Soil fauna is generally understudied compared to above-ground arthropods, and ants are no excepti... more Soil fauna is generally understudied compared to above-ground arthropods, and ants are no exception. Here, we compared a primary and a secondary forest each on two continents using four different sampling methods. Winkler sampling, pitfalls, and four types of above- and below-ground baits (dead, crushed insects; melezitose; living termites; living mealworms/grasshoppers) were applied on four plots (4 × 4 grid points) on each site. Although less diverse than Winkler samples and pitfalls, subterranean baits provided a remarkable ant community. Our baiting system provided a large dataset to systematically quantify strata and dietary specialisation in tropical rainforest ants. Compared to above-ground baits, 10–28% of the species at subterranean baits were overall more common (or unique to) below ground, indicating a fauna that was truly specialised to this stratum. Species turnover was particularly high in the primary forests, both concerning above-ground and subterranean baits and bet...
The success of social insects is largely intertwined with their highly advanced chemical communic... more The success of social insects is largely intertwined with their highly advanced chemical communication system that facilitates recognition and discrimination of species and nest-mates, recruitment, and division of labor. Hydrocarbons, which cover the cuticle of insects, not only serve as waterproofing agents but also constitute a major component of this communication system. Two cryptic Crematogaster species, which share their nest with Camponotus ants, show striking diversity in their cuticular hydrocarbon (CHC) profile. This mutualistic system therefore offers a great opportunity to study the genetic basis of CHC divergence between sister species. As a basis for further genome-wide studies high-quality genomes are needed. Here, we present the annotated draft genome for Crematogaster levior A. By combining the three most commonly used sequencing techniques—Illumina, PacBio, and Oxford Nanopore—we constructed a high-quality de novo ant genome. We show that even low coverage of long ...
The body surface of nearly all insects, including ants, is covered with a lipid layer that largel... more The body surface of nearly all insects, including ants, is covered with a lipid layer that largely consists of cuticular hydrocarbons (CHC). They fulfil several functions, the two best-studied ones being communication and protection against water loss. CHC profiles are astonishingly diverse as even a single individual can possess more than 100 different hydrocarbon molecules. Species vastly differ in their CHC composition, but also within species, CHC profiles vary among individuals of different sex, caste, fertility, age, health state, etc. This variation has been intensely studied especially in eusocial insects like ants, where differences are likely to have a signalling function. However, with so many sources of variation in CHC profiles, it is easy to lose track of which factors are more important than others, which patterns can be generalised, and which are idiosyncratic. Thus, we need a deeper understanding of how precisely different factors influence CHC variation. In this re...
Insect cuticles are covered with hydrocarbons (CHC), which serve two main functions: desiccation ... more Insect cuticles are covered with hydrocarbons (CHC), which serve two main functions: desiccation barrier and information transfer. The CHC profile of an ant carries information on its species identity, colony, caste, sex, etc. During the last years, our knowledge on causes and functions of intraspecific CHC variation has advanced drastically. Intraspecific variation is usually restricted to quantitative changes in hydrocarbon composition, whereas interspecific variation is characterized by qualitative alterations i.e. variation in substance identities and substance classes. To date, little is known about the evolutionary causes of interspecific variation. Moreover, we do not understand the role of phylogenetic or physiological constraints on CHC evolution: How fast can insects adapt their CHC profiles to e.g. new communication needs? How important are constraints for CHC evolution? Here, we analysed the CHC profiles of 37 Camponotus and 39 Crematogaster species from around the world. We searched for phylogenetic and/or physiological constraints and for selection pressures on CHC composition. CHC profiles did not appear to experience strong phylogenetic constraints. Even sister species could exhibit completely different profiles. However, we identified several physiological constraints. First, dimethyl alkanes and alkenes rarely co-occurred in the same species. Second, we found correlations between hydrocarbon chain length and the proportions of unsaturated compounds and n-alkanes. They might be caused by the need to maintain a specific viscosity of the epicuticular layer. Finally, climate strongly influenced substance class composition. Tropical rainforest species had higher amounts of unsaturated compounds than species from more arid habitats. We explain this by variance in draught stress, as less waterproofing is required in the humid tropical rainforests. The length of the hydrocarbon chains was unaffected by climate, but was higher among ants in interspecific associations. We conclude that CHC profiles can evolve quickly, but experience constraints posed by climatic conditions, interspecific interactions, and CHC viscosity.
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-... more This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
The principle of limiting similarity is one of the central assumptions of niche-based community e... more The principle of limiting similarity is one of the central assumptions of niche-based community ecology, stressing the importance of niche differentiation as the central mechanism of species coexistence
Background: Trophic interactions are a fundamental aspect of ecosystem functioning, but often dif... more Background: Trophic interactions are a fundamental aspect of ecosystem functioning, but often difficult to observe directly. Several indirect techniques, such as fatty acid analysis, were developed to assess these interactions. Fatty acid profiles may indicate dietary differences, while individual fatty acids can be used as biomarkers. Ants are among the most important terrestrial animal groups, but little is known about their lipid metabolism, and no study so far used fatty acids to study their trophic ecology. We set up a feeding experiment with high-and low-fat food to elucidate patterns and dynamics of neutral lipid fatty acids (NLFAs) assimilation in ants. We asked whether dietary fatty acids are assimilated through direct trophic transfer, how diet influences NLFA total amounts and patterns over time, and whether these assimilation processes are similar across species and life stages. Results: Ants fed with high-fat food quickly accumulated specific dietary fatty acids (C18:2n6, C18:3n3 and C18:3n6), compared to ants fed with low-fat food. Dietary fat content did not affect total body fat of workers or amounts of fatty acids extensively biosynthesized by animals (C16:0, C18:0, C18:1n9). Larval development had a strong effect on the composition and amounts of C16:0, C18:0 and C18:1n9. NLFA compositions reflected dietary differences, which became more pronounced over time. Assimilation of specific dietary NLFAs was similar regardless of species or life stage, but these factors affected dynamics of other NLFAs, composition and total fat. Conclusions: We showed that ants accumulated certain dietary fatty acids via direct trophic transfer. Fat content of the diet had no effect on lipids stored by ants, which were able to synthesize high amounts of NLFAs from a sugar-based diet. Nevertheless, dietary NLFAs had a strong effect on metabolic dynamics and profiles. Fatty acids are a useful tool to study trophic biology of ants, and could be applied in an ecological context, although factors that affect NLFA patterns should be taken into account. Further studies should address which NLFAs can be used as biomarkers in natural ant communities, and how factors other than diet affect fatty acid dynamics and composition of species with distinct life histories.
Proceedings of the Royal Society B: Biological Sciences
Chemical communication is central for the formation and maintenance of insect societies. Generall... more Chemical communication is central for the formation and maintenance of insect societies. Generally, social insects only allow nest-mates into their colony, which are recognized by their cuticular hydrocarbons (CHCs). Social parasites, which exploit insect societies, are selected to circumvent host recognition. Here, we studied whether chemical strategies to reduce recognition evolved convergently in slavemaking ants, and whether they extend to workers, queens and males alike. We studied CHCs of three social parasites and their related hosts to investigate whether the parasitic lifestyle selects for specific chemical traits that reduce host recognition. Slavemaker profiles were characterized by shorter-chained hydrocarbons and a shift from methyl-branched alkanes to n -alkanes, presumably to reduce recognition cue quantity. These shifts were consistent across independent origins of slavery and were found in isolated ants and those emerging in their mother colony. Lifestyle influenced...
Ecological communities are structured by competitive, predatory, mutualistic and parasitic intera... more Ecological communities are structured by competitive, predatory, mutualistic and parasitic interactions combined with chance events. Separating deterministic from stochastic processes is possible, but finding statistical evidence for specific biological interactions is challenging. We attempt to solve this problem for ant communities nesting in epiphytic bird’s nest ferns (Asplenium nidus) in Borneo’s lowland rainforest. By recording the frequencies with which each and every single ant species occurred together, we were able to test statistically for patterns associated with interspecific competition. We found evidence for competition, but the resulting co-occurrence pattern was the opposite of what we expected. Rather than detecting species segregation—the classical hallmark of competition—we found species aggregation. Moreover, our approach of testing individual pairwise interactions mostly revealed spatially positive rather than negative associations. Significant negative interactions were only detected among large ants, and among species of the subfamily Ponerinae. Remarkably, the results from this study, and from a corroborating analysis of ant communities known to be structured by competition, suggest that competition within the ants leads to species aggregation rather than segregation. We believe this unexpected result is linked with the displacement of species following asymmetric competition. We conclude that analysing co-occurrence frequencies across complete species assemblages, separately for each species, and for each unique pairwise combination of species, represents a subtle yet powerful way of detecting structure and compartmentalisation in ecological communities.
Background The use and partitioning of trophic resources is a central aspect of community functio... more Background The use and partitioning of trophic resources is a central aspect of community function. On the ground of tropical forests, dozens of ant species may be found together and ecological mechanisms should act to allow such coexistence. One hypothesis states that niche specialization is higher in the tropics, compared to temperate regions. However, trophic niches of most species are virtually unknown. Several techniques might be combined to study trophic niche, such as field observations, fatty acid analysis (FAA) and stable isotope analysis (SIA). In this work, we combine these three techniques to unveil partitioning of trophic resources in a tropical and a temperate community. We describe patterns of resource use, compare them between communities, and test correlation and complementarity of methods to unveil both community patterns and species’ niches. Methods Resource use was assessed with seven kinds of bait representing natural resources available to ants. Neutral lipid f...
This is an open access article under the terms of the Creative Commons Attribution License, which... more This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Terrestrial arthropods achieve waterproofing by a layer of cuticular hydrocarbons (CHCs). At the ... more Terrestrial arthropods achieve waterproofing by a layer of cuticular hydrocarbons (CHCs). At the same time, CHCs also serve as communication signals. To maintain waterproofing under different climate conditions, insects adjust the chemical composition of their CHC layer, but this may affect the communication via CHC. The detailed acclimatory changes of CHCs and how these influence their physical properties are still unknown. Here, we studied acclimation in two closely related ant species with distinct CHC profiles, Myrmica rubra and Myrmica ruginodis, in response to constant or fluctuating temperature and humidity regimes. We measured how acclimation affected CHC composition and viscosity, and the ants’ drought survival. In both species, CHC composition showed strong, predictable responses to temperature regimes. Warm-acclimated individuals had higher proportions of linear alkanes, and less methyl-branched or unsaturated CHCs. These changes coincided with higher solid content and vi...
1. Organisms from temperate zones are exposed to seasonal changes and must be able to cope with a... more 1. Organisms from temperate zones are exposed to seasonal changes and must be able to cope with a wide range of climatic conditions. Especially ectotherms, including insects, are at risk to desiccate under dry and warm conditions, the more so given the changing climate. 2. To adjust to current conditions, organisms acclimate through changes in physiology, morphology and/or behaviour. Insects protect themselves against desiccation through a layer of cuticular hydrocarbons (CHC) on their body surface. Hence, acclimation may also affect the CHC profile, changing their waterproofing capacity under different climatic conditions. 3. Here, we investigated the acclimation response of two Temnothorax ant species to different climatic conditions. We analysed CHC profiles of queens, nurses and foragers that were acclimated to different humidity × temperature regimes, and tested the beneficial acclimation hypothesis by measuring survival of workers under desiccation-heat stress. 4. Both ants possessed a species-specific CHC profile. Nevertheless, they showed similar acclimation responses concerning changes of certain CHC classes, and finally similar survival rates under desiccation-heat stress. Warm-acclimated individuals generally showed longer n-alkanes, fewer dimethyl alkanes, and more (workers) or less (queens) monomethyl alkanes. In contrast, dry conditions resulted in more n-alkanes and fewer mono-and dimethyl alkanes, but these acclimatory changes were only observed in workers and not in queens. Warm-and dry-acclimated workers survived desiccation-heat stress better, but we found no species differences. 5. Our results indicate that both ant species can plastically adjust their cuticular hydrocarbon profile, allowing them to acclimate to different climatic conditions. Although their CHC composition differs in a species-specific manner, they showed similar chemical adjustments and concomitant changes in survival rate. Hence, chemical plasticity may be critical to determine a species' climatic range and its survival under changing climatic conditions.
Soil fauna is generally understudied compared to above-ground arthropods, and ants are no excepti... more Soil fauna is generally understudied compared to above-ground arthropods, and ants are no exception. Here, we compared a primary and a secondary forest each on two continents using four different sampling methods. Winkler sampling, pitfalls, and four types of above- and below-ground baits (dead, crushed insects; melezitose; living termites; living mealworms/grasshoppers) were applied on four plots (4 × 4 grid points) on each site. Although less diverse than Winkler samples and pitfalls, subterranean baits provided a remarkable ant community. Our baiting system provided a large dataset to systematically quantify strata and dietary specialisation in tropical rainforest ants. Compared to above-ground baits, 10–28% of the species at subterranean baits were overall more common (or unique to) below ground, indicating a fauna that was truly specialised to this stratum. Species turnover was particularly high in the primary forests, both concerning above-ground and subterranean baits and bet...
The success of social insects is largely intertwined with their highly advanced chemical communic... more The success of social insects is largely intertwined with their highly advanced chemical communication system that facilitates recognition and discrimination of species and nest-mates, recruitment, and division of labor. Hydrocarbons, which cover the cuticle of insects, not only serve as waterproofing agents but also constitute a major component of this communication system. Two cryptic Crematogaster species, which share their nest with Camponotus ants, show striking diversity in their cuticular hydrocarbon (CHC) profile. This mutualistic system therefore offers a great opportunity to study the genetic basis of CHC divergence between sister species. As a basis for further genome-wide studies high-quality genomes are needed. Here, we present the annotated draft genome for Crematogaster levior A. By combining the three most commonly used sequencing techniques—Illumina, PacBio, and Oxford Nanopore—we constructed a high-quality de novo ant genome. We show that even low coverage of long ...
The body surface of nearly all insects, including ants, is covered with a lipid layer that largel... more The body surface of nearly all insects, including ants, is covered with a lipid layer that largely consists of cuticular hydrocarbons (CHC). They fulfil several functions, the two best-studied ones being communication and protection against water loss. CHC profiles are astonishingly diverse as even a single individual can possess more than 100 different hydrocarbon molecules. Species vastly differ in their CHC composition, but also within species, CHC profiles vary among individuals of different sex, caste, fertility, age, health state, etc. This variation has been intensely studied especially in eusocial insects like ants, where differences are likely to have a signalling function. However, with so many sources of variation in CHC profiles, it is easy to lose track of which factors are more important than others, which patterns can be generalised, and which are idiosyncratic. Thus, we need a deeper understanding of how precisely different factors influence CHC variation. In this re...
Insect cuticles are covered with hydrocarbons (CHC), which serve two main functions: desiccation ... more Insect cuticles are covered with hydrocarbons (CHC), which serve two main functions: desiccation barrier and information transfer. The CHC profile of an ant carries information on its species identity, colony, caste, sex, etc. During the last years, our knowledge on causes and functions of intraspecific CHC variation has advanced drastically. Intraspecific variation is usually restricted to quantitative changes in hydrocarbon composition, whereas interspecific variation is characterized by qualitative alterations i.e. variation in substance identities and substance classes. To date, little is known about the evolutionary causes of interspecific variation. Moreover, we do not understand the role of phylogenetic or physiological constraints on CHC evolution: How fast can insects adapt their CHC profiles to e.g. new communication needs? How important are constraints for CHC evolution? Here, we analysed the CHC profiles of 37 Camponotus and 39 Crematogaster species from around the world. We searched for phylogenetic and/or physiological constraints and for selection pressures on CHC composition. CHC profiles did not appear to experience strong phylogenetic constraints. Even sister species could exhibit completely different profiles. However, we identified several physiological constraints. First, dimethyl alkanes and alkenes rarely co-occurred in the same species. Second, we found correlations between hydrocarbon chain length and the proportions of unsaturated compounds and n-alkanes. They might be caused by the need to maintain a specific viscosity of the epicuticular layer. Finally, climate strongly influenced substance class composition. Tropical rainforest species had higher amounts of unsaturated compounds than species from more arid habitats. We explain this by variance in draught stress, as less waterproofing is required in the humid tropical rainforests. The length of the hydrocarbon chains was unaffected by climate, but was higher among ants in interspecific associations. We conclude that CHC profiles can evolve quickly, but experience constraints posed by climatic conditions, interspecific interactions, and CHC viscosity.
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-... more This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
The principle of limiting similarity is one of the central assumptions of niche-based community e... more The principle of limiting similarity is one of the central assumptions of niche-based community ecology, stressing the importance of niche differentiation as the central mechanism of species coexistence
Background: Trophic interactions are a fundamental aspect of ecosystem functioning, but often dif... more Background: Trophic interactions are a fundamental aspect of ecosystem functioning, but often difficult to observe directly. Several indirect techniques, such as fatty acid analysis, were developed to assess these interactions. Fatty acid profiles may indicate dietary differences, while individual fatty acids can be used as biomarkers. Ants are among the most important terrestrial animal groups, but little is known about their lipid metabolism, and no study so far used fatty acids to study their trophic ecology. We set up a feeding experiment with high-and low-fat food to elucidate patterns and dynamics of neutral lipid fatty acids (NLFAs) assimilation in ants. We asked whether dietary fatty acids are assimilated through direct trophic transfer, how diet influences NLFA total amounts and patterns over time, and whether these assimilation processes are similar across species and life stages. Results: Ants fed with high-fat food quickly accumulated specific dietary fatty acids (C18:2n6, C18:3n3 and C18:3n6), compared to ants fed with low-fat food. Dietary fat content did not affect total body fat of workers or amounts of fatty acids extensively biosynthesized by animals (C16:0, C18:0, C18:1n9). Larval development had a strong effect on the composition and amounts of C16:0, C18:0 and C18:1n9. NLFA compositions reflected dietary differences, which became more pronounced over time. Assimilation of specific dietary NLFAs was similar regardless of species or life stage, but these factors affected dynamics of other NLFAs, composition and total fat. Conclusions: We showed that ants accumulated certain dietary fatty acids via direct trophic transfer. Fat content of the diet had no effect on lipids stored by ants, which were able to synthesize high amounts of NLFAs from a sugar-based diet. Nevertheless, dietary NLFAs had a strong effect on metabolic dynamics and profiles. Fatty acids are a useful tool to study trophic biology of ants, and could be applied in an ecological context, although factors that affect NLFA patterns should be taken into account. Further studies should address which NLFAs can be used as biomarkers in natural ant communities, and how factors other than diet affect fatty acid dynamics and composition of species with distinct life histories.
Proceedings of the Royal Society B: Biological Sciences
Chemical communication is central for the formation and maintenance of insect societies. Generall... more Chemical communication is central for the formation and maintenance of insect societies. Generally, social insects only allow nest-mates into their colony, which are recognized by their cuticular hydrocarbons (CHCs). Social parasites, which exploit insect societies, are selected to circumvent host recognition. Here, we studied whether chemical strategies to reduce recognition evolved convergently in slavemaking ants, and whether they extend to workers, queens and males alike. We studied CHCs of three social parasites and their related hosts to investigate whether the parasitic lifestyle selects for specific chemical traits that reduce host recognition. Slavemaker profiles were characterized by shorter-chained hydrocarbons and a shift from methyl-branched alkanes to n -alkanes, presumably to reduce recognition cue quantity. These shifts were consistent across independent origins of slavery and were found in isolated ants and those emerging in their mother colony. Lifestyle influenced...
Ecological communities are structured by competitive, predatory, mutualistic and parasitic intera... more Ecological communities are structured by competitive, predatory, mutualistic and parasitic interactions combined with chance events. Separating deterministic from stochastic processes is possible, but finding statistical evidence for specific biological interactions is challenging. We attempt to solve this problem for ant communities nesting in epiphytic bird’s nest ferns (Asplenium nidus) in Borneo’s lowland rainforest. By recording the frequencies with which each and every single ant species occurred together, we were able to test statistically for patterns associated with interspecific competition. We found evidence for competition, but the resulting co-occurrence pattern was the opposite of what we expected. Rather than detecting species segregation—the classical hallmark of competition—we found species aggregation. Moreover, our approach of testing individual pairwise interactions mostly revealed spatially positive rather than negative associations. Significant negative interactions were only detected among large ants, and among species of the subfamily Ponerinae. Remarkably, the results from this study, and from a corroborating analysis of ant communities known to be structured by competition, suggest that competition within the ants leads to species aggregation rather than segregation. We believe this unexpected result is linked with the displacement of species following asymmetric competition. We conclude that analysing co-occurrence frequencies across complete species assemblages, separately for each species, and for each unique pairwise combination of species, represents a subtle yet powerful way of detecting structure and compartmentalisation in ecological communities.
Background The use and partitioning of trophic resources is a central aspect of community functio... more Background The use and partitioning of trophic resources is a central aspect of community function. On the ground of tropical forests, dozens of ant species may be found together and ecological mechanisms should act to allow such coexistence. One hypothesis states that niche specialization is higher in the tropics, compared to temperate regions. However, trophic niches of most species are virtually unknown. Several techniques might be combined to study trophic niche, such as field observations, fatty acid analysis (FAA) and stable isotope analysis (SIA). In this work, we combine these three techniques to unveil partitioning of trophic resources in a tropical and a temperate community. We describe patterns of resource use, compare them between communities, and test correlation and complementarity of methods to unveil both community patterns and species’ niches. Methods Resource use was assessed with seven kinds of bait representing natural resources available to ants. Neutral lipid f...
This is an open access article under the terms of the Creative Commons Attribution License, which... more This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Terrestrial arthropods achieve waterproofing by a layer of cuticular hydrocarbons (CHCs). At the ... more Terrestrial arthropods achieve waterproofing by a layer of cuticular hydrocarbons (CHCs). At the same time, CHCs also serve as communication signals. To maintain waterproofing under different climate conditions, insects adjust the chemical composition of their CHC layer, but this may affect the communication via CHC. The detailed acclimatory changes of CHCs and how these influence their physical properties are still unknown. Here, we studied acclimation in two closely related ant species with distinct CHC profiles, Myrmica rubra and Myrmica ruginodis, in response to constant or fluctuating temperature and humidity regimes. We measured how acclimation affected CHC composition and viscosity, and the ants’ drought survival. In both species, CHC composition showed strong, predictable responses to temperature regimes. Warm-acclimated individuals had higher proportions of linear alkanes, and less methyl-branched or unsaturated CHCs. These changes coincided with higher solid content and vi...
1. Organisms from temperate zones are exposed to seasonal changes and must be able to cope with a... more 1. Organisms from temperate zones are exposed to seasonal changes and must be able to cope with a wide range of climatic conditions. Especially ectotherms, including insects, are at risk to desiccate under dry and warm conditions, the more so given the changing climate. 2. To adjust to current conditions, organisms acclimate through changes in physiology, morphology and/or behaviour. Insects protect themselves against desiccation through a layer of cuticular hydrocarbons (CHC) on their body surface. Hence, acclimation may also affect the CHC profile, changing their waterproofing capacity under different climatic conditions. 3. Here, we investigated the acclimation response of two Temnothorax ant species to different climatic conditions. We analysed CHC profiles of queens, nurses and foragers that were acclimated to different humidity × temperature regimes, and tested the beneficial acclimation hypothesis by measuring survival of workers under desiccation-heat stress. 4. Both ants possessed a species-specific CHC profile. Nevertheless, they showed similar acclimation responses concerning changes of certain CHC classes, and finally similar survival rates under desiccation-heat stress. Warm-acclimated individuals generally showed longer n-alkanes, fewer dimethyl alkanes, and more (workers) or less (queens) monomethyl alkanes. In contrast, dry conditions resulted in more n-alkanes and fewer mono-and dimethyl alkanes, but these acclimatory changes were only observed in workers and not in queens. Warm-and dry-acclimated workers survived desiccation-heat stress better, but we found no species differences. 5. Our results indicate that both ant species can plastically adjust their cuticular hydrocarbon profile, allowing them to acclimate to different climatic conditions. Although their CHC composition differs in a species-specific manner, they showed similar chemical adjustments and concomitant changes in survival rate. Hence, chemical plasticity may be critical to determine a species' climatic range and its survival under changing climatic conditions.
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