Five-lined skink (Eumeces fasciatus) COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

Search effort

Great Lakes/St. Lawrence and Carolinian populations

Estimating population occurrence in Ontario is greatly aided by the Ontario Herpetofaunal Summary (OHS) database, which contains over 1,200 sightings of the species (Oldham and Weller, 2000). This database is an invaluable resource to aid in identifying the presence of the species, although population occurrence is likely to be disproportionately reported in areas that are more frequented by humans, and overall submission of data to the OHS has declined since 1995 (M. Oldham, personal communication [pers. comm.]). Species misidentification is unlikely because E. fasciatus is the only lizard species in Ontario.

Estimating population abundance in E. fasciatus is extremely difficult. Individuals are secretive and spend the greater part of their day under cover elements. As well, activity patterns change throughout the year. While adult males and females are most abundant in May, yearlings are most abundant in June (Fitch, 1954). After the breeding season, males become much less active (Fitch, 1954; Seburn, 1993) and may become more fossorial in hotter months (Seburn, 1990). Finally, individuals may shift their home range throughout the active season, resulting in over- or underestimates of the true population size. Because the habits and activity patterns of individuals change with sex and age, census data should be sorted into categories of male, female, and young (Fitch, 1954). Young are more likely to be observed than adults because of their increased numbers, increased activity throughout the season, increased movement patterns on a daily basis, and increased conspicuousness due to their brightly coloured tail.

Accurate estimates of population abundance require active searching and intensive study over the course of an appropriate time period in Ontario. In a Carolinian population in PPNP, Hecnar and M’Closkey (1998) measured activity density, which is strongly correlated with true density (see Hecnar and M’Closkey, 1998). Alternatively, Seburn (1993) and Wick (2004) used mark-recapture methods throughout the active season in a Carolinian and Great Lakes/St. Lawrence population, respectively. In both studies, marking was achieved through toe-clipping (Seburn, 1993; Wick, 2004). Although application of paint to the skin of animals is another possible marking method, this technique allows only short-term monitoring as the paint wears quickly (within 24 hours for some individuals – S. Hecnar, personal observation [pers. obs.]) and individuals will shed their skin throughout the active season. Active searching involves flipping or moving cover elements (woody debris or cover rock) that should always be replaced to their original position.

It should be noted that active searching in Great Lakes/St. Lawrence populations can result in skink fatalities, especially when performed by novice surveyors. Appropriate caution is necessary to avoid crushing individuals in the pivot point of the cover rock as it is being lifted. To minimize this risk, smaller rocks should be lifted entirely from the surface insofar as possible. When this is not possible, one surveyor should partially lift a corner of the rock, while another surveyor looks underneath the rock to check for the presence of a skink (B. Howes, pers. obs.).

Abundance

Based on records from the OHS, there are 84 Great Lakes/ St. Lawrence populations and five Carolinian populations reported since 1995 (Oldham and Weller, 2000; see Table 1). An estimate of effective population size (N_e) was generated for nine Ontario populations based on six microsatellite loci, maximum likelihood analyses (Beerli and Felsenstein, 1999; Beerli and Felsenstein, 2001), and the assumption of a typical vertebrate microsatellite mutation rate of 10^-4 per locus per generation (Table 5). The average N_e for each designatable unit was then calculated and multiplied by the population occurrence within each unit. The total N_e estimate for Ontario (and consequently Canada) is 23,839 (Howes and Lougheed, unpublished data). This approach has several sources of error: 1. estimates of population occurrence (based on the number of populations recorded in the OHS since 1995) may be inaccurate, 2. the calculated mean of N_e for sampled Great Lakes/St. Lawrence and Carolinian populations may not reflect N_e of unsampled extant populations, and 3. the likelihood method of estimating N_e may be inaccurate (e.g. Zaid et al., 2004).

Estimates of N_e produced below cannot be directly compared to estimates of census population size, because neither method has been simultaneously employed within a single population. In fact, the only population for which a census estimate exists is the population in PPNP. Seburn and Seburn (1989) roughly estimated that this population was composed of between 1,000-2,000 individuals in 1989. Accurate estimates of N_e are certainly less than the census size (Frankham, 1995). While the ratio of N_e to census population size varies among species, estimates of N_e are typically 11% of census estimates (Frankham, 1995).

Great Lakes/St. Lawrence populations

The average N_e for Great Lakes/St. Lawrence populations is 266, and the population occurrence is estimated to be 84 (Table 1) based on records from the OHS since 1995 (Oldham and Weller, 2000). Thus, the total N_e in the Great Lakes/ St. Lawrence faunal province is approximately 22,300 individuals (Howes and Lougheed, unpublished data).

Carolinian populations

The average N_e for Carolinian populations is 299 individuals, and the population occurrence is estimated to be 5 (Table 1, Table 2). Thus, the total N_e in the Carolinian faunal province is approximately 1,495 individuals (Howes and Lougheed, unpublished data). However, this total N_e estimate for the Carolinian populations is based on the assumption that the estimated average N_e for PPNP and Rondeau Provincial Park are representative of the N_e for remaining Carolinian populations. This assumption is not accurate, because the other Carolinian populations are much smaller than populations in PPNP and Rondeau Provincial Park.

Fluctuations and trends

Population density varies within a year and among years according to a variety of factors including weather conditions and successional processes. Population density was estimated at 125-250 individuals/ha (excluding hatchlings) for a Kansas population that had typical natural habitat. Population density in one area in PPNP ranged from 21 individuals/ha in 2003 and 2004 to 85 individuals/ha in 2001 (Hecnar and Hecnar, 2005). Within a year, numbers are lowest in mid-summer, just before hatching occurs. Following hatching, population density can double as it reaches a maximum (Fitch, 1954).

Cohort structure can vary greatly from year to year depending on weather conditions and various other factors. For instance, in one of his Kansas study populations, Fitch (1954) found that the two-year old cohort made up 70% of the breeding population in 1951, whereas the corresponding cohort comprised only 36% in 1950 and 58% in 1952. Any given adult cohort can be reduced by at least half within a season (Fitch, 1954).

Great Lakes/St. Lawrence populations

The occurrence of Great Lakes/St. Lawrence populations appears to fluctuate moderately based on population occurrence data in the OHS. The number of recorded populations prior to 1984 was 71. From 1984-1994, the number of recorded populations increased to 115, and from 1995-present, this number declined to 84 (Table 1; Oldham and Weller, 2000). It is difficult to assess if the recent apparent decline in occurrence of Great Lakes/St. Lawrence populations since 1995 is biologically meaningful, as the overall submission of data to the OHS has also declined during this time period (M. Oldham, pers. comm.).

Carolinian populations

The number of populations of skinks in southwestern Ontario has declined since at least 1984. Prior to 1984, 17 populations were recorded. From 1984-1994, this number declined to eight populations, and only five populations have been recorded or confirmed since 1995 (Table 1; Oldham and Weller, 2000; C. Jacobs, pers. comm.). In 2000, locations of 16 historical Carolinian populations were surveyed for E. fasciatus. Of the 16 populations surveyed, skinks were found only at PPNP, Rondeau Provincial Park and Pinery Provincial Park (Hecnar and Hecnar, 2000 in Hecnar and Hecnar 2005; S. Hecnar, pers. comm.). Several of the 16 locations no longer had suitable habitat owing to development and other anthropogenic disturbance. None of the locations on the Niagara Peninsula had skinks.

Although skink abundance within Rondeau Provincial Park appears to be stable (S. Dobbyn, pers. comm.), abundance at Pinery Provincial Park appears to have dwindled to quite low levels (S. Hecnar, pers. comm.), and this population is certainly much smaller than at PPNP or Rondeau based on surveys performed from 2002 to 2004 (B. Howes, pers. obs.). Very little information on the extant Oxley Poison Sumac Swamp population is known. The Nature Conservancy of Canada acquired the habitat in which skinks are present in 2004, and has since enhanced skink habitat by laying down artificial cover boards throughout the site (H. Arnold, pers. comm.). The amount of suitable skink habitat within Oxley Poison Sumac Swamp is small and skink sightings remain low despite considerable inventory work performed at the site (M. Oldham, pers. comm.).

The only data on a population trend available for an Ontario population are based on a population in the Carolinian population (PPNP). From 1990-1995, this population suffered a three-fold to five-fold decline in numbers. The age structure of this population appeared to be adult-biased in all five study years, suggesting that recruitment was insufficient to maintain population size (Hecnar and M’Closkey, 1998). Further investigation into this population decline indicated that a lack of suitable microhabitat (woody debris) was responsible for the absence of skinks in areas heavily used by humans. A microhabitat restoration experiment indicated that artificially placed cover elements are used by skinks and can enhance skink habitat (Hecnar and M’Closkey, 1998). The population was not only threatened by microhabitat loss, but also by systematic illegal collecting. The attractive colouration of juvenile five-lined skinks has made them desirable pets, and large-scale collecting has probably occurred in the park since at least 1989 (Hecnar and M’Closkey, 1998). Microhabitat restoration has increased skink abundance within the park since 1996, but continued active management is needed for the persistence of the species within PPNP (Hecnar and Hecnar, 2005).

Rescue effect

Great Lakes/St. Lawrence populations

Great Lakes/St. Lawrence populations separated by as little as 3-5 km showed significant genetic distinction (Wick, 2004; Howes and Lougheed, unpublished data). Wick (2004) also showed that water is an effective barrier to gene flow in the species, and that an island subpopulation in their study site had reduced genetic diversity relative to neighbouring populations that were within approximately 2 km or less. Clearly, rescue from a neighbouring population or recolonization following a local extinction event is unlikely in these populations. Rescue from United States’ populations is not possible.

Carolinian populations

The remaining Carolinian populations are extremely isolated from one another. Rondeau Provincial Park and PPNP display significant genetic differentiation, which is not surprising given the geographic distance that separates them. Natural rescue from a neighbouring population or natural recolonization following a local extinction event is virtually impossible in Carolinian populations. Rescue from the United States’ populations would not be possible owing to distance, and lack of suitable habitat to get to Ontario (e.g., they would have to cross the Great Lakes or their associated rivers).