The nature of the plant community: a reductionist view

Nitin Sekar

Ryan Bailey

Introduction

• “In this chapter we follow our reductionist approach by considering the plant-plant interactions that lead to those restrictions.”

• These plant-plant interactions will theoretically set restrictions to community development… Keep an eye out for how Bastow’s ideas could link to a BIG PICTURE of community…

• Starts with his chapter’s recurring plant/animal comparisons: “Animal ecologists think in terms of adding one species to another …” whereas where one plant species gains, another loses?

Really?

2. Interference: negative effects between plants

• Describes interference as a broad category, including competition (again, reverse from animal ecologists), allelopathy, parasitism, etc.

• Essentially, any sort of negativeeffect…

CompetitionBegon et al. (1996): “Competition is an interaction between individuals, brought about by a shared requirement for a resource in limited supply, leading to a reduction in the survivorship, growth and/or reproduction of at least some of the competing individuals concerned”

Competition must also…

- Be for “some limiting resource”- Has been described as “via reaction” (he “notes” this)

The authors do not believe in competition for space; space is a surrogate. Physical contact is “autogenic disturbance.”

More on competition…• Competitive exclusion: no two species in the same niche…

could be used to help define community, if niche space can be defined…

• No difference between plants and animals in competition, except that competition in plants affects “size before density”– the bigger, the more competitive?

• Describes savanna water competition, competition for carbon dioxide– identifying the limiting resource could help in defining community

• Cumulative competition, where results of prior competition change a plant’s ability to compete– linked to plasticity of plant phenotype

• “common species tended to be stronger competitors than rare species, and this was significant at the low level of fertility” – to animal ecologist, success is result of competition.

Allelopathy“a toxin [is] produced by plants is either leached or volatilised from their living parts” to the detriment of other species’ individuals

Most plants seem to have some sort of toxic compound…

Could allelopathy evolve via competition? Why not!? “Accidental”?

How could allelopathy, if as common as some scientists think, helpdefine plant communities? Are communities just groups of plants mutually tolerant to each others toxins?

Other forms of interference…

• Parasitism

• Pest carriers

• Red/Far Red ratio…

Parasites can infest almost any part of plant, hurt competition, etc.– surprisingly basic

Plant-plant competition through the medium of some sort of infection– some cool examples

REALLY interesting… using ratio as an indication that competition is ‘imminent’

LEAF LITTER!!!

• VERY EXCITING!

• Has it mostly evolvedfor species selection?

• Can the unique comb-ination of different leaflitter in various areas help define a commun-ity?

• “potential wider evolutionary and synecological significance…”

• Seems tied into climate and fire…• "There is some evidence that leaf litter combined from

several species tend to decompose faster than single-species litter”

The Effects of Litter…

• Has an impact on community succession…

• Deep litter can retard succussion (evidence!?)

• High nutrients can favor faster growing species

• Can block light from seedlings

• Other species depend on the nutrient combination/character of certain tree litter, etc.

• Leachate from litter could be allelopathic

•Peat, nutrient cycling, etc…

With all these effects, could leaf litter have evolved to be something much more significant than we

had ever imagined!?!...

• How could leaf litter, or anything, evolve when it benefits more than just the individual with the trait?– Kin selection?– Linked traits (excretion, ability to

metabolize/withstand)– Community selection? – “the concept is that this

reaction can lead to the evolution of secondary characters if an organism modifies its own environment, and the same induced environmental change is re-imposed for sufficient generations to serve as a significant source of selection”, and that this could lead to “selection between communities…”

• Notice how vague definition of community hinders us here.

6. Plant-plant interactions mediated by other trophic levels

Some interactions with other species seem more likely to contribute significantly to community-wide development than others…

Less significant?:- Single plant/poll-

inator interactions- Palatability/herb-

ivore defenses- Mutualisms with

insect species fordefenses…

Any arguments!?

ON THE OTHER HAND…

Some plant-plant interactions involving other species could demonstrate associations that help one set of species form a community more often than some other set… • mycorrhiza – Can help numerous plant species

sequester nutrients; a genuine symbiotic relationship that could allow participating plant species to out compete others and form a community…

•Staggered flowering times for shared pollinators – again, sharing a symbiont benefits all competitive participants

•Specialized herbivory/disease effects– influence dispersion of a plant species, forces a level of diversity

Subvention

• “Positive effects by some species on the survival and/or growth of others.” (Clements)

• Neglected until recently

• Two species normally aren’t absolutely dependent on each other for survival

--Juncus gerardii (rush) and Iva frutescens (marsh elder) example

• Benefaction: one species benefits, no harm to other

--wind and temperature shelter

--litter increases water-holding capacity

--co-evolution?--categorization of nutrient leakage

(Bastow says benefaction)--Any evidence for ultra violet light

benefaction?

• Mutualism: both species benefits• Facilitation: one species benefits, other species

is at disadvantage– Bastow gives no clear examples…

How does the “talking trees” mechanism impact our view of communities?

Are subventions accidents of evolution?

Autogenic Disturbance• Changes through the growth and death of

plant parts

• “Vegetation, by its nature, disturbs itself.”

• More predictable than allogenic disturbances so “selection for adaptation to it is more likely to occur.”

Examples

• “Gentle contact between plants can affect their growth”

• Growth of branches, etc. creates new habitat

• Crown shyness• Gravity, tree fall• Lianas Can these factors be used to define

community?

Litter-herbivore Interactions• Herbivorous grazing results in less litter

– Can enhance species richness– Bastow concludes that competition will be

more intense under heavy grazing

• Assembly rules should include litter– Strongest evidence for assembly rules is

found in uniformly and/or heavily grazed areas

• Litter-fire interactions: combination of autogenic and allogenic disturbances

• Litter-herbivore-fire interactions: grazing can decrease fire frequency, but increase fire intensity– Negative and positive processes

-How are communities shaped by these interactions

Conclusion

• “Plant community comprises continual renewal—greater than the habitat can support. The negative and positive interactions…moderate those processes.”

• For whom is this chapter written?..– Text seems designed for seminar-style

classes…– Buffet-style presentation for people interested

in the highlights of interactions…

Final Q’s…

• Do the interactions described in this chapter help us in our definition of community?

• Bastow is so critical of others who make conclusions without evidence– are his general proposals well-supported enough to include in a text book?