Smaller is better: my first publication

Not only is our communication class learning to blog, we’re also learning the basic principles of good writing.  One of the basic rules: understand the topic you’re writing about, or your writing will be confusing to yourself and your reader.

So for my first research blog, I’m going to summarize something I am very familiar with: my first and only published paper titled “within-genus size distributions in angiosperms: small is better”.  For almost a year, I worked alongside my supervisor – plant ecologist Dr. Lonnie Aarssen – to develop a little idea that turned into something surprising, exciting, and statistically significant. 

Typical field community dominated by small plants

Traditional plant competition theory states that (i) competition drives the structure of plant assemblages, (ii) plants have few changes to reduce such between-species competition (through niche differentiation), and (iii) bigger plants are better competitors.  This suggests that most natural plant assemblages – from the community to regional level – should be dominated by many large individuals from many large species.

But, empirical evidence has suggested otherwise: right-skewed plant size distributions have been noted at the community and regional level.  A similar trend has also surfaced for leaf and seed sizes.  So, if being big is supposed to be more advantageous – providing better competitive ability – then why are communities, regions, and even populations dominated by the small?

Our first thought: due to phylogeny.  Across the plant size range, maybe the distribution data are not taxonomically independent; i.e. most plants are small within an assemblage because taxa (in this case genera) having many small species are better represented than taxa having many large species.  Or maybe the smaller species belong to a variety of taxa, all having larger generic sizes (i.e. number of species) and therefore higher probability of greater representation.

So to test our hypothesis – that relatedness was controlling the skewed size distributions – we collected plant, leaf, and seed size data from online and bound regional floras for fourteen study genera.  The genera were selected without bias, including genera of different sizes (e.g. genera having twenty species versus those having hundreds of species), different growth forms (e.g. genera composed of only herbs versus those composed of trees, shrubs, and herbs), and geographical location (e.g. genera endemic to one continent versus cosmopolitan genera).  Histograms for each trait were created for each genus, and the results were remarkable.

Figure 1 from Dombroskie and Aarssen demonstrating the right-skewed
plant size distributions for a variety of genera

In almost all cases, plant, leaf, and seed sizes were significantly right skewed within genera.  This indicates that regardless of genera size, growth form, or location, these unbiased genera (likely along with those not included in the study set) are composed of more small species than large.  Even within Acacia – a genus well known for its beautiful, large, trees – most species are relatively small. 

So our initial hypotheses were incorrect.  However, the significantly right-skewed morphology distributions within a genus suggested that phylogeny and evolution were still playing major roles in plant size assemblages: over evolutionary time, there were more generic speciation events that created more small species than large (or more extinction events eliminating more large species than small).  Despite the fact that the first land plants (i.e. gymnosperms) were shrubby, speciation events throughout evolutionary time have favoured the ‘left wall’ of plant size (i.e. sizes approaching zero), even within woody genera.

Don’t get me wrong, being big has its advantages: better seed and fruit dispersal, higher pollination rates, greater support for larger seeds (which are generally more drought resistant) and leaves (which have a greater surface area for light capture), and better overall competitive ability (i.e. resource harvesting).  But, with the apparent higher origination rate of smaller species, there must be something really advantageous, and unobvious, about being small.

Figure 6 from Dombroskie and Aarssen
demonstrating potential evolutionary trend
towards smaller body size

Why being smaller might be superior:

1.   ‘Habitat availability hypothesis’ – throughout evolutionary time, low fertility and high disturbance habitats have been common.  Only the small are capable of surviving and adapting to such poor conditions because their small, resilient body size requires fewer resources for sustenance and reproduction.  Thus, speciation events favoured creation of smaller species

2.   ‘Physical-space-niche hypothesis’ – larger species are not efficient at capturing all of the resources available to them (i.e. light, soil moisture).  This leaves small, heterogeneous patches of resources (i.e. niches) leftover; perfect for a variety of smaller species to utilize.  With the evolution of a large species (and corresponding new niches), it is likely that speciation of several smaller species would follow; creating a higher small to large species speciation ratio.

3.   ‘Fecundity allocation premium hypothesis’, ‘reproductive economy hypothesis’ – although larger species may be capable of producing more seeds/offspring per individual, smaller species actually produce more offspring per unit time per unit plant size (i.e. higher fecundity allocation); smaller species have a lower reproduction size threshold and they outnumber the large species.  Thus, if smaller species are producing more offspring during more reproduction events (i.e. potential speciation events), they have increased chances to create new species that will also be small.

4.  ‘Leafing intensity premium hypothesis’ - smaller species produce smaller leaves (due to reduced support tissue and biomass).  However, if a species makes smaller leaves, it can produce more of them, based on basic biomass allocation constraints (i.e. if you produce larger leaves, you have fewer).  Producing more leaves – each associated with an axillary meristem – increases the species ‘bud bank’; these buds can be allocated to vegetative growth, reproductive growth, or dormancy.  This provides smaller species with greater potential to be adaptive to their often harsh environment, reducing extinction rates.

Our study was the first to remarkably show that within-genus size distributions are not controlling those in natural plant assemblages.  It is fairly obvious that within each genus, speciation events have favoured the production of smaller species (and/or extinction of larger species) due to some associated adaptive advantage; potentially one or all of those mentioned above.  Dr. Aarssen and I are hoping to expand this project together in January – using phylogenetic keys and data – to determine whether or not the most recently evolved species of the genus are, in fact, the smallest (i.e. evolutionary trend of producing smaller and smaller species over time).  Stay tuned for the results!

Source:
Dombroskie S, Aarssen LW (2010) Within-genus size distributions in angiosperms: small is better.  PPEES, DOI: 10.1016/j.ppees.2010.06.002.