Catherine A. Pfister

Dynamic patterns and ecological impacts of declining ocean pH in a high-resolution multi-year dataset

Increasing global concentrations of atmospheric CO2 are predicted to decrease ocean pH, with potentially severe impacts on marine food webs, but empirical data documenting ocean pH over time are limited. In a high-resolution dataset spanning 8 years, pH at a north-temperate coastal site declined with increasing atmospheric CO2 levels and varied substantially in response to biological processes and physical conditions that fluctuate over multiple time scales. Applying a method to link environmental change to species dynamics via multispecies Markov chain models reveals strong links between in situ benthic species dynamics and variation in ocean pH, with calcareous species generally performing more poorly than noncalcareous species in years with low pH. The models project the long-term consequences of these dynamic changes, which predict substantial shifts in the species dominating the habitat as a consequence of both direct effects of reduced calcification and indirect effects arising from the web of species interactions. Our results indicate that pH decline is proceeding at a more rapid rate than previously predicted in some areas, and that this decline has ecological consequences for near shore benthic ecosystems.

Chemical defense against different marine herbivores: are amphipods insect equivalents?

The Structurally similar diterpenoid alcohols pachydictyol-A and dictyol-E are produced by the brown seaweed Dictyota dichotoma. This seaweed and several related species that also produce these compounds are known to be relatively low preference foods for tropical fishes and urchins. We evaluated the effect of various concentrations of these compounds on feeding by the three common types of herbivores that co-occur with Dictyota in coastal North Carolina. Fish (Diplodus holbrooki), sea urchins (Arbacia punctulata), and a mixed species group of gammarid amphipods were offered pieces of the palatable seaweed Gracilaria tikvahiae coated with either (1) dictyol-E or pachydictyol-A dissolved in diethyl ether or (2) diethyl ether alone. Dictyol-E significantly reduced consumption by fish and urchins at concentrations of 0.5 and 1.0% of algal dry mass, but had no effect on amphipod grazing. Pachydictyol-A significantly reduced fish grazing at the relatively high concentrations of 1.0 and 1.3% of plant dry mass; at 0.5% it tended to decrease grazing, but the effect was not significant (P = .07). Pachydictyol-A had no effect on urchin grazing and significantly increased amphipod grazing. When Pachydictyol-A was fed to fish as 1.0% of food dry mass, their growth rate was reduced by a significant 48%. In feeding preference experiments with several seaweeds, Dictyota ranks low for fish and urchins but high for amphipods. This is consistent with the hypothesis that the secondary metabolites produced by Dictyota play a major role in determining its susceptibility to herbivores. The ability of amphipods to circumvent the chemical defenses (Dictyota, and the fact that the two species of algae most readily consumed by amphipods (Codium and Dictyota) were the two species least readily consumed by fish, suggest that predation and herbivory by fishes may be major factors selecting for amphipods that can live on, and eat, seaweeds that are unpalatable to fishes. Amphipods that fed on Dictyota did not appear to sequester the Dictyota metabolites; when exposed to fish predation, Dictyota-fed amphipods were eaten as readily as amphipods that had fed on an alga with no defensive chemistry. Tubicolous amphipods and other small marine herbivores that may spend significant portions of their lives on only a few plants my be under very different evolutionary constraints than the larger, more mobile herbivores that commonly moved between many plants. Several characteristics of these smaller, less mobile, and much less studied, marine herbivores suggest that they may be ecologically similar to terrestrial insects and may play a large, but presently unappreciated, role in structuring marine plant communities.

LONGEVITY CAN BUFFER PLANT AND ANIMAL POPULATIONS AGAINST CHANGING CLIMATIC VARIABILITY

Both means and year-to-year variances of climate variables such as temperature and precipitation are predicted to change. However, the potential impact of changing climatic variability on the fate of populations has been largely unexamined. We analyzed multiyear demographic data for 36 plant and animal species with a broad range of life histories and types of environment to ask how sensitive their long-term stochastic population growth rates are likely to be to changes in the means and standard deviations of vital rates (survival, reproduction, growth) in response to changing climate. We quantified responsiveness using elasticities of the long-term population growth rate predicted by stochastic projection matrix models. Short-lived species (insects and annual plants and algae) are predicted to be more strongly (and negatively) affected by increasing vital rate variability relative to longer-lived species (perennial plants, birds, ungulates). Taxonomic affiliation has little power to explain sensitivity to increasing variability once longevity has been taken into account. Our results highlight the potential vulnerability of short-lived species to an increasingly variable climate, but also suggest that problems associated with short-lived undesirable species (agricultural pests, disease vectors, invasive weedy plants) may be exacerbated in regions where climate variability decreases.

Associational plant refuges: convergent patterns in marine and terrestrial communities result from differing mechanisms

SummaryAn associational plant refuge occurs when a plant that is susceptible to herbivory gains protection from herbivory when it is associated with another plant. In coastal North Carolina, the abundance of the palatable red alga Gracilaria tikvahiae is positively correlated with the abundance of the unpalatable brown alga Sargassum filipendula during times of increased herbivore activity. To see if grazing by the sea urchin Arbacia punctulata could generate this pattern, controlled experiments were conducted in out-door microcosms and in the laboratory. Gracilaria beneath a canopy of Sargassum was eaten significantly less than Gracilaria alone. When Arbacia were excluded, Gracilaria alone grew significantly more than Gracilaria beneath Sargassum, demonstrating that Sargassum is a competitor of Gracilaria. Experiments investigating Sargassums deterrent role indicated that Sargassum decreased the foraging range of Arbacia and the rate at which it fed on Gracilaria. Additional experiments with plastic Sargassum mimics indicated that the decreased grazing on Gracilaria was not a result of Sargassum morphology, but was probably attributable to some chemical characteristic of Sargassum. The pattern of increased grazing in monocultures (only Gracilaria present) versus polycultures (both Gracilaria and Sargassum present) demonstrated in this study also has been demonstrated for plant-insect interactions in terrestrial communities. In these communities, insect density is higher in monocultures than in polycultures because insects find and immigrate to monocultures more rapidly, and once in a monoculture, they emigrate from them less often than from polycultures. In this study, urchins did not find and immigrate to monocultures more rapidly, nor did they tend to stay in them once they were found; in fact, they emigrated from monocultures of Gracilaria more rapidly than from Gracilaria and Sargassum polycultures. Increased grazing in Gracilaria monocultures resulted from increased rates of movement and feeding of individual herbivores, not from increased herbivore density as has been reported for terrestrial systems.

Harvesting Red Sea Urchins: Recent Effects and Future Predictions

The red sea urchin, Strongylocentrotus franciscanus, is a conspicuous member of subtidal communities in the north Pacific. Within the last decade, this ecologically important species has been exposed to intense harvesting for the first time ever. Analysis of population census data suggest that harvestable size urchins have rapidly declined in shallow regions while catch-per-effort and landings data suggest that divers have maintained high landings by exploiting more distant and difficult fishing areas, including deeper areas. We present a size-structured model for the red sea urchin both to estimate what previous levels of harvesting mortality were and to explore what effect future harvesting strategies might have on population trajectories. Using population census data, we explore three models: one that would result in an equilibrial population size in the absence of fishing, one that includes positive density dependence (an Allee effect), and one that incorporates realistic variability in recruitment. O...

Correctly Estimating How Environmental Stochasticity Influences Fitness and Population Growth

Increased temporal variance in life‐history traits is generally predicted to decrease individual fitness and population growth. We show that a widely used result of stochastic sensitivity analysis that bolsters this generality is flawed because it ignores the effects of correlations between vital rates. Considering the effects of these correlations (although ignoring autocorrelations), we show that the apparently simple relationship between vital rate variance and fitness can be considerably more complex than previously thought. In particular, the previously estimated negative sensitivities of fitness or population growth to variance in a vital rate can be either enhanced by positive correlations between rates or reversed by negative correlations, even to the point that variability in a rate can increase fitness or population growth. We apply this new sensitivity calculation to data from the desert tortoise and discuss its interpretation in light of the factors generating vital rate correlations.

INDIVIDUAL VARIATION AND ENVIRONMENTAL STOCHASTICITY: IMPLICATIONS FOR MATRIX MODEL PREDICTIONS

Populations are characterized by variability among individuals, a result of both intrinsic differences among individuals and environmental effects on individual per- formance. Despite the ubiquity of individual variation, its implications for population model choice are not fully understood. Population models that use state variables representing individual features such as age or size assume that these state variables can predict the population trajectory. However, state variables are often chosen based on convenience or necessity; only rarely are they tested for importance and accuracy when individuals vary. We examined whether matrix projection models, a common choice in population modeling, provide accurate predictions when individuals vary in a population. With both density- dependent and density-independent formulations, we tested whether matrix projection mod- els that used size as a state variable captured the dynamics of populations projected with an individual-based simulation (or i-state configuration model). We varied the initial size distribution of individuals, the degree to which individual growth was size dependent, the tendency for positive correlations in growth through time, and the amount of stochasticity in growth, and we asked what conditions affect the accuracy of matrix models. Stochasticity alone did not compromise the predictions of matrix models; rather, only populations with individual variation generated by strong size-dependent growth and growth correlations were poorly described by matrix models. Otherwise, matrix models captured the trajectories of populations with a fair degree of accuracy. Overall, our results provide guidance as to when and how individual variation must be included in population projections, and when a simple matrix model framework is inadequate.

Estimating Competition Coefficients from Census Data: A Test with Field Manipulations of Tidepool Fishes

Interaction coefficients between species have been estimated using both static regression models (from a single census) and dynamic regression models (incorporating population change through time). However, the output of these varied models has not been compared with the results of experimental manipulations within the same system. Using a guild of tidepool sculpins, I tested whether the competition coefficients obtained from static and dynamic regression models were consistent with the results of experimental manipulations. Field manipulations revealed a competitive effect of Oligocottus maculosus on Clinocottus globiceps growth and no effect of C. globiceps on O. maculosus growth. The result of dynamic regression models most closely matched the results of the experiments. Static regression models, in contrast, were less consistent with experimental results, sometimes even predicting interactions with a different sign from those in experiments. I show that the inconsistency in the static regression models may result from the way covariance in carrying capacities directly affects variability in equilibrium population sizes, yielding an erroneous interaction coefficient. The dynamic regression models are free of equilibria assumptions and more accurately predicted a competitive effect of O. maculosus on C. globiceps but no effect of C. globiceps on O. maculosus. These analyses illustrate why dynamically derived measures of interaction strength, rather than static ones, may better predict effects of one species on another.

Costs of Reproduction in an Intertidal Kelp: Patterns of Allocation and Life History Consequences

Reproductive investment ofthe intertidal kelp A/aria nana was manipulated experimentally to investigate costs of reproduction. Allocation patterns were examined using treatments that employed vegetative and reproductive tissue removal. Growth, sur- vivorship, size, reproductive investment, carbon, nitrogen, and phenolic secondary me- tabolites were monitored in tagged experimental plants. Further experiments investigated the contribution of reproductive tissue to drag forces on the plant in intertidal environments. Carbon, nitrogen, and phenolics were all higher in reproductive tissue (sporophylls) com- pared to vegetative tissue (the frond) in control plants. When some vegetative tissue was removed, carbon and nitrogen in the sporophylls decreased while percent phenolics re- mained unchanged. Reproductive tissue removal had no effect on plant survivorship, growth, or size compared with control plants. When the entire frond was kept clipped, sporophyll number decreased, often to the point of complete attrition. Although allocation patterns in 41Iaria nana are consistent with reproductive costs (i.e., the sporophylls are not self-supporting), there is no evidence that reproduction imposes costs on the plant that are demographically important such as growth, size, and survivorship. Instead, both repro- ductive investment and percent phenolics were positively correlated with growth and size in control plants.

The Role and Importance of Recruitment Variability to a Guild of Tide Pool Fishes

In a guild of tide pool fishes on the outer coast of Washington State, USA, recruitment varied both temporally (seasonally and interannually) and spatially (over in- tervals of 4-5 km, across a maximum of 30 km of shoreline). Using census data and mark- recapture data, I studied the demography of four species of tide pool sculpins with the goal of understanding how variability in recruitment affected adult population patterns. I found that, although adult populations reflected well the most recent recruitment pulse for the two most common species, adult population growth rate was the most sensitive to changes in survivorship rates. Similarly, analyses of the effect that variance in each demographic parameter had on variability in subsequent adult population size also showed a dispropor- tionate effect of adult survivorship on adult population sizes. In other words, high variability in a demographic estimate did not always result in high variability in population size. Negative density dependence was documented for all species, suggesting that recruitment effects are dampened by postrecruitment events. Thus, although recruitment events could be detected in the adult population for short time periods, postrecruitment events appeared to dampen the effects of seasonal recruitment pulses in these nearshore fishes.