Review: Bates, Saunders, Chopin (2005) An assessment of two taxonomic distinctness indices for detecting seaweed assemblage responses to environmental stress. Botanica Marina, 48:231-243.

Feature Paper: DOWNLOAD * Bates, Saunders, Chopin (2005) An assessment of two taxonomic distinctness indices for detecting seaweed assemblage responses to environmental stress. Botanica Marina, 48:231-243.

 

Author Abstract: We tested the efficacy of two biodiversity indices, average taxonomic distinctness and variation in average taxonomic distinctness, for indicating environmental stress in seaweed assemblages from the Bay of Fundy, New Brunswick, Canada. These indices, which measure the average number of taxonomic levels between species in a sample, offer a potential panacea for biomonitoring because their calculation requires only a species list and a regional taxonomic hierarchy, they offer a statistical framework for testing whether assemblages deviate from an expected taxonomic breadth, and previous studies involving animal assemblages have demonstrated an independence from sampling effort. However, our results were not consistent with previously published studies or with our perception of site conditions. Specifically, putatively impacted sites scored above-average taxonomic distinctness values, while sites otherwise regarded as healthy were indicated as environmentally degraded. We also demonstrate that average taxonomic distinctness values can be negatively correlated with species richness, Shannon diversity and with functional diversity. Further, increasing the breadth of the regional species list to which specific sites were compared resulted in a more conservative test of impact. We recommend that a qualitative understanding of how specific biotic assemblages respond to stress is a necessary prerequisite to use the taxonomic distinctness indices for environmental stress assessments.

 

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: This week we turn our attention to algae, an oft-maligned group of organisms in the marine environment. For those regular readers of Science Corner I have discussed algae at multiple times in the past, both from the perspective of phase shifts (whereby algae opportunistically overgrow corals and other sessile reef organisms when environmental conditions favor their growth, usually through higher nutrient inputs or physical disturbances) as well as through being a group that doesn't have the typical biodiversity patterns that corals and reef fishes do. 

This week's first paper continues the examination of taxonomic similarity that we've covered in past weeks, but applies analyses towards determining whether algae are responding to environmental stress, and therefore whether a phase shift in a reef environment may be occurring.

For those who missed the previous discussions on taxonomic diversity indices, the authors provide a good summary: "Taxonomic distinctness (TD) indices are biodiversity measures that provide a summary of the relatedness between organisms within samples from biological assemblages."

How the authors addressed assessing environmental stress was a two-part process. 

1. First, they developed an index that "incorporates the identity of species within a sample by calculating the average path length between all pairs of species, measured through a cladogram of phylogenetic relationships or a Linnaean classification."

2. Second, the authors "inferred" environmental stress "by testing for departure from an expected range of average taxonomic distinctness values calculated by resampling from a broader ‘‘master list’’ of species that could potentially inhabit the sampling region."

The authors note that their "approach is predicated on the observation that, in several types of marine invertebrate assemblages, disturbance can shift structure from a relatively diverse array of taxa to a suite of closely related species that are not representative of the potential taxonomic structure for an unimpacted site."

In other words, certain organisms are opportunistic and able to rapidly increase in numbers to the point where they overgrow other organisms and shift community compositions. When such a composition is found where a site is known to have high diversity, it could be an indicator that the system is out of balance, particularly if one knows that the environmental surveys leading to such conclusions were an accurate representation of the environmental site at the time of survey.

The authors also examined "variation in taxonomic distinctness," which relates to "the evenness of the taxonomic or phylogenetic relationships between taxa."

The authors then combined analyses of average taxonomic distinctness with variation in taxonomic distinctness to generate a "bivariate scatter plot" and then determine deviation from 95% probability contours within the scatter plots generated for each pairwise analysis.

The authors then applied their approach to surveys of algal assemblages in New Brunswick Canada because "extensive and multiple human impacts have been reported in this area." 

When the authors examined algae as an entire group rather than individual taxonomic clades, they didn't find any sites that appeared to be impacted, which went counter to their hypothesis. They then performed individual analyses on each algal clade separately: Rhodophyta, or red algae; Chlorophyta, or green algae; and Phaeophyceae, or brown algae. 

What many people don't realize is that each algal group evolved separately and independently and that each group is more distinct from each other than frogs are from fish. The brown algae are more related to bacteria, while the green algae are closer to true plants (i.e., land plants, sea grass), and red algae are actually two groups of ancient distinct organisms. All algal groups have different and distinct life histories and reproductive cycles. 

When examining individual algal groups, the authors found that the red algae exhibited deviation from 95% confidence intervals for multiple sites examined. 

While one must be careful of which taxa are grouped, since the authors note that their "results indicate that taxonomic distinctness indices may not perform equally well for all types of bioindicator groups," nevertheless the authors conclude that "average taxonomic distinctness and variation in average taxonomic distinctness offer potentially powerful tools for assessing environmental stress in situations where historical data do not exist, or exist only as species lists collected with unknown sampling effort."

Next we'll look at a good method for conducting algal surveys (which can be expanded to other benthic groups like corals, sponges, bryozoans, etc.) so that uniform sampling effort does occur.