Monthly Archives: November 2013

Signature of dispersal on community structure

This post was contributed by Rodrigo Barbosa Ferreira who is a PhD candidate in Dr. Karen Beard’s lab. Rodrigo is studying the effects of habitat fragmentation on bromeliad-dependent frogs in Brazil, and was recently awarded the CNR graduate researcher of the year. Take it away Rodrigo.

We recently discussed two papers (listed below) on the role of dispersal limitation:

  • Shurin, J. 2000. Dispersal limitation, invasion resistance, and the structure of pond zooplankton communities. Ecology. 81:3074–3086. (pdf)
  • Seidler, T. G., and J. B. Plotkin. 2006. Seed dispersal and spatial pattern in tropical trees. PLoS Biology 4:2132–2137. (pdf)

During our  discussion we attempted to answer some interesting questions such as:
– What is the driving factor structuring a given community in a given area?
– How important is dispersal for structuring communities?
– Is biological diversity an important factor preventing invasion?

The Shurin (2000) paper is an important one to discuss because the author empirically tested two fundamental paradigms in the emerging field of Invasion Ecology. Specifically, Shurin discusses both the role of dispersal and interspecific interactions on structuring the community composition of zooplankton.

Shurin (2000) begins his abstract stating that “for a species to colonize a site it must both arrive there by dispersal from another site and maintain positive population growth in the local environment”. He carried out two experiments which compared the importance of dispersal ability vs. native community resistance in structuring a community. Using zooplankton as study taxon, Shurin set up enclosures in natural ponds and manipulated several additional ponds. He added invasive species to the local communities and evaluated the establishment of invasive populations through time. In this experiment, he found that only a small portion of the introduced species became established (i.e., most introduced species became extinct). In the following year, he conducted a second experiment where he reduced the abundance of native species by filtering the enclosures. He found out an increase in the success of population establishment of the introduced species. He concludes that dispersal plays an important role determining where the species can reach or potentially colonize. However the local community diversity is the decisive factor excluding potential invaders (Shurin 2000, Fig. 2).

shurin_fig2In recent years, a consensus has emerged in the field of Invasion Ecology on the set of important processes related to species invasion. For example, now it is well recognize that propagule pressure has an important contribution to whether or not an  introduced species becomes established. Shurin mentions this in the Material and Methods section and also on the Discussion where he says “some species introduced in low numbers may have failed to invade because too few propagules were introduced”. In my opinion, a key weakness of this study was that it failed to account for propagule pressure despite of acknowledging the importance of it. In addition to the invaders abundance (that he mentioned) as a potential problem on species establishment, the number of introduction events plays also a major role in invasion. For instance, if he had conducted another introduction event in the enclosures perhaps the introduced species would have established via rescue effects.  However, as I said above, propagule pressure was not a totally recognized important factor influencing non-native species establishment at the time of this publication and therefore should not minimize the importance of these brilliant experiments.

The second paper, by Seidler and Plotkin (2006), contributed to our discussion on dispersal by demonstrating the importance of morphological traits on seed dispersal of trees in tropical region. Dispersal limitation is a potential mechanism for separating species in space and reducing competitive exclusion as stated by Seidler and Plotkin (2006). These authors investigated the spatial distribution of trees in a 50-hectare plot of primary tropical forest. By evaluating the dispersal morphologies and fruit sizes of 561 species, these authors clearly show the importance of this species traits on their spatial distribution. Animal-dispersed species exhibited significantly larger cluster sizes than species not dispersed by animals (Seidler and Plotkin, Fig. 2). This study has important theoretical implications, but I found it very interesting that the morphological traits of the plants could be linked to clustering because clustering is well known to reduce alpha diversity at small spatial scales, as well as, increase turnover and thus increase beta and gamma diversity at larger scales.  Thus this paper seems to imply a link between the individuals’ traits and the community diversity properties.