Lakeside daisy (Hymenoxys herbacea) COSEWIC assessment and update update status report: chapter 7

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Population Sizes and Trends

Population Size

The number of flowering adults per population in 2000 ranged from 3 (population SC) to an estimated 3,540,000 (population MB). The number of rosettes per population in 2000 ranged from 50 (population SC) to an estimated 202,013,200 (population MB). Effective population size (N_e, the number of rosettes in a randomly mating population with the same degree of genetic drift) was estimated for two populations on the Bruce Peninsula and averaged 4,343.4 individuals, 43% smaller than a simple census of rosettes would indicate (Campbell, 2001). The effective population sizes of the other H. herbacea populations were estimated using the N_e/N ratio of 0.43 and ranged from 22 (population GL) to 86,865,676 (population MB). An effective size of 5,000 individuals is generally recommended as a minimum target to avoid loss of both neutral genetic variation and adaptive potential and to minimize the accumulation of deleterious mutations (Lande 1993, 1994). Seven of the 38 populations had (estimated or extrapolated) values lower than this rule of thumb, suggesting that most populations are not likely to experience the negative impacts of genetic drift.

Furthermore, an elasticity analysis showed that of all demographic processes, fluctuations in the survival rate of the Juvenile 2 life stage (more than 6 leaves, no inflorescence) has the greatest impact on effective population size (Campbell, 2001). Therefore, if N_e becomes extremely low due to habitat fragmentation, conservation efforts that increase the survival rate of this life stage will likely be the most successful.

In order for a self-incompatible plant to produce offspring, a population must maintain sufficient mating type diversity. Self-incompatibility alleles are maintained through frequency-dependent selection and hence are less vulnerable to the effects of genetic drift. Byers and Meagher (1992) determined that genetic drift can severely affect mating type diversity when effective population size is smaller than 50 individuals. Only the two smallest known populations (GL, SC) had an effective size near 50 individuals (22 and 57 respectively), which indicates that genetic drift may not have a great impact on the SI allele diversity in populations as they are currently found (Campbell, 2001).

Trends

The first reported mention of H. herbacea on the Bruce Peninsula was in 1965 (U.S. Fish and Wildlife Service, 1990) and it is hypothesized to have spread across the Bruce Peninsula since then. If this is true, then the species has expanded its range remarkably quickly. However, it is probable that the plant did exist on the Bruce Peninsula prior to this first sighting. Since being recognized as a species at risk in the U.S., the intensity of searching for new H. herbacea populations has increased. From 1999 to 2000, the number of populations known to the NHIC has grown from 22 to 39. It is likely that early search efforts were not intense and communication among knowledgeable individuals poor.

Prior to 1999, little information was available on the sizes of extant populations. From 1999 to 2000, populations were observed to increase as well as decrease. In most cases the population changes were relatively small. There was no significant change in mean population size across the one-year interval and the size of each population in 1999 was strongly and positively correlated with its size in 2000 (Campbell, 2001). Three populations were observed to decline in numbers (populations SC, CPL and ML) over a 1-year period (Campbell, 2001). These three populations were relatively small initially and all experienced heavy human traffic during the past year. Populations, SC and CPL are within National Park boundaries on the Bruce Peninsula and are found on major hiking trails within the Bruce Peninsula National Park. Population ML is found in a private campground and as such experiences a large amount of traffic (both cars and humans).

One population (population EL) from the Bruce Peninsula (site #15, Figure 3) is known to have gone extinct sometime between 1995 and 1999. The habitat at this site seemed extremely wet compared to the habitat of extant populations. Otherwise there was no obvious distinction and there was little human activity. It is obvious that H. herbacea can withstand some traffic. Finally, seven populations have not been seen in at least thirteen years and hence, we don't know if any or all of them are extant or extinct. All of these are in extremely isolated locations and may explain our lack of knowledge.