To effectively balance investment in predator defenses versus other traits, organisms must accurately assess predation risk. Chemical cues caused by predation events are indicators of risk for prey in a wide variety of systems, but the relationship between how prey perceive risk in relation to the amount of prey consumed by predators is poorly understood. While per capita predation rate is often used as the metric of relative risk, studies aimed at quantifying predator-induced defenses commonly control biomass of prey consumed as the metric of risk. However,…(read more)
Predator-prey relationships define food webs and determine how much and how fast energy is transferred through ecosystems. Because most species are prey of multiple species of predators, it is essential to understand how effects of different predators combine to influence prey dynamics. Furthermore, species that are higher on the food chain often suffer greater effects from activities such as harvesting, habitat fragmentation, toxin bioaccumulation, and habitat degradation; thus understanding how decreasing predator diversity affects the rest of the ecosystem will be paramount. Ecologists have been investigating how the effects of multiple predators combine to affect foodwebs for more than 30 years. Recent reviews have shown that predators sometimes kill more and sometimes kill less prey than expected based on their independent effects. These unpredictable or emergent effects can arise, for example, when predators work cooperatively to subdue prey (leading to risk enhancement for prey) or when predators interfere with one another while foraging (leading to risk reduction prey). However, in this study we show that past models used to generate the null expectations for how predator effects should combine are biased. Previous models for predicting the combined effects of predators made two implicit (and generally unacknowledged assumptions): 1) that prey mortality rates were constant over the duration of the study; and 2) that prey density did not change over the course of the experiment. Both assumptions were likely violated in almost all studies. For example, most predators have functional response that lead to increasing prey mortality as prey density declines.
In a paper published on Sept. 23 2012 in the journal Ecology Letters, we showed that current models that predict how the effects of multiple predators combine are biased. Moreover, we show that the direction and magnitude of the bias depends on the study design and experimental duration.
This study calls into question dozens of studies and much of what we thought we had learned over the past 30+ years about how predator effects combine to influence prey mortality and other foodweb properties. Importantly, the implications of our findings are not isolated to only affecting our understanding effects of multiple predators. Indeed, our understanding of species diversity on a variety of ecological processes and ecosystem functions may be vulnerable to the same critique, which applies whenever changing densities are combined with some form of density-dependence. Finally, our paper provided a road map for moving forward that will improve our ability to understand and predict the importance of diversity on key ecological processes.
Link - Biology of Reproduction
As amniotes, mammals, reptiles, and birds form common extraembryonic membranes during development to perform essential functions, such as protection, nutrient transfer, gas exchange and waste removal. Together with the maternal uterus, extraembryonic membranes of viviparous (live-bearing) amniotes develop as an endocrine placenta that synthesizes and responds to steroid hormones critical for development. The ability of these membranes to synthesize and respond to steroid hormone signaling has traditionally been considered an innovation of placental amniotes. However, our laboratory recently demonstrated that this ability extends to the chorioallantoic membrane (CAM) of an oviparous (egg-laying) amniote, the domestic chicken, and we hypothesized that steroidogenic extraembryonic membranes could be an evolutionarily conserved characteristic of all amniotes due to similarities in basic structure, function, and shared evolutionary ancestry. In this study, we examined steroid hormone synthesis and signaling in the CAM of another oviparous amniote, the American alligator (Alligator mississippiensis). We quantified mRNA expression of a steroidogenic factor involved in the regulation of steroidogenesis (NR5A1), the key steroidogenic enzymes involved in the synthesis of progestins (HSD3B1), androgens (CYP17A1) and estrogens (CYP19A1), and the receptors involved in the signaling of progestins (PR), androgens (AR), estrogens (ESR1 and ESR2) and glucocorticoids (GR). Further, we performed protein immunolocalization for PR and ESR1. Collectively, our findings indicate that the alligator CAM has the capability to regulate, synthesize and respond to steroid hormone signaling; thus, supporting our hypothesis that the extraembryonic membranes of Amniota share an unifying characteristic, that is, the ability to synthesize and respond to steroid hormones.
Link - PNAS
More than 2 y have passed since the BP–Deepwater Horizon oil spill in the Gulf of Mexico, yet we still have little understanding of its ecological impacts. Examining effects of this oil spill will generate much-needed insight into how shoreline habitats and the valuable ecological services they provide (e.g., shoreline protection) are affected by and recover from large-scale disturbance. Here we report on not only rapid salt-marsh recovery (high resilience) but also permanent marsh area loss after the BP–Deepwater Horizon oil spill. Field observations, experimental manipulations, and wave-propagation modeling reveal that (i) oil coverage was primarily concentrated on the seaward edge of marshes; (ii) there were thresholds of oil coverage that were associated with severity of salt-marsh damage, with heavy oiling leading to plant mortality; (iii) oil-driven plant death on the edges of these marshes more than doubled rates of shoreline erosion, further driving marsh platform loss that is likely to be permanent; and (iv) after 18 mo, marsh grasses have largely recovered into previously oiled, noneroded areas, and the elevated shoreline retreat rates observed at oiled sites have decreased to levels at reference marsh sites. This paper highlights that heavy oil coverage on the shorelines of Louisiana marshes, already experiencing elevated retreat because of intense human activities, induced a geomorphic feedback that amplified this erosion and thereby set limits to the recovery of otherwise resilient vegetation. It thus warns of the enhanced vulnerability of already degraded marshes to heavy oil coverage and provides a clear example of how multiple human-induced stressors can interact to hasten ecosystem decline.