Long-term effects of weather on the cricket (Hadenoecus subterraneus, Orthoptera, Rhaphidophoridae) guano community in Mammoth Cave National Park.
One reason why the Mammoth Cave Region has such a high diversity of terrestrial cave species is that the cave "cricket" (Hadenoecus subterraneus) is a key species upon which many other cave species depend for import of food (Barr and Kuehne, 1971; Poulson, 1992). The cave cricket has a high frequency among all cave sites at all times, a high density in many cave entrance sites, and a relatively high impact per individual in terms of feces deposited per time (Poulson, 1992). Cave crickets forage outside caves at night and return to suitable cave entrances to roost, digest food and defecate. Cricket feces accumulate under roosts and are the food base for the cricket guano community (Poulson and Kane, 1981; Poulson, 1992).
In this paper we consider four hypotheses to explain the great fluctuations in the cricket guano community in White Cave and Little Beauty Cave that we observed from 1971 to 1994. The number of species and their abundances rose dramatically in the mid-1970s; many species disappeared and others crashed in the late 1980s; and by 1994 neither number of species nor abundances of remaining species had returned to the early 1970s levels. The first hypothesis that might explain these observations is that anthropogenic disturbance by cave tours caused the crickets to roost in other areas. The second hypothesis is that direct effects of weather made the guano too wet or too dry to be used by decomposers and detritivores. The third hypothesis is that direct effects of weather on cave microclimate caused crickets to move their roosts deeper into the cave so that they deposited less guano on our census areas. The fourth hypothesis is that indirect effects of weather on crickets caused changes in guano deposition rates in all areas of the cave. The proposed chain of causality for this indirect effect hypothesis is that weather affects frequency of cricket foraging outside of caves, risks of foraging affect cricket survival, both foraging and survival affect rates of cricket guano deposition and rates of guano deposition affect species of the guano community. We can conceive of no other hypotheses, such as above ground anthropogenic impacts, that could have affected cricket foraging. Over the past several decades neither land use or appearance of the forests above the caves has changed and no construction occurred anywhere near the caves.
The study sites. - Little Beauty Cave and White Cave are very small but are the most biologically rich caves in Mammoth Cave National Park in central Kentucky. Both are overlain by successional forests which have had no anthropogenic disturbances for about 75 yr. Neither cave suffers from severe winter temperature and humidity depression inside the cave due to influx of cold dry winter air because both are horizontal in orientation and no longer connected to the extensive 500 km of the Mammoth Cave System (Poulson, 1992). Both caves have a mixture of substrates which are mostly limestone rock and carbonate speleothems such as flowstone, rimstone dams and stalagmites. Little Beauty Cave is ca. 1 km S of the Joppa Ridge Motor Nature Trail. It is 45 m long front to back, from 3-10 m wide and has a total vertical relief of about 4-6 m. There are ca. 25 [m.sup.2] of guano deposits of which a third are dense. White Cave is along the Echo River Spring trail on Mammoth Cave Ridge and is gated. It is about 125 m long front to back, from 7-17 m wide and has a total vertical relief of ca. 10-12 m. There are about 75 [m.sup.2] of guano deposits of which a fifth are dense. In both caves the study areas were subsets of the dense guano areas that were both accessible and neither too wet nor too dry (Poulson, 1992) when first studied in the early 1970s.
Census of crickets and the guano community. - Community data were token on dense guano in permanent quadrats and by intensive search of larger areas. Each quadrat was 10 cm x 10 cm and the numbers of quadrats censused at each visit ranged from 15-22. Intensive searches covered 4-6 [m.sup.2] for 1-2 person-hours. The quadrats were scattered in areas with the highest densities of guano community species. These small permanent quadrats enabled us to look closely for very small species, such as mites and springtails, and the intensive searches of large areas allowed us to get abundances of larger species, such as millipedes and bristletails, that are generally uncommon. Our census areas in White Cave were in a protected area behind a flowstone curtain from 4-20 m inside the entrance and slightly above floor level. In Little Beauty Cave our census areas were from 30-35 m inside the entrance and were protected from winter temperature and humidity depression by being 3-5 m above the lowest floor level. From White Cave we have dam for occasional months in 9 yr between 1971 and 1994. From Little Beauty Cave we have data from 1972, 1973 and from 1 winter and several summer months in 1975, 1976, 1985, 1987 and 1994. We use only our June or July data for analysis for both caves because 18 mo of consecutive data from Little Beauty Cave in 1972-1973 showed that numbers of guano community species peak in June and July.
We have two indices of rates of guano deposition during our study. We did not use small plastic sheets to assay guano deposition rates because these would have disrupted parts of the guano community. We did not count crickets roosting over our census areas until the late 1980s. Our only long-term independent index of guano deposition was an indirect one based on a spider. The orb-weaving spider Meta menardi preys exclusively on crickets and so its abundance is an index of cricket densities and guano deposition rates. Meta menardi occur in White Cave but not in Little Beauty Cave. In Little Beauty Cave the relative abundance of two fungivorous mites gives an index of guano deposition rates because Ceratozetes sp. is present only on dense, fresh guano, and Belba sp. is present only if there is little or no fresh guano (Poulson, 1992).
Indices of weather favorability. - Monthly weather records for Mammoth Cave National Park were obtained from Glen Conners at the Kentucky Climate Center at Western Kentucky University and from Bobby Carson at the National Park Service Air Quality Center in Washington, D.C. To put our favorability data in context we include a Walter climatic diagram for Mammoth Cave National Park based on data from 1935-1994 [ILLUSTRATION FOR FIGURE 1 OMITTED]. Indices of favorability or unfavorability are for cricket foraging at night outside the protected microclimate of the caves. Lacking any rationale for weighting precipitation, mean minimum winter temperature and mean maximum summer temperature differently, favorability indices are simply departures from averages; inches for precipitation and degrees Fahrenheit for temperature. We determined the yearly departure from the 30-yr average precipitation and considered an above average value as favorable and a below average value as unfavorable for cricket foraging. It is known that relative humidities below 90% cause increased evaporative water loss of cave crickets at temperatures from 10 to 20 C (Studier et al., 1987; Studier and Lavoie, 1990) and that even drier and warmer conditions can desiccate the soft, moist food needed by crickets (Poulson, 1992). For temperature we determined the departure from mean minimum temperature for all days of January-March and the departure from mean maximum temperature for all days of June-August. Our rationale for using winter minimums was that values below average would not only restrict night-time foraging outside caves but also could cause influx of cold, dry air into the cave. This cold, dry air could either cause crickets to move deeper into the cave away from our census area or could dry out guano deposits and make them unusable by bacterial and fungal decomposers that are the food for the species of the guano community. However, winter mean minimum temperatures above average are above freezing and so could be favorable for cricket foraging. Our rationale for using mean summer maximums was that hot conditions due to mean maximums above average would interact with low rainfall to exacerbate droughts and that mean maximums would also be a good index of when night-time temperatures are too warm for cricket foraging. However, summer mean maximums below average could be favorable for cricket foraging.
Weather and cricket foraging. - Departures from mean minimum winter temperature were the most cyclic, departures from mean maximum summer temperature were intermediate and departures from yearly average precipitation were least cyclic [ILLUSTRATION FOR FIGURE 2 OMITTED]. From 1964 to 1971 precipitation was below average (unfavorable), winters were colder than average (unfavorable), and summers were cooler than average (favorable). From 1972 through 1976 precipitation was greater, winters were warmer and summers were cooler than average (all favorable). After 1977 conditions conducive to cricket foraging gradually deteriorated with some very cold winters, then hot summers and then a drought from 1985 through 1988. Since 1989 summers have been cooler and winters warmer than average (favorable) but precipitation has remained well below average (unfavorable).
Favorability indices quantify the weather patterns shown in Figure 2 with favorable times in black. Summing the departures from average annual precipitation, average winter (Jan.-Mar.) mean minimum temperatures, and average summer (June-Aug.) mean maximum temperatures for consecutive 5-yr periods gives net favorability (+) or unfavorability (-) indices. They are +1 for 1964 through 1968, -6 for 1969-1973, +33 for 1974-1978 (+112 for 1972-1976), -17 for 1979-1983, -40 for 1984-1988, and +54 for 1989-1993. The period from 1972 to 1976 was most favorable and the period from 1984 to 1988 was least favorable.
White Cave guano community and cricket populations. - From 1971-1994 in White Cave the number of cricket guano species and their abundances increased, then decreased and then slightly recovered [ILLUSTRATION FOR FIGURE 3 OMITTED]. Among the detritivore species the springtail Hypogastrura sp. fluctuated most dramatically and the millipede Antriadesmus fragilis only slightly less. In addition to these species the fungus gnat larva predator (Macrocera sp.), a predator of small species, and the orb-weaving spider (Meta menardi), a predator of crickets, both declined to zero in 1985 and 1989. As the cricket guano community declined, only the snail Carychium stygium, the millipede Scoterpes copei and the bristletail Litocampa cookei persisted through the late 1980s. The guano community started to recover in 1992 even though the number of crickets roosting over the guano areas censused had only increased from seven in June 1989 to 13 in June 1992. By 1994 there were 49 crickets over the guano census plot in June but the guano community still had not recovered much more than in 1992.
Little Beauty Cave guano community and cricket populations. - Compared to White Cave, we have fewer years of data on the guano community in Little Beauty Cave, but the general pattern of increase, decline and partial recovery was the same in both caves. In our small permanent quadrats the numbers of the springtail Hypogastrura peaked at 180-200 in the early 1970s, fell to 4-7 in the mid- to late 1980s and had returned to 184 in 1994. The numbers of the snail Carychium peaked at 34-44 in the early to mid-1970s, fell to 11 in the mid-1980s and had remained at 10 in 1994. For the 3 yr that we have virtually complete censuses in June, the cave cricket population was 1114 in 1973, 461 in 1987 and 562 in 1994. Over the same period the ratio of two fungivorous mites, Ceratozetes sp. to Belba sp., changed from 5/6 to 0/8 to 0/20 and back to 7/13. During this same time period the total numbers on guano of the millipede Scoterpes and the bristletail Litocampa remained at 1-2 individuals per census. We do not know whether they increased markedly in abundance as they did in White Cave in 1974, 1978 and 1981 because we have no data from Little Beauty Cave for those years.
Species biology and patterns of fluctuation in abundance. - Over our 24 yr of data collection the patterns of species abundance show a general relationship with species' generation times, mobilities and food niche breadths (Table 1). Among detritivores, the most abundant species tended to have the shortest generation time. This pattern is clear for the springtail Hypogastrura sp., the millipede Antriadesmus fragilis and the beetle Ptomaphagus hirtus but the snail Carychium stygius does not fit the pattern. During the late 1980s, species with the [TABULAR DATA FOR TABLE 1 OMITTED] fewest alternative foods had the lowest abundances. Among detritivores this pattern was also associated with low mobility as with the snails Carychium stygius and Mesodon sp., and the millipede Antriadesmus fragilis. The high mobility of adults of the predatory fungus gnat Macrocera sp. and of the orb-weaving spider Meta menardi did not compensate for their complete dependence on crickets; only M. menardi began to recover in 1992 and 1994. The springtail Hypogastrura sp. and the beetle Ptomaphagus hirtus had the most alternative food types and the shortest generation times and showed the most complete recovery by 1992 and 1994.
The mid-1980s crash of species in the cricket guano community was greater in White Cave than in Little Beauty Cave and the recovery in White Cave has been generally less (Table 1). This pattern was clear for the springtail Hypogastrura sp. and the beetle Ptomaphagus hirtus but, surprisingly, the snail Carychium stygius has recovered more in White Cave even though it dropped to a lower abundance in White than in Little Beauty Cave.
We first consider each of the four hypotheses that we outlined in the Introduction to explain the increase, decline and partial recovery of the cricket guano community in White Cave and Little Beauty Cave ([ILLUSTRATION FOR FIGURE 3 OMITTED] and Table 1). Second, we consider mechanisms by which unfavorable weather might affect cricket foraging and survival.
Effects of weather on the guano community. - We falsify the first hypothesis that people disturbed the crickets or trampled the guano community. Starting in the mid-1970s, the National Park Service ran a daily "Trog" tour with 10 children into White Cave during the summer. This could have compacted the guano or caused the cave crickets to shift their roost sites and so slowed or stopped deposition of guano on the census areas. Disturbance of the crickets seems unlikely because in the Frozen Niagara entrance to Mammoth Cave they remain at high densities next to and over the trail along which 4-6 tours of 100 people each pass every day of the year. Trampling and compaction of the guano by the Trog tour also seems unlikely because most of our census area is on angled slopes adjacent to flow-stone curtains and is accessible only by crawling over sharply sculptured rimstone dams. In addition, during our study Little Beauty Cave had virtually no traffic: only we visited the cave. Yet during this period Little Beauty had a rise, fall and partial recovery of the guano community. Thus the concurrent rise, fall and partial recovery of the guano community in both Little Beauty and White caves strongly implicates weather as a causative factor.
The data are not consistent with our second hypothesis that unfavorable weather directly affected the usability of guano by decomposers and detritivores but did not affect cricket numbers, roosting locations or guano deposition rates. Direct effects of weather on guano moisture could occur either throughout the cave at all seasons or near entrances in winter.
Year-round effects on moisture could result from major fluctuations in annual precipitation that might have immediate effects on small caves, like White and Little Beauty caves, that are only a few meters below the surface. Cricket guano can be too wet or too dry to support guano community species (Poulson, 1992). Very high precipitation will result in more water dripping onto and flowing over guano-covered formations that were previously of suitable moisture content. Dripping and flowing water leaches more nutrients from the guano (Poulson, 1992), which is already very low in caloric content due to the very high assimilation efficiency of crickets (Poulson and Kane, 1981; Studier et al., 1986). At the other extreme of direct weather effects, drought could result in guano that is too dry to be decomposed by bacteria and/or fungi which are the most important food of guano detritivores. During droughts a combination of high plant evapotranspiration with little renewal of soil water could decrease the amount of water that enters the cave, thus increasing saturation deficits and causing normally damp guano to become too dry. This seems unlikely because, during the late 1980s drought the most desiccation-intolerant troglobites, the millipede Scoterpes copei and the bristletail Litocampa cookei, were still present throughout both caves.
The other possible direct effect on guano usability is that winter inflow of cold-dry air could exacerbate drying of guano near the entrance. This could potentially explain the slower recovery of the guano community in White Cave than in Little Beauty Cave because in White Cave the guano areas censused are closer to the entrance (see Study Site description). This explanation seems unlikely for two reasons. First, the guano community crashed in Little Beauty Cave just as it did in White Cave. Second, the guano community crashed in the most protected area of White Cave in the 1980s where microclimate and guano remained damp behind a nearly solid flowstone curtain even when the more exposed guano outside the curtain seemed to be dry. The most protected area also has rimstone dams which retain water longer in summer and so help to ameliorate the microclimate and keep the guano damp. Also, in the early 1990s precipitation was still below normal but in 1994 guano community species were most abundant on the most exposed guano. In June 1994 more crickets were roosting over the most exposed guano deposits (139) than over the most protected guano deposits (49). If these differences reflect a year-long trend in cricket roosting densities and guano deposition rates, then this would be sufficient to explain the greater recovery of the guano community in the less protected area of White Cave. These differences also suggest that rate of guano deposition is the most important variable affecting the guano community.
In reference to our third and fourth hypotheses, we have evidence for a dramatic decrease and partial recovery of guano deposition rate during our study. In White Cave the obligate predator of crickets, Meta menardi, declined to zero in the 1980s and was recovering in the early 1990s. This is strong evidence that cricket numbers declined and then started to recover in White Cave. In Little Beauty Cave the change in relative abundance of two fungivorous mites (see Results) is also consistent with a decline and then partial recovery of guano deposition rates during our study. However, the mechanisms by which the rate of guano deposition changed are different for our third and fourth hypothesis.
We tentatively reject the third hypothesis that weather caused crickets to change their roost sites and thus caused a decrease in guano deposition on our census plots. Direct effects on roosting sites seem unlikely because distinct local patchiness of guano shows that crickets have used the same roost sites for a long time. Stronger evidence is that moisture conditions of the ceiling roosts over our study area remained favorable. Based on condensed moisture and dripping water, the ceiling roosting areas over both the most protected guano areas of White Cave and all the census areas in Little Beauty Cave remained near 100% relative humidity even during the late 1980s drought. Still stronger evidence against changes in roosting sites is that we searched other areas of both caves for members of the guano community and in White Cave for the obligate cricket predator Meta menardi but without success when numbers on our census plots declined dramatically in the late 1980s.
We cannot reject our fourth hypothesis that weather indirectly caused changes in overall guano deposition by its differential effects on frequency of cricket foraging and on cricket survival in summer and/or winter. The simultaneous rise and fall in abundance of guano community species in our census areas of White Cave and Little Beauty Cave and our inability to find these species deeper in the caves was clearly related to weather from 1971 to 1994. Both cricket survival and frequency of cricket foraging outside the cave will be expressed in differential rates of guano deposition under cricket roosts. Comparison of Figures 2 and 3 shows that the peak in species diversity and in abundance of each species coincided with a period in the mid-1970s of unusually favorable weather for cricket foraging and the crash in the mid- to late 1980s coincided with a period of unusually unfavorable conditions. The species least affected, Scoterpes copei and Litocampa cookei, have the lowest growth and metabolic rates of any species in the guano community (Poulson, 1992) and the broadest feeding niches (Table 1) and so persisted during low rates of guano deposition that were indirectly associated with the late 1980s drought. The slow and incomplete recovery of the more specialized species since the late 1980s suggests that the continuation of below average precipitation inhibited cricket foraging and/or decreased cricket survival more than the warmer than average winters and cooler than average summers enhanced cricket foraging frequency and/or survival.
Mechanisms of reduction in cricket foraging and survival. - Data are extensive on limits to cricket survival and foraging caused by dry and hot conditions. Studier et al. (1986) and Studier and Lavoie (1990) provided data on metabolic rate and evaporative water loss for temperatures from 9.5 to 24.5 C. The [Q.sub.10]s estimated from their metabolic rate vs. temperature curve suggest a slow increase in sensitivity to temperature with [Q.sub.10]s of 1.2 from 9.5 to 15 C, 2.5 from 15 to 20 C and 3.0 from 20 to 25 C. Lethal temperature is between 25 and 30 C. As Studier and Lavoie suggested, foraging outside of the cave at temperatures above 23 C is probably precluded mostly by the stress of desiccation because evaporative loss increases faster than metabolic rate with increased temperature. During hot summer weather, with day-time highs of 32 to 37 C, night-time temperatures often remain above 23 C and this will be stressful if saturation deficits are high. A week of observations by Leja and Poulson (1984) at Great Onyx Cave and our own observations at the Frozen Niagara entrance of Mammoth Cave show that crickets come out during hot summer weather only after day-time rains. Hot and dry days could also dry potential food so much that it is not odoriferous enough or soft enough to be found or eaten by the crickets.
We can only argue for winter limitation on cricket foraging and survival from first principles and a few natural history observations. Winter temperatures much below freezing will kill crickets that attempt to forage outside. Temperatures just below 0 C will freeze the soft food needed by the crickets and greatly reduce the volatility of odor-generating compounds and so make it hard for crickets to locate suitable food. Snow cover will certainly preclude access to most food even if temperatures are above freezing. Actually, even winter temperatures below 10 C may be a problem because our few observations and those of others (J. Meiman, pers. comm.) are that crickets come out to forage en masse after several weeks of cold weather only when temperatures rise to around 15 C and humidity is high. However, in February 1995 a few crickets came out to forage on high caloric content baits even when the temperature was at freezing after it had been much colder for the previous week or so. We infer from this that the crickets were particularly hungry and seasonal differences in removal of rat fecal baits by cave crickets is consistent with this inference. Year-round studies in several entrances to Mammoth Cave (Lavoie, 1982; Richards, 1992) and our ongoing studies in the Frozen Niagara entrance show that crickets remove piles of 100-125 rat fecal pellets in less than a week in winter but do not remove any fecal pellets in spring, summer, or early autumn. We infer that crickets were most hungry in winter, because cold and/or snowy conditions precluded foraging outside, and so fed on a food that is not preferred when foraging outside allows a wider choice of foods. Finally, it is clear that prolonged cold and or snowy weather will eventually cause mortality by precluding successful foraging. It should first affect smaller crickets which have smaller crops and so must feed outside more frequently than adults. Since crickets live at least several years (Cyr et al., 1991), mortality of young that deposit relatively little feces per individual will, after a lag of several years, result in a smaller cohort of adults and thus a great reduction in rates of guano deposition.
PERSPECTIVE AND FUTURE RESEARCH
Twenty-five years of data on the cave cricket guano community in two small caves in Mammoth Cave National Park has given us insights that we would have missed completely even with a 5-yr study. With a 5-yr study in the mid-1970s we would have missed the effects of adverse weather on cricket foraging frequency and mortality. With a 5-yr study in the late 1980s we might never have concluded that a cricket guano community existed. Finally, our study included periods that had the most and the least favorable weather possible for cricket foraging and survival and this allowed us to see differences in responses of the short-lived specialist and long-lived generalists species in the guano community to fluctuations in food supply.
A codified long-term monitoring plan would have given us more complete data but it could never have substituted for the insights generated by our long-term studies. We would not have been able to interpret the data without Poulson's 25 yr of study of the ecology and natural history of the cricket guano community species or without Studier and Lavoie's 15 yr of study of the physiological ecology of the cave cricket. We argue that a codified long-term monitoring plan is necessary but not sufficient to gain the advantages of the interplay between monitoring, research and management.
Acknowledgments. - We could not have done this research without the long-term protection of the resources provided by Mammoth Cave National Park, without the continuity of access provided early on by the negotiations of The Cave Research Foundation with local and National Park Service management, and without the increased sensitivity of the Park staff to the importance of the interplay among interpretation, research and management. The time needed to write the first drafts of this paper was provided by the Division of Science and Resource Management during Poulson's 3rd summer of tenure as a Consulting Ecologist at Mammoth Cave. Too many colleagues have helped us over the years to cite them all but we thank them collectively. Tom Kane shared the monthly burden of several years of intensive censusing in the 1970s that gave us baseline data and Gene Studier was central to the studies of cricket physiological ecology that allowed us to interpret effects of weather on cricket foraging. Finally, Liz Poulson helped with her considerable editorial skills.
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STUDIER, E. H. AND K. H. LAVOIE. 1990. Biology of cave crickets, Hadenoecus subterraneus, and camel crickets, Ceuthophilus stygius [Insecta: Orthoptera]: metabolism and water economies related to size and temperature. Comp. Biochem. Physiol., 95A:157-161.
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|Author:||Poulson, Thomas L.; Lavoie, Kathleen H.; Helf, Kurt|
|Publication:||The American Midland Naturalist|
|Date:||Oct 1, 1995|
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