TITLE PAGE - acumen - The University of Alabama
TITLE PAGE - acumen - The University of Alabama TITLE PAGE - acumen - The University of Alabama
Macroinvertebrate biomass mirrored the patterns in standing crop organic matter (Fig. 1). Similar to standing crop organic matter, macroinvertebrate biomass (29 to 346 mg dry mass m -2 ) in the cave streams from this study fall within the lower range of estimates from surface streams not impacted by anthropogenic pollution (156 to 20,206 mg dry mass m -2 ; Benke, 1993). Only two other studies to date have quantified macroinvertebrate biomass in cave streams, both of which fall within the range of estimates from this study (10 to 300 mg dry mass m -2 ; Huntsman et al., 2011 b; Chapter 3 this document). Thus, given that both organic matter and macroinvertebrate biomass were within the lower ranges of reports from surface streams, the cave streams in this study appear to fit the generalized characterization of energy-limited cave ecosystems. Population size estimates Estimating the population size of crayfish is challenging because they are potentially very mobile species and the complexity of their habitat is often high, particularly in surface streams (i.e., because of the presence of macrophytes and large woody debris). While habitat complexity is relatively diminished within many cave streams (i.e., no or little biogenic structure, prevalence of bedrock structure), cave crayfish still reside in areas that are inaccessible to humans, such as bedrock fissures or under large boulders. Thus, visuals surveys and other count-based methods will likely misrepresent population sizes of both cave and surface crayfish because a majority of the population may be unavailable for direct sampling. Rabeni et al. (1997) evaluated various methods (i.e., quadrat sampling, visual surveys, and mark-recapture) for estimating population sizes of crayfish in surface streams and found that visual surveys significantly underestimated population size when compared to mark-recapture methods using multiple sampling events. Monthly capture rates of O. australis within each cave varied 5 to 10 times and the majority of 113
crayfish captured in most months were unmarked, both of which indicate that a much larger population of crayfish was present than monthly capture rates would have shown. Thus, on any given sampling date many of the crayfish were likely in inaccessible habitats, such as in fissures or under large slabs of bedrock. Any such pattern or habitat use has clear implications for our whole-system energy budgets (see below). Molecular-based methods also suggest that visual based surveys, which are analogous to the monthly capture rates in this study, do not accurately estimate the population size of obligate cave species. Buhay and Crandall (2005) examined mitochondrial-DNA haplotypes among 43 populations of O. australis occurring throughout northeastern Alabama and eastern Tennessee, including both the Hering and Limrock populations. They found both high genetic diversity and large effective population sizes, which suggest that the population sizes of O. australis throughout its range are likely large. Thus, using visual surveys will grossly underestimate population sizes for large mobile obligate cave species like O. australis, which can lead to inaccurate estimations of ecosystem processes (e.g. secondary production and energetic demands) and conservation assessments (e.g. threatened or endangered status). Crayfish secondary production O. australis is a slow-growing species, reaching maturity in ~5 years and capable of living for ~22 years (Venarsky et al., 2012b). The slow growth of O. australis translated into estimates of secondary production and P:B values that are among the lowest recorded for either cave or surface species of crayfish (Table 6). However, the production estimates from the Tony Sinks population of O. australis are higher than those for some surface species of crayfish. Thus, while cave ecosystems do appear to be energy-limited when compared to some surface ecosystems, they are capable of supporting similar rates of crayfish production. 114
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Macroinvertebrate biomass mirrored the patterns in standing crop organic matter (Fig. 1).<br />
Similar to standing crop organic matter, macroinvertebrate biomass (29 to 346 mg dry mass m -2 )<br />
in the cave streams from this study fall within the lower range <strong>of</strong> estimates from surface streams<br />
not impacted by anthropogenic pollution (156 to 20,206 mg dry mass m -2 ; Benke, 1993). Only<br />
two other studies to date have quantified macroinvertebrate biomass in cave streams, both <strong>of</strong><br />
which fall within the range <strong>of</strong> estimates from this study (10 to 300 mg dry mass m -2 ; Huntsman<br />
et al., 2011 b; Chapter 3 this document). Thus, given that both organic matter and<br />
macroinvertebrate biomass were within the lower ranges <strong>of</strong> reports from surface streams, the<br />
cave streams in this study appear to fit the generalized characterization <strong>of</strong> energy-limited cave<br />
ecosystems.<br />
Population size estimates<br />
Estimating the population size <strong>of</strong> crayfish is challenging because they are potentially very<br />
mobile species and the complexity <strong>of</strong> their habitat is <strong>of</strong>ten high, particularly in surface streams<br />
(i.e., because <strong>of</strong> the presence <strong>of</strong> macrophytes and large woody debris). While habitat complexity<br />
is relatively diminished within many cave streams (i.e., no or little biogenic structure, prevalence<br />
<strong>of</strong> bedrock structure), cave crayfish still reside in areas that are inaccessible to humans, such as<br />
bedrock fissures or under large boulders. Thus, visuals surveys and other count-based methods<br />
will likely misrepresent population sizes <strong>of</strong> both cave and surface crayfish because a majority <strong>of</strong><br />
the population may be unavailable for direct sampling. Rabeni et al. (1997) evaluated various<br />
methods (i.e., quadrat sampling, visual surveys, and mark-recapture) for estimating population<br />
sizes <strong>of</strong> crayfish in surface streams and found that visual surveys significantly underestimated<br />
population size when compared to mark-recapture methods using multiple sampling events.<br />
Monthly capture rates <strong>of</strong> O. australis within each cave varied 5 to 10 times and the majority <strong>of</strong><br />
113
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