Predator Presence and Vegetation Density Affect Capture Rates and Detectability of Litoria aurea Tadpoles: Wide-Ranging Implications for a Common Survey Technique

Abstract

Trapping is a common sampling technique used to estimate fundamental population metrics of animal species such as abundance, survival and distribution. However, capture success for any trapping method can be heavily influenced by individuals’ behavioural plasticity, which in turn affects the accuracy of any population estimates derived from the data. Funnel trapping is one of the most common methods for sampling aquatic vertebrates, although, apart from fish studies, almost nothing is known about the effects of behavioural plasticity on trapping success. We used a full factorial experiment to investigate the effects that two common environmental parameters (predator presence and vegetation density) have on the trapping success of tadpoles. We estimated that the odds of tadpoles being captured in traps was 4.3 times higher when predators were absent compared to present and 2.1 times higher when vegetation density was high compared to low, using odds ratios based on fitted model means. The odds of tadpoles being detected in traps were also 2.9 times higher in predator-free environments. These results indicate that common environmental factors can trigger behavioural plasticity in tadpoles that biases trapping success. We issue a warning to researchers and surveyors that trapping biases may be commonplace when conducting surveys such as these, and urge caution in interpreting data without consideration of important environmental factors present in the study system. Left unconsidered, trapping biases in capture success have the potential to lead to incorrect interpretations of data sets, and misdirection of limited resources for managing species.

Removal of an exotic fish influences amphibian breeding site selection

“…We found that draining achieves this goal by increasing tadpole abundance in fish‐free ponds to over 140 times that of undrained ponds. However, a small amount of this increase may be attributed to tadpole detectability, which is increased by 4.3 times if gambusia are absent compared to present, using the method of minnow trapping (Sanders et al ). Even in cases where gambusia recolonized drained ponds prior to the beginning of the breeding season, on average the abundance of tadpoles in these ponds was still increased 7‐fold.…”

Community level impacts of invasive mosquitofish may exacerbate the impact to a threatened amphibian

“…Sanders et al (2015) compared the trapability of L. aurea tadpoles in mesocosms with and without G. holbrooki and found that tadpoles were over four times more likely to be captured when G. holbrooki was absent. Sanders et al (2015) compared the trapability of L. aurea tadpoles in mesocosms with and without G. holbrooki and found that tadpoles were over four times more likely to be captured when G. holbrooki was absent.…”

“…Furthermore, L. aurea's breeding season overlaps with peak levels of G. holbrooki abundance (Pen & Potter 1991;Pyke 2005), reinforcing the importance for a proportion of ponds to remain fishfree. Sanders et al (2015) showed that L. aurea tadpole behaviour is altered in the presence of G. holbrooki, and many prey species exhibit a generalist response to functionally similar predators (Ferrari et al 2009). Previous studies that consider the conservation of L. aurea suggest that construction of habitat mosaics provides a suitable strategy to deal with breeding habitat preference and the avoidance of G. holbrooki (Hamer & Mahony 2007;Mahony et al 2013).…”

“…The low number of L. aurea tadpoles captured during our three sampling seasons suggests that either natural L. aurea recruitment or detection probability of their tadpoles is low. Sanders et al (2015) compared the trapability of L. aurea tadpoles in mesocosms with and without G. holbrooki and found that tadpoles were over four times more likely to be captured when G. holbrooki was absent. This pattern is likely due to predator avoidance behaviours of tadpoles, such as reduced activity or increased refuge seeking activity (Feminella & Hawkins 1994;Lima 1998;Relyea & Werner 1999).…”

“…The apparent failure of adult L. aurea to avoid G. holbrooki on Kooragang does not necessarily mean tadpoles fail to exhibit anti-predator responses. Sanders et al (2015) showed that L. aurea tadpole behaviour is altered in the presence of G. holbrooki, and many prey species exhibit a generalist response to functionally similar predators (Ferrari et al 2009). Seeking shelter in the presence of a predator is a key strategy for prey survival (Lima 1998), and certain microhabitats likely provide different levels of shelter.…”