The Case of Pedunculate Oak, Winter Moth and Tits
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ORIGINAL ARTICLE OA 000 EN Functional Effects of elevated temperature on multi-species Ecology 1999 13 (Suppl.1), interactions: the case of Pedunculate Oak, Winter Moth 74–82 and Tits A. BUSE,* S. J. DURY,† R. J. W. WOODBURN,† C. M. PERRINS† and J. E. G. GOOD* *Institute of Terrestrial Ecology, Bangor Research Unit, University of Wales Bangor, Bangor, Gwynedd LL57 2UP and †Edward Grey Institute, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK Summary 1. The effects of temperature on the Oak–Winter Moth–Tit food chain were studied at Wytham Wood, Oxford, and experimentally in the controlled environment solardomes at the Institute of Terrestrial Ecology, Bangor. 2. Tree cores from Wytham indicated that mature Oaks grew best at high temperatures and rainfall, but with low caterpillar populations. Young trees grew less well at elevated temperature, probably because they lost more water than they gained. Elevated temper- atures advanced budburst, reduced foliar nitrogen and increased leaf toughness. 3. Moth eggs laid later or maintained at cooler temperatures than average required fewer heat units to hatch. Caterpillars took up to 50 days to complete growth at field temperatures but did so in only 20 days at a constant 15 °C. 4. The mass of Tit chicks at day 15 (day 1 = egg hatch) was positively correlated with temperature and negatively correlated with rainfall during the growing period. 5. At elevated temperature, budburst and moth egg hatch were synchronized, but earlier. Late feeding larvae and larvae fed on leaves from trees grown at elevated temperature produced smaller pupae. Pupal mass was unaffected when caterpillars and trees were maintained together under the same conditions. 6. Delaying egg hatch in Tits, to simulate conditions at elevated spring temperatures, resulted in reduced chick mass, body size and fledging success. This occurred because the chicks were fed later and prey quality was poorer, because the peak of caterpillar biomass was missed. 7. We predict that moth reproductive output will be retained at elevated temperatures because both leaves and caterpillars develop faster. Brood size in birds may be reduced because they cannot lay early enough to coincide with the narrower peak of food abundance. Key-words: Climate change, food web, Operophtera brumata, Parus spp., Quercus robur Functional Ecology (1999) 13 (Suppl. 1), 74–82 Introduction previous seasons. This three-step food chain, how- The relationship between Pedunculate Oak (Quercus ever, forms only a small part of a complex web of robur L.), Winter Moth (Operophtera brumata L.), interactions (Varley, Gradwell & Hassell 1973). For and Great and Blue Tits (Parus major L. and Parus example, although Oak is the preferred food plant, caeruleus L.) has been studied for many years in Winter Moth is polyphagous and can feed on a wide Wytham Wood, near Oxford, since the classic popu- range of plant species (Wint 1983); birds will feed lation studies of Varley and Gradwell (Varley 1970). their young on species other than Winter Moth cater- The interaction between these organisms is complex. pillars (Betts 1955); species other than birds, such as The suitability of one species (e.g. Oak) as a food parasitoids, will use the caterpillars as a food source resource for another (e.g. Winter Moth) will depend (Kerslake et al. 1996). The relationship between the on its quantity and nutritional quality, which are, in three species is dynamic; the nutritional quality of © 1999 British turn, affected by the current environment and the leaves and caterpillars and the nutritional require- Ecological Society performance of both food and consumer species in ments of caterpillars and birds are continuously 74 75 changing as leaves, caterpillars and birds develop Materials and methods Temperature and simultaneously. Consequently, the synchronization STUDY SITES a three-step between budburst of Oak and egg hatch of Winter woodland food Moth, and the parallel timing of leaf development The field site was Wytham Wood (National Grid chain and caterpillar growth, or caterpillar growth and Reference SP 4708; longitude 1 ° 20 ′W, latitude fledgling development, are critical to the final repro- 51°46′N) near Oxford, UK. This is an area of about ductive output of all the species. 320 ha consisting of former Ash–Maple–Hazel Current predictions suggest that global levels of coppice with Oak standards, partly invaded by atmospheric carbon dioxide will increase from the sycamore (NVC type W8a; Rodwell 1991). It is present concentration of about 360 ppmv to about typical of the woodland habitat through much of low- 500 ppmv by the year 2100, with a consequent land Britain. Because Wytham is one of the most global temperature increase of about 2 °C (IPCC intensively studied woodland sites in Britain, the 1996). Increased temperature would be expected to selection of this site for our study enabled us to reduce the nitrogen concentration and increase the include data, collected over about 30 years, on the condensed tannin content of Oak leaves (Dury et al. effects of year-to-year temperature variations on the 1998), thereby reducing the performance of the three species. For our study, elevated walkways were caterpillars which feed on them (Scriber & Slansky erected to enable us to gain access to the crowns of 1981). Elevated temperature would also accelerate several mature (around 100-year-old) Oak trees the developmental rate of caterpillars, which, conse- (Walkway Site). These trees are located at an altitude quently, would affect the breeding success of Tits of c. 160 m above sea level. The birds were studied (Perrins 1991; van Noordwijk, McCleery & Perrins intensively in this area and more generally in other 1995) whose laying date is negatively correlated parts of the wood. with temperature (Blondel 1985; Perrins & The effects of simulated elevated temperature McCleery 1989). Although there is undoubtedly an environments on entire, 4-year-old Oak trees and interaction between plants, insect herbivores and their interactions with Winter Moth were examined at predators, Stamp (1993) emphasizes that it is unclear the Climate Change Facility (solardomes) at the what the relative effects of temperature, food quality Institute of Terrestrial Ecology, Bangor. Each tree and consumers on such a system are, and to what stood in 2 cm water to prevent drought and was fed degree the three interact. Here we examine some of twice during the year with 15:30:15 nitrogen:phos- the effects of environmental change on the plant/her- phorus:potassium liquid fertilizer to provide an bivore/predator interaction, using the Oak–Winter adequate nutrient supply. The eight hemispherical Moth–Tit food chain as an example. glasshouses, each with a floor area of 10 m2, provide Our null hypotheses are, first, that elevated tem- two replicates of a 22 factorial combination of peratures will have no effect on the interaction ambient and elevated temperature (+ 3 °C) and two between Oak and caterpillars and, second, that it carbon dioxide concentrations (ambient and ambient will similarly have no effect on the interaction + 340 ppmv) (Rafarel, Ashenden & Roberts 1995). between caterpillars and birds. The overall effect There were thus two replicates of each of the combi- can best be measured as an effect on the perfor- nations of ambient temperature/ambient CO2, ambient mance and eventual reproductive output of each temperature/elevated CO2, elevated temperature/ambi- species, measured as their fecundity. The effect of ent CO2, and elevated temperature/elevated CO2. elevated temperature on each species is considered There was also a replicated treatment outside the individually before studying its effect on the inter- domes. Only the effects of the contrasting temperature action between them. treatments are considered in this paper. Several individuals and organizations have been involved in this collaborative study. The results pre- DIRECT EFFECT OF TEMPERATURE ON OAK sented therefore are taken from a variety of experi- ments and observations. All have emerged during the In 1994, the date of budburst on the Oaks in the solar- TIGER (Terrestrial Initiative in Global domes was measured as well as the duration of leaf Environmental Research) programme during the development [defined as the period when the bud is period 1993–1997. Details are already available of beginning to open and a newly hatched caterpillar can some of the methods and results emanating from enter (Varley et al. 1973), to full-size leaf]. studies of paired interactions, i.e. between Oak and Core samples were taken from the same trees as Winter Moth (Dury et al. 1998; Buse & Good 1996; Varley and colleagues had monitored, and the tree ring Buse et al. 1998) and between Winter Moth and widths measured for the period 1951–1995. The birds (Woodburn 1997). The main objective of this annual growth rates were compared with mean annual © 1999 British paper is to present an up-to-date, integrated picture temperatures for this period, recorded at the Radcliffe Ecological Society, of how these three species interact at ambient and Observatory, 5 km east of Wytham. The effect of Functional Ecology, elevated temperature, and to predict what effect elevated temperature on tree growth in the solardomes 13 (Suppl. 1), 74–82 climate change might have on this food chain. was investigated by recording the change in total 76 shoot length, total number of shoots and trunk diame- leaves from the elevated temperature treatment. The A. Buse et al. ter on nine Oak trees in each of the replicated temper- caterpillars were reared either in the laboratory at a ature treatments (36 trees in all) between the winter of constant temperature of 15 °C, where they were kept 1993 and the winter of 1994. The dry mass of leaves in individual, closed Petri dishes, or in the solardomes falling from the trees in each treatment was also at ambient or elevated temperature, where they were recorded.