The significance of optical properties in competition among visual and tactile planktivores: a theoretical study

A mathematical model describing competition for a common food resource among visual and tactile planktivores is developed. Sensitivity of the competitive regime to environmental changes are studied by analysing their effects on foraging rates and niche availability of both planktivore types. Depth, spatial, daily and seasonal variations in factors of biotic and abiotic origin may result in great changes in the competitive regime. Optical properties of the water column is of greatest importance for competition. In addition the abundance and size of prey, small scale turbulence, and the seasonal and daily variations in sun irradiance are of importance. Inherent contrast of prey organisms is likely to modify the effect from the above factors under certain circumstances only, but may together with prey size, also enhance niche differentiation among the competitors. By studying the zero growth rate isoclines for populations of tactile and visual planktivores at different depths, we show that the water column is a habitat divided into different competitive regimes. These can be thought of as relatively stable niches separated by an ecotone of rather unpredictable and ever changing competitive regime, due to changes in environmental factors. In most cases it is optimal for visual planktivores to stay at shallow depths, while tactile planktivores compete better at greater depths. However, by simulations we show that high predation pressure can make near surface waters hostile to many visual planktivores such as fish, thereby making this highly productive niche available to tactile planktivores such as jellyfish. Results are discussed in relation to naturally occurring environmental variations and the impact of anthropogenic factors. Changes in optical properties due to eutrophication might have drastic effects on the outcome of competition, and thus on the structure of marine food chains. While too little is known about the physiology, life history and behaviour of most pelagic organisms to make specific predictions, this work indicates a potential for strong competitive interactions among different functional types of planktivores.