1. Introduction
Research in animal demography has generated a substantial body of literature allowing a good understanding of how populations change over time [
1,
2,
3]. Historically, demographic information has been used in studies focusing on the context of stable population theory [
4,
5], organisms’ life tables [
6], and population biology [
7]. However, an emerging sub-discipline of the demography, the bio-demography, suggests going further, using the demographic information to understand and explain the underlying actuarial biological and ecological mechanisms acting on the populations [
8]. Current development of approaches based on maximum likelihood models allow estimating demographic parameters using recapture histories of marked individuals, because these methods consider the detection of individuals to estimate sizes and survival rates of the populations [
9,
10] .
Although mark-recapture and maximum likelihood methods have been widely used in organisms such as damselflies reviewed by [11, 12], research on Neotropical species just is beginning [
13,
14,
15,
16,
17,
18,
19,
20]. Thus, due to the scarcity of available information about the ecology and biology of Neotropical Odonata species, more studies and field observations are needed to be able to build up a “big picture” of the ecological and biological processes modelling the populations of these insects in tropical habitats [
20].
Megapodagrionidae is a pantropical family of Odonata, and includes the genus
Teinopodagrion De Marmels, 2001, an endemic genus from the South American Yungas Mountain cloud forest, extending their distribution from Venezuela to northwestern Argentina along the eastern slope of the Andes [
21]. To our knowledge, population studies of
Teinopodagrion species are inexistent, but are necessary to know and understand population biology and ecology of these tropical species, which are apparently present at very low densities in tropical ecosystems. In this paper, we present the results of a demographic and behavioral study of
Teinopodagrion oscillans (Selys) from a population inhabiting in a protected area of the Colombian Andes. The objectives of this research included: i) to estimate population size, sex ratio, survival, and life expectancy, ii) to study the area frequented by the adults, by estimating the distance from the stream in which individuals of different age classes perch, and iii) to describe patterns on coloration and general behavior.
4. Discussion
There is little biological information about most genera included in the Megapodagrionidae sensu latu, and
Teinopodagrion is not an exception. Our study is the first to compile demographic information for this genus and yielded some unexpected results. First, we were able to recapture 85% of the animals marked, a value rarely achieved with odonates. Second, males and females were very similar in all demographic parameters, which is unusual. Third, the population studied showed two clear peaks of abundance, separated by six weeks. And finally, both recapture and survival rates showed strong daily variation (
Figure 3).
Teinopodagrion oscillans shows little sexual dimorphism in body colouration, as is typical of the species of the genus (for instance, see the pictures of
Teinopodagrion meridionale De Marmels in [
28]. Males are slightly larger than females, but their wing lengths are not different (
Table 2), implying that, to maintain flight performance, wing morphology should be sexually dimorphic, given the different weight of male and female damselflies [
29]. Body and wing lengths were not correlated with date of marking. In temperate species, a decrease in body size is frequently observed along the season [
29,
30], due to time constraints and the effect of rising temperatures, and this is a general pattern in aquatic arthropods [
31]. However, in tropical environments, where seasonal limitations are scarce, body size is not expected to correlate with date [
32,
33,
34,
35].
In a recent review of mark-recapture studies with odonates, Sanmartín-Villar & Cordero-Rivera [
12] found that in general males and females differ in their recapture rates, but not in survival, provided that the study period is long enough to produce unbiased estimates. These authors also found that age at marking affected recapture rates, and also that males are recaptured in a higher proportion than females. In our population, however, both sexes and all age groups showed similar recapture rates and observed lifespan (
Table 2).
Teinopodagrion oscillans individuals show high site fidelity, and concentrate near the stream, being found on average at a distance lower than 3 m from the water. This behaviour explains the high recapture rates achieved during this study, similar to the case of another damselfly living in tropical streams [
36].
The average time between marking and last recapture was two weeks and was not significantly different between sexes. In general, observed lifespan is higher in males, and this has been explained by their behaviour, because males usually remain closer to the water than females, and, as a consequence, male recapture rates are higher e. g., [
37]). For instance, in
Hypolestes trinitatis (Gundlach), an endemic damselfly from Cuba, which has many ecological similarities with
T. oscillans, recapture rates were much higher in males [
38]. The estimated survival rate of
T. oscillans, 0.946, is one of the highest ever estimated for adult odonates, identical to the male survival rate estimated for
H. trinitatis [
38], and similar to other species with highly territorial behaviour [
12]. Some Calopterygids, Polythorids, Lestids and Thaumatoneurids have daily survival rates above 0.96 [39-41
], which translate into expected longevities of around 24 days, using the formula of Cook et al. [
42]. In our dataset the expected longevity is 18 days and the maximum observed longevity was 37 days, a value that is nevertheless lower than the maximum longevity of
Calopteryx Leach, with is about 40 days e. g., [
43].
The first nine days of adult life include the sexual maturation period in
T. oscillans. This means that the net reproductive life, for those animals surviving until maturation, is, on average, about one week (
Table 2). Sexual maturation is needed in all odonates, because females do not have mature eggs when emerge from the exuvia, and males do not have functional sperm. In general, the pre-reproductive period is longer in females than in males reviewed by [
44], but in our population this was not the case. In tropical damselflies, the maturation period can be very long, even more than one month in giant damselflies [
45], but species with similar body size as
T. oscillans have shorter maturation periods. For instance, male
Polythore mutata (McLachlan) need 12.6 days to mature [
46], and
Palaemnema desiderata Selys and
Palaemnema paulitoyaca Calvert 7-10 days [
47].
The frequency of reproductive events was low, and this could be due to some reproductive events may have not coincided with the time of our study [
48] or they took place in locations difficult to access by the observers; e. g., elevated twigs in the vegetation or quiet sites with high diversity of plants [
18].
One of the most unexpected results of our study is the large daily variation in the appearance of mature individuals (
Figure 2). Apparently, two cohorts matured with a separation of about six weeks between them, also producing a clear difference in population size between the first and the second half of the study (
Figure 2). One possible explanation for this is that these two cohorts represent two generations [
49,
50]. However, the time elapsed seems too short for larval development. Population size estimates for damselflies from tropical streams are scarce. In a study with a duration of two months of a population of
H. trinitatis, male and female abundance were estimated to show little daily variation, with slow increases or decreases, but not sudden changes [
38]. Similar patterns emerge from a population study of
Argia chelata Calvert [
51] in Costa Rica, and two populations of
Heteragrion cooki Daigle & Tennessen from Ecuador [
16]. However, a mark-recapture study of the damselfly
Ischnura pumilio (Charpentier) in Spain, also showed the appearance of two population peaks of mature adults, in this case with a separation of 10 days [
52], suggesting immigration from another population. However, we do not know of similar ecosystems close to the area of our study. Thus, the probability of migration from other places seems low.
Growth rates, timing of emergence, and flight seasons can show inter-year variation within species in the same habitat [
53]. For several Odonata species it is known that the size of last instar larvae is a complex function of temperature, time of emergence, and species identity. Cothran & Thorp [
54] for example found that in the same habitat, individuals of the same species were smaller at the warm micro-habitats, while the largest animals emerged from the cooler micro-habitats. Temperature within lentic freshwater habitats can vary broadly among microhabitats throughout the day and along of the time [
55]. In our study area, something similar probably happened with temperature due to the variety of aquatic microhabitats provided by variations in water depth, number and type of macrophytes, and turbidity caused by the mixing of reservoir water with that of the stream. Although these aspects were not measured by us, it is plausible that morphological and behavioral responses can be adapted to the varying conditions of the microhabitat where the animals live.
Our study was also unique because daily recapture and survival rates were clearly variable along the period of fieldwork (
Figure 3), despite similar sampling effort. Between 14 October and 19 November, we marked a total of 155 individuals, almost half of the total marked in the whole study, and all of them disappeared by 22 November, producing an abrupt change in population size (
Figure 3). Odonate insects strongly depend on ambient temperature to carry out their daily activities [
56,
57]. The protected area Club Náutico El Muña is in an area where temperatures vary between -2 and 25 ºC throughout the day, over the days and when comparing periods of drought and rain [
58]. Therefore, it is plausible that the number of individuals may change rapidly between days due to these temperature variations. We observed that individuals of
T. oscillans in this area are indiscriminate when selecting their perch, but on very cold days they stayed on high branches (10-12 m high), where the individuals and their numbers were difficult to track. The influence of microhabitats on
T. oscillans seems remarkable, because in addition to the low temperatures, the ecosystem was flooded by the water of the reservoir due to the rains in February 2021, strongly reducing the number of microhabitats available for oviposition and possibly the growth of larvae. Since then, only four adult individuals of
T. oscillans have been found in this habitat as of December 2023.
Our study provides crucial information for the understanding of basic population aspects of
T. oscillans, a species categorized as Least Concern because it has been reported for two protected areas, one in Colombia [
59] and the other in Venezuela [
60]. However, our findings support the idea that it is important to know how populations are affected by local conditions, to allow evidence-based categorizations and conservation decisions. Because survival and the probability of recapture were time-dependent, environmental temperature is a factor that alters the presence and size of this population, which we will continue to monitor to try to understand the dynamics that in the medium and long term affect the conservation of this species in habitats of the Andean region.