Evaluating the relative roles of positive and negative interactions in communities : shade, herbivory and physiological stress in the rocky intertidal zone

Evaluating the relative influences of positive and negative interactions in shaping communities is a major topic in modern ecology. Facilitative interactions between basal species are important in habitats with intense predation pressure or severe abiotic stresses. However, few studies address the potential for positive interactions between trophic levels to influence community structure by altering patterns of predation. I investigated whether the association between the canopy-forming alga Hedophyllum sessile and the herbivorous chiton Katharine tunicata was due to the provision of a refuge from predation, a preference for Hedophyllum as a food item, or amelioration of abiotic conditions. In a field experiment, Katharine were not affected by predation or Hedophyllum thalli, but showed a strong behavioral selection for shaded areas during summertime low tides. By providing shade, Hedophyllum controls the distribution of the system's major herbivore. In a second field experiment I evaluated the relative effects of shade and Katharina on the rest of the community. Shade had strong positive effects on a suite of consumers, increasing abundances of seven animal groups relative to unshaded areas. Shade and Katharina had quantitatively equal negative effects on the abundance of basal species, but their effects were qualitatively very different. The positive interaction between Hedophyllum and Katharina affects the entire community by altering patterns of herbivory. Such complex networks of positive, negative, direct and indirect interactions can produce deceptively simple patterns in natural systems. I used field experiments and laboratory analyses to evaluate potential physiological benefits of this positive interaction on Katharina. Levels of heat shock protein 70 isoforms in field populations were greater in summer than in winter, suggesting that Katharina are experiencing seasonal sub-lethal stress. Although shade did not affect Hsp7O levels in Katharina maintained in field enclosures, amelioration of abiotic stresses through positive biotic interactions could have direct physiological consequences for beneficiary species. These studies provide strong evidence that positive interactions between trophic levels can profoundly affect the physiology of individuals, the distribution and abundance of populations, and the structure of communities. I present a conceptual model to summarize predictions of the importance of these multi-level positive interactions in structuring communities

Appendix A. A description of methods for field tissue collection and quantification of Hsp70 isoforms using Western blot techniques.

A description of methods for field tissue collection and quantification of Hsp70 isoforms using Western blot techniques

Appendix C. A summary of univariate ANOVA analyses (by season) of Katharina abundance under experimental devices.

A summary of univariate ANOVA analyses (by season) of Katharina abundance under experimental devices

Appendix B. A summary of repeated-measures ANOVA on the effect of birds and shade on the abundance of Katharina under experimental devices.

A summary of repeated-measures ANOVA on the effect of birds and shade on the abundance of Katharina under experimental devices

Latitudinal variation in long-term stability of North American rocky intertidal communities.

Although long-term ecological stability is often discussed as a community attribute, it is typically investigated at the species level (e.g. density, biomass), or as a univariate metric (e.g. species diversity). To provide a more comprehensive assessment of long-term community stability, we used a multivariate similarity approach that included all species and their relative abundances. We used data from 74 sites sampled annually from 2006 to 2017 to examine broad temporal and spatial patterns of change within rocky intertidal communities along the west coast of North America. We explored relationships between community change (inverse of stability) and the following potential drivers of change/stability: (a) marine heatwave events; (b) three attributes of biodiversity: richness, diversity and evenness and (c) presence of the mussel, Mytilus californianus, a dominant space holder and foundation species in this system. At a broad scale, we found an inverse relationship between community stability and elevated water temperatures. In addition, we found substantial differences in stability among regions, with lower stability in the south, which may provide a glimpse into the patterns expected with a changing climate. At the site level, community stability was linked to high species richness and, perhaps counterintuitively, to low evenness, which could be a consequence of the dominance of mussels in this system. Synthesis. Assessments of long-term stability at the whole-community level are rarely done but are key to a comprehensive understanding of the impacts of climate change. In communities structured around a spatially dominant species, long-term stability can be linked to the stability of this 'foundation species', as well as to traditional predictors, such as species richness

Latitudinal variation in long-term stability of North American rocky intertidal communities.

Although long-term ecological stability is often discussed as a community attribute, it is typically investigated at the species level (e.g. density, biomass), or as a univariate metric (e.g. species diversity). To provide a more comprehensive assessment of long-term community stability, we used a multivariate similarity approach that included all species and their relative abundances. We used data from 74 sites sampled annually from 2006 to 2017 to examine broad temporal and spatial patterns of change within rocky intertidal communities along the west coast of North America. We explored relationships between community change (inverse of stability) and the following potential drivers of change/stability: (a) marine heatwave events; (b) three attributes of biodiversity: richness, diversity and evenness and (c) presence of the mussel, Mytilus californianus, a dominant space holder and foundation species in this system. At a broad scale, we found an inverse relationship between community stability and elevated water temperatures. In addition, we found substantial differences in stability among regions, with lower stability in the south, which may provide a glimpse into the patterns expected with a changing climate. At the site level, community stability was linked to high species richness and, perhaps counterintuitively, to low evenness, which could be a consequence of the dominance of mussels in this system. Synthesis. Assessments of long-term stability at the whole-community level are rarely done but are key to a comprehensive understanding of the impacts of climate change. In communities structured around a spatially dominant species, long-term stability can be linked to the stability of this 'foundation species', as well as to traditional predictors, such as species richness

Crossing Party Lines: Political Identity and Partisans’ Reactions to Violating Party Norms

The current studies examined the experiences of undergraduate political partisans who cross party lines to support a preferred, out-of-party candidate, and thus open themselves to the possibility of being misclassified as a member of a rival political party. Strongly identified partisans who endorsed an out-of-party candidate, and thus expected others to misclassify them, reported heightened threats to belonging and coherence (Study 1), unless they disclaimed rival party status by asserting their political affiliation. In Study 2, strongly identified partisans who could be misclassified were less confident in their choice of an out-of-party candidate compared to partisans who asserted their political affiliation. These results highlight the impact of identity misclassification concerns on strongly identified partisans whose personal preferences are inconsistent with party norms

Large-scale impacts of sea star wasting disease (SSWD) on intertidal sea stars and implications for recovery.

Disease outbreaks can have substantial impacts on wild populations, but the often patchy or anecdotal evidence of these impacts impedes our ability to understand outbreak dynamics. Recently however, a severe disease outbreak occurred in a group of very well-studied organisms-sea stars along the west coast of North America. We analyzed nearly two decades of data from a coordinated monitoring effort at 88 sites ranging from southern British Columbia to San Diego, California along with 2 sites near Sitka, Alaska to better understand the effects of sea star wasting disease (SSWD) on the keystone intertidal predator, Pisaster ochraceus. Quantitative surveys revealed unprecedented declines of P. ochraceus in 2014 and 2015 across nearly the entire geographic range of the species. The intensity of the impact of SSWD was not uniform across the affected area, with proportionally greater population declines in the southern regions relative to the north. The degree of population decline was unrelated to pre-outbreak P. ochraceus density, although these factors have been linked in other well-documented disease events. While elevated seawater temperatures were not broadly linked to the initial emergence of SSWD, anomalously high seawater temperatures in 2014 and 2015 might have exacerbated the disease's impact. Both before and after the onset of the SSWD outbreak, we documented higher recruitment of P. ochraceus in the north than in the south, and while some juveniles are surviving (as evidenced by transition of recruitment pulses to larger size classes), post-SSWD survivorship is lower than during pre-SSWD periods. In hindsight, our data suggest that the SSWD event defied prediction based on two factors found to be important in other marine disease events, sea water temperature and population density, and illustrate the importance of surveillance of natural populations as one element of an integrated approach to marine disease ecology. Low levels of SSWD-symptomatic sea stars are still present throughout the impacted range, thus the outlook for population recovery is uncertain

Total number of juvenile <i>P</i>. <i>ochraceus</i> (“radius” ≤ 30 mm) counted within long-term permanent plots at each site over time.

<p>For sites that were sampled > 1 time year^-1, the mean total number for that year is displayed. 2013 is indicated by a vertical line, and separates pre-SSWD years from post-SSWD years. Unshaded cells represent years when surveys were not done.</p