**4. Discussion**

In the study area the average density was 2103 plants ha−1, lower than the 3200–4633 plants ha−1 of *Dioon edule* in the center of Veracruz [4, 18], but higher than the 37 plants ha−1 of *D. purpusii* [26], 116 to 167 plants ha−1 of *D. merolae* [27] and 589 plants ha−1 of *D. holmgrenii* [28].

The absence of statistically significant differences between locations is mainly due to the parameters' wide variation observed. However, the populations with lower density coincide with other studies [27, 28], where the habitat's quality has decreased. The low quality of the habitat is due to the recurrent disturbance caused by agriculture, mainly the establishment of sugar cane fields in RN, pastures for livestock in EJ and MO, and the extraction of complete plants or their parts, like seeds and leaf harvest for tamale production [1, 29, 30]. In general, plant density and size are population attributes that indicate habitat quality [7].

Most populations presented the survival inverted "J" curve (type III) typical of shade-tolerant species [20] and most populations of *D. edule* [18], *D. purpusii* [26], and *D. angustifolium* [31]. These populations have a higher density of seedlings and juveniles with high mortality and few very long-lived adults with reduced mortality. Fewer populations have a relatively constant mortality rate throughout their life cycle, different from most *Dioon* populations. This modification could be attributed to the reduction of populations caused by habitat disturbing [32] since the natural forest was replaced with cattle pastures.

The plant's reproductive capacity is of great interest in conservation since it significantly impacts management and recovery strategies [33]. It is remarkable the high number of female plants present compared with other studies. The imbalance of females is probably related to climatic factors, soil quality, or the occurrence of fires, which in the study area is very common due to agricultural activities [18, 26, 34–38]. The plants' sex ratio average was 2: 1 (male: female) in all locations. However, the ideal ratio for *Dioon* species is 3:1 to ensure an abundant amount of pollen during female strobili receptivity [13]. There are five populations of concern that could present pollen adequacy problems during fertilization, mainly the CH locality that presented a ratio of 1:2.

The *Dioon edule* populations' distribution pattern is aggregated, which has long been attributed to the strobilus remaining attached to the plant's stem during dispersal, causing many seeds to remain close and around. Seed dispersal agents are currently being studied to identify them in the study area, including reptiles, birds, and mammals. It is relevant the evidence of asexual multiplication, which means that sprouts could be an effective strategy when sexual reproduction is difficult or in response to some stress. Like the cycad *Cycas armstrongii,* which produces sprouts after fires around the charred stems [36], facilitating rapid propagation and ensuring populations' maintenance [39].

The differences in height between reproductive and non-reproductive individuals have been attributed to the lowest growth rate of reproductive individuals due to the compression of stem tissues by the increase in weight of stored water and the production of heavy strobili [18, 40]. However, in this study, no significant differences were observed in any location, indicating that this difference does not always apply to all cycad populations.

Considering that there were significant plants' height differences among populations, it would be an indicator of the site quality. The sites of CH, GA and AB that presented the tallest plants would have the best quality since they present the lowest proportion of organic matter and smooth slopes. Diference in height and diameter of *Ceratozamia matudae* [41], *Dioon purpusii* [26] and *D. holmgrenii* [28] is attributed to disturbance conditions.

**107**

*The Endangered Species* Dioon edule *in the Sierra Madre Oriental in San Luis Potosí…*

The plants' diameter did not present significant differences between localities. It has already been observed in *Cycas micronesica* [40] between reproductive and nonreproductive individuals, which can be attributed to the manoxylic wood type with a lot of succulent tissue [42]. In the same way, the difference in the number of leaves present in reproductive and non-reproductive individuals at each location and the difference between locations can be attributed that during the adult stage, the incidence of solar radiation correlates with leaf production, but the production of strobili in female individuals stops the production of new leaves, due to the greater allocation of resources for the development of the strabo [43]. The number of leaves in reproductive adults was lower than in *Dioon holmgrenii*, but much higher in non-

As highlighted in this study, another determining factor in the development of *Dioon edule* populations is the deterioration of forests caused by agricultural activities [44]. These activities cause a lower rate of reproduction, capacity to adapt to changes, and less ability to establish themselves during the passage from one phenological stage to another, altering the age structure of the population [7]. The genetic diversity analysis tried to clear the picture of the medium and long-term repercussions of adverse factors identified when analyzing the species' population demography. However, we consider that the number of populations and molecular markers should be increased since *Dioon edule is ditributed* in various habitats with different microenvironmental conditions, causing phenotypical

The populations analyzed show low genetic diversity and an excess of homozygotes in the seedlings, juveniles, and adults. Contrary to that expected for adult plants, which were believed to have become established before the disturbances. Similar behavior was present in a study conducted in Veracruz, mentioning that populations outside the Pleistocene flora and fauna refuges located in Veracruz and Oaxaca probably suffered a bottleneck due to exposure to adverse environmental conditions generating a significant decrease in population size reflected in the low

Previous studies suggest that life history issues, such as geographic distribution area, reproduction systems, and pollen and seed dispersal systems, have essential consequences on genetic structure and genetic variability levels in natural populations [4, 6, 10, 45, 46]. These factors are especially relevant in cycads because of their distribution in particular areas. They also present vegetative reproduction

In addition to the patches of clonal sprouts, the seedling banks that form around adult plants of *Dioon edule* and trees of *Quercus laeta* and *Bursera simaruba*, act as an extensive reservoir of genetic variation. However, this eventually is lost due to the high mortality rates that this species presents in the establishment's early stages. The mortality occurs naturally in populations because seedlings and juveniles slowly develop features that allow them to tolerate adverse conditions, such as prolonged drought or insolation [48]. These conditions can be aggravated by habitat fragmentation [6, 7, 44], causing gene drift in the population and making natural selection less effective by expressing deleterious or poorly tolerant genotypes to current

Deviations in H-W were at most of the loci assessed, all tending towards a deficit of heterozygotes. The deviations may be due to the lag in the reproductive season, which can vary from four to 52 years in female plants [4]. This produces a time barrier that prevents individuals' panmictic crossing, causing groups to form that can

Five unique alleles were identified with low frequencies in the ED9 and TOM5 locus in seedlings and juveniles. These alleles' appearance could be attributed to

*DOI: http://dx.doi.org/10.5772/intechopen.96372*

differences that should be analyzed [3, 31].

levels of observed heterozygosity detected [10].

forming inside the populations [47].

environmental conditions [49].

function as independent populations.

reproductive ones [28].

#### *The Endangered Species* Dioon edule *in the Sierra Madre Oriental in San Luis Potosí… DOI: http://dx.doi.org/10.5772/intechopen.96372*

The plants' diameter did not present significant differences between localities. It has already been observed in *Cycas micronesica* [40] between reproductive and nonreproductive individuals, which can be attributed to the manoxylic wood type with a lot of succulent tissue [42]. In the same way, the difference in the number of leaves present in reproductive and non-reproductive individuals at each location and the difference between locations can be attributed that during the adult stage, the incidence of solar radiation correlates with leaf production, but the production of strobili in female individuals stops the production of new leaves, due to the greater allocation of resources for the development of the strabo [43]. The number of leaves in reproductive adults was lower than in *Dioon holmgrenii*, but much higher in nonreproductive ones [28].

As highlighted in this study, another determining factor in the development of *Dioon edule* populations is the deterioration of forests caused by agricultural activities [44]. These activities cause a lower rate of reproduction, capacity to adapt to changes, and less ability to establish themselves during the passage from one phenological stage to another, altering the age structure of the population [7].

The genetic diversity analysis tried to clear the picture of the medium and long-term repercussions of adverse factors identified when analyzing the species' population demography. However, we consider that the number of populations and molecular markers should be increased since *Dioon edule is ditributed* in various habitats with different microenvironmental conditions, causing phenotypical differences that should be analyzed [3, 31].

The populations analyzed show low genetic diversity and an excess of homozygotes in the seedlings, juveniles, and adults. Contrary to that expected for adult plants, which were believed to have become established before the disturbances. Similar behavior was present in a study conducted in Veracruz, mentioning that populations outside the Pleistocene flora and fauna refuges located in Veracruz and Oaxaca probably suffered a bottleneck due to exposure to adverse environmental conditions generating a significant decrease in population size reflected in the low levels of observed heterozygosity detected [10].

Previous studies suggest that life history issues, such as geographic distribution area, reproduction systems, and pollen and seed dispersal systems, have essential consequences on genetic structure and genetic variability levels in natural populations [4, 6, 10, 45, 46]. These factors are especially relevant in cycads because of their distribution in particular areas. They also present vegetative reproduction forming inside the populations [47].

In addition to the patches of clonal sprouts, the seedling banks that form around adult plants of *Dioon edule* and trees of *Quercus laeta* and *Bursera simaruba*, act as an extensive reservoir of genetic variation. However, this eventually is lost due to the high mortality rates that this species presents in the establishment's early stages. The mortality occurs naturally in populations because seedlings and juveniles slowly develop features that allow them to tolerate adverse conditions, such as prolonged drought or insolation [48]. These conditions can be aggravated by habitat fragmentation [6, 7, 44], causing gene drift in the population and making natural selection less effective by expressing deleterious or poorly tolerant genotypes to current environmental conditions [49].

Deviations in H-W were at most of the loci assessed, all tending towards a deficit of heterozygotes. The deviations may be due to the lag in the reproductive season, which can vary from four to 52 years in female plants [4]. This produces a time barrier that prevents individuals' panmictic crossing, causing groups to form that can function as independent populations.

Five unique alleles were identified with low frequencies in the ED9 and TOM5 locus in seedlings and juveniles. These alleles' appearance could be attributed to

*Natural History and Ecology of Mexico and Central America*

and 589 plants ha−1 of *D. holmgrenii* [28].

forest was replaced with cattle pastures.

locality that presented a ratio of 1:2.

ing populations' maintenance [39].

apply to all cycad populations.

is attributed to disturbance conditions.

In the study area the average density was 2103 plants ha−1, lower than the 3200–4633 plants ha−1 of *Dioon edule* in the center of Veracruz [4, 18], but higher than the 37 plants ha−1 of *D. purpusii* [26], 116 to 167 plants ha−1 of *D. merolae* [27]

and size are population attributes that indicate habitat quality [7].

The absence of statistically significant differences between locations is mainly due to the parameters' wide variation observed. However, the populations with lower density coincide with other studies [27, 28], where the habitat's quality has decreased. The low quality of the habitat is due to the recurrent disturbance caused by agriculture, mainly the establishment of sugar cane fields in RN, pastures for livestock in EJ and MO, and the extraction of complete plants or their parts, like seeds and leaf harvest for tamale production [1, 29, 30]. In general, plant density

Most populations presented the survival inverted "J" curve (type III) typical of shade-tolerant species [20] and most populations of *D. edule* [18], *D. purpusii* [26], and *D. angustifolium* [31]. These populations have a higher density of seedlings and juveniles with high mortality and few very long-lived adults with reduced mortality. Fewer populations have a relatively constant mortality rate throughout their life cycle, different from most *Dioon* populations. This modification could be attributed to the reduction of populations caused by habitat disturbing [32] since the natural

The plant's reproductive capacity is of great interest in conservation since it significantly impacts management and recovery strategies [33]. It is remarkable the high number of female plants present compared with other studies. The imbalance of females is probably related to climatic factors, soil quality, or the occurrence of fires, which in the study area is very common due to agricultural activities [18, 26, 34–38]. The plants' sex ratio average was 2: 1 (male: female) in all locations. However, the ideal ratio for *Dioon* species is 3:1 to ensure an abundant amount of pollen during female strobili receptivity [13]. There are five populations of concern that could present pollen adequacy problems during fertilization, mainly the CH

The *Dioon edule* populations' distribution pattern is aggregated, which has long been attributed to the strobilus remaining attached to the plant's stem during dispersal, causing many seeds to remain close and around. Seed dispersal agents are currently being studied to identify them in the study area, including reptiles, birds, and mammals. It is relevant the evidence of asexual multiplication, which means that sprouts could be an effective strategy when sexual reproduction is difficult or in response to some stress. Like the cycad *Cycas armstrongii,* which produces sprouts after fires around the charred stems [36], facilitating rapid propagation and ensur-

The differences in height between reproductive and non-reproductive individuals have been attributed to the lowest growth rate of reproductive individuals due to the compression of stem tissues by the increase in weight of stored water and the production of heavy strobili [18, 40]. However, in this study, no significant differences were observed in any location, indicating that this difference does not always

Considering that there were significant plants' height differences among populations, it would be an indicator of the site quality. The sites of CH, GA and AB that presented the tallest plants would have the best quality since they present the lowest proportion of organic matter and smooth slopes. Diference in height and diameter of *Ceratozamia matudae* [41], *Dioon purpusii* [26] and *D. holmgrenii* [28]

**4. Discussion**

**106**

#### *Natural History and Ecology of Mexico and Central America*

recurrent mutations or gene flow by pollen dispersal at great distances [50]. These alleles may benefit from genetic drift promoted by population size reduction.

AMOVA indicates a high differentiation in seedlings (FST = 0.26), probably due to the reduction in gene flow, which has led to inbreeding within the populations. Simultaneously, in adults and juveniles, the values are very similar and may be maintained by self-incompatibility that creates a dependence on pollinators to facilitate reproduction [46].

Some populations share some alleles, suggesting that they were connected by natural corridors some time ago, mainly in the vicinity of the Sierra Gorda in the northern of Querétaro and south of San Luis Potosí. However, in the current generations of seedlings, the Fst values indicate that the populations analyzed are becoming isolated. The isolation is probably caused by physical barriers associated with anthropic activities increasing in the area. This situation diminishes the habitat conditions and leads to higher mortality, affecting pollinators and favoring inbreeding and gene drift [6].
