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# Dr Stephen Gregory

Telephone +61 8 8313 1129 Postdoctoral Fellow s.gregory@adelaide.edu.au +61 8 8313 4347 Mawson Laboratories G 38a North Terrace Ecology Evolution and Landscape Science

## Biography/ Background

Stephen joined the Global Ecology Group at the University of Adelaide from the Ecologie, Systematique et Evolution lab at the Universite Paris Sud XI, where he completed his PhD.

He is working with Damien Fordham and Barry Brook to extend second generation species distribution models (those integrating spatially structured metapopulation demographic models and conventional species distribution models) to incorporate within range demographic variation caused by range limiting factors, such as species interactions and changing substrate.

For his PhD, Stephen studied Allee effects and their detection. He demonstrated the poor power of conventional methods to detect Allee effects, how measurement error and climate, among other factors complicate their detection. Furthermore, he showed how the population dynamical signal of an Allee effect can itself produce sufficient variation to complicate its detection (a sort of Catch-22 situation). To address these problems, he developed a new approach to detecting Allee effects by extending a method used by Patrick Tobin. Using his approach, Stephen detected Allee effects in a high proportion of UK bat species; an interesting result that lends support to the elusive but persistent idea that social species will be susceptible to Allee effects.

Before his PhD, Stephen was a member of WildCRU where he studied mechanisms of coexistence between a Galapagos native (Nesoryzomys swarthi) and introduced (Rattus rattus) rat. Together with Donna Harris, he found that N. swarthi has developed specific behaviours allowing it, but not R. rattus, to exploit the endemic cactus for food and nesting and so tolerate seasonal drought. Unfortunately, changing climate and El Nino patterns now threaten the cactus population, and therefore N. swarthi.

## Research Interests

• Allee effects and their dynamical consequences

• Social species and their susceptibilty to Allee effects, particularly bats

• Population modelling, including metapopulations and effects of global change

## Articles

1. Gregory, S.D., & G. Jones (2010) Bats and Allee effects: When social behaviours go batty Biologist 57:198-203 (see AbsTracT | see BiBTeX)

As social species that cooperate to improve their fitness, bats should be prone to ‘Allee effects’, or a breakdown in social behaviour in small groups with a detrimental cost for individual – and possibly population – fitness. But what are Allee effects? And what is the evidence that they will affect bats?

@ARTICLE{Gregory2010c,
author = {Gregory, S.D. and Jones, G.},
title = {{B}ats and {A}llee effects: When social behaviours go batty},
journal = {Biologist},
year = {2010},
volume = {57},
pages = {198--203}
}


2. Gregory, S.D., Bradshaw, C.J.A., Brook, B.W., & F. Courchamp (2010) Limited evidence for the demographic Allee effect from numerous species across taxa Ecology 91:2151-2161 DOI 10.1890/09-1128 (see AbsTracT | see BiBTeX)

Extensive theoretical work on demographic Allee effects has led to the latent assumption that they are ubiquitous in natural populations, yet current empirical support for this phenomenon is sparse. We extended previous single-taxon analyses to evaluate the empirical support for demographic Allee effects in the per capita population growth rate of 1198 natural populations spanning all major taxa. For each population, we quantified the empirical support for five population growth models: no growth (random walk); exponential growth, with and without an Allee effect; and logistic growth, with and without an Allee effect. We used two metrics to quantify empirical support, information-theoretic and Bayesian strength of evidence, and observed top-rank frequency. The Ricker logistic model was both the most supported and most frequently top-ranked model, followed by random walk. Allee models had a combined relative support of 12.0% but were top-ranked in only 1.1% of the time series. Accounting for local climate variation and measurement error caused the loss of top-ranked Allee models, although the latter also increased their relative support. The 13 time series exhibiting Allee models were shorter and less variable than other time series, although only 3 were non-trending. Time series containing observations at low abundance were not more likely, and did not show higher support for Allee effect models. We conclude that there is relatively high potential for demographic Allee effects in these 1198 time series but comparatively few observed cases, perhaps due to the influences of climate and measurement error.

@ARTICLE{Gregory2010b,
author = {Gregory, S.D. and Bradshaw, C.J.A. and Brook, B.W. and Courchamp, F.},
title = {{L}imited evidence for the demographic {A}llee effect from numerous species across taxa},
journal = {Ecology},
year = {2010},
volume = {91},
pages = {2151--2161}
}


3. Gregory, S.D., & F. Courchamp (2010) Safety in numbers: extinction arising from predator-driven Allee effects Journal of Animal Ecology 79:511-514 DOI 10.1111/j.1365-2656.2010.01676.x (see AbsTracT | see BiBTeX)

Experimental evidence of extinction via an Allee effect (AE) is a priority as more species become threatened by human activity. Kramer & Drake (2010) begin the International Year of Biodiversity with the important – but double-edged – demonstration that predators can induce an AE in their prey. The good news is that their experiments help bridge the knowledge gap between theoretical and empirical AEs. The bad news is that this predator-driven AE precipitates the prey extinction via a demographic AE. Although their findings will be sensitive to departures from their experimental protocol, this link between predation and population extinction could have important consequences for many prey species.

@ARTICLE{Gregory2010a,
author = {Gregory, Stephen D and Courchamp, Franck},
title = {{S}afety in numbers: extinction arising from predator-driven {A}llee effects},
journal = {Journal of Animal Ecology},
year = {2010},
volume = {79},
pages = {511--514}
}


4. Gascoigne, J., Berec, L., Gregory, S. & F. Courchamp (2009) Dangerously few liaisons: a review of mate-finding Allee effects Population Ecology 51:355-372 DOI 10.1007/s10144-009-0146-4 (see AbsTracT | see BiBTeX)

In this paper, we review mate-finding Allee effects from ecological and evolutionary points of view. We define "mate-finding" as mate searching in mobile animals, and also as the meeting of gametes for sessile animals and plants (pollination). We consider related issues such as mate quality and choice, sperm limitation and physiological stimulation of reproduction by conspecifics, as well as discussing the role of demographic stochasticity in generating mate-finding Allee effects. We consider the role of component Allee effects due to mate-finding in generating demographic Allee effects (at the population level). Compelling evidence for demographic Allee effects due to mate-finding (as well as via other mechanisms) is still limited, due to difficulties in censusing rare populations or a failure to identify underlying mechanisms, but also because of fitness trade-offs, population spatial structure and metapopulation dynamics, and because the strength of component Allee effects may vary in time and space. Mate-finding Allee effects act on individual fitness and are thus susceptible to change via natural selection. We believe it is useful to distinguish two routes by which evolution can act to mitigate mate-finding Allee effects. The first is evolution of characteristics such as calls, pheromones, hermaphroditism, etc. which make mate-finding more efficient at low density, thus eliminating the Allee effect. Such adaptations are very abundant in the natural world, and may have arisen to avoid Allee effects, although other hypotheses are also possible. The second route is to avoid low density via adaptations such as permanent or periodic aggregation. In this case, the Allee effect is still present, but its effects are avoided. These two strategies may have different consequences in a world where many populations are being artificially reduced to low density: in the first case, population growth rate can be maintained, while in the second case, the mechanism to avoid Allee effects has been destroyed. It is therefore in these latter populations that we predict the greatest evidence for mate-finding Allee effects and associated demographic consequences. This idea is supported by the existing empirical evidence for demographic Allee effects. Given a strong effect that mate-finding appears to have on individual fitness, we support the continuing quest to find connections between component mate-finding Allee effects (individual reproductive fitness) and the demographic consequences. There are many reasons why such studies are difficult, but it is important, particularly given the increasing number of populations and species of conservation concern, that the ecological community understands more about how widespread demographic Allee effects really are, and why.

@ARTICLE{Gascoigne2009,
author = {Gascoigne, Joanna and Berec, Ludek and Gregory, Stephen and Courchamp, Franck},
title = {{D}angerously few liaisons: a review of mate-finding {A}llee effects},
journal = {Population Ecology},
year = {2009},
volume = {51},
pages = {355--372}
}


5. Gregory, S.D. & D.W. Macdonald (2009) Prickly coexistence or blunt competition? Opuntia refugia in an invaded rodent community. Oecologia 159:225-236 DOI 10.1007/s00442-008-1196-6 (see AbsTracT | see BiBTeX)

Endemic Nesoryzomys swarthi and invasive Rattus rattus exist in unlikely sympatry in Galápagos as female N. swarthi suffer from competition with R. rattus. This study evaluates the role of feeding habits in facilitating their co-occurrence. Spool-and-line tracking of 85 N. swarthi and 33 R. rattus was used to quantify their selected diets, foods of which were used in captive trials of 46 N. swarthi and 34 R. rattus to quantify their preferred diets. Selected diets were compared between species and seasons using niche measures, and contrasted to preferred diets to qualify inferences about competition. Diet overlap was highest in the wet season when food - particularly fruit - abundance was highest and R. rattus diet breadth was broadest. Preferred and selected diets were marginally correlated for R. rattus but uncorrelated for N. swarthi, suggesting that R. rattus interfere with N. swarthi foraging. Diet overlap was highest between female N. swarthi and R. rattus perhaps due to female breeding requirements. Male N. swarthi avoided R. rattus preferred foods possibly to avoid aggressive encounters with R. rattus. During the dry season when foods declined and the R. rattus population crashed, diet overlap was lowest as R. rattus diet narrowed in the absence of fruits. Female, and particularly male, N. swarthi diet broadened with emphasis on acquiring Opuntia foods but, N. swarthi preferred and selected diets were uncorrelated suggesting that their foraging was inhibited by R. rattus. In conclusion, the narrower diet breadth of R. rattus in contrast to N. swarthi suggests that it may be less adapted to food acquisition at this site, particularly when fruits are absent. Year-round presence of R. rattus however appears to inhibit N. swarthi foraging for its preferred diet which instead specialise on Opuntia foods that were uneaten by R. rattus and may provide N. swarthi with a localised competition refuge from encounters with R. rattus.

@ARTICLE{Gregory2009,
author = {Gregory, Stephen David and Macdonald, David Whyte},
title = {Prickly coexistence or blunt competition? {\it Opuntia} refugia in an invaded rodent community},
journal = {Oecologia},
year = {2009},
volume = {159},
pages = {225--236}
}


6. Harris, D.B., Gregory, S.D. & D.W. Macdonald (2006) Space invaders? A search for patterns underlying the coexistence of alien black rats and Galápagos rice rats. Oecologia 149:276-288 DOI 10.1007/s00442-006-0447-7 (see AbsTracT | see BiBTeX)

The introduction and spread of the black rat Rattus rattus is believed to have caused the worst decline of any vertebrate taxon in Galápagos. However, the "extinct" Santiago rice rat Nesoryzomys swarthi has recently been rediscovered in sympatry with R. rattus providing the first exception to this general pattern of displacement. We carried out an exploratory investigation of this novel system with the aim of identifying patterns that may facilitate the apparent coexistence of the two species. We carried out an extensive survey of Santiago Island to map the current distribution of the endemic rice rat and to explore broad scale distribution-habitat associations. We then used live-trapping, radio-tracking, and spool-and-line tracking to quantify abundance-habitat correlations and to test for evidence of interspecific spatial segregation, alteration of N. swarthi activity patterns (spatial and temporal), and microhabitat partitioning. We found that N. swarthi has disappeared from part of its historical range and appears to be restricted to a 14 km stretch of the north-central coast, characterised by high density of the cactus Opuntia galapageia. In contrast, the generalist R. rattus was found at all survey sites. We found no evidence of spatial segregation, and home range size, temporal activity and density of N. swarthi did not vary with local density of R. rattus. However, pre-dawn and post-dusk N. swarthi activity levels increased with R. rattus density perhaps reflecting an increase in foraging effort necessary to compensate for the costs of interspecific exploitation or interference competition. The distribution, microhabitat selection, and abundance-habitat relations of N. swarthi suggest that the endemic cactus O. galapageia may facilitate interspecific coexistence. Further research should include a comparison of inter-seasonal resource preference and foraging activity of the two species coupled with replicated field experiments to confirm and quantify competition and to elucidate the mechanism of competitive coexistence.

@ARTICLE{Harris2006,
author = {Harris, Donna Betty and Gregory, Stephen David and Macdonald, David Whyte},
title = {Space invaders? a search for patterns underlying the coexistence of alien black rats and {Gal\'apagos} rice rats},
journal = {Oecologia},
year = {2006},
volume = {149},
pages = {276--288}
}


## Book chapters

1. Bertelsmeier, C., Bonnaud, E., Gregory, S.D. & F. Courchamp (in press) Applied Ecology. In Sourcebook in Theoretical Ecology (Alan Hastings and Louis Gross, eds.), University of California Press, California (see BiBTeX)

@INCOLLECTION{Bertelsmeierinpress,
author = {Bertelsmeier, C. and Bonnaud, E. and Gregory, S.D. and Courchamp, F.},
booktitle = {Sourcebook in Theoretical Ecology},
title = {Applied Ecology},
year = {in press},
publisher = {University of California Press},
pages = {--}
}


Entry last updated: Wednesday, 12 Sep 2012

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