Interspecific competition, predation, and the coexistence of three closely related neotropical armoured catfishes (Siluriformes - Callichthyidae)

J.H.A. Mol

Research output: Thesisinternal PhD, WU

Abstract

<p>Tropical ecosystems are renowned for their high biodiversity with many closely related species living together. Alpha diversity of tropical freshwater fishes is also extremely high, as exemplified by the cichlid fauna of the Great African lakes and the neotropical characins. Since Hutchinson in 1959 posed his often quoted question: "Why are there so many animals?", factors affecting species diversity have been important subject of study and discussion in ecology. Given the limits of the physical environment, two different biological processes are probably responsible for most of the organization of equilibrial communities: competition and predation. Critics of the view that competition and predation play a major role in structuring communities argued that several factors keep populations below a level where competition or predation could be strong. Reviews of the literature on field experiments designed to demonstrate the influence of competition and predation revealed that only 9% of the studies were conducted in the tropics.<p>The tropical rainforest is one of the most stable environments in the world. Temperature and humidity are almost constant, but important fluctuations in the water level of streams and swamps occur. Extreme conditions limiting population densities and leading to the extinction of populations are often hard to imagine. The high number of species will lead to complex biological interactions. More studies on competition and predation in the stable environment of the tropical rainforest are necessary to evaluate the importance of both processes in structuring communities.<p>A case study is presented concerning three medium-sized armoured catfishes of the family Callichthyidae: <em>Callichthys callichthys, Hoplosternum littorale</em> and <em>Hoplosternum thoracatum.</em> In order to evaluate the influence of competition and predation on the catfish communities, four factors were investigated in detail: 1) geographical distribution, 2) diet overlap, 3) reproductive seasonality and nest-site differentiation, and 4) predation.<p>In Suriname (South America), both <em>C. callichthys</em> and <em>H. thoracatum</em> occur all over the country. Both catfishes are abundant in both coastal plain swamps with standing water and rainforest creeks in the interior with running water. <em>Hoplosternum littorale,</em> on the other hand, is restricted in its distribution to coastal plain swamps. Salinity tolerance experiments demonstrated that one-week old larvae of <em>H. littorale</em> could not survive in electrolyte-poor water (e.g. rain water). The 'clear water' of rainforest creeks in the interior may be compared to slightly contaminated distilled water with very low electric conductivity. Larvae of both <em>C. callichthys</em> and <em>H. thoracatum</em> survived and developed further in rain water. The pattern of distribution in Suriname can be considered as a model of the distribution of the three catfishes in South America. In South America, <em>H. littorale is</em> not only conspicuously absent from clear water streams draining the weathered Precambrium Guyana and Brazilian Shields, but also from 'black water' streams with humic acids and also a low conductivity (e.g. the Rio Negro). Within the Amazon drainage system <em>H. littorale</em> seems restricted to 'white water' streams loaded with Andean sediments and with a higher conductivity than both clear water and black water. In Suriname, actual syntopy of the three species occurs in coastal plain swamps. In rainforest creeks of the interior <em>C. callichthys</em> and <em>H. thoracatum</em> occur syntopically. Consequently interspecific competition among the three callichthyids is possible.<p>Analysis of the stomach contents of larvae, juveniles and adults of the three armoured catfishes revealed no significant interspecific differences in the diet of larvae, juveniles and adults. However, ontogenetic differences in diet composition among larvae, juveniles and adults where significant for all three catfishes. Larvae of <em>C</em> . <em>callichthys, H. littorale</em> and <em>H. thoracatum</em> fed almost exclusively on Rotifera, Cladocera and Copepoda. The stomach contents of juveniles were more diverse and included micro-crustacea, insect larvae, aquatic insects and some detritus. Adults ingested large quantities of detritus which probably reflected the inability of large fish to seperate effectively benthic invertebrates from substrate. Chironomid larvae where found in large numbers in the stomach of adult specimens. The short alimentary tract of the three catfishes and the structure of its wall make it improbable that these fishes are able to assimilate detrital nonprotein amino acids. The posterior part of the intestine is thin-walIed and has a respiratory function. The anterior digestive portion is relatively short. Morphologically the three callichthyids are adapted to a benthic way of life and a diet of soft-bodied aquatic invertebrates. The similarity in the diet and their bottomdwelling habit provide the basis for grouping the three armoured catfishes into one ecological guild. Competitive interactions are expected to be potentially strong among members of a guild.<p>Male <em>C. callichthys, H. littorale</em> and <em>H. thoracatum</em> construct and guard a floating bubble nest in flooded swamps. Although the habit of constructing a floating bubble nest probably evolved as an adaptation to the oxygen-depleted environment of tropical standing waters, some of the characteristics of the nests of <em>C.</em><em>callichthys</em> and <em>H. thoracatum</em> may have evolved as a response to the unpredictability of the fluctuations in water level and water velocity in rainforest creeks. The conspicuous nests allow the study of the temporal pattern of breeding, the spatial distribution of the nests and the differentiation of the nest-site microhabitat among the three species. Daily surveys in coastal plain swamps revealed a similar, bimodal breeding season in the three catfishes. Nests were observed in both the short and long rainy season. The bimodal breeding pattern in Suriname is probably related to the unreliability of the rainfall in the short rainy season of December-January. In French Guiana the rains of December-January usually do not fail and an unimodal breeding season of <em>H. littorale is</em> found. Significant differences in nest macrohabitat selection were found among the three callichthyids. Nests of <em>H. littorale</em> were built in herbaceous swamps, while <em>C</em> . <em>callichthys</em> and <em>H. thoracatum</em> nested in swamp forest under trees. The two forest nesting species differed in the microhabitat at the nest site. Nests of <em>C</em> . <em>callichthys</em> were constructed in extremely shallow water and in holes of tree roots and earth. Nest densities in the selected habitat were low in <em>C.</em><em>callichthys</em> and <em>H. littorale,</em> but relatively high in <em>H</em> . <em>thoracatum.</em> Nest-site differentiation in the rainy season (the main feeding period) may prevent competitive exclusion among the three catfishes.<p>The potential predation pressure on eggs, larvae and juveniles of <em>H. thoracatum</em> was estimated by combining laboratory predation rates of 24 predator species (both invertebrates and vertebrates) with the density of the predators in the swamp as determined by chemofishing with rotenone. The contribution of a particular predator species to the total predation pressure on <em>H.</em><em>thoracatum</em> was determined to a large extent by the density of the predator in the swamp. Seemingly innocuous predators with low or moderate predation rates in the laboratory may be extremely important in the swamp due to their abundance. Smallsized fishes and aquatic invertebrates are probably major predators of early developmental stages of <em>H. thoracatum.</em> The potential predation pressure on eggs is high, but the aggressive nest guarding behaviour of the male catfish and concealment of the nests probably protects the eggs effectively from most aquatic predators. Larvae are not guarded by the male and the potential impact of the 24 predators on the larvae of <em>H. thoracatum</em> is large. Although the experiments were performed with <em>H.</em> thoracatum there is no good reason to expect important differences in predation rates on larvae of the three armoured catfishes. Even if only 2.5 % of the potential predation will be realized due to other prey available, the high mortality of early developmental stages of <em>H. thoracatum</em> (and <em>C. callichthys</em> and <em>H. littorale</em> ) in the swamp could be easily explained as the effect of predation. Potential predation pressure and the number of predators that were able to prey on <em>H. thoracatum</em> sharply declined with increasing age (size) of the juvenile catfish. The heavy armour of bony plates and stout pectoral spines protect older juveniles and adults from most potential predators. The high potential predation pressure in the swamps and the low density of juvenile catfish at the end of the rainy season suggest that predation is important in structuring these catfish communities.<p>Although larvae of the three armoured catfishes <em>C. callichthys</em> , <em>H. littorale</em> and <em>H. thoracatum</em> show differences in tolerance to electrolyte-poor water, the three species occur together in coastal swamps. Interspecific diet overlap is very high, and the three catfishes show no differentiation in their breeding season and diel pattern of activity. Ontogenetic changes in the defense mechanisms of armoured catfishes result in a situation in which both predation and competition exert control at different times in the life cycle. Predators probably kill most of the larvae and juveniles, leaving only a few individuals to escape and reach adulthood. If the number of escapes would exceed the number of adult deaths, the populations eventually become sufficiently dense to compete. However, in this situation nesthabitat segregation may prevent competitive exclusion of one <em></em> of the three species <em>.</em>
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Osse, J.W.M., Promotor
Award date15 Sep 1995
Place of PublicationS.l.
Publisher
Print ISBNs9789054854388
Publication statusPublished - 1995

Keywords

  • animals
  • relationships
  • Siluridae
  • ecology
  • animal behaviour
  • habits
  • Neotropical Region

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