Invasive Plant and Experimental Venue Affect Tadpole Performance

Abstract

Introductions of non-native predators and competitors appear to contribute to worldwide amphibian declines; however, potential negative impacts of invasive plants on habitat quality and amphibian populations have not been examined. Loss of diversity and alterations in ecosystem function associated with plant invasions may disrupt food webs, potentially leading to further declines of already threatened amphibian populations. We used a combination of small bins, mesocosms, and field experiments to examine the impacts of Eurasian purple loosestrife (Lythrum salicaria) replacing native cattails (Typha latifolia) in North American freshwater wetlands on survival, developmental rate, and diet (freshwater algae) of American toad (Bufo americanus) tadpoles. Tadpoles developed slower in L. salicaria compared to tadpoles developing in T. latifolia. This effect was consistent across experimental venues, although mesocosms showed this effect only in the second year of our study. Survival and development rates were always more variable in purple loosestrife than in cattail. In bins, tadpoles showed significantly reduced survival when raised in purple loosestrife extract and addition of leaf litter exacerbated this negative effect. Tadpole survival rates in mesocosms and field cages were not significantly different between plant species, most likely an effect of high variability among replicates. We suspect a combination of direct toxicity of high tannin concentrations in L. salicaria leaves and their indirect negative impacts on aquatic food webs are responsible for these results. Tadpole gut analyses revealed differences in algal communities among venues and between L. salicaria and T. latifolia suggesting that alterations in tadpole food quality and quantity contribute to the observed reduced tadpole performance. The replacement of native wetland plant species by L. salicaria does not represent a simple exchange of ecological equivalents and the function of invaded habitats for native species has clearly changed. While we were investigating only a single amphibian species, our results suggest that the impact of L. salicaria on ecosystem processes and aquatic food webs may be more general and likely to negatively affect other wetland species. The threats non-indigenous plants represent for amphibian populations and food webs may be underestimated, and warrant further investigation.

Stable isotopes reveal an invasive plant contributes more than native sources to anuran larvae diets

“…Such a choice may result from the tadpole's ability to recognize detritus that has higher nutritive value (Pfennig 2000). Nevertheless, purple loosestrife is known to decrease survival of larval anurans due to a high concentration of secondary compounds Brown et al 2006). Foraging theory states that organisms will make dietary choices that will increase their fitness, but frog species in this study chose to consume plant matter that is decreasing their survival rates suggesting other factors could be influencing their choices (Barrett et al Forthcoming;Pyke 1984;Milanovich et al 2016).…”

“…If tadpoles are choosing to consume purple loosestrife because of false environmental cues and their survival and fitness are decreasing due to that choice, then purple loosestrife invasions create putative ecological traps (Barrett et al Forthcoming;Battin 2004;Maerz et al 2005;Brown et al 2006;Milanovich et al 2016). An ecological trap can arise when an organism chooses a maladaptive habitat due to false environmental cues that leads to a decline in reproductive fitness and/or survival (Battin 2004;Robertson & Hutto 2006;Gilroy & Sutherland 2007).…”

“…Despite the fact that secondary compounds found in purple loosestrife lead to a lower survival rate relative to native detritus (Barrett et al Forthcoming;Maerz et al 2005;Brown et al 2006;Milanovich et al 2016), anuran larvae are choosing to consume the invasive plant. The preference for purple loosestrife we describe here may result from the higher nutrient levels found in the plant (Milanovich et al 2016), which promote faster times to metamorphosis and larger overall tadpoles (Milanovich et al 2016, Barrett et al Forthcoming).…”

“…Tadpoles need to consume a highly nutritious diet to prepare for and complete metamorphosis, which could be attracting tadpoles to choose purple loosestrife as part of their optimal diet (Kupferberg 1997). Foraging theory implies that tadpole fitness will increase when consuming optimal diets (Pyke 1984;Searle et al 2005); however, the consumption and presence of purple loosestrife has been shown to decrease tadpole survivorship (Barrett et al Forthcoming;Brown et al 2006;Cohen et al 2012;Milanovich et al 2016). Tadpoles exposed to higher concentrations of secondary compounds associated with purple loosestrife experienced changes to metamorph size and duration of the development period (Cohen et al 2012;Milanovich et al 2016).…”

Classical biological control for the protection of natural ecosystems

“…Loosestrife monocultures damage populations of specialist wetland plants, birds, amphibians, and insects (Blossey et al, 2001b;Brown et al, 2006;Maerz et al, 2005a;Schooler et al, 2009). In Oregon, purple loosestrife infestations have lowered plant and insect diversity in tufted hairgrass (Deschampsia cespitosa) communities (Schooler et al, 2006) and affected insect diversity in estuarine habitats important for juvenile salmon migration (Schooler et al, 2009).…”