Revista Catalana d’Ornitologia 21:17-28, 2005 Post-fire dynamics in Mediterranean shrublands: are bird communities structured by arthropod availability? Sergi Herrando, Lluís Brotons & Santiago Llacuna The relationship between arthropod availability and bird energy requirements was studied in Mediterranean shrublands. Specifically, we tested the hypothesis that food availability is a possible constraint in the recovery of bird communities in post-fire dynamics. We used the sweep-net sampling method to collect arthropods, and the point-census method to assess bird abundance. The censuses were carried out during the breeding and wintering seasons in a fire-free zone and in two burned zones (one burned four years before the fieldwork, the other 16 years before it). During the breeding season, we found a positive association between arthropod availability and bird requirements in all three zones. However, compared with the other two zones, the recently burned zone contained fewer birds than expected, based on arthropod availability. Thus, it seems that birds exerted significantly lower predation pressure on arthropod populations in this zone than in the other two zones, which resembled each other in this parameter. These results suggest that arthropod availability does not constrain breeding-bird numbers soon after fire; however, it could do so in later successional stages. During winter, when the studied bird species also feed on fleshy-fruits, there were no conclusive associations between arthropods and birds. Key words: arthropod availability, bird community, predation pressure, post-fire succession, burned zones, Mediterranean shrublands. Sergi Herrando, Institut Català d’Ornitologia, Museu de Ciències Naturals, Passeig Picasso s/n, 08003 Barcelona. E-mail: [email protected] Lluís Brotons, Àrea de Biodiversitat, Centre Tecnològic Forestal de Catalunya, Pujada del Seminari s/n, 25280 Solsona. Santiago Llacuna, Parc Natural del Garraf, Diputació de Barcelona, c/ Urgell 187, 08036 Barcelona. Received: 02.12.04; Accepted: 02.08.05 / Edited by Jordi Domènech. The study of avian post-fire dynamics in Medi- vegetation recovery. An association between terranean shrublands has traditionally focused food availability and bird abundance is often on bird-community turnover and its clear asso- detected during the breeding season (as a con- ciation with changes in vegetation structure (e.g. sequence of high food requirements for feeding Lawrence 1966, Prodon et al. 1984, Stanton nestlings) as well as during the winter (when 1986, Prodon et al. 1987, Vicente 1991, Pons & there may be food scarcity). However, there is a Prodon 1996, Izhaki & Adar 1997, Herrando et wide disparity of results over all periods, from al. 2003). Most of these works showed that bird strong associations between bird abundance and abundance usually increases progressively, food availability to no correlation at all (e.g. matching vegetation recovery. However, an as- Newton 1980, Martin 1987, Wiens 1989, Diaz sociation between the vegetation and the avian & Pulido 1993, Morris & Thompson 1998). The community may be due to an indirect effect of lack of a relationship may be found when these the increase in food available to birds following resources are so abundant that they do not im- 03-Arthropods.p65 17 17/02/2006, 10:22 S. Herrando et al. Revista Catalana d’Ornitologia 21 (2005) pose any constraint on the exploiters. This is a small valleys located at 100–500 m a.s.l. The common pattern when there is a second, more average annual rainfall is about 500 mm, but limiting resource for individuals, or when the karstic lithology of these hills provides only populations are in a growing stage (Begon et al. skeletic soils, which offer extremely dry condi- 1987). tions for plant communities. The vegetation of In addition, in Mediterranean ecosystems, the study area is dominated by shrubs such as studies on the post-fire dynamics of inverte- Arbutus unedo, Quercus coccifera, Pistacia brates, the main food source for many breeding lentiscus, Ulex parviflorus, Rosmarinus officinalis, passerines, have usually focused on soil arthro- Thymus vulgaris, Chamaerops humilis and Olea pods (Prodon et al. 1987, Athias-Binche et al. europaea, and by Pinus halepensis trees. These 1987, Sgardelis et al. 1995, Broza & Izhaki plant species are widespread in the natural park, 1997), whose numerical responses to fire seem regardless of the frequency of fire, which has to be very variable depending on the soil layer basically affected species abundance but not in which they live, as well as on the severity of species composition, although grasses such as the fire in terms of temperature and duration. Ampelodesmos mauritanica and Brachypodium Only a few studies have dealt with the effects retusum were more abundant in recently burned of fire on foliage invertebrates (e.g. Woinarski zones than in others (Herrando et al. 2003); this & Recher 1997, Pons 1998). As with birds, these is a common pattern in Mediterranean authors reported a fast increase in foliage-ar- shrublands (Trabaud 1981). thropod abundance in the vegetation regener- The area that is the Garraf Natural Park does ating rapidly after fire. However, despite the not now support any economic resources apart trends described for both birds and arthropods from those related to leisure activities. At the after fire, our understanding of the associations end of the nineteenth century, however, the area between these two groups remains poor. was extensively cultivated, and vineyards oc- The main aim of our study was to test the cupied the belts of terraces built into the stony hypothesis that bird communities are structured slopes of the mountains. After the phylloxera by arthropod availability. If this is true, there crisis, most of the human population migrated should be an association between arthropod to nearby urban areas, although livestock and availability and bird numbers. Since succes- timber exploitation continued until the middle sional dynamics show a progressive increase in of the twentieth century. From then on, human the number of interactions between elements activity has sharply decreased, and fire has been of the ecosystem (Margalef 1968, Odum 1969), the main form of disturbance affecting the land- we hypothesized that the association between scape and habitats. A knowledge of this histori- arthropod availability and bird requirements is cal process is essential for understanding the greater in mature unburned areas than in re- current aspect of the Garraf massif. cently burned ones. Most studies conducted on The natural park has had two extensive fires the relationship between arthropods as a food in the last decades: the first burned 10,000 ha resource and birds have, in general, focused on in 1982; and the second, which burned the area one bird species. In contrast, our study attempts again in 1994, affected 5,000 ha. The periphery to consider the entire bird community feeding of the park, about 1,000 ha, has not been dam- on shrub invertebrates. aged by fire in recent years and consists mostly of pine woods grown since the end of the tim- ber exploitation (40–50 years ago). Although Methods we could not directly reconstruct a succession from the synchronic study of these three adja- Study area and design cent zones (Prodon & Pons 1993), the histori- cal processes that occurred throughout the This study was carried out during the breeding whole massif (see above) and the similarity of and wintering seasons of 1998 in the Garraf climate, relief and plant species composition led Natural Park, situated 20 km south of the city us to assume that fire was the main factor of Barcelona (NE Iberian Peninsula). The study involved in the current physiognomy of each of area (41°15’N 1°55’E) consists of low hills and these zones. Other authors have used the syn- 18 03-Arthropods.p65 18 17/02/2006, 10:22 Post-fire dynamics in Mediterranean shrublands chronic approach to study successional post-fire to be not significant in 42 out of the 48 tests dynamics (e.g. Vicente 1991, Crête et al. 1995, carried out. Thus, we considered them as true Ding et al. 1997, Imbeau et al. 1999), and we replicates and used means between the two sam- think that our study area provided an appropri- ples for a given taxon as the most reliable indi- ate framework within which to study the rela- cator of its abundance in the given period. tionships between bird and arthropod abun- Samples were dried for 72 hours at 70°C, dance after fire. We established 18 randomly the numbers of individuals were counted, and placed survey zones: 6 within the habitat burned each sample was weighed. Next, we classified in 1982 (referred to as “F82”), 6 within the habi- arthropods to the taxonomic level of order. We tat burned in both 1982 and 1994 (“F94”), and then determined whether the different taxons 6 in the fire-free zone (”Control”). encountered had been detected in previous studies on shrubland bird diets. According to Arthropod sampling the feeding data compiled in Cramp (1988, 1992), Cramp & Perrins (1993) and Hódar We used sweep-net sampling to collect arthro- (1994), the bird species found in our study show pods from shrubs. This method is probably the a notable flexibility in diet depending on the most widely used for sampling arthropods from frequency of the different potential prey types vegetation, and it is especially suitable in eco- in the habitat. All the arthropod orders present systems where the plants of interest are small. on the studied shrubs occur in the diets of the Sweep-netting does not provide a measure of breeding birds found in the area, except absolute density; however, this is not needed Phasmida, which was very rare and accounted when examining the relationships between ar- for less than 0.01% of total arthropod biomass thropods and bird abundance (Cooper & in all zones.