RequeRimiento noRmativo de capital

ecosystems are significant and well documented (Peñuelas and Filella 2001, Walther et al. 2002, Root et al. 2003). The IPCC reported that the global average surface temperature shows an increase of 0.85 °C from 1880 to 2012 (IPCC 2014). The three ways in which plant and animals species respond to climate change are: 1) by shifting their distributions to track suitable climatic conditions; 2) by changing their phenology; 3) through in situ adaptation to altered climate conditions (Walther 2010, Bellard et al. 2012). However, the responses of individual species to climate change are also influenced by their interactions with other species (Voigt et al. 2003, Tylianakis et al. 2008), which generates uncertainty in predicting changes in the abundance and distribution of species over time under climate change scenarios (van Vliet et al. 2003, Walther 2010).

Plants are at the base of virtually all terrestrial ecosystems in providing resources for higher trophic level organisms (=consumers). Climate warming, precipitation changes and rising CO2 concentrations will alter the nutritional quality of plants and their availability for herbivorous insects (Bezemer and Jones 1998, Jun Chen et al. 2007, Tylianakis et al. 2008). Temperature is the main driver of plant phenology and there is strong evidence that recent climate warming affects plant

136

phenological events. Khanduri et al (2008) explored some of these phenological changes in more than 650 temperate species, and found the average length of the growing season has extended by 3.3 days per decade (Khanduri et al. 2008).

The effect of climate change on herbivorous insects can be direct by affecting their physiology and behaviour, or it can be indirect, through climate-induced changes in their food plants (Collinge and Louda 1989, Bale et al. 2002, Singer and Parmesan 2010). Various studies have suggested that direct effects of temperature on herbivorous insects are likely to be more important than other factors such as increased CO2 concentrations and UVB levels (Bale et al. 2002). Temperature may induce changes in insect development time, biomass, voltinism, and the extent of food-plant exploitation as well as in their geographical distributions (Bale et al. 2002). However, the responses of insect life histories to climate change are likely to be both complex and variable, depending on insect life history and the biology of the food plant. Changes in plant phenology may influence the insects associated with them depending on life-history traits that are important for establishing or affecting interactions with plants (Bale et al. 2002).

Climate change may result in phenological asynchrony between univoltine insect herbivores and their food plants (Singer and Parmesan 2010). A classic example is the winter moth (Operophtera brumata) and its only food plant, the oak tree (Quercus robur). Newly hatched caterpillars of winter moths are only able to chew and ingest soft tissues in oak buds that are about to ‘burst’ and produce leaves. Asynchrony between the time of egg hatching and bud burst has been documented in the Netherlands as a result of climate warming, especially during a critical period in early spring when winter moth eggs are hatching and oak buds are forming (van Asch and Visser 2007). Multivoltine insects feeding on short-lived annuals, like P. brassicae, may be less affected by climate change because they have many potential food-plant species that they can exploit.

The fate of parasitoids is often intimately tied to that of their hosts. Compared to their herbivorous hosts, parasitoids may be more sensitive to climate change because of their trophic position higher up in the food chain. Moreover, their relatively high degree of specialization also makes them more susceptible to increased variability in host population dynamics mediated by climate change

137 (Stireman et al. 2005, Hance et al. 2006). Host specificity and dispersal ability in parasitoids could have a strong influence on how host-parasitoid interactions respond to climate change (Berg et al. 2010). Little is known about the dispersal abilities of parasitoids (Jones et al. 1996, Elzinga et al. 2007). However, because of the more patchy distribution of their hosts, many specialized parasitoid will have difficulty in tracking their hosts (van Nouhuys and Ehrnsten 2004, Roy et al. 2011, Harvey 2015). Hosts and parasitoids may have different thermal preferences and different capacities to survive extreme temperatures under climate change scenarios (Hance et al. 2006). Therefore, climate change can affect host and parasitoid asymmetrically (in terms of development and phenology).

Climate change could alter phenological relations among tritrophic interactions if species in these trophic links respond differently in terms of their life cycles. For instance, in my study system, interactions involving P. brassicae, its natural short-lived food plants and a specialist endoparasitoid, C. glomerata, are complex in the context of life-history interactions. The consequences of climate warming on the phenology of these trophic interactions depends on how the plants and insects each respond to increasing temperatures, and how this in turn affects the availability and suitability of the resources which they exploit as food. Climate warming may change the growing time of the food plants and can increase the number of generations of the insects (Spieth et al. 2011). If the phenology of food plants of multivoltine insects advances in response to higher temperatures, climate change may result in the starvation of multivoltine insects, or else they may evolve adaptations such as aestivation (Spieth et al. 2011).

Conclusions

- The multivoltine gregarious insect herbivore P. brassicae was only marginally affected by ontogenetic, seasonal and plant species-specific variation in food-plant quality. In addition, food-plant shifts had only minor effects (both positive and negative depending on plant species) on the performance of the herbivore.

- Survival and performance of P. brassicae appears to be constrained more by quantitative than qualitative aspects of the food plant.

138

- Development of the multivoltine gregarious endoparasitoid C. glomerata was also only marginally affected by seasonal and plant-specific variation in the food-plant quality and is confronted with similar quantitative constraints as its host. - Oviposition preference of adult female P. brassicae butterflies declined with plant age (they preferred younger but smaller over older and larger plants). Female P. brassicae butterflies may thus be able to ‘anticipate’ future biomass or quality potential of the plants.

- Pre-adult experience had minor effects on oviposition preference in P. brassicae and had no effect on C. glomerata landing preference.

- Overall conclusion: insect-plant interactions are complex and dynamic. My research has shown that a better understanding of this complexity and of the mechanisms involved can be attained through exploring more intimate trait- mediated aspects of plant and insect life histories.