Publication Date:
2023-01-17
Description:
This study explores life-history strategies of recent fishes based on a data set that is two orders of magnitude larger than those used in previous studies. Trophic level, size and
productivity were taken as key traits and proxies for many other closely related traits. Size was strongly correlated with most life-history traits of fishes and also with morphological characters, behaviour, and preferred environmental conditions. Size was also a good predictor of placement on the r-K continuum. Productivity was derived from growth, age at maturity, maximum age, and fecundity data. It was positively correlated with metabolism and level of activity and was also an indicator for placement of species on the r-K continuum. It was strongly correlated with most life-history traits of fishes and also with morphological characters, behaviour, and preferred environmental conditions. It was negatively correlated with status of threat. The position of species in the food web was shown to restrict life-history options. The addition of trophic level as an orthogonal axis on the r-K continuum revealed unoccupied regions such as the combination of small size and high productivity with either herbivory or top-predatory, and the combination of very large size and very low productivity with herbivory. Discrete classes of size, trophic level and productivity were used to define 80 life history strategies. Only 50 of these strategies were used by recent species, with an exponential decline in species’ numbers from the most to the least used strategies. This decline is interpreted as an exponential increase in constraints associated with less-used strategies. Analysis of trade-off or constraint curves in life-history space revealed unoccupied areas as well as local maxima, i.e., areas occupied by more species than the surrounding space. Such a local optimum was occupied by very large top predators with very low productivity. Low-level predators of small to medium size and medium to high productivity were the three strategies used by altogether 60% of the species. Strategies used in extreme environments such as the deep and polar seas or high-altitude lakes were not ‘specialist strategies’ but rather among the 10 most-used strategies, suggesting that constraints imposed by extreme environments excluded strategies that had a high degree of inherent constraints. The number of strategies used by phylogenetic, environmental, morphological or behavioural groupings of
fishes was highly predictable from the number of species in the respective groups. A preliminary chronology of life-history strategies showed that over 2/3 of recent strategies
were invented only 200-150 million years ago during several radiations of the Actinopterygii, including small size, very large size (invented in parallel by Elasmobranchii), high productivity, and true herbivory. Phylogeny restricted the life-history options available to species with respect to size, place in the food web and productivity. There was evidence for a non-overlap of preferred life-history strategies between the two largest recent Classes, with Elasmobranchii tending towards large size and low to very low productivity, and Actinopterygii tending towards medium size with medium to high productivity. Nine selection theories were tested as to their ability to correctly predict adaptation of life-history traits in response to environmental conditions such as salinity, climate, zoogeographic realm, ocean basin, and habitat type. Predictions were 88 – 100% correct when cases where different theories predicted different adaptations were excluded. In conflicting cases, predictions by temperature theory usually prevailed over those by r-K and succession theories. Life-history strategies were examined with respect to their correlations with body shape, brain size, reproductive guild, migratory behaviour and status of threat. Productivity increased with body shape from eel-like to short and/or deep, with brain size from very small to normal and large, and with migratory behaviour from catadromous to amphidromous and non-migratory. Size decreased with migratory behaviour from catadromous to non-migratory, and with parental care from nonguarders to bearers (in Actinopterygii). Trophic level decreased with increase in brain size. Several life-history strategies were only used by migratory species. Non-threatened fishes had significantly higher productivity than threatened fishes. Life history-strategies that combined large size and low productivity contained proportionally more threatened species than other strategies. Independent estimates of abundance and distributional range of species were used as indicators of success of life-history strategies. Species showed preferences for strategies that were associated with high abundance or small to medium ranges. When abundance and range were combined into a single measure of success (Impact), most strategies were associated with impacts that were not significantly different from the overall mean. Only medium-sized low-level predators and omnivores with high productivity had significantly higher impact; these two strategies were used by 39% of the species.
Type:
Thesis
,
NonPeerReviewed
Format:
text
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