The size of the functional morphological traits of the worker and minor soldier castes of H. tenuis was mainly influenced by the type of cover and, to a lesser extent, by the ages of the pine plantations. Likewise, the intraspecific variation of 16 traits out of the 38 studied presented lower CV values, both in plantations and in gallery forests, and allowed their use to compare the effect of the change in cover type, minimizing the effect of intraspecific variability.
Variation of functional morphological traits
The FMT with more significant variability in the worker caste of H. tenuis, in the four ages of plantation and gallery forests were related to the general size of the body (head, thorax, and extremities), that is, with the food search. In contrast, the FMT with less variability were the specific characters of the oral apparatus, affecting food foraging. This observation coincides with previous studies in social insects, where intraspecific variation is significantly high in activities related to time and the active search for food between and within populations (Bockoven et al., 2015; Jandt & Gordon, 2016). Highly variable traits can improve colony fitness by extending the range of behavior (for example, foraging) and allowing better and faster responses to changes (Bockoven et al., 2015).
The FMT having the most significant variation in the soldier caste were the general size of the insect body (thorax and extremities), the shape of the head, and the size ratio of the head/right mandible; while the size of the head, the size of the pronotum and the length of both mandibles were the least variable characters. This observation agrees with previous reports in another xylophagous termite species, Cryptotermes secundus (Hill, 1925), where the size of the characters related to mechanical defense was more stable than the non-defensive morphological features (Roux et al., 2009). Another explanation relates to the soldier caste of Rhinotermitidae contribution to food exploration as a possible response to predation diminishing (Reinhard et al., 1997), since the foraging time and the time of exposure of the workers to predation is variable (Bockoven et al., 2015). In this study, we selected the minor soldiers, the most common size of the soldier; and this soldier type may be more related to the accompanying activity in the exploration of food by the workers than in the function of defense (Traniello & Leuthold, 2000). Therefore, in this soldier size, there could be a more significant intraspecific variation related to the movement characters for exploration than defensive ones. It is interesting to continue studying this species' functional and behavioral differentiation between older and younger soldiers.
The FMT of the soldier caste of H. tenuis were more variable than in the worker caste. In termites, the difference in the CV of the size of morphological traits between castes is attributed to several origins (Eggleton, 2010). First, the soldier caste of termites can be derived from all stages of workers and immature apterans, which can cause differences in final individual size polymorphisms in various species (Roisin et al., 2000). In addition, the size of the workers from which the soldiers are derived is smaller in incipient colonies (Crosland et al., 2006; Noirot, 1985), and therefore their final size is expected to be partially dependent on the development pathway (Pinzón 2007). Therefore, younger plantation workers may produce smaller soldiers. Furthermore, polymorphism in the soldier caste of termites is related to polyethism (the division of labor) and has different patterns in the genus Heterotermes (Noirot & Darlington, 2000).
Therefore, the traits that presented the lowest intraspecific CV were considered the most appropriate to estimate the type of cover's effect on the species' functional response. Traits with lower CV minimize the possible overlapping or oversizing of the functional response due to high variation (Gaudard et al., 2019). The traits considered most appropriate for comparison, including the worker caste, were: the mandibular traits (La, L1, L2, MPr, Ra, R1, R2, MP) and index two and the soldier caste: the width and length of the head, width of pronotum, length of the tibia and both mandibles, and indices 2I and 3.
Relationship of the size of morphological features with the type of land cover
Heterotermes tenuis workers from the gallery forests evaluated were larger than those from the pine plantations, regardless of the age of the plantation. This result coincides with the reduction of FMT reported in Hevea brasiliensis monocultures compared to deforested natural forest areas and is attributed to the greater variety of food resources, microhabitats, and microclimates in the latter (Liu et al., 2019). But the quality and quantity of the food resource and the environmental conditions must also be taken into account since, although the frequency of wood-eating termites increases with greater availability of food in deforested areas (Eggleton et al., 2015).
The size of the worker termites is related to physiological factors, such as nutrition and energy expenditure, and mainly to the quantity and quality of food, although the results are contrasting. The larger size of worker termites may affect their ability to absorb nutrients (Dahlsjö et al., 2015), but at the same time may favor a lower rate of energy expenditure than small and medium-sized termites (Dahlsjö et al., 2015). Additionally, large termites may have a longer food retention time by having larger intestines, which is beneficial, considering that wood is a nutritionally poor substrate (Dahlsjö et al., 2015). However, the largest workers of the Rhinotermitidae species, known as the formosan subterranean termite (Coptotermes formosanus Shiraki, 1909), consume less wood and therefore suffer higher mortality than small workers, especially in declining colonies, which produce larger and less vigorous termites (Su & Fage, 1984). On the other hand, wood-eating termite workers can benefit from smaller sizes because the smaller mandibles can grind the food into smaller pieces, improving the absorption of nutrients(Eggleton et al., 1998).
In the study area, the frequency of Heterotermes tenuis increases as the P. caribaea crops is older (Beltrán & Pinzón, 2018) compared to the surrounding natural forests (Pinzón et al., 2017). Additionally, in the study area, a positive response of the frequency of this species to the degree of decomposition and the size of the pieces of pine wood produced by pruning is known (Beltrán and Pinzón, 2017). Previous studies document how the frequency of subterranean xylophagous termites responds positively to the availability of wood resources, which they require both for food and habitat, and is also recorded that Rhinotermitidae termites prefer to feed on soft woods such as Pinus (Santos et al., 2010) and can feed on needles of this genus (Pinzón et al., 2006). The FMT of H. tenuis that was smaller in the plantations are concentrated in the mouthparts, possibly as a response to the lower resistance of the pine wood. At the same time, in the gallery forests, the food consists of heterogeneity of species having harder woods (Fernández et al., 2012), in such a way that a smaller size of the mouthparts can be adequate and imply energy savings. Alternatively, the smaller size may result of a nutritional supply from pine wood that does not allow optimal body development. Still, this study did not compare individual biomass between land uses (Pinzón et al., 2006).
On the other hand, future studies may consider the influence of intraspecific competition on the individual size of H. tenuis. Competition influences the size of offspring because competitive environments encourage the colony to invest in larger individuals in smaller numbers (Cronin et al., 2011). Furthermore, competition is relevant in organisms that tend to saturate their environments (Cronin et al., 2011), such as ants and termites. The workers of H. tenuis of the four plantation ages would have less interspecific competition compared to the workers of the relict gallery forests because the composition of xylophagous species was 24% in the pine plantations (Beltrán & Pinzón, 2018) and 42.7% in gallery forest relicts (Pinzón et al., 2012). Therefore, it is possible that H. tenuis gallery forest workers increased the size of their FMT and decreased their abundance, while plantations present smaller FMT, but a greater number of individuals.
The FMT of soldiers versus workers did not differ between the different types of land use. However, they tend to be smaller in the youngest and most mature plantations than intermediate plantations and gallery forests. The variation in the size of the soldier caste of termites is mainly in response to their predator (Eggleton, 2010) and, as also occurs in other social insects traits associated with nest defense and locomotion increase proportionally to the abundance of insects predators, because a larger size may be a better defense (Hattori et al., 2016). Likewise, ants are the most important predators of invertebrates (Sheppe, 1970) and can reduce termite densities, even if they do not specialize in predation (Lima & Pantoja, 2012). The difference in the size of some characters of the minor soldier of H. tenuis may be influenced by the frequency of predators, but this study did not include this variable.
Homogeneity of functional diversity
Intraspecific variation of H. tenuis FMT was more significant between colonies than cover types. The multinomial models improved their predictive quality and the adjustment of the Pseudo R2 when the type of coverage was used as a factor. Therefore, more significant variation is expected between colonies than cover types. This variation between colonies may be due to the range of individual phenotypes in one colony differing from the range in another (Jandt & Gordon, 2016). However, despite the high variation between colonies, its importance is relegated by the effect of the type of cover on the insect's body. Therefore, P. caribaea plantations may act as a filter for the functional diversity of H. tenuis since no relevant differences between the plantation ages regarding FMT size, especially in the worker caste were detected.
Homogenization consists of an increase in the similarity of the functional composition (Rousseau et al., 2019). This homogenization can alter some ecosystem processes such as waste decomposition and nutrient cycling (Rousseau et al., 2019), alter food webs (Simons et al., 2015), and generate losses in taxonomic diversity (Liu et al., 2016; Salas et al., 2017). Although the pine-planted areas do not correspond to transformed gallery forests, the results observed were similar to those reported for converting primary forests to rubber plantations (Liu et al., 2019; Liu et al., 2016). Rubber plantations were found to reduce the functional diversity of ants and termites, due to a loss of microhabitats, compared to primary forests. Primary forests have greater structural complexity, variety of resources, and microclimates, facilitating the coexistence of more functional features, while plantations limit specific traits. Additionally, functional diversity decreases with higher levels of disturbance (March 2013; Olden & Rooney, 2006); therefore, it is expected that pine plantations present greater disturbance than the relicts of gallery forests, a decrease in the functional diversity of both castes. Therefore, pine plantations may restrict the functional variability of H. tenuis to specific traits, with a higher incidence in the worker caste.