Dual influences of ecosystem size and disturbance on food chain length in streams

Abstract

The number of trophic transfers occurring between basal resources and top predators, food chain length (FCL), varies widely in the world's ecosystems for reasons that are poorly understood, particularly for stream ecosystems. Available evidence indicates that FCL is set by energetic constraints, environmental stochasticity, or ecosystem size effects, although no single explanation has yet accounted for FCL patterns in a broad sense. Further, whether environmental disturbance can influence FCL has been debated on both theoretical and empirical grounds for quite some time. Using data from sixteen South Island, New Zealand streams, we determined whether the so-called ecosystem size, disturbance, or resource availability hypotheses could account for FCL variation in high country fluvial environments. Stable isotope-based estimates of maximum trophic position ranged from 2.6 to 4.2 and averaged 3.5, a value on par with the global FCL average for streams. Model-selection results indicated that stream size and disturbance regime best explained across-site patterns in FCL, although resource availability was negatively correlated with our measure of disturbance; FCL approached its maximum in large, stable springs and was <3.5 trophic levels in small, fishless and/or disturbed streams. Community data indicate that size influenced FCL, primarily through its influence on local fish species richness (i.e., via trophic level additions and/or insertions), whereas disturbance did so via an effect on the relative availability of intermediate predators (i.e., predatory invertebrates) as prey for fishes. Overall, our results demonstrate that disturbance can have an important food web-structuring role in stream ecosystems, and further imply that pluralistic explanations are needed to fully understand the range of structural variation observed for real food webs.

Food webs: reconciling the structure and function of biodiversity

“…Food-chain length appears to be reduced by disturbance and increased by higher energy supply and increased ecosystem size [21][22][23]. Maximum chain length (max FCL) The maximum number of links found in any food chain in a food web [94].…”

“…Relationships between energy fluxes and biodiversity have been proposed in the past, notably that systems with larger amounts of energy entering the food web should be able to support longer food chains and hence more biodiversity [6,21], although there is the potential for interactions with ecosystem size and disturbance [22,23]. The power of integrating ecosystem functions into food-web studies is clearly illustrated by the studies that have related foodchain length to basal energy supply.…”

Habitat corridors alter relative trophic position of fire ants

“…Fragmentation may alter trophic structure via several mechanisms. Theory predicts a positive relationship between the number of trophic levels and habitat area (Schoener 1989, Holt 1993, Pimm 2002), a pattern supported by non-experimental, empirical studies (Vander Zanden et al 1999, Komonen et al 2000, Post et al 2000, Layman et al 2007a, Takimoto et al 2008, McHugh et al 2010. Although such studies provide insight on potential effects of habitat loss, confounding factors make it difficult to pinpoint the underlying mechanisms.…”

Resource availability determines food chain length in Chinese subtropical rivers

“…Although productivity generally increases FCL (Thompson and Townsend 2005;Hoeinghaus et al 2008;Doi 2012), the effects of the magnitude of ecosystem size could positively (Post et al 2000;Takimoto et al 2008;Doi et al 2009;McHugh et al 2010;Sabo et al 2010) or neutrally (Thompson and Townsend 2005;Warfe et al 2013) influence FCL. Furthermore, the effects of disturbance on FCL are highly variable and could be positive (Calizza et al 2012), neutral (Walters and Post 2008;Warfe et al 2013), or even negative (McHugh et al 2010;Sabo et al 2010).…”