Genetic Drift in Ocelots of South Texas

By Krista Ruppert, Texas A&M WFSC ‘16

 

                Ocelots, Leopardus pardalis, are considered endangered in the United States, with an estimate of less than 80 individual remaining in Willacy and Cameron counties of southern Texas. In the 20^th century, humans altered the landscape from native rangelands to other uses, causing habitat fragmentation. Now, smaller habitat parcels serve as genetic “islands,” where small populations of ocelots are blocked from accessing other populations. For example, the ocelot population at the Laguna Atascosa National Wildlife Refuge (LANWR) is in a small patch of habitat surrounded on all sides by field crops, human development, and the Laguna Madre bay. This isolation, along with comparable isolation of the only other ocelot population in the U. S. located on private lands in Willacy County, has led to genetic drift and a loss of genetic variation in ocelots in the United States, which can in turn lead to greater probability of extinction.

Genetic drift is important to consider when planning for ocelot conservation in Texas.

Photo from the US Fish and Wildlife Service.

                Threatened and endangered populations are particularly vulnerable to genetic drift and inbreeding due to their low population numbers. Genetic drift erodes genetic diversity through the loss of alleles (or alternative forms of genes) and an increase in homozygosity (wherein forms of genes are identical), and potentially can result in the reduction of adaptive variation and a decrease in population viability. More simply, when there are fewer individuals in a population, the gene pool is less diverse. As breeding occurs, each subsequent generation loses overall diversity. A loss in diversity can result in decreased fitness, as there is no selection for the most valuable traits; rather, all the traits become the same regardless of whether they are ideal or not. Decreased fitness reduces reproductive potential and may increase susceptibility to disease and other forms of mortality. A loss of genetic variation culminates with inbreeding depression, in which population numbers drastically decline as mortality increases and the birth rate decreases. Overall, individuals become less adaptable and less successful. In endangered species, this effect can be detrimental and lead to extinction. The introduction of genetic variation is pivotal to bolster populations affected by genetic drift.

This diagram explains genetic drift. Texas ocelot populations are experiencing severe genetic drift due to human-created barriers.

                While ocelots in Texas have not been examined in whether they are experiencing inbreeding depression as of yet, evidence of genetic drift has been well-documented. Janecka et al. (2007) found that the mitochondrial DNA of ocelots in Texas was far less diverse than that of ocelots in Mexico, Central America, and South America, indicating that populations in Texas are experiencing a decline in genetic variation. Janecka et al. (2008) found that ocelots in the LANWR exhibited a 23% decrease in genetic variation from 1986-1989, and determined that variation would continue to erode at a rate of 16% per generation; these data indicate a severe bottleneck, in which the population’s size and genetic variation are reduced for at least one generation, similar to that faced by the critically endangered Amur leopard. Additionally, genetic divergence between the Willacy and LANWR populations was found to be higher than the variation between the Willacy and northern Mexico populations by Janecka et al. (2011), indicating complete isolation of the LANWR. Ocelots in the two populations in Texas have not been observed having any interaction in at least 30 years, and Texas ocelots have not been observed interacting with Mexico ocelots for at least 25 years; the more severe lack of diversity in the LANWR is hypothesized to be due to a smaller number of breeding females, and thus less variation, present in the area when connections with other populations were lost. Gene flow has halted for Texas ocelots, which poses a threat to the already small populations. As the LANWR population is smaller and contained in an area of habitat that lacks potential for growth, it is more vulnerable to genetic drift and bottleneck effects.

Satellite view of the Laguna Atascosa National Wildlife Refuge. Note the agricultural development surrounding the area.

                Currently, the most highly recommended action to combat genetic drift in ocelot populations in Texas is the translocation of individuals of the same subspecies. This could be an exchange between the LANWR and Willacy populations, or the translocation of individuals from northern Mexico to populations in Texas. Individuals from Central and South America should not be used, as they may not be genetically compatible with Texas ocelots. It is estimated that a single individual introduced into a population each generation could have a huge impact on genetic diversity in Texas ocelot populations. Translocation is an ideal short-term aid to declining ocelot populations, as the restoration of native thornshrub is a slow process. Despite this, the establishment of corridors between ocelot populations in Texas and Mexico should not be neglected as it is a long-term solution to the genetic bottleneck currently being experienced by Texas ocelots. Combining short-term and long-term strategies may be the key to ensuring the survival of ocelots in Texas.

Literature Cited

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Haines, A. M., M. E. Tewes, J. E. Janecka, and L. I. Grassman. 2007. Evaluating the benefits and costs of ocelot recovery in southern Texas. Endangered Species Update 24(2):35-41.

Janecka, J. E., C. W. Walker, M. E. Tewes, A. Caso, L. L. Laack, and R. L. Honeycutt. 2007. Phylogenetic relationships of ocelot (Leopardus pardalis albescens) populations from the Tamaulipan Biotic Province and implications for recovery. Southwestern Naturalist 52(1):89-96.

Janecka, J. E., M. E. Tewes, L. L. Laack, L. I. Grassman, A. M. Haines, and R. L. Honeycutt. 2008. Small effective population sizes of two remnant ocelot populations (Leopardus pardalis albescens) in the United States. Conservation Genetics 9:869-878.

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Janecka, J. E., M. E. Tewes, L. L. Laack, A. Caso, L. I. Grassman, and R. L. Honeycutt. 2014. Loss of genetic diversity among ocelots in the United States during the 20^th century linked to human induced population reductions. PLoS ONE 9(2):e89384.

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Korn, J. M. 2013. Genetic pedigree and prey dynamics of ocelot and fine-scale movement patterns of bobcat in South Texas. Dissertation, Texas A&M University–Kingsville, Kingsville TX.

Walker, C. W. 1997. Patterns of genetic variation in ocelot (Leopardus pardalis) populations for South Texas and northern Mexico. Dissertation, Texas A&M University–Kingsville, Kingsville TX.