Comparison of two population‐level ecotoxicological endpoints: The intrinsic (r_m) and instantaneous (r_i) rates of increase

Abstract

The instantaneous rate of increase (ri) was compared to the intrinsic rate of increase (rm) as an alternative population‐level ecotoxicological endpoint. The terrestrial arthropod Acyrthosiphon pisum Harris, the pea aphid, was used as the model species and the new nicotinergic insecticide, imidacloprid, as the xenobiotic. In the first experiment, rm was generated each day from a life table for A. pisum neonates exposed to nontreated (control) potted broad bean, Vicia faba L. (variety Banner), and compared to determine the earliest rm which was not significantly different from the final rm. The rm for neonates introduced to nontreated plants was significantly different on days 6 and 7 but not significantly different between 8 to 40 d postintroduction. A second experiment examined the change in ri over time for neonates introduced at birth to nontreated broad bean and censused at 8, 11, 13, 15, 17, 19, and 21 d postintroduction. The ri for neonate populations introduced to nontreated broad bean peaked on days 11 and 19. The ri on nontreated beans was significantly different (p < 0.05) from rm on days 8 and 15 but not significantly different at 11, 13, 17, 19, and 21 d. A third experiment was conducted comparing rm and ri following 11 d exposure to a range of imidacloprid concentrations applied to broad bean. There was a high correlation (r = 0.91, n = 21, p < 0.01) between ri and rm 11 d after the start of the study, suggesting that ri can be used as a predictive measure in place of rm. There was a negative relationship, y = 0.32 – 0.43x (r2 = 0.88, n = 21), between ri and exposure to imidacloprid‐treated broad bean, suggesting that simple regressions may be utilized as tools in the assessment of xenobiotics on population growth rates.

Transgenerational Shifts in Reproduction Hormesis in Green Peach Aphid Exposed to Low Concentrations of Imidacloprid

“…For each potted plant, the instantaneous rate of population increase ( r i ) was determined as:where N t was the final number of aphids per plant, N 0 was the initial number of aphids introduced and t was the number of elapsed days during the experiment [30], [31]. Calculated r i values for each test plant were subjected to mixed model analysis of variance using Proc Mixed followed by a LSD mean separation test (α = 0.05) [29].…”

Importance of Population Structure at the Time of Toxicant Exposure

“…A measure of population growth called the instantaneous rate of increase (r G ) was used as a toxicological endpoint (Walthall and Stark 1997a). r G is a measure of population increase and is calculated with the equation…”

Insecticide-Mediated Shift in Ecological Dominance between Two Competing Species of Grain Beetles

“…The adult insects were released in each jar, and after 90 days the number of dead and live insects were recorded, as well as the average (adult) insect body mass (in samples of 20 insects per replicated jar) and grain consumption. The instantaneous rate of population growth (r i ), a robust surrogate estimator of the intrinsic rate of population growth (r m ) [19], [36], was calculated using the formula r i  = [Ln( N j / N i )]/Δt, where N j and N i are the final and initial number of live insects (in each jar), respectively, and Δt is the duration of the experiment (i.e., 90 days).…”