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Plant-Insect Interactions in the Tropics
ZOL/ENT/PLB 485 September 24, 2013
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Examples of Plant-Animal Interactions
Pollination Herbivory Seed Dispersal Seed Predation Pathogens Microbial Fungal Insect Mimicry And on, and on…
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Types of Biotic Interactions
Mutualism – both spp. benefit (but think of it as mutual exploitation) Commensalism – 1 spp. benefits, and other gets no benefit/harm Predation/Parasitism – 1 spp. benefits, and other is harmed/killed Competition – both spp. (or individuals) negatively impact the other Mutualism Commensalism Predation/ Parasitism Competition Player 1 Player 2
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+ Types of Biotic Interactions Mutualism Commensalism Competition
Mutualism – both spp. benefit (but think of it as mutual exploitation) Commensalism – 1 spp. benefits, and other gets no benefit/harm Predation/Parasitism – 1 spp. benefits, and other is harmed/killed Competition – both spp. (or individuals) negatively impact the other Player 1 Player 2 Mutualism Commensalism Predation/ Parasitism Competition +
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+ Types of Biotic Interactions Mutualism Commensalism Competition
Mutualism – both spp. benefit (but think of it as mutual exploitation) Commensalism – 1 spp. benefits, and other gets no benefit/harm Predation/Parasitism – 1 spp. benefits, and other is harmed/killed Competition – both spp. (or individuals) negatively impact the other Mutualism Commensalism Predation/ Parasitism Competition + Player 1 Player 2
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+ Types of Biotic Interactions Mutualism Commensalism Competition
Mutualism – both spp. benefit (but think of it as mutual exploitation) Commensalism – 1 spp. benefits, and other gets no benefit/harm Predation/Parasitism – 1 spp. benefits, and other is harmed/killed Competition – both spp. (or individuals) negatively impact the other Mutualism Commensalism Predation/ Parasitism Competition + Player 1 Player 2
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+ Types of Biotic Interactions Mutualism Commensalism Competition
Mutualism – both spp. benefit (but think of it as mutual exploitation) Commensalism – 1 spp. benefits, and other gets no benefit/harm Predation/Parasitism – 1 spp. benefits, and other is harmed/killed Competition – both spp. (or individuals) negatively impact the other Mutualism Commensalism Predation/ Parasitism Competition + Player 1 Player 2 B A A B Resource 1 Resource 2
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Why should we care? Important in agriculture and maintaining biodiversity Mechanisms of co-existence Origins of diversity They’re super cool! Important for the LDG “Only in the tropics…” How do these interactions affect the distribution of diversity, both locally and globally, and across temporal scales? Super cool organisms! Cool from a basic science point of view, but also… Easy to get people excited about these charismatic creatures.
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Plant-Insect Interactions and Mechanisms of Co-existence
Species “niche”: the sum of all the environmental factors acting on an organism (Hutchinson 1944) An “n-dimensional hypervolume” (Hutchinson 1957) We can consider environmental axes that act as limiting factors as “niche axes”
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Plant-Insect Interactions and Mechanisms of Co-existence
High Sunlight Soil Phosphorous Note that species diversity not only represents species richness, but can also allude to the diversity in form and function that aid co-existence Low Dry Wet Soil Moisture
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Plant-Insect Interactions and Mechanisms of Co-existence
High Sunlight Soil Phosphorous Biotic Interactions creating niche axes Low Low High Herbivore Pressure
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Plant-Insect Interactions and Mechanisms of Co-existence
Biotic interactions can act as additional niche axes Niche partitioning enables species co-existence among species Biotic Interactions creating niche axes Figure 2 from Mayfield and Levine (2010) – Ecol Letters
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Plant-Insect Interactions and Mechanisms of Co-existence
Negative density dependence Species population growth rates are limited by effects associated with high density(frequency) of individuals Competition/Crowding Predators & Pathogens Mayfield and Levine (2010)
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Plant-Insect Interactions and Mechanisms of Co-existence
Janzen-Connell Hypothesis: tree species richness is kept high due to the increased probability of mortality of seeds and seedlings growing nearer to their parent tree Negative density dependence scenario Often, predators and pathogens are specialized Janzen 1970 and Connell 1971
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Probability of Survival
Janzen-Connell Hypothesis Probability of Survival
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Janzen-Connell Hypothesis
Lots of seed/seedling mortality Less seed/seedling mortality Probability of seed dispersal decreases with increasing distance from parent Seedling Sweet Spot Figure 1 from Janzen (1970) – AmNat (w/ my colorful adaptations!)
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Plant-Insect Interactions and Origins of Diversity
Selective pressures that are the result of biotic interactions drive evolution, and ultimately speciation A Species A Population B Population A Species Species B (Selective Target) (Selective Agent)
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Ancestral state = Square flower shape
Plant-Insect Interactions and Origins of Diversity We can use a phylogenetic approach to view past evolutionary events A B Ancestral state = Square flower shape Circle flower shape
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Plant-Insect Interactions and Co-evolution
If there are reciprocal selective pressures exerted by both interactors in the relationship, you can get co-evolution Selective Target Selective Agent
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Plant-Insect Interactions and Co-evolution
Again, let’s take a look at this past evolution using a phylogenetic approach Ancestral state Ancestral state
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Plant-Insect Interactions and Co-evolution
We can see how co-evolution can drive species diversification (ie: lineage splitting), but note that it can also drive continued evolution within a lineage without leaving many descendants Note, these two scenarios are really not mechanistically different, but we may observe different patterns of species diversity today “Evolutionary Arms Race” Red Queen Hypothesis] Draw this on the board!
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Just so you know…Darwin has almost always said it first…
“The tubes of the corollas of the common red and incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty glance appear to differ in length; yet the hive-bee can easily suck the nectar out of the incarnate clover, but not out of the common red clover, which is visited by humble-bees alone” (Darwin, On The Origin of Species). Top: Bottom: Humle.jpg
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When is it co-evolution?
CAUTION! When is it co-evolution? Janzen, Daniel H When is it coevolution? Evolution 34: Just because a pair of species have traits that are mutualistically congruent, doesn’t mean they have co-evolved Parasites/predators could have evolved along with the plant they parasitize, or elsewhere, and then dispersed to their new host plant that is not “evolutionary informed” of this newly arrived predator’s tactics First, note that not all the plant-insect interactions are the result of evolutionary mechanisms, or at least these really intricate evolutionary relationships we observe in some of these interactions. We’ll be talking about these really neat plant-insect interactions, many of which are so specialized that it’s hard to believe that they aren’t the product of co-evolution, but this is a process that is hard to actually provide substantial evidence for. Evolution, is awesome, and we know that the diversity of life we see today is the result of evolution, but in most cases, the outcomes we observe are the result of past selection…and thus hard to identify. So, we’ll be looking at these natural history studies, as well as some studies that have investigated these relationships more in depth, but be wary. Which studies do you think provide substantial evidence for their conclusiosn? “…it is likely that many defense traits of plants were produced through co-evolution with animals no longer present…” (Janzen 1980)
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Just a few (very few) examples…
Inga diversification in response to herbivores Bursera Complex relationships of figs and their fig wasps Ant-Acacia relationships: The Ant Defenders!!! Lepidoptera evolution With these examples, keep in mind: How did these interactions arise? What do these interactions mean with regard to species diversity and co-existence? Is there enough evidence to support conclusions?
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Plant – Herbivore Interactions
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Plant Defenses Physical Defenses Thorns/prickles Trichomes
Toothed leaves Tough leaves Exudate/latex Compositional Defenses Chemistry Alkaloids, tannins, phenolics, cyanogenic glycosides, etc… Fiber content/nutritional content Behavioral Defenses Ant defense Timing of leafing/masting
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Inga (Fabaceae) (ie: the “pea family”) Over 300 species
Neotropical in range Recent and rapid diversification (Richardson et al. 2001) Lineage only 10 million years old Many species arising only 2 mya Variety of herbivore defense strategies
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Richardson et al Rapid diversification of a species-rich genus of Neotropical rain forest trees. Science 293: Inga Evolution
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Inga – A pairwise study in defense strategies
Coley et al Divergent defensive strategies of young leaves in two species of Inga. Ecology 86: 2633 – 2643. Question: Is there a difference in defense strategies between two closely related species of Inga? Data Collected: Herbivore-host associations Ants at EFNs Leaf size and growth rate Leaf secondary metabolites
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Inga – A pairwise study in defense strategies
Main Results: The two species compared had similar levels of herbivory There was a difference in defense strategy: Escape vs. Defense Escape (I. umbellifera) Lower levels of defense compounds Lower investment in recruitment of ants Synchronous leafing Faster leaf expansion Lower chlorophyll content “A fork in the evolutionary road” (Kricher) Defense (I. goldmanii) Opposite patterns of I. umbellifera
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Inga – Genus wide chemical defenses
Kursar et al The evolution of antiherbivore defenses and their contribution to species coexistence in the tropical tree genus Inga. PNAS 106: – Study Objectives: evaluate the evolution of antiherbivore defenses and their possible contribution to Inga coexistence Approach: 37 spp. in Panama & Peru Characterized defense mechanisms Evaluated evolution of these mechanisms in a phylo context
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Inga – Genus wide chemical defenses Main Results
Variation in antiherbivore defense In all, 13 distinct “chemotypes” Variation in leaf expansion and chlorophyll content of new leaves (Fig 2) Much variation in ant abundance and EFN visitation (20-fold difference!) Figure 2
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Inga – Genus wide chemical defenses Inga Main Results
Phylo signal in developmental traits, but no signal in ant traits or in chemical traits Species in bold are “defense” Developmental, chemical, and ant traits are orthogonal Figure 3
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Inga – Genus wide chemical defenses Main Results
Evaluation of Coexistence: NOTE: Negative values mean members in the community are similar, positive values mean they are dissimilar At both sites, the species were more different in defensive traits than expected by chance Figure 4
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Inga – Genus wide chemical defenses Main Conclusions
Inga species display much variation in all three “trait syndromes” (ie: developmental, chemical, and ant defense strategies) There is evidence of much trait convergence for chemical and ant defenses, but not for developmental defenses All three defenses are orthogonal, meaning they potentially represent 3 independent niche axes important for evolution Species co-occurring at a site are more dissimilar in defense traits than expected, suggesting niche partitioning
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Plant – Pollinator Interactions
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Figs and Fig Wasps (and their “friends”…)
Figs (Ficus – Moraceae) and their fig wasps are global in distribution There are over 750 species worldwide! Photo by Diana Durance
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“…were a human to inhabit such a place it would be an utterly dark and crowded room filled with jostling people, some of whom would be homicidal maniacs wielding sharp knives” (Kricher, paraphrasing Hamilton, 1979)
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Figs and non-pollinating wasps
Study Objectives: To evaluate the role that Idarnes, a non-pollinating fig wasp, has on the overall fitness of its host figs.
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Figs and non-pollinating wasps
Main Conclusions: Fig fitness (as measured by fruit crop production) was much lower for figs with Idarnes
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Plant – Ant Defense Interactions
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Ant-Acacia Interactions
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Ant-Acacia Interactions
Palmer et al. (2008) - Science
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Ant-Acacia Interactions
Study Objectives: To evaluate how the removal of large herbivores in an African savanna impacted the dynamics of an ant-Acacia mutualism Crematogaster mimosae : very aggressive; needs domatia C. sjostedti: less aggressive; does not use domatia, but plant stems for housing Crematogaster nigriceps: a defender; prunes axillary buds and kills apical meristems, which reduces likelihood of contact with trees occupied by hostile colonies Tetraponera penzigi, an intermediate protector; destroys its host-plants’ nectaries: a “scorched-earth” strategy to reduce competition Under natural conditions, C. mimosae is the most abundant ant symbiont, occupying ~52% of all trees at our sites, whereas C. sjostedti occupies ~16% of host plants. C. nigriceps occupies ~15% and T. penzigi occupies ~17%.
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Ant-Acacia Interactions
Figure 1 Grey bars represent presence of herbivores, white represent absence
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Figure 2 Figure 3 Figure 4
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Ant-Acacia Interactions
Main Conclusions: Removal of large herbivores in this community can greatly affect the mutualism between ants and their plants, and results in decreased fitness of the Acacia trees.
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Plant – Insect Interactions (herbivory, pollination, ant defense, oh my!)
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Lepidopterans – Heliconius & Passiflora
“Lepidopterans are (to plant species) evolutionary examples of Dr. Jekyll and Mr. Hyde” (Kricher, pg. 308)
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Heliconius & Passiflora
A Suite of Biological Interactions: Heliconia butterflies pollinate Passiflora Heliconia caterpillars are Passiflora herbivores, and can greatly reduce fitness due to folivary Passiflora has many defenses to reduce impact of herbivory by Heliconia Chemical compounds in leaves Production of extrafloral nectaries Egg mimics on leaves But…not only are the caterpillars undeterred by the chemical compounds, it is thought that these compounds are sequestered and used as a defense in adult butterflies
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Plant – Insect Interactions on a Global Scale
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Swallowtail Biodiversity
Study Objectives: Use a phylogenetic approach to investigate the evolutionary process responsible for the LDG in swallowtail butterflies (Papilionidae)
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Distributions across the globe
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Correlated Evolution Figure 2 Phylogenetic relationships of 203 swallowtail species, with evolution of their host plant associations. The tree is a 50% majority rule consensus based on Bayesian analysis, with branch lengths proportional to absolute ages. Outside the phylogeny, two coloured circles represent the different taxonomic groups as shown by the left-corner boxes. Coloured branches on the tree, as indicated in the lower right corner, map the evolution of host plant association (outgroups not shown). At host shifts, a pie chart displays the probability of each plant family. Small black squares on the phylogeny indicate node support with both BV and PP equal to or greater than 70% and 0.95 respectively. Asterisks indicate the illustrated species.
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Why should we care? Important in agriculture and maintaining biodiversity Mechanisms of co-existence Origins of diversity They’re super cool! Important for the LDG “Only in the tropics…” How do these interactions affect the distribution of diversity, both locally and globally, and across temporal scales? Super cool organisms! Cool from a basic science point of view, but also… Easy to get people excited about these charismatic creatures.
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Biotic Interactions and the LDG
Tropics have more “niche space” to occupy than do the temperate zones Tropics have higher diversification rates There has been a longer time for diversification to occur Mittelbach et al Evolution and the latitudinal diversity gradient: speciation, extinction and biogeography. Ecol Letters 10:
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Biotic Interactions and the LDG
Study Objective: Review the literature and determine if studies showed importance of interactions (a) greater at lower lats, (b) greater at higher lats, (c) no evidence of a difference Main Results: From 39 studies, found only one instance where the biotic interaction was deemed “more important” in temperate regions But, obviously this is a limited dataset, and only a review of the literature. Much more work needs to be done!
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