Environmental correlates of darkling beetle population size (Col. Tenebrionidae) on the Can ˜adas of Teide in Tenerife (Canary Islands) A. de los Santos, E.J. Alonso, E. Herna ´ndez & A.M. Pe ´rez Department of Ecology, Faculty of Biology, University of La Laguna, C/Astrofı´sico Fco. Sa´nchez, s/n. 38206, La Laguna, Sta. Cruz de Tenerife, Canary Islands, Spain (Received 15 January 2001, accepted 9 August 2001) The seasonal and yearly patterns in the adult population sizes of the darkling beetle Pimelia radula ascendens (Col. Tenebrionidae) were analysed in relation to climatic and biotic variables in a montane arid ecosystem of Tenerife. Pitfall trapping was conducted from 1988 to 2000, and capture–recapture technique and the Jolly–Seber method were used to estimate population sizes. The population had a long activity period, with seasonal patterns of activity from April to October. Activity densities and population sizes showed a similar seasonal pattern each year, reaching values of more than 13,000 individuals ha 1 during the summer and decreasing to 2000 per hectare before diapause. Minimum temperature is positively correlated to population size but not with activity density. During summer, darkling beetles showed a greater increase in population size than in activity density due to lower capture and recapture rates. Pimelia radula ascendens was the most abundant darkling beetle and their population size remained relatively stable over the study period, likely due to the absence of predation and competition, and to the environmental stability. # 2002 Elsevier Science Ltd. Keywords: montane environment; population stability; Insecta; overabun- dance Introduction Darkling beetles are a key element in arid and semi-arid environments because of their large population sizes and because of their roles in processing organic matter (Calkins & Kirk, 1973; Doyen & Tschinkel, 1974; Thomas & Sleeper, 1977; Slobodchikoff, 1978; Allsopp, 1980; Mordkovich & Afanas’ev, 1980; Thomas, 1983). Numerous studies have revealed the wide array of adaptive strategies of darkling beetles to arid environments (Knor, 1975; Allsopp, 1980; de los Santos et al., 1988). Darkling beetle communities may be strongly influenced by species tolerance limits in hot-dry climates (Cloudsley-Thompson & Crawford, 1970; Seely & Mitchell, 1987; Seely et al., 1988; Parmenter et al., 1989; Ward, 1991). Species must be specialized to 0140-1963/02/020287 + 22 $35.00/0 # 2002 Elsevier Science Ltd. Journal of Arid Environments (2002) 50: 287–308 doi:10.1006/jare.2001.0911, available online at http://www.idealibrary.com on
Transcript
Journal of Arid Environments (2002) 50: 287–308doi:10.1006/jare.2001.0911, available online at http://www.idealibrary.com on
Environmental correlates of darkling beetle populationsize (Col. Tenebrionidae) on the Canadas of Teide in
Tenerife (Canary Islands)
A. de los Santos, E.J. Alonso, E. Hernandez & A.M. Perez
Department of Ecology, Faculty of Biology, University of La Laguna,C/Astrofısico Fco. Sanchez, s/n. 38206, La Laguna, Sta. Cruz de Tenerife,
Canary Islands, Spain
(Received 15 January 2001, accepted 9 August 2001)
The seasonal and yearly patterns in the adult population sizes of the darklingbeetle Pimelia radula ascendens (Col. Tenebrionidae) were analysed in relationto climatic and biotic variables in a montane arid ecosystem of Tenerife.Pitfall trapping was conducted from 1988 to 2000, and capture–recapturetechnique and the Jolly–Seber method were used to estimate population sizes.The population had a long activity period, with seasonal patterns of activityfrom April to October. Activity densities and population sizes showed asimilar seasonal pattern each year, reaching values of more than 13,000individuals ha�1 during the summer and decreasing to 2000 per hectarebefore diapause. Minimum temperature is positively correlated to populationsize but not with activity density. During summer, darkling beetles showed agreater increase in population size than in activity density due to lowercapture and recapture rates. Pimelia radula ascendens was the most abundantdarkling beetle and their population size remained relatively stable over thestudy period, likely due to the absence of predation and competition, and tothe environmental stability.
# 2002 Elsevier Science Ltd.
Keywords: montane environment; population stability; Insecta; overabun-dance
Introduction
Darkling beetles are a key element in arid and semi-arid environments because of theirlarge population sizes and because of their roles in processing organic matter (Calkins& Kirk, 1973; Doyen & Tschinkel, 1974; Thomas & Sleeper, 1977; Slobodchikoff,1978; Allsopp, 1980; Mordkovich & Afanas’ev, 1980; Thomas, 1983). Numerousstudies have revealed the wide array of adaptive strategies of darkling beetles to aridenvironments (Knor, 1975; Allsopp, 1980; de los Santos et al., 1988). Darkling beetlecommunities may be strongly influenced by species tolerance limits in hot-dryclimates (Cloudsley-Thompson & Crawford, 1970; Seely & Mitchell, 1987; Seelyet al., 1988; Parmenter et al., 1989; Ward, 1991). Species must be specialized to
support the drought, to avoid desiccation (Cloudsley-Thompson, 1964; Smith &Whitford, 1976; Marino, 1986; Gehrken & S�mme, 1994), and to resist climaticunpredictability (Louw & Hamilton, 1972; Holm & Edney, 1973; Cloudsley-Thompson, 1975; Henwood, 1975a).
Few genera of darkling beetles have colonized arid environments, and evolved toexploit the resources efficiently in the low primary productivity of these ecosystems.This high degree of specialization, appears to be related to a low generic diversity (delos Santos, 1994). The colonization of arid environments by darkling beetles isassociated with important modifications in thermoregulation, transpiration, excretionand water absorption (Cloudsley-Thompson, 1975; El Rayah, 1970a, b; Louw &Hamilton, 1972; Henwood, 1975a, b; Crovetti et al., 1979; Seely, 1979; Slobodchik-off, 1983). Moreover, they have developed a variety of life cycles (Brun, 1970; Knor,1975; de los Santos et al., 1988) and circadian rhythms (Abushama, 1984), which hasresulted in a high degree of specialization in life-forms. Most studies emphasize thestructural modifications of body design as adaptations to the microenvironmentalmosaic of the desert lands they inhabit (Cloudsley-Thompson, 1962, Koch, 1962,1984; Medvedev, 1965; Coineau et al., 1982; Broza et al., 1983; de los Santos et al.,2000), but life-history characteristics are also important.
The life-history features of darkling beetle populations in arid ecosystems led to asuggestion that they are k-selected (sensu Pianka, 1970) relative to most insects(Gebien, 1939; Watt, 1974; Wharton, 1983; Hanrahan & Seely, 1990). Densities ofmany populations are relatively stable which suggest that darkling beetle populationsmay often be found near the carrying capacity of their environments (Doyen &Tschinkel, 1974; Calkins & Kirk, 1975; Thomas & Sleeper, 1977; Wise, 1981b). Theyare long-lived in both larval and adult stages (Knor, 1975; de los Santos et al., 1988)and are highly iteroparous, laying a single egg almost every day (Seely, 1983).
Other studies have explored their interspecies relationships within communities(Ahearn, 1970; Rickard, 1970; Keleinikova, 1971; Kuznetzov, 1971; Holm & Edney,1973; Doyen & Tschinkel, 1974; Rogers & Rickard, 1975; Slobodchikoff, 1978;Thomas, 1979, 1983; Wise, 1981a). Several workers (Seely, 1985; Polis et al., 1998;Groner & Ayal, 2001) suggest that predation can influence beetle population sizes.Seely (1985) further argues that the differential predation (males suffer increasedpredation pressure) is a major cost of reproduction and can constrain sexual selection.Predator avoidance may also influence microhabitat selection of darkling beetles(Parmenter & MacMahon, 1988). Ward & Seely (1996) showed that interspecificcompetition could affect habitat selection in two darkling beetles, and differences inforaging efficiency were a likely mechanism of co-existence that explains the distinctpreferences of these two species for tree and open habitats. The lack of evidence forinterspecific competition in insects may be a result of the greater number of studiesperformed on herbivorous and detritivorous communities (Crawford, 1991). If thereare density-dependent effects on the community structure of these generalistdetritivores, difference in foraging efficiency may be important in separating speciesniches (Crawford, 1988). Although there is considerable evidence for spatial andtemporal resource partitioning in desert darkling beetles (Whicker & Tracy, 1987;Aldryhim et al., 1992), there is no direct evidence that this is due to interspecificcompetition.
In the semi-arid zones of the Mediterranean Region, where there is a high primaryproductivity and darkling beetle diversity (de los Santos, 1994), darkling beetles with avariety of life-history features are observed (de los Santos et al., 1988). The highdegree of life history and species diversity are likely because of frequent perturbationssuch as inter-annual droughts that strongly influence community structure (de losSantos et al., 1988; de los Santos, 1992).
In the present study, we explored the relationship between temperature andpopulation size of the darkling beetle Pimelia radula ascendens Woll. in the arid
ENVIRONMENTAL CORRELATES OF DARKLING BEETLE POPULATION SIZE 289
environment of the Canadas of Teide, located at 2225 m a.s.l. in the upper level of theTenerife Island (an isolated island ecosystem). The environment may be thought of asa mosaic of microhabitat types, each fluctuating with changes in the daily or seasonalcycles of temperature and precipitation. Darkling beetles appear to wander freely, andthe movement of individuals into or out of a given area may greatly influencepopulation size and density.
We analysed the darkling beetle population using capture–recapture in pitfall trapsand obtained Jolly–Seber ( Jolly, 1965; Seber, 1965) estimates of the population sizesduring three sampling years. Pitfall catches of darkling beetle populations are widelyused to characterize beetle distribution and abundance in several arid and semi-aridecosystems, and to document the influence of ecological factors such as temperature,habitat structure, and predation (Rickard, 1970; Hinds & Rickard, 1973; Thomas,1979, 1983; de los Santos et al., 1988). Thus, darkling beetle population sizes areknown for several ecosystems (Rickard & Haverfield, 1965; Brun, 1975; Rogers &Rickard, 1975; Wise, 1981a, b; Wharton, 1983; Crist & Wiens, 1995; Fallaci et al.,1997) and have even been used to evaluate various estimators of population size(Thomas & Sleeper, 1977).
Materials and methods
Study site
Field work was undertaken in the summit scrubs of Canadas Blancas on the TeideNational Park in the Island of Tenerife at 2225 m a.s.l. (2811303200N–1613704100W).The vegetation is dominated by broom Spartocytisus supranubius (L. fil.) Webb etBerth., laburnum Adenocarpus viscosus (Willd.) Webb et Berth., and an annualmustard Descurainia bourgaeana (Fourn.) O.E. Schulz.
The climate of the area under study ((Fig. 1(a)) is dry temperate (Fernandopulle,1976) with severe winters. Mild fluctuations occurred in mean annual precipitationand mean annual temperature during the study period (Fig. 1(c)), following a normalpattern of variation for this area (Fig. 1(a)). The temperature varies little from year toyear, with the coefficient of variation (CV) of the mean annual temperature (11?11C)only 8% (n ¼ 16). The mean monthly temperatures of the coldest months (Januaryand February) and hottest months ( July and August) are 5?4–6?61C and 18?7–19?11C, respectively. The average minima and maxima of the coldest and hottestmonth are �0?41C ( January) and 26?41C ( July), respectively. Annual average rainfallis approximately 347 mm, with some substantial yearly variation (CV ¼ 58%; range70–656 mm) (Fig. 1(b)). About 98% of the annual rainfall is registered from Octoberto March and extreme drought occurs from April to September. The CV of the meanmonthly rainfall is 218% in April, 0% May–July, 234% in August (spring–summer),and 157% in autumn–winter.
Sampling methods
Pitfall trapping
Beetle populations were sampled with pitfall traps during three annual periods(1988, 1989, 2000). A grid of 25 traps was used in the first 2 years of sampling(May–December 1988 and April–July 1989). The 5� 5 grid had a 10-mtrap spacing (50� 50 m or 0?25 ha). In the third year (2000) we intensified the
Figure 1. Climatic characteristics of the study site: (a) Walter–Lieth climate-diagram, (b)mean monthly coefficient of variation of rainfall (%). and (c) relation between rainfall andtemperature during the sampling years.
290 A. DE LOS SANTOS ET AL.
sampling, selecting a more narrow sampling period ( July–August–September) thatcorresponded to the seasonal peak in the population sizes during the previousyears. We also increased the sampling size to 50 pitfall traps arranged in a50� 100-m grid. Pitfall traps were designed to increase capture efficiency by usingfunnels and trap covers (de los Santos et al., 1982a; Quintana et al., 1985; seealso Greenslade, 1964; Gist & Crossley, 1973; Luff, 1975; Thomas & Sleeper,1977; Adis, 1979; Jensen & Metz, 1979; Price & Shepard, 1980; Drach et al.,1981). Traps were processed weekly. We found that weekly processing was suffi-cient because these detritivorous beetles did not destroy or eat each otherduring trapping interval. We also assert that desiccation or food deprivationdid not influence beetles because adults lived for several months without foodand without drink in the laboratory. Finally, beetles could not crawl out ofthe trap because of the trap design and were protected from sun exposure by thetrap cover.
Captured beetles were marked by trap occasion (1988 and 1989) and individually(2000) and then released. A total of 24 marks were made on the interstrial spaces ofelytra. The marks were date-specific. Nitrocellulose paints are perhaps the mostextensively used marking materials (Evans & Gyrisco, 1960; Mitchell, 1963;Greenslade, 1964). We applied the paints in the most concentrated form by the use
ENVIRONMENTAL CORRELATES OF DARKLING BEETLE POPULATION SIZE 291
of an entomological pin (Southwood, 1978). After marking beetles were releasedwithin 2-m of each trap.
Population estimates were expressed in terms of activity density (Tretzel, 1955;Heydemann, 1957). For each week, the maximum catch constituted the sample andeach trap formed a sample unit. During the sampling period, temperatures and rainfallwere recorded using a maximum–minimum thermometer and pluviometer placed onthe soil surface.
Statistical analysis
Pitfall traps method
Population estimates were expressed in terms of activity density (Tretzel, 1955;Heydemann, 1957), which reflects the number of beetles that were more through thestudy area.
We defined:
Capture–recapture method
The following notation follows that of Seber (1982) and Jolly (1982):
Pi ¼ ni � Mi=mi
where
Mi ¼ ðri � Zi=RiÞ þ mi
We are using the Jolly–Seber method because it has been widely used for the studyof open population and it is an alternative probability model for deterministicformulations. This method, although complex, allows for survival and catchability to
N number of darkling beetle individuals in pitfall trapsn number of pitfall trapst frequency of capture (days)AD Activity density (number of animals per trap and day � 10; AD=(N� 10)/
(n� t). In this study n and t are constants, then AD=N� 10 (number of animalsper 25 traps and 7 days).
Mi is the number of marked animals in the population at the time the ith sampleis taken (i ¼ 1, ..., K; M1=0)
Pi is the total number of animals in the population at the time the ith sample istaken (i ¼ 1, ..., K )
ni is the total number of animals captured in the ith sample (i ¼ 1, ..., Kri is the number of the ni which are released after the ith sample (i ¼ 1, ..., K�1)Ri is the total of recaptured animal marked on day i and recaptured subsequently
(i ¼ 1, ..., K�1)mi is the number of marked animals in the ith sample (i ¼ 2, ... ,K )Zi is the total number of animals captured on a day previous to i, not captured at
day i, and captured again later (i ¼ 2, ..., K�1)ESPi Standard error as the square root of the variance for population estimates at
day i.
292 A. DE LOS SANTOS ET AL.
depend on the mark status of the animal, and represent a major step forward in thefield of capture–recapture analysis (Seber, 1982). Unfortunately, the notation isdifficult and the method requires large numbers of recaptures (our case) so that thesizes of the various capture classes are not too small. We would favor the Fisher andFord estimates (Fisher & Ford, 1947) in the case that true survival rate did notfluctuate greatly from day to day. Survival rate may fluctuate, however, in which casethe Jolly estimates are to be preferred. It is instructive to compare the differentestimators when using multiple catch occasions, but the Jolly–Seber estimator may bepreferable in practice (Blower et al., 1981).
Results
Capture–recapture data for males and females on each sampling occasion aretabulated in Table 1 and rearrangement data in Table 2. Hence, the first number ofeach column is ni, and the subsequent numbers are recaptures of each sampling day.The sum of all entries in columns gives sums of successive recaptures of animalsreleased on a given day (Ri). By summing the rows from the left, Zi animals areaccumulated in the columns and the mi animals are summed in the hypotenuse cells.During spring, the recapture rate is recorded during a 6-week period; however, duringsummer recaptures were recorded during a 3-week period. In subsequent weeks, therecaptures of darkling beetles in pitfall traps were rare. Table 3 summarizes thecapture–recapture analysis of population parameters according to Jolly’s model. Just afew dead individuals were found in pitfall traps during the three sampling cycles.These are individuals of post-reproductive age and they die of old age toward thesecond half of summer, principally out of the trap. Few individuals are active in thisage group. Hence, the low numbers of dead individuals were recorded in relation toconsidering the total number of animals captured in the ith sample (ni) which arereleased after the ith sample (ri).
During 1988, all the activity period was sampled but during the 1989 and 2000cycles only the first and second half, respectively, of the activity period were sampledbecause these are two different periods of the life cycles and, hence, the populationestimates can be described and related.
The maximum estimates recorded for both the population size and activity densityof P. costata consistently occurred during the same season of cycle (Fig. 2). Asexpected for this species, there was a long adult activity period from April to October(de los Santos et al., 1988). Changes in the weekly capture success are assumed toresult from changes in activity and density, which were strongly correlated with thetemperature.
The emergence of adults was quite predictable. The first beetles were captured inearly April (average soil surface temperature 111C; range=�0?5–221C). Within a fewweeks, the population had reached a high population size (2000 beetles ha�1). DuringJuly and August (average surface temperature 191C; range=8?5–311C), beetlesshowed a maximum activity and population size (10,000–13,000 beetles ha�1). Thispeak in activity corresponds to our laboratory observations, which indicated that adultemergence and surface activity were greatest at this range of temperatures. By October(average temperature is 111C; range=�1–23?51C) the beetles were no longer active atthe soil surface and entered diapause. During the cold, humid months (October–March) captures of darkling beetles in pitfall traps were rare. In the following spring,we found marked individuals that had survived from a year before.
Population dynamics can be interpreted according to the characteristic pattern of thedarkling beetle life cycle described as 2-year life cycle, with larval and adultoverwintering; reproduction occurs from May until the end of hot season and pupationin spring and early summer in the following year (Knor, 1975; de los Santos et al., 1988).
Table 1. Application of Jolly’s stochastic method to a population of darkling beetle Pimelia radula ascendens: values of captures and recaptures.Jolly’s modification of trellis type II lists recaptures of animals according to their most recent mark
25-07-00 80 4 F F F F01-08-00 75 12 3 19?8 1501?0 1682?508-08-00 163 21 9 75?9 2338?9 1235?715-08-00 117 4 18 538?5 5250?4 3129?322-08-00 149 14 13 147?4 2456?0 1144?229-08-00 64 2 12 399?0 1755?6 1332?205-09-00 86 3 3 97?0 767?2 583?912-09-00 21 F F F F F
ENVIRONMENTAL CORRELATES OF DARKLING BEETLE POPULATION SIZE 297
Figure 2. Temporal distribution during three sample years of population size (square) andactivity density (circle) of Pimelia radula ascendens population on a per hectare basis in summitscrubs of the Teide National Park.
298 A. DE LOS SANTOS ET AL.
Adult eclosion occurs from the end of July to the beginning of August, and, probably,a high mortality occurs during winter, mainly in the older generation of adults. Hence,a low population size (2000 beetles ha�1) occurs at the beginning of activity period(spring) (Fig. 2).
Figure 3 shows the linear corrrelations between activity density, population size andtemperature. For this analysis, the sample unit was the grid, not the trap. A significantpositive correlation was found between activity density and population size (r ¼0?466; po0?01). The variability is analysed in relation to changes of the activityduring summer. Hence, a highly significant positive correlation was found betweenminimum temperature and population size (r ¼ 0?677; po0?01) and a smallersignificant positive correlation was found between minimum temperature and activitydensity (r ¼ 0?384; po0?05).
The standard errors at day i (Table 3) show higher values for the estimates ofsummer. Since the probability of occurrence of certain events related to life cycleaffects the estimates, the sampling error has more to do with sample size and numbersof recaptures. The estimates of the population sizes among years fluctuated betweencharacteristic intervals for spring and summer.
Figure 3. Linear correlation analyses when the relations between population size and activitydensity (r ¼ 0?466 df. ¼ 37 po0?01) (a), population size and minimum temperature(r ¼ 0?677 df. ¼ 37 po0?01) (b), and activity density and minimum temperature (r ¼ 0?384df. ¼ 46 po0?05) (c) were examined.
ENVIRONMENTAL CORRELATES OF DARKLING BEETLE POPULATION SIZE 299
Discussion
The pitfall estimates of darkling beetle populations: ecological and biological factors
Pitfall trapping is widely used for capturing arthropods found on the ground surface.Southwood (1978) summarizing some critical studies on the efficiency of pitfalltrapping, concluded that ‘pitfall traps are of little value for the direct estimation ofpopulations or for the comparison of communities’ largely because capture success ishighly influenced by activity changes in the animals at risk due to variables such asweather, substrate, behavior, etc. Yet this is still the primary means of assessing densityof ground-dwelling insects.
The vagility of the population seems to be important among the most critical factorsin the interpretation of the pitfall captures (Briggs, 1961). The behavior of theindividuals, particularly with respect to the vagility or home range movements wouldaffect the trapability of individual beetles. Hence, trap density and capture probabilitycan be related to behavioral studies of animal movement (Skalski & Robson, 1992).Rectangular trap arrangements have the highest capture probability when animals
300 A. DE LOS SANTOS ET AL.
have low net displacements during the activity period and the population at risk oftrapping is confined to the trap area (Crist & Wiens, 1995). These conditions occurwhen individuals are relatively sedentary and occupy territories or a well-defined homerange. When individuals are transient to the trap area and populations have an openspatial structure other trap arrangements may be more efficient. Crist and Wiensfocused on the Sierpinski gasket (Mandelbrot, 1983) in the field because the transecthas low probability of recapture and therefore does not permit the use of mark-recapture estimators. In cases of high population densities as found in our study;however, a large study area could increase the absolute number of marked individualsavailable for recaptures when a high number of captured animals are marked andreleased after the ith sample. This is equivalent to the increase in the samplingintensity (Blower et al., 1981).
The variation of population size of P. radula ascendens in the Canadas of Teide fromthe spring emergence (B2000 individuals ha�1) to the first half of summer (B13,000individuals ha�1) is related to the newly emerged adults. A high vagility during thesummer may occur because vagility is related to the environmental temperature(Nicolson et al., 1984; Whicker & Tracy, 1987; Parmenter et al., 1989). As found inprevious studies (Baars, 1979a, b; Ericson, 1979; Crist & Wiens, 1995), a positiverelationship between temperatures and population sizes was shown. However, themaximum values of populations size were registered at 7?5–101C and not during thehottest weeks. If there is process of cumulative emergence of individuals, either fromoverwintering or eclosion, then we expect population density to increase regardless oftemperature. If both cumulative emergence and temperature increase between Apriland August, then the correlation between density and temperature is circumstantialand not causal. A similar phenomenon occurs with the relationship of activity anddensity. Activity density is a function of both density and temperature. With the samedensities, there will be more activity at warmer than at cooler temperatures.Therefore, the relationships among emergence, activity, density, and temperatureshould be analysed by a multi-variate analysis.
Activity density is influenced by behavioral changes as well. Possibly, the patterns ofmovement are related to temperature conditions (Crist et al., 1992). In warmconditions, a lower proportion of the marked animals was recaptured in the studyarea. Indeed, differences in population size can arise from habitat-specific residencetimes of individuals (Turchin, 1991). Hence, the increase of darkling beetleabundance with high running speeds following transient displacements (directionalpatterns of movement) influenced more the population size than the activity densitysince a lower proportion of the population was caught, the captures were generally notmarked animals, and there were lower recapture rates. Hence, with lower activitydensity (around 2000) higher population size (around 10,000 individuals ha�1) wasreached. When individuals are relatively sedentary and occupy territories or homeranges, then activity density and population size are more highly correlated. In thepopulation of P. radula ascendens, we suggest that the recapture rate declined during aperiod of time due to change in the transient behavior of the beetles, resulting in thechance of recapture decreasing with time, hence an unequal risk, and the estimatedpopulation size became stabilized.
P. radula ascendens, a large, robust and ambulatory form, reveals adaptation inrelation to a epigeic life style in edaphic environments. Indeed, Medvedev (1965)observed that species appearing on hard substrates generally have relatively long legsspecialized for ambulation. In three tenebrionid beetles (tribe Adesmiini) the averagerunning speeds were 90 cm s�1 for Onymacris plana, 23 cm s�1 for Physadesmia globosaand 3 cm s�1 for Epiphysa arenicola (Nicolson et al., 1984) and it was observed that therelative speeds of the three tenebrionid are related to leg length. Roer (1975) foundthat O. plana, a long-legged, diurnal adesmiine tenebrionid was capable of movingseveral kilometers from their release sites; conversely, Wharton (1983) observed that
ENVIRONMENTAL CORRELATES OF DARKLING BEETLE POPULATION SIZE 301
Stips stali, a short-legged nocturnal eurychorine tenebrionid, had moved an average of51 m after 8–12 months.
The high running speeds following transient displacements seems to be associatedwith low values of rates of oxygen consumption. The calculated values of rates ofoxygen consumption (VO
2 ) in darkling beetles are substantially less than those for antsand cockroaches (Jensen & Holm-Jensen 1980; Herreid, 1981; Nielsen et al., 1982).These low values of VO
2 are consistent with the ability of some darkling beetles to runat extremely high velocities. As the data indicate (Bartholomew et al., 1985), VO
2 isvirtually independent of speed above 13 cm s�1. This low rate of energy expenditureshould be advantageous to the darkling beetles because they live in an environment ofextremely low production where the food supply is both unpredictable and patchy(Seely, 1978). The necessity of food and their greater vagility might increase contactsbetween males and females (Doyen & Tschinkel, 1974), particularly for under-dispersed species (de los Santos et al., 1982b). Also, the intermittent respiration of thetenebrionid and the long periods of apnea when they are at rest, together with the factthat the abdominal spiracles open into the subelytral cavity, minimize water loss in theextremely arid environment in which they live (Buck, 1958; Cloudsley-Thompson,1964).
Comparison with other darkling beetle populations bellowing several geographical areas
We compared our estimates of population size for P. radula ascendens with those ofdarkling beetles in other arid and semi-arid environments. After standardizing theestimates of population sizes with respect to hectare area (Table 4), we observedappreciable differences among populations at the same and different sites.
The values obtained in our study (2000–13,000 individuals ha�1) bracket on arange of estimates from other studies, although past research has generally notexamined the variation in population size during an annual activity period. OnlyGillon & Gillon (1973, 1974), Brun (1975) and Fallaci et al. (1997) obtainedestimates for some monthly periods. Thomas & Sleeper (1977), Wise (1981b) andFallaci et al. (1997) commented on the constancy of population size over periods ofseveral years. Calkins & Kirk (1975) found no statistically significant yearly variationover a 5-year period. However, not all darkling beetles display such stability (e.g.Opatrini, Asidini; Rogers & Rickard, 1975; Thomas, 1979). Dramatic irruptions havebeen documented for darkling beetles populations in Mediterranean ecosystems of theLower Guadalquivir (de los Santos, 1992). Crist & Wiens (1995) showed overalldensities of Eleodes obsoleta much greater during a model validation experiment inAugust 1992 than in pitfall data from August 1990. Fallaci et al. (1997) foundconstancy of population size of Erodius siculus between several years and significantvariation in Tentyria grossa and Pimelia bipunctata.
The high abundance of the population of Pimelia radula ascendens in Tenerife, is onlysurpassed by a density of two species that are autumn emergents in a desert steppehabitat of Washington (Rickard & Haverfield, 1965). A partial explanation for thelarge difference is that the average live weight of the two species is 0?2 g, that isapproximately 4 times less than the live weight of Pimelia radula ascendens (0?85 g, n =10). However, Rogers & Rickard (1975) showed that the abundance of both speciesdeclined drastically during the last decade.
The high darkling beetle abundance in the National Park of Canadas of Teide, anisolated arid ecosystem, is interpreted in relation to lack of predation and competitionbetween congeneric species, and the high plant biomass (detritus) in an environmentwhere the detritivores carry out an important functional role. The plant biomass noteaten by herbivores and the dry remains of senescent plants fall on the soil surface,producing an accumulation of detritus during dry seasons. Most decomposition
Table 4. Size estimates of some populations and communities of darkling beetles (individuals/ha) in several geographical areas
Community 1 164,000Community 2 142,000One species 103,000
North-east of Fort Collins CO, U.S.A. Perennial grass, shrubs and cactus Crist & Wiens (1995)One species 1990 Grass 464
Bare ground 656Mixed 592Mixed 1456Cactus–shrub 1904
One species 1992 Grass 3600Cactus–shrub 4176Bare ground 2480Mixed 6080
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occurs later during the moist season through the combined attack of fungi anddetritivorous arthropods. The estimates of biomass indicate that the darkling beetles,most importantly the larval stages, play a significant role in this cycle in the arid andsemi-arid ecosystems (Thomas, 1979).
We wish to thanks Ministerio de Medio Ambiente for giving us permission to conduct theresearch on the Teide National Park. Yolanda Estefanıa provided assistance in the field. Drs C.Montes and J.P. de Nicolas gave statistical advice. Thomas O. Crist provided helpful suggestionson an early draft of the manuscript and assisted in the last redaction. Elli Groner suggested someremarks. Two anonymous referees also provided several valuable suggestions on the paper, onereferee suggested the use of mortality in the pitfall traps for obtained population estimators.
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