MacDougall et al. 2009

MacDougall et al., 2009.

Plan invasion and the niche. 2009. MacDougal, A. S., Gilbert, B and Levine, J. M. Journal of Ecology 97: 609-615.

Blog Author: Laura Segura-Hernandez
Paper Authors: 

Andrew MacDougal:

·     PhD UBC 2004

·     Associate professor in University of Guelph

·     He is interested in different topics, including: trophic island biogeography, the evolution of spatial patterns in assembling communities and global drivers of species invasions.

Benjamin Gilbert:

·     PhD University of British Columbia 2008

·     Associate professor in University of Toronto

·      I am broadly interested in how ecological processes affect population dynamics and community structure. My past research has tested the roles of environmental adaptation, species interactions, and spatial processes in promoting or reducing species diversity. I use field experiments to test predictions from ecological theory and simulation and analytic models to further explore ideas and theory. Most of my research is based in temperate or tropical plant communities, but I also work with aquatic insect and plankton communities. 

Jonathan M. Levine:

·     PhD University of California Berkley 2001.

·     Associate professor in University of California Santa Barbara

·     Levine’s work encompasses controls over the success and impacts of exotic plant invasions; species diversity and ecosystem function; mechanisms underlying rare plant persistence; determinants of plant commonness, rarity, and coexistence. Putting his work into perspective, Levine explains that the invasion of species into new biogeographic regions is a process that has regularly occurred over geologic time. Over the last millennium, however, the human-mediated transport of species across the globe has increased the rate of invasion several orders of magnitude. Levin has taught at UCSB, UCLA, and at the Imperial College in the UK. He is a member of the American Society of Naturalists, the Ecological Society of America, and the Society for Conservation Biology. 

Summary/Main points

1. Main Question: 

ØBackground: Classic niche theory proposes that the establishment and persistence of species in a habitat is limited by a set of conditions, and that if the conditions that limit a species overlap with those of other species in the community, that species will not be able to establish within the community in question. This theory has had a major impact in the study of biological invasions, as many studies are done under the hypothesis that for a species to establish in a new area, it has to have a niche that is different from the niche of other species in the area. However, this hypothesis fail to explain many successful invasion of species that share a niche with native species, leading to high impact invasion or to successful invasion that do not cause particular damages to the community. So the authors propose that studies of biological invasion should consider differences in both niche and fitness between native and invasive species.

ØMain question:The authors aim to present a framework that uses differences in both niche and fitness, as a way to analyze success and impact of biological invasions.

2. Methods:

Ødata:results compiled out of different studies regarding invasion theory  (Table 1).

Ø    

ØMethods:  They organized the results of the studies in one single table, and classified the hypothesis proposed in those studies whithin the three categories the authors proposed: Niche differences, Fitness differences or Both. They also made a figure to explain their framework (Fig 1).

3. Results: 

ØNiche versus fitness differences: He bases a lot of what he proposes on Peter Cheeson’s ideas. 

ØNiche differences: or stabilizing process. Things that would limit the species by itself, but would not limit the native competitor. When a species is able to establish successfully if it doesn’t become abundant. It usually favor coexistence of native and invaders. 

ØFitness differences: species differences lead to competitive dominance. Like dfferences in the ability of getting resources, differences in susceptibility to predation and offspring produced between species. The advantage here doesn’t depend on the invader being rare.

ØSpecies differences and the outcome of invasion: 

Ø  They propose 4 different scenarios/hypothesis concerning both niche and fitness differences and their effect in defining a successful invasion:

àScenario A: Invader has lower fitness than the resident species and difference in niche is not large enough to stop the resident from repelling the invader.

àScenario B:  Niche differences allow the invader to establish, even if its fitness is lower than the resident species. Natives and invaders coexist.

àScenario C: Invader has fitness advantage, but the niche is different enough to allow the resident to remain in the community. Natives and invaders coexist.

àScenario D: Invader has greater fitness advantage that outweighs the similarity in niche, causing the invader to displace the residents.

Ø  Related to those scenarios, the authors pointed: 

à Niche differences increase the probability that an invader can establish and increase the probability of coexistence of residents and invaders.

à Differences that increase invader fitness compared to residents fitness increase the probability of establishment  and the possible impact of the invader. 

àFor an invasion to have a high impact, it requires niche similarity and a fitness advantage in comparison to resident species. 

ØA unifying framework for species differences and invasion:

ØThis framework facilitates merging the abundant and varied hypothesis that have been proposed in relation to invasion processes. 

ØThey point how many hypothesis consider one of the differences mentioned in this framework, when in many cases difference in fitness and niche similarity are present and beneficial. 

ØThey also suggest the difference in niche and fitness depend in the temporal and spatial scale analyzed. The mention here the “novel niche hypothesis”, using as an example the antropogenic modification of landscape that can favor the establishment of certain species over other.

ØNew questions and approaches:

ØThey conjecture that the ideas proposed here are general across different systems.

ØHow can ecologist formally test the influence of niche and fitness in the interactions between invasive and resident species?

·    Using phylogenetic distance: according to Darwin’s idea, invaders benefit from greater phylogenetic distance as it minimizes overlap niche overlap, as well as it decreased their susceptibility to natural enemies but might also disfavor invaders as it might decrease their adaptations to new climates.

·   Using experimental approaches: to test the principle the niche difference causes species to limit themselves more than competitors do. For example: planting exotic species in dense patches with other invasive species and see if they grow as much as when they are in the presence of dense patches of resident species.

·    Using natural experiments of observational approaches: observing how the growth patterns change as/if the invaders becomes abundant. Or using a chronosequence study, measuring the growth of the invaders from the front towards the interior of the invasion.

4. Conclusions/Inference

·   The framework allows to group diverse hypothesis into a compact map of two variables.

·   This helps resolve the apparent conflict between classic niche theory and observation of invasions.

·   Biological invasions require future work to gain better understanding of the mechanisms that are behind them.

·   Considering coexistence of invaders and resident helps to link problems of invasion ecology to the current knowledge on how species diversity is maintained.  

5. Questions/comments: 

1.  Invasion are a very interesting topic to me. But I find that there are so many different invasive organisms that there might be too many things underlying their success. I liked how the authors managed to group all of the mechanisms proposed for plants in two principal ideas. I wonder if it would be possible to adapt this framework to systems that are not as stationary as plants. Has someone argued about using it on insects, for example? Can you think of a system where this framework will work very well? Or a system where you think it won’t work at all?

2.   I found particularly interesting how many invasions are actually “harmless”. Of course, my perception of the topic was biased because harmful invasions would naturally get more attention. But I think this make the idea of coexistence stronger, and it really seems interesting. In my opinion, this coexistence should get some attention too.