Does Trophic or Life-History Theory Predict Fish Biomass-Primary Production Relationships in the upper Gila River, NM, USA?

Authors: James E. Whitney, Keith B. Gido

Year: 2010 (xlii)

Abstract

Numerous trait classifications can be used to predict fish community responses to spatial heterogeneity in environmental conditions, including trophic guilds and life-history strategies. These trait-based approaches could be particularly useful in describing spatial variation in the abundances of native and non-native fishes in the upper Gila River of southwestern New Mexico, owing to the divergence in trophic classification and life-history strategy between natives and non-natives. Native fishes are generally herbivore-invertivores, and are representative of either periodic or opportunistic life history strategies, whereas non-native fishes are generally invertivore-piscivores, and are characteristic of the equilibrium life-history strategy. Ecological theory provides contrasting predictions of the response by trophic guilds and life-history strategies to variation in primary production. From trophic theory, it is predicted that the biomass of invertivore-piscivores (nonnatives) should increase along a gradient of primary productivity, while the biomass of herbivore-invertivores (natives) remains constant. Conversely, life-history theory predicts that the biomass of equilibrium strategists (nonnatives) should decrease with increasing primary productivity, while the biomass of periodic and opportunistic strategists (natives) increases. These predictions were tested with measurements of fish biomass and primary production across six longitudinally-positioned sites in the upper Gila River of southwestern New Mexico three times annually during 2008 and 2009. Simple linear regression was used to test the significance of relationships between trophic guild and life history strategy with primary production. Consistent with life history theory predictions, equilibrium strategist biomass demonstrated a marginally significant negative relationship with primary production in both 2008 (R²=0.60; df=5; p=0.07) and 2009 (R²=0.41; df=5; p=0.17), with opportunistic strategist biomass also demonstrating the predicted positive relationship with primary production in 2008 (R²=0.64; df=5; p=0.06) and 2009 (R²=0.69; df=5; p=0.04). The response of periodic strategist biomass to primary production did not match predictions either year. Consistent with predictions, no relationship was found between herbivore-invertivore biomass and primary production in either year. However, invertivore-piscivore biomass demonstrated the opposite pattern of prediction, showing a marginally significant negative relationship with increasing primary production in both 2008 (R²=0.60; df=5; p=0.07) and 2009 (R²=0.41; df=5; p=0.17). Application of this knowledge could aid in the conservation of native fishes and eradication of non-natives, through targeted removal efforts in areas where environmental conditions benefit the equilibrium life history strategy (i.e. low primary productivity). Biomass responses of trophic guilds and life history strategists to other environmental gradients (hydrologic variability) should be examined to gain further insight into the mechanisms governing native and non-native fish biomass.