New Publication: The relative importance of reproduction and survival for the conservation of two dolphin populations

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A team of international researchers from nine research institutions, led by Dr. Oliver Manlik, has recently published a paper in the open-access (freely available) journal Ecology and Evolution. The study provides a conservation update on the bottlenose dolphins off Bunbury and in Shark Bay, Western Australia. It assesses and compares reproductive rates, survival rates and population viability of these two populations. The title of the paper is: The relative importance of reproduction and survival for the conservation of two dolphin populations.

The paper  discusses some important questions for wildlife management: Should wildlife management focus on boosting reproduction or survival? What are the conservation options for the Bunbury population?

Citation details: Oliver Manlik, Jane A. McDonald, Janet Mann, Holly C. Raudino, Lars Bejder, Michael Krützen, Richard C. Connor, Michael R. Heithaus, Robert C. Lacy and William B. Sherwin (2016). The relative importance of reproduction and survival for the conservation of two dolphin populations. Ecology and Evolution 6(11):3496-3512. doi: 10.1002/ece3.2130

The research on the Bunbury population in this study was derived from data of Dr. Raudino’s PhD thesis and is part of the ongoing South West Marine Research Program (SWMRP).

Abstract: It has been proposed that in slow-growing vertebrate populations survival generally has a greater influence on population growth than reproduction. Despite many studies cautioning against such generalizations for conservation, wildlife management for slow-growing populations still often focuses on perturbing survival without careful evaluation as to whether those changes are likely or feasible. Here, we evaluate the relative importance of reproduction and survival for the conservation of two bottlenose dolphin (Tursiops cf aduncus) populations: a large, apparently stable population and a smaller one that is forecast to decline. We also assessed the feasibility and effectiveness of wildlife management objectives aimed at boosting either reproduction or survival. Consistent with other analytically based elasticity studies, survival had the greatest effect on population trajectories when altering vital rates by equal proportions. However, the findings of our alternative analytical approaches are in stark contrast to commonly used proportional sensitivity analyses and suggest that reproduction is considerably more important.

We show that:

  • in the apparently stable population reproductive output is higher, and adult survival is lower;
  • the difference in viability between the two populations is due to the difference in reproduction;
  • reproductive rates are variable, whereas survival rates are relatively constant over time;
  • perturbations on the basis of observed, temporal variation indicate that population dynamics are much more influenced by reproduction than by adult survival;
  • for the apparently declining population, raising reproductive rates would be an effective and feasible tool to reverse the forecast population decline;
  • increasing survival would be ineffective.

Our findings highlight the importance of reproduction – even in slow-growing populations – and the need to assess the effect of natural variation in vital rates on population viability. We echo others in cautioning against generalizations based on life-history traits and recommend that population modeling for conservation should also take into account the magnitude of vital rate changes that could be attained under alternative management scenarios.

Population trajectories and three-year reproductive rates for Bunbury and Shark Bay dolphin populations.

Population trajectories and three-year reproductive rates for Bunbury and Shark Bay dolphin populations.

 Highlights of research findings:

  • The study showed that the Bunbury dolphins produced considerably fewer calves than their counterparts in Shark Bay—while about 58% of adult females gave birth in Shark Bay only about 41% of Bunbury adult females gave birth during one three-year time period (Figure a).
  • Based on these birth and mortality rates, the study forecast that the Bunbury population would decline and is at high risk of extinction, unless dolphins from neighbouring populations migrate into the Bunbury population (Figure b).
  • In contrast the Shark Bay population was forecast to be relatively stable (Figure b).
  • The difference in viability between the Shark Bay population and the Bunbury population is due to the difference in birth rates and not morality rates between the two populations.
  • For this and other reasons, the authors conclude that in the case of the Bunbury and Shark Bay population, reproduction and not survival is the key to successful conservation actions. (This is contrary to a common generalization that for the conservation of long-lived, slow-growing animal species, including dolphins, survival is usually more important than reproduction.)
  • The forecast decline of the Bunbury population could be prevented or reverted if there is a boost in reproductive rates.
  • We suggest that “that restrictions on vessel traffic, such as vessel exclusion zones and speed limits to prevent disturbances, could have a positive effect on mating behavior, reproduction, and ultimately the viability of the Bunbury population.”

Cautionary note and outlook:

Like all models, the forecast of this population viability analysis is limited by the model assumptions and accuracy of input parameters. There are two limitations that deserve further investigation:

  • The reproductive rates that were the basis of the forecast decline of the Bunbury population was based on only one three-year time period. It is possible that reproductive rates that were used in this study have since increased and the forecast would then be more positive. The study showed, for example, that reproductive rates of the Shark Bay population vary considerably over time. Nevertheless, in twelve years of observation, the reproductive rates of the Shark Bay dolphins have never been as low as those observed for the Bunbury population. This highlights the need to continue monitoring the Bunbury dolphins, especially with respect to reproductive rates. Thanks to the support of our SWMRP partners, the research on the Bunbury dolphins is ongoing.
  • One of the model assumption of this study was that the Bunbury population is isolated from other neighboring populations. However, we know that there is some movement of dolphins in and out of Bunbury. If dolphins from neighboring populations migrate into Bunbury, this could boost the viability of the Bunbury population. In contrast, if there is a net migration out of the Bunbury population, this would make the forecast even worse. In order to investigate this, a team of researchers led by Dr. Oliver Manlik have started another project that assess past migration and gene flow into and out of the Bunbury population using genetic tools (mitochondrial DNA and microsatellite loci). For more information, see HERE.

Download the paper:

The article is freely available from Ecology and Evolution, or alternatively, if you are unable to download the article please email the lead author, Oliver Manlik, for a pdf at o.manlik at unsw.edu.au, or access a copy through Researchgate.

 Project funding and acknowledgments:

The data were accumulated over many studies whose funding included National Science Foundation (USA), NSERC Canada, National Geographic Society, WV Scott Foundation, PADI Foundation, Australian Research Council, Shark Bay Shire, and in-kind from Monkey Mia Resort, tour operators, the Shark Bay and Bunbury communities, including volunteer research assistants and the Australian Coast Radio Monitors WA Inc. Bunbury data were made possible through the support by partners of the South West Marine Research Program: Bemax Cable Sands, Bunbury Dolphin Discovery Centre, Bunbury Port Authority, City of Bunbury, Cristal Global, WA Department of Parks and Wildlife, Iluka, Millard Marine, Naturaliste Charters, South West Development Commission, WA Plantation Resources and Worsley Alumina.

We thank E. Krzyszczyk and K. Nicholson for valuable advice on the applicability of CMR methods. Important feedback also came from participants in the VORTEX-listserv and from participants in the IWC “Workshop for strategic planning of large-scale whalewatching,” Bunbury, Australia 2008, especially P. Hammond and D. Lusseau. We also thank J-M. Gaillard, N.G., Yoccoz, T.J. Regan, K. Sprogis and three anonymous reviewers for valuable comments on this study.

Other publications arising from the SWMRP:

Blog written by: Oliver Manlik.

Lars Bejder PhD
Lars Bejder PhD
Professor Lars Bejder PhD is the Research Leader of the Murdoch University Cetacean Research Unit.
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