Population regulation and diversity-stability relationships in ecological time-series

Thibaut, Loïc (2015) Population regulation and diversity-stability relationships in ecological time-series. PhD thesis, James Cook University.

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Biodiversity can provide insurance against ecosystem collapse by stabilizing assemblages that perform critical ecological functions (the "portfolio effect"). However, the extent to which this occurs in nature, and the importance of different mechanisms that generate portfolio effects, remain controversial. The overall aim of this thesis was to develop a quantitative approach to estimate the extent to which diversity stabilizes communities, and to apply this approach to herbivory on coral reefs.

In chapter 2, I examined herbivory on the Great Barrier Reef. On coral reefs, herbivory helps maintain coral dominated states, so volatility in levels of herbivory has important implications for reef ecosystems. In this chapter, I used an extensive time series of abundances on 35 reefs of the Great Barrier Reef of Australia to quantify the strength of the portfolio effect for herbivorous fishes. Then, I disentangled the contributions of two mechanisms that underlie it: compensatory interactions, and differential responses to environmental fluctuations ("response diversity"), by fitting a community-dynamic model than explicitly includes terms for both mechanisms. I found that portfolio effects operate strongly in herbivorous fishes, as shown by nearly independent fluctuations in abundances over time. Moreover, I found strong evidence for high response diversity, with nearly independent responses to environmental fluctuations. In contrast, I found little evidence that the portfolio effect in this system was enhanced by compensatory ecological interactions. My results show that portfolio effects are driven principally by response diversity for herbivorous fishes on coral reefs. I conclude that portfolio effects can be very strong in nature, and that, for coral reefs in particular, response diversity on coral reefs may help maintain herbivory above the threshold levels hypothesized to trigger regime shifts.

In chapter 3, I developed a theoretical framework to understand diversitystability relationships (DSRs) in nature, in order to overcome shortcomings in past approaches that became apparent during the work for Chapter 2. Specifically, DSRs are analysed using a variety of different population and community properties, most of which are adopted from theory that makes several restrictive assumptions that are unlikely to be reflected in nature. Here, I constructed a simple synthesis and generalization of previous theory for the DSR. I showed that community stability is a product of two quantities: the synchrony of population fluctuations, and an average species-level population stability that is weighted by relative abundance. Weighted average population stability can be decomposed to consider effects of the meanvariance scaling of abundance, changes in mean abundance with diversity, and differences in species' mean abundance in monoculture. My framework makes explicit how unevenness in the abundances of species in real communities influences the DSR, which occurs both through effects on community synchrony, and effects on weighted average population variability. This theory provides a more robust framework for analysing the results of empirical studies of the DSR, and facilitates the integration of findings from real and model communities.

Observation error is pervasive in ecological time-series and can have important implications for estimating and identifying the drivers of stability in community timeseries. In chapter 4, I used a model which explicitly accounts for observation error to assess the strength of evidence for population regulation in time-series from the Global Population Dynamics Database (GPDD). The extent to which populations in nature are regulated by density-dependent processes is unresolved. While experiments increasingly find evidence of strong density-dependence, unmanipulated population time series yield much more ambiguous evidence of regulation. In this chapter, I reexamined the evidence for density-dependence in time series of population sizes in nature, by conducting an aggregate analysis of the populations in the GPDD. I found that density-dependence is likely over-estimated when it is fitted independently for each population, even when accounting for observation error. However, in the aggregate, very strong evidence for weak, but non-zero, density-dependence remains. Rather than falling into categories of density-dependent and density-independent dynamics, differences in support for density-dependence are likely the result of differences in statistical power. My findings suggest that the observational record does indeed contain strong support for density-dependence, but that its intensity is likely weaker than is detected in laboratory and field experiments.

The findings from my latter two chapters have important implications for the assessment of diversity-stability relationships from the analysis of ecological time series. Specifically, the framework developed in chapter 3 shows that the DSR can be partitioned into an effect of synchrony and an effect of population variability. Consequently, the role and importance of several ecological drivers of the DSR can be disentangled by considering separately their effect on community synchrony and population variability. Finally, chapter 4 illustrates that observation error should be explicitly accounted for to ensure sound inferences about the processes driving fluctuations in species' abundances.

Item ID: 46590
Item Type: Thesis (PhD)
Keywords: biodiversity, coral reef fishes, coral reefs, diversity-stability relationship, ecosystem function, evenness, functional diversity, Great Barrier Reef, herbivores, herbivorous fishes, insurance hypothesis, mean-variance scaling, overyielding, phase shift, population stability, population synchrony, portfolio effect, regime shift, response diversity, species richness, stability, statistical averaging, temporal variability
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Thibaut, Loïc M., Connolly, Sean R., and Sweatman, Hugh P.A. (2012) Diversity and stability of herbivorous fishes on coral reefs. Ecology, 93 (4). pp. 891-901.

Chapter 3: Thibaut, Loïc M., and Connolly, Sean R. (2013) Understanding diversity–stability relationships: towards a unified model of portfolio effects. Ecology Letters, 16 (2). pp. 140-150.

Date Deposited: 07 Dec 2016 01:32
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 34%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 33%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 33%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960805 Flora, Fauna and Biodiversity at Regional or Larger Scales @ 50%
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