Biodiversity: Maintenance, Loss, and Consequences

Diversity is rapidly changing in modern communities due to both species introductions and extinctions, which may have a number of implications for ecosystem processes and services. The processes maintaining species diversity and the consequences of diversity change have been key areas of research in ecology, and are particularly relevant in light of current trends. In our lab, we explore how the integration of disturbance theory and facilitation theory can improve our understanding of diversity maintenance. We have also investigated the causes and consquences of realistic (observed non-random) diversity change, and are interested in the effects of both interspecific and intraspecific trait diversity.

Disturbance and Biodiversity: Feedbacks on CA Oyster Reefs

For sessile species, theory predicts that diversity can be maintained when disturbance limits the abundance of competitive dominants. This theory, however, fails to account for two facets of many ecological communities: (a) competitive dominants often create habitat that facilitates associated mobile organisms and thus overall community diversity; (b) species diversity often dictates a community’s resistance and resilience to physical disturbance.

To help reconcile these discrepancies, I experimentally demonstrated that habitat provided by a competitively dominant foundation species (the Olympia oyster) decreased with increasing disturbance. However, overall community diversity did not quantitatively change with increasing disturbance, because decreases species richness were counterbalanced by increases in species evenness, both of which are components of diversity. Given that community species richness and evenness differentially impact ecosystem functions, this research demonstrates that disturbance can change ecosystem functioning without quantitatively altering diversity.

As a compliment to my empirical work, I also collaborated with some great scientists (Byrnes, Hughes, & Stachowicz) to conceptually explore whether reciprocal feedbacks between disturbance and diversity could explain inconsistencies in the observed relationship between disturbance and diversity. Our review of published experiments showed that (i) disturbance severity can be both a cause and a consequence of local diversity in a wide range of ecosystems, and (ii) shapes of the relationship between diversity and disturbance can be quite variable. To explore how feedbacks between diversity and disturbance might alter expected patterns of diversity in nature, we developed a conceptual model that decomposed this feedback into its component relationships. Using this model, we sequentially evaluated the effect of diversity on disturbance severity, and the effect of realized disturbance on diversity loss, subsequent recruitment, and competitive exclusion. Our model suggested that feedbacks can increase mean values of richness, decrease variability, and alter the patterns of correlation between diversity and disturbance in nature.

Kimbro, D. L. and E. D. Grosholz. 2006. Disturbance influences oyster community richness and evenness, but not diversity. Ecology 87(9): 2378-2388.

Hughes A. R., J. E. Byrnes, D. L. Kimbro, and J. J. Stachowicz. 2007. Reciprocal relationships and potential feedbacks between biodiversity and disturbance. Ecology Letters 10(9): 849-864.

Bay Mouth Bar & Historical Diversity Changes

Observed diversity changes are often non-random, and can occur both within and across trophic levels. To better understand the causes and consequences of realistic, multi-trophic diversity change in a seagrass bed in Florida, we used historical ecological data sources in combination with current field surveys and manipulative experiments.
Bay Mouth Bar is a seagrass bed on the north Gulf Coast of Florida known for it’s especially diverse assemblage of large predatory gastropods. The ecologist Robert T. Paine conducted a study of this system in the late 1950’s, in which he described the predatory behavior and trophic relationships of the various gastropod species and documented strong seasonal patterns in their relative abundance. Beginning in 2010, we re-surveyed the gastropod and infaunal community on Bay Mouth Bar, interested in what changes might have occurred over the last 50 years – a period during which very little research had been done in this system. We also compiled data from a variety of published and unpublished historical sources.

Our investigations and monthly surveys of the bar revealed a number of changes since the 1950’s, including:

  • Loss of 2 of the 6 most common gastropod species
  • Changes in the relative abundance of specialist and omnivorous gastropods
  • Drastic reduction in the overall area of the bar and changes in the coverage of two seagrass species (see images below)
  • Decreased abundance of large infaunal bivalves
  • Increased abundance of top predatory horse conchs

Horse conch (top predator) preying on lightning whelk

Carnivorous gastropods on Bay Mouth Bar: Busycon contrarium (lightning whelk), Busycon spriatum (pear whelk), and Fasciolaria hunteria (banded tulip). To the upper left of the lightning whelk are Urosalpix cinerea (oyster drill) and Turbo castanea (turban snail): two smaller gastropods found on the bar.

Graduate student Tanya Rogers, after synthesizing these results, examined current and historical environmental data, and found no abrupt changes in water temperature or salinity. She also conducted a factorial field experiment examining trophic relations among the current and historical assemblages of large gastropods. The experiment revealed evidence of strong consumer pressure by the top predator on the two species that had disappeared, suggesting that increases in this predator, perhaps concentrated by decreases in habitat area, could have contributed to their disappearance. There were also strong effects of one of the vanished species on infaunal bivalves, indicating potential for a trophic cascade. However, the currently low abundance of large infaunal bivalves suggests that regional-scale processes, rather than top-down pressures, may underlie these changes, and that declines in bivalves as a food source may have simultaneously contributed to this species’ decline. These results provide a unique perspective on realistic patterns of diversity change across multiple trophic levels, and insight into interactions that may have been historically important.

Rogers, T. L., D. L. Kimbro. (2015) Causes and consequences of historical multi-trophic diversity change in an intertidal seagrass bed. Marine Ecology Progress Series ​540:13-26

Historical aerial image of Bay Mouth Bar. The grid at the NW end of the bar is the where our current study area is located. Paine’s original study area was at the SE end of the bar, just across from the channel.

Current aerial image of Bay Mouth Bar. Following a hurricane in the 1970’s, the channel closed off and longshore transport rapidly elongated the spit, eliminating most the bar.

Paine RT. 1963. Tropic relationships of 8 sympatric predatory gastropods. Ecology 44(1):63-73.