Plant-Insect Interactions in the Tropics ZOL/ENT/PLB 485 September 24, 2013

Plant-Insect Interactions in the Tropics

ZOL/ENT/PLB 485September 24, 2013

Examples of Plant-Animal InteractionsPollination HerbivorySeed DispersalSeed PredationPathogensMicrobial Fungal Insect Mimicry

And on, and on…

Types of Biotic Interactions

Mutualism

CommensalismPredation/Parasitism

Competition

Player 1 Player 2

Mutualism – both spp. benefit (but think of it as mutual exploitation)Commensalism – 1 spp. benefits, and other gets no benefit/harmPredation/Parasitism – 1 spp. benefits, and other is harmed/killedCompetition – both spp. (or individuals) negatively impact the other


Player 1 Player 2

Mutualism


Competition

+ +



Mutualism


Competition

+ +++

Player 1 Player 2



Mutualism


Competition

+ ++++

Player 1 Player 2



Mutualism


Competition

+ ++++

Player 1 Player 2

A B

Resource 1Re

sour

ce 2 B

A


Why should we care?

Important in agriculture and maintaining biodiversity

Mechanisms of co-existence

Origins of diversity

They’re super cool!

Impo

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LD

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“Only in the tropics…”

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”


http://proceedings.esri.com/library/userconf/proc99/proceed/papers/pap308/p30805.gif

Soil MoistureDry Wet

Sunl

ight

Soil Phosp

horous

High

Low


http://proceedings.esri.com/library/userconf/proc99/proceed/papers/pap308/p30805.gif

Herbivore PressureLow High

Sunl

ight

Soil Phosp

horous

High

Low


Biotic interactions can act as additional niche axes

Niche partitioning enables species co-existence among species

Figure 2 from Mayfield and Levine (2010) – Ecol Letters


Negative density dependence- Species population growth rates are limited by effects

associated with high density(frequency) of individuals

Mayfield and Levine (2010)

Competition/Crowding Predators & Pathogens

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

Janzen-Connell Hypothesis

Probability of Survival

Janzen-Connell Hypothesis

Figure 1 from Janzen (1970) – AmNat(w/ my colorful adaptations!)

Lots of seed/seedling mortality

Less seed/seedling mortality

Probability of seed dispersal decreases with increasing distance from parent

Seedling Sweet Spot

Plant-Insect Interactions and Origins of DiversitySelective pressures that are the result of biotic interactions drive evolution, and ultimately speciation

BPopulation

APopulation

ASpecies

ASpecies Species

B

(Selective Agent)

(Selective Target)

Plant-Insect Interactions and Origins of DiversityWe can use a phylogenetic approach to view past evolutionary events

Circle flower shape

Ancestral state = Square flower shape

A

A

B

Plant-Insect Interactions and Co-evolutionIf there are reciprocal selective pressures exerted by both interactors in the relationship, you can get co-evolution

Selective Target

Selective Agent

Selective Agent

Selective Target

Plant-Insect Interactions and Co-evolutionAgain, let’s take a look at this past evolution using a phylogenetic approach

Ancestral state

Ancestral state

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]

“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).

Just so you know…Darwin has almost always said it first…

Top: https://news.brown.edu/files/article_images/Darwin1.jpgBottom: https://upload.wikimedia.org/wikipedia/commons/4/41/Humle.jpg

CAUTION! When is it co-evolution?Janzen, Daniel H. 1980. When is it coevolution? Evolution 34: 611-612.

1. Just because a pair of species have traits that are mutualistically congruent, doesn’t mean they have co-evolved

2. 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

3. “…it is likely that many defense traits of plants were produced through co-evolution with animals no longer present…” (Janzen 1980)

Just a few (very few) examples…

1. Inga diversification in response to herbivores2. Bursera3. Complex relationships of figs and their fig wasps4. Ant-Acacia relationships: The Ant Defenders!!!5. Lepidoptera evolution

With these examples, keep in mind:

a) How did these interactions arise?b) What do these interactions mean with regard to

species diversity and co-existence?c) Is there enough evidence to support conclusions?

Plant – Herbivore Interactions

Plant DefensesPhysical Defenses• Thorns/prickles• Trichomes• Toothed leaves• Tough leaves• Exudate/latex

Compositional Defenses• Chemistry

o Alkaloids, tannins, phenolics, cyanogenic glycosides, etc…

• Fiber content/nutritional contentBehavioral Defenses

• Ant defense• Timing of

leafing/masting

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

Inga EvolutionRichardson et al. 2001. Rapid diversification of a species-rich genus of Neotropical rain forest trees. Science 293: 2242-2245.

Inga – A pairwise study in defense strategiesColey et al. 2005. 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 associationsAnts at EFNsLeaf size and growth rateLeaf secondary metabolites

Inga – A pairwise study in defense strategiesMain 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

Defense (I. goldmanii)

• Opposite patterns of I. umbellifera

Inga – Genus wide chemical defenses

Kursar et al. 2009. The evolution of antiherbivore defenses and their contribution to species coexistence in the tropical tree genus Inga. PNAS 106: 18073 – 18078.

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

Figure 2

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!)

Inga – Genus wide chemical defensesMain Results

IngaInga – Genus wide chemical defensesMain Results

Figure 3

Inga – Genus wide chemical defensesMain 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

Inga – Genus wide chemical defensesMain ConclusionsInga 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

Plant – Pollinator Interactions

Figs and Fig Wasps (and their “friends”…)

Photo by Diana Durance

Figs (Ficus – Moraceae) and their fig wasps are global in distribution

There are over 750 species worldwide!

http://www.youtube.com/watch?v=JfkiYfrStrU



“…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)

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.

Figs and non-pollinating wasps

Main Conclusions: Fig fitness (as measured by fruit crop production) was much lower for figs with Idarnes

Plant – Ant Defense Interactions

Ant-Acacia Interactions

http://www.youtube.com/watch?v=Xm2qdxVVRm4




Palmer et al. (2008) - Science

Ant-Acacia InteractionsStudy 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

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%.

Crematogaster nigriceps: a defender; prunes axillary buds and kills apical meristems, which reduces likelihood of contact with treesoccupied by hostile colonies

Tetraponera penzigi, an intermediate protector; destroys itshost-plants’ nectaries: a “scorched-earth” strategy to reduce competition


Figure 1

Grey bars represent presence of herbivores, white represent absence

Figure 2 Figure 3

Figure 4


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.

Plant – Insect Interactions (herbivory, pollination,

ant defense, oh my!)

Lepidopterans – Heliconius & Passiflora“Lepidopterans are (to plant species) evolutionary examples of Dr. Jekyll and Mr. Hyde” (Kricher, pg. 308)

Heliconius & PassifloraA Suite of Biological Interactions:

Heliconia butterflies pollinate PassifloraHeliconia 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

Plant – Insect Interactions on a Global Scale

Swallowtail Biodiversity

Study Objectives: Use a phylogenetic approach to investigate the evolutionary process responsible for the LDG in swallowtail butterflies (Papilionidae)

Distributions across the globe

Correlated Evolution

Why should we care?

Important in agriculture and maintaining biodiversity

Mechanisms of co-existence

Origins of diversity

They’re super cool!

Impo

rtan

t fo

r the

LD

G

“Only in the tropics…”

Biotic Interactions and the LDG

Mittelbach et al. 2007. Evolution and the latitudinal diversity gradient: speciation, extinction and biogeography. Ecol Letters 10: 315-331.

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

Biotic Interactions and the LDGStudy 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!

Documents

Plant-Insect Interactions in the Tropics ZOL/ENT/PLB 485 September 24, 2013