Griffith University, school of environment
How will increased drought affect herbivory-based insect communities in Australia’s tropical rainforests?
PhD Confirmation Report
Claire Gely
February 2017
Supervisors
Professor Brendan Mackey
Professor Nigel Stork
Abstract
Key words: insect herbivory, drought, climate change, insect diversity
Contents
SECTION 1 4
1.1 Introduction 4
1.2 Insect herbivory in tropical rainforests 4
1.3 Insect-plant interaction 7
1.4 Impact of climate change and drought on insects communities 15
1.5 Research questions 24
SECTION 2 25
2.1 Study site and drought experiment 25
2.2 Thesis chapters plan 29
2.3 Chapter 1: An examination of the effects of increased drought on insect abundance and diversity 30
2.4 Chapter 2: Ant mosaics and feeding behaviour in increased drought conditions 32
2.5 Chapter 3 : Herbivory rates in increased drought conditions 36
2.6 Chapter 4: Woodboring and feeding guilds 36
SECTION 3 38
Progress to date 38
Possible papers and conferences 38
Timetable 38
Budget 38
References 39
SECTION 1
1.1 Introduction
Understanding and predicting the impact of climate change on species and ecosystem processes in rainforests is a critical scientific challenge. Current climate change scenarios (which scenarios?is it all scenarios? Source) predict an extended dry season in Australian tropical rainforests. This PhD research project will examine how herbivorous insects respond to drought at the rainforest canopy and subcanopy levels. It will explore the impact of increased and prolonged drought on herbivory and insect community structure as water availability in plants is reduced.
The aims of this study are to examine the effects of increased drought: 1) on insects abundance and diversity; 2) on herbivory processes in relation to plant phenology, resource availability and variation in available water; 3) on ant mosaics and ants diet through variation in resource availability and food quality; 4) on different insect feeding guilds, including wood-boring insects.
1.2 Insect herbivory in tropical rainforests
Insects are the most successful and prolific animals on the planet, with an estimated ??-?? million species (source me !) on Earth. As ecologist Robert May puts it: ‘To a rough approximation, and setting aside vertebrate chauvinism, it can be said that essentially all organisms are insects’. Herbivorous insects not only make up an important part of biodiversity, they are also vital for maintaining forest ecosystem (Ewers et al., 2015) (Coley & Barone, 1996; Lowman & Rinkman, 2004; Schowalter, 2011). They play a crucial role as consumers within tropical ecosystems, since the majority of the herbivore damage occurring in the tropics is carried out by invertebrates (Coley & Barone, 1996). The loss of forest trees to insects is considerable and it is estimated that more than 10% of the plant production in tropical forests is consumed annually by herbivores (Coley, Bryant & Chapin, 1985).
Herbivory rates are higher in tropical rainforests than temperate broad-leaved forests (Coley 1998), despite the fact that tropical leaves are less nutritious and better defended (Coley 1998). Since both mature and young leaves of tropical species are better defended chemically and physically, the higher rates of herbivory in tropical forests must reflect greater overall herbivore pressure (coley 1998)
Measuring the intensity of herbivory is often difficult (Coley, Aide 1991, Comparison of herbivory and plant defences in temperate and tropical broad leaved forests, Coley & Barone, 1996, Herbivory and plant defenes in tropical forests, Schowalter TD 2000, Insect ecology, chapter 12 herbivory, academic pres, san diego). Estimates of losses due to herbivory can differ widely among methods used, therefore figures obtained for different herbivore-plant associations are in many cases difficult to compare.
The most common way to measure leaf damage is discrete sampling of leaves, usually towards the end of the growing season in order to provide a snapshot of cumulative leaf loss over the entire season. Leaf litter sampling is another way to quantify insect herbivory. The inconvenient with these methods is that it do not consider leaves that are entirely consumed or, in the case of living leaves sampling, leaves that have abscissed due to heavy herbivory damage (Lowman, 1984; De la Cruz & Dirzo, 1987, Ruiz-Guerra et al. 2010 Lois). Moreover these methods do not take into account how leaf damage is expanding as the leaf grows. One way to overcome those drawbacks is to measure herbivory over time by marking leaves and photographing (or scanning) these same leaves at a regular time interval.
This PhD research work is focused on above-ground insects. However above-ground and below-ground herbivores may compete (Bezemer, Wagenaar, Van Dam, Wackers, Interactions between above and below ground insect herbivores as mediated by the plant defense system, Tindall, Stout, Plant-mediated interactions between the rice water weevil and fall armyworm in rice). Below-ground herbivores can indirectly influence the presence of folivores. For instance, below-ground herbivory may modify plant characteristics such as production of foliar extra-floral nectar, which is well known to stimulate visits by carnivorous enemies of the folivores (Wackers, Bezemer, 2003, Root herbivory induces an above ground indirect defence).
1.3 Plant defences to herbivory
Plant species have developed nutritional, physical, and chemical defence mechanisms which prevent herbivory (Coley & Barone, 1996). Those defences may be direct such as toxins, digestibility reducers or thorns, or indirect, by attracting the natural enemies of herbivores and improving the performance of carnivorous insects. (Dicke, Bruin, 2001, Multitrophic effects of herbivore-induced plant volatiles in an evolutionary context, Strauss, 1997, Floral characters link herbivores, pollinators and plant fitness). Apart from constitutive defences, plants may also possess inducible defences. Inducible defences are generally regarded as less costly in resources, since they are only mobilized when the plant is exposed to herbivory. An additional benefit may be that they confront herbivores with a variable phenotype that reduces the possibilities for the herbivore to adapt to the plant defence (Agrawal, Karban , 1999, Why induced defences may be favored over constitutive strategies in plants, Gardner, Agrawal, 2002, Induced plant defence and the evolution of counter-defences in herbivores).
What would happen in drought?Inducible defences constantly solicited ? blabla : It is unknown how plants will cope with with herbivory attack in increased drought conditions, and if they will favour the solicitation of inducible defences against herbivory or use their energy for other functions.
Leaf toughness is a major defence mechanism against insect herbivory. Cellulose, lignine, suberin, and callose together with sclerenchymatous fibres allow a plant to be resistant to mechanical injury as well as to the tearing actin of mandibles or the penetration of piercing-sucking mouthparts and ovipositors of herbivorous insects. Coley argues that leaf toughness is the best predictor of interspecific variation in herbivory rates (Coley, 1983, Herbivory and defensive characteristics of tree species in a lowland tropical forest).
Nitrogen compounds (alkaloids, glucosinolates), terpenoids and phenolics can retard the development, intoxicate and kill herbivorous insects, as well as making the leaves unpalatable for insects. (Greene, 1989, A diet-induced developmental polymorphism in a caterpillar). Van Beek and de Groot 1986, Terpenoid antifeedants, part I, an overview of terpenoid antifeedants of natural origin / Caballero, Castanera, Ortego, Fontana, Pierro, Savona, Effects of ajugarins and related neoclerodane diterpenoids on feeding behaviour of Leptinotarsa decemlineata and Spodoptera exigua larvae). Haggstrom, Larsson, 1995, Slow larval growth on a suboptimal willow results in high predation mortality in the leaf beetle Galerucella lineola reviewed in Karban and Myers 1989 Induced plant responses to herbivory, Karban and Baldwin 1997 induced responses to herbivory).
I don’t like the structure of this paragraph which is too much a list of al the plant defences
Host-plant phenology is crucial for the performance of herbivorous insects. More than 400 000 herbivorous insect species live on roughly 300 000 vascular plant species (strong et al 1984 Insects on plants. Community patterns and mechanisms). Climate changes disrupting the synchronization between insect and plant phenology may strongly influence insect population dynamics (Feeny, 1976, Plant apparency and chemical defence/ Feeny 1970, Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars).
Paragraph on plants defences under drought.
1.4 Impact of climate change and drought on insects communities
1.4.1 Drought prediction in current CC models
1.4.2 Experimental vs observational drought data
1.4.3 Impact of drought on plants in tropical rainforests drought experiments
1.4.4 Impact on insects : drought non studied
1.5 Research questions
SECTION 2
2.1 Study site and drought experiment
2.2 Thesis chapters plan
2.3 Chapter 1: An examination of the effects of increased drought on insect abundance and diversity
2.3.1 Methods
2.3.2 Results
2.3.3 Discussion
2.4 Chapter 2: Ant mosaics and feeding behaviour in increased drought conditions
2.5 Chapter 3: Herbivory rates in increased drought conditions
2.5.1 Methods
2.5.2 Statistical analysis of herbivory rates
2.6 Chapter 4: Woodboring and feeding guilds
SECTION 3
3.1 Progress to date
3.2 Possible papers and conferences
3.3 Timetable
3.4 Budget
References
Coley, P. D., & Barone, J. A. (1996). Herbivory and plant defenses in tropical forests. Annual Review of Ecology and Systematics, 27, 305-335. doi:DOI 10.1146/annurev.ecolsys.27.1.305
Ewers, R. M., Boyle, M. J. W., Gleave, R. A., Plowman, N. S., Benedick, S., Bernard, H., . . . Turner, E. C. (2015). Logging cuts the functional importance of invertebrates in tropical rainforest. Nature Communications, 6. doi:ARTN 6836
10.1038/ncomms7836
Lowman, M. D., & Rinkman, B. H. (2004). Forest Canopies (2nd Edition ed.): Academic Press.
Schowalter, T. D. (2011). Insect Ecology: An Ecosystem Approach, 3rd Edition. Insect Ecology: An Ecosystem Approach, 3rd Edition, 1-633.