Colonization Results

What is Colonizable Habitat? In Cooloola National Park, many areas of rainforest appeared to have been destroyed in a wildfire in 1951.  While some areas regenerated into rainforest, others did not because of continual burning by loggers and, more recently, National Parks staff.  In the 1980's the Parks staff increased the interval between controlled burns and some areas now appear to be at the beginnings of the process of converting to rainforest.   This is where I carried out my work on colonization of vacant habitat.

A controlled burn moving through an area of open woodland.  This area had some rainforest seedlings present but was further from the rainforest edge than some regenerating areas.  This fire dampened off and died as it reached areas with a more advanced community of rainforest seedlings and saplings.

 

Graph showing the proportion of defecated rainforest seeds that were deposited in regenerating areas rather than rainforest.  Small numbers in blue indicate the sample size in number of seeds defecated per month.  While there was no data from February-March, all other evidence suggests that birds were facilitating colonization only between April and July.

Seasonality of Dispersal Radio-tracking of Wompoo and Rose-crowned Fruit-doves and Green Catbirds showed conclusively that these individuals were moving into regenerating habitat some of the time.  Surprisingly however, this movement was very seasonal with movements from rainforest into vacant/regenerating habitat restricted to April-July.

Diversity of Dispersal These frugivorous birds were dispersing a diversity of rainforest species to regenerating habitat, but nowhere near the diversity of rainforest species present.  Clearly birds were facilitating colonization for a limited subset of rainforest species particularly those that fruited between April and July.

Graph showing the number of seeds dispersed per month by species.  Only 9 species of rainforest seed were dispersed to regenerating habitat despite the fact that I observed these bird species feeding on over 30 species of rainforest fruit.

Does Dispersal Equal Colonization?

It is possible that having birds carry seeds to vacant habitat does not actually result in any successful colonization, or that rainforest species don't need birds to improve chances of colonization (ie. they get there without birds).  I tested this premise by looking at the seedling community in regenerating habitat.  The rainforest species that were present were disproportionately those that were dispersed by birds and which fruited between April and July.

Graph showing the numbers of seedlings surveyed in regenerating forest and the month in which they were most likely deposited.  I calculated this by looking at the fruiting phenology of each plant species.  For example, if I knew that one species fruited every year only in July, then I could assume that seedlings of that species must have germinated from seeds that were dispersed in July.

So why were birds restricting their movements into regenerating habitat between April and July resulting in a developing forest community of April to July fruiters?  The answer comes from the composition of what is already present in regenerating habitat. 

Fruit of Endiandra sieberi cut in half.  The purple-shaded tissue is the large endosperm of the seed while the thin green layer outside of it is the fat-rich pulp the birds are seeking.  Note the large size of the fruit.  I measured fruit that were up to 2.2 cm in width and 3.1 cm in length.

After rainforest in these areas was destroyed in the 1951 wildfire, a new habitat type that was more fire-tolerant replaced it.  While this habitat was dominated by Eucalyptus species and other trees with wind-dispersed seeds there was also one fleshy-fruited species that became part of the new open woodland: Endiandra sieberi.  Endiandra sieberi is a laurel (Lauraceae) that has thick bark to protect it from fire and that is found almost exclusively in open woodland habitat in Cooloola.  This species is almost never found within shady rainforest. 

However, like the many species of laurels that are found in the rainforest (Beilschmeida spp., Endiandra discolor, Litsea spp.) Endiandra sieberi produces large, fat-rich fruits that are extremely attractive to frugivores.  Birds in my study were leaving the rainforest between April to July because this is the time of the year when Endiandra sieberi fruits and they were traveling outside their normal habitat to harvest its fruit crops.  They also didn't just 'move in' to regenerating forest while Endiandra sieberi was fruiting.  Because most frugivores feed on multiple fruit species every day (presumably for nutritional reasons) and perhaps because rainforest provides better cover from predators, birds would travel back and forth between rainforest and regenerating habitat throughout the day.  In so doing, they carried many rainforest seeds with them.