Memoirs of the Museum of Victoria 56(2):33 1-337 (1997) 28 February 1997 https://doi.org/10.24199/j.mmv.1997.56.21

SPIDERS AS ECOLOGICAL INDICATORS : AN OVERVIEW FOR

Tracey B. Churchill

Arachnology, Queensland Museum, PO Box 3300. South , Qld 4101, Australia Centre, PMB 44 present address : Division of Wildlife and Ecology, CSIRO Tropical Ecosystems Research Winnellie, NT 0821, Australia

Abstract

Churchill, T.B., 1997. Spiders as ecological indicators: an overview for Australia. Memoirs of the Museum of Victoria 56(2): 331-337. Spiders operate as a dominant predator complex which can influence the structure of terrestrial invertebrate communities. The potential use of spiders as indicators of ecological to change, amongst a suite of selected taxa, is worthy of further research. Indicator taxa need be diverse and abundant, readily sampled, functionally significant, and to interact with their the environment in a way that can reflect aspects of ecological change. This paper examines proposes attributes of spiders in terms of these criteria, with an Australian perspective, and the use of families as functional groups to represent divergent foraging strategies and selec- "taxonomic tion of prey types. With such information gain, and reduced impact of the collation of impediment", the cost-benefit of surveys is enhanced to encourage the wider quantitative spider data for management or conservation purposes.

Introduction mining (Majer, 1983; Andersen, 1990, ms.). To gain a wider understanding of patterns of biod- iversity and ecological change in invertebrate have an important role to play in Invertebrates communities, however, a range of taxa need to achieving effective conservation and manage- be adopted (Beattieetal., 1993; Kitching, 1994; ment of biodiversity for three reasons: New, 1995; Noss, 1990). 1. they dominate fauna in terms of species rich- ness and abundance; potential of spiders 2. they are linked to critical ecological processes The and; In selecting a suite of taxa, arguments for choos- provide quantitative data from 3. they can ing those which are functionally important (Yen (Greenslade and Greens- small spatial scales and Butcher, 1992; New, 1994) are the most con- Butcher, 1992; Kitching. lade, 1984; Yen and vincing. Due to their ecological importance as New, 1995). 1993; Norton, 1994; dominant predators, spiders have been pro- to assess all invertebrate As it is impossible moted as one of several priority groups for pragmatic approach is to taxa, however, the research (Kitching, 1994; Yen, 1995). In terms which to focus research select major taxa on of their use as ecological indicators, spiders need 1994). efforts (New, to fulfil specified criteria, namely they must: In the case of using certain faunal groups to 1. be diverse and abundant; monitor environmental conditions, reflect and 2. be readily sampled; "indicator taxa" is frequently the term 3. be functionally significant and; and Greenslade, 1984; employed (Greenslade 4. interact with their abiotic and biotic environ- as here. In the Andersen, 1990; New, 1995), ment in a way that can reflect ecological or shifts indicator context, observed differences change (Greenslade and Greenslade, 1984; taxa can in the relative abundance of particular Andersen, 1990; Cranston, 1990; Beattie et general ecological be interpreted to reflect more al„ 1993; Yen, 1995). system. For invert- attributes or changes in a The attributes of spiders with respect to these primarily developed using ebrates, this has been criteria are reviewed below. aquatic or marine taxa to characterise water pol- quality or more specifically, the effect of Bunn, 1. Diversity and abundance lutants (e.g., reviews by Warwick, 1993; The order, Araneae, which comprises spiders, Fairweather et al., 1995). For Australian 1995; orders approaches is among the six or seven most speciose terrestrial invertebrates, parallel worldwide, with one hectare of tropical forest have been limited to the established use of ants restoration after estimated to contain 300-800 species (Codding- to evaluate processes of land

331 332 TRACEY B. CHURCHILL

ton et al., 1991). In Australia, a total of 1876 with observed changes in spider faunas having described species from 430 genera in 68 families the potential to reflect ecological impacts at has been tallied (Raven. 1988). With notable lower trophic levels, and across relatively small increases in taxonomic effort over the last eight spatial scales. years, the number of species described has risen by 26% to 2357 (R. Raven, pers. comm., Jan 3. Ease of sampling 1996). With only an estimated 30% (Davies, Due to their abundance and diverse behav- 1985) or 20% (Raven, 1988) of the fauna for- iours, spiders can be easily sampled by a range of mally described, these figures clearly demon- techniques (e.g.. Coy et al., 1993). Vagrant strate that Australia is rich in spider taxa. ground hunters are readily captured by the cost- However, the levels of richness are not unman- effective pitfall trap (Canard. 1 982; Merrett and ageable. In the north-east of Tasmania, a coastal Snazell. 1983; Churchill, 1993). Foliage dwelling heathland survey over 16 months revealed 130 taxa are more susceptible to capture bv sweep species over a maximum sampled area of 1 .2 ha net (Canard, 1982; Churchill, 1993), 'beating (Churchill. 1993). bushes (Canard, 1982; Hatley and MacMahon, Across Australia, spiders have ranged between 1980): branch clipping (Majer and Recher, the most, to the sixth most, abundant 1 invert- 988; Abbott et al., 1 992); chemical knockdown ebrate order from surveys in rainforest and (Majer and Recher, 1988; Yen and Lillywhite, Eucalyptus forest canopies using a 1 number of 990: Kitching et al., 1 993) or restricted canopy sampling methods (Majer and Recher. 1988; fogging (Basset, 1991). Spiders that are seden- Basset. 1991; Majer et al., 1990; Yen and Lillv- tary and cryptic, or conspicuous by their webs, white, 1990; Abbott etal., 1992; Coy et al., 1993; size or behaviours, are effectively sampled by Kitching et al., 1993: Kitching, 1994; Majer et visual searching and hand collection (Canard, al.. 1994). In a subtropical Queensland rainfor- 1 982; Coddington et al., 1 99 1 ; Churchill, 1 993). est tree canopy where spiders dominated the To target spiders in leaf litter, sifting and entire arthropod assemblage sampled, they were extraction techniques such as Berlese or responsible for 85% of total abundance and 65% Tullgren funnels can provide standardised and of the total biomass (Basset. 1991). quantitative samples (Canard 1979; Coyle. 1981: Coddington et al., 1991). 2. Functional significance As a predator complex, spiders are among the 4. Interaction with their abiotic and biotic most abundant and important invertebrate con- environment sumers across a range of natural and disturbed For any invertebrate taxon to be considered as habitats (Turnbull, 1973; Reichert, 1974; an indicator of ecological change, it needs to dis- Humphreys, 1988). Levels of predation upon play a sensitivity to changes in environmental the arthropod biomass of temperate forests have variables which are associated with stress and been estimated at 43.8% annual consumption disturbance (Andersen, 1990; Noss, 1990; New, (Moulder and Reichle, 1972). Spiders are often 1995). Research in the Northern Hemisphere classed as polyphagous (Reichert, 1974; Turner has revealed that habitat structure and/or associ- and Polis, 1979). yet, they include specialist ated microclimatic factors, which can be altered predators such as ant mimics and those that by many land use practices, strongly influence simulate pheromones or odours to attract cer- patterns of spider distribution (reviews by tain prey species (Stowe, 1986. Pollard et al., TurnbulJ, 1973, Uetz, 1991; Wise, 1993). Across 1987). Spiders also interact directly as competi- environmental and successional gradients the tors, mutualists. predators, and particularly as diversity and relative abundance of spider taxa prey, with higher order taxa such as birds has been shown to exhibit clear shifts (Uetz. (Gunnarsson, 1996), fish (Bleckmann and Lotz. 1976; Bultman et al., 1982; Klimes, 1987; 1987). and lizards (Schoener and Spiller. 1987) Gibson et al., 1992). The relative importance of Consequently, spider assemblages can play a different variables can change over time (Uetz. major role in ecosystem function by directly and 1979). however, with the availability of indirectly prev regulating the abundance of taxa resources another important factor (e.g., which determine rates of herbivory, pollination, Reichert, 1 974). In terms of specific responses to decomposition, soil production, nutrient cycling environmental disturbance, characteristic or energy flow (Riechert, 1974; Wise, 1993). The changes in spider faunas have been documented value of spiders as indicators relates, therefore, in Europe and America for the effects of metal to their being dominant invertebrate predators^ pollution (Bengtsson and Rundgren, 1984; .

SPIDERS AS ECOLOGICAL INDICATORS 333

Clausen, 1986), fire (Merrett, 1976), grazing spider faunas have been shown to respond faster (Gibson et al., 1992), pasture improvement to anthropogenic disturbance than vegetation

(Luff and Rushton, 1989) and clearcutting, (Klimes, 1 987), they have the potential to reveal burning, mowing and plowing (Huhta, 1971; early, and more subtle, ecological changes, Coyle, 1981; Haskins and Shaddy, 1986). which characterises the main value of an Clearly, the composition of spider communities indicator group (New, 1995). of different habitat types is affected by certain changes in environmental conditions, the chal- lenge is now to develop a predictive understand- Spider families as functional groups ing for management purposes. The information value of using certain indicator taxa is greatly enhanced if combined with a func- Australian research tional group approach. This approach has been In Australia, comparable research into spider advocated to increase an understanding of the communities has been minimal. In Western dominant processes which maintain biodiver-

Australia, Mawson (1986) studied the richness sity (Lambeck, 1 992; Walker, 1 992) and underly and diversity of the arachnid fauna in rehabili- environmental change (Andersen, 1990). The tated minesites and surrounding undisturbed fact that most spider families differ in their pri- eucalypt forest in Western Australia. The result mary foraging mode, has facilitated their classi- included recommendations for improving the fication into broad functional groups (e.g.. rehabilitation process based on the fact that a Canard, 1990; Coyle, 1991). Patterns of relative more complex habitat structure favoured a abundance of key spider families, however, are richer spider fauna. Research beyond com- here proposed as the basis of a functional group munity indices, however, is required to evaluate approach in Australia given the following:

whether Australian spider faunas display pat- 1 the paucity of ecological knowledge at lower terns of variation in relation to change in key taxonomic levels (Humphreys, 1988); environmental factors. 2. that taxonomic characters at family level With the recent expansion of multivariate have ecological relevance (see below);

techniques it is now easier to analyse complex 3. that prevailing ecological patterns in spider ecological patterns (Gauch, 1982), with indi- communities can be detected at the family cator properties of invertebrate communities level (Churchill, 1995); being determined specifically by ordination 4. an increasing demand for ecologically useful (Kremen, 1992). Consequently, these tech- data at finer spatial scales than previously niques have been applied to pitfall trap data used (Norton, 1994); to assess anthropog- derived from a 1 6 month survey of a coastal hea- 5. the need for protocols thland spider community in Tasmania, across enic change on invertebrate faunas that apply three nested spatial scales, with the minimum to various habitat types and regions (New. scale 18 X 18 m (Churchill, 1993, 1995). Corre- 1995); lation coefficients for spider vectors from 6. an urgent need to collate this information HMDS ordination (Belbin, 1991), using the cost-effectively (New, 1994; Yen, 1993); Bray Curtis association measure (Bray and and Curtis, 1957), revealed strong associations of 7. the successful development of a parallel both spider families and species (correlation approach using ant genera (Andersen, 1990: coefficients > 0.6 for 85% and 80% vectors, ms). respectively) with patterns of spatial variability As the ecology of Australian spider genera and across the community (Churchill, 1995). These species is increasingly understood, as for spiders patterns were strongly associated with changes of European heaths (Canard, 1990), this further refined. in habitat structure, particularly the mean cover approach can be of plant species. Significant correlations To distinguish families, important morpho- arrange- between changes in the abundance of taxa and logical characters relate to the size and eyes, legs and silk producing organs abiotic variables (e.g., temperature and rainfall) ment of over time were also documented at both the fam- (e.g., Davies, 1986). These anatomical features directly reflect the perception and use of import- ily and species level (Churchill, 1995). These components, including prey. results illustrate that Australian spiders can dis- ant environmental the size of particular taxa and their play a sensitivity to variation in environmental In addition, distribution within a habitat defines the factors, even at the family level. Given that spatial 334 TRACEY B. CHURCHILL

part of the prey spectrum utilised and hence, To evaluate broad scale ecological patterns in their function in the system (Canard. 1 990). For spider assemblages, family level analysis has example, the family Thomisidae, or crab been suggested (Yen, 1995) and shown to be as spiders, have evolved to be ambush hunters, effective as the use of species in Tasmanian typically cryptic and preying upon small insects coastal heath (Churchill, 1995). Since spiders attracted to flower heads (Main, 1976). More- display a sensitivity to variation in environmen- over, the spatial distribution patterns of thomi- tal parameters, even at the family level, there is a sids are significantly correlated to a high abun- opportunity to investigate, cost-effectively, gen- dance of arthropod pollinators (Turner and eral responses to various agents of ecological Polis, 1979). An observed shift in the relative change. Efforts to further refine the data set can abundance of a given family, therefore, can indi- be directed at investigating dominant taxa in cate more specifically the range of resources families which indicate the strongest trends with being altered by processes of change in the respect to the specific disturbance, or variables, system. under study. Relationships between the Additional advantages of investigating spider observed patterns with other biotic/abiotic com- communities at least to family level relate to cur- ponents can then offer an insight into the key rent concerns for rationalising the costs and ben- processes behind ecological change for more efits of surveys (Margules and Austin, 1991; specific testing. Yen, 1993). In the case of invertebrate surveys, an increasing demand for taxonomic resources Acknowledgments to identify genera and species has accentuated the decline in available expertise (Richardson The research on Tasmanian heathland spiders and McKenzie, 1992; Gaston and May, 1992). was supported by the Queen Victoria Museum This issue has been termed the "taxonomic in Launceston, an Australian Post-graduate impediment" to effective invertebrate assess- Research Award at Griffith University, and the ment (Cranston, 1990; Kitching, 1993; New, Arachnology section of the Queensland 1994), with which high costs can be associated. Museum, in Brisbane. Michael Arthur and Dr Means to circumvent this have focused on devel- Stuart Bunn, at Griffith University, assisted opment of procedures of "Rapid Biodiversity with the multivariate analyses. I am very grate- Assessment" (RBA) where specimens are taken ful to Dr Robert Raven, (Queensland Museum) to "morphospecies" or "Recognisable Taxo- for his invaluable support, and to Dr Alan nomic Units" in lieu of specific taxonomic res- Andersen, (CSIRO, Darwin), and an anony- olution (Cranston and Hillman, 1992; Kitching, mous reviewer, for comments on the manu- 1 993: Oliver and Beattie, 1 993). The separation script. of RBA procedures from taxonomy and associ- ated phylogenies (Beattie et al., 1993), however, involves an asssociated loss of biological and References biogeographic data which limits ecological Abbott, 1., Burbidge, T, Williams, M. and Van applications. Heurck, P.. 1992. Arthropod fauna of jarrah Alternatively, the use of higher taxonomic (Eucalyptus marginata) foliage in Mediterranean levels may suffice for certain survey goals as forest of Western Australia: Spatial and temporal suggested for stream invertebrate assemblages variation in abundance, biomass, guild structure and species composition. Australian Journal of (Wright et al., 1 995). To assess land degradation Ecology 17: 263-274. and restoration processes, Australian ant genera Andersen. A.N., 1990. The use of ant communities to have successfully been used as the basis of a evaluate change in Australian terrestrial ecosys- functional group approach (Andersen, 1990). In tems: a review and a recipe. Proceedings of the detecting human impacts on marine faunal com- Ecological Society of Australia 16: 347-357^ munities, responses of higher taxonomic levels Andersen. A.N., ms. Ants as indicators of ecosystem have presented restoration an advantage by operating following mining : a functional group "above the noise of natural variability" approach. Proceedings o) the Conference for Off- (Warwick, 1993). In this context, spiders can Reserve Conservation, Brisbane, 1996. Basset, Y., 1991. The taxonomic offer an additional resilience to "noise" by tol- composition of the arthropod fauna associated with an erating notable periods of starvation (Naka- Australian rainforest tree. Australian Journal of Zoology 39: mura, 1 987), to possibly provide a strong signal 171-190. when interpreted as an ecological impact lower Beattie. A.J., Majer. J. and Oliver. I., 1993. Rapid down the trophic pathway. biodiversity assessment : a review. Pp. 4-14 in: SPIDERS AS ECOLOGICAL INDICATORS 335

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