Papers by Natasha Gownaris
Ecological Applications, 2019
We developed a Hidden Markov mark–recapture model (R package marked) to examine sex‐specific demo... more We developed a Hidden Markov mark–recapture model (R package marked) to examine sex‐specific demography in Magellanic Penguins (Spheniscus magellanicus). Our model was based on 33 yr of resightings at Punta Tombo, Argentina, where we banded ~44,000 chicks from 1983 to 2010. Because we sexed only 57% of individuals over their lifetime, we treated sex as an uncertain state in our model. Our goals were to provide insight into the population dynamics of this declining colony, to inform conservation of this species, and to highlight the importance of considering sex‐specific vital rates in demographic seabird studies. Like many other seabirds, Magellanic Penguins are long‐lived, serially monogamous, and exhibit obligate biparental care. We found that the non‐breeding‐season survival of females was lower than that of males and that the magnitude of this bias was highest for juveniles. Biases in survival accumulated as cohorts aged, leading to increasingly skewed sex ratios. The survival bias was greatest in years when overall survival was low, that is, females fared disproportionality worse when conditions were unfavorable. Our model‐estimated survival patterns are consistent with independent data on carcasses from the species’ non‐breeding grounds, showing that mortality is higher for juveniles than for adults and higher for females than for males. Juveniles may be less efficient foragers than adults are and, because of their smaller size, females may show less resilience to food scarcity than males. We used perturbation analysis of a population matrix model to determine the impact of sex‐biased survival on adult sex ratio and population growth rate at Punta Tombo. We found that adult sex ratio and population growth rate have the greatest proportional response, that is, elasticity, to female pre‐breeder and adult survival. Sex bias in juvenile survival (i.e., lower survival of females) made the greatest contribution to population declines from 1990 to 2009. Because starvation is a leading cause of morality in juveniles and adults, precautionary fisheries and spatial management in the region could help to slow population decline. Our data add to growing evidence that knowledge of sex‐specific demography and sex ratios are necessary for accurate assessment of seabird population trends.
Ambio, Jan 9, 2019
This paper synthesizes current knowledge on the impacts of the Gibe III dam and associated large-... more This paper synthesizes current knowledge on the impacts of the Gibe III dam and associated large-scale commercial farming in the Omo-Turkana Basin, based on an expert elicitation coupled with a scoping review and the collective knowledge of an multidisciplinary network of researchers with active data-collection programs in the Basin. We use social-ecological systems and political ecology frameworks to assess the impacts of these interventions on hydrology and ecosystem services in the Basin, and cascading effects on livelihoods, patterns of migration, and conflict dynamics for the people of the region. A landscape-scale transformation is occurring in which commodities, rather than staple foods for local consumption, are becoming the main output of the region. Mitigation measures initiated by the Ethiopian government—notably resettlement schemes—are not adequately buffering affected communities from food insecurity following disruption to indigenous livelihood systems. Therefore, while benefits are accruing to labor migrants, the costs of development are currently borne primarily by the agro–pastoralist indigenous people of the region. We consider measures that might maximize benefits from the changes underway and mitigate their negative outcomes, such as controlled floods, irrigating fodder crops, food aid, and benefit sharing.
PLOS ONE, 2015
Lake Turkana is an understudied desert lake shared by Kenya and Ethiopia. This system is at the p... more Lake Turkana is an understudied desert lake shared by Kenya and Ethiopia. This system is at the precipice of large-scale changes in ecological function due to climate change and economic development along its major inflowing river, the Omo River. To anticipate response by the fish community to these changes, we quantified trophic diversity for seven ecological disparate species (Alestes baremose, Hydrocynus forskalli, Labeo horie, Lates niloticus, Oreochromis niloticus, Synodontis schall, and Tilapia zillii) using stable isotopes. Based on their marked morphological differentiation, we postulated that dietary niches of these species would be similar in size but show little overlap. The degree of trophic diversity varied greatly among the species studied, refuting our hypothesis regarding dietary niche size. Oreochromis niloticus and L. niloticus had the highest trophic diversity and significantly larger dietary niches than T. zillii, A. baremose and H. forskalli. Low overlap among the dietary niches of the seven species, with the exception of the synodontid catfish S. schall, is consistent with our second hypothesis.
Mussels have myriad effects on population, community, and ecosystem processes. Their aggregation ... more Mussels have myriad effects on population, community, and ecosystem processes. Their aggregation behavior is an inducible defense that links non-consumptive effects of predators to benthic spatial pattern formation. Aggregation increases intraspecific competition but can be beneficial due to lower perimeter-related predation and other risks. Mytilus edulis aggregation responses to predation threats have not been investigated outside of Europe. We studied the effects of chemical cues from heterospecifics (predators Carcinus maenas, Nucella lapillus; herbivore Littorina littorea) and conspecifics (injured and intact M. edulis) on M. edulis aggregation behavior in Maine, USA. Mussels self-organized into fractal power-law spatial patterns like those in the field. Aggregations had lower perimeter:area (P:A) ratios than singletons, despite having more complex, irregular shapes with higher fractal dimensions (D). However, with one exception, no significant differences in aggregation rate, P:A ratio, and D were observed for any chemical cue treatment when compared to no-cue controls. Our experiment revealed higher aggregation rates than reported from similar experiments, leaving little scope for additional aggregation when exposed to chemical cues. We suggest that increased aggregation in response to predation threat is context-dependent: costs outweigh benefits beyond some optimal aggregation size, and mussels in our experiment were at the upper aggregation limit beyond which more aggregation could have negative consequences. Bet-hedging with a power-law distribution of aggregation shapes and sizes may be the optimal spatial strategy, especially if predation and other risks are variable in space and time.
Oecologia, 2016
of consumer foraging behavior. Specifically, we suggest pathways by which consumer personality tr... more of consumer foraging behavior. Specifically, we suggest pathways by which consumer personality traits influence foraging activity, risk-dependent foraging, roles in social foraging groups, spatial aspects of foraging and physiological drivers of foraging, which in turn can lead to consistent individual differences in food resource use. These pathways provide a basis for generating testable hypotheses directly linking animal personality to ecological dynamics, a major goal in contemporary behavioral ecology.
PLOS ONE, 2015
Lake Turkana is an understudied desert lake shared by Kenya and Ethiopia. This system is at the p... more Lake Turkana is an understudied desert lake shared by Kenya and Ethiopia. This system is at the precipice of large-scale changes in ecological function due to climate change and economic development along its major inflowing river, the Omo River. To anticipate response by the fish community to these changes, we quantified trophic diversity for seven ecological disparate species (Alestes baremose, Hydrocynus forskalli, Labeo horie, Lates niloticus, Oreochromis niloticus, Synodontis schall, and Tilapia zillii) using stable isotopes. Based on their marked morphological differentiation, we postulated that dietary niches of these species would be similar in size but show little overlap. The degree of trophic diversity varied greatly among the species studied, refuting our hypothesis regarding dietary niche size. Oreochromis niloticus and L. niloticus had the highest trophic diversity and significantly larger dietary niches than T. zillii, A. baremose and H. forskalli. Low overlap among the dietary niches of the seven species, with the exception of the synodontid catfish S. schall, is consistent with our second hypothesis.
Ecosphere, 2014
2014. Recruitment facilitation and spatial pattern formation in soft-bottom mussel beds. Ecospher... more 2014. Recruitment facilitation and spatial pattern formation in soft-bottom mussel beds. Ecosphere 5(12):160. http://dx.
Forage fish play a pivotal role in marine ecosystems and economies worldwide by sustaining many p... more Forage fish play a pivotal role in marine ecosystems and economies worldwide by sustaining many predators and fisheries directly and indirectly. We estimate global forage fish contributions to marine ecosystems through a synthesis of 72 published Ecopath models from around the world. Three distinct contributions of forage fish were examined: (i) the ecological support service of forage fish to predators in marine ecosystems, (ii) the total catch and value of forage fisheries and (iii) the support service of forage fish to the catch and value of other commercially targeted predators. Forage fish use and value varied and exhibited patterns across latitudes and ecosystem types. Forage fish supported many kinds of predators, including fish, seabirds, marine mammals and squid. Overall, forage fish contribute a total of about $16.9 billion USD to global fisheries values annually, i.e. 20% of the global ex-vessel catch values of all marine fisheries combined. While the global catch value of forage fisheries was $5.6 billion, fisheries supported by forage fish were more than twice as valuable ($11.3 billion). These estimates provide important information for evaluating the trade-offs of various uses of forage fish across ecosystem types, latitudes and globally. We did not estimate a monetary value for supportive contributions of forage fish to recreational fisheries or to uses unrelated to fisheries, and thus the estimates of economic value reported herein understate the global value of forage fishes.
Hydrological regimes are significant drivers of fisheries production in many African Lakes due to... more Hydrological regimes are significant drivers of fisheries production in many African Lakes due to their influence on fish habitat and food availability, breeding success, and catchability. Lake Turkana, Kenya, will undergo substantial changes in hydrology due to water regulation and extraction along the Omo River in neighboring Ethiopia, which provides over 90% of its water. The objective of this study was to predict how the lake's fisheries, which provide an important livelihood and protein source in the region, will respond to hydrological change. While variations in fishing effort are poor predictors of fisheries catch in the lake, water levels and their fluctuations strongly influence fisheries production. Seasonal oscillations play a particularly important role, and with complete loss of these oscillations, the lake's predicted fisheries yield will decrease by over two thirds. The fishery is predicted to collapse at a lake level decline of 25 m, regardless of seasonal amplitude magnitude. The lake's total littoral habitat, where fisheries are currently concentrated, will increase in surface area with lake level declines of <25 m. However, the extent of productive, dynamic littoral habitat will decrease with dampening of the lake's seasonal oscillations. The most severe habitat loss will occur in the lake's Turkwel Sector, which hosts the region's highest human population densities, and North Sector, where inter‐tribal conflict over resources is common and likely to be exacerbated by lake level decline. The continued ecological functioning of Lake Turkana necessitates immediate efforts to develop and apply a water resource management plan rooted in science.
Thesis Chapters by Natasha Gownaris
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Papers by Natasha Gownaris
Thesis Chapters by Natasha Gownaris