Ecological Entomology (2003) 28,299–308

Consequences of removing a keystone herbivore for the abundance and diversity of associated with a cruciferous shrub

ADELA GONZA´ LEZ-MEGI´ A S and J O S E´ M. GO´ MEZ Departamento de Biologı´ a y Ecologı´ a,Facultad de Ciencias,Universidad de Granada,Spain

Abstract. 1. The effect of the removal of Timarcha lugens (Chrysomelidae),one of the main herbivores of Hormathophylla spinosa (Cruciferae),on the abundance of co-occurring phytophagous ,the abundance of non-phytophagous arthropods (detritivores,predators,and parasitoids),and the structure and diver- sity of the entire community,was studied for 3 years (1999–2001). 2. There was competition between T. lugens and co-occurring herbivores; the removal of T. lugens was correlated with an increase in the abundance of sap- suckers,flower-feeders,and,above all,folivores. 3. Timarcha lugens also had an indirect effect on arthropods belonging to other trophic levels; the abundance of predators increased significantly after the removal of T. lugens. 4. Community composition was affected by the experimental removal. In addi- tion,the diversity of the overall community increased after removal of T. lugens. 5. The study demonstrated experimentally that T. lugens has a significant effect not only on other belonging to the same trophic level,but also on the abundance of species belonging to higher trophic levels,and,consequently,on the entire structure and diversity of the complex community in which it is immersed. Key words. Chrysomelidae,community structure and diversity,exploitative competition, communities, Timarcha lugens.

Introduction strong effects on each other (Hunter,1992; Stewart,1996). Indeed,temporally and spatially separated competitive The overall importance of interspecific competition in struc- effects mediated by the host plant appear to be common turing insect communities is still open to debate (Damman, among phytophagous insects (Faeth & Wilson,1997; 1993; Denno et al.,1995; Stewart,1996). Whereas competi- Masters & Brown,1997). Furthermore,some guilds comprise tion is intense and frequent in some insect communities (e.g. taxonomically and ecologically distant species,which Harrison et al.,1995; Hudson & Stiling,1997; Waltz & may also be engaged in competition for shared resources Whitham,1997; Denno et al.,2000; Fisher et al.,2000),little (Davidson et al.,1984,1985; Hochberg & Lawton,1990; evidence for competition has been found in other commu- Hunter,1992; Tscharntke,1997; Suominen et al.,1999a,b; nities (e.g. Evans,1992; Faeth,1992; Marquis & Whelan, Go´ mez & Gonza´ lez-Megı´ as,2002). To obtain an accurate 1994; Cornell & Hawkins,1995; Schmitz,1998). Although idea of the actual role that competition plays in insect ecological theory assumes that competition occurs mainly communities,it may be necessary to consider the existence between members of the same guild,recent studies have of interactions between dissimilar organisms (Go´ mez & demonstrated that herbivores in different guilds can exert Gonza´ lez-Megı´ as,2002). Competition can affect not only individual- and population-level traits,such as behaviour and abundance, Correspondence: Adela Gonza´ lez-Megı´ as,School of Biology, but also community-level parameters,such as diversity, University of Leeds,Leeds LS2 9JT,U.K. E-mail: bgyagm@ composition,and structure,and sometimes the effects can leeds.ac.uk differ between levels (Connell,1983; Faeth & Wilson,1997;

# 2003 The Royal Entomological Society 299 300 Adela Gonza´lez-Megı´as and Jose´ M. Go´mez

Denno et al.,2000). Nevertheless,the differential effect of season,both new adult and older individuals are found competition at different organisation levels has seldom been throughout the summer. There are two peaks of adult studied (Denno et al.,1995). Indeed,the role of interspecific emergence,once soon after snowmelt,the other during the competition in structuring insect communities has mostly first week of August been inferred from population-level studies (Tscharntke, The study site was in the Sierra Nevada (Granada prov- 1997; Speight et al.,1999); however population dynamics ince,Spain),in an 3 ha area located at 2828 m a.s.l. and community organisation are influenced greatly by (37404500N,3 2202500W). The area is an open high- interactions within and between all trophic levels,and ana- mountain scrubland (6.4 1.4% shrub cover,15.8 12.1 lyses of pairwise interactions often fail to explain patterns of plants/20 m2) dominated by H. spinosa (95% of total plant co-existence and abundance at the community level cover),with a few individuals belonging to two other (Tscharntke,1997). These species with a pervasive influence shrubs, Reseda complicata and Sideritis glacialis,and some on the overall community composition are considered key- scattered perennial herbs. Individual shrubs occur as dis- stone species (Hunter,1992),and their removal produces a crete units,surrounded by open interspaces of bare soil and dramatic change in the associated community. schist. The work reported here investigated experimentally the effects of a potential keystone species,the Timarcha lugens Rosenhauer (Chrysomelidae),on the phytophagous Experimental design and non-phytophagous arthropod community associ- ated with the host shrub Hormathophylla spinosa (L.) The effect of T. lugens on the insect community was Ku¨ pfer (Cruciferae). Timarcha lugens is monophagous on studied by means of a removal experiment using a com- H. spinosa,feeding on its flowers,fruits,and vegetative pletely randomised design in each of 3 years (1999,2000,and tissues(Gonza´ lez-Megı´ as&Go´ mez,2001).Thisbeetlespecies 2001). In early June 1999,40 shrubs of similar size,at the consumes large quantities of plant tissue (4.5 0.6 mg per same phenological stage and early flowering,were selected. individual per day; Gonza´ lez-Megı´ as & Go´ mez,2001), Shrubs were assigned randomly to one of the following decreasing in some cases the fruit set of the plant by more treatments: (1) Timarcha exclusion: T. lugens were excluded than 30% (Go´ mez & Gonza´ lez-Megı´ as,2002). The specific selectively from 20 shrubs by hand removal of all the objective was to assess the effect of the removal of T. lugens (see Floyd,1996; Waltz & Whitham,1997,for a similar on the abundance of co-occurring phytophagous insects procedure) every 5 days throughout the experiment. Other living on the same or different parts of the shrubs,the invertebrates living in the shrubs were not disturbed by abundance of non-phytophagous arthropods (detritivores, removing the beetles using forceps. This method was used predators,and parasitoids),and the structure and diversity instead of,for example,the application of soil insecticide or of the entire insect community. Tanglefoot1 to the base of the shrubs (see,for example, Hudson & Stiling,1997),because many other herbivorous and predatory insects living in H. spinosa foliage and Materials and methods flowers are also apterous,and would thus also be excluded from experimental shrubs together with the T. lugens indivi- Study system duals. This method excluded the beetles efficiently from these plants; after the third removal period,no T. lugens remained Hormathophylla spinosa is a long-lived stunted shrub (Go´ mez & Gonza´ lez-Megı´ as,2002). (2) Twenty plants were occurring in high mountains of the western Mediterranean, not subjected to removal of T. lugens to serve as controls. from southern France to North Africa. This thorny mass- The experimental shrubs did not differ among treatments in flowering shrub is typically hemispherical in shape when either size (F ¼ 1.992,d.f. ¼ 1,38, P ¼ NS) or distance to the reproductive,bearing from 480 to over 75 000 flowers per nearest conspecific (F ¼ 2.188,d.f. ¼ 1,38, P ¼ NS). The year,arranged in inflorescences,which outgrow its surface. abundance of T. lugens during the experimental period was Timarcha lugens is a high-altitude apterous,medium- 8.9 5.6 individuals per shrub in 1999,8.9 5.5 in 2000, sized (43.1 2.07 mg dry weight, n ¼ 130) beetle endemic and 9.9 6.1 in 2001 (repeated-measures ANOVA (rm- to the Sierra Nevada mountains (Spain),occurring from ANOVA), F ¼ 0.33,d.f. ¼ 2,20, P ¼ NS). 2400 to 3200 m a.s.l. It starts to feed on H. spinosa soon after snowmelt (late June at the study site),and is active until the end of September,feeding by chewing leaves as Data collection well as flower and fruits (Go´ mez & Zamora,2000; Gonza´ lez-Megı´ as & Go´ mez,2001). Larvae emerge at the The arthropod fauna living in the foliage was examined beginning of the season,consuming young leaves. Larvae each year using the beating method (Sutherland,1996). and adults can be found sharing the same plant. Flowering Each shrub was tapped for 20 s with a wooden stick,and occurs after larvae have buried under the plant to pupate dislodged invertebrates were caught in a 20 10 cm beating (2 weeks after the emergence period),so it is only occasion- tray held beneath the shrubs. Because this method is ally possible to observe larvae eating buds (Gonza´ lez- destructive (Sutherland,1996),the arthropod fauna was Megı´ as,2001). As adults live longer than one breeding sampled twice (mid July and mid August) each year. Both

# 2003 The Royal Entomological Society, Ecological Entomology, 28,299–308 Interactions between T. lugens and co-occurring arthropods 301 periods coincided with peaks of activity of T. lugens and The effect of each response variable was studied separately with the maximum abundance of arthropods in the study for each year using one-way ANOVAs (Proc GLM; SAS, area (Gonza´ lez-Megı´ as,2001). In addition,this method has 1997). The proportional contribution of each trophic presumably negligible effects on herbivore populations, group to the community was contrasted between treatments because less than 20% of the shrub canopy was sampled for each year using one-way contingency analyses (Proc each time. Moreover,the phenological change observed in CATMOD; SAS,1997). Original data were log transformed the fauna associated with H. spinosa is not a direct effect of (for linear measurements) or angular transformed (for per- the sampling method,because it coincides with a general centages) as necessary for normalisation. phenological pattern recorded in the entire fauna (Gonza´ lez- Megı´ as,2001). The beating method was used instead of other methods because H. spinosa is a dwarf shrub covered Results completely by flowers,fruits,and thorns,and most inverte- brates associated with it are extremely tiny (see Results). All Community structure on Hormathophylla spinosa Timarcha knocked from the control shrubs during the beat- ing were returned to control shrubs. A total of 1679 individuals belonging to 83 species, All samples were taken to the laboratory,where arthro- 41 families,11 orders,and seven trophic habits were col- pods were categorised into orders and species,and their lected from the foliage of the 40 H. spinosa experimental maximum lengths were measured [L 0.1 (mm) error]. All shrubs (see Appendix). Most predator species were mites individuals were sent to specialists for identification at the (55% of relative abundance) belonging to four families, lowest taxonomic level. The species were later categorised and spiders (35%),represented mainly by the crab spider into groups based on trophic behaviour (Simberloff & Xysticus cristatus (Tomisidae,11 %). In contrast,folivores Dayan,1991). The trophic habit of each species was assigned were represented by beetles,most of them chrysomelids using the information offered by the taxonomic specialists (35%),weevils (Curculionidae,33 %),and lar- and personal observations carried out during more than vae (26%). The ant Proformica longiseta was the most 10 years of work in the study site,and the literature. The abundant flower-feeder (40%),followed in abundance by dry biomass (W,mg) of all arthropod species was estimated a weevil species (Ceutorhynchus sp.,34 %) and two other from length using allometric equations constructed speci- species of beetle belonging to the (10%). fically for each arthropod group following Ho´ dar (1996). Detritivores were mostly Collembola,the most abundant Both population-level (abundance and cumulative species being Entomobrya nevadensis,although one species biomass) and community-level (composition,diversity) of Psocoptera and one species of anobiid were found. responses to the experimental removal were analysed. Most of these arthropods were very tiny: the average Abundance was expressed as the number of individuals individual lengths and dry weights ( 1 SE) were per plant,census,and year (pooled across the two censuses 3.07 0.35 mm and 1.09 0.35 mg (n ¼ 64) for folivores, each year),while biomass was expressed as cumulative 2.38 0.07 mm and 0.31 0.02 mg (n ¼ 252) for flower- weight of all individuals caught per plant and year. The feeders,2.16 0.11 mm and 0.79 0.16 mg (n ¼ 399) for diversity of insects was assessed by four indexes (Magurran, sap-suckers,1.24 0.09 mm and 0.39 0.11 mg (n ¼ 181) 1988): richness,as the number of species collected; domi- for predators,1.51 0.04 mm and 0.42 0.03 mg (n ¼ 181) nance,as the fraction of the collection represented by the for parasitoids,and 0.84 0.02 mm and 0.02 0.01 mg most common species,the Shannon–Wiener ( H0) diversity (n ¼ 607) for detritivores. There were among-trophic index,and Hurlbert’s probability of intraspecific encounter. group differences in length (one-way ANOVA, F ¼ 70.72, All the indexes were generated by a randomisation process d.f. ¼ 5,1370, P < 0.001) and weight (one-way ANOVA, 1 using EcoSim (Gotelli & Entsminger,2000). F ¼ 8.59,d.f. ¼ 5,1370, P < 0.001).

Data analysis Removal effect on insect abundance

The effect of the experiment on both population- and The abundance of the entire assemblage living in the community-level responses was assessed by univariate foliage of H. spinosa was not affected significantly by the repeated-measures ANOVA (rmANOVA,Proc GLM; SAS, exclusion of T. lugens (Table 1). Nevertheless,the exclusion 1997),because the within-subject effect satisfied the of Timarcha had a significant effect on the abundance of Huynh and Feldt condition (by the test of sphericity). In two of the six trophic groups (folivores and predators) these analyses,the abundance/biomass of trophic groups or considered separately (the effect on omnivores was not each diversity index were introduced as dependent vari- tested due to their low abundance). The repeated-measures ables,with the presence or absence of Timarcha as the ANOVAs also showed that the abundance of every trophic between-subjects factor,and time (year) as the within- group varied among years (Table 1). subject factor. Shrubs were used as the error term for the When averaged across the 3 years,the abundance of foli- between-subjects factor; the time shrub interaction was vores was much higher in excluded plants (0.86 0.16 indi- included in the error term for the within-subject factor. viduals per census) than on control plants (0.47 0.10);

# 2003 The Royal Entomological Society, Ecological Entomology, 28,299–308 302 dl Gonza Adela ´ lez-Megı ´ sadJose and as ´ .Go M. Table 1. Repeated-measures ANOVAs on the effect of Timarcha removal on the abundance (log-transformed,estimated as number of individuals per census) and cumulative biomass (log- transformed,estimated as weight of all individuals collected per census) of each arthropod trophic group. * P < 0.05,** P < 0.01,*** P < 0.001. Plants are used as between-subject error. ´ mez Total Folivores Flower-feeders Sap-suckers Parasitoids Predators Detritivores

d.f. MS F MS F MS F MS F MS F MS F MS F #

03TeRylEtmlgclSociety, Entomological Royal The 2003 Abundance T. lugens 1 0.233 1.626 0.133 4.709* 0.278 1.804 0.110 1.730 0.066 1.535 0.317 14.347*** 0.007 0.071 Error 33 0.144 0.028 0.154 0.064 0.043 0.022 0.100 Time 2 0.855 16.884*** 1.096 29.030*** 0.339 5.007*** 1.151 25.033*** 0.322 8.938*** 0.111 3.891* 2.598 30.853*** T. lugens time 2 0.049 0.971 0.136 3.628 0.122 1.805 0.118 2.560 0.004 0.124 0.078 2.735 0.094 1.124 Error 66 0.051 0.038 0.068 0.046 0.036 0.028 0.083 Biomass T. lugens 1 0.323 1.987 0.001 0.042 0.102 2.089 0.028 1.317 0.012 0.295 0.018 0.571 0.001 0.988 Error 33 0.161 0.017 0.049 0.087 0.040 0.032 0.001 Time 2 0.195 2.134 0.020 1.402 0.113 4.136** 1.719 7.133*** 0.221 6.640** 0.024 0.667 0.009 6.289** T. lugens time 2 0.003 0.031 0.009 0.601 0.108 3.953** 0.0004 0.005 0.039 1.219 0.117 3.202* 0.001 0.356 Error 66 0.091 0.014 0.027 0.101 0.032 0.037 0.001 clgclEntomology Ecological , 28 ,299–308 Interactions between T. lugens and co-occurring arthropods 303 however the only between-treatment difference in folivore assemblage or on any of the individual trophic groups abundance was found in 1999,shortly after the removal of (Table 1); however,for flower-feeders and predators,there T. lugens (one-way ANOVA, F ¼ 5.44,d.f. ¼ 1,34, P < 0.05; were significant interactions between time and exclusion of Fig. 1). During this first year of the experiment,there was T. lugens (Table 1),indicating that the response of both also a significant between-treatment difference in the abun- groups to the exclusion of Timarcha varied among years. dance of flower-feeders (one-way ANOVA, F ¼ 4.38, Thus,whereas the cumulative biomass of flower-feeders d.f. ¼ 1,34, P < 0.05; Fig. 1),whereas the sap-sucker abun- was significantly higher in excluded shrubs than in control dance was significantly higher in excluded plants than in shrubs in 1999 (F ¼ 5.27,d.f. ¼ 1,33, P < 0.05),no effect was control plants only in 2000 (one-way ANOVA, F ¼ 4.27, found in the other 2 years. d.f. ¼ 1,34, P < 0.05; Fig. 1). Removal of T. lugens also affected the abundance of non- herbivorous species. The overall abundance of predators Effect on the composition of the arthropod community was significantly higher in excluded shrubs (1.53 0.15 individuals per census) than in control shrubs (0.83 1.13), The exclusion of T. lugens had a significant effect on the however predators were more abundant in excluded plants relative composition of the arthropod assemblage living on only in 1999 (one-way ANOVA, F ¼ 4.22,d.f. ¼ 1,34, P < 0.05) H. spinosa in all 3 years (Fig. 2). In 1999,there were and 2000 (one-way ANOVA, F ¼ 8.29,d.f. ¼ 1,34, P < 0.01; between-treatment differences (G ¼ 16.41, P < 0.01); on Fig. 1). The group of predators affected most by the exclu- excluded plants,the most frequent insects were the flower- sion of T. lugens was the predatory mites (rmANOVA, feeders (42% of the insects),whereas in control shrubs the F ¼ 7.41,d.f. ¼ 1,34, P ¼ 0.01),the overall abundance of most frequent insects were the sap-suckers (49%; Fig. 2). In which increased from 0.51 0.12 individuals per census in this year,the relative abundance of predators was higher in control shrubs to 0.98 15 individuals per census in excluded shrubs (14%) than in control shrubs (8%). In excluded shrubs. 2000,although sap-suckers were the most abundant trophic group in both treatments (37%),there were also between- treatment differences (G ¼ 13.79, P < 0.05),with the relative abundance of predators and detritivores being higher in Removal effect on insect cumulative biomass excluded shrubs (15 vs 8% and 23 vs 13% respectively; Fig. 2). In 2001,the most abundant arthropods were the There was no effect of the experimental exclusion of detritivores (more than 53 of relative abundance in both T. lugens on the cumulative biomass of the arthropod % treatments),the between-treatment difference ( G ¼ 16.55, P < 0.01) being caused by the higher proportion of para- sitoids in excluded shrubs (17 vs 9%; Fig. 2).

Effect on insect diversity

Three of the four measures of arthropod diversity showed overall differences between treatments (Table 2). Thus,rich- ness and the Shannon–Wiener index were significantly higher and the dominance index was lower in shrubs from

Fig. 1. Temporal profile of the abundance,expressed as loga- rithms,of each arthropod trophic group on shrubs with Timarcha Fig. 2. Effect of Timarcha lugens removal on the composition, lugens present and absent. Statistical differences in the abundance shown as the relative abundance (in percentage) of each trophic of the arthropods were tested using one-way ANOVAs (NS,not guild,of the entire arthropod community associated with significant,* P < 0.05,** P < 0.01). Hormathophylla spinosa during the 3 years of study.

# 2003 The Royal Entomological Society, Ecological Entomology, 28,299–308 304 Adela Gonza´lez-Megı´as and Jose´ M. Go´mez

Table 2. Repeated-measures ANOVAs on the effect of Timarcha lugens removal on the four diversity indices. *P < 0.05,** P < 0.01, ***P < 0.001. Plants are used as between-subject error. Richness Hurlbert’s PIE Shannon–Wiener Dominance

d.f MS F MS F MS F MS F T. lugens 1 49.511 5.79* 0.062 2.115 3.366 9.461** 0.310 5.307* Error 34 8.546 0.029 0.356 1.808 Time 2 18.206 4.156* 0.502 22.635*** 0.248 0.334 0.126 2.699 T. lugens time 2 23.287 5.316** 0.007 0.302 0.166 4.966 0.062 0.661 Error 68 4.380 0.022 0.235 2.891

PIE,probability of intraspecific encounter.

which T. lugens was excluded than in control shrubs (Fig. 3). intensity or in the same year. Thus,the rm ANOVAs suggest Among years,both the Shannon–Wiener index (1999: that T. lugens removal affected the abundance of the foli- F ¼ 5.04,d.f. ¼ 1,33, P < 0.05; 2000: F ¼ 6.05,d.f. ¼ 1,33, vores and flower-feeders,and this effect was immediate, P < 0.05) and richness (1999: F ¼ 5.24,d.f. ¼ 1,33, because folivore and flower-feeder abundance increased in P < 0.05; 2000: F ¼ 12.04,d.f. ¼ 1,33, P ¼ 0.001) were higher 1999 promptly after the exclusion of the chrysomelid. in 1999 and 2000 when T. lugens was excluded (Fig. 3). Con- Because T. lugens behaves both as a folivore and a flower- sequently,the dominance index was significantly lower feeder,consuming a high proportion of H. spinosa flowers when T. lugens was excluded in 1999 (F ¼ 4.83,d.f. ¼ 1,33, and fruits and thus reducing the amount of resource avail- P < 0.05; Fig. 3). The diversity of arthropods,measured by able to other flower-feeders (Go´ mez & Zamora,2000; Hurlbert’s probability of interspecific encounter,was higher Gonza´ lez-Megı´ as & Go´ mez,2001; Go ´ mez & Gonza´ lez- in the T. lugens exclusion treatment only for 2001 (F ¼ 5.14, Megı´ as,2002),it seems that in this system the competition d.f. ¼ 1,33, P < 0.05; Fig. 3). takes place very intensely between members of the same guild,as predicted by niche theory (Morris,1992; Denno et al.,1995; Harrison et al.,1995; Inbar et al.,1995; Fisher et al.,2000). Timarcha lugens might affect floral-feeders not Discussion only by reducing the shared resource directly,but also indirectly by affecting plant reproduction,because it has This experiment provided evidence for the existence of been shown repeatedly that damage to vegetative structures competition between T. lugens and the phytophagous reduces the production of flowers and fruits in many plant arthropods living on H. spinosa. The experimental removal species (Meyer,1993; Meyer & Root,1993; Mu ¨ ller-Scharer & of the chrysomelid produced an increase in the abundance Brown,1995). In this way,Harrison et al. (1995) demon- of the co-occurring herbivores in three of the nine between- strated a non-reciprocal competitive effect of a folivore treatment comparisons. The exclusion of T. lugens did not species, Tyria jacobaeae (Arctiidae),on several species of affect every kind of phytophagous insect with the same flower-feeder living in Senecio jacobaea (Compositae). Removal of T. lugens also produced an increase in the abundance of sap-suckers in 1 of the 3 years. Because these two types of organism feed on different parts of the shrub, their interaction is presumably mediated by chemical changes in the host plants,rather than by a decrease in the amount of the resources (Faeth,1988; Faeth & Wilson, 1997). After the exclusion of the main herbivore of H. spinosa (Go´ mez & Gonza´ lez-Megı´ as,2002),the plant might change nutritional characteristics or diminish the production of secondary compounds and,in turn,the abil- ity to deter attacks by sap-suckers. An increase in secondary compounds caused by folivore damage has been shown in other systems,whereas sap-suckers usually have little influ- ence on the production of secondary compounds by the plant (Denno et al.,2000; Fisher et al.,2000). The response Fig. 3. Temporal profile of the diversity of the arthropod of sap-suckers to the exclusion of T. lugens occurred during assemblage on shrubs with Timarcha lugens present and absent. the second study year,in accordance with many other stud- Statistical differences in the diversity of the arthropods were tested ies which have shown that chemically mediated effects of using one-way ANOVAs (NS,not significant,* P < 0.05, leaf-chewers on other herbivores occur late in the season or ***P < 0.001). in subsequent years (Faeth,1988; Hunter & Schultz,1995).

# 2003 The Royal Entomological Society, Ecological Entomology, 28,299–308 Interactions between T. lugens and co-occurring arthropods 305

This competitive effect has a special relevance to this study ber of heteropteran species increased from six to 10 in those because in every study year the most abundant sap-sucker plants. Furthermore,whereas all the species in control plants species (35% of relative abundance) was Psallus ibericus. were present in excluded plants,other species such as Nysius This bug species is monophagous on H. spinosa and endemic immunis and Heterocapillus perpusillus,the latter being endem- to the high-mountains of the Sierra Nevada,spending all of ic to the Sierra Nevada,appeared exclusively on excluded its life associated with this host plant. Species with a narrow plants. Also important was the increase in predator species. trophic habitat are more sensitive to competition than are Four genera of predatory mite have been found in control polyphagous species (Karban,1989; Harrison et al.,1995). plants and seven genera in T. lugens excluded plants,includ- This study also showed that the effect of T. lugens on ing individuals of Tarsolarkus, Raphignatus,and Trombidium. every herbivore guild,sap-suckers,flower-feeders,and foli- These results suggest that a single species is able to affect the vores,decreased or even disappeared during the second and, structure of the entire community by its direct and indirect above all,the third year of study. One factor provoking this effects on many different interactions,such as predation, result may be the effect that removal of T. lugens had on competition,and parasitism (Gardner et al.,1995; Waltz & predators. Thus,the abundance of predators increased after Whitham,1997; Strand & Merritt,1999). A main factor the exclusion of the focal herbivore,this effect being most favouring this striking result is the fact that T. lugens is one apparent during the second year of the experiment. Because of the main herbivores of H. spinosa in the study sites,con- the most pronounced response to removal of T. lugens was suming much of the reproductive and vegetative tissue pro- the tiny predatory mites,this increase might be an indirect duced by the host plant and decreasing plant reproductive effect provoked by the prompt increase in the abundance of output by 30% in some years (Go´ mez & Gonza´ lez-Megı´as, herbivores after the removal of T. lugens. If predators are 2002). In fact,in the few cases in which a similar effect of a prey-limited,an increase in the abundance of potential prey single insect species on the diversity of an entire community in the previous years due to the exclusion of T. lugens could has been demonstrated,the focal species also interacted provoke this indirect positive effect on predator abundance strongly with the host plant. For example,Waltz and during the second year. This type of indirect effect was Whitham (1997) found that the exclusion of Chrysomela pointed out by Connell (1983) and has been reported in confluens (Coleoptera,Chrysomelidae) from Populus sp. several other removal experiments (Pimm,1991). Moreover, increased species richness by up to 120%. The beetle removed it has been suggested that plants may amplify this effect by more than 40% of plant foliage,reducing plant suitability for attracting some predators,such as mites and parasitoids, other members of the community. A factor that might medi- through induced responses in order to defend themselves ate these strong effects of T. lugens removal may be the large against herbivores (Dicke et al.,1990; Turlings et al.,1990). difference in size between T. lugens and the other interacting This increase in predator abundance in T. lugens-excluded species. Thus,whereas T. lugens is a medium-to-large insect plants presumably caused a concomitant increase in the species (40 mg),the other species feeding on H. spinosa are all predation rate suffered by the herbivores,producing a tiny (< 1 mg),including the other species of chrysomelid,such delayed decrease in their abundance. In fact,the role of as Phyllotreta procera, Phyllotreta variipennis,and Aphthona invertebrate predators as top-down control of the abun- euphorbiae. dance of herbivores has been shown in other systems By integrating the results of the experiment,it seems that (Schmitz,1998; Dyer & Letourneau,1999). T. lugens can be considered a keystone species of the studied The experiment also showed that,due to the changes system (sensu Hunter,1992),affecting not only the perform- occurring in the abundance of several trophic groups,the ance and reproductive success of the host plant (Go´ mez & functional structure of the community was affected by the Gonza´ lez-Megı´as,2002) but also the interaction web existing removal of T. lugens. Nevertheless,these changes differed between this plant and other co-occurring arthropods. by year. Thus,the insect community in T. lugens-exclusion Furthermore,it has been shown that T. lugens has a signifi- shrubs was dominated by floral-feeders in 1999 and cant effect not only on other species belonging to the same detritivores,predators,and sap-suckers in 2000,whereas trophic level,but also on the abundance of species belonging sap-suckers dominated both years in control shrubs. to a higher trophic level and,consequently,on the entire Furthermore,the main effect occurred in 1999 and 2000; structure and diversity of the complex community (with thus,it seems that there is a short-term response of the more than 80 species involved). The findings suggest strongly arthropod community to the exclusion of T. lugens. Hudson the necessity of conducting long-term experiments,because and Stiling (1997) also showed that a species of chrysomelid, there was a significant temporal change in the response of Trirhabda baccharidis,has an important short-term effect in both herbivores and predators to the experimental removal,as the structure of the phytophagous insect community of the well as to focus on the mechanisms responsible for these effects. shrub Baccharis halimifolia (Asteraceae). Finally, T. lugens also affects the diversity of the overall community,because diversity increased after the removal of Acknowledgements this chrysomelid. There are two main reasons for the observed increase in diversity,mainly during the first 2 years We are most grateful to Dr Francisco Sa´ nchez Pin˜ ero and of the experiment. One is the presence of new species of two anonymous reviewers for their valuable comments on herbivore in the T. lugens-excluded plants. Indeed,the num- this manuscript and to Dr Jose´ A. Ho´ dar for kindly

# 2003 The Royal Entomological Society, Ecological Entomology, 28,299–308 306 Adela Gonza´lez-Megı´as and Jose´ M. Go´mez providing the regression equations. We thank Martin A. C. Gange and V. K. Brown),pp. 201–215. Blackwell Science, Pareja for linguistic advice. A.G.M. was supported by Oxford. a grant from PFPI-Junta de Andalucı´ a. The Consejerı´ a Fisher,A.E.I.,Hartley,S.E. & Young,M. (2000) Direct and de Medio Ambiente of the Junta de Andalucı´ aand indirect competitive effects of foliage feeding guilds on the the National Park provided us invaluable facilities in which performance of the birch leaf-miner Eriocrania. Journal of Animal Ecology, 69,165–176. to work in the Sierra Nevada. Dr Javier Arbea Floyd,T. (1996) Top-down impacts on creosotebush herbivores in (Collembola),Dr Victor Viraloa (mites),Dr Gloria Bastazo spatially and temporally complex environments. Ecology, 77, and Dr Jose M. Vela (Chrysomelidae),Dr Miguel A. 1544–1555. 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Appendix. Taxa collected in the experimental shrubs. Family Lowest taxonomical level Folivores Coleoptera Chrysomelidae Aphthona euphorbiae Schrank Coleoptera Chrysomelidae Phyllotreta procera Redtenbacher Coleoptera Chrysomelidae Phyllotreta variipennis Boieldieu Coleoptera Chrysomelidae Gen. sp. Coleoptera Curculionidae Sitona tenuis Rosenhauer Coleoptera Curculionidae Rhinocyllus conicus (Frielich) Coleoptera Curculionidae Entomoderus (Asparrorhinus) nov. sp. Coleoptera Curculionidae Entomoderus (Pseudorhinus) nov. sp. Lepidoptera Geometridae Larvae Prostigmata Tetranychidae Bryobia sp. Koch Flower-feeders Coleoptera Curculionidae Ceutorhynchus nov. sp. Coleoptera Malachydae Axinotarsus varius Uhagon Coleoptera Melyridae Aplocnemus andalusiacus Rosenhauer Coleoptera Melyridae Dasytes nigropunctatus Kuester Coleoptera Nitidulidae Meligethes sp. 1 Stephens Coleoptera Nitidulidae Meligethes sp. 2 Stephens Diptera Empididae Ramphoniia tennuirostris Falle´ n Diptera Fannidae Fannia scalaris Fabricius Hymenoptera Formicidae Proformica longiseta Collinwood Lepidoptera Pyralidae Gen. sp. Sap-suckers Heteroptera Lygaeidae Apterola ramburi Heteroptera Lygaeidae Macroplax fasciata Herrich-Schaefer Heteroptera Lygaeidae Nysius cymoides Heteroptera Miridae Heterocapillus perpusillus Heteroptera Miridae Macrotylus cf. atricapllus Heteroptera Miridae Macrotylus sp. Fieber Heteroptera Miridae Pachytomella sp cf. cursitans Heteroptera Miridae Psallus ibericus

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Appendix. Continued Family Lowest taxonomical level Heteroptera Miridae Strongylocoris seabrai Heteroptera Pentatomidae Sciocoris sp. Fallen Heteroptera Rhopalidae Brachycarenus tigrinus Schill. Homoptera Aphidae Gen. sp. Homoptera Aphrophoridae Gen. sp. Homoptera Cercopidae Gen. sp. 1 Homoptera Cercopidae Gen. sp. 2 Homoptera Cercopidae Gen. sp. 3 Homoptera Psyllidae Gen. sp. Omnivores Heteroptera Lygaeidae Lygaeus militaris Orthoptera Tettigoniidae Baetica ustulata Rambur Parasitoids Hymenoptera Braconidae Gen. sp. 1 Hymenoptera Braconidae Gen. sp. 2 Hymenoptera Chalcidoidea Gen. sp. 1 Hymenoptera Chalcidoidea Gen. sp. 2 Hymenoptera Chalcidoidea Gen. sp. 3 Hymenoptera Chalcidoidea Gen. sp. 4 Hymenoptera Chalcidoidea Gen. sp. 5 Hymenoptera Diapriidae Gen. sp. Hymenoptera Eulophidae Aprostocestus sp. Hymenoptera Eulophidae Necremnus folia Walker Hymenoptera Eulophidae Necremnus tidius Walker Hymenoptera Ichneumonidae Criptinae Gen. sp. Hymenoptera Pteromalidae Systasis encyrtoides Walker Hymenoptera Pteromalidae Tricomalus sp. Predators Araneae Linyphiidae Gen. sp. Araneae Oxyopidae Oxyopes sp. Latreille Araneae Philodromidae Thanatus sp. Koch Araneae Salticidae Gen. sp. Araneae Therdiidae Theridion. sp. Walckenaer Araneae Thomisidae Xysticus cristatus Clerck Coleoptera Coccinellidae Coccinella septempunctata (L.) Coleoptera Coccinellidae Scymninae Gen. sp. 1 Coleoptera Coccinellidae Scymninae Gen. sp. 2 Heteroptera Anthocoridae Anthocoris nemoralis Fabricius Heteroptera Anthocoridae Orius cf. horvathi Heteroptera Anthocoridae Orius laevigatus Heteroptera Miridae Deraeocoris serenus Prostigmata Anystidae Tarsolarkus sp. Thor Prostigmata Anystidae Anystis sp. von Heyden Prostigmata Bdellidae Biscirus sp. Thor Prostigmata Bdellidae Bdella sp. Latreille Prostigmata Bdellidae Spinibdella sp. Thor Prostigmata Raphignathidae Raphignatus sp. Prostigmata Trombidiidae Trombidium sp. Fabricius Prostigmata Trombidiidae Larvae Coleoptera Cantharidae Gen. sp. Detritivores Collembola Bourletiellidae Deuterosminthurus bicinctus (Koch) f. flava Gisin Collembola Entomobryidae Entomobrya nevadensis Steiner Collembola Entomobryidae Heteromurus major Moniez Collembola Entomobryidae Lepidocyrtus violaceus Geoffroy Collembola Entomobryidae Seira sp. Lubbock Oribatida Coleoptera Anobiidae Gen. sp. Psocoptera Gen. sp.

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