (Fowler et al., 1991).
The most representative genera in this study reflect the results of Jory & Feitosa (2020). These authors also observed Pheidole with the most significant number of species, followed by Solenopsis and Camponotus. In our study, Pheidole also showed the largest variation in the number of species between the two seasons studied, having greater activity during the rainy season. This genus is cosmopolitan, essentially pantropical and subtropical, and is generally well represented in the collection of soil ants, such as in forested areas (Leal, 2002;Bieber et al., 2006;Koch et al., 2019;Santos et al., 2017;Vasconcelos et al., 2017). Studies have shown that some Pheidole species are opportunist predators of a range of arthropods (Fernandes et al., 1994;Dejean et al., 2007;Lange et al., 2008), as well as some are associated with seed availability (Pirk et al., 2009;Lôbo et al., 2011). They can be scavengers and may explore as opportunists many other resources. Thus, a lower activity of these species is expected during the dry season, when the supply of these resources is smaller or limited, in relation to the  rainy season. The low activity which was observed may be the consequence of a selective mortality on a range of species, but more certainly reflects the reduction in foraging activity (areas explored by workers, lower frequency of them) due to a probable decrease in populations of many species.
Camponotus is more present in biomes with characteristics of open habitats, such as Caatinga (Leal, 2003) and Cerrado (Ronque et al., 2016), while Solenopsis can be found in forested areas, countryside, savannas, as well as in disturbed areas, such as urban environments . Camponotus is a genus of generalist species, many of them interacting with sucking insects. It is possible that this last characteristic is somehow responsible with some aspect of the seasonality observed on this group.
Species of the genera Atta, Cephalotes, Cyphomyrmex and Brachymyrmex occurred only at a single sampling point, all occurring exclusively during the rainy season. Since species of these genera are commonly recorded in the Caatinga and Cerrado biomes (Leal, 2003;Vasconcelos et al., 2017), we do not record activities of these ants during the dry season certainly due to lower resource availability and likely adverse environmental conditions during this period.
Two species are highlighted in this study, C. crassus and D. quadriceps. The first species is dominant in the Cerrado, mainly in vegetation, although it forages on the ground (Del-Claro et al., 2016;Lange et al., 2019). This species is exclusively foraging during the day, both in the rainy and dry seasons (Lange et al., 2019). The second is endemic to the northeast region, occurring predominantly in the Caatinga biome (Lenhart et al., 2013). We believe that the maintaining of activity of these species during the dry period can be related to their great plasticity relative to climatic gradients and opportunism to food resources. Medeiros et al. (2012) demonstrated that in the Caatinga D. quadriceps shows peaks of foraging activities in drier periods of the years, and that this foraging activity is positively associated with prey availability (Medeiros et al., 2014). Ph. radoszkowskii is a notable exception in relation to all the other species of the genus Pheidole reported in this study, as it was mostly recorded in the dry season. This species is known to occur in open and dry habitats, including sites modified by human activity (Wilson, 2003;AntWiki, 2021). This species is extremely generalist, and may even present scavenger behavior on flies (Sales et al., 2016), but this ant owes above all to its great resistance to wide variations of temperature and humidity that it is active all year round, and especially during the dry season when competition with other species must decrease.
We have recorded seven new species occurrences for the state of Piauí. Among these, B. heeri was found in a single pitfall sample during the rainy season. This omnivorous and opportunistic species has a wide geographic distribution, being found in rainforest habitats as well as in synanthropic environments along roads, in undergrowth secondary vegetation or in pastures (AntWiki, 2021). In northeastern Brazil, this ant is report-ed for the states of Bahia and Maranhão (Antmaps, 2021). Ca. atriceps, although recorded in the present study in a soil sample during the dry season, is mostly an arboreal species that occurs in different types of environments and vegetation types, being generally considered a synanthropic species (AntWiki, 2021). Koch et al. (2016) recorded the species on Caryocar brasiliense Cambess. (Caryocaraceae), a common tree in Cerrado. Apparently, this ant has the ability to also transit on the ground (Koch et al., 2019). Another arboreal species, Cr. erecta also occurs in several habitat types, including seasonal dry and wet forests, apparently being more commonly found in open and even disturbed environments than mature forest (Longino, 2003). F. maranhaoensis was recorded in pitfall samples in both seasons, demonstrating that it is indeed a common species in the region. The genus Forelius presents species that generally nest in the ground and lives in dry habitats (Cuezzo, 2000). G. moelleri which was recorded in a single pitfall sample during the dry season, is known to be an epigaeic forager (AntWiki, 2021;Koch et al., 2019). However, this ant has already been reported on vegetation in the Pantanal (Dambros et al., 2018) or associated with epiphytic bromeliads on a large tree (Erythrina genus, Leguminosae) in a cocoa plantation (DaRocha et al., 2015). The other new reports are omnivorous soil species commonly found in several Brazilian biomes: Solenopsis globularia and Pheidole diligens. The first recorded in a single sample and the second most abundant during the rainy season. These new reports point out the needs and importance of further surveys of ants in the state of Piauí, as well as highlighting the importance of developping diversity studies in ecotones.
The observation of variations in ant foraging behavior according seasonality is certainly to be interpreted from the availability of resources, which must be linked to the loss of habitat heterogeneity during the dry season, one of the main factors related to the ant abundance (Fagundes et al., 2009). Caatinga marked seasonality causes drastic seasonal changes in the productivity and availability of resources for ants (Leal et al., 2008) and the same is true for the Cerrado, especially in more open vegetation types. Ant activity and diversity can vary according to the characteristics of the habitat, such as the structure of the vegetation, since more complex phytophysiognomies ensure greater opportunities of nesting sites and prey (Albuquerque & Diehl, 2009;Gomes et al., 2010;Neves et al., 2013). Studies have shown that the richness and composition of ant assemblages can also be influenced by abiotic factors, such as litter depth, altitude, latitude, and climatic conditions, such as temperature and humidity (Vargas et al., 2007;Albuquerque & Diehl, 2009, Gibb et al., 2015. At high thermal amplitude levels associated to a seasonal drought, stressful situations can occur because these factors limit the rates of growth, acquisition of resources, and reproduction of a number of organisms (Robinet & Roques, 2010). Ants have a close relationship with climatic factors, such as temperature and humidity, and each species operates within a specific range of these ones (Andersen & Majer, 2004). Although no climatic factor was evaluated in the present study, we believe that the differentiation in these factors and consequently in the supply of resources to the ants are the main responsible for the differences in the ant activity observed here.

CONCLUSION
In this study, we observed that the activity of ant fauna in a Cerrado-Caatinga ecotone is likely influenced by seasonality (dry and rainy seasons). This is only the second study that approaches the diversity of ants in the state of Piauí. Additionally, we present seven new ant species occurrence records for the state. More studies are needed to understand the processes and factors responsible for the activity patterns of ants. As our results refer to a single sampling event at each station, further studies should evaluate more accurately the processes involved in the ants' activities in Cerrado-Caatinga ecotones, preferably considering a longer period of time.