Cactus

Newspaper article in 'Trouw' on January 3, 2020.

Natuurbeschermers op Bonaire zijn bezorgd over een plan om 1500 woningen op een voormalige plantage te bouwen. Het gebied is begroeid met kwetsbaar tropisch bos. De projectontwikkelaar begrijpt de ophef niet

Exotic herbivores are able to disrupt entire ecosystems by competing with native species and feeding on native species. Especially, on islands without natural herbivores, the impact on native plant species can be strong. This has happened in Bonaire where goats have been brought by Spanish settlers centuries ago. Nowadays, the ecosystem of Bonaire is arid which is largely due to the timber industry and a history of keeping goats. Even though the timber industry has disappeared from
Bonaire, goats still roam around the entire island and are still part of the Bonairian way of life. In Washington Slagbaai National Park, feral goats are remnants of the previous use of the park. These goats have grown to large numbers and hence have a large impact on the vegetation of the park, which include the three columnar cacti species; Pilocereus repandus, Stenocereus griseus and Pilosocereus lanuginosus. These species define the landscape of the park. More importantly, these
columnar cacti are critical for various endemic frugivorous and nectarivorous birds and bats that depend on it for food and nesting opportunities. However, as the goats have continued to increase in numbers, they have started to forage on the bark of the columnar cacti. This foraging behavior is threatening the columnar cacti and indirectly the birds and bats that depend on it.
This study investigated the effect of goats on the columnar cacti population. This was done by collecting data on distribution, damage and abundance of columnar cacti in both Washington- Slagbaai National Park and Klein Bonaire. As I expected that facilitation by microsites could prove important in an arid ecosystem with grazers, microsites and abiotic amelioration by microsites was studied as well. As expected, goats impacted the columnar cacti populations in a negative way. Using Klein Bonaire as a control area where no goats have been roaming for almost 50 years, I found significant differences when comparing columnar cacti populations with Washington-Slagbaai National Park.
The population of P. repandus in WSNP is unhealthy with only 27.7% of the population being juvenile in WSNP-Limestone. In Klein Bonaire, the population is a lot healthier with 75.2% of the population being juvenile. The WSNP population of S. griseus is healthy and most common with nearly 9000individuals documented and 86% being juvenile and 95% being juvenile in  Klein Bonaire. P. lanuginosus is the least abundant columnar cacti species in WSNP and is absent in Klein Bonaire. However, this is likely to be due to environmental stressors instead of herbivore pressure as P. lanuginosus is regenerating well with over 89% of the WSNP population being juvenile.
A significant difference on P. repandus also exists in the amount of damage between Klein Bonaire, where a mean damage of 4% was found, and Washington-Slagbaai National Park where the mean damage on P. repandus was 24%. For S. griseus, a mean damage of 19% was found in WSNP and a mean damage of 7% was documented in Klein Bonaire. However, this difference was not significant due to the small sample size. This was also the case with the proportion of dead adult S. griseus
which was 8.8% in WSNP and 0% in Klein Bonaire. As for P. repandus, the difference was significant with 10.2% of the adult P. repandus population being dead against only 1.9% in Klein Bonaire. These observations are likely to be attributed to foraging by goats as they seem to prefer P. repandus above the other two cactus species judging from the relatively high amount of trunk damage for P.
repandus (3.9% against 1% and 1.8% for S. griseus and P. lanuginosus respectively). As a result, P. repandus seems to be in direct danger, which is concerning as this columnar cactus is favored by birds, bats and people. Although the other cacti species are likely to be targeted by goats once P. repandus’ population has declined even more.
Furthermore, results show that increasing damage leads to a decrease fruit production, which links the goats indirectly with the native bats and birds which depend on the fruit in the dry season. The results on microsites of this study are in line with the theory on plant facilitation, as the more herbivory-vulnerable species P. repandus was facilitated  ignificantly more often than S. griseus, which is a more herbivory-resistant species. All these results add up to conclude that the exotic goat has been affecting Bonaire’s columnar cacti in a way which has been detrimental to its native flora as well as its fauna. Therefore, stringent measures need to be taken to eradicate goats from Washington-Slagbaai National Park.

Two sympatric species of columnar cacti on Curaçao, Netherlands Antilles, share bat pollinators, overlap in flowering phenology, and are floral homologues. Leptonycteris curasoae curasoae and Glossophaga longirostris elongata (Chiroptera: Glossophaginae) visit flowers of Stenocereus (syn. Ritterocereus) griseus and Subpilocereus (syn. Cereus) repandus on the same nights; these visits may promote interference competition between cactus species. I studied the effect of heterospecific (mixed) pollen loads on fruit and seed set, fruit and seed size and mass, germinated seeds at 5 weeks, and seedling survival at 7 months in relation to hand pollination with intraspecific pollen and natural pollination. Hand pollination seemed to limit pollen loads available for pollination. Under these conditions, natural pollination tended to produce the most fruits and seeds, and the largest fruits (but lightest seeds); mixed pollination was the least effective treatment (fruit set was significantly greater for St. griseus in natural than mixed pollination; fruit volume was greater in natural than mixed pollination for S. repandus, and seed number and fruit mass were larger in intraspecific than in mixed pollination). Aspects of natural pollination, possibly repeated visits, compensated for the negative impact of interspecific pollen loads under pollen limitation, with positive impacts on the carrying capacity of cacti for frugivores. Seed mass from natural pollination was negatively correlated with seed number only for St. griseus. Germination success was not correlated with seed mass, but seedling survival at 7 months was for S. repandus. The two species do not seem to compete through pollen interference when pollinator visits are relatively frequent.

The phenological response by three sympatric species of columnar cacti of the semi-arid island of Curaçao (Stenocereus griseus, Subpilocereus repandus, and Pilosocereus lanuginosus) to recent rainfall was investigated during 21 months. Rainfall is the climatic cue most likely to affect bud formation on Curaçao, and each species of cactus responded differently to it. Pilosocereus lanuginosus started budding immediately after rainfall, whereas Stenocereus griseus responded negatively with abortions and cessation of bud production within 2–3 weeks after rain. Subpilocereus repandus showed no response to rain within one month. Despite a long period of temporal overlap (81% overlap) between the budding/flowering activity of Subpilocereus repandus and that of Stenocereus griseus, buds and/or flowers on Stenocereus griseus appeared more than a month earlier than on Subpilocereus repandus.
The effect of plant size on phenology and how anthropogenic disturbance may affect cactus resource availability to nectar-feeders and frugivores were also examined. The larger the individual tree within a species, the more flowers were produced and the earlier the tree started to flower. Thus, the indiscriminate removal of columnar cacti for urban development can drastically affect the timing and availability of resources to threatened pollinators and other nectar-feeders, as well as to frugivores and omnivores.

Two species of columnar cacti, Subpilocereus repandus and Stenocereus griseus, are pollinated on Curacao by two species of glossophagine bats, Leptonycteris curasoae and Glossophaga longirostris (Phyllostomidae). The pollination effectiveness of the two bat species can influence the evolution of this mutualism as well as the immediate availability of resources to frugivores and omnivores. I examined the effectiveness of single-visits by L, curasoae and G. longirostris on fruit-set, seed number, and fruit size for each cactus species. Single visits of L. curasoae produced higher fruit-set and seed number in Subpilocereus repandus than did single visits of G, longirostris, but the differences were not statistically significant, possibly as a result of the small size of the L, curasoae sample. The reverse trends were observed for Stenocereus griseus, Pollination of Subpilocereus repandus by L, curasoae resulted in significantly longer fruits than did pollination by G. longirostris, During the peak of the flowering season, flowers received many visits per night. Fruit size (length, width, total mass, pulp mass) was positively correlated with the number of seeds per fruit. These results indicate that the species of bats visiting cactus flowers, as well as the number of visits to flowers, may affect pollination success, and consequently may affect the carrying capacity of the environment in terms of fruit resources for animals that feed on cactus fruits.
 
The effectiveness of two bat species as pollinators of two species of columnar cacti on Curacao.
Available from: https://www.researchgate.net/publication/239042041_The_effectiveness_of_...
[accessed Apr 16, 2015]