The ecological role of caudal lamellae loss in the larval damselfly, Ischnura posita (Hagen) (Odonata: Zygoptera).

Damselfly larvae may autotomize and regenerate any of their 3 caudal lamellae. At least one missing or regenerating lamella was evident in 50.1% of field collected Ischnura posita larvae. Lamellae loss during molting is very infrequent (1 out of 117 recorded molts). Laboratory trials indicate that conspecifics remove lamellae and that this process is density dependent. The percentage of larvae losing lamellae during 24 h trials ranged from 73.5 at the highest density tested to 17.3 at the lowest density. I. posita larvae are cannibalistic. The presence of lamellae reduces an individual's chance of being cannibalized. More than twice as many final instar lamellae-less larvae were cannibalized during 24 h trials than analogous individuals having 3 lamellae at experimental initiation. Costs are also associated with lamellae autotomy. 1) Although individuals without lamellae can swim they are more reluctant to release from a wooden stalk and swim when threatened (9% release) than are larvae with lamellae (29% release). Since swimming is part of their repertoire of anti-predator behaviors this behavioral shift should be detrimental. 2) Caudal lamellae function in O2 uptake. Trials were conducted with larvae having and not having lamellae in an experimental horizontal oxygen gradient system. Relative to larvae without lamellae, those with lamellae preferred deeper depths at PO2 values greater than 70 torr. Many lamellae-less larvae distributed themselves at the water surface throughout the range of PO2 values tested. Differential depth distribution between larvae with and without lamellae is highly significant (P < 0.01).

The effect of predation on the structure and invasibility of assembled communities.

Fifty four microcosmic communities were assembled over 4 months from a 28-species source pool of phytoplankton using nine different invasion patterns each replicated six times. Three communities from each set of replicates then were invaded with a cladoceran that feeds on phytoplankton. All communities were then treated identically for an additional 4 months. In all nine invasion categories species richness was greater in predated communities. Predation opened communities to invasion by increasing the representation of infrequently sampled species at the expense of more common species. Invasion rate was four times more influential than predation and over eleven times more important than either invasion order or the timing pattern of interspecific arrivals in determining species richness in this system of communitites.

Ecological resistance to the invasion of a freshwater clam, Corbicula fluminea: fish predation effects.

Fish predation is shown to have a twenty nine fold effect on the abundance of the invasive freshwater clam, Corbicula fluminea, in a Texas reservoir. This predation has prevented the clam from establishing the high densities commonly reported for it elsewhere. The high magnitude of the fish effect is attributed to Corbicula being an invader to this reservoir and not being able to cope well with the mix of resident fish species. In the absence of fish, colonization of the reservoir by Corbicula is spatially patchy. When fish interact with these clams, they remove sufficient numbers of individuals from dense patches to create the appearance of a spatially uniform distribution.

Testing the invulnerability of laboratory island communities to invasion.

One hundred communities were developed through the controlled introductions of microorganisms to beakers over a six month period of time. Following three months of development, a series of three separate previously unencountered species were introduced into each community. The persistences of these invaders were monitored and their relationships to invaded community complexity, composition and history of development evaluated. The null hypothesis that colonization success is independent of community complexity cannot be rejected. The rate at which species were introduced during development of these communities, as well as beaker size, influenced the invasion success of Dictyosphaerium, but not Staurastrum or Platydorina. The assembled communities were of two types: those dominated by Ochromonas and those domnated by Paramecium bursaria. Ochromonas dominated communities were invulnerable to the invasion of the above 3 species; no clear pattern of invulnerability was evident for P. bursaria communities.

The effect of temporal environmental heterogeneity on community structure: a replicated experimental study.

Periodicity, predictability and stochasticity of environmental perturbations are shown to influence the community structure that develops in microcosms. Sets of replicate, microalgal communities were subjected to different temporal patterns of rarefaction and resource resupply and their species-abundance patterns after 120 days of such manipulations were determined. Perturbations having, 1, 7, and 28 day periodicities differentially effected community structure. The predictability of these perturbations had a less profound influence on the communities which developed than the average perturbation periodicity.