Population growth correlates with increased fecundity in three-spined stickleback populations in a human-disturbed environment

Abstract

Human activity is altering the dynamics of populations through effects on fecundity, mortality and migration. An increased abundance of three-spined stickleback (Gasterosteus aculeatus) in the Baltic Sea has been attributed to a human-caused decline of top predators. However, recent research indicates that a top-down effect cannot fully explain the population growth, but the contribution of a bottom-up effect has not been investigated. Yet, anthropogenic eutrophication has increased algae biomass at the spawning sites of the stickleback and, thus, the abundance of benthic prey. We investigated if increased fecundity could have contributed to the population growth of the stickleback by analysing a two decade time series of stickleback abundance, fecundity, and body size at three spawning sites. The results show an increase in the proportion of gravid females in the populations, which correlates with the population growth. In particular, the proportion of gravid females late in the spawning season has increased, which indicates enhanced food intake at the sites during the spawning season. Thus, a bottom-up effect could have contributed to the growth of the populations by increasing the number of egg clutches females produce. These results stress the importance of considering both bottom-up and top-down processes when investigating the mechanisms behind human impact on population dynamics.

Peer Review #1 of "Mechanisms behind bottom-up effects: eutrophication increases fecundity by shortening the interspawning interval in stickleback (v0.1)"

“…This is plausible as the threespine stickleback spends the summer in shallow coastal waters where eutrophication has enhanced the growth of filamentous algae (Bäck et al 2000;Candolin 2004;Gubelit & Kovalchuk 2010;Kraufvelin et al 2006;Rinne et al 2018;Rinne et al 2011) and thereby the abundance of prey, such as gammarids and other grazers (Candolin et al 2016a;Olafsson et al 2013;, the preferred prey of the threespine stickleback in coastal waters (Candolin et al 2016a;Jakubaviciute et al 2017). In support of a bottom-up effect, the population growth of the stickleback in the Gulf of Finland correlates with increased population fecundity in terms of the proportion of gravid females in the population (Candolin & Voigt 2020). Females develop several sequential clutches of eggs during the breeding season and could profit from eutrophication in terms of increased food intake and, thus, fecundity.…”

“…Manuscript to be reviewed Candolin et al 2016b;Candolin & Voigt 2020;Olsson et al 2019). The increase has been attributed to a top-down effect from the decline of top-predators (Eriksson et al 2011;Ljunggren et al 2010).…”

“…After spawning, the female leaves the nest and the male alone cares for the eggs until hatching (Wootton 1984). Males do not benefit from increased food abundance in terms of improved body condition (Candolin & Voigt 2020), as they reduce feeding while engaged in nesting activities (Wootton 1984). Instead, males benefit from denser algal growth in term of shelter from predators and less interference from nest destroying and eggs stealing conspecifics (Candolin 2004;Candolin et al 2014;Candolin & Voigt 1998).…”

Mechanisms behind bottom-up effects: eutrophication increases fecundity by shortening the interspawning interval in stickleback

“…The abundance of the threespine stickleback, Gasterosteus aculeatus , has increased in the Gulf of Finland during the last decades, as well as in other parts of the Baltic Sea ( Bergström et al, 2015 ; Candolin, Tukiainen & Bertell, 2016b ; Candolin & Voigt, 2020 ; Olsson et al, 2019 ). The increase has been attributed to a top-down effect from the decline of top-predators ( Eriksson et al, 2011 ; Ljunggren et al, 2010 ).…”

“…This is plausible as the threespine stickleback spends the summer in shallow coastal waters where eutrophication has enhanced the growth of filamentous algae ( Bäck, Lehvo & Blomster, 2000 ; Candolin, 2004 ; Gubelit & Kovalchuk, 2010 ; Kraufvelin et al, 2006 ; Rinne et al, 2018 ; Rinne, Salovius-Lauren & Mattila, 2011 ) and thereby the abundance of prey, such as gammarids and other grazers ( Candolin, Johanson & Budria, 2016a ; Olafsson et al, 2013 ; Salovius & Kraufvelin, 2004 ), the preferred prey of the threespine stickleback in coastal waters ( Candolin, Johanson & Budria, 2016a ; Jakubaviciute et al, 2017 ). In support of a bottom-up effect, the population growth of the stickleback in the Gulf of Finland correlates with increased population fecundity in terms of the proportion of gravid females in the population ( Candolin & Voigt, 2020 ). Females develop several sequential clutches of eggs during the breeding season and could profit from eutrophication in terms of increased food intake and, thus, fecundity.…”