Effects of benthic consumer diversity on stream ecosystem function

Steven Conor Keitzer, Purdue University

Abstract

Freshwater consumer diversity is being changed at an alarming rate, the causes and ecological consequences of which are often unclear. This is particularly true for amphibian species, which are abundant but often overlooked consumers in many stream ecosystems. The goals of this study were to (1) Examine the impacts of larval salamander diversity on stream ecosystem functioning, (2) Identify potential stressors to salamander diversity, and (3) Examine more generally how changes in consumer functional diversity impact stream ecosystems. In Chapter 1, I used in-stream enclosures to examine the impacts of salamander diversity on macroinvertebrate communities in an Appalachian headwater stream. I found that larval salamanders generally reduced the abundance of macroinvertebrates, but that the more diverse salamander treatment substantially reduced macroinvertebrate abundance compared to control treatments or salamander monocultures. These effects were non-additive and suggest that niche complementarity or facilitation occurred among salamander species. Thus, alterations to salamander diversity may affect macroinvertebrate communities and have cascading effects on stream ecosystems. In Chapter 2, I examined spatial and seasonal heterogeneity in nutrient recycling by larval salamander communities in Appalachian headwater streams. Larval salamander nutrient excretion demonstrated strong spatial and seasonal patterns, contributing c. 17% of the ecosystem demand for NH4+-N prior to leaf fall but only c. 5% after leaf fall. However, in some areas of the stream where salamanders were abundant, they were capable of meeting as much as 50% of the demand for NH4+-N. This suggests that salamanders may create biogeochemical hotspots within streams, but this will vary seasonally. In Chapter 3, I examined how changes in detrital diversity may impact larval salamanders. I found that detrital composition, but not species richness, had a significant effect on salamander abundance. This appeared to be related to differences in prey availability in leaf packs, rather than to structural differences in the habitat provided by different leaf species. Thus, ongoing changes in Appalachian forest diversity may negatively impact salamander populations by disrupting stream food webs. In Chapter 4, I examined a more diverse consumer assemblage in Indiana to examine how changes in consumer functional diversity may impact several ecosystem properties. I found that increasing consumer functional diversity had an overall positive impact on ecosystem productivity, suggesting that ongoing changes in aquatic consumer communities will have a large impact on ecosystem functioning. My results suggest that consumers have a large impact on many stream ecosystem attributes and that changes in consumer diversity may disrupt stream ecosystem functioning. Changes in consumer functional diversity may be particularly important as my results demonstrate that the functional diversity of consumers has a large influence on the impact of consumers on multiple ecosystem attributes. Salamanders appear to be underappreciated consumers in headwater stream ecosystems, and their loss from these ecosystems may alter the supply of biologically important nutrients and the structure of headwater stream food webs. Salamander populations are potentially vulnerable to a multitude of stressors, and I found that changes in leaf litter inputs to streams associated with altered riparian forests might negatively impact salamander populations. More research is needed to understand the population-level effects of altered detrital quality for stream-breeding salamanders.

Degree

Ph.D.

Advisors

Goforth, Purdue University.

Subject Area

Ecology|Aquatic sciences

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