Herbivore Impacts on Marsh Production Depend upon a Compensatory Continuum Mediated by Salinity Stress

Abstract

Plant communities are disturbed by several stressors and they are expected to be further impacted by increasing anthropogenic stress. The consequences of these stressors will depend, in part, upon the ability of plants to compensate for herbivory. Previous studies found that herbivore impacts on plants can vary from negative to positive because of environmental control of plant compensatory responses, a.k.a. the Compensatory Continuum Hypothesis. While these influential studies enhanced our appreciation of the dynamic nature of plant-herbivore interactions, they largely focused on the impact of resource limitation. This bias limits our ability to predict how other environmental factors will shape the impact of herbivory. We examined the role of salinity stress on herbivory of salt marsh cordgrass, Spartina foliosa, by an herbivore previously hypothesized to influence the success of restoration projects (the scale insect, Haliaspis spartinae). Using a combination of field and mesocosm manipulations of scales and salinity, we measured how these factors affected Spartina growth and timing of senescence. In mesocosm studies, Spartina overcompensated for herbivory by growing taller shoots at low salinities but the impact of scales on plants switched from positive to neutral with increasing salinity stress. In field studies of intermediate salinities, scales reduced Spartina growth and increased the rate of senescence. Experimental salinity additions at this field site returned the impact of scales to neutral. Because salinity decreased scale densities, the switch in impact of scales on Spartina with increasing salinity was not simply a linear function of scale abundance. Thus, the impact of scales on primary production depended strongly upon environmental context because intermediate salinity stress prevented plant compensatory responses to herbivory. Understanding this context-dependency will be required if we are going to successfully predict the success of restoration efforts and the ecological consequences of anthropogenic disturbances.

Density‐mediated indirect effects from active predators and narrow habitat domain prey

“…Therefore, by increasing cordgrass root productivity, lethal ladybeetles may have profound effects on both the structure and function of salt marsh communities. Such effects may be particularly pronounced for cordgrass experiencing intermediate salinities where scale insects suppress cordgrass production (Long and Porturas ). Understanding the complex interactions between salt marsh predators, consumers, and primary producers may serve to inform restoration and management strategies.…”

“…Such efforts have experienced variable success (Langis et al 1991, Boyer and Zedler 1996, Zedler and Callaway 1999, Callaway and Zedler 2004. One factor impacting the success of restorations may be scale insect outbreaks (Boyer and Zedler 1996), as scale insects can reduce the productivity of cordgrass plants (Long and Porturas 2014). Therefore, identifying predators, such as ladybeetles, that can suppress scale insect populations and facilitate cordgrass growth may enhance marsh restoration success in southern California.…”

Habitat use is linked to resource‐specific performance of an ecologically important marsh predator

“…In each plot, we manipulated scale insects (present/absent) and pollen access (present/absent) in a 2 × 2 fully factorial design ( n = 8). In scale insect absent plots, we removed all scale insects on cordgrass stems using a soft toothbrush (Long and Porturas , Rinehart et al. ).…”

“…We manipulated collected cordgrass stems to represent one of the four habitat treatments. In the scale insect absent treatment, we removed all scale insects from cordgrass stems using a soft toothbrush (Long and Porturas , Rinehart et al. ).…”

Identifying Salt Marsh Shorelines from Remotely Sensed Elevation Data and Imagery

“…Unfortunately, salt marshes are increasingly threatened by anthropogenic factors [6][7][8][9], as well as sea level rise [10,11] and sediment deficits [12]. Marsh health can also be influenced by plant-herbivore interactions [13,14]. Given the substantial ecosystem services provided by marshes, it is important to be able to measure their extent and seaward edge at high temporal and spatial resolutions.…”