Mechanisms underlying lack of functional compensation by insect grazers after tadpole declines in a Neotropical stream

Abstract

Resilience of ecosystems to the sudden decline of large‐bodied species is dependent on characteristics of surviving guild members. However, that response may also be mediated by local habitat conditions. Here, we examine the mechanisms behind the observed lack of functional compensation in the algal‐grazing guild by insect grazers following the decline of tadpole grazers in a forested Panamanian stream. We examined: (1) shifts to the individual size distribution of insect grazers between pre‐ and post‐tadpole declines in pool and riffle habitats; (2) tadpole and insect preferences for small‐, medium‐, and large‐sized diatoms; and (3) a causal explanation for why insects did not functionally compensate for tadpole declines. The size distribution of insect grazers following tadpole declines differed between habitats, becoming uniform in pools and more right skewed toward a smaller size class in riffles. In both habitats, tadpoles selectively consumed medium‐sized diatoms but avoided the largest‐sized diatoms. In contrast, grazing insects selectively consumed small‐sized diatoms, but switched to medium‐sized diatoms after tadpole declines. Tadpole declines led to the loss of the strongest interactions between consumers and diatoms. Smaller‐bodied grazing insects could not duplicate these interactions, even with a shift in resource use, providing an explanation for the lack of functional compensation. Furthermore, tadpole declines led to different community structures in each habitat indicating that local habitat conditions mediated the response of surviving guild members. This suggests that the sudden decline of a large‐bodied species does not lead to a singular outcome for the surviving community.

Amphibian loss alters periphyton structure and invertebrate growth in montane streams

“…In our study, the reduction in periphyton biomass was more likely caused by consumption, since we did not find any differences in sediment accrual among treatments without consumers and the ones containing tadpoles or caddisflies, and the effect of stream (mainly driven by flow) affected most periphyton related variables. This suggests that stream flow was low but enough to overshadow any potential effect of tadpoles on sediment accumulation, and hence any change in bioturbation when tadpoles disappeared, contrasting with other studies where sediment accrual was reduced by tadpole presence (Barnum et al, 2022 ; Mallory & Richardson, 2005 ; Ranvestel et al, 2004 ). These differences can be due to the different conditions of neotropical streams and montane temperate streams, since differences in flow and tadpole densities between riffles and pools within the same stream have can determined the effects of tadpole loss on bioturbation (Barnum et al, 2022 ).…”

“…Abundance and richness tended to be higher in presence of only caddisflies than in treatments with tadpoles, possibly due to the more efficient grazing activity of tadpoles compared to macroinvertebrates (Colón-Gaud et al, 2010). The trend of higher Chl-c concentration in presence of caddisflies alone suggests that tadpole presence mostly reduced the abundance of diatoms, for several reasons: (1) these algae are more easily removed by bioturbation (Barnum et al, 2022;Ranvestel et al, 2004);…”

“…Abundance and richness tended to be higher in presence of only caddisflies than in treatments with tadpoles, possibly due to the more efficient grazing activity of tadpoles compared to macroinvertebrates (Colón‐Gaud et al, 2010). The trend of higher Chl‐ c concentration in presence of caddisflies alone suggests that tadpole presence mostly reduced the abundance of diatoms, for several reasons: (1) these algae are more easily removed by bioturbation (Barnum et al, 2022; Ranvestel et al, 2004); (2) they are preferred over filamentous green algae (Kupferberg, 1997); and (3) due to their tendency to present a larger size than other algae such as green algae or cyanobacteria, they are better assimilated by tadpoles and less likely to be intact after passage though tadpole gut, and therefore, less prone to recolonize the grazed area (Connelly et al, 2008; Peterson & Boulton, 1999; Steinman, 1996). This view is supported by the differences in Shannon's diversity and the indicator taxa that we found in both treatments without tadpoles.…”

“…Although the loss of amphibian species from freshwater ecosystems can be expected to cause changes in ecosystem structure and functioning, this consequence has been little explored. Only a few studies have been conducted, mainly in tropical streams, where larval amphibian losses have been shown to cause changes in algal (Barnum et al, 2022 ; Connelly et al, 2008 ) and macroinvertebrate assemblages (Barnum et al, 2022 ; Colón‐Gaud et al, 2009 ; Ranvestel et al, 2004 ), with a resulting simplification of freshwater food webs (Schmidt et al, 2017 ), a reduction of primary production (Connelly et al, 2008 ), an increase of periphyton biomass accrual (Connelly et al, 2008 ; Mallory & Richardson, 2005 ; Ranvestel et al, 2004 ) and an alteration of nutrient cycling (Whiles et al, 2013 ). There are, however, very few relevant studies from freshwater ecosystems in temperate areas, with only one reporting that periphyton accrual was limited by amphibian presence and density (Mallory & Richardson, 2005 ).…”

Nonlinear dynamics, resilience, and regime shifts in aquatic communities and ecosystems: an overview

“…Compensatory replacement of species can sometimes maintain processes in the face of change. In tropical streams, Barnum et al (2022) find that loss of tadpoles due to disease leads to an incomplete replacement of one of their functional roles-grazing on benthic diatoms. Tadpole grazing effects were not completely replaceable by compensatory responses from insects, and shifts in grazing effects were not uniform across habitats.…”