As discussed in Chapter 1, for the purpose of this assessment, we define viability as the ability of the species to sustain populations in the wild over time (in this case, 50 years). Using the SSA framework, we describe the species’ viability by characterizing the status of the species in terms of its resiliency, redundancy, and representation (the 3Rs). Using various time frames and the current and projected levels of the 3Rs, we thereby describe the species’ level of viability over time.
3.4.1. Population Resiliency
As previously described, PCC populations were delineated by genetic differentiation and separated by polygons (Figure 3.6, 3.7, and 3.16). Given the hierarchical nature of the relationship between individuals, populations, and species, we first consider resiliency at the level of an individual, then scale up to populations (patch polygons), and, ultimately, make inferences at the species-level.
Resiliency (measured at the population level) is the foundational building block of the SSA Framework; thus, for the PCC to be viable, some portion of its range must be resilient enough to withstand stochastic events. Stochastic events that have the potential to affect PCC populations include droughts, flooding, altered surface water quality, altered groundwater quality and quantity, and accumulated duff (plant material) over time. Given the data available, the metrics that were used to assess resiliency were categorized as population factors (inbreeding coefficient, isolation, and population abundance) and habitat elements (ground cover via management abilities, freshwater quality and quantity) (Table 3.7). In the next section, we discuss the methods used to estimate resiliency metrics and we explore potential causal relationships between resiliency and PCC habitat requisites.
Population Factors that Influence Resiliency
Inbreeding Coefficient: Multiple studies have shown that small populations are more likely to go extinct than large ones. Small populations are more vulnerable to chance events, such as all individuals happening to be one sex or all failing to reproduce or survive. Small populations are also less likely to be viable after an environmental disaster such as drought or flooding events (Sutherland 2000). Small populations may also suffer from the Allee effect, meaning the decline in survival rate or mean reproductive output at small populations due to a range of processes such as increased predation, reduced ability to find mates, or reduced breeding success in small groups (Sutherland 2000). The main genetic concern with a population becoming small is a loss in genetic variation and an increase in the likelihood that fertilization will be with a relative. Both result in an increase in homozygosity. A recent genetic analysis of population differentiation and clustering methods was used to assess the population structure of the PCC (Duncan et al. 2017).
Population Isolation: To promote genetic connectivity in the PCC, we must have an understanding of their potential abilities to move between populations. One working hypothesis was that ditches within the range promote movement, especially during flooding events. This idea is supported by observations of some localized movements of PCC into previously unoccupied ditches after recent flooding where they were not seen in these new locations during the next sampling event.
Because the landscape occupied by the PCC is spatially heterogeneous, it is important to understand how certain landscape features affect the PCC’s ability to move in order to meet requirements for foraging, migration, or other movement-dependent processes (Crooks and Sanjayan 2006 as cited in Duncan et al. 2017). Linear genetic distances may not best explain patterns of genetic connectivity where other factors have important roles in shaping observed genetic patterns, such as land cover or land use effects on demography. We rely on a landscape “Least Cost Path (LCP)” land cover analysis conducted by Duncan et al. (2017) to assist in determining what may affect genetic connectivity in Panama City crayfish.
Population Abundance: The size of an individual population coupled with age and sex classifications can be used as an indicator of resiliency. Within Chapter 3.3 we have summarized the years that surveys of varying levels were completed within each population or patch. Semi-terrestrial crayfish spend a significant portion of their life cycle within their burrows. Standardized methods for assessing PCC population density have not been developed but are critical to assessing PCC populations in the future. The protocol currently used for PCC monitoring typically depends on dip-net sampling when sufficient surface water is present and nondestructive evaluation of crayfish burrows. The protocol is quantitative and results in a catch per standard unit effort estimate of the population. The protocol can miss specimens in vegetation and does not sample individuals living below ground in burrows, and we currently do not have an estimate of detection probability using this protocol. We use population counts to assess the relative population size across the range of the species.
Habitat Elements that Influence Resiliency
Water Quality & Quantity: Although crayfish are facultative air breathers, moisture is required to facilitate the respiratory process (Longshaw and Stebbing 2016, p. 327). Burrowing to groundwater or access to surface water are both important habitat features needed to prevent desiccation of individuals and populations. It is believed that the PCC cannot burrow much deeper than 3 feet below the surface (Keppner and Keppner ??).
Declines in water quality are known to present a significant threat to other species of crayfish (and presumably to PCC). These declines can range from oxygen-deficient conditions resulting from algal blooms, sewage spills, or localized leaks to pollution originating from roadway runoff or chemical spills (Acosta and Perry 2001). Many substances commonly used around the home or business can be toxic to PCC and other wildlife if used or disposed of improperly. PCC often inhabit ditches and swales close or adjacent to commercial and private properties, which may affect the water quality at these sites.
We used a proxy measure of water quality and quantity based on the amount of development surrounding the population. We assumed that greater acreage in developed and unsuitable landcover types (which includes transportation and other development-related types) is correlated with declines in this habitat element.
Herbaceous Ground Cover: The PCC naturally inhabits shallow, ephemeral, freshwater wetlands that are associated with early successional wet prairie-marsh and wet pine flatwoods. These locations historically supported a native herbaceous plant community dominated by native wetland grasses and sedges with an accompanying overstory of low-density pines, and were naturally maintained by periodic wildfire. Nearly all remaining PCC habitat has been temporarily or permanently altered due to silvicultural practices, ditching, draining, exotic plant invasion, fragmentation, and/or fire exclusion/suppression, leading to unnaturally dense thickets of aggressive hardwood shrubs (e.g., titi sp. [Cliftonia monophylla and Cyrilla racemiflora], wax myrtle [Myrica cerifera], yaupon [Ilex vomitoria]).
Herbaceous vegetation is important to the PCC for food, detritus formation, and cover. Absence of vegetation increases exposure of this small crayfish to predation and reduced availability of food. Unmanaged habitat tends to overgrow with woody vegetation, which eventually eliminates herbaceous vegetation and possibly increases the transpiration rate and increases the depth of the water table during dry periods.
Suitable Habitat: Species sampling efforts and a recent landscape modeling analysis support the theory that the PCC almost exclusively relies on core and secondary soils. These soils provide the sediment structure needed for burrow construction to the water table and also support the herbaceous vegetation upon which the species relies for food and cover. Lands supporting the PCC must be of sufficient size to sustain a PCC population, but we don’t know the minimum size, as many factors influence a PCC population, including other habitat conditions. The recent work of Duncan et al. (2017) showed that all remaining populations with >800 acres of suitable habitat supporting them were genetically healthy, and population counts support this as well.
3.4.2. Species Representation
Maintaining representation in the form of genetic or ecological diversity is important to maintain the PCC’s capacity to adapt to future environmental changes. The PCC is a localized endemic historically existing within a portion of a 56 sq. mi. area. Its historic range likely created one population largely connected by core and secondary soils. The PCC’s historic range occurs within a peninsular area that is heavily developed and surrounded on 3 sides by large bodies of salt water and a creek system on the eastern side. As an endemic species it is likely the PCC has always been just one population connected through core and secondary soils until urban growth came to Panama City (incorporated in 1909), thereby beginning the phases of fragmentation and isolation. The species is now supported by 13 remaining patches or localized populations that show relatively high genetic differentiation with inbreeding coefficients ranging from 0.214 to 0.493 (Figure 3.6, Tables 3.2 and 3.3 ) (Duncan et al. 2017) and associated acreages of suitable soils ranging from 5 acres to 5,309 acres.
3.4.3. Species Redundancy
Redundancy reduces the risk that a large portion of the species’ range will be negatively affected by a natural or anthropogenic catastrophic event at a given point in time. Species that have resilient populations spread throughout their historical range are less susceptible to extinction (Carroll et al. 2010; Redford et al. 2011. The PCC historically lacked redundancy in that its historic range consisted of one population of interconnected soils, but today across the range of the species there is a distinct difference, genetically, between individual patches located in the western range versus individual patches within the eastern range (Duncan et al. 2017). While this pattern likely corresponds to patterns of fragmentation from urban development as well as some natural wetland buffers (creeks, stream bodies), it has created a scenario of the PCC now having two distinct groups of populations. The western group of populations includes 8 separate patches and the eastern group of populations includes 5 separate populations.