Diana M. T. Sharpe

Life history change in commercially exploited fish stocks: an analysis of trends across studies

Age and size at maturation have declined dramatically in many commercial fish stocks over the past few decades – changes that have been widely attributed to fishing pressure. We performed an analysis of such trends across multiple studies, to test for the consistency of life history changes under fishing, and for their association with the intensity of exploitation (fishing mortality rate). We analyzed 143 time series from 37 commercial fish stocks, the majority of which originated from the North Atlantic. Rates of phenotypic change were calculated for two traditional maturation indices (length and age at 50% maturity), as well as for probabilistic maturation reaction norms (PMRNs). We found that all three indices declined in heavily exploited populations, and at a rate that was strongly correlated with the intensity of fishing (for length at 50% maturity and PMRNs). These results support previous assertions that fishing pressure is playing a major role in the life history changes observed in commercial fish stocks. Rates of change were as strong for PMRNs as for age and size at 50% maturity, which is consistent with the hypothesis that fishing‐induced phenotypic changes can sometimes have a genetic basis.

Life history change in response to fishing and an introduced predator in the East African cyprinid Rastrineobola argentea

Fishing and introduced species are among the most important stressors affecting freshwaters and can also be strong selective agents. We examined the combined effects of commercial fishing and an introduced predator (Nile perch, Lates niloticus) on life history traits in an African cyprinid fish (Rastrineobola argentea) native to the Lake Victoria basin in East Africa. To understand whether these two stressors have driven shifts in life history traits of R. argentea, we tested for associations between life history phenotypes and the presence/absence of stressors both spatially (across 10 Ugandan lakes) and temporally (over four decades in Lake Victoria). Overall, introduced Nile perch and fishing tended to be associated with a suite of life history responses in R. argentea, including: decreased body size, maturation at smaller sizes, and increased reproductive effort (larger eggs; and higher relative fecundity, clutch volume, and ovary weight). This is one of the first well‐documented examples of fisheries‐induced phenotypic change in a tropical, freshwater stock; the magnitude of which raises some concerns for the long‐term sustainability of this fishery, now the most important (by mass) in Lake Victoria.

Little evidence for morphological change in a resilient endemic species following the introduction of a novel predator

Human activities, such as species introductions, are dramatically and rapidly altering natural ecological processes and often result in novel selection regimes. To date, we still have a limited understanding of the extent to which such anthropogenic selection may be driving contemporary phenotypic change in natural populations. Here, we test whether the introduction of the piscivorous Nile perch, Lates niloticus, into East Africas Lake Victoria and nearby lakes coincided with morphological change in one resilient native prey species, the cyprinid fish Rastrineobola argentea. Drawing on prior ecomorphological research, we predicted that this novel predator would select for increased allocation to the caudal region in R. argentea to enhance burst‐swimming performance and hence escape ability. To test this prediction, we compared body morphology of R. argentea across space (nine Ugandan lakes differing in Nile perch invasion history) and through time (before and after establishment of Nile perch in Lake Victoria). Spatial comparisons of contemporary populations only partially supported our predictions, with R. argentea from some invaded lakes having larger caudal regions and smaller heads compared to R. argentea from uninvaded lakes. There was no clear evidence of predator‐associated change in body shape over time in Lake Victoria. We conclude that R. argentea have not responded to the presence of Nile perch with consistent morphological changes and that other factors are driving observed patterns of body shape variation in R. argentea.

Exploring the effects of salinization on trophic diversity in freshwater ecosystems: a quantitative review

Salinization of freshwater ecosystems represents a potential threat to biodiversity, but the distribution of salinity tolerance among freshwater organisms and its functional consequences are understudied. In this study, we reviewed global patterns of salinity tolerance across a broad range of freshwater organisms. Specifically, we compared published data on LC50 (a metric of salinity tolerance) across climatic regions, taxa, and functional feeding groups (FFGs). We found that microinvertebrates were more sensitive to salinity than vertebrates and macroinvertebrates. Within aquatic insects, there was considerable variability in tolerance across FFGs. Specifically, scrapers, gatherers, and filterers were more sensitive on average than omnivores, shredders, and predators. Thus, we predict that increasing salinization can negatively impact trophic diversity and in turn cause overall changes in the structure and function of freshwater ecosystems. We also identified both historical exposure and taxonomic affinity as potential drivers of contemporary salinity tolerance across freshwater organisms. Finally, we found important gaps in our understanding of the potential impacts of salinization on freshwater biodiversity, particularly in regions expected to be affected by increased salinization due to climate change and secondary salinization. Understanding the differential vulnerability of freshwater taxa is critical to predicting the ecosystem impacts of salinization, and informing conservation and management decisions.

Low Predictability of Colour Polymorphism in Introduced Guppy (Poecilia reticulata) Populations in Panama

Colour polymorphism is a recurrent feature of natural populations, and its maintenance has been studied in a range of taxa in their native ranges. However, less is known about whether (and how) colour polymorphism is maintained when populations are removed from their native environments, as in the case of introduced species. We here address this issue by analyzing variation in colour patterns in recently-discovered introduced populations of the guppy (Poecilia reticulata) in Panama. Specifically, we use classic colour analysis to estimate variation in the number and the relative area of different colour spots across low predation sites in the introduced Panamanian range of the species. We then compare this variation to that found in the native range of the species under low- and high predation regimes. We found aspects of the colour pattern that were both consistent and inconsistent with the classical paradigm of colour evolution in guppies. On one hand, the same colours that dominated in native populations (orange, iridescent and black) were also the most dominant in the introduced populations in Panama. On the other, there were no clear differences between either introduced-low and native low- and high predation populations. Our results are therefore only partially consistent with the traditional role of female preference in the absence of predators, and suggest that additional factors could influence colour patterns when populations are removed from their native environments. Future research on the interaction between female preference and environmental variability (e.g. multifarious selection), could help understand adaptive variation in this widely-introduced species, and the contexts under which variation in adaptive traits parallels (or not) variation in the native range.

Galapagos Mockingbird (Mimus parvulus) Preys on an Invasive Mammal

ABSTRACT Galapagos Mockingbirds (Mimus parvulus) are opportunistic feeders that have been observed engaging in a variety of unusual predatory behaviors. Here, we report on a specific behavior that we observed: a Galapagos Mockingbird preying on an invasive mammal by repeatedly hitting it on the ground. We discuss the reasons that the mockingbird might have engaged in this behavior and the potential implications this could have for native biodiversity and conservation on the Galapagos Islands.

Urbanization erodes niche segregation in Darwin's finches

Urbanization is influencing patterns of biological evolution in ways that are only beginning to be explored. One potential effect of urbanization is in modifying ecological resource distributions that underlie niche differences and that thus promote and maintain species diversification. Few studies have assessed such modifications, or their potential evolutionary consequences, in the context of ongoing adaptive radiation. We study this effect in Darwins finches on the Galápagos Islands, by quantifying feeding preferences and diet niche partitioning across sites with different degrees of urbanization. We found higher finch density in urban sites and that feeding preferences and diets at urban sites skew heavily toward human food items. Furthermore, we show that finches at urban sites appear to be accustomed to the presence of people, compared with birds at sites with few people. In addition, we found that human behavior via the tendency to feed birds at non‐urban but tourist sites is likely an important driver of finch preferences for human foods. Site differences in diet and feeding behavior have resulted in larger niche breadth within finch species and wider niche overlap between species at the urban sites. Both factors effectively minimize niche differences that would otherwise facilitate interspecies coexistence. These findings suggest that both human behavior and ongoing urbanization in Galápagos are starting to erode ecological differences that promote and maintain adaptive radiation in Darwins finches. Smoothing of adaptive landscapes underlying diversification represents a potentially important yet underappreciated consequence of urbanization. Overall, our findings accentuate the fragility of the initial stages of adaptive radiation in Darwins finches and raise concerns about the fate of the Galápagos ecosystems in the face of increasing urbanization.